Rotation barriers in aryl- and heteroaryldi(1-adamantyl)methyl systems; the ionic hydrogenation of heteroaryldi(1-adamantyl)methanols

Abstract

[2-(3-Methylthienyl)]di(1-adamantyl)methanol is synthesized as a 1∶1 mixture of the anti and syn isomers; the latter rotates incompletely to the more stable isomer upon heating, the equilibrium constant ranging from about 5 to 10 at 150 °C, depending on the solvent. Ionic hydrogenation (deoxygenation) of the 2-(3-methylthienyl) alcohols gives a single product, the syn isomer, in which the methyl group is close to the adamantyls; the anti/syn ratio at equilibrium is substantially higher than for the alcohol. Activation energies (at 157 °C) for syn→anti rotation in this alcohol and its deoxygenation product are 32.1 and 34.4 kcal mol–1, respectively. The analogous 2-(3-methylfuryl) derivatives have much lower rotation barriers, the alcohol being isolated as the stable, anti rotamer; deoxygenation gives predominantly the corresponding methane. The anti→syn rotation barriers for (2-furyl)-, (2-thienyl)- and (thiazol-2-yl)diadamantylmethanols, measured by DNMR, are 16.3, 20.0 and 18.2 kcal mol–1, respectively. Deoxygenation of the 2-furyl alcohol or its 4-methyl derivative gives in both cases two isomers in a ratio of 2∶1, mainly the syn isomer. For the corresponding 2-thienyl alcohols the two isomers are obtained in equal amounts, while the 2-selenienyl analogue shows a preference for the anti isomer (1.8∶1). Rotation barriers (anti→syn) for the (2-furyl)-, (2-thienyl)- and (2-selenienyl)diadamantylmethanes are 16.9, 20.2 and 22.1 kcal mol–1, respectively, and about 1 kcal mol–1 higher for the 4-methyl derivatives of the first two.