Reductive dehalogenation dihalocyclopropanes

An important synthetic application of this reaction is in dehalogenation of dichloro- and dibromocyclopropanes. The dihalocyclopropanes are accessible via carbene addition reactions (see Section 10.2.3). Reductive dehalogenation can also be used to introduce deuterium at a specific site. The mechanism of the reaction involves electron transfer to form a radical anion, which then fragments with loss of a halide ion. The resulting radical is reduced to a carbanion by a second electron transfer and subsequently protonated. 

Table 23. Methods for the Double Reductive Dehalogenation of 1,1-Dihalocyclopropanes...

The alkyllithium dehalogenation of dihalocyclopropanes can be used for derivatives having heteroatomic substituents (O, N, S) and is even compatible with alcohol functions and the preparation of allene alcohols. In the latter case two or more equivalents of the alkyllithium reagent must be used (see Table 5). Improved yields of allene alcohols can be obtained by protecting the alcohol function as the trimethylsilyl derivatives. Competing processes which can occur include C-H and O-H insertion, reductive dehalogenation and intramolecular insertion of cyclopropylidene at unsaturated sites of the substituent. 

The transfonnations of gm-dihalocyclopropanes are synthetically useful because the cyclopropanes are readily prepared by the addition of dihalocarbene to olefins. In most of dehalogenation reactions to monohalocyclopropanes, the reagents are limited to metallic reductants such as organotin hydride, lithium aluminum hydride, sodium borohydride, Grignard reagent, and zinc-copper couple [1-9]. A versatile method for the reduction of gm-dibromocyclopropanes 3 with an organic reductant is achieved by use of diethyl phosphonate (commercially named diethyl phosphite) and triethylamine to give the monobromocyclopropanes 4 (Scheme 2.2) [10]. 

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