Secondary deuterium isotope effects in the solvolysis of cis- and trans-2-acetoxycyclohexyl 2,2,2-trifluoroethanesulfonates

The 2,2,2-trifluoroethanesulfonates (tresylates) of specifically deuterated cis-2-acetoxycyclohexanol (cis-1..beta..d, cis-1..cap alpha..d, cis-1..beta..'d/sub 2/) and trans-2-acetoxycyclohexanol (trans-1..beta..d, trans-1..cap alpha..d, trans-1..beta..'d/sub 2/) were solveolyzed in 97 wt percent trifluoroethanol at 93 and 55/sup 0/C, respectively, and the secondary deuterium isotope effects were measured. The solvolysis products from the trifluoroethanolysis of the unlabeled isomeric tresylates cis-1 and trans-1 were also determined. The ..cap alpha.. effect in trans-1..cap alpha..d is similar in magnitude to the effects observed in SN2 reactions (k/sub H//k/sub D/ = 1.03). The ..beta.. effects in trans-1..beta..d and trans-1..beta..'d/sub 2/ are also small (k/sub H//k/sub D/ = 0.98 and 1.04, respectively), reflecting the absence of significant hyperconjugative stabilization. These results are in agreement with a transition state structure closer to the oxonium ion intermediate than to the reactants. The results obtained in the solvolysis of the corresponding cis derivatives are significantly different. The ..cap alpha.. effect is large (k/sub H//k/sub D/ equals 1.20) indicating that ionization to the solvent-separated ion pair is rate determining, while the ..beta.. effects are ''normal'' but larger for cis-1..beta..d (1.34) than for cis-1..beta..'d/sub 2/ (1.23). On the basis of these results it was concluded that the cis derivative solvolyzes via a twist-boat transition state. The present work demonstrates the sensitivity of secondary deuterium isotope effects to structural changes of solvolytic transition states. (auth)