L,6,6a£IV-Trithiapentalenes

From 2-methyl-5-phenyl-l,6,6a IV-trithiapentalene, (5-phenyl-1,2-dithiol-3-ylidene)acetone only is obtained, and not the other possible isomer. The structure was proved by Raney nickel desulfurization (Eq. 13).44... 

Aryl-1,2-dithiolium salts are attacked by triethylamine in aqueous ethanol. Apparently one molecule of salt is first attacked by water, with loss of sulfur. The hypothetical 3-aryl-3-thioxopropanal (or its corresponding enol) would then react on another molecule of dithiolium salt to give a 2,5-diaryl-l,6,6a IV-trithiapentalene-3-carbaldehyde (Scheme 12).62... 

In like fashion, methyl ethoxycarbonyldithioacetate gives 3-ethoxycarbonyl - 2 - methylthio - 5 - phenyl -1,6,6a IV - trithiapentalene (43)46,61 and cyanothioacetamide gives 2-amino-3-cyano-5-phenyl-l,6,6a IV-trithiapentalene (44).30,64... 

However, it has been shown67-69 that the structure of the final product may depend on the experimental procedure. For example, if phenylacetylene reacts with 5-phenyl-l,2-dithiole-3-thione in xylene in the presence of hydrochloric acid, the l,3a IV,4-trithiapentalene is obtained but by using neutral dry xylene as solvent, a l,6,6a IV-trithiapentalene is formed which can also be obtained by heating the l,3aiSIV,4-isomer with phosphorus pentasulfide in tetrahydronaph-thalene (Scheme 14).67,68... 

Sulfur atoms in positions 1 and 6 in the trithiapentalene system may sometimes react as the thione sulfur atom of l,2-dithiole-3-thiones. For instance, 2,4-diphenyl- and 2,3,4-triphenyl-l,6,6a IV-trithiapentalene add methyl iodide (Eq. 34).76... 

For these compounds, the bonding of the three sulfur atoms is probably quite similar to the bonding of the same atoms in l,6,6a IV-trithiapentalenes. 

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... 

Visible and ultraviolet spectra of l,6,6a IV-trithiapentalenes have been measured for numerous compounds and have often been taken as a structural proof for this system. A SCF-MO calculation based on structure 89 has given a satisfactory explanation for the visible band, between 440 and 520 m/a, assigned to a 77 -> 77 transition.92 Ionization potentials are also correctly predicted by this calculation.92... 

Besides dithiolylidene ketones (12), which can be regarded as l-oxa-6,6a/SIV-dithiapentalenes (17), some. other systems similar to l,6,6a/SIV-trithiapentalenes have been described (18-23), and X-ray diffractometry and IR and NMR spectrometry suggest that the bonding of the three heteroatoms is somewhat similar to the bonding of the three sulfur atoms in 1,6,6a IV-trithiapentaIenes. 

Diaryl-3-formyl-l,6,6aselectively oxidized on carbon 2, and not on carbon 5 either by mercuric acetate or by peracetic acid (Eq. 15).39 A 3-alkoxycarbonyl substituent has the same orienting effect in this oxidation.45... 

These nitriles, when treated with hydrogen sulfide in pyridine and triethylamine, give 2-amino-3-cyano-l, 6,6a/S IV-trithiapentalenes (Eq. 29).48... 

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