Heteroaromatic organolithium addition to a congested ketone: conformational isomerism in (N-alkylpyrrol-2-yl)di(1-adamantyl)-methanols

Tertiary alcohols have been prepared by reaction of 2-lithio-N-methylpyrrole and 2-lithio-N-ethylpyrrole with di(1-adamantyl)ketone. The conformations of the N-methyl derivatives have been determined by single crystal X-ray diffraction studies. The N-alkylpyrrol-2-yl derivatives are synthesized as the anti isomers which upon heating undergo rotation about the sp(2)-sp(3) C-C bond to give the more stable, syn isomers with activation energies in benzene of 31.0 (Me) and 30.7 (Et) kcal mol(-1). Semi-empirical(AM1) and ab initio (3-21G//*AM1) calculations indicate that the energy difference between the two rotamers is of the order of 5 kcal mol(-1). Ionic hydrogenation of anti-(N-methylpyrrol-2-yl)diadamantylmethanol in dichloromethane-TFA-triethylsilane gives the anti isomer of (N-methylpyrrol-2-yl)diadamantyl-methane, accompanied by substantial amounts of diadamantylketone. The barrier to anti-->syn rotation in the deoxygenation product is about 4 kcal mol(-1) higher than for the corresponding alcohol.