Phosphonium thiolates, reactions

Acylation of alkylidenetriphenylphosphoranes by thiocarboxylic 5-ethyl esters 456 The reactions are carried out in a Schlenk tube, as above. To a boiling solution (which need not be filtered) of a salt-free alkylidenephosphorane (0.022 mole) in anhydrous toluene is added an 5-ethyl thiocarboxylate (0.02 mole), and the mixture is boiled for 18 h. The phosphonium thiolate, that is initially precipitated as an oil, gradually redissolves. If an unfiltered ylide solution was used, the reaction solution must be filtered hot at this stage to remove sodium... 

The formation of a P=0 bond is the driving force in the reaction of thioalkoxy-phosphonium salts, having non-nucleophilic counter anions, with carboxylates or alcoholates, thereby cirumventing the inconvenience of using thiols or thiolates in the synthesis of thiol esters or sulphides the yields are fair to excellent433,781 (reaction 235). 

The suggested mechanism involves an attack of the nucleophilic phosphorus reagent on the anomeric oxygen to afford an anomeric phosphonium ion and a thiolate counterion, which then attacks in a Sn2 reaction the anomeric position to give the thioglycoside and triafkyl phosphate or trialkylphosphine oxide. 

The method can be used preparatively. Other examples include the selective demethylation of triethylmethylammoniiun chloride with sodium benzenethiolate. A somewhat analogous reaction is observed in the reaction of alkoxytri(dimethylamine)phosphonium chloride (33) with thiolates forming a phosphine oxide and sulphide. ... 

The initial attack of PR3 on the disulphide bond is the rate-limiting step. Hydrolysis of the phosphonium salt is rapid and irreversible. The rate of reaction is enhanced by either dilute acid or dilute base at high pH hydrolysis of the phosphoium salt is rapid but at low pH the thiolate anion becomes protonated, thus suppressing the reverse reaction in the rate-limiting step. 

---