- Thermal reactions of 8-methylbicyclo[4.2.0]oct-2-enes: Competitive diradical-mediated [1,3] sigmatropic, stereomutation, and fragmentation processes
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At 275 °C, 8-exo-methylbicyclo[4.2.0]oct-2-ene (1a) undergoes a [1,3] sigmatropic rearrangement to 5-methylbicyclo[2.2.2]oct-2-enes, of which the orbital symmetry-allowed si product is only marginally favored over the forbidden sr product; that is, silsr is 2.4. Accompanying the [1,3] shift are significant amounts of epimerization and fragmentation. The 8-endo epimer 1b, which yields no [1,3] product, experiences primarily direct fragmentation and secondarily epimerization. A diradical intermediate can account for all such observations.
- Bogle, Xavier S.,Leber, Phyllis A.,McCullough, Lynne A.,Powers, David C.
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p. 8913 - 8918
(2007/10/03)
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- The bicyclo[2.2.2]octyl carbene system as a probe for migratory aptitudes of hydrogen to carbenic centers
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A series of tosylhydrazone derivatives of exo-6-substituted bicylo[2.2.2]octan-2-ones have been prepared. Thermal decomposition of the sodium salts of these tosylhydrazones gives carbene-derived products from 1,3-migration of either the C6 hydrogen (perturbed) or the C7 hydrogen (unperturbed), along with smaller amounts of alkenes derived from 1,2-hydrogen migration. The exo-6-substituent strongly activates 1,3-hydrogen migration in the case of SiMe3 and weakly activates it in the case of CH3 substitution. Thiomethoxy and carbomethoxy are weakly deactivating, while cyano and methoxy groups are strongly deactivating. B3LYP/6-31G* calculations on these substituted carbenes and transition states are in qualitative agreement with the ease of 1,3-hydrogen migration of perturbed vs unperturbed hydrogen. These experimental results and computational studies suggest carbene stabilization due to the exo-6-silyl group. They also suggest a reactant-like transition state for 1,3-hydrogen migration in which the inductive effect influences ease of migration. In the case of the exo-6-methoxy group, the inductive effect overwhelms any potential resonance-stabilizing effects.
- Creary,Butchko
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p. 1569 - 1578
(2007/10/03)
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- Orbital unsymmetrization of olefins arising from non-equivalent orbital interactions. σ-π Coupling in bicyclo[2.2.2]octenes
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We characterized experimentally the substituent effect of a 5-exo substituent on the π facial selectivities of bicyclo[2.2.2]octenes toward electrophilic oxidative reactions such as epoxidation and dihydroxylation, and we discuss the underlying orbital interactions of vicinal σ orbitals and the olefinic π orbital involved in bicyclo[2.2.2]octenes, and commonly in methylenenorbornanes. Of significance is the out-of-phase motif of these σ- π couplings. Electron-withdrawing substituents such as cyano and carboxylic acid groups unequalize the relevant σ-π coupling, leading to the observed syn-facial preference. Alkyl substituents exhibit an electron-donative perturbing effect, depending on the bicyclic ring system.
- Ohwada, Tomohiko,Uchiyama, Masanobu,Tsuji, Motonori,Okamoto, Iwao,Shudo, Koichi
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p. 296 - 306
(2007/10/03)
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- INTRAMOLECULAR DIELS-ALDER REACTION OF BICYCLODODECA-4,9,11-TRIENE
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cis-Bicyclo(6.4.0)dodeca-4,9,11-triene (5) has been synthesized and the kinetics of its intramolecular Diels-Alder reaction have been measured from 50-75 degC
- Grimme, Wolfram,Wiechers, Gerhard
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p. 6035 - 6038
(2007/10/02)
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- 222. Inductivity and Bridging in 2-Bicyclooctyl Cations. Polar Effects, Part II.
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The solvolysis rates and products of several 6-substituted 2-exo- and 2-endo-bicyclooctyl p-toluenesulfonates, 12 and 13, respectively, are reported.Inductivity, as measured by the reaction constants pI, is considerably less in the exo-series 12 (pI = -1.50) than in the corresponding 2-exo-norbornyl p-toluenesulfonates 1 (pI = -2.0).It is proposed that, for geometrical reasons, bridging of the cationic center C(2) by C(6) is not as strong in the bicyclooctane series 12 as it is in the norbornane series 1.On the other hand, inductivity is higher in the 2-endo-bicyclooctane series 13 (pI = -1.0) than in the corresponding 2-endo-norbornane series 3 (pI = 0.78), probably, because in the former case bridging of C(6) is less hindered by the departing anion.The relative yields of exo- and endo-substitution products from the series 12 and 13, are in accord with graded bridging of C(6) in the incipient bicyclooctyl cations.But almost constant bridging of C(2) by C(7) is indicated in the ionization of the 2-endo-bicyclooctane series 13.Consequently, in the free unsubstituted bicyclooctane cation C(2) is bridged symmetrically by C(6) and C(7), in contrast to the current concept of 'non-classical' two-electron-three-center bonding.
- Grob, Cyril A.,Sawlewicz, Pawel
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p. 1906 - 1917
(2007/10/02)
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- 216. Bicyclooctanes. Syntheses and Hydrolyses of 6-exo-substituted 2-exo- and 2-endo-Bicyclooctyl p-Toluenesulfonates. Part 9
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The synthesis of the title compounds and their hydrolysis products in 70percent dioxane are described.
- Grob, Cyril A.,Sawlewicz, Pawel
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p. 1859 - 1867
(2007/10/02)
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- RATE OF REACTION OF ORGANIC ACIDS WITH UNSATURATED BRIDGED BICYCLIC HYDROCARBONS
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The rate constants were determined for the addition of acetic acid and trifluoroacetic acid to 15 unsaturated hydrocarbons with the bicycloheptane, bicyclo- and bicyclooctane, and cyclohexene structures.The reactivity of the compounds
- Bobyleva, A. A.,Belikova, N. A.,Dzhigirkhanova, A. V.,Plate, A. L.
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p. 797 - 804
(2007/10/02)
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