Asymmetric induction in η2-arene complexes of pentaammineosmium(II)

A series of eta(2)-arene complexes (2a-i) of the form [Os(NH3)(5)((ArOCHRCH3)-C-1)](2+) was prepared in which the alkoxy substituent R-1 bears a potential hydrogen bond acceptor. Using an ester or amide group as the hydrogen bond acceptor, a two-point interaction was achieved between the pentaammineosmium system and the organic ligand. For these complexes, a single coordination diastereomer was observed. For compound 2a, evidence for hydrogen bonding between the carbonyl oxygen and the pentaammineosmium fragment was obtained through H-1 NMR studies and by a comparison of the rate of arene displacement for 2a compared to that of the parent complex [Os(NH3)(5)(anisole)](2+). Complex 2a undergoes stereospecific protonation to form a 4H-anisolium product 3a. Complex 3a undergoes nucleophilic addition of a silylketene acetal at C3 to generate an alkoxydiene complex (4) as a single diastereomer. Hydrolysis of 4 removes the chiral auxiliary, and subsequent oxidative decomplexation yields a cyclohexenone with a new stereogenic center.