Photorearrangement of Bishypersilyl-1,2-dione: The Wavelength Matters

We report on the synthesis, isolation and unique photoreactivity of a novel symmetric bissilyl-1,2-dione, 3. Its UV/Vis spectrum reveals two remarkable well-separated n→π* absorption bands at λmax = 637 nm (ε = 140 Lmol-1cm-1) and 317 nm (ε = 2460 Lmol-1cm-1). Irradiation at λ = 360/365 nm affords an isolable siloxyketene 4, whereas irradiation at λ = 590-630 nm leads to selective formation of siloxirane 5 by a 1,4-trimethylsilyl migration in high yield. These remarkable wavelength-dependent rearrangements are based on characteristic photochemical reaction pathways. Irradiation at 360/365 nm populates a second excited singlet state (S2) which triggers a hitherto unknown 1,3-hypersilyl migration yielding 4. At longer wavelengths (590/630 nm), the populated first excited singlet state (S1) undergoes intersystem crossing (ISC), and 5 emerges from a triplet (T1) precursor. We have established this reaction mechanism by spectroscopy (optical, in-situ IR, and NMR) and theoretical calculations. NMR and X-ray crystallography reveal the structural features of the products.