Synthesis from Hydrocarbons and Phosphenium Salts

A further method for the synthesis of cyclic compounds in the phosphinic acid series, and investigated within the last decade, is the clearly not unrelated cheletropic reaction which takes place between alka-1,3-dienes and phosphenium salts. The latter are based on dicoordinate phosphorus, [R R P ], the commonly encountered counter ion being the tetrachloroaluminate anion. Such salts are obtained in situ through the interaction of AICI3 and a hosphorus(III) chloride R R PCl, for which R = Cl, R = R2N, or R = R = 

Investigations by Cowley and coworkers and by Soottoo and Baxter have revealed that the reactions between 1,3-dienes and the phosphenium salts 123 (X = NPr 2 or Cl) proceed easily in dichloromethane at 0 °C, to give intermediate cyclic chlorophos-phonium salts (124) (compare structures 97, 119, 120 and 122), several of which were characterized spectroscopically and crystallographically. The highest yields of 124 (X = NPr 2) were achieved when R = R = Me and R = R = H, and the lowest with the reverse pattern of substitution. The hydrolysis of the salts 124 (X = NR2), using NaOH in aqueous dioxane, yields the cyclic phosphinic amides 125 (X = NR2). 

A similar cheletropic condensation occurs between a phosphenium cation and a hydra-zone from crotonaldehyde (equation 27) to give a cyclic phosphonic diamide derivative.  

Weissman and Baxter have also recorded the successful addition of a phosphenium salt to cycloheptatriene the mixture of stereoisomeric aminochlorophosphonium salts (129) then afforded a mixture of phosphinic amides (130) when hydrolysed. 

For a discussion of the mechanism of the initial cyclization process, the original papers should be consulted  

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