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

CLAUDE F. BERNASCONI; MARTIN E. ZOLOFF MICHOFF; RITA H. DE ROSSI; ALEJANDRO M. GRANADOS

JOURNAL OF ORGANIC CHEMISTRY

AMER CHEMICAL SOC

Lugar: Washington; Año: 2007 vol. 72 p. 1285 - 1293

0022-3263

The main question addressed in this paper is whether the nucleophilic substitution of the p-nitrophenoxy group in (CO)5CrdC(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 °C. A Brønsted plot of log kArO versus pKArOH is consistent with a stepwise mechanism. a 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 g pKPNP are higher than for the ester reactions. The points on the Brønsted plots for which pKArOH a a define a straight line with βnuc ) -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)5CrdC(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, π-donor, and steric effects.