STERIC AND ELECTRONIC TUNING OF MICHAEL ADDITION-REACTIONS INDUCED BY PENTACARBONYL METAL FRAGMENTS

The addition of a series of 2- and 4-substituted imidazoles to unsaturated Fischer carbene complexes of the type (CO)5MC[(OC2H5)C2C6H5] was studied as a function of temperature and pressure. A comparison of the data for the addition of the 2- and 4-substituted imidazoles enables a discussion of steric and electronic effects that control the rate of the addition process. The reported activation parameters are used to discuss the mechanism of the addition process in comparison to related addition reactions of amines and pyrazoles reported in the literature. The data in combination with kinetic isotope effect measurements clearly reflect that the reactions proceed via a two-step process with a zwitterionic intermediate.