화학공학소재연구정보센터
Journal of the American Chemical Society, Vol.115, No.26, 12526-12532, 1993
Physical Organic-Chemistry of Transition-Metal Carbene Complexes .1. Thermodynamic and Kinetic Acidities of (Co)5Cr=c(Och3)CH3 and (Co)5Cr=c(Och3)Ch2Ph in Aqueous Acetonitrile .3.
Rate constants for proton transfer from (methoxymethylearbene)pentacarbonylchromium(0), (CO)5CR=C-(OCH3)CH3, to OH- and amine bases and from (benzylmethoxycarbene)pentacarbonylchromium(O), (CO)5Cr=C(OCH3)CH2Ph, to OH-, amines, carboxylate ions, and H2O were determined in 50% acetonitrile-50% water (v/v) at 25-degrees-C. Intrinsic rate constants (k(o)) were deduced from extrapolations of Bronsted plots. For(CO)5Cr-C(OCH3)CH3 they are log k(o) = 3.70 (secondary amines) and 3.04 (primary amines) and for (CO5)Cr=C(OCH3)CH2Ph log k(o) = 1.86 (secondary amines),1.51 (primary amines), and approximately 0.8 (RCOO-). These log k(o) values are consistent with significant resonance stabilization of the respective anions. Kinetic isotope effects on the order of 2.5-3.0 for the deprotonation of either metal carbene complex by OH- and of 5.6 for the deprotonation of (CO)5Cr=(OCH3)CH2Ph by piperidine were measured. These relatively low values may indicate substantial coupling of proton transfer to heavy atom motion, including bond changes in the CO ligands. The effect of changing solvent from water to 50% acetonitrile-50% water is to induce 2-fold and 4.8-fold increases in the rate of deprotonation of (CO)5Cr--C(OCH3)CH3 by OH- and piperidine, respectively. Possible reasons for these solvent effects, including electrostatic stabilization of the transition state of the piperidine reaction, are discussed.