Isokinetic behaviors in the addition reactions of aniline nucleophiles with benzylic carbocations

Abstract

In the addition reactions of aniline nucleophiles with a benzylic carbocation, isokinetic behavior is often observed: ρX(ρnuc) vanishes and the rate becomes constant irrespective of the substituent, X, in the nucleophile when α and Y substituents on the benzylic carbon and on the ring, respectively, are kept constant (σˆα and σˆY). This means that there is a region (σY < σˆY) where thermodynamically more stable derivatives (δσX > 0) are kinetically more labile (δ log kXY > 0) so that ρX is positive, i.e. an inverse thermodynamic stability-reactivity relationship holds. The observable isokinetic point at σˆY is rationalized by the compensation effect of the intrinsic barrier, ΔG0≠, and thermodynamic driving force, ΔG°, for the specified α and Y substituents. Moreover, the two substituents, α and Y, that satisfy the condition of vanishing ρX are found to be related in a compensating manner to preserve the condition of nearly complete cancellation between the two energy terms for the vanishing ρX. The cancellation of the two energy terms is made possible by an imbalance or non-synchronization of the expression of destabilizing polar and stabilizing resonance interactions in the transition state.