Alkenes with chalcogens

As discussed in Section 1.07.3, several kinds of seleniranes and telluriranes can be synthesized by the reactions of alkenes with the atomic chalcogen generated in the flash photolysis. In the case of seleniranes, carbon diselenide or carbon oxyselenide were used as the source of the selenium atom, while dimethyltelluride was subjected to the flash photolysis to provide the reactive tellurium atoms which react with alkenes to give the expected telluriranes. 

Cycloaddition reactions also have important applications for acyclic chalcogen-nitrogen species. Extensive studies have been carried out on the cycloaddition chemistry of [NSa]" which, unlike [NOa]", undergoes quantitative, cycloaddition reactions with unsaturated molecules such as alkenes, alkynes and nitriles (Section 5.3.2). ° The frontier orbital interactions involved in the cycloaddition of [NSa]" and alkynes are illustrated in Fig. 4.13. The HOMO ( Tn) and LUMO ( r ) of the sulfur-nitrogen species are of the correct symmetry to interact with the LUMO (tt ) and HOMO (tt) of a typical alkyne, respectively. Although both... 

Introduction of two different chalcogen elements to the C-C unsaturated bond is of particular interest from both synthetic and mechanistic viewpoints. Therefore, extensive studies have been carried out on the development of the (RE)2/(R E )2 binary system without using RE-E R compounds, which are difficult to prepare. (Z)-l-Seleno-2-thio-1-alkenes are produced regio- and stereoselectively when a mixture of diaryl disulfides and diaryl diselenides is subjected to a rhodium-catalyzed reaction with alkynes (Equation (68)).193... 

Carbonylation proceeds in the presence of chalcogen compounds without poisoning Pd catalysts. Pd-catalyzed stereo- and regioselective carbonylative double thiolation of 1-octyne with diphenyl disulfide (3) afforded the (Z)-j6-(phenylthio)--unsaturated thioester 4 [2], The thioester 4 can be converted to 3-(phenylthio)-2-alkenal 5 by Pd-catalyzed reduction with HSnBu3 under mild conditions [3]. When propargyl alcohol was subjected to the carbonylation in the presence of either diphenyl diselenide (6) or disulfide 3, 3-phenylselenobutenolide 7 or 3-phenylthiobutenolide was obtained. The transformation involves isomerization of the acylpalladium intermediate 8 to 9 [4]. 

Chalcogen-based catalysts for the Baylis-Hillman reaction have been further developed. 2,6-Diphenyl-4//-chalcogenopyran-4-(thi)ones (61 X = S, Se Y = O, S) catalyse the addition of an appropriate alkene to an aldehyde in the presence of titanium tetrachloride in dichloromethane. The first step is proposed to involve Y-(rather than X-)alkylation, driven by aromatization. The resultant zwitterion (62) then attacks the aldehyde. Although (62) has not been directly observed, the reaction of methyl vinyl ketone with the selenopyranone catalyst (61 X = Se, Y = O) shows NMR evidence of an D-alkylation product and of a titanium alkoxide (63 Z = CH2CH2COMe and TiCb, respectively). 

---