Thiocarbonyl compounds thermal

Hydrazines react with thiocarbonyl compounds, such as dithioesters, to afford directly symmetrical 1,3,4-thiadiazoles via a dithioacylhydrazine intermediate, which can be isolated and converted to the thiadiazole upon treatment with an electrophilic reagent or under thermal conditions (see Section 5.10.9.1.1). Examples of the direct formation of thiadiazoles from hydrazines and thiocarbonyls can be found in CHEC-II(1996) <1996CHEC-II(4)379>. 

As indicated above, the most important of the thiocarbonyl compounds is thiocarbonyl fluoride, CF2=S. It has been synthesized by a number of routes (43), the most convenient of which for laboratory investigation is dimerization of thiophosgene followed by conversion of the dimer to 2,2,4,4-tetrafluoro- i, 3-dithietane (I) and thermal cracking of the dithietane (44). 

Thiocarbonyl compounds can be generated thermally in the gas-phase from a variety of precursors. Bock et al. [77JCS(CC)287 82CB492] have shown that pyrolysis of 1,2,4-trithiolane derivatives (81) is especially advantageous for this purpose. The fragmentation reactions were monitored by PE spectroscopy. By this method thioaldehydes including thioformaldehyde and thioketones that polymerize readily were obtained. 1,2,4-Trithiolane 4-oxide (82) yields a mixture of thioformaldehyde and thioformaldehyde oxide. 

The products of the thermolysis of 3-phenyl-5-(arylamino)-l,2,4-oxadiazoles and thiazoles have been accounted for by a radical mechanism.266 Flash vacuum pyrolysis of 1,3-dithiolane-1-oxides has led to thiocarbonyl compounds, but the transformation is not general.267 hi an ongoing study of silacyclobutane pyrolysis, CASSF(4,4), MR-CI and CASSCF(4,4)+MP2 calculations using the 3-21G and 6-31G basis sets have modelled the reaction between silenes and ethylene, suggesting a cyclic transition state from which silacyclobutane or a trcins-biradical are formed.268 An AMI study of the thermolysis of 1,3,3-trinitroazacyclobutane and its derivatives has identified gem-dinitro C—N bond homolysis as the initial reaction.269 Similar AMI analysis has determined the activation energy of die formation of NCh from methyl nitrate.270 Thermal decomposition of nitromethane in a shock tube (1050-1400 K, 0.2-40 atm) was studied spectrophotometrically, allowing determination of rate constants.271... 

The cleavage of a S—S bond has been extensively used for the preparation of thiocarbonyl compounds and this elimination can be induced thermally or by base... 

Thermal cycloreversion of the adducts can be accomplished at a convenient rate when heated in toluene under reflux. If a new diene is present in the reaction mixture, the thioaldehyde thus generated in the retro-Diels-Alder reaction may give a new adduct. Therefore, adducts 81 and 82 act as thioaldehyde or thioketone transfer reagents. These adducts dissociate reversibly on heating, thus ensuring that the concentration of the labile species remains very low. For this reason, polymerization is not a serious problem especially in the case of thioaldehydes224. The transient thiocarbonyl compounds can be trapped not only by dienes but also by 1,3-dipolar cycloadditions332 (equation 85). 

The sulfur analogues of enolates have recently received attention in the context of synthetic applications. Thiocarbonyl compounds bear a-protons which are rather acidic. Kresge et al. [120] has shown that their pKas are 10 units less than those of carbonyl compounds. Thus enethiolates are easily formed with a variety of bases, and they exhibit thermal stability [1]. They are ambident nucleophiles and the sulfur vs carbon regiochemistry has been rationalised by Anh [119] using frontier orbital treatment. 

Head-to-tail dimerization of thiocarbonyl compounds to 1,3-dithietanes (Scheme 105) occurs thermally <1977JOC2345> and photochemically <1981JOC3911>, or catalyzed either by a base <1973T2759> or a sulfonic acid <1997BCJ509>. For example, synthesis of dithietane 213 in excellent yield was performed by the room temperature dimerization of the thione 212 (Scheme 106) <2004TL7655, CHEC-III(2.18.4.2)838>. Diaryl thioke-tones are resistant to dimerization. Some thiones dimerize to 1,3-dithietanes <1975BSF(2)1670>. 

Thiocarbonyl compounds participate in a wide variety of reactions leading to cyclic compounds. In the majority of cases such cycloaddition reactions are thermally or photochemically induced and occur with a full or high degree of stereoselectivity. 

Thermal equilibration, 260, 310, 321 Thiobenzophenone, 328 Thiocarbonyl compounds. 254 Third row elements. 105. 107 Three-quantum process, 459 Through-bond interaction, 121, 292. 402, 409-10... 

High-level quantum-chemical calculations on the 3 + 2-cycloadditions of thioformaldehyde 5 -imides, S -methylide, S-oxide, and 5-sulfide have been reviewed. Theoretical studies on the 1,3-dipolar cycloaddition between thioketene 5-oxide and methyleneimine show that this reaction is concerted but non-synchronous. Adamantanethione 5-methylide reacts with thiocarbonyl compounds to produce 1,3-dithiolanes. A density-functional-theory study of the cycloaddition of the sulfine H2CSO predicts the 2 + 3-mechanism having the lowest pathway, with an activation barrier of 12.3kcalmoP. R The thermal and photochemical reactions of fluorenethione 5-oxide (69) with cyclooctyne (70) involves an initial 1,3-dipolar cycloaddition to produce the adduct (71), followed by an efficient sulfur transfer to cyclooctyne to produce the enone (72) and the dithiin (73) (Scheme 26). ° ... 

Diaryl-substituted bi(thieno[3,4-b]thiophene)s 156 were generated by intramolecular double 5-exo-dig cyclizations of bis[(aiylthiocarbonyl)thien-2-yl]acetylenes 155 [58]. For example, the reaction of 154 with Lawesson s reagent in refluxing toluene led to thiocarbonyl compound (155), thermal 5-exo-dig cycliza-tion of which furnished 156 in 82% yield (Scheme 35). 

The photocycloaddition of thiocarbonyl compounds to olefins often gives thietans. The thietan (137) formed from thiobenzophenone and acrylonitrile at 366 nm is derived from the thermal decomposition of the 1,3-dithian (136) believed to be produced via the second excited singlet state (very small amount of thietan. The thietan reacts further with thiobenzophenone to give disulphide (138). ... 

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