Stereochemical Effects in the Gas-Phase Pinacol Rearrangement. 2. Ring Contraction versus Methyl Migration in cis- and trans-1,2-Dimethylcyclohexane-1,2-diol

Article abstract of DOI:10.1021/ja00212a017

The gas-phase pinacol rearrangement of cis- and trans-1,2-dimethylcyclohexane-1,2-diols, promoted by D3⁺, CH5⁺/C2H5⁺ and t-C4H9⁺ ions, was studied by mass spectrometric and radiolytic methods in the pressure range 0.5-760 Torr.When product isomerization is inhibited, by using N(CH3)3 as a trapping reagent at high pressure, mixtures of 2,2-dimethylcyclohexanone and 1-acetyl-1-methylcyclopentane were recovered from the reaction.In methane, the trend of measured relative rates for ring contraction (k₅), methyl or hydroxyl group migration (k₆) versus the rearrangement rate of pinacol itself (k_p), is k₆(trans) = k₅(trans) > k₅(cis) > k₆(cis) > k_p.No evidence for the formation of an intermediate carbenium ion was found.Stereochemical aspects of the mechanism are discussed and compared with solution data.