Dithiol-4-ones

3-Dithiol-4-ones (Anhydro-4-hydroxy-l,3-dithiolium Hydroxides) (134) 

The dehydration of thiobenzoylthioglycolic acid (135, R = Ph, R2 = H) with acetic anhydride-boron trifluoride was initially described90 as yielding the meso-ionic l,3-dithiol-4-one (134, R1 = Ph, R2 = H), but subsequent studies91 showed that the product was in fact the 5-substituted derivative 134, R1 = Ph, R2 = COCH2SCSPh. Authentic meso-ionic l,3-dithiol-4-ones (134) have recently been prepared91,92 (85-90% yield) by the cyclodehydration of the acids (135) with acetic anhydride-triethylamine at 0°-10°. Examples include anhydro-4-hydroxy-2-phenyl-l,3-dithiolium hydroxide (134, R1 = Ph, R2 = H)91 described as scarlet needles, m.p. 113°—115° this compound is sensitive to moisture. Anhydro-4-hydroxy-2,5-diphenyl-l,3-dithiolium hydroxide (134, R1 = R2 = Ph)92 was obtained as gold lustered, deep violet needles, 

The meso-ionic l,3-dithiol-4-ones (134) participate464-77 91-93,94 in 1,3-dipolar cycloaddition reactions giving adducts of the general type 136. They show a remarkable degree of reactivity toward simple alkenes94 including tetramethylethylene, cydopentene, norbornene, and norbor-nadiene as well as toward the more reactive 1,3-dipolarophilic olefins dimethyl maleate, dimethyl fumarate, methyl cinnamate, diben-zoylethylene, JV-phenylmaleimide, and acenaphthylene. Alkynes91-93 such as dimethyl acetylenedicarboxylate also add to meso-ionic 1,3-dithiol-4-ones (134), but the intermediate cycloadducts are not isolable they eliminate carbonyl sulfide and yield thiophenes (137) directly.91-93 

Photolysis of anhydro-4-hydroxy-2,5-diphenyl-1,3-dithiolium hydroxide (134, R1 = R2 = Ph) in benzene solution yields tetraphenyl-1,4-dithiin (138, 19% yield), diphenylacetylene (16% yield), and sulfur. This result has been interpreted95 as involving the bicyclic photointermediate 139, which loses carbonyl sulfide to give the 4 -antiaromatic diphenyl thiiren (140), which is the precursor of diphenylacetylene and tetraphenyl- 1,4-dithiin (138). 

Kawamura, T. Shiba and M. Ohta, Chem. Commun., 959 (1970). 

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The vibrational spectra of 1,2-dithiole-3-thione 46 and 1,2-dithiol-3-one 47 were computed at the DFT and MP2 levels (Scheme 31) [98VS77]. Most remarkably, the uniformly scaled MP2 fundamentals are in better agreement with experimental data than the corresponding DFT frequencies. 

Dithiol-3-ones (3a) and -3-thiones (3b) give S(2p) binding energies of 164.5 and 164.7 eV respectively. Despite the small spread of values, a correlation has been made between these and attached 19F chemical shifts. 

Carbon nucleophiles may react at the sulfur-sulfur bond, or at C-3. Thus Grignard reagents react with monocyclic dithiol-3-ones (3a) and -3-imines (3c) at S-2 (74LA1261,73LA247). The product (84) of reaction of a Grignard reagent with benzo-1,2-dithiol-3-one (77a) (74T4113) could be alternately formed by attack at S-2, at C-3 or at C-5. Phosphonium ylides or other stabilized carbanions react at the 3-position of dithiol-ones... 

Alkyl- and aryl-1,2-dithiol-3-ones (3a) are made by ring synthesis from unsaturated esters (B-66MI43100) and alkylation of these affords 3-alkoxy-l,2-dithiolylium salts (35b). The parent (3a R1 = R2 = H) is best made from the thione (3b) by treatment with mercury(II) acetate (63CB2702). 

Density functional theory (DFT) and post-Hartree-Fock MP2 in conjunction with the B3LYP employing the 6-31G(d) basis set were used to predict structure and correlate assignments of the fundamental vibrational modes of 3/7-1,2-dithiole-3-one la and 3/7-1,2-dithiole-3-thione lb with experimental data <1998VSP77>. These sulfur-rich heterocycles, characterized by a long and weak S-S bond, are represented in accordance to the simple 7t-MO theory by resonance contributor 8, involving a cationic 67t-l,2-dithiolylium part and an anionic thiolate or olate part, and the exocyclic C=S bond is more delocalized than the C=0 bond. However, computational evidence suggests structures with localized bonds and relatively low aromatic delocalization, which is also supported by the low values of the dipole moments, that is, 3.54 D for la and 4.12 D for lb. 

With 2 moles of phosphorus pentachloride reacting with 1 mole of 1,2-dithiol-3-one, a 3-chloro-l,2-dithiolium hexachlorophosphate (20) is obtained. ... 

Chloro- or 3-bromo-1,2-dithiolium ions are easily hydrolyzed to 1,2-dithiol-3-ones. The same result is achieved in acetic acid, which is transformed into acetyl chloride (Eq. 12). °... 

The trivial transformation of 1,2-dithiol-3-ones into 1,2-dithiole-3-thiones by means of phosphorus pentasulfide and analogous sulfurating agents will not be treated in this section. 

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