Elsevier

Tetrahedron

Volume 70, Issues 27–28, 8 July 2014, Pages 4270-4278
Tetrahedron

[3+2] Photooxygenation of aryl cyclopropanes via visible light photocatalysis

https://doi.org/10.1016/j.tet.2014.02.045Get rights and content

Abstract

We report that Ru(bpz)32+ is an excellent sensitizer for the photooxygenation of aryl cyclopropanes upon irradiation with visible light. The effectiveness of this photocatalyst enables the synthesis of a range of five-membered endoperoxides in excellent yield with quite low (0.5 mol %) catalyst loadings even when standard household light sources are utilized.

Introduction

Cyclic peroxides are the characteristic pharmacophores of a class of biologically active compounds that exhibit a range of potent antibacterial, anticancer, and antimalarial activities.1 Five-membered endoperoxides, in particular, have also been valued as synthetic intermediates because of the ease with which their O–O bonds can be reductively cleaved to afford 1,3-diols.2 Many methods for the preparation of five-membered endoperoxides involve sequences that require pre-installation of the reactive peroxide moiety,3 which often limits the scope and yield of the reaction. An attractive alternative strategy involves the ring-expanding reaction of cyclopropanes with molecular oxygen. Vinylcyclopropanes can be induced to undergo this transformation upon reaction with phenylthiyl or phenylselenyl radicals,4 although the requirement for a vinyl substituent represents a significant limitation on the generality of this process. An arguably more general method is the photooxygenation of aryl cyclopropanes, originally developed by Mizuno and Otsuji,5 which is commonly catalyzed by organic PET sensitizers such as 9,10-dicyanoanthracene (DCA, 2).

For the past several years, our research group has been developing methods that exploit the remarkable photoredox properties of transition metal polypyridyl complexes to perform a variety of synthetically useful radical ion processes.6, 7 We have found that electron-deficient bipyrazyl and bipyrimidyl complex of Ru(II) in particular are excellent catalysts for the photochemical one-electron oxidation of electron-rich styrenes (Eq. 1), which has enabled us to develop a suite of synthetically useful reactions of the resulting radical cations.8, 9 Recognizing that the chemistry of cyclopropanes can often offer homologous reactivity to that of olefins, we have become interested in exploring radical cation reactions of electron-rich cyclopropanes (Eq. 2). Zheng recently demonstrated that Ru(bpz)32+ (1) is an effective visible light photocatalyst for the [3+2] cycloaddition of amine-substituted cyclopropanes with a variety of alkenes.10 In this manuscript, we demonstrate that Ru(bpz)32+ also catalyzes aerobic photooxygenation of electron-rich aryl cyclopropanes upon irradiation with visible light and is a markedly more effective catalyst for this transformation than DCA (Scheme 1).

Section snippets

Results and discussion

We began our initial investigations by examining the reaction of 1-(p-methoxyphenyl)-2-phenylcyclopropane (3a) under conditions based upon those we had developed for the synthesis of six-membered endoperoxides by aerobic [2+2+2] cycloaddition.11 Thus, irradiation of a solution of 3a in MeNO2 with a 23 W compact fluorescent light bulb (CFL) in the presence of 0.5 mol % Ru(bpz)3(PF6)2 under an atmosphere of O2 afforded 16% yield of the expected endoperoxide 4a (Table 1, entry 1); this compound

Conclusion

In summary, a wide range of electron-rich aryl cyclopropanes participate in aerobic [3+2] photooxygenation reactions upon irradiation with visible light in the presence of Ru(bpz)32+. In line with our previous investigations involving photogenerated alkene radical cations, we find that this photooxygenation reaction is markedly more efficient using this Ru(II) chromophore in place of DCA and enables the use of readily accessible household lighting sources in place of conventional photochemical

General information

THF and CH2Cl2 were purified by elution through alumina as described by Grubbs.12 MeNO2 was purified by distillation from MgSO4 prior to use. A 23 W compact fluorescent light bulb was used for all photochemical reactions depicted in Tables 1 and 2. Flash column chromatography was performed with Silicycle 40–63 Å silica (230–400 mesh). Ru(bpz)3(PF6)213 was prepared as previously described. Diastereomer ratios for all compounds were determined by 1H NMR analysis of the unpurified reaction

Acknowledgements

We thank the NIH (GM095666) and Sloan Foundation for funding of this research.

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