Converging Stereodivergent Reactions: Highly Stereoselective Formal anti-Markovnikov Addition of H2 O to Mixtures of Olefins

Abstract

We describe a highly diastereo- and enantioselective two-step formal anti-Markovnikov addition of H₂ O to diastereomeric mixtures of trisubstituted olefins. Our approach overcomes the limits of classical stereospecific addition reactions to olefins for the generation of adjacent stereocenters. In these stereospecific reactions, separation of olefin diastereomers is essential. Our method circumvents the need for such difficult separations by simultaneously employing both diastereomeric olefins in the organocatalytic, highly enantioselective, syn-specific Shi-epoxidation to yield diastereomeric oxiranes. The stereochemical model proposes the smallest substituent on the olefin to be stereodefining resulting in an identical enantiotopic approach for both olefin isomers on the less substituted carbon. By employing a stereoconverging epoxide hydrosilylation the identically configured center is retained, while the differing one is converted to a planar radical center that is reduced by a syn-selective intramolecular hydrogen atom transfer (HAT) from a Ti-H bond. The observed converging behavior can be attributed to a HAT-preceding directional isomerization step, that interconverts the obtained rotameric radicals which ultimately leads to high to excellent enantiomeric ratios of the final secondary alcohols.

Keywords: catalysis; convergence; epoxidation; hydrosilylation; titanium.