Key mechanistic features of the Zr-catalyzed carbomagnesation are illustrated below. Addition of EtMgCl to I leads to the formation of the derived zirconate II which undergoes site-selective cleavage, due to chelation of magnesium with the neighboring Lewis basic alkoxide (III to IV). Subsequent $\beta$-hydride abstraction followed by reductive elimination regenerates the catalyst (V) and provides the alkylmagnesium VI which is then trapped by an electrophile (e.g., O$\sb2$) to afford VII.
Catalytic asymmetric synthesis. 2,5-Dihydrofuran in equation 1 undergoes highly enantioselective ethylmagnesation (97% enantiopurity).
Catalytic kinetic resolution. As exemplified below, subjection of a racemic mixture of pyrans to asymmetric carbometallation conditions affords efficient and facile kinetic resolution. Similar results have been obtained for pyrans with a variety of substitution patterns.
Ni-catalyzed alkylations. We have been investigating a phosphine directed Ni-catalyzed addition of Grignard reagents to allylic ethers. The Lewis basic phosphine is necessary for reactivity and selectivity (eq 3). An example shown below is representative.* ftn*Please refer to the dissertation for diagrams.
