Transition states in nucleophilic substitution

Phosphoranide anions 1 - i.e. species based on pentacoordina-ted phosphorus having a lone pair as one of its five substituents -were proposed by Wittig and Maercker as early as 1967, to act as intermediates or transition states in nucleophilic substitutions at tricoordinated phosphorus ( p. Shortly afterwards in 1969, Hellwinkel brought indirect evidence for the formation of phosphoranide 3 in equilibrium with the carbanion 4, since the action of a base on phosphorane 2 gave, after acidic treatment, a mixture of 5 and 6 in proportions that depend on the experimental conditions (i). 

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