Barbier additions anionic reactions

Molander has also studied the Sml2-mediated double Barbier additions of alkyl dihalides to ketoesters.22,23 These impressive anionic-anionic, inter-molecular-intramolecular sequences require the use of Nil2 as an additive and irradiation with visible light and allow access to a range of bicyclic and tricyclic systems. The reactions proceed by reduction of the more reactive alkyl halide, intermolecular Barbier addition to the ketone, lactonisation and a second Barbier addition to the lactone carbonyl (Scheme 6.18).22... 

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. 

At the start of this work, the dual nucleophilic-electrophilic character of the a-sulfonyl carbanion 213 created some problems since this anion could react not only with the carbonyl derivative (aldehyde or ketone) but also with itself, generating via this competitive pathway some homocoupling by-products. However, a wise selection of the sulfone electrophilic substituent and the use of Barbier-type conditions considerably increased the chemoselectivity of the addition reaction and directed the addition onto the carbonyl function. Benzothiazole derivatives (hereafter denoted BT) were the first substrates to be utilized in this process. After some fine tuning, they proved to be suitable candidates, affording the desired alkenes with a reasonable efficiency (Table 3.11). 

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