Metal catalyzed coupling Grignard reagents

A general method for a-hydroxyalkylation involves addition of a Grignard reagent or an alkyllithium to a vinyl(alkoxy)silane. Metal-catalyzed coupling of the resulting a-silycarbanion with an alkyl halide (or epoxide) followed by oxidation results in a secondary alcohol. 

Base-catalyzed elimination of -acetoxy sulfones is highly stereoselective, leading to ( )-alkenyl sulfones which undergo transition metal-catalyzed coupling with Grignard reagents with retention of configuration to provide a stereoselective synthesis of trisubstituted alkenes. Either nickel(II) acetylacetonate, tris(acetylacetonato)iron(III), or iron(III) chloride can be used as the catalyst (eq 17). ... 

In the transition-metal catalyzed asymmetric Grignard coupling, Kumada et al. proposed a reaction mechanism, as shown in Scheme 4.4 [78], in which the active site is the organotransition metal compounds (R—M—R ) which can be obtained by the reaction of organotransition metal compounds with the Grignard reagent [78]. 

As in other cases of Pd- or Ni-catalyzed cross-coupling, Grignard reagents should probably be considered first in the aryl-alkenyl coupling because of their ready accessibility as the first-generation aryhnetals. As needed, other metals should then be considered as potentially superior alternatives, and Zn, B, and Sn may have been the three most widely used metals, although some other metals including Al, Si, and Cu may also prove to be useful and even superior to the others mentioned above in some cases. 

The synthesis involves the nickel-catalyzed coupling of the mono-Grignard reagent derived from 3-alkyl-2,5-diiodothiophene (82,83). Also in that year, transition-metal hahdes, ie, FeCl, MoCl, and RuCl, were used for the chemical oxidative polymerization of 3-substituted thiophenes (84). Substantial decreases in conductivity were noted when branched side chains were present in the polymer stmcture (85). 

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