Rhodium(III)-Catalyzed Annulation between N-Sulfinyl Ketoimines and Activated Olefins: C–H Activation Assisted by an Oxidizing N–S Bond

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Abstract

C-H activation under redox-neutral conditions, especially by Rh(III) catalysis, has offered attractive synthetic strategies. Previous work in redox-neutral C-H activation relied heavily on the cleavage of oxidizing N-O and N-N directing groups, and cleavable N-S bonds have been rarely used, although they may offer complementary coupling patterns. In this work, N-sulfinyl ketoimines were designed as a novel substrate for the redox-neutral coupling with different activated olefins via a Rh-catalyzed C-H activation pathway. The coupling with acrylate esters afforded 1H-isoindoles with the formation of three chemical bonds around a quaternary carbon. Furthermore, the coupling with maleimides furnished pyrrolidone-fused isoquinolines. A broad scope of substrates has been established. The mechanism of the coupling with acrylates has been studied in detail by a combination of experimental and computational methods. This coupling proceeds via imine-assisted C-H activation of the arene followed by ortho C-H olefination to afford a Rh(III) olefin hydride intermediate which, upon deprotonation, may exist in equilibrium with a Rh(I) olefin species. Cleavage of the N-S bond occurs only after C-H olefination to generate a Rh(III) imide species. DFT studies indicated that the imide group can undergo migratory insertion to produce a Rh(III) secondary alkyl which isomerizes under the assistance of acetic acid to a Rh(III) tertiary alkyl that is prone to insertion of the second acrylate.

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