1,3-dithiolium carbenes

When di-isopropylcarbodi-imide was tried in place of CSa, a (2 -F 2] cycloaddition followed by a prototropic shift occurred, giving (192). All of the CSa reactions could be satisfactorily interpreted on the basis of intermediate formation of a resonance-stabilized 1,3-dithiolium carbene (193), potentially a non-benzenoid aromatic. 

An efficient conversion of cyclobutanone to a 7-lactone ring is part of a total synthesis of prostaglandins Among the various products obtained by reaction of acetylenes with carbon disulfide are those of special interest which originate by the addition of third components to intermediate 1,3-dithiolium carbenes to form 2-subst. 1,3-dithioles... 

Dithiolium carbenes are also obtained in the reaction of perfiuorinated acetylenes with carbon disulfide and PhsP . 

The formation of compounds (54)—(56) from perfluorobut-2-yne and carbon disulphide at 100 °C has been interpreted in terms of the generation of the 1,3-dithiolium carbene (57), evidence for which has been obtained through trapping experiments,e.g.,... 

This resonance stabilization should be even more pronounced in the cyclic dithiocarbene (274), as indicated by the neutral carbene structure (274) and the ylid structures (275-280), in which the potential aromatic sextet of the 1,3-dithiolium ion is retained. Consequently, H-2 in 269 should be acidic. The analogy with the thiazolium and imidazolium ions, which have been intensively studied by Bres-... 

Hexafluoro-2-butyne and carbon disulflde react to give the tetrakis-(trifluoromethyl)tetrathiafulvalene quantitatively only in the presence of trifluoroacetic acid (70JA1412 73JA4379). The carbene initially formed is protonated the 1,3-dithiolium ion subsequently combines with the nucleophilic carbene to give the trifluoromethyl-substituted tetrathiafulvalene. 

Carbanions 128, generated from the corresponding Fischer carbene complexes with -butyllithium, reacted with 2-methylthio-l,3-dithiolium salts 123 to give heterocyclic organometallic carbenes 129. Reactions of allylic carbanions 130 afforded a mixture of mono- and diheterocyclic condensation products 131 and 132, respectively (Scheme 11) <2004TL7843>. 

For substitutions in position 3 or 5 on the 1,2-dithiole ring various mechanisms may be considered. When R (or R ) is a good electrofuge such as H, carbenes (25) are likely intermediates. " Strong acidity slows hydrogen exchange, supporting such a mechanism. A similar mechanism is possible for the thiation of 1,2-dithiolium cations by elemental sulfur in the presence of pyridine (Eq. 8). 

When a 1,2-dithiolium cation has a hydrogen atom in position 3 or 5, deuteration can be realized either with a mixture of CFjCOjD and D20 °° or one of Mc2SO and D20. As the deuteration rate diminishes when the acidity increases, a carbene mechanism has been suggested, as indicated in the preceding section. 

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. " ... 

Among the nucleophilic reagents that react with 1,3-dithiolium salts, tertiary aliphatic amines have given interesting results. Numerous TTF derivatives have been prepared by treatment of 2-unsubstituted 1,3-dithiolium salts by triethylamine or A-ethyldiisopropylamine (Section III,B,6,a).The 1,3-dithioliumcation probably first undergoes adeprotonation leading to a cyclic carbene which can also be represented by dipolar structures (Eq. 36). 

Carbenes have also been considered as intermediates in the thermal dissociation of 2-alkoxy-l,3-dithioles (Section III,B,4) and in the reactions of carbon disulfide with activated alkynes and with benzyne. Another type of deprotonation may occur with 1,3-dithiolium cations containing phenolic hydroxyls. Thus, treatment of 4-aryl-2-(4-hydroxy-phenyl)-1,3-dithiolium perchlorate with triethylamine gives 135, stabilized by its zwitterionic structure 136. ... 

The acidity of H-2 in 1,3-dithiolium cations has been observed by NMR. In CF3CO2D/D2O, H/D exchange occurs between at the 2-position, the speed being much greater for 4-aryl than for 4-methyl derivatives. A carbene intermediate is proposed (Eq. 37). 

After the discovery of the interesting electrical properties of tetrathia-fulvalene (TTF)-tetracyanoquinodimethane (TCNQ) complexes, many TTF derivatives have been prepared by reaction of 1,3-dithiolium salts with tertiary amines. This reaction has been interpreted as proceding by deprotonation of a 1,3-dithiolium cation to the corresponding carbene which in turn reacts as a nucleophile on the C-2 of another 1,3-dithiolium cation. This topic having been recently reviewed, we refer in Table 309,310 Qjjjy jQ papers subsequent to this review. ... 

Charge densities and energies calculated by Pople s method for 1,3-dithiolium cation and for the corresponding carbene were compared with those of thiazolium and imidazolium species. It is concluded that sulfur tends to facilitate ionization of hydrogen on C-2 and to increase the electro-philicity of the resulting carbene, while nitrogen has a weak ionization effect on C-2 and favors nucleophilicity of an adjacent carbene. This is confirmed by comparison of the chemical properties of these cations. 

Dithiolium cations (3) form radicals under the thermal conditions of mass spectroscopy (Section 3.11.3.4), and carbenes may be involved in the formation of l,2-dithiole-3-thiones and thieno[3,2-Z>]thiophenes by thermolysis of 2-iodo-l,2-dithiolium salts (29a) <8ILA1729>. 

The reduction by, for example, Zn, Ag, or Ti(lll) of 3-chloro-1,2-dithiolium salts (29b), probably by dimerization of carbenes, gives bis-l,2-dithiole-3-ylidenes (9) or, in some cases, acyclic products which dimerize to 1,3-dithietanes (Scheme 6) <81LA1928, 88BSF101>. Carbenes may also be involved in the formation of bis-l,2-dithiol-3-ylidenes (9) from trisalkylthio-1,2-dithiolium salts (53)... 

The three or five protons on 1,2-dithiolium salts are exchanged by deuterium under various conditions <66AHC(7)3i, 66T2ii9>. Probably carbene intermediates are involved in these, and in the conversion of 1,2-dithiolium salts (3) to the thiones (2b) by sulfur in pyridine <82AHC(3l)63,89CC1716>. 

The syntheses of bis-l,3-dithiolylidene compounds noted above, involving treatment of 1,3-dithiolium salts with bases, are thought to proceed through carbene intermediates. These may be generated in other ways, for example, by the action of carbon disulphide on acetylenes (81 R = CF3) and by the action of triphenylphosphine on the l,3-dithiole-2-thione (83 R = CF3). In the first of these two methods, more complex products. 

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