Aliphatic Claisen rearrangement transition state structure from secondary α-deuterium isotope effects

The kinetics isotope effects (KIE) at C4 and C6 in the 3,3 shift of 3-oxa-1,5-hexadiene appear to reveal that the transition state comes early, and that it involves much more bond breaking than making in contrast to the carbon 3,3 shift of relatively unperturbed dienes. At 1600C the normal KIE at C4 is 1.092 and the inverse KIE at C6 is 1.025. The ratio of the KIE to the equilibrium isotope effect at the bond breaking site is (0.09/1.27) or 1/3, while that of the bond making site is only (1.025/1.16) or < 1/6. These results seem to confirm that the transition state is early because of the exothermicity of the reaction. The transition state structure suggests that radical stabilizing substituents on C1, C4, and C6 will stabilize the transition state to a greater extent than placement of the same substituents on C2 and C5