Steric effects in the ionic hydrogenation of aryldi(1-adamantyl)-methanols to the corresponding methanes by trifluoroacetic acid and hydrosilanes or sodium borohydride

The stereoselectivity of the hydrosilane reduction of substituted di(1-adamantyl)benzy1 cations obtained by the protonation of aryldi(1-adamantyl)methanols by trifluoroacetic acid (TFA) in dichloromethane depends on both the hydrosilane and the substituent. Hydrosilanes react with meta-substituted (Me, Bu' or CF3) phenyldi(l-adamantyl)methanols to give variable amounts of the anti and syn hydrocarbons, the variations being rather more pronounced for Bu' (anti:syn = 0.7-23) than for Me (0.32-3.8) and CF3 (0.41-5.4). Almost all o-tolyldiadamantylmethanes give the anti isomer exclusively, as established by NMR spectroscopy and a single crystal X-ray diffraction study of the p-fluoro derivative. In the case of the [2-methyl-5-(tert-butyl)phenyl]diadamantylmethyl cation, however, the stereoselectivity and the rate of hydride transfer depend on the hydrosilane, the most encumbered giving hydrocarbon very slowly. Replacing the hydrosilane by sodium borohydride in these reactions results in untypical stereoselectivities. In particular, small yields of anti-(2-ethylphenyl)diadamantylmethane result from reduction of the corresponding alcohol, whereas normally only products with the syn conformation are obtained from this material.