Showing papers on "Homolysis published in 1976"
TL;DR: The homolytic substitution acquires a more significant weight in the field of the aromatic substitutions, characterized by high selectivity and versatility.
Abstract: A common aspect characterizing both the homolytic animation and the substitution of protonated hetero-aromatic bases by nucleophilic free radicals is the presence of a nitrogen atom with a positive charge. This presence determines strong polar effects whether it characterizes the radical (\(R_2 \mathop N\limits_ + H\)) or the aromatic substrate (protonated base). The awareness that the global polar effects result from the polar characteristics of both the radical and the substrate has led to new homolytic aromatic substitutions, characterized by high selectivity and versatility. Thus the homolytic substitution acquires a more significant weight in the field of the aromatic substitutions.
125 citations
TL;DR: In this paper, the authors focus on the classification of the area of free radicals in organometallic chemistry, and discuss in very general terms the experimental methods as well as the relevant literature.
Abstract: Publisher Summary The chapter focuses on the classification of the area of free radicals in organometallic chemistry, discusses in very general terms the experimental methods as well as the relevant literature. The chapter divides organometallic chemistry into the following classification: (i) organometallic paramagnetic compounds, and (ii) organometallic reactions, in which paramagnetic transient species, usually organic free radicals, are implicated in the reaction pathway. Class (i) may be divided into compounds in which the unpaired electron is localized principally on the metal M (I); or remote from the metal (11). The former belong to a wider group, the metal-centered radicals, encompassing not only, for example, the organometallic SiMe3, but also the inorganic SiC13. Corresponding paramagnetic transition metal complexes (I) are rarely named as radicals because they are often stable, and the term “radical” has traditionally had the connotation of a species of low kinetic stability. Class (ii) is conveniently separated into free radical reactions involving (a) substitution at the metal center, SH2, or bimolecular homolytic substitutions at M. Electron spin resonance spectroscopy has made a considerable impact on the study of organometallic free radical chemistry. Evidence for the participation of a free radical in an organometallic reaction, using one or more of the above methods, may still leave two major questions. The first is whether the radical pathway is a principal or a side reaction. The second is whether the mechanism of the reaction is a chain or a nonchain process.
70 citations
TL;DR: In this article, the photolysis of Mn2(CO)10 has been investigated over a range of solvents and temperatures and found to be more complicated than hitherto reported, and a temperature dependent bifunctionality of the spin-trap 2,4,6-tri-t-butylnitrosobenzene was observed.
39 citations
TL;DR: In this article, a mechanism involving a participation of sufficiently long-lived acyloxyl radicals is proposed for the iminylation reaction on the basis of the reactivity patterns in this substitution reaction.
Abstract: Irradiation of benzophenone O-acetyloxime, O-phenylacetyloxime, O-benzoyloxime, and O-(p-chlorobenzoyl)oxime in benzene, toluene, chlorobenzene, or o-xylene effected aromatic substitution on the solvent molecules by diphenylmethaniminyl radicals to give N-diphenylmethylenearylamines only when benzoxyl or p-chlorobenzoxyl radicals were generated concurrently. A mechanism involving a participation of the sufficiently long-lived acyloxyl radicals is proposed for the iminylation reaction on the basis of the reactivity patterns in this substitution reaction. o-Phenylbenzophenone O-benzoyloxime was also photolyzed in benzene to give 9-phenylphenanthridine, the intramolecular cyclization product of 2-biphenylylphenylmethaniminyl radicals.
34 citations
TL;DR: The photochemical behavior of substituted derivatives of thiochroman-4-one 1-oxide has been examined in this article, where three distinct pathways have been recognized; β-hydrogen abstraction or rearrangement to cyclic sulfenates, which then undergo further reaction by homolysis of the S-O bond.
Abstract: The photochemical behavior of a number of substituted derivatives of thiochroman-4-one 1-oxide has been examined In contrast to the analogous sulfones these sulfoxides undergo a variety of photochemical rearrangements At least three distinct pathways have been recognized; β-hydrogen abstraction or rearrangement to cyclic sulfenates, which then undergo further reaction by homolysis of the S—O bond, appearing to be particularly favorable processes In a small number of examples, photochemical deoxygenation is observed as a competing reaction Mechanisms which attempt to account for the influence of structural variations on the particular pathway followed have been proposed
22 citations
TL;DR: The available information about the reactions of epithio-compounds with nucleophilic and electrophilic agents, about the homolytic reactions of alkene sulphides, and about reactions leading to the elimination of sulphur with formation of alkenes is examined, the mechanisms of the opening of thiiran rings are discussed, and the comparative characteristics of the reactivities of O- and S-heteroanalogues of cyclopropane compounds are given as discussed by the authors.
Abstract: The available information about the reactions of epithio-compounds with nucleophilic and electrophilic agents, about the homolytic reactions of alkene sulphides, and about reactions leading to the elimination of sulphur with formation of alkenes is examined, the mechanisms of the opening of thiiran rings are discussed, and the comparative characteristics of the reactivities of O- and S-heteroanalogues of cyclopropane compounds are given. The bibliography includes 276 references.
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TL;DR: In this article, the activation enthalpies for the rate-limiting steps can be taken as a kinetic measure of the strength of the metal-metal bonds, which is consistent with initial reversible homolytic fission.
Abstract: The kinetics of the following reactions in decalinhave been studied : the complexes [MnRe(CO)10] and [Tc2(CO)10] with triphenylphosphine, [MnRe(CO)8(PPh3)2] and [Tc2(CO)8(PPh3)2] with carbon monoxide, and decomposition of [Tc2(CO)10] under oxygen. The results are consistent with initial reversible homolytic fission of the metal–metal bonds so that the activation enthalpies for the rate-limiting steps can be taken as a kinetic measure of the strengths of the metal–metal bonds. Attention is drawn to correlations between the activation enthalpies for the decacarbonyls and spectroscopic parameters related to the bond strengths.
13 citations
TL;DR: In this article, a homolytic substitution of monosubstituted benzenes by 1-adamantyl radical has been carried out and the reactivity and the polar character of the radical have been determined by examining the effects of substituents on isomer distributions and on relative reactivities.
Abstract: Homolytic substitution of monosubstituted benzenes by 1-adamantyl radical has been carried out and the reactivity and the polar character of the radical have been determined by examining the effects of substituents on isomer distributions and on relative reactivities. The results indicated that the 1-adamantyl radical has nucleophilic properties which are more pronounced than that of other more strained bridgehead radicals; it is suggested that this behaviour is attributable to the different role played by polar effects during the addition of bridgehead radicals to aromatic substrates. The syntheses of several monosubstituted 1-aryladamantanes are also described.
TL;DR: The rate constants for the S H 2 reactions of tri-n-butyltin radicals with several dialkyl selenides and tellurides have been measured by EPR spectroscopy as discussed by the authors.
TL;DR: In this paper, photolysis of 4-phenyl-1,3,2-oxathiazolylio-5oxide (1) in ethanol in the presence of oxygen leads to benzonitrile (21%) and ethyl phenyl glyoxylate (65%).
TL;DR: The mechanism of dediazoniation of arenediazonium tetrafluoroborates in 2,2,2-trifluoroethanol (TFE) is strongly dependent on the concentration of added pyridine as discussed by the authors.
Abstract: The mechanism of dediazoniation of arenediazonium tetrafluoroborates in 2,2,2-trifluoroethanol (TFE) is strongly dependent on the concentration of added pyridine. The added base complexes with the diazonium ion and diverts it to a homolytic pathway. Complex formation is indicated by the disappearance of the stretching vibration and appearance of a new band at about 1640–1690 cm−1 ascribed to the system. UV. and NMR. results support this conclusion.
Chemically induced dynamic nuclear polarization (CIDNP) experiments clearly implicate a radical-pair as an important intermediate in the decomposition of these complexes.
TL;DR: In this article, the Kinetics of the reversible reaction [Re2(CO)9(PPh3)]+ PPh3⇌[Re2CO)8(Pph3)2]+ CO in decalin have been studied in each direction and activation parameters obtained.
Abstract: The Kinetics of the reversible reaction [Re2(CO)9(PPh3)]+ PPh3⇌[Re2(CO)8(PPh3)2]+ CO in decalin have been studied in each direction and activation parameters obtained. The Kinetic behaviour is quite simple and is consistent with a ligand-dissociative mechanism. However, it is also consistent with metal migration and homolytic-fission mechanisms. The reaction of [Re2(CO)9(PPh3)] with carbon monoxide has quite different activation parameters from the reaction with triphenylphosphine and these reactions cannot, therefore, both go via simple rate-determining homolytic fission. Reaction of [Re2(CO)8(PPh3)2] with PPh3 leads to mononuclear carbonylphosphine products and is half order in [Re2(CO)8(PPh3)2]. This reaction most probably does go via reversible homolytic fission. Reactions with oxygen, nitrogen monoxide, and iodine are also described.
TL;DR: In this article, N-propyl-(Z)-6-dodecenamide was subjected to thermolytic homolysis in aqueous dioxane to yield a mixture of products from which, after zinc debromination, γ-6-Z)-dodefecenolactone was isolated.
Abstract: N-Chloro-N-propyl-threo-6,7-dibromododecanamide, prepared by the successive bromination and N-chlorination of N-propyl-(Z)-6-dodecenamide, was subjected to thermolytic homolysis in aqueous dioxane to yield a mixture of products from which, after zinc debromination, γ-6-(Z)-dodecenolactone was isolated. Some analogs of this lactone were similarly prepared.
TL;DR: The aroyloxycyclohexadienyl radicals (VIII) were detected and identified in the decomposition of dibenzoyl peroxide and bis(4-methylbenzoyl) peroxide in benzene-d6 as spin adducts of nitrosodurene as discussed by the authors.
Abstract: The aroyloxycyclohexadienyl radicals (VIII), the key intermediates iates in the homolytic aromatic aroyloxylation, were detected and identified in the decomposition of dibenzoyl peroxide, bis(4-methylbenzoyl) peroxide, and bis(4-methoxybenzoyl) peroxide in benzene-d6 as spin adducts of nitrosodurene.
TL;DR: In this paper, a general reaction of wide synthetic interest, involving homolytic δ-aminoalkylation of quinoxaline in an acidic medium, is described.
TL;DR: In this article, aryl nitromethane and manganese(III) acetate were treated with homolytic aromatic substitution involving the CH2NO2 radical.
Abstract: Treatment of aromatic hydrocarbons with nitromethane and manganese(III) acetate produces aryl nitromethanes by way of homolytic aromatic substitution involving the ·CH2NO2 radical.
TL;DR: In this article, temperature-dependent 13C-NMR spectra show a rapid equilibrium between 1 and 1′ at room temperature and above, the tautomeric form 1 being predominant.
Abstract: Thermolysis of asymmetric triazenes Ar–N=N–NH–R 1 produces anilines Ar–NH2 2, hydrocarbons R–H 3, and N-alkylated anilines Ar–NH–R 4 as the main reaction products. 1H- and 13C-CIDNP experiments show polarization of nuclei in Ar–NH2, Ar–NH–R, R–H and some additional products (especially ring alkylated anilines). These observations are in agreement with the formation of an intermediate radical pair [Ar–NH··R] which must have been formed via homolysis of the tautomeric form ArNH–N=N–R 1′. Temperature-dependent 13C-NMR spectra show a rapid equilibrium between 1 and 1′ at room temperature and above, the tautomeric form 1 being predominant. From the CIDNP spectra the signs of the 1H and 13C hyperfine couplings of aminyl radicals Ar–NH and alkyl radicals R. have been determined.
TL;DR: The authors showed that nucleophilic alkyl and acyl radicals effect homolytic substitution of acyl groups in protonated heteroaromatic compounds; the results indicate the reversibility of the addition of the acyl radical to hetero-aromatic bases.
Abstract: Nucleophilic alkyl and acyl radicals effect homolytic substitution of acyl groups in protonated heteroaromatic compounds; the results indicate the reversibility of the addition of the acyl radicals to heteroaromatic bases.
TL;DR: In this paper, the unexpected preferential formation of the less stable n-alkyl radical is tentatively ascribed to steric constraints in a 5-coordinate intermediate, which is not the case in this paper.
TL;DR: In this paper, the pyrolysis of N-benzyl-p-toluidine resulted in the same neutral products together with ammonia, toluene, bibenzyl, trans-stilbene, o-toluidine, and 2,7-dimethyl-9-phenylacridine.
Abstract: Heating of N-benzyl-o- and p-toluidines resulted in migration of the benzyl group to the corresponding toluidine nucleus. The pyrolysis of N-benzyl-o-toluidine produced ammonia, toluene, bibenzyl, trans-stilbene, o-toluidine together with 4-amino-3-methyldiphenylmethane and 2,3-diphenyl-7-methylindole. N-benzyl-p-toluidine resulted into the same neutral products together with ammonia, p-toluidine, 2,3-diphenyl-5-methylindole together with 2,7-dimethyl-9-phenylacridine. Using phenol or quinoline as solvents, the normal rearranged products were accompanied with 2- and 4-benzylphenols and 2- and 4-benzylquinolines. It was concluded that the pyrolysis of the secondary amines proceeds through their homolytic fission to benzyl and the corresponding toluidino free radicals. Homolysis of some primary products was also observed.
TL;DR: In this paper, the authors show that the thermal decomposition of dibenzoyl peroxide and bis(4-substituted benzoyl) peroxides in diphenyl ether or anisole affords phenyl benzoate and 4-substantituted benzoates, respectively, arising from ipsosubstitution in the aryl ethers by corresponding benzoxyl radicals.
Abstract: Thermal decomposition of dibenzoyl peroxide and bis(4-substituted benzoyl) peroxides in diphenyl ether or anisole affords phenyl benzoate and 4-substituted benzoates, respectively, arising from ipso-substitution in the aryl ethers by the corresponding benzoxyl radicals.
TL;DR: In this article, Butoxyl, ethoxyl or trimethylsiloxyl radicals attack at sulphur in thietan or 2-methylthietan to bring about rapid homolytic ring-opening.
Abstract: t-Butoxyl, ethoxyl, or trimethylsiloxyl radicals attack at sulphur in thietan or 2-methylthietan to bring about rapid homolytic ring-opening.
TL;DR: In this article, the 2,4-di-t-butyl-6,6-bis(organosilyl)cyclohexadienyl radicals, the intermediate radicals in homolytic ipso-substitution and silylation, respectively, are generated and characterized by esr spectroscopy.
Abstract: Seven 2,4-di-t-butyl-6,6-bis(organosilyl)cyclohexadienyl and two 2,4-di-t-butyl-6-organosilylcyclohexadienyl radicals, the intermediate radicals in homolytic ipso-substitution and silylation, respectively, are generated and characterized by esr spectroscopy. Spectral features, especially the large 29Si coupling constants are discussed.