Sulfonates leaving group formation

In contrast to phosphorus esters, sulfur esters are usually cleaved at the carbon-oxygen bond with carbon-fluorine bond formation Cleavage of esteri nf methanesulfonic acid, p-toluenesidfonic acid, and especially trifluoromethane-sulfonic acid (tnflic acid) by fluoride ion is the most widely used method for the conversion of hydroxy compounds to fluoro derivatives Potassium fluoride, triethylamine trihydrofluoride, and tetrabutylammonium fluoride are common sources of the fluoride ion For the cleavage of a variety of alkyl mesylates and tosylates with potassium fluoride, polyethylene glycol 400 is a solvent of choice, the yields are limited by solvolysis of the leaving group by the solvent, but this phenomenon is controlled by bulky substituents, either in the sulfonic acid part or in the alcohol part of the ester [42] (equation 29)... 

It should be mentioned here that if no other leaving group is present, sulfonyl can act as its own leaving group in hydroxide- or alkoxide-catalyzed elimination from sulfones. Carbanion formation is not involved in this but the promotion of the ionization of a C—H bond by the sulfonyl group is seen at the /1-carbon rather than the a-carbon, e.g. equation 21. 

Polar C=Y double bonds (Y = NR, O, S) with electrophilic carbon have been added to suifinic acids under formation of sulfones. As in the preceding section one must distinguish between carbonyl groups and their derivatives on the one hand, and carboxylic acids (possessing leaving groups at the electrophilic carbon) on the other. Aldehydes " of sufficient reactivity—especially mono-substituted glyoxals - —and their aryl or arylsulfonyl imines have been added to suifinic acids (in a reversible equilibrium) to yield a-hydroxy or a-amino sulfones the latter could also be obtained from the former in the presence of primary amines (equation 26). 

The above studies show that dithionite reduction of mitosenes results in the formation of lOa-sulfite esters as well as sulfonates. The presence of the excellent sulfite-leaving group at the mitosene 1 Oa-position suggests that alkylation reactions at this position could still occur. The subsequent sulfite to sulfonate rearrangement results in loss of alkylation capability by this position. 

The alkoxide ion reacts with the substrate in an SN2 reaction, with the resulting formation of the ether. The substrate must bear a good leaving group. Typical substrates are alkyl halides, alkyl sulfonates, and dialkyl sulfates, i.e. 

A reaction which is related to hydroxy-de-diazoniations is the formation of aryl trifluoromethylsulfonic esters (aryl triflates, ArOSC CFs) which became widely used reagents because of their leaving-group properties. The classical method of synthesis by esterification of phenols with trifluoromethane-sulfonic anhydride or -sulfonyl halide is, however, not applicable for the preparation of aryl triflates bearing a (free)... 

Substitution of primary and secondary alcohols by SN2 reactions is generally effected according to two protocols, as shown in Scheme 1. The most common approach involves the formation of a sulfonate ester which is an excellent leaving group. In the second method, a direct conversion from the alcohol can be achieved by a transient species, usually an alkoxyphosphonium salt. 

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