Thermolysis Rate Constants Research Articles

Capture and Release of Singlet Oxygen in Coordination-Driven Self-Assembled Organoplatinum(II) Metallacycles.

Singlet oxygen (1O2), as an important active reagent, has found wide applications in photodynamic therapy (PDT), synthetic chemistry, and materials science. Organic conjugated aromatics serving as hosts to capture and release singlet oxygen have been systematically investigated over the last decades. Herein, we present a [6 + 6] organoplatinum(II) metallacycle by using ∼180° dipyridylanthracene donor and ∼120° Pt(II) acceptor as the building blocks, which enables the capture and release of singlet oxygen with relatively high photooxygenation and thermolysis rate constants. The photooxygenation of the metallacycle to the corresponding endoperoxide was performed by sensitized irradiation, and the resulting endoperoxide is stable at room temperature and can be stored under ambient condition over months. Upon simple heating of the neat endoperoxide under inert atmosphere at 120 °C for 4 h, the resulting endoperoxide can be reconverted to the corresponding parent form and singlet oxygen. The photooxygenation and thermolysis products were characterized by NMR spectroscopy and electrospray ionization time-of-flight mass spectrometric analysis. Density functional theory calculations were conducted in order to reveal the frontier molecular orbital interactions and reactivity. This work provides a new material platform for singlet oxygen related promising applications.

Features of the thermal decomposition of diacylic diperoxides in various organic solvents

Thermal decomposition of diacylic diperoxides of various structures was studied: R 1 -C(O)O-O-C(O)-R 2 -C(O)-O-O(O)C-R 1 .It has been shown that the frontier radicals R 1 of the diperoxide molecule influence the rate of the thermal decomposition of diperoxide. The located R 2 radicals that are between peroxide groups practically do not affect the rate of decomposition of peroxide groups. Thethermolysis of didekanoildiperoxyadipinate in various organic solvents has been studied. It has been shown that the primary homolytic dissociation of the peroxide group is accompanied by secondary reactions of chain-inducted decomposition. The reaction medium affects the rate of both primary homolytic dissociation and secondary induced decomposition processes. Correlation equations between the rate constants of the reactions in study and the physicochemical parameters of the solvents were suggested. The correlation equation for effective thermolysis rate constants and the basic physicochemical parameters of solvents at 338 K is the following: k=51.0872+(0.0840±0.0067)В-(0.7854±0.1528)E T - (0.2273±0.0335)V M (3) N=19; R=0.9548; S=±2.6880; F=1.2519. The basicity, electrophilicity and molar volume of solvent have effect of thermolysis process. The influence of polarization, polarizability and Hilderbrand's function on the thermal decay process is negligible.Correlation equations for effective rate constants for other temperatures are similar. The suggested correlation equations relate the parameters of the transition state of the thermolysis process and the physic-chemical parameters of solvents. Keywords : thermolysis, solvent effects, correlations equations, diperoxide, activation energy.

ESR spin trapping determination of the hydroperoxide concentration in polyethylene oxide (PEO) in aqueous solution

Initial hydroperoxide (ROOH), which exists even in freshly produced polymers, acts as an initiator for the oxidative degradation of the polymer. Electron spin resonance (ESR) with the spin trapping agent 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was used to investigate the presence of ‘initial ROOH’ in a polyethylene oxide (PEO) sample. The degradation of PEO was investigated by heating or irradiating mixtures of PEO/DMPO/H2O with or without dissolved oxygen by ultraviolet-visible (UV-vis, λ > 250 nm) and visible (λ > 350 nm) light. The concentration of ‘initial ROOH’ and the photolysis and thermolysis rate constants were estimated from the concentrations of the DMPO–H, DMPO–C–PEO, and DMPO–OH spin adducts. Knowledge of the oxidative degradation mechanism of polymers allows the ‘initial ROOH’ concentration to be used as a quality control indicator for polymers.

PH-Sensitive C–ON Bond Homolysis of Alkoxyamines of Imidazoline Series: A Theoretical Study

The pH-dependent kinetics of C-ON bond homolysis reactions of the four alkoxyamines (viz., the derivatives of 2-(4-(dimethylamino)-2-ethyl-5,5-dimethyl-2-(pyridin-4-yl)-2,5-dihydro-1H-imidazol-1-oxyl and 2-(2-carboxyethyl)-5,5-diethyl-2,4-dimethyl-2,5-dihydro-1H-imidazol-1-oxyl) in water solution have been scrutinized using DFT calculations (M06-2X/6-311++G(2df,p) level of theory with the PCM model). On the basis of computations, the experimental results obtained before (J. Org. Chem. 2011, 76, 5558) have been rationalized. The concentration dependence of all possible protonated forms of the four alkoxyamines was obtained from pH measurements. The contributions of particular protonated forms into the overall rate constants of thermolysis were estimated using the DFT calculated Gibbs free energies Δ(r)⁰G of C-ON bond homolysis reactions. The computations revealed that the observed rate constants of thermolysis of the two species at several pH values are dominated by decomposition reactions of two or even three protonated forms. The observed trends in reactivity of the alkoxyamines were mainly attributed to destabilization of the radical products of C-ON bond scission reactions. A linear correlation between the sum of radical stabilization energies (RSEs) of products of thermolysis and the calculated Gibbs free energies of reactions was found for various protonated forms of the species studied. Apart from this, the linear correlation exists between the relative RSE and nitrogen hyperfine constants aN of various protonated forms of the nitroxide radical products.

Thermolysis kinetics of diethyl 2,3‐dicyano‐2,3‐di(p‐substituted phenyl)succinates

An experimental approach was developed to determine the intrinsic thermolysis rate constants of the central carbon–carbon bond during the dl/meso isomerization of diethyl 2,3‐dicyano‐2,3‐di(p‐substituted phenyl)succinates (G=H, Me, OMe, Cl, and NO2) at temperatures ranging from 80 to 120 °C. The obtained rate constants are significantly affected by the polarity of the para substituents, in sharp contrast to their negligible effects on the dl/meso isomerization equilibrium constants. Moreover, the substituent effects on the activation enthalpies can be linearly correlated with the Hammett substituent resonance constants and the homolytic dissociation enthalpies (bond dissociation energies) of the benzylic C–H bonds of ethyl 2‐cyano‐2‐(p‐substituted phenyl)acetates. Copyright © 2011 John Wiley & Sons, Ltd.

Thermal decomposition of aliphatic peroxy acids

The thermal decomposition reactions of aliphatic peroxy acids containing from 8 to 16 carbon atoms in a molecule were studied. It was found that the carbon radical length had no effect on the thermal stability of peroxide groups. The apparent rate constants of thermolysis of peroxydecanoic acid in various solvents and the activation energies of the test reaction were found. The thermal degradation of peroxy acids involved secondary reactions of induced chain degradation in addition to the primary homolysis of the peroxide group. The rate constants of induced chain degradation were found.

Synthesis and thermolysis rate constants of diastereomeric oxadiazoline sources of acetoxy(methoxy)carbene — Reaction of acetoxy(methoxy)carbene with isocyanates

Oxidation of the methoxycarbonylhydrazone of p-methoxyacetophenone affords both the cis- and trans-2-acetoxy-2-methoxy-5-(p-methoxyphenyl)-5-methyl-Δ3-1,3,4-oxadiazolines (also known as corresponding 2,5-dihydro-1,3,4-oxadiazoles) as well as methyl 1-acetoxy-1-(p-methoxyphenylethyl)diazenecarboxylate. The three isomers were separated and identified by spectroscopic means. Methyl 1-acetoxy-1-(p-methoxyphenylethyl)diazenecarboxylate is the major product from oxidation in dichloromethane. Oxidation in acetic acid did not afford the oxadiazolines but gave the diazenecarboxylate and, in addition, 1-(p-methoxyphenyl)ethyl acetate. Attempts to isomerize the diazenecar boxylate to the oxadiazolines by acid catalysis were not successful. Thermolysis of the oxadiazolines at 50.4 °C occurred with approximately the same rate constant (ca. 3.6 × 10–5 s–1) to afford acetoxy(methoxy)carbene, which rearranges to methyl pyruvate by acetyl transfer. The carbene, which reacts with relatively unhindered isocyanates to transfer the methoxy carbonyl group to carbon and the acetyl group to nitrogen, can be considered an acyl anion equivalent in that reaction.Key words: acetoxy(methoxy)carbene, diazene, oxadiazoline, isocyanate, (acetylamino)oxoacetate.

Thermal decomposition mechanisms of tert-alkyl peroxypivalates studied by the nitroxide radical trapping technique

The thermolysis of a series of tert-alkyl peroxypivalates 1 in cumene has been investigated by using the nitroxide radical-trapping technique. tert-Alkoxyl radicals generated from the thermolysis underwent the unimolecular reactions, beta-scission, and 1,5-H shift, competing with hydrogen abstraction from cumene. The absolute rate constants for beta-scission of tert-alkoxyl radicals, which vary over 4 orders of magnitude, indicate the vastly different behavior of alkoxyl radicals. However, the radical generation efficiencies of 1 varied only slightly, from 53 (R = Me) to 63% (R = Bu(t)()), supporting a mechanism involving concerted two-bond scission within the solvent cage to generate the tert-butyl radical, CO(2), and an alkoxyl radical. The thermolysis rate constants of tert-alkyl peroxypivalates 1 were influenced by both inductive and steric effects [Taft-Ingold equation, log(rel k(d)) = (0.97 +/- 0. 14)Sigmasigma - (0.31 +/- 0.04)SigmaE(s)(c), was obtained].

Kinetics and mechanism of thermolysis of 1-phenyl-2-pyridylethanes

Kinetics and mechanism of thermolysis of 1-phenyl-2-(4-pyridyl)ethane have been studied and the thermolysis rate constants of its two isomers have been calculated. The calculated and experimentally determined thermolysis rate constants of 1-phenyl-2-(3-pyridyl)ethane agree well. Compared with 1,2-diphenylethane, the introduction of heteroatom, nitrogen into meta position accelerated the thermolysis slightly, but retarded it into para- and ortho-position.

Correlation between structure and reactivity of diarylmethanes as coal-related model substances

The kinetics of diarylmethanes thermolysis has been studied in excess tetralin and under hydrogen pressure. It has been shown that the logarithms of thermolysis rate constants for the most diarylmethanes are proportional to the sums of the reactivity indices of carbon atoms connected with the methylene group, i.e. log k = f ( ϵN t ). This conformity accords with the mechanism of ipso-attack as the limiting stage of the whole process. But two diarylmethanes do not submit to this conformity: 9-benzylanthracene reacts with a rate ten times greater than the predicted one and diphenylmethane transforms with much lesser rate as compared to the predicted one. These deviations have been explained. 9-Benzylanthracene is easily hydrogenated before destruction, i.e. in this case a much weaker bond, C alk-C alk, than the C ar-C alk bond is ruptured. Diphenylmethane is disproportionated into diphenylmethyl and benzylcyclohexadienyl radicals. The former is less active and transforms relatively slowly into fluorene, and the latter is the more active but it undergoes exchange in the presence of tetralin giving back diphenylmethane and cyclohexadienylic radical of tetralin. This is the reason for the relative inactivity of diphenylmethane.

Synthesis and thermolysis of tetrasubstituted cyclopropenes

Photolysis of 5-benzyl-2-methoxy-2,5-dimethyl-Δ3-1, 3,4-oxadiazoline and of the 5,5-dibenzyl analogue with 300-nm light afforded 1-phenyl-2-diazopropane and 1,3-diphenyl-2-diazopropane, respectively. The diazoalkanes were intercepted, in situ, with dimethyl acetylenedicarboxylate to afford 3-benzyl-4,5-bis(methoxycarbonyl)-3-methyl-3H-pyrazole and 3,3-dibenzyl-4,5-bis(methoxycarbonyl)-3H-pyrazole, respectively. Those pyrazoles are short-lived under the reaction conditions and undergo two major reactions. Photolysis prior to rearrangement affords the corresponding 3,3-dialkyI-1,2-bis(methoxycarbonyl)-cyclopropenes. Thermal 1,5-benzyl migration converts the two 3H-pyrazoles in part into the corresponding 4H-pyrazoles, which undergo photolysis to 2,3-dialkyl-1,3-bis(methoxycarbonyl)cyclopropenes. Thermolysis of the 3,3-dialkyl-1,2-bis(methoxycarbonyl) cyclopropenes affords conjugated dienes, presumably through the sequence cyclopropene → vinyl carbene → diene. The stereochemistry of the dienes was determined and a mechanism consistent with that stereochemistry is proposed. The 2,3-dialkyl-1,3-bis(methoxycarbonyl)cyclopropanes are very stable under conditions that convert isomeric 3,3-dialkyl-1,2-bis(methoxycarbonyl)cyclopropenes to conjugated dienes. It is proposed that the effect of substitution pattern on the thermolysis rate constants is the result of combined ground state and transition state factors.

Effect of the structure of coal-related model ethers on the rate and mechanism of their thermolysis. 3. Additive effect of substituents

In a previous communication 1 the effect of substituents in C 6H 5CH 2OC 6H 4X ethers on the rate of thermolysis was reported and a correlation was shown between the rate constants of thermolysis and the Hammet constants of the substituents. Additional experiments have demonstrated that introduction of the substituents into the ring at the methylene group exerts a similar effect and introduction of the substituents into both rings produces an additive effect.

Effect of the structure of coal-related model ethers on the rate and mechanism of their thermolysis: 2. Effect of substituents in the C 6H 5CH 2OC 6H 4X structure

The pathways of conversion were studied and the rate constants of thermolysis for meta- and para-substituted benzyl aryl ethers were measured under conditions close to the initial stages of coal liquefaction (hydrogen pressure, 8.5 MPa, 275–350 °C; solvent: tetralin). The destruction of ethers is accompanied by rearrangement into the corresponding derivatives of benzylphenol that amount to 20–25 % of the reaction products. The electron-donating substituents increase the thermolysis rate whereas the electron-accepting substituents decrease it. The effect of substituents on thermolysis rate is described by the Hammet equation with ϱ constant at 325 °C equal to −2.75.

Bifunctional initiators, II. Synthesis and characterization of 5,5′‐azo‐bis‐(5‐cyanocaproyl)‐benzoyl diperoxide

Abstract5,5′‐Azo‐bis‐(5‐cyanocaproyl)‐benzoyl diperoxide was synthesized starting from 5,5′‐azo‐bis‐(5‐cyanohexanoic) acid, obtained in its turn from 5‐keto‐hexanoic acid, hydrazine sulphate and sodium cyanide.5,5′‐Azo‐bis‐(5‐cyanohexanoic) acid was then converted into acid chloride and condensed with perbenzoic acid. The structures of intermediates and of the product were elucidated by means of elemental analysis and IR and NMR spectral measurements.In order to settle the bifunctional character of the synthesized initiator the thermolysis rate constants for azo and peroxy groups, energies of activation and half‐life times were settled. By comparing the kinetic data with those obtained in 4,4′‐azo‐bis‐(4‐cyanovaleryl)‐benzoyl diperoxide thermolysis the decomposition rate was found to increase and the bifunctional character to enhance with increasing length of the radical connecting the azo and peroxy groups.