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Cyclopropanes constitute a pivotal molecular scaffold in medicinal and agrochemical research and find broad applications in marketed drugs and other bioactive compounds. Their synthesis commonly relies on metal-catalyzed carbene transfer reactions that necessitate the utilization of stereochemically defined olefin starting materials, which in turn requires a high stereochemical fidelity in the olefin synthesis step. Herein, we report on a photocatalytic strategy that allows the use of a mixture of the E- and Z-isomers of such olefins and gives access to a single isomer of the cyclopropane product in a stereoconvergent reaction. Experimental and theoretical data suggest the intermediacy of a triplet carbene intermediate that readily reacts with either isomer of the olefin. The intersystem crossing from triplet to singlet state proceeds in a diastereoselective fashion and can rationalize this stereoconvergent reaction. The application of this process was then examined with a diverse set of stereochemical mixtures of double- or triple-substituted olefins that readily undergo cyclopropanation disregarding of the stereochemical fidelity of the used olefin.
The high ring strain and unique geometric features of cyclopropanes and the development of novel synthesis strategies is of continuing interest in chemistry [
]. Only in late 2021, the FDA emergency use authorization of Paxlovid showed the high relevance of cyclopropanes in modern drugs and the continuous need in the development of novel and efficient synthesis methods for cyclopropanes [
]. The vast majority of these build on the reactivity of a metal-bound or free singlet carbene that undergoes stereospecific cycloaddition reactions with olefins that in turn requires olefination methods with high fidelity on the stereochemical outcome [
]. Such approach would reduce synthesis demands in the olefination reaction and could be used to rapidly construct cyclopropanes in a stereoselective fashion. To achieve such stereoconvergent cyclopropanation reaction, the key reaction step of cycloaddition should occur in a step-wise fashion, which can be achieved, e.g. using metal-bound or free triplet carbene intermediates. In this context, the groups of de Bruin [
] recently reported on Co-based catalyzed cyclopropanation reactions of styrene that involve a metal-bound radical carbene intermediate (Scheme 1b). In a related context, our group reported that simple acceptor-only diazoalkanes would undergo the formation of a free triplet carbene under photocatalytic conditions to achieve gem-difluoroolefination reactions [
We hypothesized that such photocatalytic approach are a feasible strategy to achieve stereoconvergent cyclopropanation reactions of E/Z mixtures of 1,2-disubsituted olefins, which can be readily obtained in a classic, unselective Wittig olefination.
2. Results and discussion
To achieve this goal, we first studied the reaction of E-β-methyl styrene (8a) with ethyl diazoacetate (EDA, 6a) in the presence of different photocatalysts under irradiation with blue LEDs. We observed formation of 9a as a single diastereomer in 84% yield when using 4-CzIPN as catalyst (Table 1, entry 1). To proof our above hypothesis on stereoconvergent cyclopropanation reactions, we next studied the reaction of Z-β-methyl styrene (8b) and observed formation of the same product 9a in slightly reduced yield (Table 1, entry 2). No significant differences were observed when investigating a 1:1 mixture of E- and Z- β-methyl styrene, which led to formation of 9a as a single diastereomer in 83% yield (Table 1, entry 3). Further studies concerned the use of different photocatalysts that were found either incompatible with the present reaction conditions (Table 1, entry 4) or led to a mixture of diastereomers (Table 1, entry 5). For further optimization, we also studied different reaction parameters such as solvent, concentration, or stoichiometry, yet none of these changes increased the reaction efficiency (for details please see Table S1 in ESI). Finally, control reactions concerned the reaction in the dark or under thermal conditions, yet either no reaction or decomposition of EDA (6a) was observed (Table 1, entries 6,7). Moreover, when studying the dark reaction in the presence of carbene transfer metal catalyst (e.g. Rh2(OAc)4 and FeTPPCl) only a moderately selective cyclopropanation could be observed at best (Table 1, entries 8,9. This observation underlines the efficiency of photocatalytic carbene transfer reactions.
Table 1Optimization of reaction conditions.
a Reaction conditions: In an oven-dried test tube 8a (0.6 mmol, 3 equiv.), 6a (0.2 mmol, 1 equiv.) and 4-CzIPN (3 mol%) were dissolved in 1 mL dry and degassed CH2Cl2 under argon atmosphere, irradiated with blue LEDs (40 W × 2, 467 nm) for 16 h at room temperature; all the further changes and their corresponding outcomes have been noted in the Table 1.
b Isolated yields. dec = decomposition of EDA (6a). 4-CzIPN = 1,2,3,5-tetrakis(carbazole-9-yl)-4,6-dicyanobenzene; FeTPPCl = 5,10,15,20-Tetraphenyl-21H,23H-porphine iron(III) chloride; CoPc = Cobalt(II)-phthalocyanine.
With the optimized conditions in hand, we next embarked on studies of cyclopropanation reactions using a diverse set of β-methyl styrenes, which were employed as a mixture of E- and Z-isomers of varying composition (Scheme 2a, and Table S4 for d.r. of styrenes). Different alkyl groups, electron-donating, or electron-withdrawing groups in the para- and meta-positions had little effect on the reaction outcome and the corresponding triple-substituted cyclopropanes (9b-g) were obtained in high isolated yield and high diastereoselectivity. Only in the case of ortho-substitution of the aromatic ring, a significant reduction of the product yield was observed (9h-k), which may be related to steric hindrance due to the ortho substituent. The nitro-group was identified as a limitation of this method and a detrimental effect on the diastereoselectivity was observed (9l and 9m). In the course of these studies, we could observe that heterocycles are compatible and 9n and 9o were obtained in good yield as a single diastereoisomer.
Next, we examined different β-alkyl styrenes, which smoothly reacted in excellent diastereoselectivity under the present reaction conditions (Scheme 2b). Simple alkyl chains of varying chain length and substitution were well-tolerated in the present reaction (11a-e). Most notably, even a cyclopropyl substituent proved compatible and the bis-cyclopropane 11d was obtained in good yield, however only in moderate diastereoselectivity. A limitation lies within the compatibility of Lewis-basic carbonyl groups, as shown in example 11e. We then embarked on the evaluation of an additional aromatic substituent at the olefin, yet, only a low yield of cyclopropane 11f was obtained when using stilbene. Importantly, when the second phenyl substituent is located more remote, the cyclopropanation proceeds well (11g, 11h). As part of these studies, we examined cyclic olefins dihydronaphthalene and indene smoothly reacted to cyclopropane 11i and 11j, respectively. To further probe the robustness and versatility of this approach, we also examined triple- and tetrasubstituted styrenes. Remarkably, the tetra-substituted cyclopropanes 11k and 11l could be obtained in good yield as a single diastereoisomer from triple-substituted olefins. A limitation however lies within the use of fully-substituted olefins, internal aliphatic olefins, or electron-poor olefins, such as β-nitro styrene or cinnamic ester. In this case, the cyclopropanation did not occur – instead decomposition of ethyl diazoacetate (6a) was observed, which we assume to be related to the steric or electronic influence of the aromatic ring.
We next examined the application of different alkyl diazoacetates (Scheme 3). Simple alkyl and aryl esters were tolerated in this photocatalytic cyclopropanation reaction, and cyclopropanes 12a and 12b were obtained in high yield. A limitation of this method lies within the use of diazoacetates bearing a benzylic or remotely aryl-substituted ester group. In these cases, we consistently observed a reduced diastereoselectivity (12c-e). Importantly, this method can be extended to the synthesis of tetra-substituted cyclopropane 12f, which was obtained as a single diastereoisomer in good isolated yield. Finally, we examined diazoacetates derived from natural products and examples derived from terpenes or a protected sugar were well-tolerated to access the corresponding cyclopropanes (12g-i).
For an understanding of the reaction mechanism, we performed a set of control experiments. An on/off experiment suggests that constant irradiation with light is needed for the reaction to proceed (Scheme 4a). Fluorescence quenching data is further indicative of a reaction mechanism that involves either an electron or energy transfer process with ethyl diazoacetate as reaction partner (Scheme 4b). Only a weak fluorescence quenching was observed using β-methyl styrene, which is also reflected in an isomerization reaction of either E- or Z-β-methyl styrene under photocatalytic reaction conditions (Scheme 4c, for details, please see ESI, Table S2) [
]. We further probed a radical trapping experiment with TEMPO, DMPO, and 1,4-cyclohexadiene. While a minor reduction of the reaction yield was observed with DMPO and 1,4-cyclohexadiene, the reaction was completely inhibited in the presence of TEMPO which we assume that this is related to the strong absorption of TEMPO in the visible light (Scheme 4d). Furthermore, we examined radical clocks using cyclopropyl-substituted styrenes. α-Cyclopropyl styrene 13 underwent smooth cyclopropanation (14) of the olefin and no by-products from ring opening of the α-cyclopropane ring was observed (Scheme 4e). As described above (Scheme 2b), the cyclopropyl unit of β-cyclopropyl styrene also remained stable (11d) under reaction conditions.
The above data is suggestive of a reaction mechanism that involves a stepwise reaction mechanism (Scheme 5). We therefore consider that the photoexcitation of the 4-CzIPN (PC) catalyst results in a triplet sensitization [
] of ethyl diazoacetate (6a) to initially give a triplet carbene intermediate (16). This triplet carbene undergoes addition reaction with the styrene component via addition to the β-carbon atom. Rapid C–C bond rotation followed by intersystem crossing and cyclization gives the cyclopropane product (9a) and results in the high stereochemical fidelity of this process. The persistence of the cyclopropane ring in the α-position (Scheme 4e) also suggests a rapid, subsequent intersystem crossing that is followed by cyclization to the cyclopropane product.
In summary, we herein report on the photocatalytic reaction of styrenes in a stereoconvergent reaction. In the presence of a cheap and readily available organic photocatalyst, a diverse set of double- or triple-substituted olefins undergoes smooth cyclopropanation disregarding of the stereochemistry or the stereochemical fidelity of the used olefin.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
RMK thanks the German Science Foundation of RWTH Aachen Foundation for financial support. CE acknowledges the Fonds der Chemischen Industrie for a Kekulé scholarship. The authors acknowledge Adithyaraj Koodan for the synthesis of starting materials.
The Supporting Information is available free of charge: Experimental details and spectroscopic data for all products.
Appendix A. Supplementary data
The following is the Supplementary data to this article: