Physical organic chemistry of transition metal carbene complexes. 25. Kinetic and thermodynamic acidities of substituted (methylthiophenylcarbene)pentacarbonyl tungsten(0) and (Benzoxymethylcarbene)pentacarbonyl tungsten(0) in aqueous acetonitrile. Evidence for transition state imbalances.

A kinetic study of the reversible deprotonation of substituted (methylthiophenylcarbene)pentacarbonyltungsten(0) ((CO)(5)W=C(SC(6)H(4)Z)CH(3)) and of substituted (benzoxymethylcarbene)pentacarbonyltungsten(0) ((CO)(5)W=C(OCH(2)C(6)H(4)Z)CH(3)) by primary aliphatic and secondary alicyclic amines in 50% MeCN-50% water (v/v) at 25 degrees C is reported. From the dependence of the deprotonation rate constants on amine basicity and on carbene complex acidity (variation of Z), Brønsted beta(B) and alpha(CH) values, respectively, were obtained. The alpha(CH) values were found to be smaller than the beta(B) values. These results indicate a transition state imbalance in which the loss of the carbene complex stabilizing pi-donor effect of the OCH(2)Ar and SAr groups lags behind the proton transfer. These findings confirm a previously formulated hypothesis as to how pi-donor groups attached to the carbene carbon of carbene complexes can affect transition state imbalances and mask the experimental manifestation of such imbalances. It is also shown that the transition state structure of the reactions examined in this work is subject to changes with changing amine basicity and carbene complex acidity; these changes can be expressed by p(xy)() cross correlation coefficients, which are positive.