Epoxidation With sulfonium ylids

So what has this got to do with cyclopropanes If sulfur ylids react with enones either the epoxide 74 or the cyclopropane 76 may be formed.19 The general rule is that sulfonium ylids from 69 give epoxides but sulfoxonium ylids give cyclopropanes 76. 

We could compare sulfonium ylids with the carbenoids we discussed in Chapter 40—both are nucleophilic carbon atoms carrying a leaving group, and both form three-membered rings by insertion into n bonds. Sulfonium ylids are therefore useful for making epoxides from aldehydes or ketones other ways you have met of making epoxides (Chapters 20 and 45) started with alkenes that might be made with phosphorus ylids. 

There have been several linear (as opposed to convergent) syntheses of racemic LTA4 methyl ester. All employ a sulfonium ylid at or near the last step to generate the epoxide by condensation with an aldehyde. It is conceivable that an optically active ylid would induce optical activity in the epoxide-forming... 

Sulfonium ylids are therefore useful for making epoxides from aldehydes or ketones other ways you have met of making epoxides (Chapter 19) started with alkenes that might themselves be made with phosphorus ylids. 

Conversion of aldehydes to terminal epoxides has been achieved with 95% ee, through methylene transfer from a chiral sulfonium ylid. ° ... 

The simplest sulfur ylids are formed from sulfonium salts 69 by deprotonation in base. These ylids react with carbonyl compounds to give epoxides.18 Nucleophilic attack on the carbonyl group 70 is followed by elimination 71 of dimethylsulfide 72 and formation of the epoxide 73. You should compare diagram 71 with diagram 23 in chapter 15. The phosphonium ylid reacted by formation of a P-0 bond and an alkene in the Wittig reaction. The sulfonium compound reacts by formation of a C-O bond 71 as the S-O bond is much weaker than the P-0 bond. The sulfonium salt 69 can be reformed by reaction of 72 with Mel. 

Sulfoxonium ylids react with unsaturated carbonyl compounds in the same way as the stabilized ylids that you have met already do—they form cyclopropanes rather than epoxides. The example below shows one consequence of this reactivity pattern-—by changing from a sulfonium to a sulfoxonium ylid, high yields of either eposjde or cyclopropane can be formed from an unsaturated carbonyl compound (this one is the terpene carvone). 

The alkylation of the sulfide 96, the formation of the ylid 99, and the reaction with the aldehyde are all carried out in one operation. The sulfide is a good nucleophile for alkyl halides and forms the sulfonium salt 98. This gives the ylid 99 with NaOH as a convenient base in aqueous tert-butanol. The ylid selectively attacks the aldehyde to give the betaine 100 that closes to the epoxide and releases the sulfide 96 for the next round. 

It is known that dimethyl sulfide (MeSMe) reacts with 1-bromobutane to form a sulfonium salt. Draw it. It is also known that treatment of this salt with butyllithium leads to an ylid (draw it) however, when this yhd reacts with acetone, the product is an epoxide (l-butyl-2,2-dimethyloxirane) rather than an alkene. The coproduct of this reaction is dimethyl sulfide. Suggest a mechanism that accounts for formation of this epoxide and MegS from the yhd. 

Phase transfer catalysis, which proved extremely useful in classical ylid reactions with both phosphonium and sulfonium salts (Ref, 8, 42-45), was first used with a polymer by Farrall, Durst and Frechet in 1978 (Ref, 15) according to scheme 4. In this reaction, the polymeric sulfonium salt (IX), which is suspended in a dichloromethane solution of the carbonyl compound, is treated with aqueous sodium hydroxide in the presence of tetrabutyl ammonium hydroxide to give over 95% yield of the desired epoxide together with a polymeric by-product (X) which can be recycled and reused repeatedly without any loss of activity. In contrast, the same polymeric reagent (IX) used under classical conditions affords lower yields of epoxides and loses its activity rapidly on repeated recycling. This last observation shows clearly that phase transfer catalysis may contribute significantly to the prevention of side reactions in some modifications of polymers. 

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