Alkyl sulfonates alkene preparation from

Trifluoromethanesulfonates of alkyl and allylic alcohols can be prepared by reaction with trifluoromethanesulfonic anhydride in halogenated solvents in the presence of pyridine.3 Since the preparation of sulfonate esters does not disturb the C—O bond, problems of rearrangement or racemization do not arise in the ester formation step. However, sensitive sulfonate esters, such as allylic systems, may be subject to reversible ionization reactions, so appropriate precautions must be taken to ensure structural and stereochemical integrity. Tertiary alkyl sulfonates are neither as easily prepared nor as stable as those from primary and secondary alcohols. Under the standard preparative conditions, tertiary alcohols are likely to be converted to the corresponding alkene. 

Literature procedures for the synthesis of hydroperoxides include the preparation from hydrogen peroxide (via reaction with alkyl halides, -phosphites, -sulfonates, alkenes,... 

Alkenes are prepared from alcohols either by direct dehydration or by de-hydrohalogenation of intermediate alkyl halides to avoid rearrangement we often select dehydrohalogenation of halides even though this route involves an extra step. (Or, sometimes better, we use elimination from alkyl sulfonates.)... 

In both SnI and Sn2 reactions, the leaving group is the halogen of an alkyl halide or the sulfonate group of a sulfonate ester. Both alkyl halides and sulfonate esters are prepared from alcohols. In Chapter 10, alcohols were prepared by the hydration reaction of alkenes, by oxymercuration-demercuration of alk-enes, or by hydroboration of alkenes. Other methods can be used to prepare alcohols, and they will be discussed at a later time. This section will describe several of the reactions used to convert alcohols to halides or sulfonate esters. 

PREPARATION OF ALKENES FROM HALOALKANES AND ALKYL SULFONATES BIMOLECULAR ELIMINATION REVISITED... 

Trialkylboranes react rapidly and in high yields with a-halo ketones,a-halo esters, a-halo nitriles, and a-halo sulfonyl derivatives (sulfones, sulfonic esters, sulfonamides) in the presence of a base to give, respectively, alkylated ketones, esters, nitriles, and sulfonyl derivatives. Potassium tert-butoxide is often a suitable base, but potassium 2,6-di-tert-butylphenoxide at 0°C in THF gives better results in most cases, possibly because the large bulk of the two tert-buXy groups prevents the base from coordinating with the R3B. The trialkylboranes are prepared by treatment of 3 mol of an alkene with 1 mol of BH3 (15-16). With appropriate boranes, the R group transferred to a-halo ketones, nitriles, and esters can be vinylic, or (for a-halo ketones and esters) aryl. " °... 

A number of methods for the preparation of vinyl and allyl sulfones are available [1U9, 110], and the syntheses of vinyl sulfones from alkenes has been reviewed [116]. A simple one-step procedure of wide applicability makes use of a palladium-catalysed cross-coupling reaction between aryl and alkyl sulfonyl chlorides and substituted vinyl and allyl stannanes... 

The same authors also report on the reaction of alkenes with chlorosulfuric acid in the absence of dioxane <1999EJ091 >. Thus, treatment of various alkenes at —40°C with chlorosulfuric acid results in the formation of, r-alkyl hydrogen sulfates (Scheme 16). From the results presented it appears that the rcr-alkyl hydrogen sulfates are transformed into a mixture of internal trans- and m-/3-sultones upon warming to 0 °C. Subsequent sulfonation of these alkenes by S03 then yields the corresponding /3-sultones. Again isolation yields were not reported but the reactions appear to proceed in a quantitative manner. The same paper also describes the use of acetyl sulfate and trifluoroacetyl sulfate for the preparation of /3-sultones, but mixtures appear to result. 

The Julia olefination reaction is highly regioselective and ( )-stereoselective, providing a valuable alternative to the Schlosser reaction for making rrans -disubstituted olefins. The reaction involves condensation of a metalated alkyl phenyl sulfone with an aldehyde to yield a P-hydroxysulfone, which is then subjected to a reductive elimination to produce the alkene. There are limitations to the preparation of tri- and tetra-substituted alkenes via the sulfone route because the P-alkoxy sulfones derived from addition of the sulfone anion to ketones may be difficult to trap and isolate or they may revert back to their ketone and sulfone precursors. 

Alkyl phenyl sulfones (piCj,27) are nearly as acidic as esters hence they are readily deprotonated by n-BuLi, LDA in THF, or EtMgBr in THF to give a-metalated sulfones. Their reaction with aldehydes gives a mixture of diastereomeric P-phenylsul-fone alkoxide adducts. Reductive elimination of the benzenesulfinate moiety from the adduct to produce the alkene is usually slow. To minimize side reactions, the hydroxyl group is first converted to an acetate, benzoate, mesylate, or /7-toluenesul-fonate and then treated with an excess of sodium amalgam [Na(Hg), prepared by adding small pieces of sodium to mercury] in methanol to furnish the trans-dlk n P... 

Desulfonylation. The facile detachment of the arenesulfonyl moiety from allylic benzothiazol-2-yl sulfones with the use of NaBHjCN is crucial to the preparation of alkenes by chain homologation based on alkylation of benzothiazol-2-yl methallyl sulfone.-Transposition of the double bond occurs during the desulfonylation. 

---