Sulphonium salts rearrangement

Sulphonium salts, rearrangement of 561-566 allyl 562-564 non-allyl 564-566 Sulphonium ylid, intermediate in... 

Irradiation of sulphonium salts (308) in acetonitrile solution has been shown to lead to lateral-nuclear rearrangement and the gg formation of l-(methylthio)-2-substituted alkylnaphthalenes (309). The quantum yield of product formation varies between 0.24 and 0.1 but bond cleavage to yield 1-naphthyl methyl sulphide also occurs and MeCONH-CHg-R is a by-product of the migrating group and the solvent. The steric crowding in r.tris(trimethylsilyl )methyl] benzene (310) is considered to be the origin of its photolability and on irradiation it efficiently yields (311), (312), and five... 

The Stevens rearrangement of sulphonium salts is known to proceed through the intermediacy of the corresponding sulphonium ylid . There appears to be two distinct mechanistic pathways, depending upon the structure of the ylid, leading to rearranged product. Rearrangement of... 

A second type of rearrangement involves ylids derived from non-allyl sulphonium salts. Baldwin and coworkers have reported that rearrangement of the sulphonium ylid 115 in toluene at reflux temperatures occurs by a radical pair mechanism (equation 94), in which the benzyl group migrates with predominant retention of configuration to yield 116. 

Thompson and Stevens , in their first paper on the rearrangement of sulphonium salts, reported obtaining the sulphide 116 upon treatment of 117 with sodium methoxide. However, more recent work has shown ... 

The Stevens rearrangement of a sulphonium salt involves treatment of the salt with base and leads to migration of a group from sulphur to an adjacent carbon atom (equation 91). Analogous Stevens rearrangement of ammonium salts and the related Wittig rearrangement of ethers are well known. 

The sulphonium salts used in the Stevens rearrangement need not be prepared initially from a thiol however, this is feasible and is often the case. This method, therefore, allows the conversion of a thiol to a sulphonium salt, followed by rearrangement with concomitant carbon—carbon bond formation. Removal of the sulphur moiety following rearrangement permits, in efiect, a thiol to function in a reaction that leads to bond formation between two R groups that originally were attached to sulphur (equation 92). 

Bu OK in THF. Further studies of the formation of sulphides from rearrangements of sulphonium salts provide more details of the repression of the rearrangement by adjacent substituents capable of causing delocalization of charge from the intermediate ylide. 

The thio-Claisen rearrangements of allylic phenyl sulphides, allyl thienyl sulphides, addition products of acetylene derivatives and allyl mercaptan, and dimethylallyl 2-indolyl sulphonium salts have been described. Of particular interest is a recent paper reporting the quantitative formation of (62) by the thermal rearrangement of the sulphoxide (61). The second step in the mechanism (Scheme 3) proposed for this reaction... 

Several groups earlier had demonstrated that even stabilized sulphonium ylides, e.g. phenacylides, would undergo 1,5-rearrangement. Baldwin and Erickson found that even the aromatic sulphonium salt (21), when treated with butyl-lithium at — 40 °C, would undergo 1,5-rearrangement leading to destruction of the aromatic system. 

The formation of S-halogeno-sulphonium salts from the halogen and a sulphide is in competition with an a-halogenation-oxidation reaction which leads to sulphonyl chlorides when water is present. The initial attack at sulphur may be followed by a Pummerer rearrangement to the adjacent carbon atom, and in the case of unsymmetrical sulphides the regioselectivity is determined by the relative electron density at the a-carbon atoms. ... 

The other main direction of investigation involves the treatment of succinimido- or hydantoinyl-sulphonium salts [e.g. (50) and (51)] with tertiary amines, which affords interesting [2,31- or U,3]-sigmatropically rearranged products e.g. (52)]. The amino-sulphonium salt (51), upon treatment with a carboxylic acid, forms the corresponding carboxy-sulphonium salt as an intermediate, which then reacts with amines to form the acid amides.The synthesis of 9-alkyl- or of 9-phenyl-thioxanthene-iV-(toluene-/7-sulphonyl)sulphimides (53) has been reported. The configuration of (53) has been determined by H n.m.r. 

Quaternary thiazolium salts that possess a 2-chloroethyl group, e.g. (76), are rapidly rearranged by aqueous base to thietan derivatives, e.g. (77), presumably via an intermediate bicyclic sulphonium salt. ... 

The standard method for the conversion of a-thio-esters into a,/3-unsaturated esters involves oxidation to the corresponding sulphoxide followed by pyrolysis. An alternative approach, which avoids the use of an oxidant, is by formation of a sulphonium salt using CFjSOaCHzSiMea followed by ylide formation using caesium fluoride and rearrangement (Scheme 44). This method can run into difficulties if the starting thio-ester already contains an olefinic bond. 

Propargylic sulphonium salts (239) on treatment with base rearrange to allenic thioethers (240) a competitive reaction is isomerization of the acetylene to the allene sulphonium salt (241) followed by an analogous sigmatropic rearrangement to the acetylenic thioether (242). There is no isomerization of (240) to (242) under the reaction conditions. ... 

Vilsmaier has reported that succinimido-sulphonium salts may undergo [1,2]- and [2,3]-sigmatropic rearrangement, giving JV-methylthiomethyl succinimide and the appropriate methylthiomethyl ether, respectively. 

Charged dithiiran intermediates have been suggested to be involved in the rearrangement of 6-phenylthioacetamidopenicillin sulphoxide j-nitrobenzyl ester, " the anodic oxidation of em-disulphides (thioketals), " and in the decomposition of a sulphonium salt containing a (phenylthio)methyl substituent. ... 

The Diels-Alder-type cycloaddition between 1,3-dithienium fluoroborate (106), obtained from 1,3-dithian and trityl fluoroborate, and 1,3-dienes formed the basis of a new and efficient synthesis of A -cyclopentenones. Simple 1,3-dienes such as butadiene, isoprene, or 2,3-dimethylbutadiene reacted rapidly with (lOQ to give (107) (107b) was obtained stereospecifi-cally from (106) and isoprene in 95% yield. Treatment of the sulphonium salts (107) with butyl-lithium at -78 °C gave an ylide which rearranged on warming to the cyclopropyl-dithian (108). The latt , v/hea heated further to 200 C, underwent the well-known vinylcyclopropane to cyclopentene transformation. Hydrolysis of the dithian thus obtained furnished cyclopen-tenones. The overall yield of 3-methylcyclopent-3-enone from (106) and isoprene was greater than 83%. 

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