Thiopyran-2-thione

Hydroxy-THISs add to the C-C bond of diphenylcyclopropenethione (181. Inner salts without substituents in 5-posnion react similarly with diphenylcyclopropenone (Scheme 10) (4, 18). Pwolysis of the stable adducts (9) leads to rupture of the R-C-CY bond. Subsequent ring closure yields 10. When Y = O. 10 eliminates COS. producing 2-pyridone. When Y = S. 10 is isolated together with its isocyanate extrusion product, a thiopyran-2-thione (18). 

H NMR data for the adduct between CgoFis and 277-thiopyran-2-thione, generated in situ from tetrathiofulvene (TTF), are shown in Figure 34 the magnitude of J s A 6.6 Hz is unusual as this is normally ca. 10 Hz <2003CEJ2008>. 

Figure 34 1H NMR data for the adduct between C6oF18 and 2/-/-thiopyran-2-thione.

Figure 35 1H NMR data for anti and syn iminium iodides derived from 2H-thiopyran-2-thione 145.

Thiopyran-2-thione 334, derived from tetrathiafulvalene by loss of CS2, undergoes a [2+2] cycloaddition with the fullerene C60F18 involving the more electron rich 5,6-double bond (Equation 63) <2003CEJ2008>. 

The cycloaddition of CS2 to the 1,4-dilithio species obtained from derivatives of 1,4-diiodobuta-l,3-diene affords thiopyran-2-thiones. However, formation of substituted thiophenes through cleavage of one of the C=S bonds is a significant competing reaction (Equation 173) <2002TL3533>. It appears that bulkier substituents on the terminal C atoms favor thiopyranthione formation <2003ARK155>. 

Thiopyran-2-thiones can also be synthesized through the reaction of alkynic dienophiles with the disulfide 502 (Equation 174) <1996BCJ2091>. 

Acetophenones react with 3,3-dichloropropenal to give 1-aryl-5,5-dichloropenta-2,4-dienones 505 which on treatment with thiourea afford 6-aryl-2/7-thiopyran-2-thiones (Scheme 188) <2000MC77>. 

Reaction of 4-fluoro5-5-(l,l,2,2-tetrafluoroethyl)-3//-dithiole-3-thione with 2equiv of Na2S affords 3,4-difluoro-2-mercapto-l,6,6aA-trithiapentalene after an acidic work-up. Further reduction with Na2S gives 3,5-difluoro-4-mercapto-2//-thiopyran-2-thione 507 in >30% overall yield. Methylation of the Na salt of the pentalene affords an 1 1 mixture of methylsulfanyl derivatives of a 2//-thiopyran-2-thione 508 and 4//-thiopyran-4-thione 509 (Scheme 190). DFT calculations predict the stability sequence for the thiones is 507 > 508 > 509 <2006JFC(127)774>. 

Treatment of 3-(2-oxocyclopentyl)propanoic acid with Lawesson s reagent gives 3,4-dihydrocyclopenta[A]thiopyran-2-one 535 together with smaller amounts of the thiones, cyclopenta[3]thiopyran-2-thione and a dihydrocyclopen-tathiopyran-2-thione (Equation 191) <1992DOK354>. 

Enamines treated with 5-unsubstituted l,2-dithiole-3-thiones (3b) form 2//-thiopyran-2-thiones (85) (77BSF1142) in a reaction which involves initial attack at C-5 of the dithiole (Scheme 13), and anions of acenaphthenone react at the 5-position of dithiolethiones, eventually forming acenaphthothiopyran-2-thione derivatives (73BSF3334). 

Diynes react with CS2 and isothiocyanates in a Ru-catalysed [2+2+2] cycloaddition to yield cyclopenta[c]thiopyran 2-thiones and 2-imines respectively <02JA28>. Cycloaddition... 

A number of thiopyran-2-thiones have been alkylated with a-halogeno-ketones yielding thiopyrylium cations of the type 163 (74BSF1356 80BSF(2)427 84AP938 86MI3 87FES465],... 

A useful reaction is the treatment of thiopyran-2-thiones with peracetic acid to form 2-unsubstituted thiopyrylium ions. Although the reaction actually involves reduction of the ring system, the exocyclic sulfur atom is oxidized and eliminated as sulfate (70CJC3388 74CJC3021). For example, 2,4-diphenylthiopyrylium ion (154) has been prepared from the thiopyran-2-thione 164. The reaction does not succeed with thiopyran-4-thiones thus reaction of 2,6-diphenyl-4//-thiopyran-4-thione [122 (R = = Ph)] with peracetic acid gives the thiopyran-4-one 140... 

Chlorothiopyrylium ions can also be formed in situ. Thus the thiopyry-lium ion 267 has been prepared by reaction of 2,6-diphenyl-4//-thiopyran-4-one [140 (Z = S, R = Ph)], POCIj and the appropriate anilino derivative (83HCA2165). An analogous reaction occurs between the thiopyran-2-thione 164, POCI3 + PCI5, and 2,4-diphenylthiophene as an activated aromatic compound [77JCS(P1)1511]. 

Thia-thio- a-pyrone 1,2-Dithiopyrone 2 H-Thiopyran- 2-thione... 

Starting from enamines of methyl ketones (30), 4,6-disubstituted thiopyran-2-thiones (8) are obtained ... 

The pK value has so far been determined only for the protonation of 2H-thiopyran-2-thione.88 The protonation takes place ontheexo-cyclic atom, giving a structure resembling a derivative of the pyrylium (X = 0) or thiopyrylium (X = S) ion. Comparative measurements have also been carried out for trithione (Table XIV). It is evident that... 

H-thiopyran-2-thione, which is more basic than trithione, exhibits a higher 77 electron density on the exocyclic sulfur. It would be more correct, of course, to compare the difference in the 77 electron energies of the protonated and the nonprotonated forms. However, for conjugated systems these quantities are interrelated,107 and it is consequently possible to estimate the order of basicity of structurally related compounds by means of the 77 electron densities. The expected order of increasing basicity of the following four substances is (the 77 electron densities are given in parentheses) ... 

Cobaltacyclopentadienes react with CS2 to form thiopyran-2-thiones, and with methyl isothiocyanate to form pyridine-2-thiones, respectively, in two little-explored analogs of the processes described above. 

Whereas the former condensation with retention of the 1,2-dithiole ring required the presence of a hydrogen atom as R, this is not necessary in the formation of thiopyran-2-thiones, as shown in Table III. 

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