Phototransformations of bridgehead-disubstituted dibenzobarrelenes. Interesting rearrangements of dibenzosemibullvalene intermediates derived from 9-(Hydroxyalkyl)-10-methoxy-substituted dibenzobarrelenes

Abstract

The photochemistry of 11,12-dibenzoyl-9,10-dihydro-9-(hydroxymethyl)-10-methoxy-9,10-ethenoanthracene (7a) and 11,12-dibenzoyl-9,10-dihydro-9-(1-hydroxyethyl)-10-methoxy-9,10-ethenoanthracene (7b) has been studied through steady-state photolysis, product analysis, and laser flash photolysis. Irradiation of 7a in benzene, methanol, or acetone gave 69-72% yields of a dibenzopentalene ketone 11a, arising through a dibenzosemibullvalene precursor. Irradiation of 7b, which exists in equilibrium with its cyclic form 7b', gave a mixture of the dibenzopentalene ketone 11b (41%) and the dibenzopentalenopyran derivative, 16b' (26%). The photochemistry of 11,12-dibenzoyl-9,10-dihydro-9,10-dimethoxy-9,10-ethenoanthracene (17) has been reinvestigated. Irradiation of 17 in benzene and methanol gives a 90% yield of an isomeric pentacyclic product 24, formed through the rearrangement of a dibenzosemibullvalene precursor. Irradiation of 17 in aqueous methanol gives a mixture of the dibenzopentalene ketone 22 (40%) and a pentacyclic methanol adduct 21 (34%). The structures of 7b', 11a,b, 16b', 21, and 24 were confirmed through X-ray crystallographic analysis. The 308 nm laser flash photolysis of 7a,b in benzene results in the formation of their triplets (φ^T = 0.55-0.76). These triplets possess short lifetimes (0.45-0.75 μs) and are 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 (0.26-4.7) × 10⁹ M^-1 s^-1.