An efficient synthesis of (Z)-γ-fluoroallylphosphonates using a base-promoted deconjugation of (E)-γ-fluorovinylphosphonates, and its utility as fluoroolefin-containing building block

A three-step synthesis of γ-fluoroallylphosphonates starting with α,β-unsaturated aldehydes is described. Treatment with diethyl phosphite in the presence of KF gives α-hydroxyallyl phosphonate in excellent yield; DAST deoxofluorination produces the corresponding γ-fluorovinylphosphonate through a SN2′ mechanism, and finally, a base-promoted double bond migration leads to the desired γ-fluoroallylphosphonate. γ-Fluoroallylphosphonate is a useful building block in the synthesis of fluoroolefins. An exploratory study yielded excellent yields of (Z)-diethyl 1-benzyl-3-fluoro-2-butenylphosphonate and (Z)-diethyl 1,3-difluoro-2-hexenylphosphonate. Treatment of (E)-diethyl 3-fluoro-2-hexenylphosphonate with LiN(TMS)2 and benzaldehyde in THF led to the preferential formation of syn-(Z)-diethyl 3-fluoro-1-(hydroxybenzyl)-2-butenylphosphonate.