1.3- dithiol-2-ylidene compounds

Scheme 8.1 Redox of (a) TTF, (b) 1,3-dithiol-2-ylidene compounds, and (c) a dendraiene-type polymer with DT rings.

Chloro-l,2-dithiol-3-ones (e.g., 71a) and 4,5-dichloro-l,2-dithiol-3-one (71b) react with Grignard reagents on sulfur with subsequent ring closure of the open-chain intermediates.12lb The dithiethanes were isolated in 57-97% yields. Both cis and trans isomers were formed. Dichloro compound 71b gives via thioketene derivatives l,3-dithiol-2-ylidene compounds in 12-75% yield. Mixtures of stereoisomers were obtained12 lb (Scheme 8). 

Some of the recently synthesized l,3-dithiol-2-ylidene compounds of the type 2461 are strongly basic towards perchloric acid and yield the perchlorates (247i). However, on attempted recrystallization from ethanol these acids (247i) revert to the neutral compounds.i53d... 

The silver(I) complexes with the tetrakis(methylthio)tetrathiafulvalene ligand have been reported, the nitrate salt presents a 3D structure with an unprecedented 4.16-net porous inorganic layer of silver nitrate,1160 the triflate salt presents a two interwoven polymeric chain structure.1161 The latter behaves as a semiconductor when doped with iodine. With a similar ligand, 2,5-bis-(5,5,-bis(methylthio)-l,3,-dithiol-2 -ylidene)-l,3,4,6-tetrathiapentalene, a 3D supramolecular network is constructed via coordination bonds and S"-S contacts. The iodine-doped compound is highly conductive.1162 (Methylthio)methyl-substituted calix[4]arenes have been used as silver-selective chemically modified field effect transistors and as potential extractants for Ag1.1163,1164... 

Relatively stable l,2-dithiole-3-thiones, such as the 5-aryl-substituted compounds, react with 1-acenaphthenone24 or 1-tetra-lone21 in the presence of sodium hydroxide, giving a 2-(l,2-dithiol-3-ylidene)-l-acenaphthenone (or tetralone) (Eq. 7). 

Phosphorus oxychloride reacts with l,2-dithiol-3-ones, giving 3-chloro-l,2-dithiolium ions. If this reaction is carried out in the presence of an active methylene compound, a condensation follows immediately, giving a dithiolylidene derivative. For instance, 5-phenyl-l,2-dithiol-3-one reacts with phosphorus oxychloride and benzoylacetonitrile, giving benzoyl-(5-phenyl-l,2-dithiol-3-ylidene)-acetonitrile (Eq. 10).29... 

Aryl-l,2,4-dithiazole-3-thiones react with phenylacetylene giving two compounds, namely, A-(4-aryl-l,3-dithiol-2-ylidene) thiobenz-amide (52) and 2,5-diaryl- 1,6,6a IV-trithia-3-azapentalene, which may also be considered as AT-(5-aryl-l -dithiohS-ylideneJthiobenz-amide (53). This compound is converted into the corresponding benzamide (54) by mercuric acetate oxidation, the opposite reaction being realized with phosphorus pentasulfide.79... 

Methyl iodide adducts of 2,4-diphenyl- or of 2,3,4-triphenyI-l,6,6a IV-trithiapentalene react with aniline giving a compound which can be considered as a-(l,2-dithiol-3-ylidene)aldimine (59) or perhaps also as a-(isothiazolin-5-ylidene)thioketone (60).76... 

These attributions of carbonyl frequencies based on solvent effects have been challenged, in the case of 5-methyl-l,2-dithiol-3-ylidene-acetone (96) on the basis of some results concerning a nonketonic compound 98.22... 

UV and Visible Spectra of Various a-(l,2-DiTHioL-3-YLiDENE)CARBONYL Compounds... 

The 1,2-dithiole system and related compounds possess two adjacent sulfur atoms in a five-membered ring. The types are known respectively as 1,2-dithiolanes (1), 1,2-dithioles (2), l,2-dithiol-3-ones (3a), l,2-dithiole-3-thiones (3b), 3-iminodithioles (3c), l,2-dithiol-3-ylidenes (3d) and 1,2-dithiolylium (4) ions. 

IR spectra have played an important role in the structural study of the class of compounds described in this chapter as l-oxa-6,6aA4-dithiapentalenes and often named in the literature as a-(l,2-dithiol-3-ylidene) ketones. The prominent feature of the IR spectra of these compounds is the absence of the usual carbonyl band in the 1620-1720 cm-1 range and the presence of two or more strong bands in the 1500-1610 cm-1 range, a fact considered as the result of a short distance O-S interaction. 

The starting material, which can also be considered as an a- (1,3 - dithiol-3 -ylidene) thioketone, is obtained by reacting alkynes with l,2-dithiole-3-thiones. These compounds when heated with elemental sulfur or sulfur-containing catalysts rearrange to l,6,6aA -trithiapentalenes as shown in equation (21). A mechanism has been suggested for this reaction (72CC540) but it should be noted that in the reaction of alkynes on l,2-dithiole-3-thiones or on 1,2,4-dithiazole-3 -thiones the final l,6,6aA 4- trithiapentalene is often obtained directly <7lAHC(l3)l6l,P. 191,199). 

A-Propargyl dithiocarbonates (xanthates) 533 bearing a variety of substituents were convenient precursors of 1,3-dithiole-2-ylidene derivatives 534 by reaction with active methylene compounds involving a [3,3]-sigmatropic rearrangement (Equation 64) <1995TL5171>. In all cases except dibenzoylmethane, a mixture of regioisomers was obtained, with double bond exo or endo. 

In the last 12 years, the chemistry of 1,2- and 1,3-dithiole compounds has developed considerably for several reasons. One reason is that bis(l,3-dithiol-2-ylidene), often called tetrathiafulvalene, and its derivatives are important components of organic metals. Another is that the discovery of l,6,6a/l -trithiapentalenes has brought attention to their partial bonding with quasi-covalent bonds whose bonding character, in terms of usual covalent bonds, is between zero and unity. 

As indicated in Section II,A,2,a, (5-aryl-l,2-dithiol-3-ylidene)malonic acids (14) give, by decarboxylation and protonation, 3-aryl-5-methyl-1,2-dithiolium cations (36). In the presence of aromatic amines, two types of compounds may be obtained37, resulting from the condensation of 36 with the amine, and 38, resulting from the condensation of 36 with its conjugate base (cf. Ref. 1 p. 83). 

A special type of nucleophilic attack on 1,2-dithiolium compounds is the pyridine-catalyzed rearrangement of 3-[(l,2-dithiol-3-ylidene)methyl]-l,2-... 

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. ... 

Activated methylenes may react with one or two 1,3-dithiolium ions (Scheme 30). When the methylene compound condenses with an equimolar amount of 1,3-dithiolium ion, the adduct 170 is easily oxidized and the final product is a 1,3-dithiol-2-ylidene derivative of the starting methylene compound. Some results are given in Table Starting from 4-phenyl-1,3-dithiolium cation and tropolone, a compound of type 170 is obtained, but halogenotropolones give mixtures of compounds 170 and 171. ° ... 

The mass spectra of a series of methyl and phenyl substituted compounds have been studied.287 A general feature of these mass spectra is that the base peak is normally due to the molecular ion. 5-Monosubstituted compounds (e.g., 181) show intense peaks corresponding to M — HS2 (182), which can be formed by the fragmentation shown, such ions are also found in the mass spectra of a-(l,3-dithiole-3-ylidene)thioketones and have been used in the discussion of the structure of these compounds.204... 

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. ... 

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