Additions to Unsaturated Phosphonium Salts

Additions to Unsaturated Phosphonium Salts. In a review of nucleophilic substitution at acetylenic carbon, attention has been drawn to the synthetic potential of alkynyl-phosphonium salts, e.g. (140), which have received little study so far.  

The rates of acid-catalysed cyclization of certain alkynylphosphines to the cyclic diphosphonium salts (141) have been studied, and a mechanism has been proposed 

The addition of nucleophiles to prop-2-ynyltriphenylphosphonium bromide (142) proceeds via the allenylphosphonium salt (143) to give the substituted vinylphos-phonium salts (144). The salts formed by addition of primary amines to (143) exist, in many cases, exclusively in either the enamine (145) or imine (146) tautomers, 

The addition of butanol or diethylamine to the allenylphosphonium salt (148) is reported to form the substituted allylphosphonium salts (149), which undergo acid hydrolysis to form (150).  

A delicate balance of steric and electronic effects at phosphorus controls the course of decomposition of the phosphonium betaines (151) that may be generated in protic solvents. Electron-withdrawing heteroaryl substituents e.g. 2-furyl and 2-thienyl) promote intramolecular collapse of the betaine to form the normal Wittig products, as also does enclosure of the phosphorus in the ring-strained dibenzophospholium system. Except in such cyclic systems, both electron-donating and bulky groups e.g. o-tolyl and t-butyl) reduce the rate of intramolecular collapse and allow a dehydra- 

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