Dithiolium salts

The synthesis of 1,2-dithiolium salts from /3-diketones has been modified by the use of diacetyl disulphide in place of hydrogen disulphide. Precursors of the general type (30), where Y = O, S, (OEt)a, or N+MCg, and Z = OH, Cl, or OEt, are treated with the diacyl disulphide and a strong acid. This versatile synthesis has been applied to the preparation of a wide range of alkyl- and aryl-substituted dithiolium salts. 

3-Methylthio-l,2-dithiolium perchlorates may be obtained directly from keten mercaptals, for example (31), by the action of phosphorus pentasulphide and subsequent treatment of the insoluble residue with perchloric acid. Similar treatment of /3-keto-amides (32) yields 3-arylimino-l,2-dithiole perchlorates (27 R = Ar, X = CIO4). The method succeeds with amides (32 R = alkyl) and thus serves as a route to the previously inaccessible 5-alkyl-l,2-dithiol-3-imines. Demethylation of 3-methylthio-5-phenyl-l,2-dithiolium cation, to give 3-phenyl-l,2-dithiole-3-thione, occurs when the iodide is heated in xylene. 

Ri =s R2 = Ph). 4-Aryl-l,2-dithiolium salts of type (36 R = Me or Ph, R = H) yield 1,2-dithiolylidene ketones (37) when heated in ethanol, but this reaction fails when the dithiolium salt has a 5-amino-substituent (36 R = NRa). The 5-amino-compounds have been found to give 1,3-dithiole derivatives on treatment with phosphorus pentasulphide in pyridine. The structure of one of the products (38) was established by conversion into the 6a-thiathiophthen (39), which was also prepared by sulphurization of the dithiolylidene ketone (37 R = Ar = Ph, R = MeaN) obtained by the triethylamine-acetic acid route. 

The unstable yellow salt obtained by the action of hydrogen sulphide and hydrogen chloride on the triketone (40) has been assigned the dithiolium salt structure (41). On treatment with bases it yields the intensely coloured meso-ionic compound (42), which is also obtained by the 

The effect of varying degrees of alkyl substitution (mono-, di-, and tri-) in 1,2- and 1,3-dithiolium salts on electronic spectra, charge-transfer spectra with iodide anion as donor, and n.m.r. spectra, has been studied, and the data have been compared with the results of SCF MO calculations.  

Details have been given of methods for the preparation of 1,2-dithiolium tri-iodides and iodides by the action of hydrogen sulphide and iodine on 1,3-diketones, and the diacetyl disulphide method has been applied to l-acylindan-2-ones, yielding indeno-1,2-dithiolium salts (22), which can be deprotonated to indeno-1,2-dithioIes (23). A mechanistic study has been 

2-Dithiolylidene derivatives of malonic acid (26) decarboxylate in perchloric acid-acetic acid to give 5-ary 1-1,2-dithiolium perchlorates. Vilsmeier salts of type (27) are produced by condensation of 5-methyl-1,2-dithiolium salts with dimethylthioformamide, and a similar trans-structure (28) is formed when 6-methyl-l,6a-dithia-6-azapentalene (see Chapter9) is protonated. 

M6tayer, G. Duguay, and H. Quiniou, Bull. Soc. chim. France, 1972, 4576. 

Non-empirical MO calculations for 1,2- and 1,3-dithiolium cations suggest that the d-orbitals are involved in bonding only to a trivial extent. A series of papers has appeared describing CNDO calculations of the electronic spectra of 1,2-dithiolium cations, and presenting a theoretical treatment of the species produced by cathodic reduction of dithiolium salts, particularly the radical form (32). Flash photolysis of 2,5-diphenyl-1,2-dithiolium salts in ethanol yields two products, of which the less stable has the absorption spectrum of the radical (32 R = Ph), and the longer-lived product appears to be the ring-opened anion (33) thus, the reaction is a photoreduction involving solvent. 

Oxidation of dithiomalonamides with hydrogen peroxide in the presence of hydrochloric acid leads to 3,5-diamino-4-chloro-l,2-dithiolium chlorides (20). The formation and deacetylation (or debenzoylation) of 3,5-diaryl-4-acyloxy-l,2-dithiolium salts have been described, and an earlier investigation of the mass spectra of the thermolysis products of dithiolium salts has been extended to include 3-alkylthio- and 3-arylthio-derivatives. Studies on the photochemical formation of 1,2-dithiolyl radicals and dithioketonate anions from dithiolium salts have been continued.  

Dimeric products (21), previously obtained electrochemically, have been prepared by reduction of 1,2-dithiolium salts with zinc. These products oxidize, either in air or with selenium dioxide, to highly coloured products of type (22). 

The reactions of 3-chloro-l,2-dithiolium salts (23) feature in a number of papers. With secondary amines, the products are 3-amino-l,2-dithiolium salts (24), which undergo an interesting reaction with the carbanion of nitromethane to give thiophen derivatives (25). Aromatic amines, phenols, and phenol ethers are substituted at thepam-position by 3-chloro-5-phenyl-l,2-dithiolium cations. The product obtained with phenol can be deprotonated to the quinonoid compound (26), and 3-arylamino-l, 2-dithiolium salts (27 = aryl or PhCHg) 

Ring-opening reactions of dithiolium salts with amines continue to receive attention. The behaviour of 3-styryl derivatives deuteriated in various positions/ and of 3-ethoxy-1,2-dithiolium salts/ has been studied. Further examples have been provided of the formation of isothiazoles and pyrazoles by the action of ammonia and hydrazine, respectively. 3,5-Di-isopropyl-l,2-dithiolium cations are deprotonated by aqueous ethanolic ammonia to give the dithiole (29).  

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The reaction of 1,2-dithiolanes with 2- and 4-picolyllithium has been examined <96PS(112)101> and the reactions of thioanhydrides such as 94 with both thiols <95JOC3964> and amines <96TL5337> have been reported. Treatment of 1,2-dithiolium salts with lithium or thallium cyclopentadienide results in formation of a variety of bi-, tri- and tetracyclic products <96LA109>. Reaction of 95 with trimethyl phosphite gives some of the desired coupling product but also the phosphonates 96 <96PS(109)557>. 

The first recorded attempt to prepare a thio derivative of a /1-diketone resulted in the isolation of the colourless dimer (57 R = R = Me) from the reaction of acetylacetone and hydrogen sulfide in hydrochloric acid.243 Similar dimers with various R and R groups have been reported244 and the structure (57) has been confirmed from NMR and MS data.245 Other attempts to prepare dithio-/ -diketones (58) yielded 1,2-dithiolium salts (59).246... 

Aryl-1,2-dithiolium salts are attacked by triethylamine in aqueous ethanol. Apparently one molecule of salt is first attacked by water, with loss of sulfur. The hypothetical 3-aryl-3-thioxopropanal (or its corresponding enol) would then react on another molecule of dithiolium salt to give a 2,5-diaryl-l,6,6a IV-trithiapentalene-3-carbaldehyde (Scheme 12).62... 

Dithiolium salts, particularly with aromatic substituents, readily form isothiazoles on treatment with ammonia58-62 and a mechanism (Scheme 21) has been proposed by Olofson et al. °... 

A variant of this procedure is the reaction of 1,3-diketones or 1,3-monothiodiketones with P4S10 and treatment with acid.14b However, here again only 3,5-disubstituted 1,2-dithiolium salts with at least one aryl substituent can be made. 

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). 

Suitable oxidizing agents include bromine (in benzene) and sulfuryl chloride (in ether/glacial acetic acid). The scope of this method has not yet been fully explored, and appears to be capable of further modification and expansion. Though laborious in comparison with other routes, it makes possible the preparation of certain 1,2-dithiolium salts that cannot be obtained, for example, by the extremely simple Klingsberg method (Section II, A, 2,b). 

Whereas trithiones (some of them at least) were until recently obtainable only at great preparative cost, a number of simple and very productive syntheses have been developed in the past fewyears.47b The oxidative elimination of the thione sulfur from the trithiones has therefore become the most important method of synthesizing 1,2-dithiolium salts (Table II). The substituents Rx and R2 may be... 

According to whether the intermediate 28 dissociates by path A or by path B, the product is a 1,2-dithiolium salt in which the original exocyclic residue X has been replaced by Z (29a) or has been retained as XY (29b). 

Chlobo-Substituted 1,2-Dithiolium Salts (32) from Baumann-Fbomm Disulfides and Tbithiones ... 

The literature contains many more examples of the preparation of 3-substituted 1,2-dithiolium salts of the type 29b, in which the exo-cyclic residue of 27 may be 0, S, NR, or CRR. 

The numerous addition compounds of the trithiones with salts of heavy metals (Cu, Ag, Au, Zn, Cd, Hg, Mn, Fe, Co, Ni, Al, Sn, Pb, Sb, Bi, Pt, Pd2 80,80-a2), most of which are sparingly soluble and characteristically colored, have also been formulated by various authors as 1,2-dithiolium salts. 

The same uncertainty exists with regard to the bonding in the addition products of trithiones with Cl2, Br2, and I2,82 84 and with the acid chlorides S02C12, S0C12,83, 84 and S2C12.88 The relatively high melting points of all these crystalline adducts, many of which decompose very readily, but which have very definite compositions, can be taken as evidence for the 1,2-dithiolium salt structures (e.g., 41 or 42) however, physical measurements that could conclusively establish the saltlike character have not so far been carried out. 

The trithione -methides (3-methylene-1,2-dithioles) are iso-77-electronic with the heptafulvenes,71 and possess a basicity comparable to that of the latter. Some true 3-alkylatSd 1,2-dithiolium salts can be obtained by protonation of compounds 52. This method of preparation is rather narrowly limited, however, by the fact that the simple derivatives of 52 (as in the hydrocarbon series) are evidently very unstable and have not as yet been described. These dithiafulvenes become stable, easily handled compounds only when they contain aryl or typical acceptor residues (R and/or R = CN, C02R) in the 6-position.53, 72 This substitution, however, again as in the hydrocarbon series,73 lowers the basicity to such an extent that the dinitrile (52) (Rj = R2 = H, R = R = CN), for example, is not appreciably protonated even in pure trifluoroacetic acid.52... 

Protonation does occur, however, in the case of the 1,2-dithiaful-valenes (54), which are iso-w-electronic with the hydrocarbon sesqui-fulvalene74 the favored product is the cation that is most stable in the particular case in question, i.e., the 3-alkyl- (55) or 3-alkenyl-1,2-dithiolium salts (56 and/or 57)45 53 (Tables IV and V). 

The 1,2-dithiolium salts are electron-deficient compounds, and as such are very reactive and unselective towards nucleophilic reagents. Qualitative considerations based on the simple resonance theory, as well as refined quantum mechanical approximations (Section II, C, 1), indicate that the resonance hybrid is most closely described by the carbonium and sulfonium structures (1 and 60), whereas the decet (61) and long-bond (62) structures appear to be of little importance. As would be expected for a charge distribution indicated in 1, protons attached in positions 3 and 5 experience a relatively weak electron screening effect (Section II, C, 4).79c... 

Reactions with Nucleophilic Reagents In close parallel with the oxidation of tropilidene derivatives by tropylium salts,81 for example, 1,2-dithiolium salts containing a single substituent in position 4 or 5, e.g. 70, can capture a hydride anion from the doubly substituted, highly strained leuco bases (71), thus oxidizing the latter to the more highly substituted and thermodynamically more stable salts (73).82... 

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 4-phenyl-1,2-dithiolium salt (86a) is decomposed by ethoxide ion24 the only product that can be isolated is the sulfurization product 4-phenyltrithione. 89 In the case of the 3,5-disubstituted... 

The action of SH- or SR- on 1,2-dithiolium salts is similar to that of OH- or OR-. Thus the 3,5-diamino- 1,2-dithiolium salt (15) is very rapidly decomposed by SH-, the only reaction product identified again being dithiomalonamide.9... 

Diphenylisothiazole (99) can be obtained by this method in a yield of about 50%.24 The same result has also been found for the 4-phenyl-1,2-dithiolium salt (88% yield of 4-phenylisothiazole).94 The course of this reaction evidently depends mainly on the solvent. In benzene, for example, only small yields of 4-phenylisothiazole are obtained, the principal product isolated being the bisdithiolyl... 

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... 

There are at present no detailed data available as to whether and to what extent the formation of isothiazoles from unsymmetrically monoaryl-substituted 1,2-dithiolium salts is stereoselective, or how the ratio of the isomera can be influenced by variation of the substitution. 

Some of these aminothiones react further with excess amine thus the cis isomer of the malonic dialdehyde dianil (107) has been obtained by the reaction of the 4-phenyl-1,2-dithiolium salt (105) with two moles of aniline.24... 

Dithiolium salts (111) with no substituent in position 3 readily undergo a single-step reaction with dimethylaniline to form the new 1,2-dithiolium salts (113). The thermodynamically less stable salt (111) again acts as an oxidizing agent, as discussed earlier.23... 

It is not known yet whether the 1,2-dithiolium salts are sufficiently electrophilic to attack even benzene derivatives containing less efficient electron-donating groups. In any case trithionium salts, with their already weaker electrophilic character, are unable to substitute phenols70 recently, substitution of phenolate has been reported.98b... 

No reactions of either 1,2-dithiolium salts or trithionium salts with hydroxylamine have been described in the literature. The explanation in the case of the trithionium salts is probably the fact that the expected condensation products, i.e., the oximes of the Baumann-Fromm disulfides, can be obtained via the trithiones themselves2 this is one of the few examples in which trithiones react with carbonyl reagents without undergoing cleavage of the 1,2-dithiole ring. 

The behavior of 96 towards hydrazines, as towards mercaptans (leading to 97) and primary amines (leading to 118), is abnormal in that the nucleophilic attack by the base takes place in position 5. The reaction (both the product and the probable mechanism of which are reminiscent of the action of hydrazines on 1,2-dithiolium salts) leads to the methylmercaptopyrazole (122).59... 

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