Phototransformations of bridgehead-substituted dibenzobarrerenes. Steady state and laser flash photolysis studies of 9-benzoyl- and 9-(a-hydroxybenzyl)-substituted dibenzobarrelenes

Abstract

The photochemistry of 9-benzoyl- and 9-(α-hydroxybenzyl)-substituted dibenzobarrelenes (1 and 8) was investigated by steady state photolysis, product analysis and laser flash photolysis. The irradiation of 1 in benzene gave a 91% yield of the dibenzosemibullvalene 6, and its structure was confirmed by X-ray crystallographic analysis. The 9-(α-hydroxybenzyl)-substituted dibenzobarrelene (7) exists in the cyclic carbinol form 8, as revealed via X-ray structural determination. The irradiation of 8 in benzene gave a 52% yield of 4H-pyranodibenzopentalene 15. X-Ray single-crystal diffraction studies confirmed the structure of 15. The 308 nm laser pulse excitation of 1 and 8 in benzene gave rise to transient phenomena which could be attributed to the triplets, characterized by broad absorption spectra and relatively short lifetimes (0.46-0.83 µs). These transients were quenched by oxygen, 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO), 4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxy (HTEMPO) and ferrocene at rates in the range (0.23-4.05) × 10⁹ M^-1 s^-1. The formation of the observed photoproducts from 1 and 8 was rationalized in terms of di-p-methane rearrangements leading to the dibenzosemibullvalenes 6 and 12 and the subsequent transformation of 12 to give 15.