| Summary: | This article describes the development of a nickel-catalyzed regio- and diastereoselective formal [3+2] cycloaddition between <i>N</i>-substituted indoles and donor–acceptor cyclopropanes to synthesize cyclopenta[<i>b</i>]indoles. Optimized reaction conditions provide the desired nitrogen-containing cycloadducts in up to 93% yield and <i>dr</i> 8.6:1 with complete regioselectivity. The substrate scope showed high tolerance to various substituted indoles and cyclopropanes, resulting in the synthesis of six new cyclopenta[<i>b</i>]indoles and the isolation of five derivatives previously reported in the literature. In addition, a mechanistic proposal for the reaction was studied through online reaction monitoring by ESI-MS, allowing for the identification of the reactive intermediates in the Ni(II) catalyzed process. X-ray crystallography confirmed the structure and relative endo stereochemistry of the products. This method enables the fast and efficient construction of fused indolines from readily accessible starting materials.
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