HYDROGEN-DEUTERIUM FRACTIONATION FACTORS FOR HYDROGEN-SP(2) 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-d4, to cycloheptadiene has a kinetic isotope effect of 1.08 +/- 0.02/D when extrapolated to 25-degrees-C. To the extent that the transition state resembles two allyl radicals, the kinetic effect is the fractionation factor for deuterium between sp2 allyl radical carbon and sp2 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. The corrections are moderate, altering the previously determined extent of bond breaking and bond making by up to 50% only when there is little bond making, but do not alter the relative extents of bond making and breaking in the large number of compounds examined.