Cross‐interaction constants as a measure of the transition state structure. Part 15. Kinetic isotope effects in the S N 2 reactions of 2‐phenylethyl derivatives with deuterated aniline nucleophiles

Kinetic isotope effects (KIE) were determined for the reactions of 2‐phenylethyl and 1‐methyl‐2‐phenylethyl benzenesulphonates with deuterated aniline nucleophiles in acetonitrile at 65.0 °C. The results are in good agreement with the transition state (TS) structures proposed based on the sign and magnitude of the cross‐interaction constants, pxz , between the substituents in the nucleophile (X) and the leaving group (Z). In the reactions of 2‐phenylethyl derivatives, all three reaction pathways, k r , k t and k Δ , were found to be contributing competitively; the inverse secondary KIE observed with a stronger nucleophile changed into the primary KIE with a weaker nucleophile owing to the predominant contribution of the four‐centre TS in the k f path. For the reactions of the 1‐methyl‐2‐phenylethyl series, the k r path played a major role, the contribution from the front‐side nucleophilic attack, k f , being negligible. In both reaction series, the aryl participation was important for the p ‐CH 3 O‐substituted substrate.