Entropic intermediates and hidden rate-limiting steps in seemingly concerted cycloadditions. Observation, prediction, and origin of an isotope effect on recrossing.

An unusual intramolecular kinetic isotope effect (KIE) in the reaction of dichloroketene with cis-2-butene does not fit with a simple asynchronous cycloaddition transition state, but it can be predicted from trajectory studies on a bifurcating energy surface. The origin of the KIE is related to a high propensity for transition state recrossing in this system, with heavier masses recrossing less. The KIE can also be predicted by a statistical model that treats the cycloaddition as a stepwise mechanism, the rate-limiting second step being associated with an entropic barrier for formation of the second carbon-carbon bond. The relevance of this stepwise mechanism to other asynchronous but seemingly concerted cycloadditions is suggested by examination of organocatalytic Diels-Alder reactions.

Experimental evidence for heavy-atom tunneling in the ring-opening of cyclopropylcarbinyl radical from intramolecular 12C/13C kinetic isotope effects.

The intramolecular (13)C kinetic isotope effects for the ring-opening of cyclopropylcarbinyl radical were determined over a broad temperature range. The observed isotope effects are unprecedentedly large, ranging from 1.062 at 80 degrees C to 1.163 at -100 degrees C. Semiclassical calculations employing canonical variational transition-state theory drastically underpredict the observed isotope effects, but the predicted isotope effects including tunneling by a small-curvature tunneling model match well with experiment. These results and a curvature in the Arrhenius plot of the isotope effects support the recently predicted importance of heavy-atom tunneling in cyclopropylcarbinyl ring-opening.

Isotope effect, mechanism, and origin of catalysis in the decarboxylation of mandelylthiamin.

The mechanism of decarboxylations in water and the catalysis of mandelylthiamin (MTh) decarboxylation by pyridinium ions is explored. It has been recently proposed that a decarboxylation step forming an intermediate molecule/CO(2) pair is reversible and that pyridinium ions catalyze the MTh decarboxylation by trapping the intermediate, preventing reversion to MTh. A calculation of the barrier for the back reaction goes against this proposal, as the diffusional separation of CO(2) would be on the order of 10,000 times faster. A comparison of predicted and experimental isotope effects for a series of decarboxylations including the MTh reaction shows in each case the absence of significant reversibility of the decarboxylation step. An alternative origin of the pyridinium catalysis of decarboxylation is proposed, based on the formal binding of the pyridinium ions to the decarboxylation transition state.