Physical Organic Chemistry of Transition Metal Carbene Complexes. 13.¹ Kinetics of Proton Transfer from (5-Methyl-2-oxacyclopentylidene)pentacarbonylchromium(0) and Hydrolysis of Its Conjugate Anion in Aqueous Acetonitrile

Abstract

The pK a = 13.41 of the title compound (5), determined by a kinetic method, is about 1.3 units lower than the pK a of (2-oxacyclopentylidene)pentacarbonylchromium(0) (1) in 50% MeCN−50% water at 25 °C. The acidifying effect of the methyl group is attributed to its stabilizing effect on the CC double-bond resonance structure of the anion (5 - ). The rate constant for deprotonation of 5 by OH- is about the same as for deprotonation of 1, despite the higher acidity of 5. This means that the intrinsic rate constant for proton transfer from 5 is significantly lower than that from 1. This reduction in the intrinsic rate constant is mainly the result of the imbalanced nature of the transition state which deprives the transition state from the stabilizing effect of the methyl group. Based on precedents, the most likely mechanism for the hydrolysis of 5 involves rate-limiting protonation of 5 - concerted with metal−carbon bond cleavage. Even though the relatively small kinetic solvent isotope effect cannot rigorously exclude an alternative nucleophilic substitution mechanism, a comparison of the reactivity of 5 and 1 allows one to rule out this latter mechanism.

Kinetic Isotope Effects in the Study of Organometallic Reaction Mechanisms

“…The mechanisms of hydrolysis of Fischer carbene complexes have been extensively studied by the group of Bernasconi . Kinetic studies carried out with carbene complexes with ionizable α-carbons 525 , 526 , 527 , 528 , 529 , and 530 revealed that the hydrolysis (50% MeCN−50% water) is pH dependent (a general base-catalyzed reaction) and is subject to a substantial kinetic solvent isotope effect ( k H2O / k D2O ) larger in magnitude as the pH of the medium increases, which is consistent with a rate-limiting proton transfer (Figure ).…”

Physical Organic Chemistry of Transition Metal Carbene Complexes. 23.1 Kinetic and Thermodynamic Acidities of Cationic Benzothienyl- and Selenylcarbene Complexes of Rhenium in Aqueous Acetonitrile

“…Over the past decade, we have reported a number of kinetic and thermodynamic investigations of the reversible deprotonation of Fischer carbene complexes. − Most of the complexes studied were of the type 1H where M is Cr or W; CH 3 X with X = O or S acts as a π-donor, while R and R‘ are H, alkyl, or aryl groups. The majority of the studies were performed in 50% MeCN−50% water (v/v) at 25 °C; − , one investigation was carried out in water, another one in pure acetonitrile, and one in various MeCN−water mixtures . The most important findings can be summarized as follows.…”

Physical Organic Chemistry of Transition Metal Carbene Complexes. 15.1 Kinetic and Thermodynamic Acidities of (Methylthiomethoxycarbene)pentacarbonyl Complexes of Chromium and Tungsten in Aqueous Acetonitrile

“…Very little additional quantitative work on acidities was performed until 1989, when we published the first kinetic proton-transfer study on a Fischer carbene complex ( 2 -Cr) , Most of these data have been obtained in 50% MeCN−50% water, a solvent chosen because it is essentially aqueous but allows the carbene complexes to be soluble enough for such determinations. Virtually all carbene complexes investigated so far are of the general structure 1 -M where M = Cr, W, and, in one case, Mo; R was either Me or Et while R‘ and R‘ ‘ included H, Me, n -Pr, and Ph.…”