1,2-Dithiolium ions/salts

Another characteristic of electrophilic reactions of pseudoazulenes is the application of numerous cations as the electrophile, for example, diazonium salts and Vilsmeier- Haack s reagent (see Table VI), tropylium ion,135 triphenylmethyl cation,"4 pyrylium ion,119 and dithiolium ion.166 Very stable cations are formed (e.g., 120) addition of base releases the substituted pseudoazulene (see example in Eq. 10). Generally reactions of this type are thermodynamically favored (see also Section IV,B). The site of substitution... 

Until a few years ago, however, the cations 1 and 2 had aroused little interest. In 1947 Liittringhaus and Bottcher2 recognized the products of the reaction of l,2-dithiole-3-thiones ( trithiones ) with alkyl esters of inorganic acids as 3-alkylmercapto derivatives of the 1,2-dithiolium system these authors explained certain apparently unusual physical and chemical properties of these salts by resonance stabilization of the 1,2-dithiolium system.8 Thus these trithionium salts were the first derivatives of the 1,2-dithiolium ion to be identified with certainty. 

The 3-mercapto-1,2-dithiolium salts (39a) also suffer deprotonation on addition of water. This behavior is the basis of a simple and effective method of purification, which is particularly useful for aryl-substituted trithiones. 48 The tendency towards charge neutralization in the alkylmercapto-1,2-dithiolium ions is also the reason for the relative ease of removal of the alkyl group (cf. thermal dealkylation, transmethylation), as well as for the extremely labile bonding of other groups such as acyl—, —S02—, —SO—, —S2C1, and halogen to the exocyclic S atom of the corresponding 3-mercapto-1,2-dithiolium derivatives. 

It should be mentioned at this point that the ease of alkylation of the isotrithiones to isotrithionium salts (251) is not due to any specific stability of the 2-methylmercapto-1,3-dithiolium ion, in which the C=C double bond takes part in the mesomeric system and contributes a pair of it electrons to the aromatic sextet. As was recently shown by Wizinger et aZ.,138 Gompper,157 and Mayer and Schafer158 the dihydroisotrithiones (257) can also be readily alkylated by heating with alkyl halides, dimethyl sulfate, or ethyl p-toluene-sulfonate. Results obtained so far indicate that the reactivity of these dihydroisotrithionium salts towards C, 0, N, and S bases corresponds closely to that of the isotrithionium salts. ... 

Most of the known salts without fused rings, on the other hand, are very unstable. Unfortunately, no quantitative studies have so far been carried out on the thermodynamic stability of the 1,3-dithiolium ion, and in view of the limited experimental data available it is impossible to draw any definite conclusions regarding the dependence of stability on the nature of the substituents. It is clear, however, that here the nature of the anion is of vital importance thus whereas 4-(p-methoxyphenyl)-isotrithionium iodide is stable, the pbenyl derivative can be isolated only as the monomethyl sulfate. 

Aldehydes condense readily with 3-methyl-l,2-dithiolium ions, giving styryl derivatives (68). Vilsmeier salts (69) are obtained by reacting dimethylformamide, dimethylthioformamide, A-methyl-A-phenyl-thioformamide, or dimethylformamide diethylacetal with dithiolium compounds having a-methylene groups. 

Monocyclic 3-ethoxy-1,2-dithiolium ions are easily hydrolyzed to the corresponding dithiolone. The same compounds are obtained with anhydrous ethanol, which is transformed into diethyl ether." With 3-ethoxy-l,2-benzodithiolium ion, hydrolysis gives only small quantities of 1,2-benzo-dithiol-3-one, the major product being 2,2 -dithiobis(benzoic) acid." Vilsmeier salts such as 26 are vinylogs of amino-1,2-dithiolium ions (27) and, accordingly, are attacked by nucleophiles. Hydrolysis of 26 is a good preparation of a-(l,2-dithiol-3-ylidene) aldehydes, as shown in the case of 28. Methanolysis may be also realized by heating in methanol. ... 

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. 

Thus the leuco bases expected from the reaction of aryl-substituted 1,2-dithiolium salts with tertiary aromatic amines can only occasionally be obtained as such (e.g., 19b). The 1,2-dithiole derivatives (19) generally suffer rapid loss of hydride ion by the unreacted 1,2-dithiolium salt or by added oxidizing agent.23 It has recently been shown24 that 1,2-dithiole derivatives can be isolated only if the ring is stabilized by several aryl substituents. The compounds are then reasonably stable, and can be reconverted into the starting salts by the action of strong acids (Section II, B, 3). 

Like other cations, such as the tropylium ion, the 1,2-dithiolium ring is stable towards electrophilic reagents. Thus the salts remain unchanged on storage for long periods in concentrated sulfuric acid, for example.43... 

A characteristic property of nearly all 1,2-dithiolium salts is their extremely high sensitivity towards aqueous bases. Monoaryl derivatives decompose almost instantaneously in the presence of hydroxide ions, with liberation of elementary sulfur. Any explanation that may be given is subject to a great deal of uncertainty in view of the speed and complexity of the decomposition. It appears plausible, however,... 

The exocyclic C=C double bond of the 1,4-dithiafulvenes (263) can be selectively hydrogenated under controlled conditions in an acidic medium (Zn/glacial acetic acid), the product being 295. The ease with which the postulated intermediate 1,3-dithiolium salts (264) accept the hydride ion has been adduced as evidence that the resonance stabilization of these ions cannot be very strong.159... 

Mass spectra have been obtained from various 3-alkylthio- and 3-arylthio-1,2-dithiolium iodides thermolyzed in the ion source of the mass spectrometer, The spectra from alkylthio salts can be rationalized by assuming the primary formation of a 1,2-dithiolyl radical. The 3-arylthio compounds give a bis(l,2-dithiol-3-ylidene), probably through formation of a carbene intermediate. ... 

A study of alkyl- or aryl-substituted 1,2-dithiolium salts leads to similar conclusions. One group of these salts undergoes thermolysis to a dithiolyl radical, and in the mass spectrum the parent ion is the dithiolyl ion, which often loses a hydrogen atom. However, sometimes the parent ion is stable enough to be observed in the spectrum. This is the case for 3,5-diphenyl-l,2-dithiolium bromide. Other salts, such as 3- or 4-phenyl-1,2-dithiolium bromide, probably first expel a proton, giving a carbene intermediate. " ... 

Unlike 3/f-l,2-dithioles, numerous 2i/-1,3-dithioles (115) have been isolated and are rather stable. By reaction with trityl salts, they can lose a hydride ion, giving 1,3-dithiolium salts representative examples are listed in Table Yields are fair to good. This elimination is favored by... 

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. ... 

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, ° ... 

Alkylthio-1,2-dithiolium salts (17) also lose alkyl groups to form thiones (2b) under the conditions of mass spectroscopy. For other 1,2-dithiolium salts (3), radicals are initially formed whose masses correspond to the molecular ions. Certain 1,2-dithiolium salts (3) form 1,2-dithiole-3-thiones under mass spectrometry conditions, presumably by thermal degradation <80SRl>. 

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