Physical organic chemistry of transition metal carbene complexes. 19. Kinetics of reversible alkoxide ion addition to substituted (methoxyphenylcarbene)pentacarbonylchromium(0) and (methoxyphenylcarbene)pentacarbonyltungsten(0) in methanol and aqueous acetonitrile
Cf. Bernasconi et L. Garcia-rio, Physical organic chemistry of transition metal carbene complexes. 19. Kinetics of reversible alkoxide ion addition to substituted (methoxyphenylcarbene)pentacarbonylchromium(0) and (methoxyphenylcarbene)pentacarbonyltungsten(0) in methanol and aqueous acetonitrile, J AM CHEM S, 122(16), 2000, pp. 3821-3829
Rate and equilibrium constants for the nucleophilic addition of MeO- in met
hanol and in 90% MeCN-10% MeOH, of HC=CCH2O- and CF3CH2O- in 50% MeCN-50% w
ater, and of OH- in 50% McCN-504b water (rate constants only) to Fischer ca
rbene complexes of the type (CO)(5)M=C(OMe)C6H4-4-Z (M = Cr and W) are repo
rted. The reactions lead to the formation of tetrahedral adducts of the typ
e (Co)(5)(M) over bar-C(oMe)(OR)C6H4-4-Z. The kinetics of general acid cata
lyzed loss of MeO- from the MeO- adducts by phenol, LF-bromophenol, and 3,5
-dichlorophenol, to regenerate the carbene complex, were also investigated.
Hammett rho and rho(n) values as well as approximate beta(nuc) (beta(nuc)(
n)) values were determined. They suggest that the transition state for alko
xide ion addition to the carbene complexes is imbalanced in the sense that
charge delocalization into the CO ligands lags somewhat behind C-O bond for
mation. This finding is consistent with the fact that: the intrinsic rate c
onstants for these reactions are substantially lower than those for alkoxid
e 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 bon
d cleavage.