Sulphonyl group nucleophilicity

Despite the aversion of the sulphonyl group to function as a nucleophile, there are a limited number of literature reports that deseribe sueh a process . The first of these does not lead to a reduetion at sulphur and is thus not direetly relevant, but the report by Braverman and Duar deseribes reaetions in whieh an acetylenie sulphinate ester ean undergo a [2,3] sigmatropie rearrangement to an allenie t-butyl sulphone. Reaetion of this sulphone with the appropriate eleetrophile, for example bromine, gave a y-sultine, formation of whieh requires the partieipation of the sulphonyl group in an intramoleeular cyclization process. The reactions are outlined in equation (40). 

Vinylsulphonyl chlorides undergo a similar reaction. The sulphonyl group activates the double bond to nucleophilic attack with loss of a chlorine to give a sulphene30. It was not possible to distinguish whether a two-step process via a zwitterion or a concerted process was involved. 

Aromatic systems need to be activated towards nucleophilic attack to enable halogen exchange to occur (see Chapter 9) and the sulphonyl group has been employed as a disposable activating group [45] (Figure 3.3b). Addition of triphenyltin fluoride is claimed to be beneficial [46]. 

The above reactions involve the nucleophilic attack of a sulphonamide jS-carbon at the carbon atom a to the sulphonyl group. A radical analogue of this reaction is known132,133. 

A variety of y-sultams have been prepared via metallation (lithiation) of iV-(2-bromo-alkyl)-alkanesulphonamides (equation 195)251. Lithiation occurs at the methylene carbon a to the sulphonyl group to give the resonance-stabilized carbanion-like species, which then undergoes nucleophilic attack on the 2-bromoalkyl carbon atom. Thus cyclization is achieved via C—C bond formation (Scheme 16). 

KOMe [93% (all- )]. Notably, /-BuOK does not promote the elimination in this case. The mechanism of this process is different from that with the alkoxysulphones. The initial step is formation of a vinyl sulphone 50. Isomerization of this intermediate to the allylic sulphone 48 via 51 is effected by Michael addition and elimination of methanol. The ineffectiveness of t-BuOK is explained by this mechanism. This bulky reagent cannot abstract the proton from the a-carbon of the sulphonyl group because this is embedded in a sterically highly crowded environment. Moreover, r-BuOK does not participate in the Michael addition because of its weaker nucleophilic character. 

Ring Substitution of Aryl Sulphones.—The influence of the sulphonyl group on reactions of substituted phenyl sulphones has been studied in relation to nucleophilic substitution of p-h dogenophenyl aryr or trifluoromethyl aryl sulphones/ by phenoxide anions and other species. 

Nucleophilic additions to carbonyl groups lead to alcohols which on dehydration, furnish alkenes70,71. This two-step protocol has been extremely useful for diene and polyene synthesis with wide variation in the carbonyl substrate and the nucleophilic addendum. Diene synthesis using aldol-type condensation as well as phenyl sulphonyl carbanion (the Julia reaction) are also discussed in this section. 

A methyl group attached to the double bond should decrease the rate of the nucleophilic attack by increasing the electron density and the steric interactions at the double bond. In addition, an a-methyl group should decrease the overall rate even for highly basic nucleophiles, by blocking the elimination-addition route. Substituent effects in a-aryl-sulphonyl-j8-haloethylenes bear out this prediction. The rate retardation by an a-methyl group, more pronounced for the less reactive... 

There are relatively few examples of the syntheses of aliphatic sultones by this mode of cydization. It relies on internal nucleophilic attack of oxygen at sulphonyl sulphur which has a good leaving group (e.g. a sulphonyl halide group). A recently reported case involves the cydization of co-hydroxy-1-alkanesulphonyl chlorides (equation 30)51. This method... 

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