Dithiolium, synthesis

Tlie interest in the preparation and use of dithiolium salts in connection with the synthesis of TTF derivatives led to the development of a new uses of heteroaromatic cations in organic synthesis. Based on that, a new carbonyl olefination for the synthesis of numerous heterofulvalenes was developed (77S861). For example, 2-dimethoxyphosphinyl-l,3-benzodithiole was deprotonated with butyllithium in THF at -78°C and the resulting phosphonate carbanion reacted with 9-alkyl-acridones to give the dithia-azafulvalenes of type 45 (78BCJ2674) (Scheme 15). 

The synthesis of the bis-l,3-dithiolium radical cation (TTF+) in 1970 [1] enabled dramatic growth in the field of molecular conductors in the decades thereafter. TTF and several of its homologues are depicted in Scheme 1. The field of low dimensional molecular metals was further motivated by the discovery of the TTF-TCNQ charge-transfer complex in 1973 [2, 3]. Seven years later, superconductivity was induced in the cation-radical salt, (TMTSF)2PF6, upon application of 12 kbar pressure [4]. Shortly thereafter, superconductivity below 1.4 K was observed at ambient pressure in the perchlorate analog [5]. 

Another reaction that can be formally grouped with those discussed above is the sulfurization of dithiolium salts with sulfur in pyridine,7 9d 8 9 in which the sulfur also acts as the oxidizing agent. This reaction enjoys a certain importance as a trithione synthesis. Nitro-substituted trithiones such as 94 cannot be obtained by the usual methods, since the nitro groups are reduced under the drastic reaction conditions required. These nitro derivatives can, however, be readily obtained by the method outlined in Scheme 4. 

Judging from these early results, it appears that the reaction of 1,2-dithiolium salts with ammonia offers an elegant synthesis of certain substituted isothiazoles that are difficult to obtain by other methods.98 The reaction of the unsubstituted 1,2-dithiolium salt with NH8, however, does not yield the parent isothiazole system. Moreover, in the case of the 3(5)-methyl salts, the abstraction of a methyl proton activated by the positive charge of the ring successfully competes with this reaction (Section II,B,4).14a... 

This pyrazole synthesis, which undoubtedly profits from the thermodynamic stability of the end products, gives excellent yields (Table VI). Its range of application is limited, however, by the fact that the reaction of 3-substituted unsymmetrical dithiolium salts with substituted hydrazones leads to mixtures of isomers which can be separated only with difficulty. On the other hand, it does permit the preparation of a single 4,5-disubstituted pyrazole from a 3,4-disub-stituted salt and a substituted hydrazine. 

When benzo-l,3-dithiolium tetrachlorozincate is heated in acetic anhydride, a 30% yield of dibenzotetrathiafulvalene (286) is obtained. Compound 286 is also formed, together with benzoisodithione, during the pyrolysis of the spiro compound (287), and its structure has been confirmed by independent synthesis from 1,2-benzenedithiol and tetrachloroethylene. 

Based on standard procedure, a sequence of reactions on a series of 5-aryl-l,2-dithiole-3-thiones 62 led, via 5-aryl-3-methylthio-l,2-dithiolium iodides 63, <1961JA2934, 1970BSF3076>, 2-alkyl-5-arylisothiazole-3-thiones 64 <1974CJC1738> and 2-alkyl-3-alkylthio-5-arylisothiazolium iodides 65, to synthetically useful thioaroylketene 3",iV-acetals 66 (Scheme 4) which were employed as precursors in an efficient synthesis of 3-alkylamino-5-arylthio-phenes 67 having various substituents X at C-2 <2000JOC3690>. 

The repetition of the whole transformation, occurring on the remaining isopropyl group in the dithiolium salt 218, affords the bis-dithiolium salt 220, which cyclizes by further reaction with S2CI2 to 3,5-dichloro-bis-dithiolium salt 221 as the key intermediate for the synthesis of bis[l,2]dithiolo[l,4]thiazines 14 and bis[l,2]dithiolopyrroles 124 or 15. 

The final stage of the synthesis of thiophene-functionalized fluorenes 121 involved the condensation of lithio derivatives of compounds 120 with the 1,3-dithiolium salt 119, prepared almost quantitatively from 118 by A-methylation (Scheme 9) <1999J(P2)505>. 

In the reviewed period, most work was connected with synthesis of 1,3-dithioles and 1,3-dithiolanes. Only a few contributions on the synthesis of 1,3-dithiolium ions, mesoionic l,3-dithiol-4-one and -thione derivatives were reported during this time. 

This section deals with the synthesis of 1,3-dithiole and 1,3-dithiolane rings. No investigations on ring synthesis of 1,3-dithiolium ions and mesoionic 1,3-dithiole derivatives were reported in this category. 

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

Owing to their positive charge, 1,2-dithiolium cations are, of course, almost unreactive to electrophiles. On the other hand, their conjugate bases, such as 3-alkylidene-l,2-dithioles, react easily with electrophilic reagents such as aromatic aldehydes, dimethylformamide, and nitrous acid. Some of these reactions are useful for the synthesis of partially bonded compounds and have been described, for papers prior to 1970, in a previous review (Ref 2 pp. 181, 187, 193). 

The discovery of the unusually high solid-state electrical conductivity of the charge-transfer complex of TTF with TCNQ has prompted extensive investigations of 1,3-dithiolium salts, an important class of intermediates for the synthesis of tetrathiafulvalene derivatives. 

Dithiolium salts react with the phosphoranes 178. When excess triethylamine is added in the reaction mixture, a TTF derivative is obtained. In a first step, 178 undergoes nucleophilic addition to the 1,3-dithiolium cation. Then triethylamine brings about elimination of triphenylphosphine and formation of TTF (Scheme 31). This synthesis allows the preparation of... 

Dithiols, 1,3-dithiolium salts, synthesis and reactivity of 82MI31. 

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