Photochemical and thermal rearrangements of a benzoylnaphthobarrelene-like system

Demuth, Martin ; Amrein, Walter ; Bender, Christopher O. ; Braslavsky, Silvia E. ; Burger, Ulrich ; George, Manapurathu V. ; Lemmer, Dieter ; Schaffner, Kurt (1981) Photochemical and thermal rearrangements of a benzoylnaphthobarrelene-like system Tetrahedron, 37 (19). pp. 3245-3261. ISSN 0040-4020

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8-Benzoyl-9-deuterionaphtho[de-2.3.4]bicyclo[3.2.2]nona-2,6,8-triene (12a)rearranges in a photochemical di-π-methane-type process to the l-benzoylatho[de-[2.3.4]tricyclo[,9]nona-3,7-dienes 14a-c.The dihydro derivate 13a and the hydroxypheoylmethyl analogs 21a and 22a undergo similarly regioselective rearrangements to 15a+c, 23a-c, and 24a, respectively. At 298 K the primary photoreaction directly occus from the S1(n,π) and T2(n,π) states, and it proceeds from T1(π,π) and from S2(π,π) either directly or via T2. At lower temperature on direct irradiation. S1→T intersystem crossing and triplet reaction compete with reaction from the singlet. The rearrangement 12a→14a-c proceeds along three reaction paths evolving from the two primary photochemical processes of naphthyl-vinyl and vinyl-vinyl bonding in β-position to the CO (12→25+29). Two ground-state triplet diradical intermediates such as 25 and 27 have been shown to intervene consecutively-for the first time in di-π-methane photochemistry. Each has been characterized by ESR and IR, and the second one additionally by fluorescence and fluorescence excitation, and by laser flash photolysis. The failure of products 14a-c to interconvert photochemically is ascribed to efficient energy dissipation through thermally reversible pbotocleavage of the 3-membered ring. Compounds 12 and 14 thermally interconvert in the dark which constitutes the first example of a ground-state counterpart of a di-π-methane photorearrangement. The thermal reaction includes a path with highly regioselective (and possibly concerted) product formation competing with a stepwise process causing positional scrambling. The sequence 12→14 (photochemically; φ = 1.0 at 366 nm and 298 K) and an electrophile-catalyzed reversal 14→12 in the dark is a model of a chemical light energy storage cycle which can be conducted without loss of reactants.

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