Oxyphosphonium salts

Recent mechanistic studies have shown that the many combinations of alcohols, carboxylic acids and solvents cannot be correctly described by a uniform mechanism. In certain cases the reaction appears to involve a pentavalent dialkoxyphos-phorane 10 as an intermediate, which is in equilibrium with oxyphosphonium salt 8 4 ... 

FIGURE 2.20 Intermediates and their fate in benzotriazol-l-yl-oxyphosphonium salt-mediated reactions. Indirect evidence (Figure 2.21) is not compatible with the tenet that the precursor of the peptide is the benzotriazolyl ester (path C). The evidence indicates that the peptide originates from the acyloxyphosphonium cation (path B). Conversion of this intermediate into the oxazolone (path E) can account for the epimerization that occurs during segment couphngs. 

Camp, D., Jenkins, I. D. The mechanism of the Mitsunobu esterification reaction. Part I. The involvement of phosphoranes and oxyphosphonium salts. J. Org. Chem. 1989, 54, 3045-3049. 

The nascent alkoxyphosphonium ion is a much better cr-bond electrophile than the alcohol, so in in the last part of the reaction mechanism, the carboxylate O attacks the electrophilic C of the oxyphosphonium salt in Sn2 fashion, displacing PI13PO to give the product... 

In the second part of the mechanism, the carboxylate deprotonates the alcohol, which then displaces N from P in 8 2 fashion, giving an oxyphosphonium salt and converting the alcohol C into a much better electrophile. 

Condensation of alcohols and acidic components (NuH) on treatment with dialkyl azodicarboxylates and trialkyl- or triarylphosphines occurring primarily with inversion of configuration via the proposed intermediary oxyphosphonium salts ... 

Introduction. The combination of triphenylphosphine and N-halosuccinimide was applied by Hanessian to the preparation of primary halides from carbohydrate precursors. This method is related to a number of other methods for hydroxyl substitution via oxyphosphonium salts as described in a comprehensive review. ... 

BENZOTRIAZOL-1 -YL-OXYPHOSPHONIUM AND CARBENIUM SALT-MEDIATED REACTIONS... 

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