Silanones 4 + 3 cycloaddition reactions

In analogy to the reaction of decamethylsilicocene 41 with carbon dioxide leading to the generation of intermediary silanone 44, silicocene 41 was allowed to react with carbon oxysulfide under very mild conditions (—78 °C/toluene) to give the corresponding 1,3,2,4-dithiadisiletane derivative 81, a formal head-to-tail [2 + 2] cycloaddition reaction product of the initially formed silanethione 80 (Scheme 29)41b. 

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. 

A siloxetane 526 as an intermediate from a [2 + 2] cycloaddition of silene 241 with acetone has been formulated by lshikawa134. It extrudes a silanone equivalent to give the vinyl ether 527. The second regioisomeric silene 242 generated together with 241 by photolysis of 240 undergoes an ene reaction instead (equation 179)134. 

The pathway of the reaction of 1 with the above heterocumulenes is discussed as follows (see Scheme 2) The first intermediate is a highly reactive [2+1] cycloaddition product or its ring-opened isomer, which easily loses CO or in the case of isothiocyanates the corresponding isonitrile RNC to give the silanone or silathione 2. The formation of CO was proved in the reaction of 1 with CO2 by reaction with the iron complex (HsC5)Fe(SMe2)3 BF4 [7]. In the reaction of 1 with isothiocyanates the formation of RNC was proved using NMR spectroscopy. The intermediates of the type Cp 2Si=X (X = O, S) are not stable and stabilized via different routes to the final products. In the reaction of 1 with CO2 the double bond species Cp 2Si=0 is transformed by CO2 to the formal [2+2]... 

In the case of the reaction of 1 with COS the intermediate silathione dimerizes to the dithiadisiletane derivative III. In the reaction of 1 with RNCS the silathione does not dimerize, but it is trapped by the respective isothiocyanate in form of the formal [2+2] cycloaddition product IV. The intermediates Cp 2Si=X (X = O, S) (2) could not be isolated but derivatized by trapping reactions. In the presence of t-butyl methyl ketone the silanone (Cp 2Si=0) reacts in an ene-type reaction [9] to give the addition product 5 the silathione (Cp 2Si=S) is transformed in a first step to an addition product analogous to 5, in a further step this species reacts with 1 in an oxidative addition process to form the final product 4 (see Scheme 2). 

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