Tunneling Assists the 1,2-Hydrogen Shift in N-Heterocyclic Carbenes

摘要

At room temperature, 1,2-hydrogen-transfer reactions of N-heterocyclic carbenes, like the imidazol-2-ylidene to give imidazole is shown to occurr almost entirely '(>90 %) by quantum mechanical tunneling (QMT). At 60 K in an Ar matrix, for the 2, 3-dihydrothiazol-2-ylidene →thiazole transformation, QMT is shown to increase the rate about 105 times. Calculations including small-curvature tunneling show that the barrier for intermolecular 1,2-hydrogen-transfer reaction is small, and QMT leads to a reduced rate of the forward reaction because of nonclassical reflections even at room temperature. A small barrier also leads to smaller kinetic isotope effects because of efficient QMT by both H and D. QMT does not always lead to faster reactions or larger KIE values, particularly when the barrier is small.