Lithium halocarbenoids

Siegel, H. Lithium Halocarbenoids Carbanions of High Synthetic Versatility. 106, 55-78 (1982). 

Shubin, V. G. Contemporary Problems in Carbonium Ion Chemistry II. 1161117, 267-341 (1984). Siegel, H. Lithium Halocarbenoids Carbanions of High Synthetic Versatility. 106, 55-78 (1982). Steudel, R. HomocycUc Sulfur Molecules. 102, 149-176 (1982). 

Lithium Halocarbenoids — Carbanions of High Synthetic Versatility... 

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. 

The synthetic applications of halocarbenoids are mainly determined by the framework bearing the carbenoid center. This article describes the different kinds of synthetic transformations that can be achieved by the use of alkylidene, a-heterosubstituted, cyclopropylidene, vinylidene, and allylidene lithium halocarbenoids. Their particuliar value in organic synthesis results from various rearrangement reactions of the primary adducts formed by reaction of the carbenoid with the electrophile. 

Since the preparative applications of lithium halocarbenoids have strongly increased in recent years, it is the purpose of this review to demonstrate the synthetic significance of these intermediates. 

Scheme 1 Acceptor and donor reactivity of lithium halocarbenoids...

Formally, lithium halocarbenoids are obtained by replacing a-hydrogen atoms in alkyllithium compounds by halogenes. Thus, we can derive from methyllithium monohalo-, dihalo- and trihalolithiomethanes... 

The types of reactions exemplified by Eqs. (9) and (10) are characteristic of all types of lithium halocarbenoids (see also below) ... 

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 interesting application of lithium halocarbenoids at temperatures above —80 °C involving substitution of halogen by alkylanions (cf. Scheme 1) and subsequent alkylation of the resulting carbanion has been reported by Duhamel59) (reaction sequence (54)). 

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). 

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