Synthesis, structure and isomerization of A₂Si=SiB ₂-type tetrakis(trialkylsilyl)disilenes

Two novel A₂Si=SiB₂-type tetrakis(trialkylsilyl) disilenes: 3(A= t-BuMe₂Si, B = i-Pr₂MeSi) and 4 (A= t-BuMe₂Si, B= i -Pr₃Si) were synthesized in good yields. X-ray structural analysis revealed that both 3 and 4 have a twisted Si=Si double bond with a planar arrangement around each of the unsaturated silicon atoms. The twist angle around the Si=Si double bond is remarkably dependent on the substituents and amounts to 27.95(3)° for highly congested disilene 4. Disilene 3 underwent gradually a formal dyotropic rearrangement of trialkylsilyl substituents to give an equilibrium mixture of 3 and (E)- and (Z)-isomers of the corresponding ABSi=SiAB-type disilenes (2E and 2Z ). The activation energy for the isomerization of 3 to 2Z and/or 2E was found to be much smaller than that for the corresponding isomerization of a tetraaryldisilene system.