Showing papers on "Homolysis published in 1971"
TL;DR: In this paper, a new method of homolytic alkylation of heteroaromatic bases is described, in which the silver-catalysed oxidative decarboxylation of acids by peroxydisulphate is used as a source of alkyl radicals.
347 citations
TL;DR: In this paper, a quantitative study concerning the nucleophilic character of the dioxanyl radical, carried out by measuring the relative rates of attack on 4-substituted quinolines, revealed in detail all the features of nucleophilicity substitutions.
84 citations
TL;DR: In this paper, the synthesis of homolytic substitution of pyridine, quinoline and acridine by redox systems is discussed, and the synthetic aspects and the mechanism of the reduction of oxaziranes and homolyty substitution are discussed.
71 citations
TL;DR: In this article, the stochastic properties of dialkylmanganese species were examined and the decomposition process was shown to proceed principally by α,β-elimination of an hydrido, and the resulting mixture was catalyzed in a remarkable way by reduced Fe, Co, Ni, Ag and Pd.
57 citations
TL;DR: In this article, a β-fragmentation reaction was proposed for α-hydroperoxy-tetrahydropyranyl ethers annelated to cyclododecane.
Abstract: Vicinal α-hydroperoxy-tetrahydropyranyl ethers annelated to cyclododecane are smoothly converted by homolytic fission in about 75% yield to a mixture of the macrocyclic lactones 3, 4, and 5. Based on this observation, an economically feasible synthesis of Exaltolide1 (>65% yield) has been developed. The mechanism of this new β-fragmentation reaction is discussed.
39 citations
TL;DR: In this article, the authors considered photolysis of bis(trimethylsilyl)mercury with benzene or toluene to give the normal products of homolytic aromatic substitution.
35 citations
TL;DR: In this article, the authors investigated the kinetics of the autoxidation of a series of n-, s-, and t-butylboron compounds in iso-octane at 30°.
Abstract: The kinetics of the autoxidation of a series of n-, s-, and t-butylboron compounds in iso-octane at 30° have been investigated. The reactions of tri-n- and tri-s-butylboranes (and of triphenylborane) were normally controlled by the rate of diffusion of oxygen into the solution but proceeded at measurable rates in the presence of pyridine which reduced the concentration of the boranes by complexation.In every case, the kinetics were compatible with a homolytic chain reaction involving the following steps.[graphic omitted]In the early stages of the reaction, initiation is by the direct reaction between borane and oxygen, but, as the reaction proceeds, the unimolecular decomposition of the alkylperoxyborane, or its bimolecular reaction with alkylborane become more important.Values for the ‘oxidizability’ of the different boranes at the same rates of initiation [Kp/(2Kt)1/2], and of the rate constants for the bimolecular homolytic substitution by the alkylperoxy radicals at the boron centre (Kp) are listed. These latter processes are influenced by counterbalancing electronic and steric effects in the alkyl groups, and by the presence of oxygen ligands on the boron, but in suitable cases they are considerably faster than the corresponding reaction at hydrogen, which is the corresponding propagation step in the autoxidation of a hydrocarbon.
34 citations
31 citations
29 citations
TL;DR: The silicon-substituted organomercurials (Me3SiCCl2)2Hg, Me3SiCl2Hg bond homolysis, which is a minor side reaction at 120°, became the major process at 220°, and evidence indicates that the trimethylsilyldichloromethyl radicals formed couple and abstract hydrogen and chlorine as discussed by the authors.
25 citations
TL;DR: In this paper, a photolysed di-t-butyl peroxide was used in the presence of a variety of organometallic compounds (RM) in an e.s.r. cavity.
Abstract: When di-t-butyl peroxide is photolysed in the presence of a variety of organometallic compounds (RM) in an e.s.r. cavity, the alkoxyl radicals which are formed induce an SH2 reaction at the metal centre, and the e.s.r. spectrum of the displaced alkyl radical R· can be observed [equation (i)]. The rates of these SH2 reactions for a variety of trialkyl- [graphic omitted] boranes and trialkylboroxines have been studied by causing the organoboranes to compete with cyclopentane for reaction with the t-butoxyl radicals [equation (ii)], and monitoring the relative concentrations of the alkyl and cyclopentyl radicals by e.s.r. The reactions are very fast (ki= 105–107 M–1 s–1) and are characterised by low activation energies (0–5 kcal mol–1), and A factors generally in the range 107–108.
TL;DR: The nucleophilic character of acyl radicals is also confirmed by the higher reactivity of the 6-nitro-derivative of benzothiazole in the 2-position as discussed by the authors.
Abstract: Benzothiazole is acylated by acyl radicals obtained from aldehydes selectively in the 2-position. The nucleophilic character of acyl radicals is also confirmed by the higher reactivity of the 6-nitro-derivative. The high reactivity is useful in syntheses; this reaction can also be used as a diagnostic criterion for revealing the presence of acyl radicals in the oxidation of aldehydes with various oxidizing agents (O2, CeIV, CrVI, or MnVI).
Book•
01 Jan 1971
TL;DR: The tri-tert-alkylchromium system as discussed by the authors undergoes stepwise fragmentation on warming, and at low temperatures a homolytic process leads to the exclusive formation of alkane and 1-alkene.
TL;DR: In this article, a free radical chain mechanism involving bimolecular homolytic substitution by the butylthiyl radical at boron and addition to the olefin was found.
Abstract: Trialkylboranes react with butane-1-thiol to give dialkylbutylthioboranes, R2BSBun, by a free radical chain mechanism involving bimolecular homolytic substitution by the butylthiyl radical at boron. By competing the trialkylborane with oct-1-ene for the butylthiyl radical relative rates of SH2 attack at boron and addition to the olefin were found. The reversibility of the addition of the thiyl radical to the olefin was taken into account. The occurrence of the SH2 process was also shown by the observation of the e.s.r. spectrum of the displaced alkyl radical when a solution containing a trialkylborane and a dialkyl or a diaryl disulphide was photolysed in the cavity of an e.s.r. spectrometer.The relative reactivities of the trialkylboranes and oct-1-ene were shown to be in the order Bun3B > Bui3B > Bus3B > oct-1-ene > Bun2BSBun. The absolute rate coefficients for alkylthiyldealkylation were of the same magnitude (105–107 M–1 s–1) as those found for the SH2 reactions of alkylperoxyl and alkoxyl radicals with organoboranes.
TL;DR: In this article, the bimolecular homolytic substitution type of free radicals is investigated at a metallic center, particularly where the displaced group is an alkyl radical.
Abstract: Reactions of the bimolecular homolytic substitution type take place readily at a metallic centre, particularly where the displaced group is an alkyl radical. This provides a new context for investigating the behaviour of free radicals and is the key to the understanding of many organometallic reactions.
TL;DR: In this article, the reaction of tertiary alcohols with mercuric oxide bromine involves formation and homolytic decomposition of alkyl hypobromite followed by cyclization of γ-bromo alcohols.
Abstract: Metallic silver initiates short-chain homolytic decomposition of tertiary alkyl hypochlorites and hypobromites. Evidence is presented that the reaction of tertiary alcohols with silver oxide – mercuric oxide – bromine involves formation and homolytic decomposition of alkyl hypobromite followed by cyclization of γ-bromo alcohols. A convenient synthesis of tricyclo[4.3.1.03,8]decan-4-one (4-proto-adamantanone) is described.
TL;DR: In this article, the synthesis of the title compound is described, and its merits as a spin trap are outlined, as well as its properties as a spinner and a spin-trap.
Abstract: The synthesis of the title compound is described, and its merits as a spin trap are outlined.
TL;DR: In this article, a mixture of an aliphatic ketone and a trialkylborane is irradiated with u.v.s. light, and the ketone triplet brings about a bimolecular homolytic substitution at the boron center, and superimposed e.g. spectra of the two radicals which are formed can be observed; the ultimate products of the reaction result from the coupling of these radicals.
Abstract: If a mixture of an aliphatic ketone and a trialkylborane is irradiated with u.v. light, the ketone triplet brings about a bimolecular homolytic substitution at the boron centre, and the superimposed e.s.r. spectra of the two radicals which are formed can be observed; the ultimate products of the reaction result from the coupling of these radicals. R1R2CO*+ BR33→ R1R2ĊOBR32+ R3·→ productsBy monitoring the intensities of the spectra when two different boranes, or a borane and an olefinic quencher, compete for reaction with acetone triplets, or when substitution and fragmentation reactions of di-isopropyl ketone triplet compete, the above mechanism was established, and kinetic parameters for the substitution reaction were derived. Typically, the reaction between triplet acetone and tri-n-butylborane has a rate constant of 7 × 106M–1 s–1 at 20°.
TL;DR: In this paper, the u.v. photolysis (Λ= 185 nm) of 2 M aqueous solutions of ethylene glycol was studied at 22°C.
Abstract: — –The u.v. photolysis (Λ= 185 nm) of 2 M aqueous solutions of ethylene glycol was studied at 22°C. Products (quantum yields) are hydrogen (0.204) formaldehyde (0.194), glycolaldehyde (0.08), methanol (0.074), glycerol (0.06), erythritol (0.03), acetaldehyde (0.02), 3,4-dihydroxybutanal (0.01) and succinaldehyde (0.001). With increasing temperature the yields of all products remain essentially unchanged except that of acetaldehyde (φ= 0.32 at 90°C) which is formed in a chain reaction. The photolysis of (CD2OH)2 yields 91% HD, indicating that the most important primary process is the homolytic splitting of the O-H bond. The resulting oxy radicals fragmentate largely into formaldehyde and CH2OH radicals. Molecular fragmentation processes yielding hydrogen and glycolaldehyde, as well as formaldehyde and methanol, are discussed in the proposed decomposition scheme.
TL;DR: In this article, substituent and solvent effects on decomposition rates were investigated for the case of Arenediazothiolates (Ar-NN-SR; R alkyl or aryl) and showed that the primary step consists in the formation of a radical pair rather than multiple fission.
Abstract: Arenediazothiolates (Ar-NN-SR; R alkyl or aryl) undergo homolysis on heating. This follows from substituent and solvent effects on decomposition rates; product formation is interpretable on a homolytic basis. Substituent as well as solvent viscosity effects suggest that the primary step consists in the formation of a radical pair rather than multiple fission . In contrast to the situation at high concentration hardly any induced decomposition takes place at the low concentrations used for kinetics.
TL;DR: In this paper, a mechanism for propene production in reaction systems in which triethylborane and methyl radicals are present, and an estimate for the rate of homolytic substitution at boron by tertiary butoxyl radicals, are also given.
Abstract: The homolytic substitution at boron and hydrogen-abstraction reactions of methyl radicals with triethylborane have been studied in the gas phase: the values of log (k/ml mol–1s–1) are respectively 10·60 – 5200/4·58 T and 12·38 – 7780/4·58 T. A mechanism for propene production in reaction systems in which triethylborane and methyl radicals are present, and an estimate for the rate of homolytic substitution at boron by tertiary butoxyl radicals, are also given.
TL;DR: The homolytic dissociation of dinitrogen trioxide in a number of organic solvents has been studied and relevant thermodynamic data are reported in this article, where it is shown that the stability of trioxide varies considerably and can be correlated with the donor properties of the solvent.
Abstract: The homolytic dissociation of dinitrogen trioxide in a number of organic solvents has been studied and relevant thermodynamic data are reported. The stability of dinitrogen trioxide varies considerably and can be correlated with the donor properties of the solvent.
TL;DR: In the triplet state, the radicals R21COBR22 and R2 were obtained by a bimolecular homolytic substitution as mentioned in this paper, where the radicals were given by trialkylboranes (R32B).
Abstract: In the triplet state, ketones (R21CO) will react with trialkylboranes (R32B) by a bimolecular homolytic substitution to give the radicals R21COBR22 and R2.
TL;DR: In this paper, the reaction of cyano-radicals, produced by the photolysis of cyanogen iodide, with aromatic compounds afforded aromatic nitriles, was determined for a variety of monosubstituted benzenes and the results indicated that the cyano radical preferably attacks relatively electronegative sites.
Abstract: The reaction of cyano-radicals, produced by the photolysis of cyanogen iodide, with aromatic compounds afforded aromatic nitriles. The isomer ratios and the relative reactivities were determined for a variety of monosubstituted benzenes and the results indicated that the cyano-radical preferably attacks relatively electronegative sites. The partial rate factors for the meta- and para-positions correlate with σ+ to give a slope of –0·42. It is concluded that cyano-radical possesses a slight electrophilic character.
TL;DR: In this article, a mechanistic approach was proposed to rationalize the effect of added nitrobenzene on the nature of the products from the thermal decomposition of benzoyl peroxide in benzene.
Abstract: A mechanistic scheme is suggested which rationalises the effect of added nitrobenzene on the nature of the products from the thermal decomposition of benzoyl peroxide in benzene. A small fraction of the nitro-compound is reduced to nitrosobenzene, which scavenges phenyl radicals to form diphenyl nitroxide. The nitroxide radicals efficiently oxidise phenylcyclohexadienyl radicals to biphenyl, and thus prevent side-reactions (e.g., dimerisation), which would give high molecular weight products. The nitroxide is regenerated by oxidation of diphenylhydroxylamine by molecular benzoyl peroxide. Evidence in support of this scheme is evaluated, and several complicating features are discussed.
TL;DR: In this paper, electron transfer from paraquat cation radical to t-butoxide ion was used to study rates of homolytic fission in organic molecules, and showed that the reaction affords a novel and convenient method of studying rates of femtosecond fission.
Abstract: t-Butoxy radicals generated by photolysis of di-t-butyl peroxide, or thermolysis of di-t-butyl hyponitrite, are quantitatively reduced to t-butoxide ion by electron transfer from paraquat cation radical; the reaction affords a novel and convenient method of studying rates of homolytic fission in organic molecules.