Dynamic NMR as a Nondestructive Method for the Determination of Rates of Dissociation. XVII. Dissociation of the N–B Bond in the Coordinated Form of 2-[2-(Dimethylaminomethyl)phenyl]-4,4-diphenyl-1,3,2-dioxaborolane and Related Compounds

Abstract

The rates of dissociation of the N–B bonds in the title compounds were reexamined by the dynamic NMR technique to get further insight into the mechanism of dissociation. Entropies of activation were fairly large positive to suggest that the dissociation of a bond of ionic characters is the rate-limiting step. The rates of dissociation were generally larger in nonpolar solvents than in polar solvents, suggesting that the stabilization of the ground state by solvation was important in determining the activation energy for dissociation. Diethyl ether, tetrahydrofuran, and acetone gave larger rate constants of dissociation than those expected from their polarity. Participation of solvent molecules in the dissociation is discussed on the basis of these results. Introduction of various substituents indicated that steric effects as well as electronic effects affect the N–B bond energy.