Generation of Lithium Halocarbenoids

Lithium halocarbenoids are species in which lithium and one, two, or three halogen atoms are linked to the same carbon atom. A great deal of the chemistry of these carbenoids was developed by Kobrich and his co-workers [1] who were the first to generate these species in solution. Useful synthetic applications were found several years later by a number of groups, which could take advantage of the considerable progress in the field of polar organometallic chemistry. 

Although some successful syntheses have been realized at relatively high temperatures with transient carbenoids and very reactive trapping reagents like carbonyl compounds and chlorotrimethylsilane, halocarbenoid chemistry is in general performed below — 80 °C with preformed carbenoids. 

The review by Siegel [2] covers most of the halocarbenoid chemistry. The procedures in this chapter exemplify the methods and techniques used in the generation of lithium halocarbenoids and in their subsequent functionalization reactions in accordance with the scope of this volume, only carbenoids in which the carbanionic centre is spMike ( C(Li)Hal) are considered. 

Kobrich G (1972) Angew Chemie 84 557 (1972) Angew Chemie Int ed (Engl) 11 473 

Siegel W (1982) Lithium Halocarbenoids, Carbanions of High Synthetic Versability, Topics in Current Chemistry 106 55, Springer-Verlag 

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Seyferth et al. [1] give well-detailed descriptions for the generation of a number of lithium halocarbenoids from geminal dihalocyclopropanes. In their procedures, a solution of BuLi in hexane is added to a mixture of the dihalo compound and THF kept between — 90 and — 100 °C. According to Villieras [2], lithium bromide... 

Lithium halocarbenoids are no more subject of mechanistic interest only. Improvement of preparative techniques in the last ten years made them to valuable synthetic intermediates which are stable in the temperature range between —130 and —70 °C. They are generated from readily available starting materials and give high yields of adducts on reaction with electrophiles. 

Vinylidene lithium halocarbenoids may be generated by halogen/lithium exchange from the corresponding dihaloolefins. In addition, there is another route to these intermediates involving metallation of monohaloolefins, since the acidity of the vinylic proton is sufficiently high (Eq. (49) 3)). 

An extension of the synthetic use of vinylidene lithium halocarbenoids is the method of Zweifel60) who succesfully generated a vinylidenecarbenoid bearing a p-hydrogen atom, as outlined in Eq. (55). Usually, attempts to prepare these intermediates resulted in the elimination of hydrogen halide with the formation of the corresponding acetylene 3). 

Villieras and coworkers have contributed a considerable number of synthetic methods employing lithium halocarbenoids. An early example was the alkylation of dibromomethyllithium and higher homologs (Scheme 15). The anion (15) is easily generated from 1,1-dibromoalkanes by deprotonation with LDA in THF at low temperature. Alkenes can then be generated simply by treating the products with Bu"Li in EtjO, Alkylation of dichloromethyllithium with primary alkyl halides followed by reaction of the dichloroalkanes with Bu"Li represents a method for the preparation of 1 -alkynes. ... 

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