4//-l,3-Dithiins

At the MP2 level of theory, the half-chair conformer of 4//-l,3-dithiin 55 is 2.0kcalmoP more stable than the boat conformer (Equation 4) <1998JCC1064> and has a calculated twist angle of 33.2° the relative stability of the halfchair conformer was attributed to absent lone-pair-lone-pair repulsions and a decrease of torsional strain owing to an... 

A [5 - 2 + 2 + 1] fragmentation followed by cyclization forming a new five-membered ring was observed by FVP studies of 2-propenyl-l,3-dithiolan 1,1-dioxide (79) (95H1967). The reaction mixture consists of four products thiophene (26%), 2,5-dihydrothiophene (80,34%), 4-methyl-2-propenyI-4//-l,3-dithiine (20%), and 2,6-dimethyl-2//,6//-l,5-dithiocine (20%). The last two compounds are formed by [4 + 2] or [4 + 4] dimerization of the intermediate 2-butenethial. Formation of 80 involves a 1,5-H shift of the as-butenethial, followed by cyclization. 

Acyclic alkenic sulfur compounds have also been obtained via pyrolytic rDA reactions. A general route to thioacrylamides utilizing rDA reactions under FVP conditions has been developed. The rDA reaction of the appropriate adducts yields thioacrylamide, V-methylthioacrylamide and NA -dimethyl-thioacrylamide in approximately quantitative yield (equation 13). The high reactivity of these a,3-unsaturated thioamides had previously limited their availability. a,3-Unsaturated acyclic chio-ketones such as (17) were prepared via a rDA reaction using a FVP technique. These highly reactive species dimerize to 4//-l,3-dithiins (18) on warming to-60 °C (equation 14). ... 

Whereas 1,4-dioxin is planar, 1,4-dithiin 173 and 1,4-oxathiin are significantly distorted from planarity [98JST11]. For 173 a planar nonequilibrium structure was computed to be destabilized by about 2.5 kcal/mol with respect to the boat con-former. The half-chair and boat conformations of 3,4-dihydro-l,2-dithiin, 3,6-dihydro-1,2-dithiin, 4//-l,3-dithiin, and 2,3-dihydro-1,4-dithiin 174 were investigated at the MP2/6-31G level [98JCC1064] (see the structures of the corresponding dioxines in Scheme 114). The half-chair conformers are stabilized with respect to the corresponding boat conformers, and the most stable structure was found to be the half-chair conformer of 174. 

Di-2- propenyl disulphide, Di-2-propenyl trisulphide, di-2-propenyl-tetrasulfide, 2- di-propenyl thiocyanate, 3//-l,2-dithiolene, 2-vinyl-4//-l,3-dithiin, 4//-l,2,3-trithiin, 5-methyl-l,2,3,4-tetrathiane and A(Af-di-2-propenyl thiourea as major compound... 

The parent 1,2-dithiolylium ion (4) is readily prepared by treatment of l,2-dithiole-3-thione (3b R = R = H) with hydrogen peroxide in acetic acid (65JCS32). The method may be applied to the alkyl and aryl derivatives with equal success. For cations with 3- and 5-substitution the acid catalyzed reactions of j8-dicarbonyl compounds with hydrogen disulfide or equivalent are best (80AHC(27)l5i), whereas the benzo-1,2-dithiolylium ion (172) and related compounds are best prepared by ring contraction of benzo-l,3-dithiins (171) (63LA(661)84>. 

Thiophosphorsaure -O.O-di-ethylester-0-(4-ethoxycar-bonyl-2H,6H-l,3-dithiin-5-ylester) E2, 694 (Cl - O —En)... 

The formation of naphthothiete (34) from dithiol 41 and naphthothiadiazine (42) is preceded by biradical intermediate 43 as proven by the photolysis of naphthothiadiazine (42) in carbon disulfide. After 15 min irradiation alongside naphthothiete (34) obtained in a 52% yield there also formed in a 22% yield naphtho[l,8-<7e]-2,4-dihydro-l,3-dithiin-2-thione (44) resulting from the reaction of biradical 43 with carbon disulfide (Scheme 7). 

AUyl methyl suUide Dimethyl trisulfide AUyl methyl trisulfide 3-Vinyl-4-H-l, 3-dithiin AUyl methyl disuUide... 

Diene moieties, reactive in [2 + 4] additions, can be formed from benzazetines by ring opening to azaxylylenes (Section 5.09.4.2.3). 3,4-Bis(trifluoromethyl)-l,2-dithietene is in equilibrium with hexafluorobutane-2,3-dithione, which adds alkenes to form 2,3-bis-(trifluoromethyl)-l,4-dithiins (Scheme 17 Section 5.15.2.4.6). Systems with more than two conjugated double bonds can react by [6ir + 2ir] processes, which in azepines can compete with the [47t + 27t] reaction (Scheme 18 Section 5.16.3.8.1). Oxepins prefer to react as 47t components, through their oxanorcaradiene isomer, in which the 47r-system is nearly planar (Section 5.17.2.2.5). Thiepins behave similarly (Section 5.17.2.4.4). Nonaromatic heteronins also react in orbital symmetry-controlled [4 + 2] and [8 + 2] cycloadditions (Scheme 19 Section 5.20.3.2.2). 

Thianthrene is numbered as shown in 1 using current Chemical Abstracts numbering before 1937, the numbering shown in 2 was used in Chemical Abstracts. The ring positions ortho to sulfur have been termed a, and the others are termed y3. Alternative names for thianthrene, found more in older literature, are dibenzo-1,4-dithiadiene, dibenzo-1,4-dithiin, di-o-phenylene sulfide, and diphenylene disulfide. The literature also contains references to o-thianthrene this is dibenzo[c,e][l,2]dithiin, (3). 

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). 

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