The Catalytic Enantioselective [1,2]-Wittig Rearrangement of Allylic Ethers

Abstract

The catalytic enantioselective [1,2]-Wittig rearrangement of allylic ethers is currently unknown. This process constitutes a recognised challenge as it is traditionally considered to arise from a non-concerted reaction pathway via formation and recombination of radical pairs. This manuscript demonstrates a catalytic enantioselective solution to this challenge, and shows that [1,2]-Wittig products are generated via an alternative reaction cascade to traditional dogma. The developed process employs a chiral bifunctional iminophosphorane catalyst to promote an initial enantioselective [2,3]-sigmatropic rearrangement. A subsequent base promoted, stereoconvergent, ionic fragmentation-recombination that proceeds with high enantiospecificity and retention of configuration, formally equivalent to a Woodward-Hoffmann forbidden thermal [1,3]-sigmatropic rearrangement, generates [1,2]-Wittig products in up to 97:3 er. This unique chirality transfer process will have broad implications for fundamental stereocontrol in organic transformations.