α-Fluorobenzylphosphonate esters are conveniently made by treating α-hydroxybenzylphosphonate esters with diethylaminosulphur trifluoride. The reaction is not subject to steric impedance and is extended to the α-fluorination of an α-hydroxybenzylphosphine oxide. For α-hydroxyallyl and α-hydroxycinnamylphosphonates, the replacement of the hydroxy group by fluorine proceeds via an SN2′ rearrangement to give the γ-fluoroalk-1-enylphosphonates exclusively. Dehydration rather than substitution occurs in the case of alcohols of secondary alkylphosphonates.
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Blackburn et al. (1986) studied this question.