Sulfites, Sulfates and Sulfonates

Conversion of alcohols to sulfonate esters is a way of labilising them to nucleophilic substitution and elimination. The common leaving groups are arenesulfonates, particularly p-toluenesulfonate (tosylate), methanesulfonates (mesylate) and trifluromethanesulfonate (triflate) they are introduced by reaction of the acid chlorides (or, in the case of the trifluoromethanesulfonates, acid anhydrides) in a basic solvent such as pyridine. Traditionally, the reactions are carried out in pyridine as solvent, but both this solvent and the liberated 

Alkanesulfonyl chlorides can also react by an initial elimination in water, mesyl chloride hydrolyses below pH 6.7 by an S 2 reaction of water at sulfur, between pH 6.7 an 11.8 by capture of the sulfene by water and above pH 11.8 by capture of the sulfene with hydroxide ion. An extended study of mesylations of phenols catalysed by various pyridines revealed a competition between the sulfene pathway and a general base-catalysed attack of the phenol, which was dependent on the pAa of both phenol and pyridine, with no mention of an A-mesylpyridinium ion. However, formation of an A-triflylpyr-idinium ion occurs immediately pyridine and triffic anhydride are mixed and this is probably the triflating species. Reaction via an A-sulfonyl quaternary ammonium ion therefore becomes important with more basic amines and more electrophilic sulfonylating reagents. 

compared with around -2.5 for methanesulfonic acid and -3 for benzene-sulfonic acid with a maximum /lig value for S 2 reactions of -0.5 (for methyl transfer), the triflates will be at least 10 times more reactive than tosylates, etc. 

Nature is generally reluctant to carry out classical, central S- 2 reactions, with the exception of methylations - glycosyl- and prenyl-transfer reactions are very S Nl-like. A much preferred route for displacements of sugar substituents is oxidation by a tightly bound nicotinamide cofactor of a p-hydroxyl, elimination of the leaving group, introduction of the substituent in a Michael addition and reduction, as dealt with in Section 6.8. 

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