Silanethiones formation

Thermolysis at 180°C of dithiaphosphadigerminane 1929 leads to transient 2-thia-l,3-digermetane 20, which gives a [4] —> [2 H- 2] decomposition with formation of both dimethylgermene 15 and dimethylgermanethione 21 [Eq. (4)]. Note that a silene and a silanethione were also simultaneously obtained by West et al in a rather similar decomposition by photolysis of a 3-thia-1,2-disiletane,30 a four-membered ring with a Si-Si-S-C linkage. 

Silanethione 80 was also postulated as an intermediate in the reaction of silicocene 41 with isothiocyanates (room temperature/toluene/16 h for methyl isothiocyanate and 65 °C/toluene/5 h for phenyl isothiocyanate), which resulted in the formation of the corresponding intermolecular [2 + 2] cycloadducts (82 and 83) of 80 with the isothiocyanates (Scheme 30)41a b. Under even more drastic conditions (100°C/toluene/20 h), the reaction of 41 with phenyl isothiocyanate gave the five-membered heterocycles 84, which is most likely produced by a second attack of the silicocene 41 on the initially formed [2 + 2] cycloadduct 83 (Scheme 30). 

These results are most likely interpreted in terms of the photochemical cycloreversion of the thiadisiletane 106 leading to the formation of the silene 109 and silanethione 110, though both of them were not isolated. 

The reaction of silylenes 83 and 85 with the chalcogens S, Se, Te resulted in the formation of the respective four-membered heterocycle 102 (Scheme 8) <1996JOM211, 1998JA12714>. For the reaction of silylene 83 with 1 equiv of sulfur low-temperature NMR studies suggest the formation of silanethione 103 which then dimerizes. Reaction of excess sulfur with silylene 83 results in the formation of compound 104 with simultaneous release of the diimine ligand. 

As in the case of extrusion of dimethylsilanone, Mc2Si=0 (10), in the thermolysis of certain silaketenes , a similar type of silanethione (Me2Si=S 112) extrusion was postulated in the flash vacuum pyrolysis of bis(trimethylsilyl)thioketene (113) and (dimethylsilyl)(trimethylsilyl)thioketene (121) as shown in Schemes 37 and 38. In both cases, the formation of all the reaction products (compounds 115-120 for the pyrolysis of 113 shown in Scheme 37 and compounds 118, 120,123 and 124 for the pyrolysis of 121 shown in Scheme 38) can be mechanistically rationalized by processes each initiated by isomerization of the starting thioketenes via a 1,2-shift of a trimethylsilyl group to the corresponding a-thioketocarbenes 114 and 122. Under the pyrolytic reaction conditions used (700 or 768 °C) the intermediate silanethione 112 underwent ready oligomerization to give its dimer 120 and/or trimer 124. 

In 1989 Jutzi and coworkers reported the reaction of decamethylsilicocene 41 with tri- -butylphosphine selenide in benzene at room temperature leading to almost quantitative formation of 1,3,2,4-diselenadisiletane derivative 138, a head-to-tail [2 + 2] cycloaddition reaction product of the initially formed silaneselone 137. The intermediacy of silaneselone 137 was supported by the fact that the reaction in the presence of 2,3-dimethyl-l,3-butadiene resulted in the formation of the corresponding [2-1-4] cycloaddition reaction product 139 (Scheme 41). As in the cases of silanone 44 and silanethione 80, the ligands on silicon undergo a haptotropic rearrangement from jj -CsMes in 41 to ij -CsMes in 138 or 139. Apparently, silaneselone 137 is not kinetically stable enough to be isolated under normal conditions. 

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