Silanethione dimerization

In contrast, the reaction of silicocene 41 with carbon disulfide did not give the reaction products derived from the expected silanethione 80 instead a surprising multistep reaction product 89 was isolated (Scheme 31)41b. In the preliminary communication413, the authors misinterpreted the structure of this reaction product as being a simple dimer 90 of the initial intermediate, thiasiliranethione 85, but the X-ray crystallographic analysis later revealed that 89 is the actual product, as shown in Scheme 3141b. 

The reaction of 59 with sulfur is somewhat more complicated (Scheme 19). At low temperatures, an intermediate is observed with a 29Si NMR chemical shift of +122.6 ppm this deshielded value is consistent with the silanethione, 69. Wanning of the solution produces the 4-ring dimer, 70. But when the reaction is carried out at room temperature this intermediate is not observed, and the major product is the trisilicon compound 71 in which a molecule of diimine has been lost from the central silicon374. The structure of 71 is somewhat similar to that of solid silicon disulfide. 

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. 

Little is known about the reactivity of silanethiones. The reactions that have been invoked to explain the results observed so far and listed above are (i) nucleophilic attack on Si=S leading to an addition, in particular addition of Si-O and Si-S bonds, and (ii) nucleophilic attack on Si=S leading to cycloaddition, in particular 2 4- 2 dimerization and cycloaddition to a silene. 

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. 

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