Rotational isomerism of disubstituted benzenes in the alkylphenyldi(1-adamantyl)methanol series

Abstract

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 ¹H and ¹³C 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–Bu₃SnH 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.