Iridium-Catalyzed Asymmetric Hydrogenation of Carbocation Precursors via Wagner-Meerwein Rearrangement.

Carbocation rearrangement is a powerful tool for converting a simple precursor into a complex molecular scaffold. However, controlling the stereoselectivity of a reaction that involves carbocation rearrangement is challenging and remains elusive. In this study, we demonstrate a novel iridium-catalyzed Wagner-Meerwein rearrangement and asymmetric hydrogenation of carbocation precursors (1-(aryl)-1-(1-methylcyclobutyl/cyclopentyl) ethan-1-ol) for the synthesis of various optically active gem-dimethyl cycloalkanes. Hence, enantiopure gem-dimethyl-containing compounds are important motifs found in many natural products, and some are FDA-approved drugs. Our methodology starts with an iridium-catalyzed formal deoxygenation of tertiary alcohols to generate a tertiary carbocation that triggers the Wagner-Meerwein rearrangement via the ring expansion and alkyl migration cascade to furnish a stable tertiary-benzyl carbocation. Sequential olefination and in situ asymmetric hydrogenation provide access to various gem-dimethyl chiral cycloalkanes in excellent yield (> 99%) and enantioselectivity (ee up to > 99%). Otherwise, establishing a high yield and enantioselectivity in a saturated cyclic hydrocarbon next to a sterically hindered gem-dimethyl group would not be possible by conventional methods.