Separation of Ring Polar and Resonance Effects on the Rate Constants for Uncatalyzed N-Arylidenepropylamine Formation in Methanol
作者:Jean Toullec、Didier Milin
DOI:10.1021/jo00089a033
日期:1994.5
Rate constants are reported for the reaction of a series of 19 ring-substituted benzaldehydes with propylamine in methanol and in O-d-methanol at different temperatures and of three of them with 2-methoxyethylamine and 2,2-dimethoxyethylamine. The large direct solvent kinetic isotope effects (KSIE) (2.1-2.9), as well as the Delta H* (4.7-7.6 kcal mol(-1)) values vary smoothly with substituent. Delta S* (ca. -45 cal mol(-1) K-1) is almost independent of substituent. These data indicate that there in no change in the rate-limiting step, and since the large KSIE and \Delta S*\ can be related to significant solvation of the leading OH- group at the transition state, they agree with a mechanism involving cu-amino alcohol formation in a fast preequilibrium followed by rate-limiting OH- detachment yielding iminium ions. When corrected for parallel hemiacetal formation, the second-order rate constants are accounted for by the Young-Jencks equation with p(n) = 0.08 and p(r) = 0.57, this equation allowing a separation of the direct resonance effects from the polar effects of the substituted ring. The very low p(n) value indicates that the polar effects for the rate-limiting OH- detachment are almost compensated by those on the constant of the alpha-amino alcohol-formation preequilibrium. In contrast, the relatively large p(r) coefficient, which means that the overall rate constants depend almost exclusively on resonance, is due to predominance of the direct resonance effects on the preequilibrium. The KSIE and Delta S* data, as well as the beta(nucl) parameters (close to unity) observed when the reacting amine was changed (slopes of the straight lines obtained by plotting the logarithm of the rate constants vs the pK(a) of the ammonium ions) favor a late transition state. In contrast, the low p(r) value for the rate-limiting step (ca. -0.5) shows that the transition state is not iminium ion-like.