120217-04-1Relevant articles and documents
Photocyclization of 2,4,6,2′,4′,6′-hexaalkylbenzils
Wagner, Peter J.,Park, Bong-Ser,Sobczak, Martin,Frey, Joseph,Rappoport, Zvi
, p. 7619 - 7629 (2007/10/02)
Three of the title compounds - the hexamethyl-, hexaethyl-, and hexaisopropylbenzils - all photocyclize both in solution and as solids to 5,7-dialkyl-2-(2′,4′,6′-trialkylphenyl)-2-hydroxy-1-indanones. At wavelengths 4. Moreover, AM1-level semiempirical calculations suggest that a simple exothermic hydrogen transfer can convert the 1,4-biradical triplet dienol to the same 1,5-biradical formed by δ-hydrogen abstraction. The 1,5-biradical has two major conformations, one leading to Z product and an internally OH-O=C hydrogen bonded one leading to E product. The AM1 computations suggest that the two conformations are of comparable energy and thus implicate 1,5-biradicals as the major precursors to hydroxyindanone products. Stern-Volmer quenching studies indicate a triplet decay rate of 5 × 106 s-1 for the hexaisopropylbenzil. The known behavior of structurally similar monoketones predicts such a rate for δ-hydrogen abstraction but a much slower rate for γ-hydrogen abstraction. However, relative quantum efficiencies parallel those for benzocyclobutenol formation from 2,4,6-trialkylbenzophenones (iPr and Et ～0.3, Me ～0.03). The hexa-tert-butylbenzil undergoes very low quantum yield formation of 3,3-dimethyl-5,7-di-tert-butyl-1-indanone and 2,4,6-tri-tert-butylbenzaldehyde, presumably by δ-hydrogen abstraction and highly efficient radical cleavage of the resulting 1-aroyl-1-indanol.
