The gas-phase pinacol rearrangement of cis- and trans- 1,2dimethylcyclohexane1 , 2-diols, promoted by D3+C, H5'/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 the measured relative rates for ring contraction (k5)m, ethyl or hydroxyl group migration (k6)v ersus the rearrangement rate of pinacol itself (kp),is k6(trans) = k5(trans) 3 k5(cis) > k6(cis) 3 kF No evidence for the formation of an intermediate carbenium ion was found. Stereochemical aspects of the mechanism are discussed and compared with solution data.
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|Titolo:||Stereochemical effects in the gas-phase pinacol rearrangement. 2. Ring contraction versus methyl migration in cis- and trans-1,2-dimethylcyclohexane-1,2-diol|
|Data di pubblicazione:||1988|
|Appartiene alla tipologia:||01a Articolo in rivista|