Physical organic chemistry of transition metal carbene complexes. 24. Thermodynamic and kinetic acidities of phenyl-substituted (benzylmethoxycarbene)pentacarbonylchromium(0) complexes. Is there a transition-state imbalance?

A kinetic study of the reversible deprotonation of phenyl-substituted (benzylmethoxycarbene)pentacarbonylchromium(0) complexes by OH(-) and by a series of primary aliphatic and a series of secondary alicyclic amines in 50% MeCN-50% water (v/v) at 25 degrees C is reported. Brønsted alpha(CH) values (dependence on carbene complex acidity) and beta(B) values (dependence on amine basicity) were determined. According to current notions about proton transfers involving carbon acids activated by pi-acceptors, alpha(CH) was expected to substantially exceed beta(B), the result of transition-state imbalances that are characteristic of such reactions. However we find that alpha(CH) and beta(B) have essentially the same values, which are close to 0.5. It is shown that these findings do not indicate the absence of an imbalance but rather suggest that the manifestation of the imbalance is masked by the pi-donor effect (3H-Z <--> 3H-Z(+/-)) of the methoxy group.