Pitfalls in assessing the α-effect: Reactions of substituted phenyl methanesulfonates with HOO-, OH-, and substituted phenoxides in H2O

Abstract

Toward resolving the current controversy regarding the validity of the α-effect, we have examined the reactions of Y-substituted phenyl methanesulfonates 1a-1l with HOO-, OH-, and Z-substituted phenoxides in the gas phase versus solution (H2O). Criteria examined in this work are the following: (1) Brønsted-type and Hammett plots for reactions with HOO-and OH-, (2) comparison of βlg values reported previously for the reactions of Y-substituted phenyl benzenesulfonates 2a-2k with HOO- (βlg = -0.73) and OH- (βlg = -0.55), and for those of 1a-1l with HOO- (βlg = -0.69) and OH- (βlg = -1.35), and (3) Brønsted-type plot showing extreme deviation of OH- for reactions of 2,4-dintrophenyl methanesulfonate 1a with aryloxides, HOO-, and OH-, signifying extreme solvation vs different mechanisms. The results reveal significant pitfalls in assessing the validity of current interpretations of the α-effect. The extreme negative deviation by OH- must be due, in part, to the difference in their reaction mechanisms. Thus, the apparent dependence of the α-effect on leaving-group basicity found in this study has no significant meaning due to the difference in operating mechanisms. The current results argue in favor of a further criterion, i.e., a consistency in mechanism for the α-nucleophiles and normal nucleophiles. © 2010 American Chemical Society.