STEREOCHEMICAL EFFECTS IN THE GAS-PHASE PINACOL REARRANGEMENT OF CIS-1-METHYLCYCLOHEXANE-1,2-DIOL AND TRANS-1-METHYLCYCLOHEXANE-1,2-DIOL

The gas-phase pinacol rearrangement of cis and trans-1-methyl-1,2-cyclohexanediols, promoted by D3+ and CnH5+ (n = 1,2), was studied by the radiolytic method in the pressure range 100-760 Torr. Under all conditions, 2-methylcyclohexanone is the predominant product, arising from both substrates via different pinacol rearrangements and successive fast isomerization of the corresponding primary intermediates, e.g., O-protonated 1-methyl-1-cyclopentanecarboxaldehyde. This conclusion is based on kinetic anal. of competition expts. with pinacol as ref. substrate, carried out at high pressure (760 Torr) with or without added base (NMe3, 3 Torr), showing that the pinacol rearrangement rates are markedly dependent on the stereochem. features of the diol. Accordingly, the trans diol rearranges more rapidly than the cis isomer, which in turn isomerizes faster than pinacol, indicating that anti-periplanar CH2 migration to the vicinal tertiary C-OH2+ center in trans diol is over five times faster than H migration in cis diol. Anal. of the relative migrating ability of the different CH2 moieties in trans diol allowed exclusion of appreciable anchimeric assistance in these gas-phase pinacol rearrangements. The results are compared with relevant gas-phase data and with those concerning the same substrates in acidic soln.