A novel photorearrangement of a bridgehead-disubstituted dibenzobarrelene. Steady state and laser flash photolysis studies of a 9-hydroxymethyl-10-methyl-dibenzobarrelene

Abstract

Results of a photochemical study based on steady state photolysis and nanosecond laser flash photolysis of a bridgehead-disubstituted dibenzobarrelene 1 are presented. The product isolated from steady state photolysis reveals that the dibenzosemibullvalene, formed initially through a di-π-methane rearrangement, undergoes further rearrangements leading to an oxadibenzopropallane carbinol 9. The structures of 1 and 9 were confirmed by X- ray crystallographic analysis. The 308 nm laser pulse excitation of 1 in benzene gives rise to transient phenomena which can be attributed to its triplet (φ^T=0.93), characterized by broad, diffuse absorption spectra and a short lifetime (0.53 μs). This transient is quenched by oxygen, 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO), 4-hydroxy- 2,2,6,6-tetramethylpiperidinyl-1-oxy (HTEMPO), ferrocene and β-carotene at rates in the range (0.23-5.9) × 10⁹ M^-1 s^-1.