Photoisomerisation of dibenzobarrelenes - a facile route to polycyclic synthons

Triplet state mediated di-p-methane rearrangements of dibenzobarrelenes give a variety of interesting synthons, formed as primary and secondary photoproducts. These synthons could find use for the synthesis of complex synthetic targets. This tutorial review highlights the photoisomerisation of some bridgehead substituted dibenzobarrelenes and the products derived from them. Selected examples of photoisomerisations proceeding through a tri-π-methane pathway are also included

Biradical intermediates in the photoisomerization of dibenzodihydropentalenofurans to dibenzosemibullvalenes

The photoisomerization of a few substituted dibenzodihydropentalenofurans to the corresponding dibenzosemibullvalenes is reported. Steady-state photolysis of the dibenzodihydropentalenofurans 3a-d gave the corresponding dibenzosemibullvalenes 2a-d in nearly quantitative yields. The quantum yields of this photoisomerization were found to be in the range 0.17-0.26. Laser flash photolysis studies of the dibenzodihydropentalenofurans 3a-e showed transients, with absorption maxima around 410 nm and decaying by first-order kinetics. The lifetimes were in the range 14-30 μs in degassed benzene at 25°C. These transients were readily quenched (trapped) by molecular oxygen, and the Stern-Volmer quenching constants were found to be in the range (2.45-3.17) × 109 M-1 s-1. As a representative example, the 1,3-biradical intermediate from 3e was trapped by molecular oxygen to give the corresponding endoperoxide 11e. The transients were weakly quenched by triplet/radical quenchers such as 2,2,6,6-tetramethylpiperinyl-1-oxy (TEMPO) and 4-hydroxy-2,2,6,6-tetramethylpiperinyl-1-oxy (HTEMPO), and the quenching constants are found to be in the range (1.09-3.19) × 106 M-1 s-1. The decay rates of the transients were found to be temperature dependent and obeyed the Arrhenius equation. For example, the activation energy of the transient from 3a was ~4.5 kcal mol-1 and the Arrhenius preexponential factor log(A/s-1) for the decay of the transients was ~7.5. On the basis of our studies, these transients were assigned as the ground-state triplet biradicals, generated by the cleavage of the C-O bond of the starting dibenzodihydropentalenofurans

Steady state photolysis of bridgehead disubstituted dibenzobarrelenes and thermal isomerization of their photoproducts

The photorearrangements of a few bridgehead disubstituted dibenzobarrelenes 5a-c and thermal transformations of their primary photoproducts 8a-c are described. Irradiation of 9-acetyl-10-methyl-substituted dibenzobarrelene 5a in benzene gave a mixture of the regioselective 8b-acetyl substituted dibenzosemibullvalene 8a (74%), involving a di-π-methane rearrangement and a novel polycyclic ketone 11 (7%), through δ-hydrogen abstraction. Irradiation of the disubstituted dibenzobarrelenes 5b and 5c, under identical conditions, gave the corresponding dibenzosemibullvalene derivatives 8b (85%) and 8c (81%), respectively. Interestingly, compound 8a underwent a facile reaction with methanol to give the acetal derivative 15. Thermolysis of the semibullvalene derivative 8a at 180-200°C for 2 h gave a dibenzopentalenofuran 12a, in 92% yield. Similarly, thermal transformations of 8b and 8c, under analogous conditions, gave the corresponding pentalenofurans 12b and 12c, respectively, in nearly quantitative yields. The kinetics of the thermal isomerization reactions were studied and the activation energies of 8a-c to 12a-c have been found to be 21.04, 23.42 and 24.74 kcal mol-1, respectively. The structures of 5a, 8c, 11 and 15 were established unambiguously through X-ray crystallographic analysis

Photochromic dibenzobarrlenes: long-lived triplet biradical intermediates

Upon exposure to UV light, the disubstituted dibenzobarrelene derivative 1a turns green in the solid phase and reverts back to its original pale-yellow color within several hours in the dark. The lifetime of the colored species in degassed benzene at room temperature is 37 ± 2 s (Ea for decoloration is 14.5 ± 0.7 kcal mol-1 and log A is 8.92 ± 0.5 s-1) and highly sensitive to molecular oxygen; the Stern-Volmer quenching constant is 6.9 ± 0.2 × 108 M-1 s-1. Similarly, the disubstituted dibenzobarrelenes 1b and 1c exhibited pink coloration when exposed to UV light in the solid phase. On the basis of combined experimental and theoretical evidence, it is proposed that upon photoexcitation the excited singlet state of 1a undergoes rapid intersystem crossing to its triplet state, followed by intramolecular δ-H abstraction, to yield the triplet biradical intermediate 32. Upon prolonged irradiation, 2 undergoes cyclization to the alcohol 3, which affords the enone 4 as the final photoproduct. The δ-H abstraction on the triplet-state potential energy surface, calculated at the B3LYP/6-31G∗ level of density functional theory (DFT), has an activation energy of 18.5 kcal/mol. Further, the absorption spectrum of the triplet biradical 32, obtained from time-dependent DFT calculations, displays an intense absorption maximum at 670 nm, which is in good agreement with the observed absorption peak at 700 nm. The molecular-orbital analysis of the triplet diradical 32 suggests that its long-wavelength absorption involves the transition of the unpaired electron from the comparatively localized benzyl-type HOMO to the extensively conjugated benzoyl-type LUMO. The present experimental and theoretical results strongly support the intervention of a long-lived triplet biradical 32 in the photochromism of appropriately substituted dibenzobarrelenes

Photoisomerization of bridgehead monosubstituted dibenzobarrelenes and interesting thermal isomerization of their photoproducts

Phototransformations of a few bridgehead monosubstituted dibenzobarrelenes, 4a-d, and thermal isomerizations of their primary photoproducts, 7a and 11a-c, are described. Irradiation of 4a in benzene gave a mixture of regioisomeric products, 7a (8b-isomer) and 11a (4b-isomer), in 20 and 70% yields, respectively. In contrast, irradiation of 4b, under identical conditions, yielded only the corresponding 4b-substituted dibenzosemibullvalene, 11b, in 95% yield. Similarly, irradiation of the nitro- (4c) and chloro- (4d) substituted dibenzobarrelenes gave exclusively the corresponding dibenzosemibullvalenes, 11c (81%) and 11d (81%), respectively. The formation of regioselective products in these systems has been attributed to the relative stabilities of the diradical intermediates involved in these transformations. Interestingly, the dibenzosemibullvalene 7a, on refluxing in xylene, gave the corresponding dibenzopentalenofuran 9a (94%). Thermolysis of 11a and 11c in o-dichlorobenzene and, likewise, of 11b in xylene, yielded 94-97% of the corresponding dibenzopentalenofurans 12a, 12c, and 12b, respectively. Activation energies for the thermal isomerizations of 7a to 9a, 11a to 12a, and 11b to 12b have been found to be 21.57, 25.97, and 17.93 kcal/mol, respectively. The structures of 11a, 11c, 11b, and 12b were established unambiguously through X-ray crystallographic analysis, whereas the structures of the different photoproducts, 7a, 11a-d, 12a-c and 9a, have been arrived at on the basis of spectral data and analytical results