Photochemistry of carbonyl-bridged dibenzocyclodecadienes

Abstract

The photolyses of 5,6,7,12,13,14-hexahydro-6,13-methanodibenzo[A,F]cyclodecen-15-ones 1, 2 and 3 have been investigated. After Norrish type 1 cleavage, diester 1 gave the alkenes E-5 and Z-5via loss of carbon monoxide and hydrogen transfer, while 2 and 3 gave ketenes 10a and 10b, respectively, by transannular migration of an α-hydrogen, which was quenched with MeOH, giving the monoester 6 and diesters cis-11 and trans-11, respectively. Photolysis of deuteriated 2 and 3 revealed that the hydrogen transfer is an intramolecular process and the Norrish type 1 cleavage is the rate-determining step (K_H/K_D= 1.09 ± 0.05). Naphthalene was effective in quenching the photolysis of 1, 2 and 3 at –70 °C, but quenching was incomplete at 5 °C. It was postulated that the two annulated benzene rings might circumscribe the carbonyl group by stereodynamic inversion and that the naphthalene molecule could not come close enough for the energy transfer.