1.3- Dithioles 1.4- dithiafulvenes

There is a long standing interest in the chemistry and the properties of cyclic compounds containing sulfur atom in modern material chemistry due to their redox chemistry. In particular, the focus has been on dithiole derivatives, e.g., dithiafulvenes and tetrathiafulvalenes, since the finding of metallic conductivity and low temperature superconductivity in radical cation salts. The quite low oxidation potentials of 1,4-dithiin compounds have been reported, recently [109]. On the other hand, thioketene dimers (2,4-bis(alkyli-dene)-l,3-dithietane) have been known for more than 100 years and synthesized by various methods [110-115]. The structure of these dimer compounds is similar to that of the redox-active sulfur compounds therefore, the potential electronic property of the thioketene dimer moiety is considerably attractive with the aim of application to a new and better -donor. 

It has been shown (64JHC163) that a-halo ketones readily form 1,3-dithioles with gem-dithiols. A convenient alternative pathway for the synthesis of certain l,3-dithiol-2-ylidene derivatives (295) is the reaction of a-halo ketones with dithioacids of type (294) which exist as a,(3-unsaturated gem-dithiols (65JOC732). For the synthesis of dithiafulvene derivatives via (4,5-diethoxycarbonyl-l,3-dithiolyl)tributylphosphonium tetrafluoroborate, see (79JOC930). 

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... 

Electrochemical oxidation of l,3-dithiol-2-one and -2-thione gave behavior consistent with the formation of dimers which were subsequently oxidized to radicals of uncertain structure. Chemical oxidation of the isoelectronic dithiafulvenes 151 (R = phenyl, 4-chlorophenyl, 4-tolyl, or anisyl) led to dimeric exocyclic C—C-coupled products, indicating the intermediacy of the radical 152. 5o... 

The attack of the acid (154) on the readily polarizable 1,2-dithiafulvene (155) corresponds to the extremely ready addition of electrophilic reagents to the simple and vinylogous heptafulvene derivatives, which are iso-n-electronic with 155. The opening of the dithiole rings in 156 and 158 under the pressure of the carbanionoid electron pair liberated by the proton abstraction and of the free electron pair on the sulfur, as well as the elimination of elementary sulfur and the intramolecular electrophilic attack of the mercaptide ion (157) on the 5-position to form 158, are simply the typical reactions of 1,2-dithioles that have already been discussed (Section II, B, 3). The reactivity of the 3-methyl group in 154 finds many parallels in the ease of condensation of the methyl-substituted pyridinium, pyrylium, thiopyrylium, and tropylium salts, and particularly... 

Alkynyl alkanedithioates (49) yield dithiafulvenes (50) on treatment with bases, via diaIkylidene-l,3-dithioles. The indene derivative (51) and other dithiafulvenes, including isothiathiophthens (see Chapter 7), have been prepared by condensations involving 2-methylthio-l,3-dithiolmm salts. 

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