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

Hyperconjugation and the angular dependence of .beta.-deuterium isotope effects

“…Good correspondence is obtained for ds-2-acetoxycyclohexyl-6,6-d2 tresylate, a substrate which presumably solvolyzes via a twist boat transition state. 29 The ß effects in the menthyl and cholestanyl systems where a chair like transition state is more likely13•18 are better accommodated by assuming a CH3/H rate ratio of 106. This puts the chair conformer of the cyclohexyl cation in the same relative position with respect to its -methyl derivative, as the isopropyl cation relative to tert-butyl.7 2-Adamantyl.…”

“…An effect of this magnitude (/c(H/D) = 1.34) was actually observed in the trifluoroethanolysis of m-2-acetoxycyclohexyl-2-(f1 tresylate, a system where competing rate determining participation (or elimination) of the eclipsed deuterium is apparently prevented. 29 The extraordinary high electron demand of the 7-norbornyl cation was explained in terms of noninteraction of the cationic center with the ribbon orbitals of the underlying cyclohexane skeleton.46 Since a similarly high CH3/H effect was observed in 2-adamantyl derivatives47 where the noninteracting orbital argument does not hold, a greatly reduced solvation and the absence of hy-Sunko, Szele, Hehre j Angular Dependence of ß-Deuterium Isotope Effects perconjugative stabilization of the respective secondary cation should also be considered as the cause for these high CH3/H effects.47•48 It remains to be seen how isotope effects determined in solvents of high ionizing power and low nucleophilicities correlate with effects determined in the gas phase. Work in this direction is in progress.…”