Reactions with formation of tricoordinated phosphorus

Lastly, mono- or bis-phosphonium salts can be cleaved to phosphines when a negative charge is created at a suitable position either with nucleophiles792,793 or using a base to abstract labile hydrogen atoms540,701,794-798 (reaction 243). In the case of 1.2-ethylene, 

2-vinylene, l,4-but-2-enylene or buta-l,4-dienylene bisphosphonium salts, the nature of the other substituents on the phosphorus strongly influences the reduction to phosphine species with or without cleavage of the bridge364,700,713. 

TABLE 25. Synthesis of cyclic compounds from vinylic phosphonium salts and carbonyl-containing nucleophiles 

As expected, buta-l,3-dienylphosphonium salts add ketonic nucleophiles in the exposition792. A butadienylphosphonium salt is also the intermediate accounting for the 

Allenyl- and propargyl-phosphonium salts have also been used as precursors of heterocyclic compounds in the presence of various functionalized nucleophiles279, via the intermediacy of vinylphosphonio compounds however, an ylide extrusion has been sometimes observed815,816, depending on the nature of the functions in the nucleophile (reaction 250). Allenylphosphonium salts are able to add such weak CH acids as in ketones, even in absence of basic catalysts817,818, without any modification of the keto group (reaction 251). 

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