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 pi -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 o f tertiary and secondary 5-substituted (X) 2-adamantyl cations highlig hts 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 compare d with the corresponding data for the silicon analogue (6, Y = CH2; X = SiMe3). Low electron demand coupled with the stereoelectronic requir ement of double hyperconjugation appears the most likely explanation, rather than a long-range steric factor, for the total lack of stereose lectivity in the Cl- capture of the 2-methyl-5-(trimethylsilyl)adamant -2-yl cation (1, Y = CH2; X = SiMe3) in NO2CH3 as solvent.