| Autor: |
F. Bernasconi, Claude, Bhattacharya, Santanu, J. Wenzel, Philip, M. Olmstead, Marilyn |
| Zdroj: |
Organometallics; August 2006, Vol. 25 Issue: 18 p4322-4330, 9p |
| Abstrakt: |
The synthesis and characterization of Cp(NO)(PPh3)Re(2,5-dimethyl-3-thienyl)carbene(3H(S)), a novel Fischer carbene complex, is described. 3H(S) undergoes reversible deprotonation with a pKaof 2.64 in 50% MeCN−50% water (v/v). Rate constants for the deprotonation of 3H(S) by carboxylate ions and by primary aliphatic and secondary alicyclic amines were determined. A major focus of this paper is a comparison of the kinetic and thermodynamic acidities of 3H(S) with those of Cp(NO)(PPh3)Re(2-thienyl)carbene, 1H(S). The pKaof 1H(S) (2.51) is virtually the same as that of 3H(S). This appears to be the result of a fortuitous cancellation of acidity-reducing and acidity-enhancing factors, a conclusion supported by a computational study of gas phase acidities. An analysis of Brønsted plots indicates that the intrinsicrate constants for proton transfer are about 2 orders of magnitude lower for 3H(S) than for 1H(S). This reduction is the result of steric effects and of electronic effects connected with the imbalanced nature of the transition state. |
| Databáze: |
Supplemental Index |
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