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The influence of pressure on [2 +-21 cycloaddition reactions of a,P-unsaturated Fischer carbene complexes (M = Cr, W) with 3,4-dihydro-2H-pyran was studied. The cycloaddltion was found to be sensitive toward pressure and showed a remarkable rate acceleration independent of the metal center. The temperature, pressure, and solvent dependence of these reactions was

The temperature and pressure dependence of the addition of pyrazole to the carbene complex (CO),WC[(OC,H,)(C,C,H,)] in a series of solvent mixtures (acetonitrile/toluene) were studied, and the corresponding activation parameters are reported. The solvent dependence data enables the estimation of the radius and dipole moment of the transition state, following different theoretical treatments. The results are in good agreement and demonstrate that the transition state is significantly more polar than the reactant state, and leads to the formation of a zwitter-ionic intermediate.

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