Enantioselective photochemical reactions within the confined cavities of supramolecular assemblies
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Abstract
Compared to bulk solvents, reactions in the confined spaces of supramolecular self-assemblies feature rate acceleration, high efficiency and substrate selectivity. These advantages lead to efficient catalytic efficiency and excellent selectivity in enantioselective supramolecular photochemical transformations. During the last few years, enantioselective supramolecular photocatalysis has developed into one of the most powerful strategies to construct enantioenriched chiral compounds. In this review, the recent advances of enantioselective photochemical reactions taking place within the confined spaces of supramolecular assemblies are summarized, with an emphasis on the specific catalytic modes and chemical transformations. Organization of the data follows a subdivision according to supramolecular host and reaction type. At last, the current limitations and the future research orientation of this research field are discussed.
Keywords
INTRODUCTION
Enantioselective photochemical reaction represents a powerful and intriguing protocol to construct enantioenriched chiral compounds[1-3]. In this regard, photochemical reactions take place in a chiral environment. Generally, there are two ways to realize enantioselective photochemical reactions. One is using a chiral photosensitizer (template) which can simultaneously initiate the photochemical reaction and induce the chirality of the terminal product. The other is a synergistic strategy where a photochemical process is mediated by an achiral photosensitizer and chiral transfer is controlled by another chiral catalyst (template). Nevertheless, some inherent challenges remain in the stereo-control of enantioselective photochemical reactions. For instance, the short lifetime and weak intermolecular interactions in electronically excited states complicate chiral transfer between a chiral template (catalyst) and substrate. Meanwhile, organic molecules in excited states have relatively high reactivity, leading to undesired transformations without interacting with the chiral template (catalyst).
Recently, supramolecular catalysis has generated increasing interest, whereby catalytic reactions can occur inside confined cavities - supramolecular containers and assemblies (hosts)[4-13]. In 2010, Meeuwissen et al. highlighted that the implementation of supramolecular assemblies into transition-metal catalysis and organocatalysis exhibits promising catalytic potential beyond enzyme mimics[14]. In contrast to bulk solvents, reactions in nanovessels feature (1) rate acceleration; (2) high efficiency; (3) substrate selectivity, which can be controlled at the microscopic level, as a consequence of the recognition and energetic stabilization of substrates (guests) and transition states within well-confined cavities[15-20].
Supramolecular hosts assemble from multiple components via weak intermolecular interactions (e.g., van der Waals, hydrogen bonding, metal ion coordination, π-stacking and hydrophobic interactions), which can serve as well-organized tunable catalytic systems[21]. Structurally, the host exhibits a certain degree of skeletal rigidity, and the inner lipid-soluble cavities can accommodate organic guests. This advantage creates a unique environment for the subsequent photochemically chiral transfer, avoiding the unfavorable impacts of external factors, i.e., solvent polarity, temperature and acidity/basicity, on the asymmetric induction. Supramolecular host-guest interactions can alter photophysical properties of guest molecules through the combined non-covalent forces and steric constraints. The non-covalent host and guest interactions in the ground state will complement the transient and weak excited-state interactions[22]. The confinement of chiral supramolecular hosts plays a key role in pre-organizing the substrate by the manipulation of the orientation and conformation/configuration of the guest and inducing the process of enantio-differentiation and photochirogenesis[23]. In light of these advantages, the rational design of novel supramolecular catalysts or platforms and studies of their applications in the asymmetric photocatalysis are a topic of great interest across academia and industry.
Over the past two decades, a plethora of supramolecular hosts have been developed to initiate the enantioselective photochemical reactions, including native and functionalized macrocycles, metal-organic cages (MOCs), metal-organic frameworks (MOFs), and covalent organic frameworks (COFs). Asymmetric photocatalytic transformations, such as photodimerization, photocyclization, photoisomerization, photocycloaddition, functionalization, and photooxidation reactions, have been well established using these supramolecular hosts. Table 1 summarized the hosts and enantioselective photoreactions discussed in this review. Reactions mediated by biomacromolecules (e.g., serum albumin and nucleic acids)[24,25] and liquid crystals[26,27] will not be discussed here. In this context, the review will provide a comprehensive summary of the recent advances in enantioselective photochemical reactions within the confined cavities of supramolecular assemblies, with an emphasis on the specific catalytic modes and types of chemical transformations. Due to different structural and physiochemical properties, these supramolecular hosts have diverse functions in enantioselective photocatalysis. In order to help the readers understand the unique advantages of supramolecular strategy in photochirogenesis, we will try to elucidate the specific supramolecular effects of the hosts exerted on the reactions. Organization of the data follows a subdivision according to supramolecular host and reaction types. The current challenges and potential research orientations of this research field will also be discussed.
Summary of the hosts and enantioselective photoreactions discussed in this review
| Host type | Cavity | Reaction type | Guest category | Key role of cavity | Stereochemical outcome [ref.] |
| Macro-cycles | Native CDs | Photodimerization Photocyclization | ACs; naphthalenes Cyclic ketenes | Pre-organization Rigid environment | See Table 2 14: 33% ee[45]; 16: 59% ee[46]; 20: 45% ee[48] |
| Modified CDs | Photodimerization Photocyclization | ACs Adamantyl acetophenone | Pre-organization Chiral auxiliary | See Table 2 cis-18: 20% ee; trans-18: 9% ee[47] | |
| OA | Photodimerization | ACs | Pre-organization | 2: up to 67% ee; 3: up to 56% ee[41] | |
| Photosensitizing hosts | Photoisomerization Photoaddition | Cyclooctene; cyclooctadiene Diphenylpropene | Activate substrate Activate substrate | Refer to[44] 22: 26% ee[50] | |
| MOCs | [Pd6(RuL3)8]28+ | Photodimerization | 2-Naphthol | Stabilize product | 25: up to 58% ee[56] |
| (MOC-1) | Unsymmetrical acenaphthylene | Energy transfer | 27: up to 88% ee[57] | ||
| Photocycloaddition | Active olefins | Heteromolecular binding | 33: up to 99% ee[61] | ||
| [Pd6L4]12+ (MOC-2) | Photocycloaddition | Fluoranthene and maleimide | Pre-organization | 30: 50% ee[59] | |
| MOFs | MOF-3 (M = K) | Photodimerization | ACs | Multivalent interactions | 47: up to 79% ee[65] |
| MOF-4 (M = Zn) | α-alkylation of aldehydes | Saturated aldehyde | Electron transfer and stereo-induction | 52: up to 92% ee[66] | |
| MOF-5 (M = Zn) | β-arylation of aldehydes | Saturated aldehyde | Electron transfer and stereo-induction | 58: up to 52% ee[67] | |
| MOF-6s (M = Zn, Zr, Ti) | α-alkylation of aldehydes | Saturated aldehyde | Electron transfer and stereo-induction | 52: up to 87% ee[68] | |
| MOF-7,8,9,10 | See Table 3 | ||||
| COFs | COF-11, COF-12 (with chiral additive) | α-alkylation of aldehydes | Saturated aldehyde | Electron transfer and stereo-induction (exogenous) | 65: up to 94% ee[79] |
| COF-13, COF-14 (with chiral additive) | α-alkylation of aldehydes | Saturated aldehyde | Electron transfer and stereo-induction (exogenous) | 73: up to 94% ee[80] | |
| COF-15 (chiral COF) | α-benzylation of aldehydes | Saturated aldehyde | Photothermal conversion and stereo-induction | 65: up to 94% ee[82] | |
| COF-16s (chiral COF) | α-alkylation of aldehydes | Saturated aldehyde | Electron transfer and stereo-induction | 65: up to 93% ee[83] | |
| COF-17 (chiral COF) | Photooxidation | Methylphenylsulfide | Energy transfer and stereo-induction and phase transfer | 83: up to 99% ee[84] | |
| COF-18 (chiral COF) | Henry reaction | Benzylic alcohol and nitromethane | Photothermal conversion and stereo-induction | 86: up to 98% ee[85] | |
| COF-19 (chiral COF) | A3-coupling | Benzaldehyde and aromatic alkyne and secondary amine | Photothermal conversion and stereo-induction | 90: up to 98% ee[85] | |
| COF-20 (chiral COF) | Strecker reaction | Benzaldehyde and trimethylsilyl cyanide and secondary amine | Photothermal conversion and Lewis acid catalysis and stereo-induction | 94: 94% ee[86] | |
| COF-21 (with enzyme) | Oxidative Mannich reaction | 2-Arylindole | Energy transfer and stereo-induction (exogenous) | 96: 86% ee[87] |
ENANTIOSELECTIVE PHOTOREACTIONS WITHIN MACROCYCLES
Macrocycles, especially composed of repeated glucose units, are the earliest supramolecular hosts to be used for asymmetric photoreactions. This kind of macrocycle features visible-light transparency, flexible modification and chiral cavities. In this section, enantioselective photodimerization, photoisomerization and several other types of photochemical transformations within macrocyclic hosts will be discussed.
Photodimerization within macrocyclic hosts
The selective [4 + 4] photodimerization of arenes within macrocycles is a long-standing research topic in supramolecular photochemistry. In this context, cyclodextrins (CDs) are always introduced as the chiral hosts. Anthracenes (ACs) are the most selected substrates to evaluate the photochirogenic ability and catalytic performance of a chiral supramolecular macrocycle[28]. Generally, the irradiation of 2-anthracenecarboxylate will afford four configurational isomers [Scheme 1], among which syn-9,10:9’10’-head-to-tail dimer 2 (syn-HT) and anti-9,10:9’10’-head-to-head dimer 3 (anti-HH) are chiral, while anti-9,10:9’10’-head-to-tail (anti-HT) and syn-9,10:9’10’-head-to-head (syn-HH) cyclodimers 1 and 4 are achiral. Studies have also shown that the chiral slipped cyclodimers 5 and 6 (anti-5,8:9’,10’-HT, syn-5,8:9’,10’-HT) can also be generated[29]. In this part, the related work in this field will be presented in chronological order.
Scheme 1. The products of [4 + 4] photodimerization of 2-anthracenecarboxylate.
Native CDs and modified CDs
As shown in Scheme 2, CDs are cyclic oligosaccharides containing D-glucose units linked via α-1,4 glycosidic bonds. Three kinds of native CDs, α-CD (n = 6), β-CD (n = 7), and γ-CD (n = 8), are formed with different numbers of glucose units. CDs bear two portals after assembling into a truncated funnel: a wider portal containing secondary hydroxyl groups and a narrow one comprising primary hydroxyl groups. Guest molecules will enter the hydrophobic cavity of the host upon complexation, while polar or charged functionalities reside outside. CDs feature chiral cavities, ready availability, good solubility in water as well as transparence in ultraviolet-visible (UV-vis) regions. The chiral confinement of CDs leads to a hydrophobic environment, substrate proximity, increased local concentration, and substrate pre-organization, which have been widely used to promote the reactivity and manipulate the selectivity[30]. Accordingly, CDs and modified CDs have become the foremost hosts for asymmetric [4 + 4] photodimerization of AC derivatives [Scheme 3]. For a clear comparison, the stereochemical outcomes of photodimerization of AC within native and modified CDs have been summarized in Table 2.
Scheme 2. The chemical structures of native CDs (top) and stereo-induction in the photodimerization of 2-ACs within CDs (bottom). CDs: Cyclodextrins; ACs: anthracenes.
Scheme 3. Representative modified CDs employed in enantioselective photodimerization of 2-ACs. CDs: Cyclodextrins; ACs: anthracenes.
Stereochemical outcomes of photodimerization of 2-ACs within native and modified CDs in Scheme 3
To achieve excellent stereo-control, rigid and flexible caps or cationic side arms were always installed into
In 2008, Yang et al. synthesized α-CD-appended AC to investigate the stereodifferentiating photodimerization within γ-CD (CD-3) and cucurbit[8]uril (CB[8])[34]. Interestingly, the head-to-tail (HT) dimers were formed in 98% selectivity within CD-3. Particularly, the syn-HT photodimer was generated in 68% yield with 91% ee, which were much higher than the reaction with unmodified AC carboxylate (32% ee, 44% yield). An inversion of HT/head-to-head (HH) selectivity was also observed using CB[8], affording exclusively the HH photodimers (99% selectivity). This study indicated that the outside interactions can influence the asymmetric photochemical reaction by manipulating the ground-state complexation and the excited-state reactivity.
Shortly after, a non-sensitizing metallosupramolecular host (CD-4) consisting of diamino-γ-CD combined with Cu(ClO4)2 was exploited by Ke et al. to promote the photodimerization of AC[35]. In this study, the anti-HH dimer formed in 51%-52% yields with 64%-70% ees within CD-4 at -50 °C in aqueous methanol. Mechanistically, the CD-4 facilitates the generation of a HH-oriented 1:2 host-guest complex and discourages the generation of the diastereomeric syn-HH complex.
Luo et al. studied the photodimerization of naphthalene derivatives in native γ-CD [Scheme 4][36]. Unlike AC, naphthalene 7 underwent a photodimerization and produced the anti-HH cubane-like product, which usually could not be obtained under thermal reaction conditions. In this study, the anti-HH photodimerization product 8 was generated in 48% (aqueous solution) and 34% (solid state) ee under ambient temperature and pressure.
Scheme 4. Enantioselective photodimerization of naphthalene derivatives.
In 2011, Yang et al. developed an enantioselective photochemical protocol to realize the acceleration and excellent stereo-control in the photodimerization of 2-anthracenecarboxylate tethered to an α-CD scaffold[37]. In this protocol, two AC molecules tethered to one α-CD (CD-5) were accommodated in the cavity of γ-CD or CB[8]. The anti-HH dimer was selectively obtained in 98% yield with 99% ee using γ-CD as the host, while 97% yield and 98% ee were recorded in achiral CB[8].
To better manipulate the stereochemical outcomes of [4 + 4] photodimerization of ACs, Yao et al. synthesized several diamino-γ-CD and diguanidino-γ-CD hosts[38]. In the photodimerization mediated by diguanidino-γ-CD (CD-6), they found the ee and chiral sense of the final products were dynamic functions of reaction temperature and the co-solvent. The anti-HH dimerization product was generated with moderate enantioselectivity in aqueous methanol at -70 °C (64% ee), while the antipodal isomer was formed in 72% yield with excellent enantioselectivity at -85 °C in aqueous ammonia (86% ee). However, the diamino-γ-CD host did not exhibit this remarkable stereo-control behavior.
The above studies mainly focused on the modifications of larger-in-cavity γ-CD to mediate photodimerization of ACs and deliver classical chiral 9,10:9’10’-cyclodimer products. The slipped chiral cyclodimers have long been overlooked. In 2018, Wei et al. disclosed that chiral slipped 5,8:9’,10’-cyclodimers 5 and 6 were produced by β-CD via higher-order 2:2 complexation [Scheme 5][29]. The authors also investigated the photophysical and structural aspects of the complexation-mediated transformation. Later, Ji et al. designed and prepared a range of sulfur-linked or arene-spacer-tethered β-CD dimers (CD-7) for manipulating the enantio- and regioselectivities of the photochemical dimerization of 2-AC[39]. In this system, two nonclassical 5,8:9’,10’-cyclodimers (5 and 6) and four stereoisomeric classical 9,10:9’10’-cyclodimers were generated via a photoreactive 1:2 complex. Wherein syn-9,10:9’10’-HT dimer could be obtained in 98% yield with > 99% ee.
Scheme 5. The formation of slipped cyclodimers via 2:2 host-guest complexation.
The enantiomeric forms of CD compounds are not readily available. Therefore, preparation of the enantio-isomers of chiral products through the induction of opposite chirality is not relevant in CD-mediated transformation. Variation of external factors, such as temperature, solvent, irradiation wavelength, pH and additives, and modification of CD frameworks may lead to chirality inversion of CD-mediated asymmetric photochemical reactions. For instance, Kanagaraj et al. harnessed two types of γ-CD derivatives, videlicet, bisquinoline-modified γ-CD (CD-8) and its N-methylated derivative (CD-9) to catalyze the photodimerization of 2-anthracenecarboxylic acid[40]. By adjusting the pH, the ee of the anti-HH dimer was inverted from 25% to -64% (within CD-8) and 41% to -76% (within CD-9), respectively.
Other macrocyclic hosts
Apart from above CD-based hosts, organic macrocyclic octa acid (OA) is also suitable for enantioselective photodimerization of 2-AC. In 2020, Wei et al. found that OA can act as both a capsule and a cavitand[41]. As shown in Scheme 6, the AC molecules were oriented with the two COO- groups facing water and the two AC rings buried within the hydrophobic cavity of the cavitand. Interestingly, photodimerization with this cavitand favored a HH dimer product. In addition, when guests were tethered with a chiral auxiliary (α-CD-AC), achiral OA can also induce this photodimerization reaction with good diastereoselectivity.
Scheme 6. Enantioselective photodimerization of α-CD-ACs mediated by OA. CD: Cyclodextrin; ACs: anthracenes; OA: octa acid.
Photoisomerization within macrocyclic hosts
Photoisomerization of (Z)-cyclooctene 9 and (Z, Z)-1,3-cyclooctadiene 11 can also take place inside the cavity of macrocyclic supramolecular hosts, furnishing the desired chiral products (R/S)-(E)-cyclooctene 10 and (R/S)-(E, Z)-1,3-cyclooctadiene 12, respectively [Scheme 7][42,43]. In this context, modified oligosaccharide supramolecular hosts, such as aryl-derived CD hosts and functionalized cyclic nigerosylnigerose (CNN), have been introduced to mediate this transformation. Due to the fact that neither cyclooctene nor cyclooctadiene can absorb the light, functionalization of these macrocyclic hosts with various photosensitizers is essential. These modified macrocycles can serve as a chiral photosensitizer to activate the substrate and manipulate the enantioselectivity simultaneously. Compared with the transformation in bulk solvents, enantioselective photoisomerization of cyclooctene/cyclooctene within supramolecular cavities can proceed in the aqueous environment and achieve promising enantioselectivity under relatively mild reaction conditions. Related works in this research field have been nicely summarized by Genzink et al.[44]. Therefore, we will not highlight this aspect herein.
Scheme 7. Enantioselective photoisomerization of (Z)-cyclooctene 9 and (Z, Z)-1,3-cyclooctadiene 11.
Other transformations within macrocyclic hosts
Several other types of enantioselective photochemical reactions mediated by macrocycles have also been studied, including [2 + 2] photocycloaddition reaction, photocyclization and difunctionalization of alkenes. For instance, Koodanjeri et al. employed CDs in the photocyclization of achiral tropolone alkyl ethers 13 to yield a bicyclo[3.2.0] product 14 bearing two contiguous chiral centers (up to 33% ee) [Scheme 8A][45]. Subsequently, the same group reported a β-CD-mediated photocyclization of achiral N-alkyl pyridones 15 to 2-azabicyclo[2.2.0]-hex-5-en-3-ones 16 with up to 59% ee in the solid state [Scheme 8B][46]. In 2009, they further studied the chiral induction by β-CD in intramolecular photocyclization of carbonyl compounds[47]. Taking the photocyclization of adamantyl acetophenone 17 as an example, trans-cyclobutanol and cis-cyclobutanol 18 were obtained in 9% and 20% ee, respectively, in the presence of chiral benzylamine-derived β-CD [Scheme 8C]. Moreover, Mansour et al. utilized β-CD to mediate the photochemical electrocyclization of 1,3-dihydro-2H-azepin-2-one 19 [Scheme 8D][48]. The authors found that no enantioselectivity was recorded in solution. However, solid-state [2 + 2] photocycloaddition followed by reduction delivered the terminal (1R,5R)-2-azabicyclo[3.2.0]heptan-3-one 20 in up to 79% isolated yields and with up to 45% ee.
Scheme 8. Other asymmetric transformations within macrocyclic CD-based hosts. CD: Cyclodextrin.
On the other hand, Fukuhara et al. reported an enantioselective photochemical addition of methanol to 1,1-diphenylpropene (DPP) initiated by a cyanonaphthalene-modified β-CD (CDnp), affording anti-Markovnikov adducts[49]. They further investigated the competitive addition of water and methanol to DPP in the presence of CDnp. In this study, the ee of water addition product 22 was up to 26% [Scheme 8E][50].
The above examples show that CD-based macrocycles are promising chiral hosts for enantioselective photodimerization of 2-anthracenecarboxylates and photoisomerization of cycloolefins. Asymmetric [2 + 2] photocycloaddition reaction, photocyclization and difunctionalization of alkenes using CDs also work, albeit with lower enantioselectivity. The chiral confinement of CD hosts plays a significant role in the pre-organization of the substrates in the ground state, which leads to not only the closer substrate proximity but also selective spatial arrangement of the guest molecules.
ENANTIOSELECTIVE PHOTOREACTIONS WITHIN MOCS
Different from macrocycles, MOCs are assembled from metals and organic ligands through coordination[51-53]. Recently, some chiral MOCs have been successfully introduced into enantioselective photocatalysis. In these studies, MOCs serve as efficient nanoreactors to utilize the light energy and induce the asymmetric photochemical reactions. This section will discuss the recent progress in enantioselective photodimerization and photocycloaddition reactions using chiral MOCs.
Photodimerization within MOCs
In 2014, Li et al. constructed a heterometallic [Pd6(RuL3)8]28+ MOC host (MOC-1), which incorporated photoredox-active and stereogenic RuL3 units and supramolecular RuL3/Pd photo-hydrogen-evolving (PHE) moieties[54]. Further endeavors allowed them to obtain the homochiral and photoactive MOC-1 (Δ/Λ-MOC-1) with a chiral coordination space[55]. Subsequently, Guo et al. reported a seminal work about regio- and enantioselective 1,4-homocoupling of 2-naphthol derivatives to form 4-(2-hydroxy-1-naphthyl)-1,2-napthoquinone in the confined chiral-control cavity of Δ/Λ-MOC-1 [Scheme 9][56]. In most cases, the dimerization of 2-naphthols will generate relatively stable 1,1’-homocoupling product 1,1’-bi-2-naphthol (BINOL), with unstable 1,4-homocoupling 25 as a side product. Under the optimized reaction conditions, 1,4-homocoupling product 25 was obtained in 54% (Δ-MOC-1) and 58% (Λ-MOC-1) ee with 5 mol% of Δ/Λ-MOC-1. Based on some spectral studies and theoretical calculations, a plausible mechanism was proposed. Upon the irradiation of blue light-emitting diodes (LEDs, 453 nm), MOC-1 was excited to the excited state
Scheme 9. Regio- and enantioselective 1,4-homocoupling of 2-naphthol derivatives within Δ/Λ-MOC-1. MOC: Metal-organic cage.
The [2 + 2] photodimerization of olefins has been extensively studied, which represents a feasible and efficient approach in the construction of cyclobutanes. Kindled by the pioneering work, Guo et al. reported an unprecedented [2 + 2] enantioselective photodimerization of an unsymmetrical acenaphthylene derivative 26 mediated by enantiopure Δ/Λ-MOC-1[57]. The dimerization of 26 gave anti-HH product 27 both within Δ-MOC-1 (86% ee) and Λ-MOC-1 (-88% ee) with high regio-, stereo-, and enantioselectivity. In this transformation, the chiral confinement of Δ/Λ-MOC-1 played a pivot role in governing the stereochemical outcome. The combination of multiple functions, including the pre-organization of substrates in the ground state, the regulation of excited-state stereochemistry as well as the transfer of triplet energy and chirality, have been verified by experimental results and computational studies. As shown in Scheme 10, they have established the possible simplified cage models to deduce the most stable configurations of reactants and products associated with the cage to explain the high enantioselectivity.
Scheme 10. Enantioselective [2 + 2] photodimerization of acenaphthylene derivatives within Δ/Λ-MOC-1. MOC: Metal-organic cage.
Photocycloaddition within MOCs
In 2006, Yoshizawa et al. prepared a self-assembled coordination cage (M6L4) by mixing an exo-tridentate ligand and an end-capped PdII ion in water[58]. They found that the Diels-Alder reaction between AC and phthalimide occurred inside of this M6L4 cage with excellent regioselectivity. Subsequently, replacing the ethylenediamine with chiral diamines on each Pd center formed the chiral MOC (MOC-2) [Scheme 11][59]. They used MOC-2 as a chiral host for the [2 + 2] photocycloaddition reaction of fluoranthene 28 with maleimide 29. To their delight, the modification at the periphery of MOC-2 was sufficient to produce considerable asymmetric induction. Upon the irradiation of 360 nm light, cyclobutane product 30 was obtained in 75% yield with 50% ee. MOC-2 exhibited excellent guest binding capability, which worked as the prerequisite for the pre-organization of substrates at the ground state. Based on the calculation results, the authors ascribed the high regioselectivity to the steric effect within the confined cavity rather than orbital control. The control experiments suggested that the steric profile of the diamine ligand was correlated with the ee of the product. This study utilized a chiral diamine ligand to assemble a photoactive supramolecular coordination cage. The diamine ligand first coordinated with the Pd center and then induced the chiral deformation of the cavity. Further investigations revealed that the larger steric bulk of the N-substituent (R) increased the tilt angle of the triazine ligand, resulting in the enhancement of enantioselectivity.
Scheme 11. Enantioselective [2 + 2] photocycloaddition of fluoranthene with maleimide within MOC-2. MOC: Metal-organic cage.
Based on the previous studies on the applications of MOC-1 in the enantioselective 1,4-homocoupling, [2 + 2] photodimerization and Diels-Alder reaction[60], Ruan et al. explored its catalytic potential in other chemical transformations. Recently, they successfully utilized Δ-MOC-1 to catalyze the asymmetric [2 + 2] photocycloaddition of chalcone and cinnamate derivatives with high levels of chemo-, regio-, diastereo-, and enantioselectivities [Scheme 12][61]. In comparison with well-established photocycloaddition between active and inactive olefins, the formation of two different diradical intermediates renders the selective [2 + 2] cross-photocycloaddition of two photoactive olefins with similar reactivities a considerable challenge, since it may result in a complex outcome for photocycloaddition with the generations of multiple undesired isomers. This chiral cage photoreactor has enabled the enantioselective [2 + 2] cross-photocycloaddition via multilevel-selectivity control to promote the formation of syn-HH isomer that is sterically and thermodynamically unfavorable in traditional conditions. Under the optimal conditions, a syn-HH isomer was obtained in good yield with excellent ee (39 examples, up to 86% yield and 99% ee). In this study, they obtained two single crystals of [43]18 ⊂ Λ-MOC-1 and [44]2 ⊂ rac-MOC-1-Zn complexes, both of which accommodate two same substrates in each pocket via π-π stacking interactions. It suggested that the distance (4.8 Å) or angle (75°) between the C=C bonds of the two same molecules is indeed unfavorable for homocoupling, which accounts for the preferential heterocoupling of two different substrates. Moreover, the controlled excited-state processes and dynamic exchange between the reactant and product within the cage reactor facilitated the formation of syn-HH cross coupling products. This study further demonstrated the advantages of supramolecular photochemical strategy in manipulating the chemoselectivity and stereoselectivity.
Scheme 12. Multilevel-selective [2 + 2] cross-photocycloaddition of α, β-unsaturated carbonyl compounds within Δ-MOC-1. Reproduced with permission from[61]. Copyright 2024, American Chemical Society. MOC: Metal-organic cage.
ENANTIOSELECTIVE PHOTOREACTIONS WITHIN MOFS
MOFs are a class of nanomaterials that feature high specific surface area and porosity, and easy functionalization[62-64]. In recent years, some chiral MOFs have also been designed and employed in enantioselective photochemical reactions. In this part, asymmetric photodimerization, functionalization of aldehydes, and other intermolecular couplings within MOFs will be introduced.
Photodimerization within MOFs
In 2021, an elegant enantioselective [4 + 4] photodimerization of 1-AC- was reported by Chen et al.
Scheme 13. Enantioselective [4 + 4] photochemical dimerization of 1-AC within γ-CD containing MOF-3. Reproduced with permission from[65]. Copyright 2021, American Chemical Society. AC: Anthracene; CD: cyclodextrin; MOF: metal-organic framework.
Asymmetric functionalization of aldehydes within MOFs
In 2012, Wu et al. prepared a pair of enantiomeric Zn-based MOFs (Δ/Λ-MOF-4) via incorporating (L)- or
Scheme 14. Enantioselective α-alkylation of aliphatic aldehydes within Zn-based MOF-4. MOF: Metal-organic framework.
The catalytic performance of MOFs shows strong correlations with their topological structures. As shown in Scheme 15, Xia et al. took advantage of an interpenetrated homochiral MOF (MOF-5) containing the photoredox and asymmetric catalytic units to achieve the enantioselective β-arylation of aldehydes (ee up to 52%)[67]. X-ray single-crystal analysis showed the generation of the twofold interpenetrated coordination polymer, which shortens the space distance between two active units and, therefore, accelerates the electron transfer between the oxidized photosensitizer and enamine intermediate. The authors postulated a possible mechanism to explain the functions of interpenetrated MOF-5 in photocatalytic β-carbonyl activation. Initially, aldehydes 57 diffused into the channels of MOF-5 and condensed with (L)-PYI to form the enamine intermediate. Under the irradiation of visible light, 1,4-dicyanobenzene 56 was adsorbed by MOF-5, followed by the generation of radical anion and oxidized MOF-5+. Subsequently, the enamine was oxidized by MOF-5+ through SET to form an enaminyl radical cation intermediate, which increased the acidity of the allylic C–H bonds. Therefore, the deprotonation took place at the β-position of aldehyde. The 5πe-β-enaminyl radical intermediate was then coupled with a 1,4-dicyanobenzene radical anion to generate the cyclohexadienyl anion, which was hydrolyzed to the terminal β-arylation product 58 and released (L)-PYI to re-enter the catalytic cycle.
Scheme 15. Enantioselective β-arylation of aliphatic aldehydes with p-dicyanobenzene mediated by interpenetrated MOF-5. MOF: Metal-organic framework.
In the same year, Zhang et al. constructed a series of chiral crystalline MOFs (X-MOF-6, X = Zn, Zr, Ti) via a chiral photoredox ligand coordinated with different metal ions (Zn2+, Zr4+ and Ti4+), and successfully used in visible-light-induced enantioselective α-alkylation of 3-phenylpropionaldehyde and cis-6-nonenal
Scheme 16. Visible-light-mediated asymmetric α-alkylation of aldehydes within chiral X-MOF-6. Reproduced with permission from[68]. Copyright 2017, American Association for the Advancement of Science. MOF: Metal-organic framework.
Other asymmetric intermolecular couplings within MOFs
MOFs can also serve as the carriers for photocatalysts or/and chiral catalysts to promote the asymmetric photochemical reactions. As shown in Scheme 17, this kind of MOF can be classified into three types based on its roles in photocatalytic asymmetric reactions: (1) chiral MOF loaded with photocatalyst (MOF-7)[69]; (2) achiral MOF loaded with enzyme (MOF-8)[70]; (3) supporting frameworks for photocatalyst and chiral catalyst (MOF-9, MOF-10)[71,72]. For a clear comparison, their applications in the photocatalytic asymmetric intermolecular couplings have been summarized in Table 3.
Scheme 17. MOFs loaded with photocatalyst or/and chiral catalyst employed in the photocatalytic asymmetric intermolecular couplings. (A) Reproduced with permission from[69]. Copyright 2019, American Chemical Society; (B) Reproduced with permission from[70]. Copyright 2024, American Chemical Society; (C) Reproduced with permission from[71]. Copyright 2021, American Chemical Society; (D) Reproduced with permission from[72]. Copyright 2024, Royal Society of Chemistry. MOFs: Metal-organic frameworks.
The applications of MOF carriers in photocatalytic asymmetric intermolecular couplings
| MOF | Substrate | Reaction | Role | Outcome |
| TiO2@MOF-7[69] | Benzyl alcohols and methyl acrylate | Sequential MBH reaction | MOF-7: chiral induction; TiO2: photocatalyst | Up to 99% yield; 99% ee |
| WGL@MOF-8[70] | (a) Tetrahydroisoquinoline and ketones; (b) 2-arylindoles and acetone | (a) Dehydrogenation coupling; (b) Oxidative Mannich reaction | MOF-8: photocatalyst; WGL: chiral induction | Up to 65% yield; 70% ee |
| Ru(bpy)3@MOF-9/ proline[71] | 2-Arylindoles and ketones | Oxidative Mannich reaction | MOF-9: supporting framwork; proline: chiral induction; Ru(bpy)3: photocatalyst | Up to 88% yield; 99% ee |
| Au@MOF-10/proline[72] | Aromatic aldehydes and acetone | Aldol reaction | MOF-10: supporting framwork; proline: chiral induction; Au NPs: photocatalyst | Up to 81% yield; 91% ee |
ENANTIOSELECTIVE PHOTOREACTIONS WITHIN COFS
COFs are a class of porous and tunable crystalline polymers constructed by strong covalent bonds with 2D or 3D network topologies, which have shown great potential as heterogeneous photocatalysts[73-78]. In general, COFs used for asymmetric photochemical reactions can be divided into two types: achiral and chiral. Achiral COFs function as supramolecular photocatalysts and are used in combination with chiral organocatalysts, while chiral COFs (CCOFs) containing both photosensitizer and chiral moieties can mediate the light-energy conversion and manipulate the stereoselectivity. This part will discuss the recent works about the designs and applications of COFs in enantioselective photochemical transformations, including α-functionalization of aldehydes, photooxidation of thioethers, and other asymmetric intermolecular couplings.
Asymmetric α-functionalization of aldehydes within COFs
In 2020, Kang et al. designed a pair of 3D COFs (COF-11, COF-12) with a rare twofold-interpenetrated ffc topology through condensations between rectangular and trigonal monomers [Scheme 18][79]. Triphenylamine was selected as the functional molecule, since it not only exhibits unique photophysical and redox properties but also represents an important hole-conducting molecule. COF-11 was obtained from tetraamine (ETTA) and trialdehyde (NBC), while COF-12 was from tetraaldehyde (ETBC) and triamine (BADA). The authors proposed that both COFs adopted a twofold interpenetrated ffc topology with the C2/m space after simulations. The two COFs tethered with a chiral amine organocatalyst could catalyze the asymmetric α-alkylation of aldehydes, which exhibited similar catalytic performances in reactivity and enantioselectivity. Under the optimized conditions, the desired α-alkylation products 65 were formed in good yields with excellent enantioselectivity (up to 88% yield, 94% ee).
Scheme 18. Highly enantioselective α-alkylation of aldehydes within 3D COFs. Reproduced with permission from[79]. Copyright 2020, Royal Society of Chemistry. COFs: Covalent organic frameworks.
Li et al. also synthesized two robust tetrahydroquinoline-linked (QH)-COFs (COF-13, COF-14) visible light photocatalysts and used in the enantioselective photochemical α-alkylation of aldehydes [Scheme 19][80]. The tetrahydroquinoline linkage plays an important role in enhancing the stability and widening the absorption spectrum of COFs. Up to 94% ee was obtained using this kind of COF by merging with a chiral secondary amine, and the catalytic efficiency is similar to that of conventional Ru catalyst [Ru(bpy)3]Cl2·6H2O (80% yield, 91% ee). In this study, the authors also proposed a possible catalytic mechanism. Under visible light irradiation, one electron was transferred from the conduction band (CB) of COFs to α-bromocarbonyl substrate 72, thus leading to the formation of the alkyl radical intermediate. Then, aldehyde 50 condensed with the chiral secondary amine organic catalyst to form an enamine intermediate, followed by the addition of the electron-deficient alkyl radical. Subsequently, the generated α-amino radical delivered one electron to the hole in the VB of QH-COFs and formed an imine cation. Hydrolysis of the imine intermediate afforded the terminal product and regenerated the catalyst to finish the cycle. Additionally, Zhou et al. also synthesized an achiral photoactive COF by the condensation of 1,3,6,8-tetrakis(4-formylphenyl) pyrene with amino/cyano-containing monomers. By merging with a chiral amine catalyst, the asymmetric α-alkylation of aldehydes proceeded quite well under visible light irradiation[81].
Scheme 19. Visible-light-induced enantioselective α-alkylation of aldehydes within QH-COFs merging with a chiral secondary amine. Reproduced with permission from[80]. Copyright 2020, Elsevier. QH-COFs: Tetrahydroquinoline-linked covalent organic frameworks.
Regarding asymmetric photochemical reactions, achiral COFs usually served as photocatalysts to harvest light and chiral induction usually required an additional chiral catalyst. The construction of CCOFs by merging photoactive and chiral units into a single framework is a promising strategy. In 2022, Ma et al. designed and synthesized a chiral multifunctional COF (COF-15) through the condensation of chiral BINOL-phosphate with Cu(II)-porphyrin-derived monomers, and successfully introduced in the enantioselective α-benzylation of aldehydes [Scheme 20][82]. COF-15 bears both Brønsted (phosphate) and Lewis [Cu(II)] acidic catalytic sites, chiral confinement space, and photothermal conversion (PTC) properties. Upon 420 nm light irradiation, (R)-COF-15 could catalyze the α-benzylation of aldehydes with alkyl halides in good yields with excellent enantioselectivity (ten examples, up to 98% yields, 94% ees). The PTC behavior of COF-15 was also investigated, and results indicated that a significant temperature increase was observed when a solution of (R)-COF-15 in MeOH was irradiated under 420 nm light for 18 min. Interestingly, the configuration of the terminal product could be easily reversed by tuning the chirality of MOF, without any loss of yield and ee.
Scheme 20. Photoinduced thermally-driven enantioselective α-benzylation of aldehydes catalyzed by (R)-COF-15. Reproduced with permission from[82]. Copyright 2022, Royal Society of Chemistry. COF: Covalent organic framework.
Recently, He et al. has successfully synthesized a series of CCOFs (COF-16a-16f) with photoactive porphyrin moieties as knots, and secondary-amine-based chiral functional groups were fixed on the benzoimidazole linkers [Scheme 21][83]. Among these CCOF photocatalysts, COF-16a exhibited the best results in the enantioselective α-alkylation of aldehydes 50 in high yields (up to 97%) with good enantioselectivity (up to 93% ee) under visible light irradiation. The good results benefited from the activation of bromide 64 by porphyrin units and aldehydes by chiral secondary amines. The authors also investigated the physical and chemical properties of these COFs. Broad absorption region and excellent stability and recyclability also showed their advantages as metal-free catalytic platforms in enantioselective photocatalysis.
Scheme 21. Visible-light-mediated enantioselective α-alkylation of aldehydes catalyzed by COF-16. COF: Covalent organic framework.
Asymmetric photooxidation within COFs
In 2022, Kan et al. designed and constructed a photoactive and hydrotropic CCOF (COF-17) bearing a photosensitizer, chiral moiety, phase transfer group and linkage, which was successfully applied in the enantioselective photooxidation of sulfides to sulfoxides in water [Scheme 22][84]. This homochiral COF-17 was synthesized via asymmetric A3-coupling polymerization of 2,5-dimethoxyterephth-aldehyde, quaternary ammonium bromide-modified phenylacetylene and 5,10,15,20-tetrakis(4-aminophenyl)porphyrin. The key active units, photosensitive porphyrin, chiral propargylamine linkage and amphipathic quaternary ammonium bromide, were introduced into the CCOF supramolecular catalyst. Under the irradiation of 660 nm light, (R)-COF-17 could catalyze the oxidation of substituted methylphenylsulfides 82 to the corresponding (R)-methylphenylsulfoxides 83 in water with molecular oxygen in air as the oxidant (81%-96% yields, 77%-99% ee). Interestingly, biologically active (R)-modafinil could also be obtained using this protocol.
Scheme 22. Enantioselective aerobic photooxidation of sulfides into sulfoxides catalyzed by COF-17. COF: Covalent organic framework.
Other asymmetric intermolecular couplings within COFs
Photoactive CCOFs integrated with metal nanoparticles (M NPs) are also suitable for enantioselective photochemical reactions. For instance, Ma et al. designed two kinds of M NP (M = Au, Pd)-loaded and Cu(II)-porphyrin-containing homochiral COFs (COF-18, COF-19), which exhibited excellent PTC and asymmetric catalytic performance in visible-light-induced thermally-driven enantioselective Henry and A3-coupling [Scheme 23][85]. For instance, Henry reaction between benzylic alcohols 84 and nitromethane 85 within COF-18 could generate the desired nitroaldols 86 in high yields (up to 99%) with excellent enantioselectivity (ee of up to 98%). Meanwhile, A3-coupling of benzaldehyde derivatives 87, aromatic alkynes 88 and secondary amines 89 within COF-19 also took place and delivered the products in good results (yield of up to 98%, ee of up to 98%). According to the experimental results, the loading of M NPs into COFs would not affect the PTC efficiency. The confinement of CCOFs played a crucial role in suppressing the racemization of the terminal products under high temperatures.
Scheme 23. Asymmetric visible-light-induced thermally-driven Henry and A3-coupling reactions within chiral COFs loaded with metal nanoparticles. Reproduced with permission from[85]. Copyright 2019, Springer Nature. COFs: Covalent organic frameworks.
Ma et al. employed a versatile homochiral COF (COF-20) synthesized by the condensation of Cu(II)-porphyrin-derived monomers [5,10,15,20-tetrakis(4-aminophenyl)porphyrin-Cu-(II) (Cu-TAPP)] and chiral (R)-4,4’-dialdehyde-6,6’-dichloro-2,2’-diethoxy-1,1’-binaphthalene [(R)-BINOL-DA][86]. COF-20 possessed multiple active sites, including chiral templating (CT), Lewis acid (LA) and PTC sites, which ensured its excellent performance in catalyzing the asymmetric Strecker reaction (98% yield, 94% ee). They successfully used COF-20 to synthesize the (S)-CIK 94, a key intermediate in the synthesis of antithrombotic drug (S)-clopidogrel [Scheme 24]. The detailed physical and chemical property studies of COF-20 were also conducted. This study further demonstrated the promising potential of CCOFs as a green and facile synthesis platform for chiral drug preparation and discovery.
Scheme 24. Visible-light-induced asymmetric synthesis of drug intermediate (S)-CIK catalyzed by COF-20. Reproduced with permission from[86]. Copyright 2020, American Chemical Society. COF: Covalent organic framework.
In the last few years, the combination of photocatalysis and biocatalysis has attracted much research interest in asymmetric synthesis. Merging photoactive COFs with enzymatic catalysts is an effective strategy to promote enantioselective photochemical reactions. Recently, Jin et al. developed a COF-based photoenzymatic platform for asymmetric oxidative Mannich reaction [Scheme 25][87]. In this study, a photoactive M-porphyrin-containing COF (M-COF-21; M = H, Zn, Cu, Ni) served as a porous carrier to encapsulate wheat germ lipase (WGL) and generated a heterogeneous photobiocatalyst
Scheme 25. Asymmetric oxidative Mannich reaction catalyzed by a heterogeneous photobiocatalyst WGL@M-COF-21. Reproduced with permission from[87]. Copyright 2022, American Chemical Society. WGL: Wheat germ lipase; COF: covalent organic framework.
CONCLUSION AND OUTLOOK
In the review, we have summarized the recent advances in enantioselective photochemical reactions within the confined spaces of supramolecular assemblies including macrocycles, MOCs, MOFs, and COFs. This research field has flourished over the past two decades, accompanied with the development of new types of supramolecular hosts with excellent catalytic and stereo-controlled performance. However, there are still some limitations in supramolecular enantioselective photocatalysis. For instance: (1) The types of chiral hosts for enantioselective photochemical reactions are not diverse. Macrocycles are largely limited to CDs, and chiral organocatalysts immobilized into MOFs and COFs are mainly based on chiral secondary amines and BINOL backbones; (2) The reaction types are quite narrow. CDs-initiated reactions are mainly focused on photodimerization of 2-ACs. Most of the asymmetric reactions mediated by MOCs, MOFs and COFs are [2 + 2] photocycloaddition reactions and α-alkylation of aldehydes; (3) Only a few guest molecules can be accommodated very well in the rigid cavities of chiral hosts, leading to the low substrate diversities.
To enrich the library of chiral hosts for enantioselective photochemical reactions, other modified metal complexes and organic photosensitizers could be used to construct new types of chiral supramolecular hosts. Meanwhile, some other chiral privileged structures, e.g., chiral hydrogen bonding moieties (urea, thiourea), N-heterocyclic carbenes and chiral tertiary amines, can be incorporated into the hosts, or as cooperative catalysts in supramolecular enantioselective photochemical reactions. By doing so, growing reaction substrates can be introduced, and therefore, the reaction types could be further expanded. On the other hand, dual activation models by the combination of supramolecular catalysis with transition metal or LA catalysis are also potential strategies. Future advances can be expected along these lines.
DECLARATIONS
Acknowledgments
We thank M.Sc. Xiao-Yu He (Wuhan University) for the helpful discussions and for proofreading the final version of this manuscript. We also express our gratitude to all leading chemists and co-workers involved in the development of enantioselective supramolecular photocatalysis.
Authors’ contributions
Wrote the draft manuscript: Su YW, Pan C, Sun TY
Initiated and supervised the work, and revised the manuscript: Zou YQ
Availability of data and materials
Not applicable.
Financial support and sponsorship
Financial support by the start-up funding from Wuhan University (Nos. 691000002 and 600460026) and Taikang Center for Life and Medical Sciences (No. 692000007) is gratefully acknowledged.
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2024.
REFERENCES
1. Brimioulle R, Lenhart D, Maturi MM, Bach T. Enantioselective catalysis of photochemical reactions. Angew Chem Int Ed Engl 2015;54:3872-90.
2. Yao W, Bergamino EAB, Ngai MY. Asymmetric photocatalysis enabled by chiral organocatalysts. ChemCatChem 2022;14:e202101292.
3. Jiang C, Chen W, Zheng WH, Lu H. Advances in asymmetric visible-light photocatalysis, 2015-2019. Org Biomol Chem 2019;17:8673-89.
4. Zarra S, Wood DM, Roberts DA, Nitschke JR. Molecular containers in complex chemical systems. Chem Soc Rev 2015;44:419-32.
5. Morimoto M, Bierschenk SM, Xia KT, Bergman RG, Raymond KN, Toste FD. Advances in supramolecular host-mediated reactivity. Nat Catal 2020;3:969-84.
6. Ramamurthy V, Gupta S. Supramolecular photochemistry: from molecular crystals to water-soluble capsules. Chem Soc Rev 2015;44:119-35.
7. Ramamurthy V, Sivaguru J. Supramolecular photochemistry as a potential synthetic tool: photocycloaddition. Chem Rev 2016;116:9914-93.
8. Gao W, Zhang H, Jin G. Supramolecular catalysis based on discrete heterometallic coordination-driven metallacycles and metallacages. Coordin Chem Rev 2019;386:69-84.
9. Liu Y, Xuan W, Cui Y. Engineering homochiral metal-organic frameworks for heterogeneous asymmetric catalysis and enantioselective separation. Adv Mater 2010;22:4112-35.
10. Hao Y, Lu YL, Jiao Z, Su CY. Photocatalysis meets confinement: an emerging opportunity for photoinduced organic transformations. Angew Chem Int Ed Engl 2024;63:e202317808.
11. Ballester P, Wang QQ, Gaeta C. Supramolecular approaches to mediate chemical reactivity. Beilstein J Org Chem 2022;18:1463-5.
12. Ham R, Nielsen CJ, Pullen S, Reek JNH. Supramolecular coordination cages for artificial photosynthesis and synthetic photocatalysis. Chem Rev 2023;123:5225-61.
13. Olivo G, Capocasa G, Del Giudice D, Lanzalunga O, Di Stefano S. New horizons for catalysis disclosed by supramolecular chemistry. Chem Soc Rev 2021;50:7681-724.
15. Hong CM, Bergman RG, Raymond KN, Toste FD. Self-assembled tetrahedral hosts as supramolecular catalysts. Acc Chem Res 2018;51:2447-55.
16. Jing X, He C, Zhao L, Duan C. Photochemical properties of host-guest supramolecular systems with structurally confined metal-organic capsules. Acc Chem Res 2019;52:100-9.
17. Pascanu V, González Miera G, Inge AK, Martín-Matute B. Metal-organic frameworks as catalysts for organic synthesis: a critical perspective. J Am Chem Soc 2019;141:7223-34.
18. Zhang Q, Catti L, Tiefenbacher K. Catalysis inside the hexameric resorcinarene capsule. Acc Chem Res 2018;51:2107-14.
19. Wang MX. Nitrogen and oxygen bridged calixaromatics: synthesis, structure, functionalization, and molecular recognition. Acc Chem Res 2012;45:182-95.
20. Fang Y, Powell JA, Li E, et al. Catalytic reactions within the cavity of coordination cages. Chem Soc Rev 2019;48:4707-30.
21. Qin B, Yin Z, Tang X, et al. Supramolecular polymer chemistry: from structural control to functional assembly. Prog Polym Sci 2020;100:101167.
22. Vallavoju N, Sivaguru J. Supramolecular photocatalysis: combining confinement and non-covalent interactions to control light initiated reactions. Chem Soc Rev 2014;43:4084-101.
24. Nishijima M, Wada T, Mori T, Pace TC, Bohne C, Inoue Y. Highly enantiomeric supramolecular [4 + 4] photocyclodimerization of 2-anthracenecarboxylate mediated by human serum albumin. J Am Chem Soc 2007;129:3478-9.
25. Wada T, Nishijima M, Fujisawa T, et al. Bovine serum albumin-mediated enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylate. J Am Chem Soc 2003;125:7492-3.
26. Ishida Y, Kai Y, Kato SY, et al. Two-component liquid crystals as chiral reaction media: highly enantioselective photodimerization of an anthracene derivative driven by the ordered microenvironment. Angew Chem Int Ed Engl 2008;47:8241-5.
27. Ishida Y, Matsuoka Y, Kai Y, et al. Metastable liquid crystal as time-responsive reaction medium: aging-induced dual enantioselective control. J Am Chem Soc 2013;135:6407-10.
28. Ji J, Wei X, Wu W, Yang C. Asymmetric photoreactions in supramolecular assemblies. Acc Chem Res 2023;56:1896-907.
29. Wei X, Wu W, Matsushita R, et al. Supramolecular photochirogenesis driven by higher-order complexation: enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylate to slipped cyclodimers via a 2:2 complex with β-cyclodextrin. J Am Chem Soc 2018;140:3959-74.
31. Nakamura A, Inoue Y. Supramolecular catalysis of the enantiodifferentiating [4 + 4] photocyclodimerization of 2-anthracenecarboxylate by gamma-cyclodextrin. J Am Chem Soc 2003;125:966-72.
32. Nakamura A, Inoue Y. Electrostatic manipulation of enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylate within gamma-cyclodextrin cavity through chemical modification. inverted product distribution and enhanced enantioselectivity. J Am Chem Soc 2005;127:5338-9.
33. Yang C, Nakamura A, Wada T, Inoue Y. Enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylic acid mediated by gamma-cyclodextrins with a flexible or rigid cap. Org Lett 2006;8:3005-8.
34. Yang C, Mori T, Origane Y, et al. Highly stereoselective photocyclodimerization of alpha-cyclodextrin-appended anthracene mediated by gamma-cyclodextrin and cucurbit[8]uril: a dramatic steric effect operating outside the binding site. J Am Chem Soc 2008;130:8574-5.
35. Ke C, Yang C, Mori T, Wada T, Liu Y, Inoue Y. Catalytic enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylic acid mediated by a non-sensitizing chiral metallosupramolecular host. Angew Chem Int Ed Engl 2009;48:6675-7.
36. Luo L, Liao GH, Wu XL, Lei L, Tung CH, Wu LZ. Gamma-cyclodextrin-directed enantioselective photocyclodimerization of methyl 3-methoxyl-2-naphthoate. J Org Chem 2009;74:3506-15.
37. Yang C, Ke C, Liang W, et al. Dual supramolecular photochirogenesis: ultimate stereocontrol of photocyclodimerization by a chiral scaffold and confining host. J Am Chem Soc 2011;133:13786-9.
38. Yao J, Yan Z, Ji J, et al. Ammonia-driven chirality inversion and enhancement in enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylate mediated by diguanidino-γ-cyclodextrin. J Am Chem Soc 2014;136:6916-9.
39. Ji J, Wu W, Liang W, et al. An ultimate stereocontrol in supramolecular photochirogenesis: photocyclodimerization of 2-anthracenecarboxylate mediated by sulfur-linked β-cyclodextrin dimers. J Am Chem Soc 2019;141:9225-38.
40. Kanagaraj K, Liang W, Rao M, et al. pH-controlled chirality inversion in enantiodifferentiating photocyclodimerization of 2-antharacenecarboxylic acid mediated by γ-cyclodextrin derivatives. Org Lett 2020;22:5273-8.
41. Wei X, Raj AM, Ji J, et al. Reversal of regioselectivity during photodimerization of 2-anthracenecarboxylic acid in a water-soluble organic cavitand. Org Lett 2019;21:7868-72.
44. Genzink MJ, Kidd JB, Swords WB, Yoon TP. Chiral photocatalyst structures in asymmetric photochemical synthesis. Chem Rev 2022;122:1654-716.
45. Koodanjeri S, Joy A, Ramamurthy V. Asymmetric induction with cyclodextrins: photocyclization of tropolone alkyl ethers. Tetrahedron 2000;56:7003-9.
46. Shailaja J, Karthikeyan S, Ramamurthy V. Cyclodextrin mediated solvent-free enantioselective photocyclization of N-alkyl pyridones. Tetrahedron Lett 2002;43:9335-9.
47. Kaliappan R, Ramamurthy V. Chiral photochemistry within natural and functionalized cyclodextrins: chiral induction in photocyclization products from carbonyl compounds. J Photoch Photobio A 2009;207:144-52.
48. Mansour AT, Buendia J, Xie J, et al. β-cyclodextrin-mediated enantioselective photochemical electrocyclization of 1,3-dihydro-2H-azepin-2-one. J Org Chem 2017;82:9832-6.
49. Fukuhara G, Mori T, Wada T, Inoue Y. The first supramolecular photosensitization of enantiodifferentiating bimolecular reaction: anti-Markovnikov photoaddition of methanol to 1,1-diphenylpropene sensitized by modified beta-cyclodextrin. Chem Commun 2006;28:1712-4.
50. Fukuhara G, Mori T, Inoue Y. Competitive enantiodifferentiating anti-Markovnikov photoaddition of water and methanol to 1,1-diphenylpropene using a sensitizing cyclodextrin host. J Org Chem 2009;74:6714-27.
51. Dong J, Liu Y, Cui Y. Supramolecular chirality in metal-organic complexes. Acc Chem Res 2021;54:194-206.
52. Jin Y, Zhang Q, Zhang Y, Duan C. Electron transfer in the confined environments of metal-organic coordination supramolecular systems. Chem Soc Rev 2020;49:5561-600.
53. Pan M, Wu K, Zhang J, Su C. Chiral metal–organic cages/containers (MOCs): from structural and stereochemical design to applications. Coordin Chem Rev 2019;378:333-49.
54. Li K, Zhang LY, Yan C, et al. Stepwise assembly of Pd6(RuL3)8 nanoscale rhombododecahedral metal-organic cages via metalloligand strategy for guest trapping and protection. J Am Chem Soc 2014;136:4456-9.
55. Wu K, Li K, Hou YJ, et al. Homochiral D4-symmetric metal-organic cages from stereogenic Ru(II) metalloligands for effective enantioseparation of atropisomeric molecules. Nat Commun 2016;7:10487.
56. Guo J, Xu YW, Li K, et al. Regio- and enantioselective photodimerization within the confined space of a homochiral ruthenium/palladium heterometallic coordination cage. Angew Chem Int Ed Engl 2017;56:3852-6.
57. Guo J, Fan YZ, Lu YL, Zheng SP, Su CY. Visible-light photocatalysis of asymmetric [2+2] cycloaddition in cage-confined nanospace merging chirality with triplet-state photosensitization. Angew Chem Int Ed Engl 2020;59:8661-9.
58. Yoshizawa M, Tamura M, Fujita M. Diels-alder in aqueous molecular hosts: unusual regioselectivity and efficient catalysis. Science 2006;312:251-4.
59. Nishioka Y, Yamaguchi T, Kawano M, Fujita M. Asymmetric [2 + 2] olefin cross photoaddition in a self-assembled host with remote chiral auxiliaries. J Am Chem Soc 2008;130:8160-1.
60. Chen J, Wu X, Huang S, et al. Catalytic enantioselective cycloaddition transformation of tricyclic arenes enabled by a dual-role chiral cage-reactor. ACS Catal 2024;14:3733-41.
61. Ruan J, Li Z, Yin C, et al. Enantioselective [2+2] cross-photocycloaddition enabled by a chiral cage reactor via multilevel-selectivity control. ACS Catal 2024;14:7321-31.
62. Xiao JD, Jiang HL. Metal-organic frameworks for photocatalysis and photothermal catalysis. Acc Chem Res 2019;52:356-66.
63. Wang J, Wang C, Lin W. Metal–organic frameworks for light harvesting and photocatalysis. ACS Catal 2012;2:2630-40.
64. Qiu X, Zhang Y, Zhu Y, et al. Applications of nanomaterials in asymmetric photocatalysis: recent progress, challenges, and opportunities. Adv Mater 2021;33:e2001731.
65. Chen XY, Chen H, Đorđević L, et al. Selective photodimerization in a cyclodextrin metal-organic framework. J Am Chem Soc 2021;143:9129-39.
66. Wu P, He C, Wang J, et al. Photoactive chiral metal-organic frameworks for light-driven asymmetric α-alkylation of aldehydes. J Am Chem Soc 2012;134:14991-9.
67. Xia Z, He C, Wang X, Duan C. Modifying electron transfer between photoredox and organocatalytic units via framework interpenetration for β-carbonyl functionalization. Nat Commun 2017;8:361.
68. Zhang Y, Guo J, Shi L, et al. Tunable chiral metal organic frameworks toward visible light-driven asymmetric catalysis. Sci Adv 2017;3:e1701162.
69. Hu YH, Liu CX, Wang JC, Ren XH, Kan X, Dong YB. TiO2@UiO-68-CIL: a metal-organic-framework-based bifunctional composite catalyst for a one-pot sequential asymmetric Morita-Baylis-Hillman reaction. Inorg Chem 2019;58:4722-30.
70. Wang S, Liu W, Wang J, et al. Mechanochemical encapsulation of enzymes into MOFs for photoenzymatic enantioselective catalysis. ACS Mater Lett 2024;6:2609-16.
71. Liu W, Yang Y, Yang X, et al. Template-directed fabrication of highly efficient metal-organic framework photocatalysts. ACS Appl Mater Interfaces 2021;13:58619-29.
72. Kushnarenko A, Zabelina A, Guselnikova O, et al. Merging gold plasmonic nanoparticles and L-proline inside a MOF for plasmon-induced visible light chiral organocatalysis at low temperature. Nanoscale 2024;16:5313-22.
74. Yang L, Wang J, Zhao K, et al. Photoactive covalent organic frameworks for catalyzing organic reactions. Chempluschem 2022;87:e202200281.
76. Ding SY, Wang W. Covalent organic frameworks (COFs): from design to applications. Chem Soc Rev 2013;42:548-68.
77. He T, Zhao Y. Covalent organic frameworks for energy conversion in photocatalysis. Angew Chem Int Ed Engl 2023;62:e202303086.
78. Kang X, Stephens ER, Spector-Watts BM, et al. Challenges and opportunities for chiral covalent organic frameworks. Chem Sci 2022;13:9811-32.
79. Kang X, Wu X, Han X, Yuan C, Liu Y, Cui Y. Rational synthesis of interpenetrated 3D covalent organic frameworks for asymmetric photocatalysis. Chem Sci 2019;11:1494-502.
80. Li C, Ma Y, Liu H, et al. Asymmetric photocatalysis over robust covalent organic frameworks with tetrahydroquinoline linkage. Chinese J Catal 2020;41:1288-97.
81. Zhou Z, Li L, Dai L, Liu H, Li Y, Li P. The synthesis of highly crystalline covalent organic frameworks via the monomer crystal induction for the photocatalytic asymmetric α‐alkylation of aldehydes. J Polym Sci 2024;62:1621-8.
82. Ma HC, Sun YN, Chen GJ, Dong YB. A BINOL-phosphoric acid and metalloporphyrin derived chiral covalent organic framework for enantioselective α-benzylation of aldehydes. Chem Sci 2022;13:1906-11.
83. He T, Liu R, Wang S, et al. Bottom-up design of photoactive chiral covalent organic frameworks for visible-light-driven asymmetric catalysis. J Am Chem Soc 2023;145:18015-21.
84. Kan X, Wang JC, Chen Z, et al. Synthesis of metal-free chiral covalent organic framework for visible-light-mediated enantioselective photooxidation in water. J Am Chem Soc 2022;144:6681-6.
85. Ma HC, Zhao CC, Chen GJ, Dong YB. Photothermal conversion triggered thermal asymmetric catalysis within metal nanoparticles loaded homochiral covalent organic framework. Nat Commun 2019;10:3368.
86. Ma HC, Chen GJ, Huang F, Dong YB. Homochiral covalent organic framework for catalytic asymmetric synthesis of a drug intermediate. J Am Chem Soc 2020;142:12574-8.
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Su, Y. W.; Pan C.; Sun T. Y.; Zou Y. Q. Enantioselective photochemical reactions within the confined cavities of supramolecular assemblies. Chem. Synth. 2024, 4, 63. http://dx.doi.org/10.20517/cs.2024.64
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