Transition state geometry in super-ene reactions

Abstract

A pseudopericyclic transition state (TS) involving nonlinear H transfer is proposed for the superene reaction (eq 1), characterized by a temperature-independent isotope effect (^h/^d = 2.863). Ancillary evidence for rapid, reversible formation of a four-membered cyclic charge-transfer complex is also presented to account for the observed regio-and stereoselectivity of the reaction.

Temperature dependence of the primary kinetic hydrogen isotope effect as a mechanistic criterion

“…It has been found that the reaction course is considerably different from that in the regular ene. 37 Thus, the superene reaction described by eq 7 is char- over a long temperature range, which is indicative of a bent TS. Ancillary evidence supports the rapid, reversible formation of a four-membered charge-transfer complex between the superene reactants.…”

Anatomy of ene and Diels–Alder reactions between cyclohexadienes and azodicarboxylates

“…Kwart proposed the terminology of 'bent transition states' for superene reactions on the basis of kinetic isotope effects. 24 Precision Density Measurements.-These were carried out as previously described,' yielding the following partial molar volumes in toluene (cm3 mol-I): cyclohexa-1,4-diene (98.0), DEAD (153.4), 2 (217.4); cyclohexa-1,3-diene (98.6), 1 (217.6).…”