Silylation Barbier reactions

In their studies on the use of silyl-substituted ligands as thermal and oxidative stabilizers of transition metals, Pannell and collaborators [79] show that tram- -trimethylsilyl-3- / -allyl(trimethyl)tin can be prepared in an approximate 50% yield by the Barbier reaction of a trialkyl or triarylmetal chloride with magnesium metal and l-trimethylsilyl-3-chloroprop-l-ene. With nickel(II) chloride, magnesium, a trace of mercuric(ll) chloride, and 2-trimethylsilyl-3-chloroprop-l-ene, the thermally and oxidatively stable bis( / -2-trimethylsilylallyl)nickel derivative is prepared in 36% yield. The unsubstituted derivative is thermally unstable and ignites in air. 

Functionally substituted benzylic, allylic, and vinylic compounds containing alkoxides, esters, ethers, nitriles, or amides can be reacted with halosilanes under Barbier conditions using HMPT to yield C- and O-silylated products, 1,2- or 1,4-addition products, as well as reductive dimers. Radical and anionic intermediates are postulated, based on SET reactions from the metal, and multiple silated species can be obtained. The use of the TMSCl-Mg-HMPT system has been extensively investigated by Galas group [85] at the University of Bordeaux, and their work has greatly advanced the science of the Barbier reaction with silanes. 

The bicyclic 8,8-dimethyl-8-silyl-bicyclo[3.2.1]octa-2,6-diene is the only product isolated from the Barbier reaction of cylohepta-2,4,6-triene with dimethyldichlorosilane [104]. 

The same polymeric arenes that served as metallation catalysts in equation 119 can also be used for silylation in Barbier-type reactions (equation 131). The polymer is presumably converted to a lithium arene adduct that activates metallic lithium for metallation of the halogenated substrates, before addition of an electrophile to achieve the synthetic goal. Equations 132-135 illustrate some of the cases investigated. The products can be characterized by the usual spectroscopic methods . 

A similar reaction presumably underlies one of the most remarkable transformations yet achieved by reductive lithiation with catalytic DBB the transformation of a nitrile into an organolithium with cleavage of a C-C bond.99 The electrophile needs to be present in the reaction mixture (Barbier conditions), but silylation or reaction with a carbonyl compound competes successfully with deprotonation of the nitrile starting material ... 

Coupling reactions of alkyl, alkenyl, and aromatic halides with halosilanes under Barbier conditions are well known. The partial silylation of geminal polychlorinated compounds... 

---