Deuterium isotope effect positive charge substituents

The consistancy of the observed /4-factors, the magnitude of the activation entropies, the a correlations in the substituent effects at the a- and /8-carbon positions, the collective influence of all three substitutable centers on the reaction rates, the importance of charge stabilization in the transition state, and the primary deuterium isotope effects on the alkyl acetic acid ester decomposition, all favor the concerted polar 6-center transition state shown below (IV). However, an alternative possibility involving intimate ion-pair formation has been proposed by Scheer et al.. ... 

When imidazole or methanol was introduced to the dichloromethane solution of 36, instantaneous decomposition of 36 and the epoxide formation were observed. Thus, in the presence of either methanol or imidazole, the rate-determining step in the reaction of 14 and olefin was changed to the formation of 36. Under these conditions, secondary deuterium kinetic isotope effects on epoxi-dation were examined by a- and /3-deuterio-p-chlorostyrenes. For both the a- and the /3-positions of styrene, kn/fco = 1 was observed. The isotope effect and substituent effect on the formation of 36 suggest that both the a- and /8-carbons remain planar (sp hybridized) at the transition state and that a positive charge forms on the a-carbon. Accordingly, the formation of an olefin cation radical by an electron transfer from the olefin to 14 is indicated in the formation of 36 (Scheme XX). 

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