1.4- Dithiins oxidation

Among the reactions of 1,4-dioxins and 1,4-dithiins, oxidation of their tetraphenyl and dibenzo derivatives is important. This reaction leads to the formation of radical cations 7 and dications 8... 

Oxidation of B Ahr s Salt with iodine or thionyl chloride gives tetracyano-l,4-dithiin [2448-55-7] (11), l,4-dithiin-2,3,5,6-tetracarbonitrile (62,63). The dithiin loses sulfur at 210°C to give tetracyanothiophene [4506-96-1] (12), 2,3,4,5-thio-phenetetracarbonitrile, and adds sulfur under the influence of sodium ethoxide to give the isothia2ole [4656-27-3] (13) (63). 

Dicyano-l,2-dithiete [53562-16-6] (14) is thought to be an intermediate when BAhr s Salt is oxidi2ed (64,65). If the oxidation is carried out in the presence of vinyl ethers, dihydro dithiins can be obtained in yields up to 60%. 

Tile preparation of beiizo-l,2-dithiete (264) had been claimed by oxidation of 1,2-benzenedithiol (25JIC318). However, later work has shown that the reaction product was probably a polymeric mixture (61JOC4782). Subsequently, compound 265 was irradiated to give a mixture of CO, sulfur, and dithiin and thiophene derivatives, which could, at least in part, be explained by the formation of 266 (72JHC707). Results of the thermolysis of 267 were also rationalized in terms of the intermediacy of o-dithiobenzo-quinone (the tautomer of 264) (78JOC2084). 

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). 

Tricyclic dithiine derivatives of tetrathiafulvalene (TTF) 32 have been prepared for their increased electropolymerization potential <2000CC1005>. The effect of different tricyclic heterocycles upon the redox properties of TTF analogues was explored, and showed that the furan derivative 33 had an unusual nonplanar conformation that allowed for the attainment of higher oxidation states at relatively low oxidation potentials <2004JMC2822>. 

Lithiation of the dibromodithienyl compound 464 and reaction with sulfur and subsequent oxidation yielded the tetramethyl substituted dithieno[3,Z-c.Z, 3 - ][l,2]dithiin 465 (Equation 126) <1997T7509>. 

The behavior of dithiins, highly characteristic of the annelation with BCO units, are demonstrated by the results of their one-electron oxidation as described below. 

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.

The 1,4-dithiin annelated with two BCO units 21 also undergoes ready one-electron oxidation with 1.5 equivalents of SbF5 in CH2C12 to give radical cation salt 21 SbF6" in 67% yield as brown-colored single crystal, which was stable... 

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... 

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. 

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. 

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>. 

Reactions of cyclopentadienyl- and (pentmethylcyclopentadienyl)iron dicarbonyl 2-alkynyl complexes as well as cyclopentadienylmolybdenum tricarbonyl 2-alkynyl complexes with 4,5-diphenyl-3,6-dihydro-l,2-dithiin 1-oxide 111 were shown to yield transition metal-substituted five-membered ring thiosulfinate esters 112 in moderate to excellent yields (Scheme 27) <19910M2936, 1989JA8268>. These reactions are formal [3-1-2] cycloadditions. When... 

Another effect of the high steric hindrance in dithiin 25 due to the 3,3 -dibornane skeleton is the fact that this dithiin can be oxidized readily (with 1 mol of t-chloroperbenzoic acid (MCPBA) at 0°C) to sulfoxide 26 <1995T13247, 1994TL1973> (Scheme 30), a stmcture which is remarkably stable in contrast with the usual characteristics of sulfoxides in the dithiin series. The six-membered ring interconversion of the sulfoxide 26, slowed obviously by the steric bulk hindrance, was investigated by dynamic NMR (AG ca. 10-11 kcalmoP ). With an excess of MCPBA at elevated temperature, the sulfoxide 26 is oxidized to the sulfone 27 parallel treatment with oxygen, however, failed to yield the same compound. 

Simple side-chain reactions of 1,2-dithiin diols have been conducted. Besides the formation of esters, ethers (R = Me, Et, 7-Pr, cyclopropyl, Ph, pyridyl, cyclopentyl), and thioethers (R = H, TBDMS R = 4 -(4-hydroxyphenyl)-l//-tetrazole-5-thiol), selective oxidation of the primary alcohol groups in the presence of the 1,2-dithiin heterocycle could be readily achieved (Scheme 36) <1995JME2628, 1994SL201>. Additionally, amides, ureas, and carbamates of the dithiin diol were synthesized <1995JME2628>. 

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