- The Mystery of the Benzene-Oxide/Oxepin Equilibrium—Heavy-Atom Tunneling Reversed by Solvent Interactions
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The equilibrium between benzene oxide (1) and oxepin (2) is of large importance for understanding the degradation of benzene in biological systems and in the troposphere. Our studies reveal that at cryogenic temperatures, this equilibration is governed by rare heavy-atom tunneling. In solid argon at 3 K, 1 rearranges to 2 via tunneling with a rate constant of approximately 5.3×10?5 s?1. Thus, in a nonpolar environment, 2 is slightly more stable than 1, in agreement with calculations at the CCSD(T) level of theory. However, if the argon is doped with 1 % of H2O or CF3I as typical hydrogen or halogen bond donors, respectively, weak complexes of 1 and 2 are formed, and now 2 is tunneling back to form 1. Thus, by forming non-covalent complexes, 1 becomes slightly more stable than 2 and the direction of the heavy-atom tunneling is reversed.
- Prado Merini, Melania,Sander, Wolfram,Schleif, Tim
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- Reaction of O(3P) atoms with benzene
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The gas-phase reaction of O(3P) atoms with benzene was investigated in a flow system in the pressure range of 50-100 mbar and a temperature of 295 ± 2 K with a focus on the product formation. O2 concentrations in the carrier gas were in the range of (7.7-84) × 1014 molecule cm-3. The primary stable products detected were phenol, benzene oxide/oxepin and a not identified compound with the probable composition C5H6O. The yields of phenol and benzene oxide/oxepin were 0.12 ± 0.02 and 0.26 ± 0.06, respectively, being not affected by the experimental conditions. For benzene oxide/oxepin, a rapid consecutive reaction with O(3P) atoms was observed with a rate coefficient of k(295 K) = (1.1 ± 0.1) × 10-10 cm3 molecule-1 s-1 measured at a pressure of 100 mbar. A substance with the formula C6H6O2 (likely oxepin 4,5-epoxide or oxepin 2,3-epoxide), the isomers of muconaldehyde, as well as formic acid, acrolein and trans-butenedial were identified as products of the reaction of O(3P) atoms with benzene oxide/oxepin.
- Berndt, Torsten,Boege, Olaf
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p. 391 - 403
(2007/10/03)
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- ?-Facial stereoselectivity in the Diels-Alder reactions of benzene oxides
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The Diels-Alder reactions of N-phenylmaleimide and dimethyl acetylenedicarboxylate with benzene oxide (1,3,5-cyclohexatriene 1,2-oxide, 3) and its more substituted derivatives 1,2-dimethyl-1,3,5-cyclohexatriene 1,2-oxide (7) and 10-oxatricyclodeca-2,4-diene (11) in a kinetic manner gave exclusively products of addition anti to the plane-nonsymmetrical oxygen.The structures of the adducts were determined unequivocally by nuclear Overhauser enhancements in their 1H nuclear magnetic resonance spectra and by X-ray crystallographic methods.The ?-facial stereoselectivity was rationalized in terms of unfavorable orbital interactions, steric hindrance between the dienophile and the syn face of benzene oxide, and ?-donation by the oxygen. Key words: cycloaddition, Diels-Alder, syn-anti, ?-facial stereoselectivity, benzene oxide.
- Gillard, James R.,Newlands, Michael J.,Bridson, John N.,Burnell, D. Jean
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p. 1337 - 1343
(2007/10/02)
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- Photochemical Transformations, 64. The 3? -> 3?-Route to Oxepines/Benzene Oxides
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The 7-oxanorbornadiene skeleton (8a), its 2,3-bis(trifluoromethyl)derivative (8b), and several 2,3-dicarboxylic esters with potential dipolarophilic groups at C-1 (16a-d, 17a-c) were synthesized and selectively transformed into the 3-oxaquadricyclanes (18a,b, 19a-d, and20a-c) through sensitized (8a,b) or direct (16a-d, 17a-c) photoexcitation.In the thermolysis of these oxaquadricyclanes a(C6D6) = 32.6 +/- 0.3 kcal/mol (32.2 +/- 1.4 kcal/mol); lg A = 15.8 (14.5)> CF3(CH3CO2) substituents at C-2(3) favor the scission of the opposite (neighbouring) cyclopropane bonds.Competition experiments with dimethyl acetylenedicarboxylate as external dipolarophile support the stepwise mechanism of the oxaquadricyclane -> oxepine conversion.The intramolecular carbonyl ylide-interception cannot compete with oxepine formation in 19a-d but is efficient in 20 a-c.The unusually low activation barrier (Ea(C6D6) = 23.5 +/- 1 kcal/mol; lg A = 11.6) and singular specifity in case of 20a is interpreted in terms of a concerted ?2 + ?2 + ?2>-cycloaddition to the bishomofuran unit of the oxaquadricyclane skeleton.
- Prinzbach, Horst,Bringmann, Horst,Markert, Juergen,Fischer, Gerhard,Knothe, Lothar,et al.
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p. 589 - 615
(2007/10/02)
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- On the cis-Glycol and the Epoxide of Benzvalene
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By means of standard methods, benzvalene (1) can be transformed into the cis-glycol 2, from which derivatives such as the ditosylate 3, the cyclic carbonate 4, and the cyclic orthoacetates 5 and 6 are readily prepared.Irradiation causes thiophenol to add across the central bond of the bicyclobutane moiety of 2 with formation of the thioether 8.The solvolysis of 3 in aqueous acetone leads to endo,endo-tricyclo2,6>hexane-3,5-diol (11). - The epoxide 18 of benzvalene (1) may be synthesized starting either from the orthoesters 6 via the trans-chlorohydrin acetate 17 or directly from 1 with N-benzoylpercarbamidic acid, the latter reaction being the better one.Initially, the thermolysis of 18 produces 2-oxabicyclohepta-3,6-diene (19) which in part isomerizes to oxepin (20)/benzene oxide (21).In the thermal decomposition of 3,4-dideuterated 18, the labels end up in positions 5 and 6 of 19.If 18 is thermolyzed in high concentration 19, and 21 undergo Diels-Alder addition to a considerable extent to form the pentacyclic product 22.Epoxide 18 adds acetic acid and, under irradiation, thiophenol resulting in 1:1 adducts of the tricyclo2,6>hexane type (29 and 28, respectively).On slight warming, the diol monoacetate 29 rearranges to the corresponding bicyclohex-2-ene derivative 30, which is converted into its bicyclohex-2-ene isomer 31 at 130 deg C.Depending on the solvent and the temperature, LiAlH4 reduces 18 either to almost pure tricyclo2,6>hexan-3-ol (32) or to a 1:1 mixture of 32 and endo-tricyclo2,6>hexan-3-ol (33).On the basis of the NMR data of the corresponding methyl ethers 34 and 35, a structure assignment in the literature has to be revised.
- Christl, Manfred,Leininger, Hartmut,Kemmer, Petra
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p. 2963 - 2987
(2007/10/02)
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