Asymmetric dearomatization of η2-arene complexes:: Synthesis of stereodefined functionalized cyclohexenones and cyclohexenes

A nonracemic eta(2)-arene complex (2) was prepared from (R)-(+)-methyl-2-phenoxypropionate and pentaammineosmium(II) with high coordination diastereoselectivity (>9:1). Complex 2 was then treated with HOTf, an acetal, or a Michael acceptor to generate 4H-oxonium complexes at low temperature. These products were then combined with a nucleophile (silylketene acetal or hydride) to form stable alkoxydiene complexes. Further reaction of these complexes with triflic acid yielded isolable enonium complexes, and subsequent hydrolysis formed substituted cyclohexenone complexes. Upon oxidative decomplexation, these materials provided substituted cyclohexenones isolated with high ee's (80-85%). The reaction of cycloenonium complexes with hydride led to the formation of cyclohexenyl ether complexes, and their oxidative decomplexation yielded substituted cyclohexenyl ethers with de and ee values > 90%. Treatment of a cyclohexenyl ether complex with triflic acid gave an isolable Jr-allyl complex. The reaction of this material with a nucleophile followed by decomplexation afforded cis-1,4,5-trisubstituted cyclohex-2-enes with ee's ranging from 80 to 90%. The increase in the steric bulk of the alkyl group of the alkoxy chiral auxiliary (methyl vs isopropyl) led to a more complete transfer of chirality.