Sulfonates as leaving groups

For electrophiles such as Me3SiCH2X strong ground-state destabilization has been observed for X = 4-nitrobenzoate[130]. For X= halide, on the other hand, this ground-state destabilization is significantly smaller, and it may therefore be advisable to choose carboxylates or sulfonates as leaving groups when alkylations with a-silyl-substituted electrophiles are to be performed. 

Diene synthesis with p,y-ethylenesulfones as acetylene equivalents Sulfones as leaving groups. 

As in part (a), identify the nucleophilic anion in each part. The nucleophile replaces the p-toluene-sulfonate (tosylate) leaving group by an SN2 process. The tosylate group is abbreviated as OTs. 

The preparation of tosylate and other sulfonate esters for use as leaving groups in nucleophilic substitution reactions (see Section 8.9) employs the reaction of a sulfonyl chloride (an acid chloride of a sulfonic acid) with an alcohol. Another example is shown in the following equation. Note the similarity of this reaction to the reaction of an acyl chloride with an alcohol to form an ester. 

As indicated below, the weakest bases (that is, the anions derived from the strongest acids) are indeed the best leaving groups. The / -toluene-sulfonate (tosylate) leaving group is very easily displaced, as are I" and Br, but Cl" and F" are much less effective as leaving groups. 

One of the reasons that tosylates, or related sulfonate esters, are employed as leaving groups rather than, say, chlorides is that from an enantiomerically pure alcohol, with a stereogenic carbon that is bonded to OH, the tosylate is also enantiomerically pure since the bond between oxygen and the stereogenic carbon is not broken. 

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