Tosylate, as leaving group

Nucleophilic aliphatic radiofluorination with tosylate as leaving group. 

Since tosyl group and bromide have the same efficiency as leaving groups, the synthesis in good yield of heptakis-6-thioglycosyl- cyclodextrin (17 e, 17f) from the heptakis(6-bromo-6-deoxy)- cyclodextrin (15c) needs, as expected, the sodium salts of 1-thiogalactose (16c, 16d) and high temperature (Scheme 6) [27]. 

Tosylate and other alkoxy groups have been used as leaving groups (equations 1144b, 115146 and 116147). 

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. 

The cyclodehydration reactions of hydroxyalkyloxyphenols using a tosylate <88MI 609-03) or imidate ester <93TL7483> as leaving group are also available. 

On p. 344 you saw a tosylate (we just called it a sulfonate ester then) being displaced by acetate in an Sn2 reaction. Acetate is not a very good nucleophile, and it is a testament to the power of the sulfonate esters that they are willing to act as leaving groups even with acetate, which is usually too weak to react by 5 2. 

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