Phosphonium salts, carbon-phosphorus bond formation

Reaction of the bromotrifluoromethane/tris(diethylamino)phosphane system with benzalde-hyde leads to the formation of the phosphonium salt 19, which decomposes at 140 C, affording the bromophenylethanc 20 and not the alcohol. Due to the ease of formation of a phosphorus compound with a P = 0 bond, in this case the intermediate carbenium ion reacts with the bromide anion, giving rise to the final reaction product. The reaction is general in character pcrfluoroalkyl bromides with different carbon-chain lengths (from 2 to 6) have been used as reactants. 

In addition to iodonium, sulfonium and selenonium compounds, onium salts of bromine, chlorine, arsenic, and phosphoras are also stable and can act as sources of cation radicals as well as Bronsted acids, when irradiated with light. Performance of diaryl chloronium and diaryl bromonium salts was studied by Nickers and Abu. Also, aryl ammonium and aryl phosphonium, and an alkyl aryl sulfonium salt were investigated. It appears that the general behavior of these materials is similar to diphenyl iodonium and triphenyl sulfonium salts. These are formations of singlet and triplet states followed by cleavages of the carbon-onium atom bonds and in-cage and out of cage-escape reactivity. The anions of choice appear to be boron tetrafluoride, phosphorus hexafluoride, arsenic hexafluoride, and antimony hexafluoride. 

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