Thietanes from thiocarbonyl compounds

The formation of thietanes from thiones and olefins has been less exploited for synthetic purposes than the corresponding oxetane-forming reaction. It should be remarked that thiocarbonyl compounds very often undergo efficient photoreactions from the second excited singlet state S2 U4). One interesting synthetic application is found in the photochemical preparation of quinolines from N-thioamides (4.84)498). The primary photochemical step is assumed to be the intramolecular thietane formation. 

Studies with other thiocarbonyl compounds reveal a different reactivity pattern from that of thiones in the presence of olefins. 0-Alkyl thiobenzoates do not form dithiane products. Instead, if an olefin carries no bulky substituents, the intermediate diradical cyclizes directly to the thietane ring (Eq. 4). The excited (n, 71 ) state of thiophosgene reacts with alkenes to... 

A silver ion-mediated desulfurization of thiocarbonyl compounds 163 or 164 followed by condensation with benzohydroxamic acid in the presence of excess triethylamine affords, respectively, dioxazole derivatives 165 or 166 (Scheme 25) <2002H(57)143>. Other reports concern the synthesis of l,4-(oxa/thia)-2-azoles 170-173 from unusual four-atom components such as the sulfur diimides 167, the thietan sulfimide 168, and the carbimidoyl chloride 169 <1996CHEC-II(4)530>. 

Similarly to carbonyl compounds (Section 6.3.1), thiocarbonyl compounds abstract hydrogen upon irradiation however, both n,7t and n,n excited states are reactive and the hydrogen atom can be added to either the sulfur (Table 6.17, entry 1) or carbon (entry 2) atoms of the C=S bond. Aliphatic and aromatic thiocarbonyl compounds can also undergo photocycloaddition to unsaturated compounds from both singlet or triplet excited states to form thietanes (analogously to the Paterno Biichi reaction see Section 6.3.2) (entry 3) or 1,4-dithianes. On the other hand, fragmentation of the S C bond is a typical primary process observed in excited sulfones and sulfonates (entry 4), followed by efficient SO2 extrusion from the radical intermediate. 

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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