Asymmetric Induction in η2-Arene Complexes of Pentaammineosmium(II)

A series of η 2 -arene complexes ( 2a − i ) of the form [Os(NH 3 ) 5 (ArOCHR 1 CH 3 )] 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 1 H NMR studies and by a comparison of the rate of arene displacement for 2a compared to that of the parent complex [Os(NH 3 ) 5 (anisole)] 2+ . Complex 2a undergoes stereospecific protonation to form a 4 H -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.