Thermal decomposition of thiones of selected N‐, O‐ and S‐heterocycles under flash vacuum thermolysis conditions was investigated. In the case of six‐membered 4H‐3,1‐benzoxathiin‐4‐thione 6, the course of the reaction depended on the substitution pattern at C(2) (Scheme 3). Thus, the 2‐unsubstituted derivative 6a led to the unstable product 2, which upon treatment with MeOH was converted quantitatively into methyl 2‐mercaptobenzoate (7). The analogous thermolysis of the 2,2‐dimethyl derivative 6b yielded 2‐methyl‐4H‐1‐benzothiopyran‐4‐thione (8) as a sole product. In the case of thiophthalide derivatives 15, a thermal rearrangement in the gas phase leading to the corresponding benzo[c]thiophen‐1(3H)‐ones 16 in high yields was observed (Scheme 6). Unexpectedly, thionation of 1,3‐oxathiolan‐5‐one 17 with Lawesson's reagent under standard conditions led to 1,2‐dithietane derivative 19, which, after the gas‐phase thermolysis, underwent a ring enlargement to yield 3H‐1,2‐dithiole 20 (Scheme 7). The six‐membered 4H‐1,3‐benzothiazine‐4‐thione 21 was shown to give three products: phenanthro[9,10‐c]‐1,2‐dithiete (22), 3H‐1,3‐benzodithiole‐3‐thione (23), and N‐(3H‐1,2‐benzodithiol‐3‐ylidene)prop‐2‐en‐1‐amine (24) (Scheme 8). The latter is the product of the initial reaction, whereas 22 and 23 are postulated to be formed as secondary products of the conversion of the intermediate 6‐(thioxomethylene)cyclohexa‐2,4‐diene‐1‐thione (26) (Schemes 9 and 10).
The mechanism of the reaction of 4H-3,1-benzoxathiin-4-thione (3) under flash vacuum thermolysis conditions has been studied by UV-photoelectron spectra. It was shown that in the first step of the reaction at 4008 3 underwent the Schçnberg -Newman -Kwart rearrangement to give 4H-1,3-benzodithiin-4-one (6). Increasing of the temperature to 6508 resulted in the elimination of thioformaldehyde and the formation of benzothiet-2-one (1). Subsequent elevation of the temperature prompted presumably the ring opening of 1 to (6-thioxocyclohexa-2,4-dien-1-ylidene)methanone (7) (at 6808), which at 7008 eliminated CO and underwent a Wolff-type ring contraction to give (cyclopenta-2,4-dien-1-ylidene)methanethione (2). UV-Photoelectron spectra were recorded at different steps of the reaction and analyzed considering the theoretical evaluation of ionization potentials.Introduction. . 1 is stable only below À 208 in the solid state; above this temperature, rapid polimerization, as well as partial di-and trimerization takes place. Above 8008, the loss of CO from 1 and a Wolff-type ring contraction to give (cyclopenta-2,4-dien-1-ylidene)methanethione (2) were mentioned [4] (Scheme 1).It is noteworthy that the analogous 2H-naphtho[2,3-b]thiet-2-one was successfully isolated and identified at room temperature [4]. Both thiet-2-ones were characterized by IR,
5 ) as an intermediate, has been postulated. UV photoelectron spectra of 2 at variable temperature flash vacuum thermolysis have been recorded and analyzed considering the theoretical evaluation of ionisation potentials.
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