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The direct irradiation of the monomethoxybarrelene 8 gives cyclooctatetraene product 18 (Φ = 0.075) via the anti-Epiotis aromatic-vinyldiester [2π+2π] bridging route, and two semibullvalenes 16 (Φ = 0.075) and 17 (Φ = 0.075) from di-π-methane rearrangements initiating from vinyl-vinyldiester bridging. Benzophenone sensitization yields 16 (Φ =0.11) and 17 (Φ = 0.11). The dimethoxybarrelene 9 gives only semibullvalene 10, this resulting from di-π-methane rearrangement involving the vinyl and vinyldiester bridges, upon direct (Φ = 0.009) or benzophenone sensitized irradiations (Φ = 0.38). 10 undergoes a vinylcyclopropane rearrangement to give semibullvalene 11 upon direct or acetone sensitized irradiations (Φ = 0.008).

The direct irradiation of 5-cyano-2,3-benzobicyclo[4.2.0]octa-2,4,7-triene (18) led to 6-cyanobenzocyclooctatetraene (17; Φ = 0.00075), 6-cyanobenzosemibullvalene (14; Φ = 0.0003), and 2-cyanonaphthalene (Φ = 0.00015). The triplet excited state of 18, generated by sensitization with a variety of ketone sensitizers (e.g., Michler's ketone and acetophenone), showed no unimolecular reactivity. Compound 18 does not participate in Zimmerman di-π-methane rearrangement; the potential product from such a transformation, 8-cyanobenzosemibullvalene (19), was, however, isolated from direct (Corex filter; Φ = 0,0010) irradiation of the semibullvalene 14. Deuterium labelling experiments suggest that 14 derives from a mechanism involving initial C4—C8 bonding of 18, and that cyclooctatetraene 17 is probably formed by simple electrocyclic opening of the cyclobutene ring of 18. Keywords: photochemistry, rearrangement mechanism, di-π-methane, semibullvalenes.

The direct irradiation of 6-cyanobenzocyclooctatetraene (7) led to 5- and 8-cyano-2,3-benzobicyclo[4.2.0]octa-2,4,7-triene (9 and 8), isolated in 21 and 63% yields, respectively. On triplet sensitized irradiation 8 gave 1- and 6-cyanobenzosemi-bullvalenes (4 and 12; Φ = 0.006 and 0.088, respectively) and naphthalene (Φ = 0.012). The direct irradiation of 8 yielded 12 (Φ = 0.050), naphthalene (Φ = 0.015), 7 (Φ = 0.007), 7-cyanobenzocyclooctatetraene (13; Φ = 0.007), and 5-cyano-7,8-benzotetracyclo[3.3.0.02,4.03,6]oct-7-ene (14; Φ = 0.004). Deuterium labelling studies suggest that 12 derives from Zimmerman di-π-methane rearrangement of 8, and that photoproduct cyclooctatetraene 7 is not formed by simple electrocyclic opening of the cyclobutene ring of 8.

The direct photolysis of 2-cyanobenzobarrelene (4) yields 5- and 7-cyanobenzocyclooctatetraene [Formula: see text] and 1-cyanobenzosemibullvalene [Formula: see text], whereas the acetophenone sensitized irradiation of 4 yields only 8 [Formula: see text]. Deuterium labelling studies suggest that 7 derives from an initial 2Π + 2Π cycloaddition between the vinyl and vinylcyano bridges of 4, whereas 8, resulting from direct or sensitized irradiations of 4, derives from di-Π-methane rearrangement involving initial vinyl–vinylcyano bridging. The observed initial bonding preferences of the 2Π + 2Π cycloadditions and di-Π-methane rearrangement of 4 are in accord with predictions based upon CNDO-CI calculations of changes in atom–atom interaction energies between possible bridging sites in singlet and triplet excited states of 4.

The direct irradiation of 7-cyanobenzocyclooctatetraene (5) led to 6- and 7-cyano-2,3-benzobicyclo[4.2.0]octa-2,4,7-triene (7 and 8; Φ7 = Φ8 2,4.03,6]oct-7-ene (17, Φ = 0.03). Upon sensitized irradiation, 7 gave 6 (Φ = 0.68) and 2-cyanonaphthalene (Φ = 0.007), while 8 gave 16 (Φ = 0.01) and naphthalene (Φ = 0.52). Deuterium labelling studies suggest that 6 and 16 derive from Zimmerman di-π-methane rearrangements.

La photolyse directe est sensibilisee du benzo-2,3-dimethyl-7,8-bicyclo[4•2•0]octa-2,4,7-triene 1 conduit au dimethyl-6,7-benzotricyclo[3•3•0•0 2,8 ]octa-3,6-diene 2 et au naphtalene 3. Etudes de marquage suggerant que 2 derive d'un rearrangement di-π-methane de Zimmerman et que dimethyl-1,8-tetrahydro-1a,2,7,7a-metheno-1,2,7-cyclopropa-1H-naphtalene n'est pas un compose intermediaire en route a 2

Deuterium labeling studies are consistent with the initial photochemical 2π + 2π closure of singlet excited dimethyl 1,4-dihydro-1,4-ethenonaphthalene-2,3-dicarboxylate (4) proceeding via bridging between the vinyl and vinyl diester moieties and not via the vinyl–benzo bridging mode favored by Grovenstein and by the Epiotis polar pericyclic theory. The possible involvement of the charge-transfer excited state of 4 in the reaction is discussed.

The direct (Φ6 = 0.04) or acetone sensitized (Φ6 = 0.51) photolysis of 2-cyano-1,4-dihydro-1,4-ethanonaphthalene (5) yields 1-cyanobenzotricyclo[3.3.0.02,8]octa-3-ene (6). Deuterium labelling studies suggest that 6 derives from a Zimmerman di-π-methane rearrangement and not from the McCullough 1-cyanocyclohexene rearrangement. The observed rearrangement and regioselectivity are in accord with predictions based upon CNDO-CI calculations of changes in atom–atom interaction energies on excitation to the S1 and T1 excited states of 5.

8-Benzoyl-9-deuterio-naphtho [de-2.3.4]bicyclo [3.2.2]nona-2,6,8-triene (1) rearranged quantitatively in a photochemical di-π-methane-type process to 2-, 6-, and 9-deuteriated 1-benzoyl-naphtho [de-2.3.4]tricyclo [4.3.0.02,9]nona-2, 6-diene (8a–c). The phenylhydroxymethyl analogue 2 underwent a similar regioselective rearrangement to 9a–c. The rearrangement 1 → 8a–c is proposed to proceed along three reaction paths evolving from two primary photochemical processes of naphthylvinyl and vinyl-vinyl bonding (1 → 3 + 6). Evidence for a competition between several paths and involvement of biradical intermediates derives from changes in the isotopomeric composition with temperature, and from laser flash detection (λexc 353 nm) of a transient. The dependence of the quantum yield for product formation from 1 on excitation wavelength and sensitizer triplet energy leads to the conclusion that reaction to the primary biradicals occurs directly from the S1 (n, π*) and T2 (n, π* ) states, and that reaction from T1 (π, π*) and from S2 (π, π*) proceed either directly or via T2.

1,6- and 1,8-Dimethylbenzotricyclo[3.3.0.02,8]octa-3,6-diene (16 and 14) are the major semibullvalene products formed from acetophenone sensitized irradiations of the barrelenes 2,6- and 2,3-dimethyl-1,4-dihydro-1,4-ethenonaphthalene (37 and 22), respectively (Φ37→16 = 0.65; Φ22→14=0.57). The direct (Φ14 = 0.017) or chlorobenzene sensitized (Φ14 = 0.12) irradiation of 14 yields 2,3-benzo-5-methyl-6-methylenebicyclo[3.3.0]octa-2,7-diene (17), a transformation typical of methylcyclopropanes. Gas phase thermolysis (420 °C/6 Torr) of 14 gives 6,7-dimethylbenzocyclooctatetraene (15). The direct irradiation (Φ39 = 0.003) or the gas phase thermolysis (395 °C/6 Torr) of 16 gives 6,9-dimethylbenzocyclooctatetraene (39). The rearrangement 14 → 17 is in accord with predictions based upon CNDO-CI calculations of changes in atom–atom interaction energies on excitation to the S1 or T1 excited states of 14.

1,6- and 1,8-Dimethylbenzotricyclo[3.3.0.02,8]octa-3,6-diene (16 and 14) are the major semibullvalene products formed from acetophenone sensitized irradiations of the barrelenes 2,6- and 2,3-dimethyl-1,4-dihydro-1,4-ethenonaphthalene (37 and 22), respectively (Φ37→16 = 0.65; Φ22→14=0.57). The direct (Φ14 = 0.017) or chlorobenzene sensitized (Φ14 = 0.12) irradiation of 14 yields 2,3-benzo-5-methyl-6-methylenebicyclo[3.3.0]octa-2,7-diene (17), a transformation typical of methylcyclopropanes. Gas phase thermolysis (420 °C/6 Torr) of 14 gives 6,7-dimethylbenzocyclooctatetraene (15). The direct irradiation (Φ39 = 0.003) or the gas phase thermolysis (395 °C/6 Torr) of 16 gives 6,9-dimethylbenzocyclooctatetraene (39). The rearrangement 14 → 17 is in accord with predictions based upon CNDO-CI calculations of changes in atom–atom interaction energies on excitation to the S1 or T1 excited states of 14.