Separation of Ring Polar and Resonance Effects on the Rate and Equilibrium Constants for Methoxide Ion-Promoted Addition of Methanol to N-Benzylideneanilines Substituted at the Benzylidene Moiety
作者:Jean Toullec、Sadjia Bennour
DOI:10.1021/jo00089a032
日期:1994.5
Rate and equilibrium constants for the title reaction (alpha-amino ether formation) at 25 degrees C in methanol are reported for 16N-benzylideneanilines (BAs) substituted at the benzaldehyde moiety. In contrast, to a previous report dealing with BAs substituted at the aniline residue, it is shown that an uncatalyzed pathway competes with the methoxide ion-catalyzed reaction, these two processes corresponding to rate-limiting attachment of the lyate ion to the C=N bond of iminium ion and free imine, respectively. For the latter reaction pathway, the lifetimes of the amide-ion intermediates are long enough to allow a stepwise mechanism. The equilibrium constants for the overall reaction are correlated by the two-parameter Young-Jencks (YJ) equation [log K or log k = rho(n) sigma(n) + rho(n) sigma(n) + C] allowing separation of the overall polar effect of the substituted ring (p(n) = 0.87), including resonance-induced polar (RP) effects, from the direct resonance (DR) effect due to conjugation with the C=N group (p(r) = 0.54). For the forward methoxide ion-catalyzed process, the polar and DR contributions to the substituent effects on the second-order rate constant have the same sign (p(n) = 1.79 and p(r) = 0.28, respectively), but these have opposite signs (p(n) = 0.92 and p(r) = -0.25) for the reverse reaction. The contribution of the DR effects for the reverse rate constants, which make the points lie above the log k-sigma(n) line, stems from gain in resonance at the negatively charged transition state. The validity and significance of the YJ treatment are discussed.