Isotope Effects and Heavy-Atom Tunneling in the Roush Allylboration of Aldehydes

摘要

Intermolecular ¹³C kinetic isotope effects (KIEs) for the Roush allylboration of p-anisaldehyde were determined using a novel approach. The experimental ¹³C KIEs fit qualitatively with the expected rate-limiting cyclic transition state, but they are far higher than theoretical predictions based on conventional transition state theory. This discrepancy is attributed to a substantial contribution of heavy-atom tunneling to the reaction, and this is supported by multidimensional tunneling calculations that reproduce the observed KIEs.