Sulfur-Wittig reaction

A computational study has examined the effects of substituents on the sulfur-Wittig reaction of H2S=CH2 with a range of formyl (0=CH-R) substrates R = H, F, Cl, Me, OMe, NMe2 and... 

The Peterson reaction has two more advantages over the Wittig reaction 1. it is sometimes less vulnerable to sterical hindrance, and 2. groups, which are susceptible to nucleophilic substitution, are not attacked by silylated carbanions. The introduction of a methylene group into a sterically hindered ketone (R.K. Boeckman, Jr., 1973) and the syntheses of olefins with sulfur, selenium, silicon, or tin substituents (D. Seebach, 1973 B.T. Grdbel, 1974, 1977) illustrate useful applications. The reaction is, however, more limited and time consuming than the Wittig reaction, since metallated silicon derivatives are difficult to synthesize and their reactions are rarely stereoselective (T.H. Chan, 1974 ... 

Similar to phosphur ylides, sulfur ylides 1 and 2 possess the nucleophilic site at the carbon atom and the pendant leaving group at the heteroatom (sulfur). Different from the Wittig reaction, the Corey-Chaykovsky reaction does not lead to olefins. 

Reaction of the tri- -butylphosphine adduct 180 with compound 157 leads to cycloaddition across the two sulfur atoms to give corresponding 2-tri- -butylphosphoranylidene-l,3-dithiole 181, which can be trapped by in situ Wittig reaction with benzaldehyde to give 182 (Scheme 21) <1994CC2603, 1997T2261>. 

Ylides of other elements have been used much less commonly than sulfur ylides in cyclopropanations. Rather, other ylides are better known for their uses in other types of reactions, the best example being the use of phosphonium ylides in the Wittig reaction with carbonyl compounds to give alkenes. Nonetheless, some cases of cyclopropanations have been reported with phosphonium ylides and the related arsenic derivatives. Examples are given in Table 9. 

But how are we to make the epoxide 61 The obvious route is by epoxidation of the alkene 63. The alkene 63 could be made by a Wittig reaction (chapter 15) on the ketone 64 or directly by sulfur ylid chemistry (chapter 30). 

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. 

The mechanism of the Wittig reaction of phosphoranes with carbonyl compounds has fascinated chemists and remains a matter of discussion. Though the sulfur equivalent is very much less used, this reaction is known... 

Sulfonium salts react in several ways. They may behave as a leaving group, undergoing substitution by a nucleophile or fragmenting with the formation of an alkene. However, the most important reaction of sulfonium salts involves the formation of an ylide in the presence of a base. The carbanion of this sulfur ylide is stabilized by the adjacent positively charged sulfonium ion. The reaction of the carbanion with a carbonyl group parallels that of a phosphonium ylide in the Wittig reaction. However, the decomposition of the intermediate dipolar species is different and leads to the formation of an epoxide (oxirane) rather than an alkene. 

Quinazoline-4(3// )-thione condenses with ethyl bromocyanoacetate " and ethyl 2-chloro-3-oxobutanoate in the presence of sodium ethoxide in ethanol with sulfur extrusion to form ethyl 2-cyano-2-(quinazolin-4-ylidene)acetate (5, R = CN) and ethyl 2-(quinazolin-4-ylidene)-acetate (5, R = H), respectively. It has been suggested that the initial adduct of quinazoline-4(3//)-thione and a methylene compound is deprotonated to give the corresponding ylide which undergoes electrocyclic closure to a thiirane derivative and then desulfurization. A C - C double bond is formed between the methylene carbon atom and the electrophilic carbon C4, providing an alternative to the Wittig reaction. 

Sulfur Ylide Epoxidations/Wittig Reactions on Epoxides... 

Hurd, C. D. Wittig reaction. Quarterly Reports on Sulfur Chemistry 969, 4,159-227. 

Schlosser, M., Oi, R., Schaub, B. The Wittig reaction 30 years later. Phosphorus and Sulfur and the Related Elements 1983, 18, 171-174. 

Maryanoff, B. E., Reitz, A. B. Delving into the Wittig reaction stereochemistry and mechanism. Stereochemical idiosyncrasies and mechanistic implications. Phosphorus and Sulfur and the Related Elements 1986, 27,167-189. 

McKenna, E. G., Walker, B. J. The mechanism and stereochemistry of Wittig reactions of phosphonium ylide-anions. Phosphorus, Sulfur Silicon Relat. Elem. 1990, 49-50, 445-448. 

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