Bromenium-catalysed tandem ring opening/cyclisation of vinylcyclopropanes and vinylcyclobutanes: a metal-free [3+2+1]/[4+2+1] cascade for the synthesis of chiral amidines and computational investigation.

We present a detailed study of a [3+2+1] cascade cyclisation of vinylcyclopropanes (VCP) catalysed by a bromenium species (Br(δ+)-X(δ-)) generated in situ, which results in the synthesis of chiral bicyclic amidines in a tandem one-pot operation. The formation of amidines involves the ring-opening of VCPs with Br-X, followed by a Ritter-type reaction with chloramine-T and a tandem cyclisation. The reaction has been further extended to vinylcyclobutane systems and involves a [4+2+1] cascade cyclisation with the same reagents.