Kinetics of the Reactions of [p-Nitrophenoxy(phenyl)carbene]pentacarbonylchromium(0) with Aryloxide Ions, Hydroxide Ion, and Water in Aqueous Acetonitrile. Concerted or Stepwise?

Abstract

The main question addressed in this paper is whether the nucleophilic substitution of the p-nitrophenoxy group in (CO)5Cr=C(OC6H4-4-NO2)Ph (1-NO2) by a series of substituted phenoxide ions is concerted or stepwise. Rate constants, kArO, for these substitution reactions were determined in 50% MeCN-50% water (v/v) at 25 degrees C. A Brønsted plot of log kArO versus pKa(ArOH) s consistent with a stepwise mechanism. This contrasts with reactions of aryl oxide ions with p-nitrophenyl acetate and with similar acyl transfers which are concerted. The reason for the contrast is that the tetrahedral intermediates formed in the reactions of 1-NO2 are much more stable than those in acyl transfers and the intrinsic barriers to their decomposition are higher than for the ester reactions. The points on the Brønsted plots for which pKa(ArOH) > or = pKa(PNP) define a straight line with beta(nuc) = -0.39, suggesting that bond formation has made very little progress at the transition state and that partial desolvation of the nucleophile is part of the activation process. The hydrolysis of 1-NO2 and of the unsubstituted analogue (1-H) has also been studied over a wide pH range, providing rate constants for nucleophilic attack by hydroxide ion (kOH), by water (kH2O), and by general base-catalyzed reaction with water (kB). Furthermore, kH2O values were obtained for the hydrolysis of (CO)5Cr=C(OC6H4X)Ph (1-X) as a byproduct of the reactions of 1-NO2 with aryl oxide ions. Structure-reactivity relationships for these reactions are discussed in terms of inductive, pi-donor, and steric effects.