Sulphonyl Carbanions

Another approach to an oxidative desulphonylation reaction is to oxidize an a-sulphonyl carbanion with an oxidizing agent that is also nucleofugal. An example of this was presented by Little and Sun Ok Myong203 who used a molybdenum peroxide complex (M0O5.Pyridine.HMPA) as the oxidant. However, this reagent is expensive and... 

Sulphonylbenzofurans 638 a-Sulphonyl carbanions 760, 766, 767, 797, 813, 951, 961 acylation of 627-636 addition to unsaturated bonds 636-642 alkylation of 627-636, 781 halogenation of 1058-1060 Michael addition of 642-649 miscellaneous reactions of 653-655 Ramberg-Backlund reaction of 649-653, 1057, 1058... 

Subsequently, Paquette and Johnson used LAH reductions to convert strained thietane or thiolane derivatives to their respective sulphides, generally in good yields. Whitney and Cram described the LAH reduction of a chiral derivative of benzothiophene sulphone, as outlined in equation (24). The authors also noted the formation of hydrogen gas and suggested that their results were consistent with those of Bordwell as outlined in equation (22), namely that reduction takes place by the formation of an aluminium oxide and hydrogen gas. In this case, the reduction clearly cannot involve the formation of an a-sulphonyl carbanion and it is unlikely that any C—S bond cleavage and reformation could have occurred. 

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. 

Both processes occur through an a-sulphonyl carbanion and the results indicate that this species is asymmetric and racemizes more slowly than it... 

Two explanations were originally put forward to account for the asymmetry of sulphonyl carbanions. In the first [146] it was assumed that the carbanion is sp3 hybridized and pyramidal, for example, as shown in (XI) and (XII) and that the rate of pyramidal inversion was slower than the rate of protonation. This would explain why isotope exchange occurs more rapidly than loss of optical activity. An alternative explanation has been suggested in which the carbanion is assumed to be planar and sp2 hybridized. Overlap between the p-orbital with its pair of electrons and sulphur d-orbitals may be involved. A symmetric structure like (XIV) will not explain the results but an asymmetric structure (XIIII) in which asymmetry results from restricted rotation about the carbon—sulphur bond has been proposed [148]. Further, the kinetic results require that de-deuteration or protonation must occur preferentially from one side of... 

In another most elegant experiment Corey and Lowry [149], assuming a planar sulphonyl carbanion with restricted rotation, set out to examine in which direction preferential protonation and deprotonation involving (XIII) occurs. The two possibilities for deprotonation in the directions syn or anti to the sulphone oxygen atoms are shown in (93) and (94), respectively, viz. 

A simple and short strategy for the synthesis of 4-amino-2,3-dihydroisothiazole-1,1-dioxide derivatives was reported, which was based on the a-sulphonyl carbanion mediated ring closure of a,a-disustituted a-aminonitriles <97TL4835>. 

Exposure of p-acetoxysulphones, which are readily accessible from an a-sulphonyl carbanion and an aldehyde, to an excess amount of potassium alkoxide results in elimination of both functional groups in a one-pot reaction, introducing unsaturation in two carbon-carbon bonds. As a consequence, acetylenes or dienes are obtained, depending on the aldehyde (Equations 4 and 5) [15,16]. 

Attempts to generate the a-sulphonyl carbanion at higher temperatures led to extensive anionic polymerisation. Both cis- and trans-Y-monohydroxylated prenyl synthons (or their equivalents)... 

Sulphonyl carbanions react with a variety of carbonyl systems. With isocyanates, methylsulphonyl carbanion underwent a diacylation to afford a diamido-derivative (79), whereas the corresponding sulphinyl carbanion... 

Becker and Gosselck have reported the addition of sulphonyl carbanions to dimethylvinylsulphonium ion to give cyclopropanes. Interestingly, the sulphinate ion was not displaced in these reactions. Instead, the initial adduct (83) underwent a proton transfer, converting a sulphonium ylide into a sulphonyl ylide, and the latter apparently... 

Sulphones Used in Syndiesis.— The range of examples of reactions of sulphones discussed in the preceding section makes clear the scope for applications in synthesis. Most applications are based on sulphonyl carbanions, and are covered in the next Chapter, but ring syntheses are illustrated by a novel synthesis of naphthalene that involves the condensation of o-formylbenzyl p-tolyl sulphones... 

Intramolecular alkylation, acylation, and carbonyl addition by cr-sulphonyl carbanions, followed by reductive removal of the sulphur function, has also received considerable attention as a means of generating medium-sized rings, such as ( )-recifeiolide (61) (note the use of a r-allylpalladium complex as the electrophile see Scheme 6), cycloundecanone (82) (Scheme 7), and neocembrene. Other polycyclic systems, such as ( )-zizaene, potential dodecahedrane precursors, ... 

Alkylation of anions (88) and (89), followed by pyrolysis, provides novel methods of synthesis of carbonyl compounds and polycyclic hydrocarbons (including steroids), respectively. A synthesis of polycyclic hydrocarbons related to that based on the anion (89) involves alkylation of a benzocyclobutenyl-sulphonyl carbanion. Alkylation of a thietan 1,1-dioxide carbanion (90), followed by photolysis of the product, affords cyclopropanes. ... 

All-/ra s-geranylgeraniol and other olefins have been synthesized through the reaction of allylic sulphonyl carbanions (or other functionalized cr sulphonyl carbanions) with Tr-allylpalladium complexes. -Other applications of allylic sulphonyl anions include syntheses of the marine hexaprene mokupalide, nerol and isogeraniol, solanesol, labda-7,14-dien-13-ol, Icr-hydroxy-tachysterolj and certain alkynes, and allylic alcohols. At low temperatures, allylsulphonyl carbanions (91) will undergo y- and well as or-addition to Michael acceptors. The presence of a y-keto-group also promotes y-substitution of allylsulphonyl carbanions. ... 

Miscellaneous reactions of cr-sulphonyl carbanions with electrophiles include processes involving arylsulphonylmethyl methyl ether anions, cr-sulphinyl sulphone anions, perfluorobutylsulphonylmethylmagnesium halides, nucleophilic substitution in aromatic nitro-compounds by cr-halogenosulphonyl... 

E- l-Benzenesulphonyl-2-(trimethylsilyl)ethylene (151) is a useful equivalent of acetylene for Diels-Alder reactions (Scheme 74). It can also function as an equivalent of monosubstituted acetylenes because it is possible to alkylate the Diels-Alder adduct via an a-sulphonyl carbanion before elimination. 

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