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.
Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842, USA.
Ollie M
OM
Gonzalez-James
Eugene E
EE
Kwan
Daniel A
DA
Singleton
D000475:Alkenes / Q000737:chemistry*; D002247:Carbon Isotopes / Q000737:chemistry; D003500:Cyclization; D004000:Dichloroethylenes / Q000737:chemistry*; D019277:Entropy; D007659:Ketones / Q000138:chemical synthesis* / Q000737:chemistry; D007700:Kinetics; D015394:Molecular Structure; D011789:Quantum Theory
D000475:Alkenes; D002247:Carbon Isotopes; D004000:Dichloroethylenes; D007659:Ketones; C039586:dichloroketene; C021639:2-butene