Influence of compression upon kinetic isotope effects for S(N)2 methyl transfer: A computational reappraisal

Secondary α-D₃ kinetic isotope effects (KIEs) have been calculated using ab initio HF and B3LYP methods with 6-31G and 6-31G* bases for four methyl transfer reactions of the general type R₃N + CH₃-NR₃⁺. Comparison of R = CH₃ (B) with R = H (A) serves to investigate the effect of varying the nucleophile and nucleofuge by alkylation of the entering and leaving amine moieties. The changes in KIE, transition-structure (TS) looseness, and energy barrier do not accord with the generalization of Wolfe and co-workers (J. Am. Chem. Soc. 1993, 115, 10147). Reactions C and D are intramolecular methyl transfers between the bridgeheads of inside-methylated [1.1.1]cryptand and 1,7-diazabicyclo[5.5.5]heptadecane, respectively, in which the N···N distances are significantly smaller than in B. Comparison of B with either C or D serves to investigate the effect of compression along the N-C-N axis. The energy barriers for C and D are markedly lower than for B, and, although their TS's are tighter than for B, their reactant complexes (RC's) are even tighter. Progress from RC to TS in the compressed systems is accompanied by a decrease in strain. The inverse α-D₃ KIEs for C and D are dominated by zero-point energy changes, but the contribution from the CH stretching modes is less inverse than that for B. The more inverse B3LYP KIEs for C and D arise because there is a significant increase in the relaxed valence force constant for bending the H-C(α)-N(lg) angle in going from RC to TS. These calculated results are consistent with Schowen's compression hypothesis for enzymatic methyl transfer.