Ester decomposition as a two-center synchronous reaction

Experimental data on the molecular decomposition of esters with various structures into an olefin and the corresponding acid in the gas phase are analyzed in terms of the intersecting parabolas method. Enthalpies and kinetic parameters characterizing this decomposition have been calculated for 33 reactions. Ester decomposition is a concerted two-center reaction characterized by a very high classical potential barrier of thermoneutral reaction (148–206 kJ/mol). The totality of reactions examined is divided into eight classes. Activation energies and rate constants have been calculated for 38 reactions using the kinetic parameters obtained. The activation energies and rate constants of the reverse bimolecular reaction of acid addition to olefins have been calculated by the intersecting parabolas method. Factors in the activation energy of ester decomposition and formation reactions are discussed.     

An analysis of the molecular concerted decomposition of nitroalkanes by the method of intersecting parabolas

The molecular concerted decomposition of nitroalkanes was analyzed using the model of two intersecting parabolas. Parameters describing the activation energy of decomposition as a function of the enthalpy of the reaction were obtained. These parameters were used to calculate the activation energies of unstudied reactions of the decomposition of RNO₂ and the activation energies and rate constants for reverse combination reactions between olefins and nitrous acid with the formation of the corresponding nitroalkanes. The concerted decomposition of nitroalkanes was also analyzed using another two-center model, in which decomposition was treated as two single-stage events, the transfer of H from the C-H bond to the O atom of the nitro group and the dissociation of the C-N bond accompanied by a shift of electrons in a five-membered reaction center. This approach and two elementary event models (a model of two intersecting parabolas and a model of the superposition of three parabolas) were used to calculate the spectrum of activation energies, in which each overall reaction event was characterized by an activation barrier of its own.     

Decomposition reactions of azo compounds,as studied by quantum chemistry methods and the parabolic model

Decomposition reactions of azoalkanes of different structure were studied by quantum chemistry methods (MP2/6-311++G** calculations) and by the method of three intersecting parabolas (M3IP). The MP2 method was used to obtain the transition-state geometries, the bond lengths in the molecules under study, and the activation energies. Possible mechanisms of decomposition are discussed. Concerted decomposition of branched azoalkanes was shown to be the most probable mechanism of the process. The M3IP method was used to calculate the kinetic and thermodynamic parameters of concerted decomposition of azoalkanes and to determine and evaluate the main factors affecting the activation energy (E). The stabilization energy of the radical being formed in the decomposition reaction is one of the key factors determining the concerted mechanism. The kinetic parameters calculated by the two independent methods are in good agreement.

     

Reactivity of fluoroalkanes in reactions of coordinated molecular decomposition

Experimental results on the coordinated molecular decomposition of RF fluoroalkanes to olefin and HF are analyzed using the model of intersecting parabolas (IPM). The kinetic parameters are calculated to allow estimates of the activation energy (E) and rate constant (k) of these reactions, based on enthalpy and IPM algorithms. Parameters E and k are found for the first time for eight RF decomposition reactions. The factors that affect activation energy E of RF decomposition (the enthalpy of the reaction, the electronegativity of the atoms of reaction centers, and the dipole–dipole interaction of polar groups) are determined. The values of E and k for reverse reactions of addition are estimated.     

Kinetics and thermodynamics of the exchange reactions of peroxy radicals with hydroperoxides

The enthalpies and equilibrium constants of the exchange reactions of peroxy radicals with hydroperoxides of various structures are calculated. The experimental data on the reactions of hydrogen atom abstraction by the peroxy radicals from the hydroperoxides are analyzed, and the kinetic parameters characterizing these reactions are calculated using the intersecting parabolas method. The activation energies and rate constants for nine reactions of H atom abstraction by a peroxy radical from the OOH group of a peroxide are calculated using the above parameters. The geometric parameters of the transition states for the reactions are calculated. The low triplet repulsion plays an important role in the fast occurrence of the reactions. The polar interaction in the transition state is manifested in the reactions of the peroxy radicals with hydroperoxides containing a polar group.     

Kinetic parameters and geometry of the transition state of acyl radical decarbonylation

Experimental data on acyl radical decomposition reactions (RC^·O → R^· + CO, where R = alkyl or aryl) are analyzed in terms of the intersecting parabolas method. Kinetic parameters characterizing these reactions are calculated. The transition state of methyl radical addition to CO at the C atoms is calculated using the DFT method. A semiempirical algorithm is constructed for calculating the transition state geometry for the decomposition of acyl radicals and for the reverse reactions of R^· addition to CO. Kinetic parameters (activation energy and rate constant) and geometry (interatomic distances in the transition state) are calculated for 18 decomposition reactions of structurally different acyl radicals. A linear correlation between the interatomic distance r ^#(C…C) (or r ^#(C…O)) in the transition state the enthalpy of the reaction (δH _e) is established for acyl decomposition reactions (at br _e = const). A comparative analysis of the enthalpies, activation energies, and interatomic distances in the transition state is carried out for the decomposition and formation of acyl, carboxyl, and formyl radicals.     

Polar effect and geometry of the transition state in the reactions of ozone with C-H bonds of polar molecules

A large body of experimental data on the reactions of ozone with C-H bonds of polar molecules in the liquid and gas phases is analyzed in the framework of the intersecting parabolas model. The reactions are considered as the abstraction reaction O₃ + RH → HOOO^. + R^.. The contribution from the polar effect to the activation energy of such reactions is calculated. This contribution is ?6.8 kJ/mol for the reactions of ozone with aliphatic alcohols, and is ?8.1, ?11.7, ?6.8, and ?2.2 kJ/mol for the reactions of ozone with ketones, ethers, 1,3-dioxolanes, and 1,3-dioxanes, respectively. The contribution is insignificant in the reactions of ozone with aldehydes. The interatomic distances in the transition state of these reactions r ^#(C…H) and r ^#(O…H) and the angle between the C…H and O…H bonds are calculated. For the reactions in polar solvents, the contribution from solvation to the activation energy is calculated. In most of the systems considered, this contribution is insignificant (from ?1 to ?3 kJ/mol). The reactions involving ozone are compared to the reactions of peroxy radicals with the same classes of compounds.     

Role of Radical Elimination Reactions with Concerted Fragmentation in the Chain Decomposition of Alkyl Nitrates

The reactions X? + HCR₂ONO₂ → XH + R₂C=O + ?NO₂ are very exothermic due to the cleavage of the weak N?O bond and the formation of the energy-intensive C=O bond. The quantum chemical calculation of the transition state of these reactions for X? = Et? and EtO? used as examples showed that they actually proceed in one elementary act as eliminations with concerted fragmentation. The kinetic parameters were estimated within the framework of the intersecting parabolas model; the parameters allow the calculation of the activation energy and rate constant from the enthalpy of the above reaction. For a series of reactions involving the Et?, EtO?, RO?₂, and ?NO₂ radicals, on the one hand, and a number of alkyl nitrates, on the other, their enthalpies, activation energies, and rate constants were calculated. Based on the data obtained, new kinetic schemes of the chain decomposition of alkyl nitrates involving eliminations with fragmentation were proposed.     

A novel type of free radical reactions: isomerization of a radical with concerted fragmentation

A comparison of experimental data and results of the rate constant calculations by the method of intersecting parabolas (MIP) showed that abstraction of H atom by a peroxyl radical from the C_α-H bond in hydroperoxide is accompanied by concerted fragmentation of the molecule. The complicated character of the elementary event is due to high exothermicity of the reaction. The kinetic parameters of isomerization with fragmentation of peroxyalkyl, peroxyalkoxyl, and peroxyperoxyl radicals were calculated within the framework of the MIP method. The enthalpies, activation energies, and rate constants for a series of isomerization reactions of peroxyl radicals with concerted fragmentation were also obtained from the MIP calculations. Factors influencing these reactions are analyzed.     

Generation of radicals and antimalarial activity of dispiro-1,2,4-trioxolanes

The kinetic schemes of the intramolecular oxidation of radicals generated from substituted dispiro-1,2,4-trioxolanes (seven compounds) in the presence of Fe²⁺ and oxygen were built. Each radical reaction was defined in terms of enthalpy, activation energy, and rate constant. The kinetic characteristics were calculated by the intersecting parabolas method. The competition between the radical reactions was considered. The entry of radicals generated by each compound into the volume was calculated. High antimalarial activity was found for 1,2,4-trioxolanes, which generated hydroxyl radicals. The structural features of trioxolanes responsible for the generation of hydroxyl radicals were determined.     

Molecular decomposition of polyolefins: Kinetic parameters and geometry of the transition state

The experimental data on the molecular decomposition of olefins ( 1 -pentens) of various structures to two olefins in a gas phase were analyzed by means of the parabola intersection method. The enthalpies and kinetic parameters characterizing such decomposition were calculated for eighteen reactions. Decomposition of olefins representing two-centered concerted re action was found to be characterized by a very high classic potential barrier of thermoneutral reaction (197.4 kJ/mol). The kinetic parameters (activation energy and velocity constant) of fifteen reverse reactions of formation of olefins from two alkenes were calculated using the parabola-intersection method. The factors affect ing the activation energy of the reactions of olefin decomposition and formation are discussed. Quantum chemical calculations of the transition state energy and geometry for six reactions of olefin decomposition were performed.     

Competition between mono-and bimolecular reactions of artemisinin alkoxyl radicals

The competition between intramolecular and bimolecular reactions of alkoxyl radicals formed from artemisinin was theoretically analyzed. The enthalpies of these reactions were calculated. The activation energies and rate constants of reactions of intramolecular hydrogen atom transfer, decyclization, and decomposition of alkoxyl radicals of artemisinin and several its derivatives, as well as the activation energies and rate constants of reactions of these radicals with the C-H, S-H, and O-H bonds in biological substrates and their analogs were calculated by the intersecting parabolas method The fastest reactions of artemisinin alkoxyl radicals were identified. The full kinetic scheme of transformation of these radicals was proposed. Artemisinin radicals with the free valence on the carbon atom are predominantly formed due to the transformation of the artemisininoxyl radicals. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1502–1510, September, 2006.     

Triflic acid catalyzed reductive coupling reactions of carbonyl compounds with O-, S-, and N-nucleophiles

Highly efficient metal-free reductive coupling reactions of aldehydes and ketones with a range of nucleophiles in the presence of triflic acid (1-5 mol%) as the catalyst are presented. The reactions can be performed at ambient temperature without exclusion of moisture or air. A range of symmetrical and unsymmetrical ethers were obtained by this method in high yields and short reaction times. For the first time, the influence of additional functionalization has been studied. Furthermore, the formation of thioethers from ketones (by addition of unmodified thiols) and of sulfonamides from either aldehydes or ketones has been achieved under catalytic conditions.     

Topological analysis of transition states of the concerted molecular decomposition of haloalkanes and alcohols with HHal and HOH elimination

The quantum chemical modeling and topological analysis of transition states of the concerted molecular decomposition of haloalkanes and alcohols with the elimination of HHal and HOH were performed for the ten compounds. The possibility of formation of two types of transition states was mentioned, and their electronic structures were studied. The influence of the alkyl substituent at the C atoms on the binding energy and the nature of the transition state was shown. The decomposition activation energies were calculated using the method of density functional theory (DFT/B3LYP/6-311++G**) and the intersecting parabolas method. The calculated results were compared, and their agreement was shown.     

The reactivity of phenoxyl radicals of bioantioxidants in the abstraction reactions

The enthalpies, activation energies, and rate constants for the reactions of 15 phenoxyl radicals derived from natural bioantioxidants with hydroperoxides, C-H bonds of linoleic acid, SH-groups of l-cysteine, and O-H bonds of α-tocopherol (60 reactions) were calculated. The activation energies were calculated using the model of intersecting parabolas. The interatomic distances in the reaction sites of the transition states of the studied reactions were calculated. The factors affecting the reactivity of these radicals are discussed. The activation energy of the reaction of oxygen with the O-H bond of the 1,2-dihydroxybenezene semiquinone radical was estimated.     

Reactions of alkoxy and peroxy radicals formed upon the decomposition of hydroxyperoxide groups in the artemisinin derivatives

The enthalpies of intramolecular reactions of alkoxy and peroxy radicals formed from polyatomic artemisinin hydroperoxides and of their bimolecular reactions with C—H, S—H, and O—H bonds of biological substrates were calculated. The activation energies and rate constants of these reactions were calculated using the intersecting parabolas method. The decomposition of artemisinin hydroperoxides can initiate the cascade of intramolecular oxidation reactions involving radicals R·, RO·, HO·, HO₂·, and RO₂·. The main sequences of transformation of these radicals were established. The oxidative destruction of the artemisinin peroxy derivatives generates radicals RO₂·, HO·, and HO₂· in an amount of 4.5 radicals per peroxide derivative molecule on the average. The kinetic scheme of oxidative transformations of the hydroperoxide with four OOH groups and radicals formed from it was constructed using this radical as an example.     

The energy and geometric characteristics of the transition state in reactions of RO<Stack><Subscript>2</Subscript><Superscript>•</Superscript></Stack> with carbonyl compound C-H bonds

The energy and geometry of the transition state in reactions of the ethyl peroxyl radical with ethane, ethanol (its α and β C-H bonds), acetone, butanone-2, and acetaldehyde were calculated by the density functional theory method. In all these reactions (except EtO_2/? + ethanol α C-H bond), the C…H…O reaction center has an almost linear configuration (φ = 176° ± 2°); polar interaction only influences the r ^≠ (C…O) interatomic bond. In the reaction of EtO_2/? with the ethanol α C-H bond, it is the O-H…O H-bond formed in the transition state that determines the configuration of the reaction center with the angle φ(C…H…O) = 160°. The results were used to estimate the r ^≠ (C…H) and r ^≠ (O…H) interatomic bonds in the transition state by the method of intersecting parabolas and the contribution of polar interaction to the activation energy of reactions between peroxyl radicals and aldehydes and ketones.     

Vinyl polymerization versus [1,3] O to C rearrangement in the ruthenium-catalyzed reactions of vinyl ethers with hydrosilanes

Two reactions, vinyl polymerization and [1,3] O to C rearrangement of vinyl ethers, are investigated in the ruthenium-catalyzed reaction with hydrosilanes. The reaction pathways are dependent on the substituents of the vinyl ether, in particular, those of the alkoxy group. Primary-, secondary-, and tertiary-alkyl vinyl ethers, ROCHCH₂, are polymerized with ease to give the corresponding polymer in good yields. When R is electron-donating benzyl groups, the reaction does not give the polyvinyl ether but results in [1,3] O to C rearrangement to give the corresponding aldehyde, RCH₂CHO in moderate to good yields. The rearrangement selectively proceeds when vinyl ethers having α-substituents are used as the starting materials to give the corresponding ketones in high yields. With catalytic amounts of hydrosilanes, the rearrangement gives ketones or aldehydes selectively. In sharp contrast, use of excess amounts of hydrosilanes leads to the rearrangement followed by reduction of the formed carbonyl group to give the corresponding silyl ethers in good yields. Nature of catalytically active species is discussed.     

Efficient synthesis of substituted vinyl ethers using the Julia olefination

Julia olefination between alpha-alkoxy sulfones 2a-c and a wide variety of ketones or aldehydes afforded substituted vinyl ethers in 46-90% yields. Sulfones 2a-c were readily prepared in two steps from commercially available reagents in 68-80% yields. Optimization revealed that the nature of the base, the solvent, and the temperature were crucial to obtaining the desired vinyl ethers. [reaction: see text]     

Radical Abstraction Reactions with Concerted Fragmentation in the Chain Decay of Nitroalkanes

Reactions of the type X? + HCR₂CH2NO₂ → XH + R₂C=CH₂ + N?O₂ are exothermic, due to the breaking of weak C–N bonds and the formation of energy-intensive C=C bonds. Quantum chemistry calculations of the transition state using the reactions of Et? and EtO? with 2-nitrobutane shows that such reactions can be categorized as one-step, due to the extreme instability of the intermediate nitrobutyl radical toward decay with the formation of N?O₂. Kinetic parameters that allow us to calculate the energy of activation and rate constant of such a reaction from its enthalpy are estimated using a model of intersecting parabolas. Enthalpies, energies of activation, and rate constants are calculated for a series of reactions with the participation of Et?, EtO?, RO?₂, N?O₂ radicals on the one hand and a series of nitroalkanes on the other. A new kinetic scheme of the chain decay of nitroalkanes with the participation of abstraction reactions with concerted fragmentation is proposed on the basis of the obtained data.