Alkyl halides formation with phosphorus reagents

FIGURE 7.44 Formations of alkyl halides with phosphorus reagents. 

These reactions may be considered to be a method of obtaining 1,3,2,5-dioxaborataphosphoniarinanes with different substituents at carbon and phosphorus atoms of the ring. Comparing the properties of cyclic oxyalkyl-phosphines and boryloxyalkylphosphines, it should be noted that in both cases the reaction with alkyl halides results in the formation of a tertiary phosphonium salt. The reaction with electrophilic reagents such as diphe-nylchlorophosphine and diphenylchloroborane proceeded quite differently [Eq. (100)]. 

These reagents are usually prepared by treatment of a phosphine with an alkyl halide and successive addition of base, like BuLi or NaHMDS. Reaction with the ketone takes place by attack of the carbanionoid carbon of the ylide form on the electrophilic carbon of the carbonyl group via a four-membered heterocyclic transition state, the oxaphosphetane 35. The driving force for this transformation is provided by formation of the very strong phosphorus-oxygen bond. Subsequent collapse of the oxaphosphetane furnishes the desired exomethylene group under retention of geometry. 

Compared with the typical Wittig reaction, the Wittig-type reaction under catalytic conditions provides a way to couple aldehydes with tosylhydrazones for the preparation of substituted stilbenes with good tran -selectivity. The method also avoids the separate preparation of phosphorus ylide. Thus, this method provides an alternative pathway of C=C bond formation and extends the scope of substrates from alkyl halides to various aldehydes. However, more than 1 equiv. of phosphoms reagent is stiU needed. This method has not been well applied to ketmie olelination. 

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