PHOTOTRANSFORMATIONS OF BRIDGEHEAD-SUBSTITUTED DIBENZOBARRELENES - STEADY-STATE AND LASER FLASH-PHOTOLYSIS STUDIES OF 9-BENZOYL-SUBSTITUTEDAND 9-(ALPHA-HYDROXYBENZYL)-SUBSTITUTED DIBENZOBARRELENES
Sa. Kumar et al., PHOTOTRANSFORMATIONS OF BRIDGEHEAD-SUBSTITUTED DIBENZOBARRELENES - STEADY-STATE AND LASER FLASH-PHOTOLYSIS STUDIES OF 9-BENZOYL-SUBSTITUTEDAND 9-(ALPHA-HYDROXYBENZYL)-SUBSTITUTED DIBENZOBARRELENES, Journal of photochemistry and photobiology. A, Chemistry, 103(1-2), 1997, pp. 69-73
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.