Physical organic chemistry of transition metal carbene complexes.: 29.: Kinetics of reactions of [Ethoxy(phenyl)carbene]pentacarbonylchromium(0) and [Ethoxy(phenyl)(Cr(CO)3)carbene]pentacarbonylchromium(0)with water, OH-, and amines.: Mechanistic changes induced by the Cr(CO)3 group

A kinetic study of the aminolysis of (CO)(5)Cr=C(OEt)C6H5 (7-Cr) and (CO)(5)Cr=C(OEt)-C6H5(Cr(CO)(3)) (8-Cr) by five primary aliphatic amines and of the hydrolysis of 8-Cr in 50% MeCN-50% water (v/v) at 25 degreesC is reported. The introduction of the Cr(CO)(3) group has only a minor effect on the electrophilic reactivity of the carbene carbon; that is, the nucleophilic attack by OH- and the amines on 7-Cr and 8-Cr occurs at similar rates. However, the Cr(CO)(3) group exerts a strong influence on the mechanisms of both types of reactions, which results in changes of what steps are rate limiting. In the hydrolysis reaction these changes lead to a more complex rate versus pH profile for 8-Cr compared to the previously reported one for 7-Cr. In the aminolysis the effect is to change the mechanism of general base catalysis from rate-limiting leaving group departure for the reactions of 7-Cr to rate-limiting proton transfer for the reactions of 8-Cr. We provide a detailed analysis of the reasons for these changes.