1.3- Dithioles 1.3- dithiolium salts

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

Hydrolysis of chlorinated dithiolium salts of this type (Section II, B, 3) yields chlorinated l,2-dithiol-3-ones which have recently attracted attention because of their fungicidal properties.13... 

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). 

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... 

The reactions of 3-halogenated dithiolium salts with H2S are again of preparative value the mercapto-l,2-dithiole derivatives (89) cannot be isolated, but lose HC1 to form the trithiones (90).11... 

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. 

So far, only fused-ring 1,3-dithiolium salts have been synthesized in this way. The reason for this is that cis-ethylene- 1,2-dithiols were, until recently, difficult to obtain. Since, however, the cis-dithiol (208) can now be synthesized by the action of phenyl lithium or sodium alkoxide on isodithione (207),134 or by the cleavage of the cis-dibenzyl thioether (209),135 simple 1,3-dithiolium salts should now also be readily obtainable by this one-stage method. 

In isotrithiones of the type 252, the —M and —I effects of the substituents in positions 4 and 5 oppose the formation of a positive charge in the ring, and alkylation of these compounds is consequently impossible.134 1501 This corresponds to the failure of the similarly substituted 1,3-dithioles (236) to undergo oxidation to the 1,3-dithiolium salts. 

On the basis of this charge distribution, these compounds [cf. the discussion of the fused-ring analogs (Section III, A, 1, a) and of the 3,5-diamino-l,2-dithiolium salts (Table XI)] should be regarded as salts of 2-imino-l,3-dithiole rather than as 1,3-dithiolium salts. 

The nitro-substituted fluorenes 125 and 126 containing 1,3-dithiole units were synthesized by condensation of carbanions derived from fluorenes 122 with the dithiolium salts 123 or aldehydes 124. Selected examples of final products are shown in Scheme 10 <1998CC819, 1999JOC6937>. 

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>. 

Merocyanines containing a proaromatic donor unit (l,3-dithiol-2-ylidene moiety) and a proaromatic acceptor (1,3-diethylthiobarbituric acid) 150-152 were obtained in reactions of 147 with dithiolium salt 138 or aldehydes 149 and 150, respectively (Scheme 13) <20030L3143>. 

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>. 

The reaction of substituted 4-(2-hydroxyaryl)-2-(T,T-dialkylamino)-l,3-dithiolium salts 166 (NR2 = pyrrolidine, piperidine, morpholine) with sodium sulfide in EtOH at room temperature afforded the corresponding 1,3-dithiole-2-thiones 167, whereas reaction in boiling EtOH led to formation of 2-hydroxyacetophenones 168 (Scheme 16) <2003SC3071>. 

Reduction of 2-iminium-l,3-dithioles 169 with NaBH4 and futther deamination using HPE gave 2-dimethylamino-1,3-dithioles 170 and the 1,3-dithiolium salts 171, in turn (Scheme 17) <2004EJ01455>. 

Amino-l,2-dithiolium salts are obtained from either / -oxocarbox-amides or ) -oxocarbonitriles by reaction with phosphorus penta-sulfide, followed by addition of perchloric acid (Scheme 6). These cations, which may also be considered as iminium salts, are deprotonated to the corresponding l,2-dithiol-3-imine by ammonia. ... 

Reaction of some isothiazolium salts (9) with hydrogen sulfide leads to unstable compounds which have been described as bis(l, 2-dithiol-3-yl) sulfides (10). These compounds are converted into 1,2-dithiolium salts (11) by strong acids such as hydrogen chloride or iodide, or perchloric acid. ... 

An example of the formation of 1,2-dithiolium salts by protonation of an alkylidene-l,2-dithiole is the decarboxylation of 1,2-dithiol-3-ylidene malonic acids (14) in the presence of perchloric acid. In this particular case, a strongly acidic reaction medium is necessary for obtaining satisfactory yields because the corresponding 3-aryl-5-methylene-l,2-dithiole is unstable and undergoes an autocondensation reaction leading to an a-(thiopyran-2-ylidene)thioketone (cf. Ref. 1 p. 83). 

Some 2-alkylidene-l,3-dithioles are converted to 1,3-dithiolium salts by protonation with perchloric acid. These salts tend to lose a proton, and weak bases are sufficient to displace the equilibrium completely toward the neutral species. This is given as proof of the low resonance energy of the dithiolium cation (Eq. 34). ... 

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... 

A similar and convenient route to 2-unsubstituted-4-aryl-1,3-dithiolium salts (122) is by the action of acids in ethanol upon 2-amino-4-aryl-l,3-dithioles (121). Yields are generally higher than 90%. °... 

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. ... 

Unlike the 1,2-dithiolium ring, the 1,3-dithiolium ring is stable in strong basic medium. Thus, salts 145 react with alcoholates giving 2-alkoxy-l,3-dithioles. Even under milder conditions, by-dissolution in methanol, ethanol, or 1- or 2-propanol, 1,3-dithiolium salts form 2-alkoxy-l,3-dithioles... 

Dialkylamino and 2-ethylthio-I,3-dithiolium salts are cleaved by hydrogen sulfide,or sodium hydrosulfide,giving l,3-dithiole-2-thiones in excellent yields. Sodium sulfide and sodium hydrosulfide react with 1,3-benzodithiolium salts in methanol giving the sulfide 158 and the dithiole 159. 

More recent UV measurements on 2-unsubstituted 1,3-dithiolium salts were carried out in acidic medium and were found to be quite different from previous results in ethanol solution. It was shown that UV spectra taken in ethanol were not the spectra of 1,3-dithiolium salts but were those of 2-ethoxy-1,3-dithioles formed by nucleophilic attack of ethanol on C-2 in the cation. 

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