1,2-Dithiolium salts, 3-methylthio

An important route to 1,4,2-dithiazines (136) involves the ring expansion of 1,3-dithiolium salts (76JPR127). Thus reaction of 2-methylthio-l,3-dithiolium salts with azide ion gives an unstable intermediate which rapidly loses nitrogen to give 1,4,2-dithiazines (Scheme 33). It is not known whether the reaction involves a nitrene intermediate or whether the [1,2] shift is concerted with loss of nitrogen. The latter possibility seems more likely. 

Cleavage of the S-S bond in4-aryl-3,5-bis(methylthio)-l,2-dithiolium salts occurs on treatment with a cyclopentadienide species which results in the formation of an ot,(3-unsaturated dithioester 518. Deprotonation by excess reagent initiates an intramolecular cyclization which yields a 5/7-cyclopenta[A]thiopyran-4-thione (Scheme 202) <1996LA109>. 

Carbanions 128, generated from the corresponding Fischer carbene complexes with -butyllithium, reacted with 2-methylthio-l,3-dithiolium salts 123 to give heterocyclic organometallic carbenes 129. Reactions of allylic carbanions 130 afforded a mixture of mono- and diheterocyclic condensation products 131 and 132, respectively (Scheme 11) <2004TL7843>. 

The 1,3-dithiole derivative 141 was prepared in the reaction of 2-methylthio-l,3-dithiolium salt 140 with phenylma-lononitrile. Similarily, the reaction of 140 with the corresponding l,3-dimethyl-5-phenylbarbituric acid or 1,3-diethyl-5-phenylthiobarbituric acid gave quinonoid derivatives 142 and 143, respectively (Equations 3 and 4) <2005JA8835>. 

The condensation of 2-methylthio-l,3-dithiolium salts 144 (2equiv) with the thienoquinoxaline derivative 145 in the presence of potassium ft-ry-butoxide as a base gave rt-extended dihydrothienoquinoxaline-TTF derivatives 146 (Equation 5) <2005JOC768>. 

The l,3-dithiol-2-ylidene derivatives 162 containing an azine spacer were prepared via reaction of 2-methylthio-l,3-dithiolium salts 138 or 160 with a hydrazone 161 in the presence of pyridine (MeOH, 0 °C to rt) (Equation 6) <2001JMC374>. 

Methylthio-l,2-dithiolium salts are useful intermediates for synthesis and are generally obtained by reaction of methyl iodide or of dimethyl... 

The ability of 1,3-dithiolium salts to add hydride ions has been widely studied. 2-Unsubstituted salts as well as 2-alkyl or 2-aryl, 2-amino, and 2-methylthio derivatives are easily reduced to give 1,3-dithioles in excellent yields (Eq. 38). Reducing agents are sodium hydrosulfide in ethanoF sodium borohydride in methanol, ethanol, or tetrahydrofuran - lithium aluminium hydride or deuter-ide in ether or sodium borodeuteride in acetonitrile. ... 

Reactions of 2-methylthio-1,3-dithiolium salts with active methylene compounds have been extensively reported and reviewed. They are performed in refluxing ethanol, acetic acid, or acetonitrile, pyridine being used as catalyst. Reagents frequently used include ethyl cyanoacetate, acetyl-... 

The reduction of 1,2-dithiolium salt 158 with sodium borohydride followed by oxidation yields the 4,5-bis(methylthio) derivative 159 in 95% yield (Scheme 34)... 

Vilsmeier-Haack formylation of 2,5-diaryl-6a-thiathiophthens leads to the 3-formyl compounds, which are also obtainable from 3-aryl-l,2-dithiolium salts by treatment with triethylamine, and details have been given for the bromination of 2,5-disubstituted thiathiophthens. Nitration of 2-methylthio-5-phenyl-6a-thiathiophthen gives the 3-nitro-compound in poor yield, and attempted nitrosation experiments lead, in several examples, to rearranged structures of type (19), although the first stage of these reactions is presumed to be electrophilic attack on the 3-position. 

Various routes to 1,2-dithiolylidene ketones (45) have been described. These include the reaction of 3-methylthio-l,2-dithiolium salts with sodium benzoylacetate, the treatment of 3-phenacylthio-l,2-dithiolium salts (43 = R = Ph, R = H) with triethylamine and decomposition of... 

Further examples have been given of the reaction of 1,2-dithiolium salts (both 3,4-dialkyl and 3,5-dialkyl) with ammonia to give isothiazoles, and the formation of ring-opened products, (45) and (46), from 3-aryl-l,2-dithiolium perchlorates and excess of dimethylamine has been reported. The reactions of 3-methylthio-l,2-dithiolium salts with amines continue to be of interest. With primary aliphatic amines, 5-aryl-3-methylthio-l,2-dithiolium iodides give j8-alkylaminodithioacrylates (47) and isothiazoline-3-thiones (48) as well as the demethylated compounds (l,2-dithiole-3-thiones). Similar products are obtained with secondary amines, except that the isothiazole derivatives are replaced by 3-dialkylamino-l,2-dithio-lium iodides (49), the n.m.r. spectra of which indicate restricted rotation... 

A new route to isothiathiophthens has been reported, involving the reaction of 2-methylthio-l,3-dithiolium salts with enamines, and treatment of the intermediate products (23) with hydrogen sulphide. More complex examples, e.g. (24), have also been prepared. 

Reduction of the 1,3-dithiolium cation with hexacarbonylvanadate anion yields the colourless dimer (46), which can be oxidized to tetrathiafulvalene with manganese dioxide. Reduction of 2-methylthio-l,3-dithiolium salts with sodium borohydride and treatment of the resulting 2-methylthio-l,3-dithiole with fluoroboric acid provides a convenient small-scale preparation of 1,3-dithiolium fluoroborate. The method has been extended to the preparation of l-selena-3-thiolium and 1,3-diselenolium fluoroborates. For large-scale preparation of the 1,3-dithiolium cation, the route involving peroxy-acid oxidation of l,3-dithiole-2-thione and isolation of the hexafluorophosphate is recommended. This method fails with selenium-containing compounds. ... 

Reactions of 2-methylthio-l,3-dithiolium salts with sodium azide have been investigated the products include 3-methylthio-l,4,2-dithiazines and sul-phenimides (47), from which 2-amino-1,3-dithiolium salts and other related compounds can be prepared. 

The Vilsmeier salt synthesis has been extended to the preparation of 2-methylthio-6a-thiathiophthens (15 R = H or Me). Full details have been published of the route to dithiolylidene ketones and thiathiophthens by way of 3-phenacylthio-l,2-dithiolium salts and related compounds. The 2-dimethylamino-6a-thiathiophthen (16), prepared by this method, is also obtained when the dithiolium salt (17) is treated with phosphorus pentasulphide, and the resulting mixture of isothiathiophthens (18 R = H, R = Ph) and (18 R = Ph, R = H) heated with thioacetamide. 

Condensations involving 3-methylthio-1,2-dithiolium salts have been used to prepare compounds (41) and (42). The spectroscopic properties of the former suggest that the quinonoid form (41a) makes a more important... 

Other synthetic applications of 3-methylthio-l,2-dithiolium salts are illustrated by the preparation of dithiolylidene ketones of types (44) and (45). The previously reported route to dithiolylidene ketones, from 3-phenacylthio-l,2-dithiolium salts, has been described in detail." ... 

Dithioles. Diselenoles. and Dltelluroles.- Treatment of ethyl benzoylacetate with Lawesson s reagent (188) gives the l,2-dithiole-3-thione (189) 197 the latter reacts with arenediazonium fluoroborates to yield the 1,2-dithioliua salts (190 R = aryl). The action of -methylbenzylidenemalononitrile, PhMeC C(CN)2, on the methylthio-analogue (190 R = Me) leads to a mixture of the condensation product (191) and the thieno[3,2-b]thiopyranimine (192).199 The mesoionic dithiolium oxide (193) has been prepared from -dichloroacetone and sulphur, followed by methylation. 

Condensation of 3,5-dimethyl-l,2-dithiolium perchlorate with methylthio-1,2- or -1,3-dithiolium cations has also been reported and leads to the intensely coloured methine salts (50) and (51)/ ... 

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