Silyl sulfonates

Silicon presents an attractive option among eledrophilic activating and dehydrating agents of hemiacetals because of the wide commercial availability of eledrophilic silicon sources. The two main classes of silicon electrophiles used, namely silyl halides and silyl sulfonates, have been demonstrated to promote a variety of glycosylations including some examples of oligosaccharide synthesis. 

Tetrahydropyranilation of alcohols is an important process for the protection of hydroxyl groups. For this purpose pyridinium p-toluenesulfonate and bis(trimethyl-silyl) sulfonate have been frequently employed as mild catalysts. The apphcation of EGA catalysis is an effective alternative to protection and deprotection of hydroxyl groups The combinations, CH2CI2—LiClO4—Et4NC104 and MeOH—LiClO4 are applicable for the protection and deprotection step, respectively (34 35). 

Some [hydroxy(tosyloxy)iodo]perfluoroalkanes, RFI(OH)OTs, were prepared from RfI(02CCF3)2 by reaction with either a sulfonic acid, e.g.p-TsOH, or a silyl sulfonate, e.g. Me3SiOTf [30,31]. Several organic acids derived from phosphorus... 

Another type of reactivity is due to the enhanced nucleophilicity of the hydroxyl group of o-iodosobenzoic acid. For example, o-iodosobenzoic acid reacts with silyl triflate or other silyl sulfonates to afford O-substituted sulfonates [56]. 

Terminal aikenes. /8-Silyl sulfones on treatment with fluoride ion undergo elimination to the corresponding alkene. This reaction, coupled with a -alkylation of sulfones, is useful for synthesis of terminal olefins and 1,3-dienes. 

At the end of the chapter we briefly consider some reactions such as the fluoride-induced elimination of 3-silyl sulfones, which are not reductive eliminations but which have been included because of their relation to the Julia alkenation. 

The fluoride-induced elimination of P-trimethylsilyl sulfones to give terminal alkenes was first reported in 1979. The p-silyl sulfone (120 Scheme 42), prepared by the alkylation of the readily available crystalline sulfone (119), undergoes elimination on warming with TBAF in THF to give 1-decene in 80% yield. A second alkylation-elimination sequence can then be performed to give a 1,1-dialkyle-thylene derivative (121). Thus, the sulfone (119) may serve as a synthon for CH2=CH" or in... 

A major detraction to the p-silyl sulfone route to alkenes is its narrow scope, owing to the paucity of methods for preparing the requisite p-silyl sulfones. The method has so far been largely restricted to monosubstituted and 1,1-disubstituted ethylene derivatives because the P-silyl sulfones have usually been most conveniently and economically prepared by the alkylation of a sulfone anion with iodomethyl-trimethylsilane. Unfortunately higher haloalkylsilanes are difficult to prepare and their alkylation chemistry remains unexplored. 

Sammes and cowoikers devised a promising solution to the synthesis of substituted P-silyl sulfones, which has been exploited in a new approach to the antibiotic bicyclomycin (Scheme 44). In their approach the P-silyl sulfone (125) was prepared by conjugate addition of the lithium enolate of the mono-imino ether derivative (123) of a dioxopiperazine to the unsaturated P-silyl sulfone (124). After oxidative cyclization to the bicyclic system (126), the latent alkene functionality was unleashed in 89% yield on treatment with TBAF in THF at room temperature to give the basic skeleton of bicyclomycin. 

Nonfunctionalized alkenes such as terminal alkenes are not available on similar treatment of, for example, 2-sulfonylalkanes with bases. However, these have been obtained on alkylation of a-metallo sulfones with trimethylsilylmethyl iodide and subsequent reaction of the resulting -silyl sulfone with... 

At lower temperature, treatment of this silylated sulfone epoxide 6 with 0.8 equivalents of potassium mrr-butoxide for 4 hours gave a new product, (la,7a,8a)-l-trimethylsilyl-2-thiabicyclo[6.1.0]nonan-7-ol 2,2-dioxide (9), together with unrearranged sulfone epoxide 10 arising from protodesilylation, and the desilylated bicyclic hydroxy sulfone 7 in the ratio 78 16 6. 

Cyclopentene derivatives with unsaturated appendages are also available via this general annulation strategy representative examples are presented in Scheme 15. In these several transformations desulfonylation is achieved using base-induced -elimination, via the fluoride-promoted elimination of 3-silyl sulfones, and by means of Brook rearrangement chemistry according to the method of Reich. In summary, a wide variety of complex functionalized cyclopentenes are available using this versatile [4 1]... 

Alkyl radicals have been used to abstract hydrogen atom from C-H bonds at secondary carbon centers. For instance, Fuchs has developed a self-immolative elimination of aryl sulfones [84]. The or/Ao-silylated sulfone gives upon treatment with tin hydride and AIBN a primary alkyl radical that abstracts a hydrogen atom in a... 

Acetylene equivalent. As a dienophile for the Diels-Alder reaction, it incorporates into the adducts two hetero substituents on trigonal carbons. After conjugate reduction with Li AIH4 the )8-silyl sulfone moiety is susceptible to fluoride ion-initiated fragmentation. 

AJkenylsilanes and alkenyl sulfones. 1,1-Disilylepoxides undergo an unusual rearrangement, whereas p-silyl sulfones lose the sulfonyl group on treatment with LDA to give alkenylsilanes. - ... 

Alternative Desulfonylation/C-C Bond-formiiig Procedures. The foregoing discussion has focussed on reductive methods for removing sulfones and, in the case of the Juba alkenation, desulfonylation is accompanied by the formation of a new C-C bond. Another method which accomphshes desulfonylation with the concomitant construction of a C=C bond entails fluoride-induced elimination of -silyl sulfones. 

Alternative Routes to Substituted (2-Phenylsulfonylethyl)-trimethylsilanes. Although the fluoride-induced elimination is a reliable and efficient method for synthesizing alkenes, the requisite silyl sulfone precursors are often better prepared by methods which avoid the use of (2). For example, eq 3 illustrates an alternative synthesis of 2,2-dialkyl-2-(phenylsulfonylethyl)trimethyl-silanes involving alkylation of a lithio sulfone with (iodomethyl)-trimethylsilane. ... 

Figure 2.7 shows the different steps of the synthesis procedure. The commercial polysulfone Udel (Amoco) was dissolved in chloroform at a temperature of 25 °C and subsequently treated with trimethylsilyl chlorosulfonate (Aldrich) to produce a silyl sulfonate polysulfone. The amount of the intermediate product was cmitrolled by the molar ratio of the sulfonating agent to the polymer repeating unit and varied from 2 to 2.3. The reaction time was 24 h at a temperature of 25 0.1 °C. The obtained silyl sulfonate polysulfone polymer was then treated with a 30 wt% sodium methoxide/ methanol solution (2.26 mmol g of polysulfone) at 25 °C for 1 h, which was used to cleave the sUyl sulfraiate moieties, yielding the sodium... 

The organolithium reagent (1) also reacts with a wide variety of other electrophiles, including silyl chlorides to provide bis(silyl)methane derivatives, and nitriles to provide -sUyl amines after in situ reduction of the intermediate imine derivative. a-Silyl epoxides are opened to provide the substituted vinylsilane. Reaction of (1) with arenesulfonyl fluorides provides a-silyl sulfones, key intermediates for the preparation of vinyl sulfones. Reaction of the lithium reagent (1) with alu-... 

The synthesis of a-fluorovinylsulfones by the use of the Peterson olefination has been described by Usuki et al. who reacted silyl lithiums 86 generated in situ from the corresponding ot-fluoro-a-silyl sulfones 85 and TBSCl, with carbonyl compounds 87. They found that the reaction proceeded in better yield when an aldehyde was used rather than a ketone. The phenylsulfonyl moiety could be reductively removed using a sodium amalgam in NaH2P04-NaHP04. 

Sulfonates s. Silyl sulfonates. Sulfonic acid esters a-Sulfonation of carboxylic acid esters 23, 577 N-Sulfonation... 

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