Steric and Stereoelectronic Effects in the Hydrogenolysis and Birch Reduction of Some Hindered Tertiary-Benzylic Carbinols

3-(4'-Methoxyphenyllpentan-3-ol (3a) and 3-(4'-methoxypheny1)-2,4-dimethylpentan-3-ol (3b) underwent catalytic hydrogenolysis over 10% palladium/charcoal at moderate temperatures and pressures. The more hindered tertiary-benzylic carbinols 3-(4'-methoxypheny1)-2,2,4-trimethylpentan-3-ol (6), 3-(4'-methoxyphenyl)-2,2,4,4-tetramethylpentan-3-ol (3c), 1-(4'- methoxyphenyl )-2,2,6,6-tetramethylcyclohexan-l-ol (8) and 1-(1',1'-dimethylethy1)-6-methoxy- 2,2-dimethyl-l,2,3,4-tetrahydronaphthalen-1-ol (10) were completely resistant to hydrogenolysis, even under vigorous conditions. While the hindered tertiary-benzylic carbinols (6),(8) and (10) readily underwent Birch reduction, the analogous di-t-butyl anisyl carbinol (3c) was unchanged. The failure of (3c) to undergo Birch reduction is probably due to a hitherto unrecognized stereoelectronic effect: the C-OH bond of (3c) is constrained to lie more or less in the plane of the benzene ring, and addition of an electron to the benzene ring of the derived oxyanion (31) is inhibited in this conformation.