- Nitroxyl radical addition to pentafulvenones forming cyclopentadienyl radicals: A test for cyclopentadienyl radical destabilization
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Photochemical Wolff rearrangements in alkane solvents of the 6-diazo-2,4-cyclohexadienones 4 and 13-15 give pentafulvenone (1), 2,3-benzopentafulvenone (2), dibenzopentafulvenone (3), and 2,4-di-tert-butylpentafulvenone (16), as identified by conventional
- Allen,Porter,Tahmassebi,Tidwell
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- On the absolute photoionization cross section and threshold photoelectron spectrum of two reactive ketenes in lignin valorization: Fulvenone and 2-carbonyl cyclohexadienone
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We report the absolute photoionization cross section (PICS) of fulvenone and 2-carbonyl cyclohexadienone, two crucial ketene intermediates in lignin pyrolysis, combustion and organic synthesis. Both species were generated in situ by pyrolyzing salicylamide and dectected via imaging photoelectron photoion coincidence spectroscopy. In a deamination reaction, salicylamide loses ammonia yielding 2-carbonyl cyclohexadienone, a ketoketene, which further decarbonylates at higher pyrolysis temperatures to form fulvenone. We recorded the threshold photoelectron spectrum of the ketoketene and assigned the ground state (X+2A′′ ← X1A′) and excited state (?+2A′ ← X1A′) bands with the help of Franck-Condon simulations. Adiabatic ionization energies are 8.35 ± 0.01 and 9.19 ± 0.01 eV. In a minor reaction channel, the ketoketene isomerizes to benzpropiolactone, which decomposes subsequently to benzyne by CO2 loss. Potential energy surface and RRKM rate constant calculations agree with our experimental observations that the decarbonylation to fulvenone outcompetes the decarboxylation to benzyne by almost two orders of magnitude. The absolute PICS of fulvenone at 10.48 eV was determined to be 18.8 ± 3.8 Mb using NH3 as a calibrant. The PICS of 2-carbonyl cyclohexadienone was found to be 21.5 ± 8.6 Mb at 9 eV. Our PICS measument will enable the quantification of reactive ketenes in lignin valorization and combustion processes using photoionization techniques and provide advanced mechanistic and kinetics insights to aid the bottom-up optimization of such processes.
- Bodi, Andras,Hemberger, Patrick,Pan, Zeyou,Van Bokhoven, Jeroen A.
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p. 3655 - 3663
(2022/02/21)
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- Contrasting Photolytic and Thermal Decomposition of Phenyl Azidoformate: The Curtius Rearrangement Versus Intramolecular C-H Amination
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The decomposition of phenyl azidoformate, PhOC(O)N3, was studied by combining matrix isolation spectroscopy and quantum chemical calculations. Upon UV laser photolysis (193 and 266 nm), the azide isolated in cryogenic noble gas matrices (Ne and Ar, 2.8 K) decomposes into N2 and a novel oxycarbonylnitrene PhOC(O)N, which was identified by matrix-isolation IR spectroscopy (with 15N labeling) and EPR spectroscopy (|D/hc| = 1.620 cm-1 and |E/hc| = 0.024 cm-1). Subsequent visible-light irradiation (532 nm) causes rearrangement of the nitrene into phenoxy isocyanate PhONCO with complex secondary fragmentation (PhO· + ·NCO) and radical recombination species in matrices. The observation of PhONCO provides solid evidence for the Curtius rearrangement of phenyl azidoformate. In sharp contrast, flash vacuum pyrolysis (FVP) of PhOC(O)N3 at 550 K yields N2 and exclusively the intramolecular C-H amination product 3H-benzooxazol-2-one. FVP at higher temperature (700 K) leads to further dissociation into CO2, HNCO, and ring-contraction products. To account for the very different photolytic and thermal decomposition products, the underlying mechanisms for the Curtius rearrangement (concerted and stepwise) of PhOC(O)N3 and the intramolecular C-H amination of the nitrene in both singlet and triplet states are discussed with the aid of quantum chemical calculations using the B3LYP, CBS-QB3, and CASPT2 methods.
- Wan, Huabin,Xu, Jian,Liu, Qian,Li, Hongmin,Lu, Yan,Abe, Manabu,Zeng, Xiaoqing
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p. 8604 - 8613
(2017/11/24)
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- Ketene-Ketene interconversion. 6-Carbonylcyclohexa-2,4-dienone-hepta-1,2,4, 6-tetraene-1,7-dione-6-oxocyclohexa-2,4-dienylidene and wolff rearrangement to fulven-6-one
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6-Carbonylcyclohexa-2,4-dienone (1) has been generated by flash vacuum thermolysis (FVT) with Ar-matrix isolation of methyl salicylate (7), 2-phenylbenzo-1,3-dioxan-4-one (8), phthalic peranhydride (9), and benzofuran-2,3-dione (11) and also by matrix photolysis of 9, 11, and 2-diazocyclohepta-4,6-dien-1,3-dione (12). In each case, FVT above 600 °C results in decarbonylation of 1 and Wolff rearrangement to fulven-6-one (13) either concertedly or via open-shell singlet 6-oxocyclohexa-2,4-dienylidene (18). Ketenes 1 and 13 were characterized by IR spectroscopy. Photolysis of matrix-isolated 1 at 254 nm also results in the slow formation of 13. The sequential formation of ketenes 1 and 13 from 7 has also been monitored by FVT-mass spectrometry, and 13 has been trapped with MeOH to afford methyl 1,3-cyclopentadiene-1- and -2-carboxylates 15 and 16. FVT of methyl salicylate-1-13C 7a revealed a deep-seated rearrangement of the 13C-labeled 1a to hepta-1,2,4,6-tetraen-1,7-dione (17a) by means of electrocyclic ring opening followed by a facile 1,5-H shift and recyclization prior to CO-elimination and ring contraction to 13C-labeled 13. The rearrangement mechanism is supported by M06-2X/6-311++G(d,p) calculations, which predict feasible barriers for the FVT rearrangements and confirm the observed labeling pattern in the isolated methyl salicylate 7a/7b and methyl cyclopentadienecarboxylates 20 and 21 resulting from trapping of 13 with MeOH.
- Koch, Rainer,Blanch, Rodney J.,Wentrup, Curt
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p. 6978 - 6986
(2014/08/18)
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- Mechanistic aspects of ketene formation deduced from femtosecond photolysis of diazocyclohexadienone, o-phenylene thioxocarbonate, and 2-chlorophenol
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The photochemistry of diazocyclohexadienone (1), o-phenylene thioxocarbonate (2), and 2-chlorophenol (3) in solution was studied using time-resolved UV-vis and IR transient absorption spectroscopies. In these three cases, the same product cyclopentadienyl ketene (5) is formed, and two different mechanistic pathways leading to this product are discussed: (a) rearrangement in the excited state (RIES) and (b) a stepwise route involving the intermediacy of vibrationally excited or relaxed carbene. Femtosecond UV-vis detection allows observation of an absorption band assigned to singlet 2-oxocyclohexa-3,5- dienylidene (4), and this absorption feature decays with an ~30 ps time constant in hexane and acetonitrile. The excess vibrational energy present in nascent carbenes results in the ultrafast Wolff rearrangement of the hot species. IR detection shows that photoexcited o-phenylene thioxocarbonate (2) and 2-chlorophenol (3) efficiently form the carbene species while diazocyclohexadienone (1) photochemistry proceeds mainly by a concerted process.
- Burdzinski, Gotard,Kubicki, Jacek,Sliwa, Michel,Réhault, Julien,Zhang, Yunlong,Vyas, Shubham,Luk, Hoi Ling,Hadad, Christopher M.,Platz, Matthew S.
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p. 2026 - 2032
(2013/04/10)
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- IR and UV matrix photochemistry and solvent effects: the isomerization of diazocyclohexadienones (ortho quinone diazides) - detection of molecules with the 1,2,3-benzoxadiazole structure. A UV/Vis and IR absorption and UV photoelectron spectroscopic investigation
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6-Diazo-2,4-cyclohexadienone and derivatives with fluoro, chloro, methyl, tert-butyl and methoxy substituents have been investigated by UV/Vis absorption, IR absorption and UV photoelectron spectroscopy.Spectral results obtained in the gas phase, in an argon matrix at 10 K and in n-hexane solution at room temperature reveal an equilibrium with the respective 1,2,3-benzoxadiazole isomers, thus disproving current textbook opinions.The 1,2,3-benzoxadiazole structure is derived from the agreement of observed and calculated vertical ionization energies, characteristic IRand UV/Vis absorption bands as well as selective IR and UV photochemical transformations.The relative concentration of the respective 1,2,3-benzoxadiazole in equilibrium with the diazoketone isomer strongly depends on the substituents and on solvent effects.The diazoketone structure is stabilized by hydrogen bonding and polar interactions.The most stable 1,2,3-benzoxadiazole in this study, the 5,7-di-tert-butyl derivative, is at least 6.3 kJ mol-1 more stable than its diazocyclohexadienone valence isomer, whereas 2,3,4,5-tetrafluoro-6-diazo-2,4-cyclohexadienone and 3-methoxy-6-diazo-2,4-cyclohexadienone did not isomerize to a notable extent.Energetic considerations for the stabilization of 1,2,3-oxadiazoles are discussed and compared with experimental and theoretical findings.
- Schweig, Armin,Baumgartl, Horst,Schulz, Reinhard
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p. 135 - 172
(2007/10/02)
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- Cyclopentadienylidene. A Matrix Isolation Study Exploiting Photolysis with Unpolarized and Plane-Polarized Light
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Diazocyclopentadiene (1) was photolyzed in N2, CO, and other low-temperature matrices.The resulting carbene, cyclopentadienylidene (2), was characterized by its UV and IR spectra, and its thermal dimerization and reaction with CO were observed.Photolysis of 1 with plane-polarized light gave matrices exhibiting linear dichroism.Comparison of dichroic IR and UV spectra revealed that the photolysis proceeds via an excited A1 state of the diazo compound.Plane-polarized irradiation of the corresponding ketene (4) in CO matrices resulted in photoreorientation of the molecules of 4 without significant loss.
- Baird, Mark S.,Dunkin, Ian R.,Hacker, Nigel,Poliakoff, Martyn,Turner, James J.
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p. 5190 - 5195
(2007/10/02)
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