1.4- Dithiin, structure

In THF, (53) with tetraphenylcyclopentadienone and with tetramethylpiperidine, as base, gave the imexpected dithiin (86) in low yield (Equation (2)) together with the product of reaction of the aryne with the very hindered amine used. The dithiin structure (86) is interesting since the aryne appears to have been incorporated twice. A possible pathway for its formation is proposed in <89PS(43)261,91JCS(P1)317>. 

W. Schroth, E. Hintzsche, H. Jordan, T. Jende, R. Spitzner and I. Thondorf, 1,2-Dithiins and precursors. 17. Synthesis and properties of thieno annellated 1,2-dithiins - structural influence on color. Tetrahedron, S3, 7509-7528... 

The results are critically dependent on the level of theory. However, a stepwise mechanism with closed shell structures along the reaction path was found to be lower in energy than a concerted reaction. An all-cw conformer of 172 is reported to be a transition state rather than an intermediate. Similarities of the conformational isomers of the intermediate 2-butenedithial 172 with the dinitrosoethylenes discussed in Section IV,c are evident. 3,6-Diamino-substituted dithiins are predicted to be more stable in the open-chain bisthioamide structure [95JST51]. The... 

The photochemical behavior of a number of substituted derivatives of thiochroman-4-one 1-oxides has been examined by Still and coworkers192-194. These authors also report that rearrangement to cyclic sulfenates, with subsequent reaction by homolysis of the S—O bond, appears to be a particularly favorable process. For example, ultraviolet irradiation of a solution of 8-methylthiochroman-4-one 1-oxide (133) in benzene for 24h afforded a single crystalline product which was assigned the disulfide structure 134 (equation 54). More recently, Kobayashi and Mutai195 have also suggested a sulfoxide-sulfenate rearrangement for the photochemical conversion of 2,5-diphenyl-l,4-dithiin 1-oxide (135) to the 1,3-dithiole derivatives 136 and 137 (equation 55). 

The structures of 1,4-dithiins such as dithieno[2,3- 3, 2 -f]-l,4-dithiin have been shown previously to be nonplanar they are folded along the S-S axis <1984CHEC(4)973>. Brisse et al. have shown that iV,iV -dimethyl-l,4-dithiin-l,2 4,5-tetracarboxamide 18 and its diselenine analogue are planar (Figure 3) <2000AXC190>. 

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 17. 1,4-Dithiin 21 and the molecular structure of its radical-cation salt 2f+SbF6 determined by X-ray crystallography. The observed lengths (A) of Cl-S and C1-C2 in 2f+SbF6 are 1.72(1) and 1.31(2) to be compared with 1.324(3) and 1.760(2) determined for neutral 21, respectively. Also shown is the thianthrene radical cation 23 +. 

Henning et al. (2006) used spectroelectrochemical and DFT methods to follow conformational transition of 3,6-diphenyl-l,2-dithiin by one-electron oxidation. The primary cation-radical is flattened partially. This cation-radical was fixed at 223 K. Heating up to 293 K provided this cation-radical with an additional energy. It resulted in the formation of an entirely planar structure with complete spin delocalization within the molecular framework. The transition is depicted in Scheme 6.22. 

The synthesis of 1,4-dithiines from arenes and heteroarenes has been known for more than 100 years. The reaction of quinoline and sulfur monochloride gave dithiodiquinoline 152 (R = R = H) (1896JPR340). The structure of such compounds has been correctly identified only recently (1976PJC785, 1997JCR(S)435 Scheme 80). 

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>. 

In total contrast, the unusual dithiin 25 shows a marked tendency for the insertion of sulfur with the formation of the diborneo-l,2,3-trithiepine 120. The structure of 120 was elucidated by X-ray crystallography and the seven-membered ring interconversion studied by dynamic H NMR (AG = 13.0kcalmoP ) <1995T13247>. 

Vinyl-3,6-dihydro-l,2-dithiin 2-oxide has been isolated as one of the main components from garlic Allium sativum) <2001MI867>. Its structure was elucidated by NMR and MS. 

In this chapter, the structures and chemistries of 1,3-dioxins, 1,3-oxathiins, and 1,3-dithiins are described, including both their fully saturated forms (1, 7, and 13) as well as their benzo analogs (6, 11, 12, and 17). The formally fully unsaturated monocyclic structures (4, 9, 10, and 16) contain only one endocyclic double bond with further unsaturation being accomodated by exocyclic double bonds (2, 3, 5, 8, 14, and 15), for example, by the introduction of a carbonyl group. Well known and intensively studied are the Meldrum s acid derivatives 18 and 19. In addition, 1,3-dioxane, 1,3-oxathiane, and 1,3-dithiane moieties can be part of spiro structures as well as hi- and tricyclic analogs. And finally, both the structures and chemistries of the corresponding sulfoxides and sulfones are also reported. 

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