1.3- Dithiins

Standard procedures. The benzo[6]thieno[/]thiepin derivatives (31) and (32) were obtained by reaction of the thiepinone (30) with the appropriate amines in benzene in the presence of titanium tetrachloride. 

Sindelaf, J. MetySova, J. Mety , J. PomykaCek, and M. Protiva, Coll. Czech. Chem. Comm., 1970, 35, 3721. 

4-Dithiepin (16) has been synthesized and the derived anion shown not to be aromatic. A study of the electronic spectra of the tricyclic compounds (17) (X = CHg, NH, O, CO, S, or Se) appears to indicate a non-planar (boat) conformation for these systems.  

Theoretical aspects of thiepins were reported earlier (see Vol. 3, p. 755). A general method for evaluating resonance energies has been successfully applied to thiepin.  

2-Dithiins.— The cyclic structure of 1,2-dithiins has been further substantiated by a determination of the heat of formation of 3,6-diphenyl-l,2-dithiin.  

The reaction of the disulphide C1C(E)=C(E)SSC(E)=C(E)SCN(E = COaMe) with sodium thiophenolate unexpectedly yielded the dithiin (18). Addition of sulphene or phenylsulphene to 3-aminovinyl-thiones afforded 1,2-dithiin 1,1-dioxides (see Vol. 3, p. 238) or their dihydro-derivatives. Oxidation of 1-thiochromene with selenium dioxide yielded the novel heterocycle (19). 

Photochemically generated diatomic sulphur added to l,2-bis(methylene)-cyclohexane to afford a 3,6-dihydro-l,2-dithiin. The dihydrodithiin (20) was formed by thermal rearrangement of a 1,3-dithiin. Addition of disulphur monoxide to 2,3-disubstituted buta-1,3-dienes yielded the rather unstable dihydrodithiin 1-oxides (21). 

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When treated with sodium disulfide in ethanol, deca-2,4,6,8-tetrayne (53) gives dithiafulvene 54 rather than 1,2-dithiins 55 (67CB107). 

The addition of benzyl and t-butylmercaptans to diacetylene and its symmetric disubstituted homologs 57 affords l,4-di(benzylthio)- (59) or l,4-di(t-butylthio)-buta-1,3-dienes (61), respectively, from which 1,2-dithiins 55 are formed (65ZC352 67AG685 85KGS1443 96T12677 96USP5453500). 

In contrast with the normal behaviour of aliphatic thioketones, 3-exo,3 -exo-(lR,l R)-bithiocamphor cannot exist as enethiol, since the latter is obtained by reduction of its 1,2-dithiine, immediately stabilized by 1,5-prototropic rearrangement.6... 

Tetracyanoethylene (TCNE)175,176 reacts with thiobenzophenones to yield 2,3-dihydrothiophene, thiophene, and 1,2-dithiin derivatives, thus depending on temperature. Okuma suggested a mechanism176 for the formation of 2,3-dihydrothiophenes through [2+2] and [4+2] sequential cycloadditions. 

The one-electron oxidation of 1,2-dithiin 20 with 1.5 equivalent of SbCl5 under vacuum at room temperature gave a bright yellow solution that exhibited a nine-line ESR signal. The optimized structure obtained by theoretical calculations (B3LYP/6-31G(d)) for the radical cation 20 + was the one with a... 

Figure 15. Two-electron oxidation of 1,2-dithiin 20 and 1,4-dithiin 21 with the lH NMR chemical shifts (ppm) of the bridgehead protons.

Anti-aromatic 1,2-dithiins 179 display properties opposite to those of 1,4-dithiins 180, whose dications show aromatic stabilization. Unlike other antiaromatic compounds, the 1,2-dithiin derivatives, with eight jr-electrons (such as 181 and 182), appear in... 

In wet acetonitrile, the oxidation of diaryldisuUrdes [119] and dialkyl disulfides (Aik 7 f-Bu) [120] affords the corresponding aryl and alkylthiosulfonates in good synthetic yields (Eq. 15). Thus, the oxidation of a cyclic disulfide, dibenzo(c,e)-1,2-dithiin (1,1) does not affect the S—S bond and results in a corresponding thiosulfonate, dibenzo(c,e)-l,2-dithiin-l,l-dioxide(Scheme 27) [121]. Such oxidized products can form in wet acetonitrile as well as in a dry solvent, but in the latter case this is probably a result of disproportionation of the primarily... 

A thiophene ring can also be produced from two methylene groups. Here 2,5-dicarbethoxy-3,4-dicyanomethylthiophene 19 reacted with sulfur mono-chloride to give tetrasubstituted thieno[3,4-c]thiophene 20 in moderate yield (2002JCX72453). A mechanism for the thiophene 20 formation was proposed and 1,2-dithiine derivative 21 was likely to be an intermediate (Scheme 10) because sulfur monochloride gave higher yields of 20 than SCl2. At the next step (21 20), sulfur monochloride apparently acted as an oxidant. 

Heterocycles with two sulfur atoms obtained from sulfur monochloiide, for example, 1,2-dithioles, 1,2,3-dithiazoles and 1,2-dithiines, are the most anticipated compounds because they are obtained by a direct insertion of two sulfur atoms during heterocyclic molecule construction. But even in that case some... 

Only two examples of the 1,2-dithiine synthesis from biphenyls are known (1996NJC1031), although this transformation is the expected one. Dibenzodithiin 155 was formed in a reaction of 3,3, 4,4, 5,5 -hexamethylbiphenyl 154 with sulfur monochloride at low (0-5 °C) temperature. At room temperature the main product was bis[l,2]dithiine 156. Surprisingly, monodithiine 155 did not convert to bisdithiine 156 after treatment with S2CI2, and this probably implies that the addition of two sulfur monochloride molecules to biphenyl 154 took place simultaneously (Scheme 82). 

Other heterocyclic compourtds containing four sulfur atoms - tetrathiocines 196, 197 - were synthesized from activated aromatic compounds, in particular 1,2-dialkoxybenzenes or 2,3-dialkoxynaphthalenes, and sulfur monochloride in acetic acid in fairly good yields (1989PS111 Scheme 97). Biphenyl 154 treated with S2CI2 under the same conditions yielded 1,2-dithiines (see Section 4.3). 

Dithiins, Partially and Fully Saturated Analogs References... 

As in the case of the 1,2-dioxins, the 1,2-dithiins exist in various states of saturation, oxidation, and benzoannelation (cf. Scheme 1, 17-27) and they have been studied in detail both theoretically and experimentally. Not only were the conformations of the ring and attached substituents investigated, but the valence isomerism of 1,2-dithiin by both NMR and high-level ab initio molecular orbital (MO) calculations and the dithiol/disulfide equilibrium by MP2 calculations were also examined. The latter equilibrium has been applied successfully as a luminescent molecular switch (cf. Section 8.10.2.1). Finally, as a very interesting 1,2-dithiin derivative, the synthesis, structure, and reactivity of the (-l-)-camphor-derived analog 25 and its sulfoxide 26 and sulfone 27 have been reported. Both the synthesis and the antimalarial activity of the 2,3-dioxabicyclo[3.3.1]nonane pharmacophore 28, which contains the 1,2-dioxane moiety, have been reviewed recently <2006BML2991>. 

Recently, synthetic methods for preparing 1,2-dioxins and their benzo- and dibenzofused derivatives <2004SOS(16)13> and 1,2-dithiins <2004SOS(16)39> have been reviewed. 

Figure 1 Calculated ring current effects of 1,2-dloxln, 1,2-oxathlln, and 1,2-dithiin (in comparison with the ring current effects of cyclobuta-1,3-dlene, benzene and the anisotropic effect of buta-1,3-dlene) shielding isochemical shielding surface (ICSS) of -0.1 ppm, gray, and deshielding ICSS of 0.1 ppm, dark gray.

By irradiation with visible light (436 nm, 2 h) in an argon matrix at 25 K, 1,2-dithiin was transformed into s-trans-Z-s-fir-2-butenethial 31, which is twisted by ca. 40° away from planarity (Scheme 4) <1996JA4719>. 

The rotational spectrum of 1,2-dithiin was measured using a pulsed-beam microwave spectrometer in the 8-18 GHz range <1996JSP(180)139> by Stark effect measurements, the electric dipole moment was also determined (/ta = 1.85 D). The molecule proved to be of C2 symmetry with a twisted conformation about the S-S bond and a C-S-S-C dihedral angle of 53.9... 

In spite of the absence of a typical chromophore, 1,2-dithiin is a bright reddish-orange color. Absorption maxima were found at 451 (2.75 eV), 279 (4.36 eV), and 248 nm (5.00 eV), and the colored band was assigned to a A excitation <1991JST(230)287>. The main reason for the colored absorption of 1,2-dithiin is the low HOMO-LUMO gap of the KS orbitals which amounts to only 3.6 eV (HOMO = highest occupied molecular orbital LUMO = lowest unoccupied molecular orbital KS = Kohn-Sham) <2000JMM177>. By comparison, saturated 1,2-dithiane is colorless (290 nm). 

The spectroscopic data of a number of 1,2-dithiin derivatives (Scheme 13) are summarized in Table 2. Comparison of the H NMR spectroscopic data of the dithiins 51 with those of the corresponding thiophenes shows that both the a- and ft- protons are significantly shielded in 51, reflecting the presence of the ring current effect in the thiophenes <2000JA5052>. 

Table 2 Selected spectroscopic data for 1,2-dithiins <2000JA5052>...

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