Kinetics of radical heterolysis reactions forming alkene radical cations

Rate constants for heterolytic fragmentation of beta-(ester)alkyl radicals were determined by a combination of direct laser flash photolysis studies and indirect kinetic studies. The 1,1-dimethyl-2-mesyloxyhexyl radical (4a) fragments in acetonitrile at ambient temperature with a rate constant of k(het) > 5 x 10(9) s(-1) to give the radical cation from 2-methyl-2-heptene (6), which reacts with acetonitrile with a pseudo-first-order rate constant of k = 1 x 10(6) s(-1) and is trapped by methanol in acetonitrile in a reversible reaction. The 1,1-dimethyl-2-(diphenylphosphatoxy)hexyl radical (4b) heterolyzes in acetonitrile to give radical cation 6 in an ion pair with a rate constant of k(het) = 4 x 10(6) s(-1), and the ion pair collapses with a rate constant of k < or = 1 x 10(9) s(-1). Rate constants for heterolysis of the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(diphenylphosphatoxy)ethyl radical (5a) and the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(trifluoroacetoxy)ethyl radical (5b) were measured in various solvents, and an Arrhenius function for reaction of 5a in THF was determined (log k = 11.16-5.39/2.3RT in kcal/mol). The cyclopropyl reporter group imparts a 35-fold acceleration in the rate of heterolysis of 5a in comparison to 4b. The combined results were used to generate a predictive scale for heterolysis reactions of alkyl radicals containing beta-mesyloxy, beta-diphenylphosphatoxy, and beta-trifluoroacetoxy groups as a function of solvent polarity as determined on the E(T)(30) solvent polarity scale.     

Acid-, base-, and lewis-acid-catalyzed heterolysis of methoxide from an alpha-hydroxy-beta-methoxy radical: models for reactions catalyzed by coenzyme B12-dependent diol dehydratase

[Reaction: see text].A model for glycol radicals was employed in laser flash photolysis kinetic studies of catalysis of the fragmentation of a methoxy group adjacent to an alpha-hydroxy radical center. Photolysis of a phenylselenylmethylcyclopropane precursor gave a cyclopropylcarbinyl radical that rapidly ring opened to the target alpha-hydroxy-beta-methoxy radical (3). Heterolysis of the methoxy group in 3 gave an enolyl radical (4a) or an enol ether radical cation (4b), depending upon pH. Radicals 4 contain a 2,2-diphenylcyclopropane reporter group, and they rapidly opened to give UV-observable diphenylalkyl radicals as the final products. No heterolysis was observed for radical 3 under neutral conditions. In basic aqueous acetonitrile solutions, specific base catalysis of the heterolysis was observed; the pK(a) of radical 3 was determined to be 12.5 from kinetic titration plots, and the ketyl radical formed by deprotonation of 3 eliminated methoxide with a rate constant of 5 x 10(7) s(-1). In the presence of carboxylic acids in acetonitrile solutions, radical 3 eliminated methanol in a general acid-catalyzed reaction, and rate constants for protonation of the methoxy group in 3 by several acids were measured. Radical 3 also reacted by fragmentation of methoxide in Lewis-acid-catalyzed heterolysis reactions; ZnBr2, Sc(OTf)3, and BF3 were found to be efficient catalysts. Catalytic rate constants for the heterolysis reactions were in the range of 3 x 10(4) to 2 x 10(6) s(-1). The Lewis-acid-catalyzed heterolysis reactions are fast enough for kinetic competence in coenzyme B12 dependent enzyme-catalyzed reactions of glycols, and Lewis-acid-catalyzed cleavages of beta-ethers in radicals might be applied in synthetic reactions.     

Kinetic results implicating a polar radical reaction pathway in the rearrangement catalyzed by alpha-methyleneglutarate mutase

alpha-Methyleneglutarate mutase (MGM) catalyzes the rearrangement of 2-methyleneglutarate to 3-methylitaconate (2-methylene-3-methylsuccinate). A putative mechanism for the MGM-catalyzed reaction involves 3-exo cyclization of the 2-methyleneglutaric acid-4-yl radical to a cyclopropylcarbinyl radical intermediate that ring opens to the 3-hydroxycarbonyl-2-methylenebutanoic acid-4-yl radical (3-methylitaconic acid radical). Model reactions for this mechanism were studied by laser flash photolysis kinetic methods. alpha-Ester radicals were produced by 266 nm photolysis of alpha-phenylselenyl ester derivatives. Rate constants for cyclizations of the (Z)-1-ethoxycarbonyl-4-(2,2-diphenylcyclopropyl)-3-buten-1-yl radical ((Z)-8a) and (E)- and (Z)-1,3-di(ethoxycarbonyl)-4-(2,2-diphenylcyclopropyl)-3-buten-1-yl radicals ((E)- and (Z)-8b) were determined. The ester group in (Z)-8a accelerates the 3-exo cyclization in comparison to the parent radical lacking an ester group by a factor of 3, an effect ascribed to a polarized transition state. The ester groups at C3 in radicals 8b slow the 3-exo cyclization reaction by a factor of 50. The rate constant for cyclization of the 2-methyleneglutaric acid-4-yl radical is estimated to be k approximately 2000 s(-1) at ambient temperature. When coupled with the estimated partitioning of the intermediate cyclopropylcarbinyl radical, the overall rate constant for the conversion is estimated to be k approximately equal to 1 x 10(-3) s(-1), which is much too small for any radical reaction and several orders of magnitude too small for kinetic competence for the MGM-catalyzed process. The possibility that the radical reaction in nature involves an unusual mechanism in which polar effects are important is discussed.     

Efficient production of enol ether radical cations by heterolytic cleavage of beta-mesylate radicals

[reaction: see text] alpha-Methoxy-beta-mesyloxy radicals were produced in laser flash photolysis reactions, and yields of enol ether radical cations formed by heterolytic fragmentation of the mesylate group were determined. The mesylate heterolysis reaction is faster than heterolyses of phosphate and bromide groups in analogous radicals and highly efficient in medium-polarity solvents.     

Laser flash photolysis kinetic studies of enol ether radical cations. Rate constants for heterolysis of alpha-methoxy-beta-phosphatoxyalkyl radicals and for cyclizations of enol ether radical cations

Two series of enol ether radical cations were studied by laser flash photolysis methods. The radical cations were produced by heterolyses of the phosphate groups from the corresponding alpha-methoxy-beta-diethylphosphatoxy or beta-diphenylphosphatoxy radicals that were produced by 355 nm photolysis of N-hydroxypryidine-2-thione (PTOC) ester radical precursors. Syntheses of the radical precursors are described. Cyclizations of enol ether radical cations 1 gave distonic radical cations containing the diphenylalkyl radical, whereas cyclizations of enol ether radical cations 2 gave distonic radical cation products containing a diphenylcyclopropylcarbinyl radical moiety that rapidly ring-opened to a diphenylalkyl radical product. For 5-exo cyclizations, the heterolysis reactions were rate limiting, whereas for 6-exo and 7-exo cyclizations, the heterolyses were fast and the cyclizations were rate limiting. Rate constants were measured in acetonitrile and in acetonitrile solutions containing 2,2,2-trifluoroethanol, and several Arrhenius functions were determined. The heterolysis reactions showed a strong solvent polarity effect, whereas the cyclization reactions that gave distonic radical cation products did not. Recombination reactions or deprotonations of the radical cation within the first-formed ion pair compete with diffusive escape of the ions, and the yields of distonic radical cation products were a function of solvent polarity and increased in more polar solvent mixtures. The 5-exo cyclizations were fast enough to compete efficiently with other reactions within the ion pair (k approximately 2 x 10(9) s(-1) at 20 degrees C). The 6-exo cyclization reactions of the enol ether radical cations are 100 times faster (radical cations 1) and 10 000 times faster (radical cations 2) than cyclizations of the corresponding radicals (k approximately 4 x 10(7) s(-1) at 20 degrees C). Second-order rate constants were determined for reactions of one enol ether radical cation with water and with methanol; the rate constants at ambient temperature are 1.1 x 10(6) and 1.4 x 10(6) M(-1) s(-1), respectively.     

p-Nitrobenzenesulfenate esters as precursors for laser flash photolysis studies of alkyl radicals

A series of p-nitrobenzenesulfenate esters was used in laser flash photolysis (LFP) studies to generate alkoxyl radicals that fragmented to give the (2,2-diphenylcyclopropyl)methyl radical. Rate constants for the beta-scission reactions increased as a function of the carbonyl compound produced in the fragmentation reaction in the order CH2O < MeCHO < Me2CO < PhCHO < Ph2CO and increased with increasing solvent polarity. For alkoxyl radicals that fragment to produce benzaldehyde and benzophenone, the beta-scission reactions are faster than 1,5-hydrogen atom abstractions when the incipient carbon radical is as stable as a secondary alkyl radical, and this entry to carbon radicals can be used in LFP kinetic studies.     

Solvent polarity effects and limited acid catalysis in rearrangements of model radicals for the methylmalonyl-CoA mutase- and isobutyryl-CoA mutase-catalyzed isomerization reactions

The kinetics of reactions of models for the intermediate radicals formed in the methylmalonyl-CoA mutase- and isobutyryl-CoA mutase-catalyzed rearrangements were studied by laser flash photolysis methods. The aldehyde-containing model analogous to the propanal-3-yl radical reacted via 3-exo cyclization with rate constants that varied with solvent polarity (k in the range 2 x 105 to 1 x 107 s-1). The analogous methyl ketone-containing radical reacted 2 orders of magnitude less rapidly, and the ethylthiocarbonyl-containing radical analogue reacted too slowly for kinetic measurements. No acid catalysis was observed in acetic acid, but the CF3CO2H-complexed radicals reacted 1 order of magnitude faster than the uncomplexed radicals. The results indicate that catalysis of the 3-exo radical cyclizations of the radicals formed in the enzymes by hydrogen bonding to an acid, so-called "partial protonation", is not adequate for acceleration of the reactions to the point of kinetic competence. A dissociative mechanism for the radical rearrangements in nature is considered as an alternative.     

Product studies and laser flash photolysis on alkyl radicals containing two different beta-leaving groups are consonant with the formation of an olefin cation radical.

1-Bromo-2-methoxy-1-phenylpropan-2-yl (3) and 2-methoxy-1-phenyl-1-diphenylphosphatopropan-2-yl (4) were generated under continual photolysis from the respective PTOC precursors in a mixture of acetonitrile and methanol. The radicals undergo heterolytic fragmentation of the substituent in the beta-position to generate the olefin cation radical (5). Z-2-Methoxy-1-phenylpropene (15) is the major product formed in the presence of 1,4-cyclohexadiene, and is believed to result from hydrogen atom transfer to the oxygen of the olefin cation radical, followed by deprotonation. Laser flash photolysis experiments indicate that reaction between 5 and 1,4-cyclohexadiene occurs with a rate constant of approximately 6 x 10(5) M(-1) s(-1). 2,2-Dimethoxy-1-phenylpropane (18) is observed as a minor product. Laser flash photolysis experiments place an upper limit on methanol trapping of 5 at k <1 x 10(3) M(-1) s(-1) and do not provide any evidence for the formation of reactive intermediates other than 5. The use of two PTOC precursors containing different leaving groups to generate a common olefin cation radical enables one to utilize product analysis to probe for the intermediacy of other reactive intermediates. The ratio of 15:18 is dependent upon hydrogen atom donor concentration, but is independent of the PTOC precursor. These observations are consistent with the proposal that both products result from trapping of 5 that is formed via heterolysis of 3 and 4.     

Kinetic studies of a fast, reversible alkene radical cation cyclization reaction

The radical cation formed by mesylate heterolysis from the 1,1-dimethyl-7,7-diphenyl-2-mesyloxy-6-heptenyl radical was studied in several solvents. Computational results suggest that the initially formed acyclic radical cation is a resonance hybrid with partial positive charge in both double bonds of 1,1-diphenyl-7-methyl-1,6-octadiene (10). Thiophenol trapping was used as the competing reaction for kinetic determinations. The acyclic radical cation rapidly equilibrates with a cyclic distonic radical cation, and thiophenol trapping gives acyclic product 10 and cyclic products, mainly trans-1-(diphenylmethyl)-2-(1-methylethenyl)cyclopentane (11). The rate constants for cyclization at ambient temperature were k = (0.5-2) x 10(10)(s-1), and those for ring opening were k = (1.5-9) x 10(10)(s-1). Laser flash photolysis studies in several solvents show relatively slow processes (k = (2.5-260) x 10(5)(s-1) that involve rate-limiting trapping reactions for the equilibrating radical cations. In mixtures of fluoroalcohols RfCH2OH in trifluoromethylbenzene, variable-temperature studies display small, and in one case a negative, activation energies, requiring equilibration reactions prior to the rate-limiting processes. Fast equilibration of acyclic and cyclic radical cations implies that product ratios can be controlled by the populations of the acyclic and cyclic species and relative rate constants for trapping each.     

Compatible injection and detection systems for studying the kinetics of excess electron transfer

[reaction: see text] A design for fast kinetic studies of electron transfer in radical anions is reported. alpha-Hydroxy radicals formed by 355 nm laser flash photolysis of alpha-phenacyl alcohols are deprotonated under basic conditions to give ketyl radical anions that serve as electron injectors in inter- and intramolecular electron-transfer reactions. The 2,2-diphenylcyclopropyl group serves as a reporter. When an electron is injected and transferred such that spin character is adjacent to the reporter, cyclopropyl ring opening gives a readily detected diphenylalkyl radical.     

Rate constants for anilidyl radical cyclization reactions

[reaction: see text] N-Aryl-5,5-diphenyl-4-pentenamidyl radicals (3) were produced by 266 nm laser-flash photolysis of the corresponding N-(phenylthio) derivatives, and the rate constants for the cyclizations of these radicals were measured directly. The 5-exo cyclization reactions were fast (k(c) > 2 x 10(5) s(-1)), and radicals 3 generally behaved as electrophilic reactants with a Hammett correlation of rho = 1.9 for five of the six radicals studied. However, the p-methoxyphenyl-substituted radical 3f cyclized much faster than expected from the Hammett analysis. Variable temperature studies of parent radical 3a (aryl = phenyl) gave an Arrhenius function with log k = 9.2 - 4.4/2.3RT (kcal/mol). The rate constant for the reaction of p-ethylphenyl-substituted anilidyl radical 3b with Bu(3)SnH at 65 degrees C was k(T) = 4 x 10(5) M(-1) s(-1).     

Heterolytic cleavage of a beta-phosphatoxyalkyl radical resulting in phosphate migration or radical cation formation as a function of solvent polarity

[formula: see text] The 2-(diethylphosphatoxy)-2-(p-methoxyphenyl)-1,1-dimethylethyl radical (1) reacted to give the benzylic radical product from phosphate migration or a radical cation (or a mixture of the two) as a function of solvent. Smooth acceleration in rates of reactions of 1 in solvents of increasing polarity and consistent entropies of activation indicate that radical 1 reacts by common mechanism irrespective of the final products formed, specifically by initial heterolysis to a radical cation-phosphate anion pair.     

Studies of the gas phase reactions of linalool, 6-methyl-5-hepten-2-ol and 3-methyl-1-penten-3-ol with O3 and OH radicals

The reactions of three unsaturated alcohols (linalool, 6-methyl-5-hepten-2-ol, and 3-methyl-1-penten-3-ol) with ozone and OH radicals have been studied using simulation chambers at T ～ 296 K and P ～ 760 Torr. The rate coefficient values (in cm(3) molecule(-1) s(-1)) determined for the three compounds are linalool, k(O3) = (4.1 ± 1.0) × 10(-16) and k(OH) = (1.7 ± 0.3) × 10(-10); 6-methyl-5-hepten-2-ol, k(O3) = (3.8 ± 1.2) × 10(-16) and k(OH) = (1.0 ± 0.3) × 10(-10); and 3-methyl-1-penten-3-ol, k(O3) = (5.2 ± 0.6) × 10(-18) and k(OH) = (6.2 ± 1.8) × 10(-11). From the kinetic data it is estimated that, for the reaction of O(3) with linalool, attack at the R-CH═C(CH(3))(2) group represents around (93 ± 52)% (k(6-methyl-5-hepten-2-ol)/k(linalool)) of the overall reaction, with reaction at the R-CH═CH(2) group accounting for about (1.3 ± 0.5)% (k(3-methyl-1-penten-3-ol)/k(linalool)). In a similar manner it has been calculated that for the reaction of OH radicals with linalool, attack of the OH radical at the R-CH═C(CH(3))(2) group represents around (59 ± 18)% (k(6-methyl-5-hepten-2-ol)/k(linalool)) of the total reaction, while addition of OH to the R-CH═CH(2) group is estimated to be around (36 ± 6)% (k(3-methyl-1-penten-3-ol)/k(linalool)). Analysis of the products from the reaction of O(3) with linalool confirmed that addition to the R-CH═C(CH(3))(2) group is the predominant reaction pathway. The presence of formaldehyde and hydroxyacetone in the reaction products together with compelling evidence for the generation of OH radicals in the system indicates that the hydroperoxide channel is important in the loss of the biradical [(CH(3))(2)COO]* formed in the reaction of O(3) with linalool. Studies on the reactions of O(3) with the unsaturated alcohols showed that the yields of secondary organic aerosols (SOAs) are higher in the absence of OH scavengers compared to the yields in their presence. However, even under low-NO(X) concentrations, the reactions of OH radicals with 3-methyl-1-penten-3-ol and 6-methyl-5-hepten-2-ol will make only a minor contribution to SOA formation under atmospheric conditions. Relatively high yields of SOAs were observed in the reactions of OH with linalool, although the initial concentrations of reactants were quite high. The importance of linalool in the formation of SOAs in the atmosphere requires further investigation. The impact following releases of these unsaturated alcohols into the atmosphere are discussed.     

Lewis acid catalysis in heterolysis reactions of glycol ether radicals mimicking diol dehydratase-catalyzed reactions

Zinc bromide-catalyzed heterolysis reactions of glycol ether radicals were studied by laser flash photolysis methods, which gave the binding constants and catalytic rate constants for fragmentation. The Lewis acid-catalyzed heterolysis reactions mimic a putative reaction pathway in diol dehydratase-catalyzed reactions and are potentially useful polar processes for incorporation into conventional radical chain reaction sequences. [reaction: see text]     

The O-neophyl rearrangement of 1,1-diarylalkoxyl radicals. Experimental evidence for the formation of an intermediate 1-oxaspiro[2,5]octadienyl radical

A product study on the reactivity of a 1,1-diarylalkoxyl radical bearing 2,2-diphenylcyclopropyl groups in the para-positions has been carried out. The exclusive formation of a product deriving from cyclopropyl ring-opening has been observed, indicating that 1,1-diarylalkoxyl radicals exist in equilibrium with a bridged 1-oxaspiro[2,5]octadienyl radical. This represents the first experimental evidence in support of the stepwise nature of the O-neophyl rearrangement of 1,1-diarylalkoxyl radicals.     

A new kinetic approach to the evaluation of rate constants for the spin trapping of superoxide/hydroperoxyl radical by nitrones in aqueous media

A new kinetic approach to the evaluation of rate constants for the spin trapping of superoxide/hydroperoxyl radical by nitrones in buffered media is described. This method is based on a competition between the superoxide trapping by the nitrone and the spontaneous dismutation of this radical in aqueous media. EPR spectra are recorded as a function of time at various nitrone concentrations, and kinetic curves are obtained after treatment of these spectra using both singular value decomposition and pseudo-inverse deconvolution methods. Modelling these curves permits the determination of the rate constants k(T) and k(D) for the superoxide trapping and the adduct decay reactions, respectively. Kinetics parameters thus obtained with six nitrones, namely the 2-ethoxycarbonyl-2-methyl-3,4-dihydro-2H-pyrrole N-oxide (EMPO), the 5-diethoxyphosphoryl-5-methyl-3,4-dihydro-5H-pyrrole N-oxide (DEPMPO), the 5,5-dimethyl-3,4-dihydro-5H-pyrrole N-oxide (DMPO), the 1,3,5-tri[(N-(1-diethylphosphono)-1-methylethyl)-N-oxy-aldimine]benzene (TN), the N-benzylidene-1-ethoxycarbonyl-1-methylethylamine N-oxide (EPPN), and the N-[(1-oxidopyridin-1-ium-4-yl)methylidene]-1-ethoxycarbonyl-1-methylethylamine N-oxide (EPPyON), indicate that cyclic nitrones trapped superoxide faster than the linear ones. However, the low k(T) values obtained for compounds show that there is still a need for new molecules with better spin trapping capacities.     

Substituent and solvent effects on the beta-heterolysis reaction of beta-hydroxy arylethyl radicals

Time-resolved conversion of a series of beta-hydroxy arylethyl radicals with electron-donating and -withdrawing aromatic substituents to their corresponding styrene radical cation via heterolytic loss of the beta-hydroxy leaving group was examined with nanosecond laser flash photolysis. In all cases, the reaction was catalyzed by added perchloric acid. Radicals 2a-d reacted via a pre-equilibrium protonation mechanism in acidic 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), and measuring rate constants for radical cation formation as a function of acid content allowed for the determination of absolute rate constants ranging from 3.6 x 10(6) to 3.8 x 10(7) s(-1) for the loss of water from the protonated beta-hydroxy arylethyl radicals 2a-d, as well as the acidity constants, pKa approximately 1.5 (in HFIP), for the protonated radicals. The 4-methoxy-substituted beta-hydroxy arylethyl radical 2e reacted by rate determining protonation in HFIP with a second-order rate constant of k(H+) = 7.8 x 10(8) M(-1) s(-1). However, in acetonitrile, 2,2,2-trifluoroethanol, and mixtures of these two solvents, 2e reacted by pre-equilibrium protonation, allowing for solvent effects on the rate constant for loss of water from the protonated radical 2e to be determined. With use of these data, substituent electronic effects on the kinetics of the beta-heterolysis reaction are discussed. Differences in the effect of solvent on the rate constant for loss of water from the protonated beta-hydroxy arylethyl radicals and other beta-substituted arylethyl radicals are also discussed.     

Free-radical-induced chain breakage and depolymerization of poly(methacrylic acid): equilibrium polymerization in aqueous solution at room temperature

Hydroxyl radicals, generated by ionizing radiation in N2O saturated aqueous solutions, abstract H atoms from poly(methacrylic acid) at the methyl and methylene groups, and radicals 1 and 2 are formed, respectively. The reactions of the poly(methacrylic acid) radicals were investigated by pulse radiolysis (using optical and conductometric detection), EPR, product analysis, and kinetic simulations. The conductometric detection allowed us to measure the rate of chain scission and monomer release. Under conditions in which the polymer is largely deprotonated, the primary radical 1 abstracts a hydrogen (k= 3.5 x 10(2)s(-1)) from the methylene group, and this yields the more stable secondary radical 2. This radical undergoes chain scission by beta-fragmentation (k= 1.8 s(-1)), and the terminal (end-of-chain) radical 3 is formed. The polymer radicals terminate only slowly (2k= 80 dm3mol(-1)s(-1)). This allows effective depolymerization (depropagation) to take place (k=0.1 s(-1)). The yield of monomer release is higher than the original radical yield by up to two orders of magnitude. Once monomer is formed, it reacts with 3 (propagation, k= 15 dm3mol(-1)s(-1)), and a situation close to an equilibrium radical polymerization is approached. From these data, the equilibrium monomer concentration is calculated at 6.7 x 10(-3) mol dm(-3) at room temperature. The standard entropy of propagation is estimated at -185 to -150 J mol(-1)K(-1). Because the monomer reaches concentrations in the millimolar range, the *OH radicals increasingly react with monomers (results in oligomerization) rather than with the polymer. This effect is reflected by, for example, a lowering of chain-scission yields upon prolonged irradiation. In acid solutions, the decay of the polymer radicals becomes much faster (estimated at about 10(7)dm3mol(-1)s(-1) at pH3.5), and monomer release is no longer observed.     

Rate coefficients for the reaction of OH with a series of unsaturated alcohols between 263 and 371 K

Rate coefficients for the gas-phase reactions of OH radicals with four unsaturated alcohols, 3-methyl-3-buten-1-ol (k1), 2-buten-1-ol (k2), 2-methyl-2-propen-1-ol (k3) and 3-buten-1-ol (k4), were measured using two different techniques, a conventional relative rate method and the pulsed laser photolysis-laser induced fluorescence technique. The Arrhenius rate coefficients (in units of cm(3) molecule(-1) s(-1)) over the temperature range 263-371 K were determined from the kinetic data obtained as k1 = (5.5 +/- 1.0) x 10(-12) exp [(836 +/- 54)/T]; k2 = (6.9 +/- 0.9) x 10(-12) exp [(744 +/- 40)/T]; k3 = (10 +/- 1) x 10(-12) exp [(652 +/- 27)/T]; and k4 = (4.0 +/- 0.4) x 10(-12) exp [(783 +/- 32)/T]. At 298 K, the rate coefficients obtained by the two methods for each of the alcohols studied were in good agreement. The results are presented and compared with those obtained previously for the same and related reactions of OH radicals. Reactivity factors for substituent groups containing the hydroxyl group are determined. The atmospheric implications for the studied alcohols are considered briefly.     

Efficient syntheses of ethyl 4-cyano-5-hydroxy-2-methyl-1H-pyrrole-3-carboxylate and ethyl (2Z)-(4-cyano-5-oxopyrrolidin-2-ylidene)ethanoate

Ethyl 2-chloroacetoacetate and its 4-chloro isomer react with cyanoacetamide in the presence of the mild, nonnucleophilic base, triethylamine under stoichiometric conditions to give high yields of ethyl 4-cyano-2-hydroxy-2-methyl-5-oxopyrrolidine-3-carboxylate and ethyl (4-cyano-2-hydroxy-5-oxopyrrolidin-2-yl)acetate, respectively. These, under acid-catalyzed dehydration conditions, afforded ethyl 4-cyano-5-hydroxy-2-methyl-1H-pyrrole-3-carboxylate and ethyl (2Z)-(4-cyano-5-oxopyrrolidin-2-ylidene)ethanoate, respectively. Similarly, the 4-chloro isomer reacted with ethyl cyanoacetate to give the novel product, diethyl 2-cyano-4-oxohexanedioate. The use of triethylamine enables access to a whole new library of pyrrole derivatives from easily accessible, commercially available starting materials. The reactions described in this Letter enable access to libraries of important pyrrole systems in any of the isotopically enriched forms.