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 = 637 nm ( = 140 mol dm cm) and 317 nm ( = 2460 mol dm cm). Whereas excitation of 3 at = 360/365 nm affords an isolable siloxyketene 4 in excellent yields, irradiation at = 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 a hitherto unknown 1,3-hypersilyl migration the population of a second excited singlet state (S, = 317 nm, a rare case of anti-Kasha reactivity), the siloxyrane 5 emerges from the first excited triplet state (T S = 637 nm). These distinct reaction pathways can be traced back to specific energy differences between the S, S and T, 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, IR, NMR spectroscopy and theoretical calculations.