Sulfonium salts reactions with alkenes

The reaction of ylides with alkenes proceeds in two steps according to the MIRC reaction mechanism. Dimethylsulfonium trimethylsilylmethylide (2), generated in situ from the corresponding sulfonium salt 1 with ec-butyllithium, reacts with various a,/l-unsaturated ketones to cyclopropanate the activated C-C double bond (see Table 2). The resulting l-acyl-2-trimethylsilylcyclopropanes 3 are isolated as mixtures of diastereomers. 

As Ph2S is a better leaving group that Me2S, the diphenylsulfonium salt is expected to be more reactive. Tlie same trend was observed in the just described copper-catalysed reaction of sulfonium ylides with alkenes (see Section 4.4.1). 

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. 

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. 

The reaction of LVII" and some related heterocycles with a large number of nucleophiles has been investigated thoroughly. Among the nucleophiles, which with few exceptions form sulfonium salts by their reaction with LVII", are water [194, 195], carboxylate ions [196, 197], pyridine [198], organometallic compounds like dialkylmercury and dialkylzinc [199], and Grignard reagents [200], alkenes and alkynes [201], and various aromatic compounds [202-204]. 

Sulfones and sulfoxides containing at least one b-hydrogen atom undergo a bimolecular elimination reaction (E2) an treatment with alkoxides, leading preferentially to the formation of the least substituted alkene (Hofmann s rule) (Scheme 37). The reaction resembles the analogous eliminations of quaternary ammonium and sulfonium salts (see Chapter 6, p. 83). 

Warming the sulfonium salt (5) may cause elimination of a p-hydrogen to yield a mixture of the sulfide and the alkene, i.e. route (iv) in Scheme 3. On the other hand, proton abstraction from (5) by treatment with a strong base gives the sulfur ylide (6) which can undergo molecular rearrangement to form the sulfide (7). Sulfonium salts are unstable in basic media as a result of their tendency to undergo nucleophilic substitution reactions. 

Chiral epoxides frequently play a key role as intermediates in organic synthesis and the development of methods for the catalytic asymmetric synthesis of such compounds therefore remains an area of intensive research. Methods have focused principally on the asymmetric electrophilic oxidation of alkenes and good enantioselectivity has been achieved [1]. An alternative to oxidative processes for the synthesis of epoxides is the reaction of sulfur ylides with aldehydes and ketones [2,3,4,5,6]. Sulfur ylide epoxidation is a carbon-carbon bond forming reaction and is complementary to oxidative methods. The standard conditions for this reaction utilize the original Corey method treatment of a sulfonium salt with a strong base in the presence of or followed by the addition of an aldehyde... 

Unlike the corresponding phosphonium salts, addition of sulfonium salts to aldehydes results, not in the alkene products, but in the formation of epoxides (see Section 1.1.5.2). However, sulfones can be used to prepare alkenes, by way of the a-metallo derivatives, in what is termed the Julia olefination (alkenylation). Addition of the organometallic species to an aldehyde or ketone gives a p-hydroxy sulfone which, in the form of its 0-acyl or 0-sulfonyl derivative, undergoes reductive cleavage with, for example, sodium amalgam in methanol to form the alkene. The reaction is regioselective and can be used to prepare mono-, di- and trisubstituted alkenes (2.91). 

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. 

Other Nucleophiles. DMTSF and Et3N-3HF react with alkenes to yield products formally of a MeSF addition. Yields are in excess of 80% and the reaction proceeds rapidly. " Homoal-lylic sulfides cychzed in the presence of DMTSF. The resulting sulfonium salt was successfully demethylated with Mc3N to yield 3-methylthiotetrahydrothiophenes. ... 

The camphor-derived P-hydroxyl-sulfonium yhde proved to be unusually efficient for the cyclopropanation of electron-deficient alkenes. For example, the reaction of salt 21b with a,P-unsaturated nitrile affords the desired cyclopropane in 61% yield and 96% ee. In comparison, the corresponding tetrahydrothiophene and dimethyl sulfide-derived sulfonium salts (23 and 24), which lack the hydroxyl group, only gave a trace amount of the desired cyclopropane (Scheme 20.21). 

The less polar nature of disulfides and sulfenamides makes the addition of these reagents to alkenes inefficient. Although thiolates can be generated under reducing conditions,the activation of disulfides and sulfenamides is routinely performed by Lewis acids to enable the introduction of new u bonds other than C—S bonds. As a consequence, the stereospecificity and reactivity observed in the Lewis-acid-promoted addition of disulfides and sulfenamides to alkenes is similar to that of sulfenylation by sulfenyl halides and thi-osulfonium salts. Likewise, the accepted reaction mechanism includes the coordination of Lewis acid, resulting in a thio-sulfonium-like species that can further react with alkene either directly or indirectly by way of dithiosuhonium ions. 

The other set of reactions which allow the transformation of sulfides and selenides to alkenes require the intermediary formation of a sulfonium or selenonium salt, and their treatment with a base such as KOH or Furthermore, conjugated dienes, trienes and polyenes have been pro-... 

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