Theoretical investigation of mechanism of antioxidant action of sterically hindered phenols

Heats of reaction describing the mechanism of antioxidant action of phenols in polyolefins have been calculated by the MNDO quantum-chemical method. It has been shown that reactions producing substituted quinones will proceed the most readily. The influence of substituents on the heat of reaction has been investigated.Vilnius University. Materials, Tambov. Translated from Teoreticheskaya i 8ksperimentaI'naya Khimiya, Vol. 27, No. 2, pp. 165–170, March–April, 1991. Original article submitted February 16, 1990.     

Palladium-catalyzed sequential indole synthesis using sterically hindered amines

A palladium catalyst derived from a bulky N-heterocyclic carbene ligand enabled a modular synthesis of indoles bearing sterically hindered N-alkyl or N-aryl substituents through a reaction sequence comprising an intermolecular N-arylation and an intramolecular hydroamination.     

Electron spin resonance studies on the oxidation mechanism of sterically hindered cyclic amines in TiO2 photocatalytic systems

A sterically hindered cyclic amine, 4-hydroxy-2,2,6,6-tetramethylpiperidine (HTMP), is converted to the corresponding aminoxyl radical (nitroxide radical), 4-hydroxy-2,2,6,6-tetramethyl piperidine 1-oxyl (TEMPOL radical) as a result of a photocatalytic reaction in TiO2 aqueous suspension. The time profile of the radical formation and the effect of additives, such as SCN-, I-, methanol, and H2O2, on the initial formation rate were measured in order to elucidate the reaction mechanism. The experimental observations indicated that the direct photocatalytic oxidation of HTMP followed by reaction with O2 is the dominant process in the formation of TEMPOL radicals. Electrochemical measurements showed that HTMP is oxidized at 0.7 V (vs NHE), which is consistent with the proposed mechanism. The possibility of other processes, involving reactions with singlet molecular oxygen, superoxide radical, and hydroxyl radical, were excluded from the reaction mechanism.     

Functional derivatives of sterically hindered amines. 1. Glycidyl compounds

Four polymerizable glycidyl derivatives of sterically hindered amines were prepared from derivatives of 2, 2, 6, 6-tetramethyl-4-hydroxypiperidine and 14, 16-dioxo-7, 15-diazadispiro [5, 1, 5, 3] hexadecane. One of them is a stable nitroxyl derivative.     

Regiospecific oxidation by DDQ of unhindered alkyl groups in sterically hindered aromatic amines

DDQ oxidises an unhindered activated alkyl group to a carbonyl or a hydroxymethyl group depending on the nature of the substitution, in a sterically hindered aromatic amine.     

Detailed mechanism of radical high polymerization of sterically hindered dialkyl fumarates

The radical polymerization kinetics and mechanism of sterically hindered dialkyl fumarates (DRF) bearing various ester alkyl groups are described comprehensively. The overall polymerization reactivity of DRF, the initiation mechanism and the reactivity of the primary radicals in the polymerizations with azo initiators, the determination of the propagation and termination rate constants by means of electron spin resonance spectroscopy, the propagation mechanism and the microstructure of the polymers, and the chain rigidity of poly(DRF) and bimolecular termination process are discussed.     

Solid-phase synthesis of aryl, heteroaryl, and sterically hindered alkyl amines using the Curtius rearrangement

An efficient method for the solid-phase synthesis of aryl amines, heteroaryl amines, and sterically hindered alkyl amines has been developed. The key step in this process was the formation of resin-bound carbamates (B) by the Curtius rearrangement of aryl carboxylic acids with Wang resin providing the trapping hydroxyl group. N-Alkylation reactions of B gave secondary amines in good yield. Some biaryl amines, which are found widely in biologically active substances, were also prepared by the Suzuki reaction of resin-bound carbamates of 2-iodoaniline (16) or 3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline (21). The developed methods can be applied to the preparation of libraries containing aryl, heteroaryl, and sterically hindered alkyl amine structures as the pharmacophores.     

High-pressure transesterification of sterically hindered esters

A mild, rapid, and efficient method for the solvolysis of sterically hindered esters under high pressure is described. Transesterification is carried out in the presence of DBU at room temperature and at a pressure of 10 kbar to give quantitative conversions within short reaction times. The substrates examined included aromatic and aliphatic esters of sterically hindered alcohols and phenols. An optically pure benzyl ester of phenylalanine was chosen to study racemization of the amino acid esters under high-pressure reaction conditions.     

Self-addition of a sterically hindered alkynylselenolate

[structure: see text] The unprecedented formation of a complex macrocyclic selenium-carbon ring system, which is initiated by the action of coordinating reagents upon a sterically hindered alkynylselenolate, has been observed. The crystal structure of the product could be obtained and shows a rigid tricyclic arrangement consisting only of selenium and sp(2)-hybridized carbon atoms. This reactivity stands in contrast to the corresponding unsubstituted alkynylselenolates and is an example where a bulky substituent destabilizes an adjacent unsaturated pi-system.     

Aquametric microdetermination of sterically hindered ketones

Conclusions The previously developed aquametric micromethod for determining carbonyl compounds also gives reliable results when most of the sterically hindered ketones studied by us are analyzed. The determination error is ±0.3%.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 942–944, April, 1972.     

Reactivity of a sterically hindered Fe(II) thiolate dimer with amines and hydrazines

The sterically hindered Fe(II) thiolate dimer Fe(2)(mu-STriph)(2)(STriph)(2) (1; [STriph](-) = 2,4,6-triphenylbenzenethiolate) reacts with primary amines ((t)BuNH(2), aniline) and N(2)H(4) to form the structurally characterized addition complexes Fe(STriph)(2)(NH(2)(t)Bu)(2), Fe(2)(mu-STriph)(2)(STriph)(2)(NH(2)Ph)(2), and Fe(2)(mu-eta(1):eta(1)-N(2)H(4))(2)(N(2)H(4))(4)(STriph)(4) in high yield. Chemical and NMR spectroscopic evidence indicate that the binding of these nitrogen donors is labile in solution and multispecies equilibria are likely. With arylhydrazines, 1 catalytically disproportionates 1,2-diphenylhydrazine to aniline and azobenzene, and it rearranges 1-methyl-1,2-diarylhydrazines to give, after treatment with alumina, mononuclear, trigonal bipyramidal Fe(III) complexes of composition Fe(ISQ)(2)(STriph), where [ISQ](-) denotes an appropriately substituted bidentate o-diiminobenzosemiquinonate ligand. Complex 1 shows no reaction with hindered 1,2-dialkylhydrazines (isopropyl or tert-butyl) or tetrasubstituted 1,2-dimethyl-1,2-diphenylhydrazine.     

Photoisomerization of sterically hindered polymethine dyes

Conclusions In sterically hindered polymethine dyes the photoisomerization process, with the formation of sterically hindered trans and di-cis forms, successfully competes with the nonradiative degradation of the energy on the higher vibrational sublevels of the ground state.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1926–1930, August, 1977.     

Claisen rearrangement of sterically hindered N-alkenylindolines

Conclusions Claisen rearrangement of N-(1-methyl-2-butenyl)-2-methyl-2-ethylindoline occurs under mild conditions upon treatment with acids at 20°C. Tricyclic compounds, namely, 6,6-dimethylperhydroeyclopent[g]-2-methyl-2-ethylindoline and 8-methyl-8-ethylpyrrolidino[h, i]-3-methyl-2-ethylindole, were obtained from 7-(1-methyl-2-butenyl)-2-methyl-2-ethylindoline.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 613–618, March, 1987.