Alpha-hydroxyalkyl Radicals Research Articles

Effects of Azomethine Derivatives of 4,6-Di-tert-Butyl-2,3-Dihydroxybezaldehyde on Radiation-Induced Reactions Involving Alkyl and Alpha-Hydroxyalkyl Radicals

Effects of Azomethine Derivatives of 4,6-Di-tert-Butyl-2,3-Dihydroxybezaldehyde on Radiation-Induced Reactions Involving Alkyl and Alpha-Hydroxyalkyl Radicals

Effects of Azomethine Derivatives of 4,6-Di-tert-Butyl-2,3-Dihydroxybezaldehyde on Radiation-Induced Reactions Involving Alkyl and Alpha-Hydroxyalkyl Radicals

The effects of azomethine derivatives of 4,6-di-tert-butyl-2,3-dihydroxybenzaldehyde on the formation of radiolysis products of deaerated n-hexane and ethanol have been studied. It was shown that the catechol moiety in the tested compounds did not significantly affect their reactions with carbon-centered radicals. The aldimine group exhibited high activity as an α-hydroxyalkyl radical scavenger and low activity as a scavenger of alkyl radicals. The introduction of additional radical-inhibiting groups –OH, –SH, –NO2, and =N–NH– into the side fragment of a molecule led to a significant increase in the antiradical activity withrespect to various types of carbon-centered radicals.

Isomer-Selective Study of the OH Initiated Oxidation of Isoprene in the Presence of O2 and NO. I. The Minor Inner OH-Addition Channel

We report isomer-selective kinetics and mechanistic details for the hydroxyl radical-initiated oxidation of isoprene, in the presence of O(2) and NO, employing complementary experimental and theoretical techniques. Using a recently demonstrated photolytic route to initiate isomer-selective kinetics in OH-initiated oxidation of unsaturated hydrocarbons via the UV photolysis of iodohydrins, the photolysis of 1-iodo-2-methyl-3-buten-2-ol results in a single isomer of the possible four OH-isoprene adducts, specifically the minor channel associated with OH addition to one of the inner carbon atoms. Employing both the laser-photolysis/laser-induced fluorescence (LP/LIF) technique and time-dependent multiplexed photoionization mass spectrometry, we find clear experimental evidence supporting the prompt rearrangement of the initially formed beta-hydroxyalkyl radicals to alpha-hydroxyalkyl radicals, in agreement with Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation predictions. We have determined a rate constant of (3.3 +/- 0.5) x 10(-11) cm(3) molecule(-1) s(-1) for molecular oxygen to abstract a hydrogen atom from the alpha-hydroxyalkyl radical to form 4-penten-2-one and HO(2). This reaction provides a mechanistic route to C(5) carbonyl species as first-generation end products for the addition of hydroxyl radical to isoprene in the presence of O(2) and NO.

Addition−Elimination in the Reaction of α-Hydroxyalkyl Radicals with 3,5-Pyridinedicarboxylic Acid and Nicotinic Acid: Example of Inner Sphere Organic Electron Transfer

Reactions of alpha-hydroxyalkyl radicals with 3,5-pyridinedicarboxylic acid (3,5-PDCA) and nicotinic acid (NA) were studied at appropriate pHs in aqueous solutions by pulse radiolysis technique. At pH 1, CH(3)C*HOH and *CH(2)OH radicals were found to react with 3,5-PDCA by rate constants of 2.2 x 10(9) and 5.1 x 10(8) dm(3) mol(-1) s(-1), respectively, giving radical adduct species. The adduct species formed in the reaction of CH(3)C*HOH radicals with 3,5-PDCA underwent unimolecular decay (k = 9.8 x 10(4) s(-1)) giving pyridinyl radicals. Reaction of (CH(3))(2)C*OH, CH(3)C*HOH, and *CH(2)OH radicals with NA at pH 3.3 gave the adduct species which subsequently decayed to the pyridinyl radicals. At pH 1, wherein NA is present in the protonated form, (CH(3))(2)C*OH radicals directly transfer electrons to NA, whereas CH(3)C*HOH and *CH(2)OH radicals react with higher rate constants compared with those at pH 3.3, initially giving the adduct species which subsequently undergo elimination reaction giving pyridinyl radicals. Reactions of alpha-hydroxyalkyl radicals with 3,5-pyridinedicarboxylic acid and nicotinic acid are found to proceed by an addition-elimination pathway that provides one of the few examples of organic inner sphere electron-transfer reactions. Rate constant for the addition reaction as well as rate of elimination varies with the reduction potential of alpha-hydroxyalkyl radicals.

Reactions of 1-Hydroxy-1-methylethyl Radicals with NO2-: Time-Resolved Electron Spin Resonance

The reaction of the alpha-hydroxyalkyl radical of 2-propanol (1-hydroxy-1-methylethyl radical) with nitrite ions was characterized. A product of the reaction was assigned as the adduct nitro radical anion, [HO-C(CH(3))(2)NO(2)](*-). This radical was identified using time-resolved electron spin resonance (TRESR). The radical's magnetic parameters, the nitrogen hyperfine coupling constant (a(N) = 26.39 G), and its g-factor (2.0052) were the same as those of the nitro radical anion previously discovered in (*)OH spin-trapping experiments with the aci-anion of (CH(3))(2)CHNO(2). Production of [HO-C(CH(3))(2)NO(2)](*-) was determined to be 38% +/- 4% of the reaction of (CH(3))(2)C(*)-OH with nitrite. The reason why this fraction was less than 100% was rationalized by invoking the competitive addition at oxygen, which forms [HO-C(CH(3))(2)ONO](*-), followed by a rapid loss of (*)NO. Furthermore, by taking this mechanism into account, the bimolecular rate constant for the total reaction of (CH(3))(2)C(*)-OH with nitrite at reaction pH 7 was determined to be 1.6 x 10(6) M(-1) s(-1), using both decay traces of (CH(3))(2)C(*)-OH and growth traces of [HO-C(CH(3))(2)NO(2)](*-). This correspondence further confirms the nature of the reaction. The reaction mechanism is discussed with guidance by computations using density functional theory.

Method of Rapid Assessment of Photocatalytic Activities of Self-Cleaning Films

An ink, comprising the redox dye resazurin (Rz) and the sacrificial electron donor glycerol, is shown to be capable of the rapid assessment of the photocatalytic activities of self-cleaning films. In the key initial stage of photocatalysis the ink changes from blue to pink. Prolonged irradiation bleaches the ink and eventually mineralizes it. The kinetics of the initial photoinduced color change is studied as a function of UV irradiance, [glycerol], [Rz], and temperature. The results reveal an apparent approximate quantum yield of 3.5 x 10(-3) and an initial rate, r(i), which increases with [glycerol] and decreases with [Rz]. It is proposed that the reduction of Rz, dispersed throughout the thick (ca. 590 nm) indicator film, may take place either via the diffusion of the dye molecules in the ink film to the surface of the underlying semiconductor layer and their subsequent reaction with photogenerated electrons and/or via the diffusion of alpha-hydroxyalkyl radicals, produced by the oxidation of the glycerol by photogenerated holes, or hydroxy radicals, away from the surface of the semiconductor into the ink film and their subsequent reaction with the dye molecules therein. The decrease in r(i) with [Rz] appears to be due to dimer formation, with the latter impeding the reduction process. The activation energy for the initial color-change process is low, ca. 9.1 +/- 0.1 kJ mol(-1) and not unlike many other photocatalytic processes. The initial rate of dye reduction appears to be directly related to the rate of destruction of stearic acid. The ink can be applied by spin-coating, stamping, or writing, using a felt-tip pen. The efficacy of such an ink for assessing the photocatalytic activity of any photocatalytic film, including those employed on commercial self-cleaning glasses, tiles, and paving stones, is discussed briefly.

Tetrabutylammonium Decatungstate‐Photosensitized Alkylation of Electrophilic Alkenes: Convenient Functionalization of Aliphatic CH Bonds

Tetrabutylammonium decatungstate (TBADT, 2 x 10(-3) m) is an effective photocatalyst for the alkylation of electrophilic alkenes (0.1 m, alpha,beta-unsaturated nitriles, esters, ketones) by alkanes, alcohols, and ethers. The products are in most cases obtained in >70 % isolated yields, through an experimentally very simple procedure. The kinetics of the radical processes following initial hydrogen abstraction by excited TBADT in deoxygenated MeCN have been studied. In the absence of a trap, back hydrogen transfer from reduced tungstate is the main pathway for alkyl radicals, while alpha-hydroxyalkyl radicals are oxidized to ketones by ground-state TBADT. With both radical types the reaction ceases at a few percent conversion. However, trapping by electrophilic alkenes is followed by reduction of the radical adduct and regeneration of the catalyst, which allows the alkylation to proceed up to complete alkene conversion with the mentioned good yields of products. With a nucleophilic (alpha-hydroxyalkyl) radical, alkylation is efficient (Phi = 0.58) and can also be carried out when degassing is omitted, the only difference being a short induction period. With a less reactive (cyclohexyl) radical, the quantum yield is lower (Phi = 0.06) and the reaction is considerably slowed in aerated solutions, but the chemical yield remains good.

The OH-Initiated Oxidation of 1,3-Butadiene in the Presence of O2 and NO: A Photolytic Route To Study Isomeric Selective Reactivity

We report the study of the isomeric selective OH-initiated oxidation of 1,3-butadiene in the presence of O2 and NO using the LP/LIF technique. The photolysis of monodeuterated 1-iodo-3-buten-2-ol provides only one of the possible OD-butadiene adducts, the minor addition channel product, simplifying the oxidation mechanism. We find, based on analysis of OD time-dependent traces that prompt rearrangement of initial beta-hydroxyalkyl radicals to alpha-hydroxyalkyl radicals occurs in agreement with RRKM/ME theoretical predictions. We report a rate constant of (3.3+/-1.0) x 10(-11) cm3 molecules(-1) s(-1) for deuterium abstraction from the alpha-hydroxyalkyl radical at 298 +/-2 K. Our approach demonstrates the feasibility of isomeric selective kinetic studies of the OH-initiated oxidation of unsaturated hydrocarbons.

Biomimetic alcohol oxidations by an iron(iii) porphyrin complex: relevance to cytochrome P-450 catalytic oxidation and involvement of the two-state radical rebound mechanism

The systematic oxidation reactions of a wide range of alcohols have been carried out by using an iron porphyrin complex in order to understand their relation to cytochrome P-450 enzymes and to have a practical application to organic synthesis. The iron porphyrin complex catalyzed efficiently alcohol oxidation to the respective carbonyl compound via a high-valent iron-oxo porphyrin intermediate ((Porp)Fe=O+). Several mechanistic studies such as isotope 18O labeling, deuterium isotope effect, linear free energy relationship, and ring-opening of radical clock substrate, have suggested that the alcohol is oxidized by a sequence of reactions involving an alpha-hydroxyalkyl radical intermediate and oxygen rebound to form the gem-diol, dehydration of which yields the carbonyl compounds. Moreover, it has been proposed that a two-state reactivity mechanism can also be adopted for alcohol oxidation reactions in iron porphyrin model systems as exhibited by P-450 enzymes.

Oxidation potentials of alpha-hydroxyalkyl radicals in acetonitrile obtained by photomodulated voltammetry.

Oxidation potentials, E1/2(ox) of alpha-hydroxyalkyl radicals of the type R(1)R(2)C(*)OH (denoted K(1)H(*)) have been obtained in acetonitrile by photomodulated voltammetry. The values of E1/2(ox) increase as the R(1) and R(2) groups are changed from alkyl to aryl and, in particular, strong electron-withdrawing functionalities such as CN and CF3. Using rate data available in the literature for the pinacol photoexchange reaction K + K(1)H(*) --> KH(*) + K(1), it is found that as the difference in the standard potential of the ketone K, EK degrees and the oxidation potential of K(1)H(*), E1/2(ox), increases there is a modest increase in the exchange rate constant, k(ex). This indicates that even if some charge transfer may occur between the hydroxyalkyl radical and the ketone in the transition state, it is certainly not to the extent of a complete electron transfer. If the exchange reaction is treated as a simple hydrogen atom transfer process within the Marcus model, the intrinsic barrier is found to be 8-13 kcal mol(-1) due to the changes occurring in bonds, hybridizations, and bond angles. Finally, acid dissociation constants for K(1)H(*) are provided by means of a thermochemical cycle.

Photoinduced chain reactions of alcohols in the presence of diphenyliodonium ion pairs with cyanometallates--steady state UV/visible spectroscopic and pulse radiolysis studies.

Diphenyliodonium ions (Ph2I+) form donor-acceptor ion pairs with suitable cyanometallates such as [Mo(CN)8]4-, [W(CN)8]4-, [Ru(CN)6]4- and [Os(CN)6]4-. Such ion pairs are characterized by new spectroscopic transitions due to second-sphere interactions between donor ([M(CN)x]4-, x=6, 8) and acceptor (Ph2I+) ions. Photochemical excitation of these ion-pair charge-transfer (IPCT) states leads to efficient electron transfer reactions that yield short-lived diphenyliodyl radicals (Ph2I*) and oxidized cyanometallates ([M(CN)x]3-). Diphenyliodyl radicals decay to iodobenzene and phenyl radicals. This very convenient source for generating phenyl radicals was applied to the photoinduced chain oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones, respectively. However, unexpected side reactions led to undesired chain-terminating reactions. Adduct formation of diphenyliodonium ions with alpha-hydroxyalkyl radicals was verified by pulse radiolysis studies. These relatively longlived adducts give rise to chain-terminating reactions because of interactions with [M(CN)x]4- complexes that lead to oxidized cyanometallates [M(CN)x]3- upon regeneration of the starting alcohols.

Addition of e-aq and H atoms to hypoxanthine and inosine and the reactions of alpha-hydroxyalkyl radicals with purines. A pulse radiolysis and product analysis study.

The reactions of hydrated electrons e-aq with hypoxanthine and inosine were followed using pulse radiolysis methods. In a neutral solution the electron adduct of inosine is immediately protonated at the heteroatoms of the purine ring by water (k >> 2.5 x 10(6)s-1) to give In(N,O-H).. These N,O-protonated intermediates have a single absorption maximum at 300 nm. In basic solution the protonation of the electron adduct of inosine by water leads to other intermediate products with an absorption maximum at 350 nm. These intermediates are believed to be the C-protonated electron adducts of inosine (In(N,O-H).). In (N,O-H). and In(C-H). differ strongly in their ability to reduce p-nitroacetophenone (PNAP). In(N,O-H). are strong reductants and reduce PNAP quantitatively to PNAP.-. Based on the pH dependence of PNAP.- yields, two types of tautomers of In(C-H). could be distinguished. One of the tautomers can reduce PNAP, albeit with slower rate than In(N,O-H)., the other tautomer has no reducing properties. The latter is the one with the higher pKa and therefore thermodynamically more stable. The absorption spectrum of the intermediates produced in the reaction of e-aq with hypoxanthine at neutral pH is very similar to that of In(N,O-H). with a maximum at 300 nm. However, no build-up at 350 nm was observed in basic solution as in the case of the electron adduct of inosine. The reaction of H atoms with inosine produces in basic solution intermediate radicals with the same absorption spectrum as the C-protonated electron adducts of inosine. It is suggested that both the reactions of e-aq and H. with inosine in basic solution produce the same radical, namely the H-adduct of inosine (In(C-H)) with the highest pKa. alpha-Hydroxyalkyl radicals were found to react very slowly with purine bases and nucleosides in neutral to basic solutions. In acidic solution their reactivity increases and a number of rate constants were determined by pulse radiolysis measurements at pH 0.4. The intermediates from the reaction of 2-hydroxy-2-propyl radicals with inosine could be observed pulse spectrometrically in neutral and in basic solutions. In basic solution this reaction leads to intermediates with the same absorption maximum at 350 nm as that of the H-adduct of inosine. Furthermore, the yield of acetone was found to increase strongly in basic pH.(ABSTRACT TRUNCATED AT 400 WORDS)