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

[2-(3-Methylthienyl)]di(1-adamantyl)methanol is synthesized as a 1 : 1 mixt ure of the anti and syn isomers; the latter rotates incompletely to the mor e stable isomer upon heating, the equilibrium constant ranging from about 5 to 10 at 150 degrees C, depending on the solvent. Ionic hydrogenation (deo xygenation) 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. Act ivation energies (at 157 degrees C) for syn --> anti rotation in this alcoh ol and its deoxygenation product are 32.1 and 34.4 kcal mol(-1), respective ly. The analogous 2-(3-methylfuryl) derivatives have much lower rotation ba rriers, the alcohol being isolated as the stable, anti rotamer; deoxygenati on gives predominantly the corresponding methane. The anti --> syn rotation barriers for (2-furyl)-, (2-thienyl)- and (thiazol-2-yl)diadamantylmethano ls, measured by DNMR, are 16.3, 20.0 and 18.2 kcal mol(-1), respectively. D eoxygenation of the 2-furyl alcohol or its 4-methyl derivative gives in bot h cases two isomers in a ratio of 2 : 1, mainly the syn isomer. For the cor responding 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.