The reaction of diethyl alkylphosphonates with chlorotrimethylsilane in the presence of lithium diisopropylamide leads to the corresponding diethyl 1-lithio(trimethylsilyl)alkylphosphonates which are cleanly halogenated (X = F, Cl, Br, I) with electrophilic reagents and then deprotected to give pure diethyl a-monohalogenoalkylphosphonates. The reaction has been extended by the onepot conversion of triethyl phosphate into diethyl a-monohalogenobutylphosphonates in 85-97% overall yields.Over the years, a number of methods have been described for the preparation of a-monohalogenated methyl 2,3 or alkylphosphonates. In the majority of cases each halide is introduced in the a-position to phosphonates either by a specific method or with an appropriate reagent through oxidative or reductive processes (F, 4,5 Cl, 6-10 Br, 6,11,12 I 6,10 ). There are no known methods for the conversion of alkylphosphonates to a-monohalogenated alkylphosphonates by a common route to the four halides. The importance of a-halogenated alkylphosphonates as precursors of more elaborated compounds or as reagents in organic synthesis has been well recognized in recent years. 2,3,13 For example, they play an evergrowing role in the preparation of phosphonates bearing different functional groups. 3,14 Our continuing interest in the use of a-monohalogenated alkylphosphonates 4 ( Figure) prompted us to explore the possibility to develop a general method giving access to these compounds under mild conditions. At the same time we should like to demonstrate how easily and selectively any single chosen halogen can be introduced in the a-position to alkylphosphonates.Recently, we reported a new and efficient method for the conversion of diethyl benzylphosphonates to a-monohalogenated diethyl benzylphosphonates based on the electrophilic halogenation of a-phosphorylated-a-silylated carbanions. 1 This mild and versatile one-pot procedure, which results in overall yields of 68-97% of pure amonohalogenobenzylphosphonates has now been further developed for the conversion of diethyl alkylphosphonates 1 into a-monofluoro-, monochloro-, monobromoand monoiodoalkylphosphonates 4. These two procedures are complementary with respect to the reactivity of phosphorylated carbanions toward chlorotrimethylsilane (TMSCl). Furthermore, the extension of this approach gives us once more the possibility to promote electrophilic halogenating reagents such as hexachloroethane, dibromotetrachloroethane and dibromotetrafluoroethane which combine availability, cost, efficiency and ready elimination of byproducts.A variety of starting diethyl alkylphosphonates 1, with short or long carbon chain, substituted or unsubstituted, are involved in the reaction sequence. They are easily available in high yields either by thermal reaction between triethyl phosphite and the corresponding alkyl halides (Michaelis-Arbuzov reaction) 15-18 or by anionic reaction at low temperature between triethyl phosphate and the corresponding alkyllithiums (phosphate-phosphonate conversion). 19 Once these star...
