Generation of Unsaturated silicon species from acylpolysianes and investigation on their chemical behaviors

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1.Isomerization of acylpolysilanes of importance in silicon chemistry as a convenient method to produce silenes. However, little is known for the mechanism of the isomerization. To know about the mechanism, we determined the kinetic parameters of the thermal isomerization of variously substituted acylpolysilanes, as being ΔH^≠=25.7-29.3 kcal mol^<-1> and ΔS^≠=-11.5--17.5 cal mol^<-1> K^<-1>. The negatively large activation entoropies and the fact that introduction of an electron-withdrawing group on the carbonyl carbon facilitated the isomerization, clearly indicated the intramolecular concerted mechanism. The molecular orbital calculations on a model system also suggested the four-centered transition state. The silenes thermally generated from acylpolysilanes always gave [2+4] adducts as the major products by the reactions with 2,3-dimethylbutadiene. In addition to these, silenes from acylpolysilanes bearing an aryl group on the carbonyl carbon gave ene-adducts as the minor products. However, in contrast to this, silenes from alkyl-substituted acylpolysilanes afforded [2+2] cycloadducts as the minor products.2.Thermal and photocheical isomerization of 2,2-bis(acyl)trisilanes and 1,2-bis(acyl)disilanes was investigated, and it was found that these reactions gave products arising from silene intermediates.