Rate constants for the reactions of alkyl radicals with 1,4-cyclohexadiene (original) (raw)
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The diastereofacial selectivity in reactions of a series of alkyl-substituted cyclohexyl radicals has been investigated. In additions of cyclohexyl radicals to alkenes, it has been found that only substituents bound at the olefinic center being attacked by the radical influence the equatorial-axial selectivity. Substituents bound to the radical center or axial substituents-beta to the radical center lead to increased axial attack. Equatorial beta-substituents or axial gamma-substituents increase the amount of equatorial attack. The same trends are observed for halogen and hydrogen abstraction reactions; the amount of axial reaction product is usually somewhat higher than in the addition reactions. The stereoselectivities can be explained with steric and torsional effects very similar to those suggested for nucleophilic addition reactions to cyclohexanones. A MM2 force field has been parameterized to gain further insight into the stereochemistry of the reaction.
Gas-Phase Reactivity of Protonated 2-, 3-, and 4-Dehydropyridine Radicals Toward Organic Reagents
Journal of Physical Chemistry A, 2009
To explore the effects of the electronic nature of charged phenyl radicals on their reactivity, reactions of the three distonic isomers of n-dehydropyridinium cation (n = 2, 3, or 4) have been investigated in the gas phase by using Fourier-transform ion cyclotron resonance mass spectrometry. All three isomers react with cyclohexane, methanol, ethanol, and 1-pentanol exclusively via hydrogen atom abstraction and with allyl iodide mainly via iodine atom abstraction, with a reaction efficiency ordering of 2 > 3 > 4. The observed reactivity ordering correlates well with the calculated vertical electron affinities of the charged radicals (i.e., the higher the vertical electron affinity, the faster the reaction). Charged radicals 2 and 3 also react with tetrahydrofuran exclusively via hydrogen atom abstraction, but the reaction of 4 with tetrahydrofuran yields products arising from nonradical reactivity. The unusual reactivity of 4 is likely to result from the contribution of an ionized carbene-type resonance structure that facilitates nucleophilic addition to the most electrophilic carbon atom (C-4) in this charged radical. The influence of such a resonance structure on the reactivity of 2 is not obvious, and this may be due to stabilizing hydrogen-bonding interactions in the transition states for this molecule. Charged radicals 2 and 3 abstract a hydrogen atom from the substituent in both phenol and toluene, but 4 abstracts a hydrogen atom from the phenyl ring, a reaction that is unprecedented for phenyl radicals. Charged radical 4 reacts with tert-butyl isocyanide mainly by hydrogen cyanide (HCN) abstraction, whereas CN abstraction is the principal reaction for 2 and 3. The different reactivity observed for 4 (as compared to 2 and 3) is likely to result from different charge and spin distributions of the reaction intermediates for these charged radicals.
Electron transfer induced dissociations of 2- and 4-alkyl cyclohexadienones
Tetrahedron, 2003
Several 2-and 4-alkylcyclohexadienones were prepared and shown to accept electrons to produce ketyl radical anions that dissociated rapidly at room temperature to release carbon-centered radicals and an aromatic phenoxide type anion. In the PET process with benzyl-substituted cyclohexadienones, initiated with triethylamine, the benzyl radicals dimerised or abstracted an H-atom from solvent. In electrochemical reductions, and in reductions with alkali metals in liquid ammonia, the benzyl radicals were further reduced to anions. q
The Journal of Organic Chemistry, 1987
Theoretical and photochemical methods have been used to probe the structures, electronic distributions, and solution-phase chemical properties of allene cation radicals. Results of SCF-level (UHF) calculations show that the linear-45O-twisted structure of the parent allene cation radical represents the minimum energy geometry and that this species has high positive charge density at the central carbon and large odd electron density at the terminal carbons. In comparison, the linear-nontwisted structure serves aa the low energy geometry for the 1,l-dimethylallene cation radical. Charge and odd electron density in this system are localized in the dimethyl-substituted C=C a-bond. The electron-transfer photochemistry of allenes with the electron acceptor, 2-phenyl-1-pyrrolinium perchlorate (6), is in accord with these representations of allene cation radicals. Photoadditions of allene to 6 in methano1,give the allenyl-, propargyl-, and (methoxyally1)pyrrolidines 15-17. Analogous adducts are formed in 1,l-dimethylallene, tetramethylallene, and 1,2-cyclononadiene photoadditions to 6. Mechanisms for these photoadditions involving generation of intermediate allene cation radicals by electron transfer to the singlet excited state of 6 are presented. Product formation occurs by deprotonation or methanol addition to the intermediate allene cation radicals followed by coupling of the resultant neutral radical pairs. Support for this proposal comes from studies of photoadditions of the allylsilane 34, allenylsilane 41, and propargylsilanes 42 and 43 to pyrrolinium salt 6. These processes occur through documented pathways involving sequential electron-transfer-desilylation processes. Finally, photoadditions of allenes to 6 in acetonitrile follow 2 + 2 and 4 + 2 cycloaddition routes, the nature of which depends upon the allene used. The chemo-and regiochemical selectivities of these processes are discussed in terms of solvent polarity dependent partitioning of singlet exciplex intermediates to cycloadducts or radical ion pairs. (21) Concentrations of the allenes are sufficient (based upon fluorescence quenching rate data) to insure that all singlets of 6 are captured. Chemistry of Allene Cation Radicals Scheme VI1
Kinetic Studies on the Reactions of Hydroxyl Radicals with Cyclic Ethers
Rate coefficients for the gas-phase reactions of hydroxyl radicals with a series of alkoxy esters of structure RC(O)O(CH 2 ) n OR′, where R ) H, CH 3 , R′ ) CH 3 , C 2 H 5 , and n ) 1-2, have been determined with use of relative and absolute rate methods. Relative rate measurements were performed in a Teflon reaction chamber at 298 ( 2 K and atmospheric pressure. Absolute rate measurements were made with pulsed laser photolysislaser induced fluorescence over the temperature range 263-372 K at pressures of ∼100 Torr. The kinetic data are used to derive Arrhenius expressions for the reactions and tropospheric lifetimes for the alkoxy esters. The reactivity of the alkoxy esters is discussed in light of the current understanding of the atmospheric chemistry of oxygenated organic compounds.
Chemical Physics Letters, 1986
The absolute rate constants for the gas-phase H-atom abstraction by hydroxyl radicals from cyclohexane and ethane have been determined at room temperature. OH radicals were produced by pulse radiolysis of an H,O-Ar mixture, and the decay of OH was followed by monitoring the transient light absorption around 309 nm. The rate constants were found to be k = (5.24*0.36)x10-I2 and (2.98&0.21)X10-'3 cm3 molecule-' s-' for cyclohexane and ethane, respectively. These results are compared with literature data.
Helvetica Chimica Acta, 2006
Reactions of isopropyl and of undecyl radicals with meta-and para-substituted toluenes are reported. The results demonstrate that the reactivities of toluenes are due to both benzyl-H abstraction and addition of the alkyl radicals to the aromatic ring. Relative reactivities yield curved Hammett plots, consistent with kinetic data reported by Dütsch and Fischer. Abstractions and ring additions occur with comparable rates, but opposite Hammett slopes. Addition is favored by electron-withdrawing and abstraction by electron-donating substituents. The effects of substituents on the dissociation energies of benzyl CÀH bonds are shown to be the major factor influencing reaction rates for benzyl-H abstraction by alkyl radicals.
Chemical Physics, 1998
Reaction of cyclohexenyl radicals with O has been studied by a combination of pulsed laser photolysis and 2 photoionization mass spectrometry. These radicals could be generated either by photolyzing 3-bromocyclohexene or by the two step reaction of carbon tetrachloride photolysis followed by the H atom abstraction reaction of Cl atom with cyclohexene. The equilibrium between cyclohexenyl radicals, O and cyclohexenylperoxy radicals could be observed at a 2 temperature of 361 K. An analysis of the temporal signal of cyclohexenyl radicals was used to evaluate both the forward and reverse rate constants as well as the equilibrium constant. Use of the latter together with an estimated D S 0 for the reaction yields a C-O bond energy of 80.4 " 4.2 kJ mol y1 in the RO adduct. q 1998 Elsevier Science B.V. All rights reserved. 2 1 y1 the R-O bond dissociation energy. 2 ) Corresponding author. 0301-0104r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž . PII: S 0 3 0 1 -0 1 0 4 9 8 0 0 0 2 3 -8 ( ) R. Zils et al.r Chemical Physics 231 1998 303-313 304