A SECONDARY C-11/C-14-KINETIC ISOTOPE EFFECT IN THE BASE-CATALYZED PROTOTROPIC REARRANGEMENT OF 1-METHYLINDENE TO 3-METHYLINDENE

The secondary 11C/14C kinetic isotope effect (KIE) has been determined to 1.010 ± 0.005 for the base-catalyzed rearrangement of 1-methylindene (1) to 3-methylindene (2) isotopically substituted in the methyl group in the solvent benzene at 20 °C by using the tertiary amine DABCO as catalyst. The result is discussed in connection with previously reported values of the corresponding primary 2H and secondary β-2H3 KIEs (5.03 and 1.103, respectively). The isotope effects are consistent with a late TS, structurally close to the ion-pair intermediate postulated in the rate-determining proton abstraction step of the reaction. The influence of hyperconjugation and rehybridization on the magnitude of the secondary carbon KIE is discussed. © 1990, American Chemical Society. All rights reserved.