Carbon-oxygen bond cleavage reactions by electron transfer. 1. Electrochemical studies on the formation and subsequent reaction pathways of cyanoanisole radical anions (original) (raw)
Related papers
Journal of the American Chemical Society, 2008
Aryloxenium ions 1 are reactive intermediates that are isoelectronic with the better known arylcarbenium and arylnitrenium ions. They are proposed to be involved in synthetically and industrially useful oxidation reactions of phenols. However, mechanistic studies of these intermediates are limited. Until recently, the lifetimes of these intermediates in solution and their reactivity patterns were unknown. Previously, the quinol esters 2 have been used to generate 1, which were indirectly detected by azide ion trapping to generate azide adducts 4 at the expense of quinols 3, during hydrolysis reactions in the dark. Laser flash photolysis (LFP) of 2b in the presence of O 2 in aqueous solution leads to two reactive intermediates with λ max 360 and 460 nm, respectively, while in pure CH3CN only one species with λmax 350 nm is produced. The intermediate with λ max 460 nm was previously identified as 1b based on direct observation of its decomposition kinetics in the presence of N 3 -, comparison to azide ion trapping results from the hydrolysis reactions, and photolysis reaction products (3b). The agreement between the calculated (B3LYP/6-31G(d)) and observed time-resolved resonance Raman (TR 3 ) spectra of 1b further confirms its identity. The second intermediate with λ max 360 nm (350 nm in CH3CN) has been characterized as the radical 5b, based on its photolytic generation in the less polar CH 3CN and on isolated photolysis reaction products (6b and 7b). Only the radical intermediate 5b is generated by photolysis in CH 3CN, so its UV-vis spectrum, reaction products, and decay kinetics can be investigated in this solvent without interference from 1b. In addition, the radical 5a was generated by LFP of 2a and was identified by comparison to a published UV-vis spectrum of authentic 5a obtained under similar conditions. The similarity of the UV-vis spectra of 5a and 5b, their reaction products, and the kinetics of their decay confirm the assigned structures. The lifetime of 1b in aqueous solution at room temperature is 170 ns. This intermediate decays with firstorder kinetics. The radical intermediate 5b decomposes in a biphasic manner, with lifetimes of 12 and 75 µs. The decay processes of 5a and 5b were successfully modeled with a kinetic scheme that included reversible formation of a dimer. The scheme is similar to the kinetic models applied to describe the decay of other aryloxy radicals.
Theoretical Study of the Reactivity of Phenyl Radicals Toward Enol Acetates
Journal of the Brazilian Chemical Society, 2021
Arylation reactions are an important class of reactions and allow the synthesis of natural and synthetic products. Despite the efficient, but high cost and toxic methodologies involving transition metals, radical arylations have gained importance after the advent of photoredox catalysis. Arylation of enol acetates is an important tool for obtaining aryl ketones but the scope of the reaction is limited to the pattern of substitution at phenyl radical and α-carbon of the enol. Theoretical calculations ((U)BHandHLYP/6-311G**) show that the polar effect is the key factor in this reaction. A good correlation of calculated rate constants with field effect explained why phenyl radicals with electronwithdrawing groups react faster toward enol acetate. The presence of alkyl groups at α carbon at the enol showed some influence of enthalpic effect but strong influence of steric effect, evidenced by great correlations with Taft and Charton parameters. Finally, substitution at β carbon showed no significant effect at reaction rates.
Ab Initio Study of Radical Reactions: Cyclization Pathways for the Butylbenzene Radical (II)
The Journal of Physical Chemistry A, 2001
Ab initio density functional theory calculations are presented on some model reactions involved in coke formation during the thermal cracking of hydrocarbons. The reactions under consideration are different cyclization pathways for the butylbenzene radical, which can lead to a further growth of the coke layer. This study enables us to gain more microscopic insight into the mechanistic and kinetic aspects of the reactions. Special attention is paid to the exact treatment of internal rotations and their impact on the kinetic parameters. Pre-exponential factors are very sensitive to the accuracy of constructing the microscopic partition functions. In particular, the relative importance of cyclization toward five and six-membered rings is studied on the basis of the calculated rate constants and concentration profiles of the reactants. The influence of the size of the ring and of the relative stability of the primary and secondary butylbenzene radical on the cyclization reaction is discussed. The activation energy for the formation of six-membered rings is approximately 30 kJ/mol lower than that for five-ring formation. The predicted values for the kinetic parameters enable us to validate some basic assumptions on coke formation. The calculations as presented here are especially important for complex reaction schemes, for which experimental data are not always available.
The Journal of Organic Chemistry, 2013
The reaction of N-allyl-N-(2-halobenzyl)-acetamides and derivatives was investigated in liquid ammonia under irradiation with the nucleophiles Me 3 Sn − , Ph 2 P − and O 2 NCH 2 − . Following this procedure, novel substituted 2acetyl-1,2,3,4-tetrahydroisoquinolines and substituted 2-acetyl-2,3,4,5-tetrahydro-1H-benzo[c]azepines were obtained in good yields. These reactions are proposed to occur through the intermediacy of aryl radicals, which by intramolecular 6-exo or 7-endo attack to a double bond cyclize to give aliphatic radicals, which react along the propagation steps of the S RN 1 chain cycle to afford the cyclic substituted compounds as main products. The reactions were modeled with DFT methods, which provide a rational understanding that relates the product distribution to the structure of the aliphatic radicals proposed as intermediates and the kinetic of their formation. | J. Org. Chem. 2013, 78, 4719−4729 ] = 3.33 × 10 −3 M. Irradiation time = 4 h. h Byproducts 22a (12%) and 23a (23%). i Byproducts 22b and 23b, not quantified.
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.
Tetrahedron, 1992
Key Wordr~ sequential radical reactions; 3 +2 cycloadditions; S-phenylthio-3-pentenyl radicals, thiohydmxamic esters of 6-phenylthio4hexenoic acids. Abstmct: Sequential radical a&iition/5-exo-cyclization&elimination reactions, accomplished by therm&y or photolyticalty induced decompositions of 0-acyl derivatives of N-hydroxypyridine-2-thione or other thiohydroxamic esters of 6-phenylthio-4-herenoic acids II in the presence of an excess of radicophilic ole~ns, afforded the 2vinylcyclopentanc den'vatives 13 in 5090% yields, while decompositions of thiohydroxamic esters or 6-phenylthio-4-hexynoic acids 24, under the same experimental conditions and in the presence of electron deficient oleflts, affordedthecorre.vponding2-vinylidenecyclopentane (2-o-alleniccyclopentane)derivatiws25 @I-72%). However, when &compositions of thiohydroxamic esters 39 were carried out in boiling toluene solution without rodicophilic olefins. 3-e-co-cyclization took place and the corresponding 2-vinylcyclopropane der&tives 40 were obtained in 43-60% yields. Tandem sequence of radical addition/cyclization reaction represents a simple and very useful methodology for annulation of polysubstituted cyclopentane rings. te3 Starting from two appropriate unsaturated synthons 1 and 2, by this sequence of reactions, 3 + 2 cycloaddition takes place involving two carbon-carbon bonds formations. 134*5 In addition to the tandem sequences4*6-'6. higher sequential radical transformations were Scheme 1. systematically investigated by designing the appropriate substrates, selection of reaction conditions and reagents, and following sequences such as additionlcyclizationladdition, 1*2 addition/cyclization/atom transfer17 and fragmentation/addition/cyclization'8 were successfully applied in the synthesis of complex polycyclic compounds. t6 In all of these sequential radical reactions several radical intermediates were involved in the propagation step of these radical chain reactions. The reactivities of the intermediary radicals involved in the key sequence, e.g. addition/cyclization, have to be in accordance with the reactivity of the corresponding unsaturated bonds,"
RSC Adv., 2015
Absolute experimental and theoretical rate constants are determined for the first time for the reaction of 3hydroxy-3-methyl-2-butanone (3H3M2B) with OH radicals as a function of temperature. Experimental studies were carried out over the temperature range of 277 to 353 K and the pressure range of 5 to 80 Torr, by using a cryogenically cooled cell coupled to the PLP-LIF technique. OH radicals were generated for the first time from the photodissociation of the reactant 3H3M2B at 266 nm and the OH formation yield in 3H3M2B photolysis at 266 nm was measured under our experimental conditions. In addition, the reaction of 3H3M2B with OH radicals was studied theoretically by using the Density Functional Theory (DFT) method under three hydrogen abstraction pathways. According to these calculations, H-atom abstraction occurs more favourably from the methyl group adjacent to the hydroxyl group with a small barrier height. The calculated theoretical rate constants are in good agreement with the experimental data over the temperature range of 278 to 1000 K. No significant temperature dependence can be observed although a very slight effect was observed within the error bars.