[33558-14-4] · C19H15F2P · Difluoromethylenetriphenylphosphorane · (MW 312.30)
(utilized for the introduction of =CF2 into organic compounds)
Alternate Name: triphenylphosphonium difluoromethylide [34034-84-9].
Preparative Methods: prepared in situ by the reaction of lithium or Sodium Chlorodifluoroacetate with Triphenylphosphine2-6 (or, analogously, with Tri-n-butylphosphine7,8); by the reaction of Dibromodifluoromethane with 2 equiv of tertiary phosphines;6,9 or by the dehalogenation of bromodifluoromethyltriphenylphosphonium bromide with tertiary phosphines9 or metals.10
Treatment of sodium or lithium chlorodifluoroacetate and triphenylphosphine with aldehydes or perfluoroalkyl aryl ketones gives the corresponding 1,1-difluoroalkenes in 52-78% yields (eq 1).1-6,11 Monoglyme (MG), diglyme (DG), and triglyme (TG) have been employed as solvents in the above reaction. In DMF, DMSO, dimethylacetamide, tetramethylurea, and N-methyl-2-pyrrolidone, reaction of sodium chlorodifluoroacetate and tributylphosphine with ketones bearing a-protons affords the corresponding 1,1-difluoroalkenes in 35-46% yields.7,8 However, Burton reported that less than 10% of 1,1-difluoroalkenes were obtained from the reaction of sodium chlorodifluoroacetate and tributylphosphine with non-activated ketones9 and a different type of reaction was observed with activated ketones.12
Difluoromethylenation of aldehydes and ketones has also been accomplished in 81-87% yields by the reaction with dibromodifluoromethane and 2 equiv of triphenylphosphine6 or Hexamethylphosphorous Triamide9 in DG, TG, and DMF solvents (eq 2). Alternatively, bromodifluoromethyltriphenylphosphonium bromide or bromodifluoromethyltris(dimethylamino)phosphonium bromide,10 generated from dibromodifluoromethane and triphenylphosphine or tris(dimethylamino)phosphine, can be treated with carbonyl compounds and 1 equiv of the corresponding phosphine1 or metals (Zn, Cd, and Hg)10 to obtain 1,1-difluoroalkenes (eq 3).
Donald J. Burton & Weiming Qiu
University of Iowa, Iowa City, IA, USA