|Ph.D Student||Zborovsky Lieby|
|Subject||Geometric Isomers of Stable Silenes and their|
|Department||Department of Chemistry||Supervisors||? 18? Yitzhak Apeloig|
|Dr. Dmitry Bravo-Zhivotovs|
Silenes, compounds with a silicon-carbon double bond (i.e., R2Si=CR2), were considered to be unstable reaction intermediates for more than a century. However, with the ground-breaking isolation of the first silene in 1981, the chemistry of these compounds developed rapidly. Nevertheless, although much progress was achieved, much of the chemistry of silenes is not fully explored. For example, a basic and widely studied reaction for alkenes as the E/Z isomerization was not yet described for silenes. This is probably due to the small number and variety of available stable silenes. In particular, no asymmetrically substituted silenes which are suitable for the study of the E/Z isomerization reaction were known.
In this work we synthesized the first asymmetrically substituted silene of the type (R3Si)(R'3Si)Si=CHR and studied its E/Z isomerization both experimentally and computationally.
(tBu2MeSi)(tBuMe2Si)Si=CH(1-Ad) (1) was obtained in the reaction of heteroleptic silyllithium (tBuMe2Si)2(tBu2MeSi)SiLi (2) with (1-Ad)C(O)H in benzene. 1 was obtained predominantly as the E isomer (E:Z ratio of 95:5).
Irradiation of the obtained mixture of 1 at 250 or 350 nm allowed the enrichment of the reaction mixture with the less thermodynamically favorable Z isomer resulting in a photostationary 1-E:1-Z mixture of 53:47. Heating the resulting photostationary mixture at 65-87ºC shifts the reaction mixture composition towards the more stable E isomer until it reaches the thermodynamical equilibrium of 1-E:1-Z of 97:3.
The kinetics of the 1-Z/1-E interconversion was studied at 65-80°C and the activation parameters of isomerization were determined. The Z/E isomerization has an experimental activation barrier (Ea) of 24.4 kcal/mol and the entropy of activation (ΔS≠) is -13 e.u. The strongly negative ΔS≠ suggests a highly ordered transition state. Quantum mechanical calculations (at BP86-D3/def2-TZVP(-f)//BP86-D3/def2-TZVP(-f)) were used in order to study the isomerization mechanism. Based on these calculations, and the experimental findings, we conclude that the most plausible mechanism for the isomerization of 1 is the migration-rotation mechanism via a silylene intermediate rather than the classical rotation mechanism operative in alkenes, which for 1 has Ea=29.6 kcal/mol.
In conclusion, in this work we describe for the first time the synthesis of the first two geometrical isomers of a stable silene. It is also the first reported study of the kinetics and mechanism of the EàZ isomerization of a silene finding that the isomerization mechanism which occurs viamigration-rotation is very different from the rotation isomerization mechanism of alkenes.