Synthesis and Chemistry of Trifluoroacetyl Hypofluorite with Elemental Fluorine. A Novel Method for Synthesis of &α-Fluorohydrins

Article abstract of DOI:10.1021/jo01292a025

Sodium trifluoroacetate reacts with fluorine in the presence of traces of water or HF to give mainly trifluoroacetyl hypofluorite, CF3COOF (1).This uncommon reagent was reacted in situ with a number of stilbenes and with diphenylacetylene.The oxygen-bound fluorine clearly acts as an electrophile, since Markownikoff addition of 1 to the double bond was observed.However, in cases where the Hammett constant ?_p⁺ of the ring substituent is low, as in 4-chloro- (15) or 4-methyl- (32) stilbenes, some of the regioselectivity is lost.Usually only syn adducts were found except in cases where the 1-fluoro carbocation is stabilized, as in the case of trans-4-methoxystilbenes (23), or more than usual sterically hindered, as in trans-2-(carbomethoxy)stilbene (20).The stereoselectivity achieved in the reaction of 1 with stilbenes was compared with the stereoselectivity of fluoroxyperfluoroalkanes, R_fOF, with the same olefins.In the light of this comparison it seems to us that the oxygen-bound fluorine in 1 is more electrophilic in character than in R_fOF compounds.The reaction products of diphenylacetylene (28) with 1, which are in sharp contrast to the products of the parallel reaction of 28 with CF3OF, support this observation.The strong electrophilic character of the oxygen-bound fluorine of 1 is also demonstrated by aromatic electrophilic fluorination taking place on the activated ring of 4-methoxystilbene (23).In almost all cases the 1-fluoro-2-hydroxy (or 2-trifluoroacetoxy) compounds adopt the more stable gauche conformation as evident from their NMR (¹H and ¹⁹F) spectra.In the case of steric disturbance, however, as in 21b and 22b, a deviation from the gauche conformation was observed and discussed.The 1-fluoro-2-trifluoroacetoxy compounds were readily hydrolyzed to the corresponding α-fluorohydrins, thus opening a new route for the synthesis of this important group.