Diastereofacial Selectivity in Reactions of 5-Substituted (X) Adamant-2-yl Cations: Manifestations of Electron Demand

A series of 5-substituted (X) 2,2-dimethoxyadamantanes (4, X = F, Cl, Br, COOCH3, OCH3, CH3, C6H5, p-NO2C6H4, p-BrC6H4, SiMe3, and SnMe3) as well as 2-methylene-5-tert-butyladamantane (6, Y = CH2; X = C(CH3)3) have been synthesized and characterized. Reduction of the ketals 4 under ionic conditions with triethylsilane and phenylsilane provide π-facial diastereoselectivities for hydride trapping of 5-substituted (X) 2-methoxyadamant-2-yl cations (1, R = OCH3 ↔ 5). A comparison of this data with known diastereoselectivities for nucleophilic capture of tertiary and secondary 5-substituted (X) 2-adamantyl cations highlights that stereoselectivity in these systems is a function of electron demand. Diastereoselectivities for the hydrochlorination of 6 (Y = CH2, X = C(CH3)3) in CH2Cl2 and NO2CH3 have been determined and compared with the corresponding data for the silicon analogue (6, Y = CH2; X = SiMe3). Low electron demand coupled with the stereoelectronic requirement of double hyperconjugation appears the mos...