142635-93-6Relevant articles and documents
On the mechanism of the zirconium-catalyzed carbomagnesation reaction. Efficient and selective catalytic carbomagnesation with higher alkyls of magnesium
Hoveyda, Amir H.,Morken, James P.,Houri, Ahmad F.,Xu, Zhongmin
, p. 6692 - 6697 (2007/10/02)
In zirconium-catalyzed carbomagnesation of alkenes, the dramatic enhancement in reactivity and selectivity induced by an internal Lewis base is such that higher order alkylmagnesium halides, normally significantly less reactive than EtMgCl, may be used efficiently. In reactions with n-BuMgCl, two secondary carbon stereogenic centers are formed with excellent levels of stereocontrol (4, >95%). Experimental data are presented which demonstrate that the observed levels of selectivity and reactivity are not simply the result of regioselective insertion of the alkene into the zirconacyclopropane. A general mechanism scheme for carbomagnesation of bicyclic substrates 1 is presented. Noteworthy mechanistic issues are the following: (1) Excess alkylmagnesium halide is required for high levels of regioselectivity and substrate reactivity. We suggest that the influence of the Grignard reagent steins from generation of the zirconate complex 12 (R = H, Me, or Et), which may be more susceptible to ligand exchange than its zirconacyclopropane precursor 7. Subsequent formation of the zirconocene complex of the reacting alkene would then lead to the high levels of reactivity and selectivity observed in reactions where excess alkylmagnesium halide is present. (2) Deuterium labeling experiments demonstrate that the resident Lewis base strictly controls and reverses the mode of metallacyclopentane cleavage. The heteroatom binds and delivers magnesium to initiate a highly regioselective metallacyclopentane cleavage, effecting a Mg-Zr exchange with inversion of configuration.
