The bromination of 3-bromo-6,7-benzobicyclo[3.2.1]octa-2,6-diene

The bromination of 3-bromo-6, 7-benzobicyclo [3.2.1] octa-2, 6-diene at ?50°C has been found to give only one product, the tribromide(7) produced via Wagner-Meerwein rearrangement with accompanying aryl migration. The bromination at 0°C produced nonrearranged tribromides beside the rearranged product. The structures of the products were determined by means of spectral data. The addition mechanism is discussed in terms of exo- and endo-attack.     

Photochemical formation of novel pyrrolo[3,2-b]-6,7-benzobicyclo[3.2.1]octa- 2,6-diene

The first synthesis of 1,4,9,10-tetrahydro-4,9-methanobenzo[4,5]cyclohepta[1,2-b]pyrrole (11) was achieved by the photochemical intramolecular [2 + 2] cycloaddition of N-phenoxycarbonyl- (5a) and N-ethoxycarbonyl-2-[2-(2-vinylphenyl)]pyrrole (6a), respectively, followed by basic hydrolysis of the isolated N-substituted 1,4,9,10-tetrahydro-4,9-methanobenzo[4,5]cyclohepta[1,2-b]pyrroles (10a, 10b). Some competitively formed products were also isolated.     

The Bromination of 3-Bromo-6,7-benzobicyclo [3,2,1] octa-2,6-diene and characterization of the products

The bromination of 3-bromo-6,7-benzobicyclo [3.2.1] octa-2,6-diene at -50°C has been found to give only one product, the tribromide 8 produced via Wagner-Meerwein rearrangement with accompanying aryl migration. The bromination at 0°C produced nonrearranged tribromides beside the rearranged tribromide and the ketone 12. The structures of the products were determined by ¹H-, ¹³C-NMR data and single X-ray structural analysis. The addition mechanism is discussed in terms of exo- and endo-attack.     

Relative reactivities of bicyclo[3.2.1]octa-2,3-diene and 1,2-cycloheptadiene with conjugated dienes and styrene

Bicyclo[3.2.1]octa-2,3-diene (2) and 1,2-cycloheptadiene (3), generated by treatment of the corresponding dichlorides 4 and 5 with magnesium, were found to undergo cycloaddition reactions with 2,3-dimethylbutadiene, styrene, and 1,3-cyclopentadiene. 2, but not 3, was also found to undergo a (2 + 2) cycloaddition reaction with cis-pentadiene. The relative reactivities of 2 and 3 with cis-pentadiene, 2,3-dimethylbutadiene, styrene, and 1,3-cyclopentadiene at 60° in THF were found to be: 0·18, —; 1·0, 1·0; 0·60, 7·5; and 4·0, 150.     

The bromination of bicyclo[3.2.1]octa-2,6-diene with N-bromosuccinimide

The bromination of bicyclo[3.2.1]octa-2,6-diene (3) by NBS does not follow the familiar free-radical course but proceeds through the cyclopropylcarbinyl cation 7. 7 can be trapped by addition of small amounts of methanol. The bicyclo[3.2.1]octa-2,6-dien-4-yl radical is involved in the reduction of exo-6-bromotricyclo [3.2.1.0^2,7]oct-3-ene by tributyltin hydride.     

Role of singlet diradicals in reactions of 2-carbenabicyclo[3.2.1]octa-3,6-diene

The generation of 2-carbenabicyclo[3.2.1]octa-3,6-diene (1) results in the formation of C(8)H(8) hydrocarbons endo-6-ethynylbicyclo[3.1.0]hex-2-ene (4), semibullvalene (5), and 5-ethynyl-1,3-cyclohexadiene (6), and C(8)H(10) hydrocarbons bicyclo[3.2.1]octa-2,6-diene (7), tricyclo[3.2.1.0(4,6)]oct-2-ene (8), and tetracyclo[3.3.0.0(2,8)0(4,6)]octane (9). Focus is placed on three mechanistic pathways for the formation of the C(8)H(10) hydrocarbon fraction: (a) abstraction of hydrogen by triplet carbene 1T to produce an equilibrating set of monoradicals, (b) interconversion of triplet carbene 1T into tricyclic triplet diradical 19T and tetracyclic triplet diradical 20T, and (c) interconversion of singlet 1S with analogous singlet diradical 19S and 20S. Ab initio calculations at the (U)B3LYP/6-311+G(3df,2p)//(U)B3LYP/6-31G(d,p) and broken spin symmetry UBS B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p) levels rule out choices (a) and (b) and are consistent with the singlet diradical process.     

Photochemistry of bridged cycloheptadienones : Di-π-methane photorearrangement of 6,7-benzobicyclo[3.2.1]octadienone and related dienes

A series of 6,7-Benzobicyclo[3.2.1]octadienes (10–13) have been prepared. Direct and sensitized irradiation of the dienes gave di-π-methane rearrangement products (14–17). Quantum yields for the direct and sensitized rearrangements were recorded and compared. Substituent effect and structure-multiplicity relationships are discussed.     

The Electronic Spectrum of Tricyclo[3.3.0.0.2,6]octa-3,7-diene

The unusual electronic spectrum of the title compound is shown to be due to the strong interaction between the π-orbitals of the double bonds and the Walsh-orbitals of the fourmembered ring.     

Thermolysis of 2-phosphabicyclo[2.2.2]octa-5,7-dienes: Generation and trapping of P-methyl- and P-phenylphosphaethene

P-Methyl- and P-phenylphosphaethene were generated in solution under very mild conditions in virtually quantitative yield by a thermally-induced extrusion of the phosphorus-containing bridge from appropriately substituted 2-phosphabicyclo[2.2.2]octa-5,7-dienes. The phosphaethenes so formed, the simplest yet produced by conventional methods, are trapped readily in [2+4] cycloadditions with conjugated dienes.     

Bicyclo[3.2.1]octa-3,6-dien-2-yl cation: a bishomoantiaromate

Antiaromatic compounds with a closed loop of 4n p-electrons are relatively unstable and often difficult to study. We report in this article the synthesis of alcohols 2-(4'-fluorophenyl)bicyclo[3.2.1]octan-2-ol 11, 2-(4'-fluorophenyl)bicyclo[3.2.1]oct-3-en-2-ol 12, and 2-(4'-fluorophenyl)bicyclo[3.2.1]octa-3,6-dien-2-ol 13 and their transformations into corresponding carbocations 14-16, respectively, in a superacidic medium (FSO3H/SO2ClF) at -120 degrees C. Cations 14-16 are characterized by NMR analysis (1H, 13C, 19F), and 15 and 16 are further characterized by quenching in NaOCH3/H3COH at -120 degrees C. The relative stabilities of 14-16 are determined experimentally by 19F NMR spectroscopy. Cation 16 is found to be experimentally less stable than cation 15 by 3.7 kcal/mol. DFT calculations (structure and energy: B3LYP/6-31G(d); NMR: B3LYP/6-311+G(2d,p)) are performed for alcohols 11-13 and bicyclo[3.2.1]octyl cations 6, 7, 9, 14-16, 26, 28, and 30. In the case of 11-16, data from DFT calculations is in good agreement with experimental data. Because 6,7-dimethylenebicyclo[3.2.1]oct-3-en-2-yl cation 26 is more stable than cation 7 by 1.69 kcal/mol, the inductive effect of sp(2)-hybridized carbon atoms C6 and C7 in carbocations 6 and 16 cannot be the reason for the destabilization of 6 relative to 7 and 16 relative to 15. Destabilization of 6 relative to 7 and 16 relative to 15 and the calculated NICS of 6 (+4.17 ppm) and 16(+3.3 ppm) document that 6 and 16 are bishomoantiaromates.     

The Radical Cations of Bicyclo[2.2.1]hepta-2,5-diene (8,9,10-Trinorborna-2,5-diene) and bicyclo[2.2.2]octa-2,5-diene (2,3-Dihydrobarrelene). An ESR and ENDOR study

The radical cation of bicyclo[2-2.1]hepta-2,5-diene (8,9,10-trinorborna-2,5-diene; 1 ) in CF₂ClCFCl₂, and CF₃CCl₃ matrices and that of bicyclo[2.2.2]octa-2,5-diene (2,3-dihydrobarrelene; 2 ) in CFCl₃ and CF₃CCl₃ matrices have been studied by ESR and ENDOR spectroscopy. For ${\bf 1}^{+ \atop \dot{}}$, the coupling constants of the olefinic, methano-bridge, and bridgehead protons are ?0.780 ±0.005, +0.304±0.002, and ?0.049±0.002 mT, respectively. The hyperfine tensor for the methano-bridge protons is axial, A_x = +0.263 ± 0.002 and A_y = +0.386 ± 0.002 mT, while that for the olefinic protons is orthorhombic, A_x = ?0.594 ± 0.005, A_y= ?0.913 ± 0.005, and A_z = ?0.834 ± 0.005 mT (x parallel to C? H- z parallel to 2p_π axis). For ${\bf 2}^{+ \atop \dot{}}$, the coupling constants of the olefinic, ethano-bridge, and bridgehead protons are ?0.68 ± 0.01, +0.162 ± 0.005, and ?0.108 ± 0.005 mT, respectively. The hyperfine data for ${\bf 1}^{+ \atop \dot{}}$ and ${\bf 2}^{+ \atop \dot{}}$ fully support the presentation of their singly occupied orbitals as antisymmetric combinations, b₂(π), of the two bonding ethene π-MO's.