PHOTOTRANSFORMATIONS OF BRIDGEHEAD-SUBSTITUTED DIBENZOBARRELENES - STEADY-STATE AND LASER FLASH-PHOTOLYSIS STUDIES OF 9-BENZOYL-SUBSTITUTEDAND 9-(ALPHA-HYDROXYBENZYL)-SUBSTITUTED DIBENZOBARRELENES

The photochemistry of 9-benzoyl- and 9-(alpha-hydroxybenzyl)-substitut ed dibenzobarrelenes (1 and 8) was investigated by steady state photol ysis, 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-( al pha-hydroxybenzyl)-substituted dibenzobarrelene (7) exists in the cycl ic carbinol form 8, as revealed via X-ray structural determination, Th e irradiation of 8 in benzene gave a 52% yield of 4H-pyranodibenzopent alene 15, X-Ray single-crystal diffraction studies confirmed the struc ture of 15, The 308 nm laser pulse excitation of 1 and 8 in benzene ga ve rise to transient phenomena which could be attributed to the triple ts, characterized by broad absorption spectra and relatively short lif etimes (0.46-0.83 mu s). These transients were quenched by oxygen, 2,2 ,6,6-tetramethylpiperidinyl-1-oxy (TEMPO), 4-hydroxy-2,2,6,6-tetrameth ylpiperidinyl-1-oxy (HTEMPO) and ferrocene at rates in the range (0.23 -4.05) X 10(9)M(-1)s(-1). The formation of the observed photoproducts from 1 and 8 was rationalized in terms of di-pi-methane rearrangements leading to the dibenzosemibullvalenes 6 and 12 and the subsequent tra nsformation of 12 to give 15.