Physical Organic Chemistry of Transition Metal Carbene Complexes. 19.¹ Kinetics of Reversible Alkoxide Ion Addition to Substituted (Methoxyphenylcarbene)pentacarbonylchromium(0) and (Methoxyphenylcarbene)pentacarbonyltungsten(0) in Methanol and Aqueous Acetonitrile

Rate and equilibrium constants for the nucleophilic addition of MeO - in methanol and in 90% MeCN−10% MeOH, of HC⋮CCH 2 O - and CF 3 CH 2 O - in 50% MeCN−50% water, and of OH - in 50% MeCN−50% water (rate constants only) to Fischer carbene complexes of the type (CO) 5 M C(OMe)C 6 H 4 -4-Z (M = Cr and W) are reported. The reactions lead to the formation of tetrahedral adducts of the type (CO) 5 M̄−C(OMe)(OR)C 6 H 4 -4-Z. The kinetics of general acid catalyzed loss of MeO - from the MeO - adducts by phenol, 4-bromophenol, and 3,5-dichlorophenol, to regenerate the carbene complex, were also investigated. Hammett ρ and ρ n values as well as approximate β nuc ( ) values were determined. They suggest that the transition state for alkoxide ion addition to the carbene complexes is imbalanced in the sense that charge delocalization into the CO ligands lags somewhat behind C−O bond formation. This finding is consistent with the fact that the intrinsic rate constants for these reactions are substantially lower than those for alkoxide ion addition to carboxylic esters. Another type of imbalance is seen for the transition state of the general acid catalyzed MeO - departure from the MeO - adducts. Here proton transfer to the leaving group is ahead of C−O bond cleavage.