Mechanism of the Thermal Z⇌E Isomerization of a Stable Silene; Experiment and Theory.

The E and Z geometric isomers of a stable silene (tBu2 MeSi)(tBuMe2 Si)Si=CH(1-Ad) (1) were synthesized and characterized spectroscopically. The thermal Z to E isomerization of 1 was studied both experimentally and computationally using DFT methods. The measured activation parameters for the 1Z⇌1E isomerization are: Ea =24.4 kcal mol-1 , ΔH≠ =23.7 kcal mol-1 , ΔS≠ =-13.2 e.u. Based on comparison of the experimental and DFT calculated (at BP86-D3BJ/def2-TZVP(-f)//BP86-D3BJ/def2-TZVP(-f)) activation parameters, the Z⇌E isomerization of 1 proceeds through an unusual (unprecedented for alkenes) migration-rotation-migration mechanism (via a silylene intermediate), rather than through the classic rotation mechanism common for alkenes.