ROTATIONAL-ISOMERISM OF DISUBSTITUTED BENZENES IN THE ALKYLPHENYLDI(1-ADAMANTYL)METHANOL SERIES

Addition of di(1-adamantyl) ketone to the appropriate tolyllithium compounds gives all five positional and rotational isomers of tolyldi(1-adamantyl)methanol, as evidenced by H-1 and C-13 NMR spectroscopy. Synthesis of the ortho-substituted derivatives gives essentially the anti isomer (anti/syn = 11.6), the meta-derivatives mainly syn (anti/syn = 0.77). Thermal equilibration converts anti,ortho almost exclusively to syn,ortho while the anti/syn ratio is virtually unchanged for the meta derivatives. The rotation barriers for the ortho- and meta-derivatives are 39 and 27 kcal mol(-1), respectively. The anti/syn ratio for the synthesis of the corresponding meta-(tert-butyl)-substituted alcohols increases with the reaction time. Thermal equilibration indicates that the anti Isomer is about 0.3 kcal mol(-1) more stable than the syn, due to attractive interactions between the tert-butyl group and the adamantyls, whereas the syn,meta-tolyl is 0.2 kcal mol(-1) the more stable. Molecular mechanics calculations slightly exaggerate the stability of the anti rotamers. Butyllithium-catalysed rotation of the meta-substituted alcohols favours the anti-isomers in both cases, much more for Bu(t) than for Me. When the meta-substituted alcohols are converted to the corresponding methanes by the (COBr)(2)-Bu(3)SnH procedure, rotamerically mixed products are obtained. Again the syn,meta-tolyl and the anti,meta-(tert-butyl)phenyl derivatives are the more stable, by about 0.15 kcal mol(-1) in both cases. Reaction of anti,ortho-tolyldiadamantylmethanol gives a single methane, the syn rotamer. The rotation barrier for the meta-tolyldiadamantylmethanes is less than 1 kcal mol(-1) greater than that for the corresponding alcohol. The mechanism of organolithium addition to ketones is discussed.