Photorearrangements of bridgehead-aryl-substituted dibenzobarrelenes. Steady-state and laser flash photolysis studies

Abstract

Examples of a novel di-{pi}-methane rearrangement of 9-aryl-substituted dibenzobarrelenes containing 1,2-dibenzoylalkene moieties (1a-f) are presented. Steady-state irradiation of the dibenzobarrelenes 1a-f in benzene gives the regioselectively formed dibenzopentalenofurans, 5a-f, in excellent yields (74-90%). The structure of the bromo derivative 5f was confirmed through X-ray crystallographic analysis. Controlled irradiation of 1b, a representative example, at low temperatures suggests that the dibenzopentalenofurans (5a-f) are formed through the thermal isomerization of the corresponding dibenzosemibullvalenes 6a-f, which, in turn, arise through a di-{pi}-methane type of rearrangement. Laser flash photolysis (337.1 nm) of 1a-f in benzene gives rise to transient phenomena, attributable to dibenzobarrelenes triplets ({phi}{sup T} = 0.2-0.5). These transients are characterized by broad and diffuse absorption spectra and possess short lifetimes (0.13-0.19 {mu}s). They are readily quenched by oxygen, HTEMPO (4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxy), ferrocene, and {beta}-carotene, with rate constants in the limit of diffusion control (10{sup 8}-10{sup 10} M{sup {minus}1} s{sup {minus}1}).