Meledathu C. Sajimon

Photoisomerisation of dibenzobarrelenes—a facile route to polycyclic synthons

Triplet state mediated di-pi-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-pi-methane pathway are also included.

Substituent Effects on Regioselectivity in the Photorearrangement of a Few Naphthobarrelenes

The photorearrangement of a few 9,10-disubstituted-naphthobarrelenes to the corresponding naphthosemibullvalenes has been examined. It was observed that the regioselectivity in these photoisomerizations depends on the relative stabilities of the diradical intermediates. AM1 semi-empirical calculations are in support of this view. Thermal transformation of the naphthosemibullvalenes gave the corresponding water added naphthopentalenofurans, in nearly quantitative yields.

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.

Regioselectivity in dibenzobarrelene photorearrangements: photoproducts derived from 9-substituted-dibenzobarrelenes

Irradiation of the 9-formyl-, 9-acetyl-, 9-nitro- and 9-tent-substituted dibenzobarrelenes gave the corresponding 4b-substituted dibenzosemibullvalenes, (2a) (8b,8c-dibenzoyl-4b,8b,8c,8d-tetrahydrodibenzo[a,f]-cyclopropa[cd]pentalene-4b-carbaldehyde, C 31 H 20 O 3 ), (2b) (4b-acetyl-8b,8c-dibenzoyl-4b,8b,8c,8d-tetrahydrodibenzo[a,f]cyclopropa[cd]pentalene, C 32 H 22 O 3 ), (2c) (8b,8c-dibenzoyl-4b-nitro-4b,8b,8c,8d-tetrahydrodibenzo[a,f]cyclopropa[cd]pentalene, C 30 H 19 NO 4 ) and (2d) (8b,8c-dibenzoyl-4b-tert-butyl-4b,8b,8c,8d-tetrahydrodibenzo[a,f]cyclopropa[cd]pentalene). The product (2d) gave the corresponding dibenzopentalenofuran derivative, (3d) (10c-benzoyl-6b-tert-butyl-2-phenyl-10b,10c-dihydro-6bH-dibenzo[a,f]furo[2,3,4-cd]pentalene, C 34 H 28 O 2 ). The structures of (2a), (2b), (2c) and (3d) have been established unequivocally via X-ray crystallographic analysis. The molecular structures of (2a), (2b), and (2c) incorporate the basic dibenzosemibullvalene skeleton, which contains the 6:5:5:6:3 fused ring system with normal bond lengths and bond angles. The crystal structures of (2a), (2b), and (2c) have intermolecular hydrogen-bonding interactions (< 2.60 A) which are almost linear around the H atoms. The structure of (3d) shows a 6:5:5:6:5 fused ring system and does not indicate hydrogen-bonding-type intermolecular interactions.

Ring-closing photoisomerization of some 2,6-diarylstyrenes

Abstract The structure, spectroscopy and photochemical reactions of three symmetrical 2,6-diarylstyrenes (aryl = phenyl, 2-furyl and 2-thiophenyl) have been investigated. The ground-state structures are highly nonplanar, having large aryl-phenyl and vinyl-phenyl dihedral angles. All three diarylstyrenes have broad UV absorption bands attributed to allowed, delocalized π,π* (highest occupied molecular orbit–lowest unoccupied molecular orbit) transitions. The 2-furylstyrene is weakly fluorescent, with a large Stokes shift attributed to a change in geometry from the nonplanar ground state to a more planar singlet state. Irradiation in fluid solution results in efficient conversion of the diarylstyrenes to cyclized 9,10-dihydrophenanthrene and 4,5-dihydronaphthofuran or thiophene products, thus extending the scope of the 2-vinylbiphenyl photocyclization reaction to heterocyclic analogs. Irradiation at low temperatures in glassy media permits observation of the UV absorption spectra of the unstable primary photoproducts. Upon warming of the glass, these intermediates undergo rapid hydrogen migration to form the stable dihydroarene products.