Zirconium catalysed carbomagnesiations of terminal alkenes

The zirconium catalysed ethylmagnesiation of a range of terminal alkenes by diethylmagnesium and ethylmagnesium bromide has been studied. Both mono- and 1,4-dimagnesiated species are produced. We have delineated conditions for the formation of either reagent, and shown that they can be usefully functionalised with a variety of electrophiles.The results of deuterium labelling studies has implicated a zirconocene η2-ethylene complex as an intermediate common to the formation of both the mono and dimagnesiated reagents. A deuterium isotope effect for the β-hydride transfer process, a key step in the proposed mechanism for the formation of the monomagnesiated reagent, has been determined.Kinetic studies have shown that the reaction between η2-ethylene zirconocene and the alkene is rate limiting and that Lewis bases (L, the alkene substrate and/or the solvent) inhibit the reaction by trapping the catalyst as the "ate" complex [Cp2Zr(CH2=CH2)Et]-[EtMgL]+.Mass spectrometry has been used to monitor the evolution of ethane as the reaction proceeds and to determine a secondary kinetic isotope effect for the insertion of an alkyne into a deuteriated zirconacyclopropane.A general mechanism for the carbomagnesiation of alkenes and the dependence of the nature of the products formed upon the reaction conditions, the substrate, and the organomagnesium reagent employed, has been discussed, and the work of other research groups reviewed.The range of chiral zirconium catalysts presently available, and their reported applications to organic transformations has been reviewed. The preparation of two such complexes was undertaken, and has been discussed. The application of a chiral ansa-zirconocene catalyst to the ethylmagnesiation of allylpyrrolidine furnished a product with poor enantio-purity (<8% ee).