Sulfonate Esters as Substrates in Nucleophilic Substitution

Sulfonic acids such as methanesulfonic acid and p-toluenesulfonic acid are strong acids, comparable in acidity with sulfuric acid. 

Alkyl sulfonates are derivatives of sulfonic acids in which the proton of the hydroxyl group is replaced by an alkyl group. They are prepared by treating an alcohol with the appropriate sulfonyl chloride. 

Alcohol Sulfonyl chloride Sulfonate ester Hydrogen chloride 

These reactions are usually carried out in the presence of pyridine. 

Alkyl sulfonate esters resemble alkyl halides in their ability to undergo elimination and nucleophilic substitution. 

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Methanesulfonates. The most common use of methanesulfonyl chloride is for the synthesis of sulfonate esters from alcohols. This can be readily accomplished by treatment of an alcohol with mesyl chloride in the presence of a base (usually Triethy-lamine or Pyridine). The methanesulfonates formed are functional equivalents of halides. As such they are frequently employed as intermediates for reactions such as displacements, eliminations, reductions, and rearrangements. Selective mesylation of a vicinal diol is a common method of preparation of epoxides." Alkynyl mesylates can be used for the synthesis of trimethylsilyl allenes. Oxime mesylates undergo a Beckmann rearrangement upon treatment with a Lewis acid. Aromatic mesylates have been used as substrates for nucleophilic aromatic substitution. Mesylates are more reactive than tosylates toward nucleophilic substitution, but less reactive toward solvolysis. 

An advantage that sulfonate esters have over alkyl halides is that their preparation from alcohols does not involve any of the bonds to carbon. The alcohol oxygen becomes the oxygen that connects the alkyl group to the sulfonyl group. Thus, the configuration of a sulfonate ester is exactly the same as that of the alcohol from which it was prepared. If we wish to study the stereochemistry of nucleophilic substitution in an optically active substrate, for example, we know that a tosylate ester will have the same configuration and the same optical purity as the alcohol from which it was prepared. 

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