Quantum-chemical Treatment of Cyclization and Recyclization Reactions: XXVI. Effects of Substituents in the Thermal Rearrangement of Methylenecyclopropane

Basing on the coupled perturbation Hartree-Fock theory an effect of substituents on the thermal rearrangement of methylenecyclopropane derivatives was considered. The effect is described in the second order of the theory. The activation energy depends both on the first and second power of the parameter characterizing the substituent, and introduction of several groups as a rule cannot be described in the framework of an additive scheme and requires accounting for the interactions between the substituents. The results of computation are in good agreement with the results of kinetic measurements.     

Neighboring-Group Effects and Rearrangements in Reactions of Cyclopropylmethyl,Cyclobutyl, and Homoallyl Systems

The neighboring-group effect produced by a double bond in the homoallyl position or by a cyclopropane or cyclobutane ring leads to interesting rearrangements, which are discussed in this paper mainly from the point of view of mechanism, but also from that of preparative work. The question of the postulated non-classical carbonium ions acquires a special significance in this connection.     

Reactions of Heterocumulenes with Organometallic Reagents: X. Quantum-Chemical Study of the Reaction of Aliphatic Isothiocyanates with Lithiated Allenes. Intramolecular Rearrangements of 1,3,4-Azatrienes

Study of the potential energy surface for the reaction of isothiocyanates with -lithiated allenes showed that the formation of 1,3,4-azatriene occurs in one step through a four-center transition state. [1,5]-Sigmatropic rearrangements of 1,3,4-azatrienes and electrocyclizations of the resulting conjugated 1,3,5-azatriene systems lead to formation of dihydropyridine ring. Pyrrole structures are more likely to arise from protonation of 3-alkoxy-1,3,4-azatriene.     

二氟沙星激发态氧化损伤氨基酸和脱氧鸟苷酸的激光光解研究

The reactions of triplet-state difloxacin (DFX) with various amino acids and deoxyguanylic acid in aqueous media were studied using laser flash photolysis. Tryptophan, tyrosine, cysteine, and 2'-deoxyguanosine-5'-monophosphate (dGMP) were found to completely quench the triplet state of DFX in aqueous solution, the corresponding second-order rate constants being 1.97×10⁸, 1.48×10⁸, 1.72×10⁸, and 6.92×10⁷ dm³·mol^－1·s^－1. The quenching mechanism involves electron transfer to the photoexcited triplet state of DFX from the tryptophan, tyrosine, cysteine, and dGMP moieties, followed by fast protonation of the resulting DFX anion radical.     

Quantum-Chemical Study of Silicon Halide Adducts with Nitrogen-containing Donors: III. Energy Effects of Ammonolysis and Complex-Formation Reactions in Silicon Tetrahalide-Ammonia Systems

Thermodynamic characteristics of hydrogen halide elimination from SiX4-NH3 systems were calculated by the B3LYP density functional method. The role of adducts and oligomeric forms of compounds containing Si-N bonds, as intermediates in the chemical vapor deposition of silicon nitride, was considered. It was shown experimentally that the reaction between silicon tetrachloride and ammonia in nonaqueous solvents involves ammonolysis.     

Mechanism of Reactions of Thiols and Disulfides with Dihaloalkanes: A Quantum-Chemical Study

The potential surfaces of the reactions of thiols and disulfides with haloalkanes in the gas phase and with participation of a hydrizine molecule were studied quantum-chemically. The stereoelectronic characteristics of the prereaction complexes and reaction products were studied. The transition states were localized. The four-center reaction mechanism is preferable. The increased activation barrier in exchange of thiols with chloroalkanes is the major factor responsible for formation of unsymmetrical chlorinated sulfides.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 1, 2005, pp. 89–95.Original Russian Text Copyright © 2005 by Shagun, Deryagina, Korchevin, Russavskaya.     

Reactions of Substituted Boryl Radicals with Nitroalkanes. EPR, Kinetic, and Product Studies

The radical-initiated reaction of amine-boranes and phosphine-boranes, LBH(3) (L = R(3)N, R(3)P) with aliphatic nitro compounds has been investigated in order to explore the possibility of reducing tertiary nitroalkanes to the corresponding hydrocarbons. In all the examined cases boroxy nitroxides, RN(O(*))OBLH(2), resulting from the addition of ligated boryl radicals, LBH(2)(*), to an oxygen atom of the nitro group were detected and characterized by EPR spectroscopy. This reaction occurs at room temperature with a rate constant of 1.5 x 10(7) M(-)(1) s(-)(1) for LBH(2)(*) = Me(3)NBH(2)(*) and RNO(2) = Me(3)CNO(2). The boroxy nitroxides from tertiary nitroalkanes decay by a fragmentation reaction occurring with cleavage of the nitrogen-oxygen bond, rather than of the carbon-nitrogen bond as would be required for the reduction to the corresponding alkane to take place. The Arrhenius parameters for this fragmentation have been determined in few cases.     

Synthesis,Reactions and Applications of Phosphorus Containing Carbon Centered Radicals

A general method for synthesis of C_n-C (n=1, 2...) phosphonate bonds involving a reaction of 1-diethoxy-phosphorylalkan-1-, -2-and -3-yl radicals 1,3,6 with alkenes 4, a new example of functional group interconversion in 1-heterosubstituted phosphonates and synthesis of useful phosphoroorganic compounds and methylenomycin B are described.     

Visible‐Light‐Mediated Generation of Nitrogen‐Centered Radicals: Metal‐Free Hydroimination and Iminohydroxylation Cyclization Reactions

The formation and use of iminyl radicals in novel and divergent hydroimination and iminohydroxylation cyclization reactions has been accomplished through the design of a new class of reactive O‐aryl oximes. Owing to their low reduction potentials, the inexpensive organic dye eosin Y could be used as the photocatalyst of the organocatalytic hydroimination reaction. Furthermore, reaction conditions for a unique iminohydroxylation were identified; visible‐light‐mediated electron transfer from novel electron donor–acceptor complexes of the oximes and Et₃N was proposed as a key step of this process.     

Aryl Diazonium Salt‐Triggered Cyclization and Cycloaddition Reactions: Past,Present, and Future

Aryl diazonium salts occupy a privileged role in synthetic chemistry owing to their ready availability and versatile reactivity. While their applications in accessing diversely functionalized arene derivatives via denitrogenation‐coupling and reduction/addition reactions have been well recognized by practitioners in both academia and industry, recent renaissance in chemical transformations of retaining the key N₂‐unit has emerged as a powerful technique to construct various N‐heterocycles. This review covers the history and latest advances in cyclization and cycloaddition reactions using aryl diazonium salts as N₂‐annulation synthons. The scope, applications, and opportunities in exploring new chemical space by this sustainable strategy are summarized and discussed.     

Thiol-Ene Cationic and Radical Reactions: Cyclization,Step-Growth,and Concurrent Polymerizations for Thioacetal and Thioether Units

Thiol-ene cationic and radical reactions were conducted for 1:1 addition between a thiol and vinyl ether, and also for cyclization and step-growth polymerization between a dithiol and divinyl ether. p-Toluenesulfonic acid (PTSA) induced a cationic thiol-ene reaction to generate a thioacetal in high yield, whereas 2,2′-azobisisobutyronitrile resulted in a radical thiol-ene reaction to give a thioether, also in high yield. The cationic and radical addition reactions between a dithiol and divinyl ether with oxyethylene units yielded amorphous poly(thioacetal)s and crystalline poly(thioether)s, respectively. Under high-dilution conditions, the cationic and radical reactions resulted in 16- and 18-membered cyclic thioacetal and thioether products, respectively. Furthermore, concurrent cationic and radical step-growth polymerizations were realized using PTSA under UV irradiation to produce polymers having both thioacetal and thioether linkages in the main chain.     

Reactions of Heterocumulenes with Organometallic Reagents: XXV. Quantum Chemical Study of the Substituent Effects in Alkyl [(Buta-2,3-dienimidoyl)sulfanyl]acetates on Their Cyclization to Thiophene Derivatives

Russian Journal of Organic Chemistry - Base-catalyzed low-temperature transformation of alkyl [(buta-2,3-dienimidoyl)sulfanyl]acetates [1-aza-1,3,4-trienes generated from lithiated alkoxy(or...     

Implication of Sensitive Reactions to Ignition of Methyl Pentanoate: H-Abstraction Reactions by H and CH3 Radicals

Methyl pentanoate(MP) was identified as a potential candidate. To facilitate the application of MP with high efficiency in engines, a comprehensive understanding of combustion chemical kinetics of MP is necessary. In this work, the H-abstraction reactions from MP by H and CH₃ radicals, critical in controlling the initial fuel consumption, are theoretically investigated at the DLPNO-CCSD(T)/CBS(T-Q)//M06-2X/cc-pVTZ level of theory. The multistructural torsional(MS-T) anharmonicity is characterized using the dual-level MS-T method; the HF/3-21G and M06-2X/cc-pVTZ methods are chosen as the low- and high-level methods, respectively. The conventional transition state theory(TST) is employed to calculate the high-pressure limit rate constants at 298-2000 K with the Eckart tunneling correction. Our calculations indicate that the hydrogen atoms of the methylene functional group are easier to be abstracted by H and CH₃ radicals. The multistructural torsional anharmonicities of H-abstraction reactions MP+H/CH₃ are significant within the temperature range investigated. The tunneling effects are more pronounced at low temperatures, and contribute considerably to the rate constants below 500 K. The model from the work of Diévart et al. is updated with our calculations, and the simulations of the updated model are in excellent agreement with the reported ignition delay time of MP/O₂/Ar and MP/Air mixtures. The sensitivity analysis indicates that the H-abstraction reactions, MP+H=CH₃CH₂CHCH₂C(=O)OCH₃/CH₃CHCH₂CH₂C(=O)OCH₃+H₂, are critical in controlling the initial fuel consumption and ignition delay time of MP.