Dual visible-light and NHC-catalyzed radical relay trifunctionalization of unactivated alkenes
Abstract
Radical trifunctionalization of unactivated alkenes remains rare and challenging, although they can provide a robust tool for the construction of molecules with high added value from simple materials. This work presents the relay dual N-heterocyclic carbene organocatalytic and visible-light photocatalytic multi-component trifunctionalization of alkyl alkenes via the merger of remote 1,4-cyano migration and alkylacylation. The method features a broad substrate scope and good compatibility of diverse functional groups. Density functional theory calculations were also carried out to rationalize the origin of this reaction. The cooperative N-heterocyclic carbene and photoredox catalysis enabled reductive single-electron transfer reaction of acyl azolium species and subsequent radical-radical cross-coupling, allowing for the facile construction of three new C−C bonds in one-pot reactions with high regioselectivity.
Keywords
INTRODUCTION
The assembly of molecular frameworks by multi-step relay carbon-carbon bond cleavage and formation in a controlled and efficient manner has been at the heart of organic synthesis. Alkenes are readily available and inexpensive feedstocks that have been widely used for that purpose. Difunctionalization of alkenes presents numerous mature methods for accessing diverse-ranging molecules with high added value and structural complexity[1-6]. In comparison, the trifunctionalization of alkenes, particularly unactivated alkenes, is less explored and still poses a challenge. Recently, the appearance of radical functionalization of unactivated alkenes through the strategy of remote functional group migration (FGM)[7-10] provides an exceptional chance for the construction of trifunctionalized derivatives that are not accessible by other reactions [Scheme 1A]. Therefore, the discovery of new protocols for FGM reactions remains an important target and continues to be highly desirable for molecular assembly.
Scheme 1. Radical-mediated trifunctionalization of unactivated alkenes. FGM: Functional group migration; NHC: N-Heterocyclic carbene.
N-Heterocyclic carbene (NHC) catalysis[11-17] has emerged as one of the most powerful catalytic strategies in the field of organocatalysis, usually by means of a polarity-reversal mechanism. With the rapid development of photocatalysis [18-21], recent advances in single-electron transfer (SET)-based radical reactions have further broadened its reaction modes[22-28], offering otherwise inaccessible strategies compared to traditional
As a continuation of our studies on radical NHC-catalysis, our purpose herein is to realize the first reductive radical organocatalytic FGM reactions to fulfill trifunctionalization of hexenenitriles 3[70,71] via remote cyano migration[72-75]. Compared to the previous work using the NHC oxidative radical strategy, the substrate scope can enlarge to in-stock carboxylic acids and much more radical precursors. Thus, this protocol features a compatible dual catalytic system, mild reaction conditions, readily available substrates, excellent regioselectivity, and capability of late-stage functionalization, which will be favorable for molecular assembly [Scheme 1C].
RESULTS AND DISCUSSION
To start this work, commercially available sodium trifluoromethanesulfinate 1a was used as the trifluoromethyl radical precursor to investigate the feasibility of the reaction with 4-chlorobenzoic acid 2a and hexenenitrile 3a using DCE as the solvent, Cs2CO3 as a base, and Blue LEDs as light sources [Figure 1]. It is worth noting that the acid can be activated to produce benzoylimidazole 2a’in situ with
Figure 1. Optimization of the reaction conditiona. aUnless otherwise noted, 2a (0.4 mmol, 2.0 equiv.) and CDI (0.4 mmol, 2.0 equiv.) in DCE (1 mL) were typically stirred for 2 h. Then, the above solution was added to the mixture of 1a (0.4 mmol, 2.0 equiv.), 3a (0.2 mmol, 1.0 equiv.), NHC catalyst (0.03 mmol, 15 mol%), photocatalyst (0.01 mol, 5 mol%), and Cs2CO3 (0.4 mmol, 2.0 equiv.) in DCE (2 mL) which was irradiated under Blue LED typically for 24 h; bIsolated yields based on 2a, the dr was around 1/1 for all cases, and the dr was determined by 1H NMR; cThe reaction was conducted without N2 protection; d1 mmol scale; eK2CO3 as a base; ftriethylamine as a base; gDBU as a base. CDI: 1,1’-carbonyldiimidazole DBU; NHC: N-Heterocyclic carbene.
After screening different photosensitizers, it was found that the desired product 4a was produced in 72% yield with [Ir(dtbbpy)(ppy)2]PF6 as the photocatalyst (entry 3). Other photocatalysts, such as fac-Ir(ppy)3, 4CzIPN, and Br-4CzIPN, were inferior to the [Ir(dtbbpy)(ppy)2]PF6 (entries 4-7). Subsequently, we screened different NHC precursors, and NHC A was selected as the optimal organocatalyst to promote the reaction. Other NHCs B-D did not improve the reaction yields (entries 8-10). The nitrogen atmosphere protection was also essential for this reaction. The yield dropped to 45% when the reaction was conducted in the air (entry 11). Moreover, a scale-up (1 mmol) reaction was taken to afford product 4a in maintained yield (entry 12). Remarkably, only a trace of byproduct 4a’ was observed along the optimization process. At last, bases were screened, and a slightly decreased yield was observed with K2CO3. Organic bases were not suitable for this transformation.
With the optimal condition in hand, the generality of substrates was explored. We initially tested different acids 2 [Scheme 2]. Substituted benzoic acids bearing either halide or electron-donating groups at para positions of the benzoic acid were well compatible with the optimal condition, giving the products 4a-e in comparable yields. Slightly decreased yields were tracked when benzoic acids bearing electron-withdrawing groups 4f-h. Benzoic acids bearing meta substitution were also well tolerated under the standard conditions 4i-j. However, the more sterically hindered 2-methylbenzoic acid was not suitable for this reaction. Either
Scheme 2. Substrate scope for trifunctionalization of unactivated alkenes. CDI: 1,1’-carbonyldiimidazole.
The feasibility of installation of other radical precursors to the hexenenitrile 3d was also studied [Scheme 2, bottom]. Several sulfinate salts bearing a fluorinated alkyl group displayed good compatibility with the present system 4aa-ad. Sulfinate salts bearing a phenylsulfonyl substituted methyl group were also used as effective substrates, enabling the trifunctionalization of 3d to give product 4ae in an acceptable yield. However, sulfinate salts, such as sodium methanesulfinate, sodium ethanesulfinate, and other listed aliphatic sulfinates, did not deliver the desired products. Density functional theory (DFT) calculations were then taken into account for the possible reasons [Scheme 3]. To our interests, the activating energy of the corresponding radical added to the hexenenitrile significantly contributes to the success of the reaction. The reactions failed to yield the desired product when the activating energy was above 14.1 Kcal/mol. These findings might be helpful for prediction of the reactivity of other sulfinate salts.
To highlight the utility of this transformation, we undertook the derivatization of the ketone 4a [Scheme 4]. The alcohol 5 was synthesized using LiAlH4 as redundant in the yield of 83%. The cyano group within 4a can transform to amide to yield compound 6 with a mixed acid system (HOAc and H2SO4). In addition, the reaction of 4a with hydrazine could form hydrazone 6 in 80% yield.
Next, DFT calculations have been conducted to further prove the potential reaction mechanism and origin of the regioselectivity for 1,4-cyano migration [Figure 2].
The combination of NHC-A (C1) generated from precatalyst A under basic conditions with benzoylimidazole 2a’ leads to NHC-bound acyl azolium C2. The SET process between C2 and CF3SO2Na proceeds smoothly to give transient trifluoromethyl radical and persistent NHC-bound ketyl radical C3. Subsequent addition of trifluoromethyl radical to hexenenitrile 3a produces intermediate S2 through the transition state TS1 (ΔG‡ = 9.8 kcal/mol). Whether the incident of 1,4-CN migration of TS1 occurs would decide the two possible pathways (path A, purple line vs. path B, blue line) for the radical-radical coupling. As described in the blue line, the direct radical-radical coupling of C3 with S2 yields intermediate S5 through transition state TS4 (ΔG‡ = 17.0 kcal/mol). The collapse of S5 gives birth to product 4a’ and NHC-A for the next catalytic cycle. In path A (purple line), the intramolecular radical addition of S2 to the cyano group forms a five-member imine radical intermediate S3 through transition state TS2 (ΔG‡ =
Based on these results, it can be concluded that the dual photocatalytic and NHC-promoted SET between sodium trifluoromethanesulfinate 1a and acyl azolium intermediate SA could simultaneously produce the NHC-derived radicals C3 and CF3 radicals. CF3 radicals were subsequently triggered by hexenenitriles 3 to obtain S2. The generated radical intermediate S2 may undergo remote 1,4-cyano migration via a cyclic intermediate S3 to yield radicals S4. The radical-radical cross-coupling with the persistent ketyl radical C3 and S2 or S4 would produce intermediate S5 and S7, respectively, which finally afford adducts 4a and byproduct 4a’ with the loss of NHC for the next catalytic cycle [Scheme 5].
CONCLUSIONS
To sum up, we have described a novel protocol for trifunctionalization of unactivated hexenenitriles via merged NHC organocatalytic and photocatalytic radical relay alkylacylation. This reaction offers a generalizable and efficient strategy for molecular framework assembly, which involves multi-step C−C bond cleavage and formation by remote 1,4-cyano migration and alkylacylation. Theoretical calculations were used to both support the possible mechanism and figure out the origin of the regioselectivity for 1,4-CN migration.
DECLARATIONS
Authors’ contributions
Supervised the synthesis process: Feng J
Responsible for data collection and analysis: Feng JQ, Li L, Wang J, Ni A
Conducted calculations: Wei Z
Led the design and paper-writing efforts: Du D, Feng J
Availability of data and materials
NMR data for all the new compounds can be found in Supplementary Materials, and the data supporting the findings of this study are available within its Supplementary Materials.
Financial support and sponsorship
This work was supported by the National Natural Science Foundation of China (Nos. 21572270, 21702232), the “Double First-Class” University Project of China Pharmaceutical University, and the College Students’ Innovative Entrepreneurial Training Plan Program.
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.
Supplementary Materials
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How to Cite
Feng, J. Q.; Li, L.; Wang, J.; Ni, A.; Wei, Z.; Du, D.; Feng, J. Dual visible-light and NHC-catalyzed radical relay trifunctionalization of unactivated alkenes. Chem. Synth. 2024, 4, 5. http://dx.doi.org/10.20517/cs.2023.43
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