Hydrogen−deuterium fractionation factors for hydrogen-sp2 carbon bonds in olefins and allyl radicals
Thermally induced 3,3-shift of trans-1,2-divinylcyclopropane, 1, and trans- 1,2-bis(2',2'-dideuteriovinyl)-cyclopropane, 1-d 4 , to cycloheptadiene has a kinetic isotope effect of 1.08±0.02/D when extrapolated to 25 o C. To the extent that the transition state resembles two allyl radicals, the kinetic effect is the fractionation factor for deuterium between sp 2 allyl radical carbon and sp 2 olefinic carbon. AM1 calculations on allyl radicals reveal a similar fractionation factor. The new value for the FF of allyl radical requires a correction to previously developed linear free energy relationships for kinetic and equilibrium isotope effect in 3,3-shifts and Diels-Alder reactions