1.2- Dithiolylium ions

The 1,2-dithiolylium and 1,3-dithiolylium ions (115) and (116) are iso-rr-electronic with the tropylium ion, from which they may be formally derived by replacing double bonds by sulfur atoms. Various calculations and structural data demonstrate that the rings are stabilized by tt-electron delocalization. 

Active methylene compounds can add to 1,3-dithiolylium ions to give 2-substituted 1,2-dihydro-1,3-dithioles (206). Again, addition is often followed by oxidation (to 207). Alternatively, further addition can occur (to 208) (80AHC(27)151). In this reaction, (205) can be CH2(CN)2, CH2(COMe)2 or even MeCOMe. Somewhat similar reactions are shown by 1,3-diarylimidazolium ions. 

Such reactions are easy in cationic derivatives for example, in the 1,2-dithiolylium series (699), substituted cyclopentadienyl ion gives fulvene derivatives (700) (66AHC(7)39). 2-Methylthio groups in 1,3-dithiolylium ions are substituted by primary amines or secondary amines (80AHC(27)l5l), and similar reactions are known for 2-alkylthiothiazoles. 

This chapter deals with 1,3-dithiole compounds such as 1,3-dithiolylium ions (1), mesoionic l,3-dithiol-4-ones (2), 1,3-dithioles (3), 1,3-dithiolanes (4) and the tetrathiaful-valene system (5). During the last 15 years the chemistry of 1,3-dithiole compounds has developed considerably. One reason is that tetrathiafulvalene and its derivatives serve as donors in organic charge-transfer salts which exhibit the electrical properties of quasi-one-dimensional metals. For the preparation of such organic metals, 1,3-dithiolylium cations serve as useful synthetic intermediates. 

Quantum mechanical calculations for the 1,3-dithiolylium ion, benzo-1,3-dithiolylium ion, l,3-dithiol-2-one and l,3-dithiole-2-thione have been carried out with the simple LCAO MO method <66AHC(7)39). According to these calculations, the lowest electron density was found at the 2-position of the 1,3-dithiolylium cation, and the C(4)—C(5) bond order corresponded approximately to that of an isolated C—C double bond. A delocalization energy of 105 kJ mol-1 has been calculated for this cation. For the benzo-l,3-dithiolylium ion it was found that nucleophilic attack should take place in the 2-position but electrophilic substitution at the 4-position. 

On the other hand, a non-empirical calculation of the 1,3-dithiolylium ion using linear combinations of Gaussian orbitals with and without d-orbitals showed that almost all of the positive charge is shared by the sulfur and hydrogen atoms, whereas the carbon at the... 

Furthermore, the influence of alkyl substituents on the UV transitions of 1,3-dithiolylium ions has been studied by the PPP method and compared with experimental values the agreement was good (71T4705). These compounds possess two UV absorption maxima of approximately equal intensities. The long wavelength band corresponds to a transition with a transition moment in the direction of the two-fold symmetry axis of (1), whereas the v r transition of the second band is polarized perpendicular to the first one (71T4705). 

In 2-p-substituted phenyl derivatives of 4-phenyl-l,3-dithiolylium ions a linear relationship exists between the 5-H resonance signals and the Hammett [Pg.816]

The H-5 resonance of 2-phenyl-l,3-dithiolylium-4-olate (Table 4) appears at significantly higher field than in the 1,3-dithiolylium ion. This is in agreement with the delocalization of the negative charge on 0-4 and C-5 which leads to an increased shielding at C-5. 

The C-2 resonance signal of the 1,3-dithiolylium ion at 8 179.5 appears at higher field than in the 1,3-dithiolanylium ion (8 221.2) whereas the opposite holds for the C-4 resonances. These data reflect the greater delocalization of the positive charge in the... 

This section is divided into reactions of fully unsaturated rings like 1,3-dithiolylium ions, mesoionic 1,3-dithiolones, l,3-dithiol-2-ones and l,3-dithiole-2-thiones, and reactions of the saturated 1,3-dithiolanes. As a consequence of the positive charge in 1,3-dithiolylium ions, the main reactions of this class of compounds consist of nucleophilic attack at the 2-position, whereas the mesoionic 1,3-dithiolones undergo cycloaddition reactions. Reactivity of the benzo ring in benzo-l,3-dithiole and related systems has not been studied to any extent. 

Since 1,3-dithiolylium ions are positively charged, attack of electrophiles at the ring carbons is very rare. One example, where the mesoionic 1,3-dithiolone is acylated at the 5-position in situ during its preparation, occurs in the preparation of (29) from its precursor (28) (78CB2021). 

Several 1,3-dithiolylium salts have been prepared by hydride removal from 2H-1,3-dithioles. Naturally, this elimination is favored by electron-releasing substituents in the 2-position, whereas 4,5-dicyano-l,3-dithiole remains stable toward oxidizing agents. The reaction of the 1,3-dithioles (30) with trityl salts to produce the 1,3-dithiolylium ions (31) offers one such example <80AHC(27)15l). 

The electron-rich para-position of arylamines, phenols, and phenol ethers react at C-2 of 1,3-dithiolylium ions (54) leading to compounds of type (55). When both para-positions are blocked, as in 7V,7V,7V, /V -tetramethyl-l,4-phenylenediamine, then ortho attack is observed <80AHC(27)151). 

The attack of the cation (51) on (56) finds parallels in many reactions of 1,3-dithiolylium ions with 1,1-disubstituted alkenes and with cinnamic acid derivatives (59HCA1733, 63HCA2167). 

It is known that protons of a methyl group in the 4- or 5-positions of the 1,3-dithiolylium ions are relatively inert towards strong bases, however, the amide proton in the cation... 

On the other hand, reaction of the l,3-dithiole-2-thione (21) with peracetic acid in acetone at -40 °C produces the parent 1,3-dithiolylium ion (1) (74JOC2456). 

On the other hand, the addition of pyridine to the 2-methylthio-1,3-dithiolylium ion (195) results in transmethylation instead of nucleophilic attack at the 2-position. The reaction products are (21) and (196) (66AHC(7)39). 

Electrochemical reduction of the 2-ethylthio-l,3-dithiolylium salts (344) in acetonitrile gives the corresponding dimers (345) which can then pyrolyze to the tetrathiafulvalene derivatives (24) in nearly quantitative yields (74JA945). When one considers the total yield, this method is superior to the base-catalyzed dimerization of 1,3-dithiolylium ions. However, for large-scale synthesis, the convenience may be questionable. 

Methyl-l,2-dithiolyliums react with aldehydes to give styryl derivatives and with DMF to give Vilsmeier salts, and on nitrosation form the bicyclic products 598. 2-Alkyl groups in 1,3-dithiolylium ions also react with aromatic aldehydes to give 599 and with DMF to give 600. 

This chapter deals with the synthesis, reactivity, and characterization of five-membered heterocycles containing two ring sulfur atoms (1,3-dithiole derivatives) and is a review of the literature in the period 1995-2006. Previous reviews covered the literature till 1982 (CHEC(1984)) <1984CHEC(6)813> and 1995 (CHEC-II(1996)) <1996CHEC-11(3)607>. Among 1,3-dithiole compounds, 1,3-dithiolylium ions 1, mesoionic l,3-dithiol-4-ones 2, mesoionic 1,3-dithiole-4-thiones 3, 1,3-dithioles 4, 1,3-dithiolanes 5, and the tetrathiafulv alene (TTF) system 6 as a special class of compounds are discussed. 7l-Extended tetrathiafulvalenes (7t-exTTFs) containing more than one conjugated multiple bond between two 1,3-dithiole moieties are also discussed, in conjunction with 6. 

In the previous reviewing periods (CHEC(1984), CHEC-II(1996)) theoretical methods were applied in order to understand certain chemical and physical properties of 1,3-dithiole compounds, especially 1,3-dithiolylium ions, 1,3-dithiol-2-one and l,3-dithiole-2-thione. Since 1995, theoretical methods have been applied, mainly to a particular class of compounds, TTFs, which as electron donors easily form charge-transfer (CT) complexes. These complexes show a wide variety of electronic behaviors leading to semiconductor, metal-like, or superconductor properties. 

A number of publications of substantial significance have appeared in literature since the preparation of this chapter up to October 2007. These concerned mostly 1,3-dithiolanes, 1,3-dithioles and TTF systems. No new investigations were reported on 1,3-dithiolylium ions, mesoionic 1,3-dithiol-4-ones and mesoionic l,3-dithiole-4-thiones. 

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