Ylids, sulfur formation

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. 

An original observation was made with the sterically hindered 2,2,4,4-tetramethyl-3-thioxocyclobutanone and arylazides [254]. Dispiro-l,3,4-di-thiazoles were obtained in 67-83% yields. Their formation was explained by a 1,3-dipolar cycloaddition, elimination of N2 and formation of a novel thiocarbonyl ylid, which underwent dipolar cycloaddition with cyclobuta-nethione. The other outcome possible for the ylid was cyclisation to a thia-ziridine and sulfur extrusion to produce the imine, as previously reported. 

Treatment of a sulfoxide, particularly one with an anion-stabilizing substituent to help ylid formation, produces cations reactive enough to combine with nucleophiles of all sorts, even aromatic rings. The product is the result of electrophilic aromatic substitution (Chapter 22) and, after the sulfur has been removed with Raney nickel, is revealed as a ketone that could not be made without sul-... 

A Lewis acid (SnCl4) is used to remove the oxygen from the sulfoxide and the ketone assists ylid formation. The sulfur atom stabilizes the cation enough to counteract the destabilization by the ketone. The Lewis acid is necessary to make sure that no nucleophile competes with benzene. 

These two reports clearly demonstrate that thiophene can form ylids, despite the considerably reduced nucleophilicity of the sulfur atom and that stable thiophenium ylids might be found. This was subsequently confirmed with the observation that the photolysis of dimethyl diazomalonate in thiophene result in the formation of 15, which was isolated in low yield as a stable crystalline solid (77JOC3365). It was subsequently demonstrated by my own group that thiophenium bis(alkoxycarbonyl)methylides could be formed in high yield using rhodium(II) acetate-catalyzed addition of diazomalonate... 

There can now be little doubt that the reaction of singlet carbenes and car-benoids with thiophene proceeds by attack of the carbene at the ring sulfur atom to generate the S,C-ylid. However, only in the special cases indicated above do these ylids enjoy any real stability. In the majority of cases, other products usually result from rearrangement of the intermediate ylids and the nature of the products formed is remarkably sensitive to both steric and electronic effects. Broadly speaking, six major reaction pathways have been observed (1) 2-substituted thiophene formation, (2) 2H-thiopyran formation, (3) formation of derivatives of 2-thiabicyclo[3.1.0]hex-3-ene, (4) 3-substituted thiophene formation, (5) oxathiocin formation, and (6) carbenic fragmentation. 

The generally accepted mechanism for the reaction between sulfur ylids and aldehydes or ketone is formation of 76, with displacement of the Me2S leaving... 

Reaction of the naphthothiin 229 with benzyne produces acenaphthylene in high yield via formation and Stevens rearrangement of the sulfur ylid 230 (Scheme 22).115... 

However, unlike the triaryIsulfonium salts, these compounds undergo reversible photoinduced ylid formation rather than homolytic carbon-sulfur bond cleavage. Because the rate of the thermal back reaction is appreciable at room temperature, only those monomers that are more nucleophilic than the ylid will polymerize. Epoxides, vinyl ethers, and cyclic acetals undergo facile cationic polymerization when irradiated in the presence of dialkylphenacylsulfonium salts as photoinitiators. 

Some 2,3-epoxyamides of aldonic acids have been prepared by reaction of acyclic aldose derivatives with sulfur ylids (Scheme 1). 5y -epoxidation of some racemic 2-benzyloxy-4-alkenamides by way of iodohydrin formation and subsequent base treatment has afforded epoxides such as 2. Epoxidation of hex-2-enopyranosyl phosphonates has been effected with H202/sodium tungstate. ... 

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