Wavelength-dependent rearrangements of an α-dione chromophore: a chemical pearl in a bis(hypersilyl) oyster

The symmetric bissilyl-dione 3 reveals two well-separated n → π* absorption bands at λ_max = 637 nm (ϵ = 140 mol⁻¹ dm³ cm⁻¹) and 317 nm (ϵ = 2460 mol⁻¹ dm³ cm⁻¹). Whereas excitation of 3 at λ = 360/365 nm affords an isolable siloxyketene 4 in excellent yields, irradiation at λ = 590/630 nm leads to the stereo-selective and quantitative formation of the siloxyrane 5. These remarkable wavelength-dependent rearrangements are based on the electronic and steric properties provided by the hypersilyl groups. While the siloxyketene 4 is formed via a hitherto unknown 1,3-hypersilyl migration via the population of a second excited singlet state (S₂, λ_max = 317 nm, a rare case of anti-Kasha reactivity), the siloxyrane 5 emerges from the first excited triplet state (T₁via S₁λ_max = 637 nm). These distinct reaction pathways can be traced back to specific energy differences between the S₂, S₁ and T₁, an electronic consequence of the bissilyl substited α-dione (the “pearl”). The hypersilyl groups act as protective ‘‘oyster shell”, which are responsible for the clean formation of 4 and 5 basically omitting side products. We describe novel synthetic pathways to achieve hypersilyl substitution (3) and report an in-depth investigation of the photorearrangements of 3 using UV/vis, in situ IR, NMR spectroscopy and theoretical calculations.