Photo-induced E/Z Isomerization of Stable Silenes

Lieby Zborovsky, Chemistry, Technion-Israel Institute of Technology, Haifa, Israel

The photochemically promoted E⇌Z isomerization of alkenes is a very important class of reactions in chemical and biological processes and it has long been a subject to intense research. In contrast to alkenes, the chemistry of their silicon analogues, silenes (R₂Si=CR₂) and disilenes (R₂Si=SiR₂), is much less investigated.

In this work, we describe for the first time the stereo-specific formation of stable silenes (1) and demonstrate their photoinduced Z⇌E isomerization. We report the preparation of a novel type of stable asymmetric, hydrogen substituted silenes (tBuMe₂Si)(tBu₂MeSi)Si=C(H)R (1) (R=1-Ad, 1,3,5-triisopropylphenyl). 1 is obtained in a one-pot reaction via sila-olefination of (tBuMe₂Si)₂(tBu₂MeSi)SiLi with RC(=O)H in benzene at room temperature. Formation of 1 is stereospecific, giving the Z-isomer (1a) as the sole product. Upon irradiation with a full-spectrum light at room temperature, 1a is quantitatively transformed after 10 hours to the E-isomer (1b). We conclude that the Z-isomer (1a) is the kinetic product, and that it transforms photochemically to the thermodynamically more stable, E-isomer (1b). The mechanism of the Z to E isomerization was computationally investigated, and will be discussed.