Photocyclization of 2,4,6,2’,4’,6’-Hexaalkylbenzils

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 <370 nm these primary photoproducts undergo secondary photocleavage to ketoaldehydes. The hexaethylbenzil produces only the (Z)-hydroxyindanone in the solid but a 2/1 Z/E ratio at low conversion in solution. The solid-state reactivity and the preference for formation of the (Z)-hydroxyindanone from the hexaethylbenzil suggest that much of the reaction involves 6-hydrogen abstraction followed by coupling of the 1,5-biradical. The formation of (E)-hydroxyindanones from o-ethylphenyl diketones had been attributed to stereospecific rearrangement of a photodienol formed by y-hydrogen abstraction. However, none of the H/D exchange of benzylic hydrogens expected of a photodienol occurs in methanol-d₄. 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.