Highly Regio- and Enantioselective Formal [3 + 2]-Annulation of Indoles with Electrophilic Enol Carbene Intermediates

Chiral cyclopentane-fused indolines are synthesized with high regio- and enantiocontrol by formal [3 + 2]-annulation reactions of indoles and electrophilic enol carbenes. High enantioselectivity and exclusive regiocontrol occurred with enoldiazoacetamides using a less sterically encumbered prolinate-ligated dirhodium(II) catalyst in reactions with N-substituted indoles without substituents at the 2- or 3-positions via a selective vinylogous addition process. In this transformation, donor–acceptor cyclopropenes generated from enoldiazoacetamides serve as the carbene precursors to form metal carbene intermediates.