Barbier-type cross coupling

Indium mediated Barbier-type cross coupling between carbonyl compounds and allyl halides proceed efficiently under solvent-free conditions. No apparent competing pinacol-coupling or homo-coupling of the allyl halide was observed. The reactions were found to be mediated also by zinc, tin, bismut and copper [45]. 

The less stable trifluoromethyl zinc compounds [48] can be used as a source of nucleophilic trifluoromethyl fragments either in the isolated form or generated in situ by sonication of perfluoroalkyl iodides with zinc in DMF or THF. Zinc perfluoroorganyls find application in Barbier-type reactions [49], palladium-catalyzed cross-coupling reactions [50], or hydroperfluoroalkylations of acetylenes or olefins [51] (Scheme 2.119). 

Of interest also is the aqueous Reformatsky cross-coupling reaction between a-halo ketones or aldehydes and carbonyl compounds in the presence of zinc, tin and indium. Like the Barbier-type reactions, indium was found to be the most efficient metal and it strongly reduced undesirable side reactions such as the reduction of the halide [134]. As usual in aqueous reactions implicating carbonyl compounds, lanthanum(III) triflate promoted the reaction [146]. 

Carbon-carbon bond formation plays an essential role in chemical syntheses. In this regard, the development of transition metal-catalysed C-C bond-forming reactions has attracted much interest in recent years and titanium complexes have been found to be highly effective as catalysts for cross-couplings, hydroaminoallq lations, Barbier-type reactions and oligomerisation reactions. 

---