Chauvin’s mechanism

Some of these intermediates are analogous to those proposed by Chauvin in olefin metathesis ( Chauvin s mechanism ) [36]. They can be transformed into new olefins and new carbene-hydrides. The subsequent step of the catalytic cycle is then hydride reinsertion into the carbene as well as olefin hydrogenation. The final alkane liberation proceeds via a cleavage of the Ta-alkyl compounds by hydrogen, a process already observed in the hydrogenolysis [10] or possibly via a displacement by the entering alkane by o-bond metathesis [11]. Notably, the catalyst has a triple functionality (i) C-H bond activation to produce a metallo-carbene and an olefin, (ii) olefin metathesis and (iii) hydrogenolysis of the metal-alkyl. 

The most important difference between Chauvin s mechanism for olefin metathesis and the mechanism for alkane metathesis is that the latter applies itself to the reverse reaction of cleavage of alkanes by methane (which has no single C-C bond, see below) and, especially, it is based on a metal hydrido-carbene in equi-Ubrium with a metal-alkyl. 

Mangohas raised an objection to the Chauvin mechanism. His analysis and calculation based on basic principles of thermodynamics indicates that more cyclopropane should be present in metathesis reactions than has been observed, i.e, at equilibrium 20% ethylene converts to cyclopropane. However, arguments that Mango s analysis is in error have been presented. Grubbs notes that the formation of cyclopropane is a chain-termination step and, since the initiation of metal carbenes is very slow compared to the catalytic reaction itself, the concentration of cyclopropane cannot be greater than the metal carbene. Grubbs concludes that the Chauvin mechanism is not inconsistent with thermodynamic calculations and remains as the mechanism most compatible with a large body of other experimental... 

A common mechanism is to devalue other aspects of the person whom one would otherwise envy.99 Thus the stereotype that beautiful blondes are dumb may be due to other women s envy rather than to male chauvinism. Among the strategies for "restoring equality" found in a French village, that of introducing additional criteria of... 

Casey and Burkhardt reasoned that the metathesis mechanism influenced by Chauvin and Herisson s work was sufficient to explain their results. Complex 13 is interesting in that the pentacarbonyl substitution makes it similar to a traditional Fischer carbene complex, yet it also possesses Schrock character because both substituents at Ccarbene are hydrocarbon fragments. 

Tsong, T. Y. Liu, D. S. Chauvin, F. Astumian, R. D. Resonance electroconformational coupling A proposed mechanism for energy and signal transduction by membrane proteins. Biosci. Rep. 9, 13-27. 

W. S. Knowles, R. Noyori, K. B. Sharpless enantioselective catalysis) 2005 Y. Chauvin, R. H. Grubbs, R. R. Schrock (alkene metathesis) 2007 G. Ertl chem. processes on solid surfaces/mechanisms of heterogeneous catalysts) 2010 R. F. Heck, E. Negishi, A. Suzuki Pd Catalyzed cross coupling reactions). 

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