Showing papers on "Homolysis published in 1977"
95 citations
28 citations
27 citations
TL;DR: In this paper, it was shown that the C6-addition radicals of pyrimidines and the C2-additions radicals of adenine derivatives need a polar environment to stabilize.
Abstract: In pyrimidines with an unsubstituted C5 = C6 bond, H-addition via an anionic stage, the “ionization path”, occurs preferentially at position C6 and direct hydrogen addition, the “excitation path”, at position C5. In adenine derivatives, the “excitation path” yields C8-addition radicals and the “ionization path” C2-addition radicals. However, when the molecule is doubly protonated, C8-addition radicals are preferentially formed by protonation of the electron adduct. In pyrimidine crystals, the C5-addition radicals transform into C6-addition radicals upon irradiation with light of λ 400 nm. In purine crystals, the C8-addition radicals can also be transformed into C2-addition radicals in this way. In “van der Waals crystals”, this transformation is reversible upon storage at room temperature. In “polar crystals”, the transformation is irreversible. This indicates that the C6-addition radicals of pyrimidines and the C2-addition radicals of adenine derivatives need a polar environment to be stabilized. INDO calculations support this conclusion. While the “ionization path” could not be detected in “van der Waals crystals” both paths have been observed in “polar crystals”. These observations bear out the view that, in nonpolar environments, ionization is followed by geminate ion recombination with eventual subsequent homolytic dissociation of atomic hydrogen.
24 citations
TL;DR: In this paper, the spin adducts L(OC) 4 MnN(O)C(CH 3 ) 3 were identified as the spin-adducts of the spin electron spin resonance spectroscopy.
22 citations
TL;DR: The presence of aroyl oxyl radicals in the reaction medium was demonstrated by thermal decomposition of benzoyl peroxide in the presence of aromatic acids in this article, leading to two isomers, 7-aroyloxy- and 5-aroyloxy-derivative, whose structures were assigned on the basis of the ESR hfccs and of the 1H NMR spectra of corresponding amines.
21 citations
TL;DR: In this paper, the reactions of indolinone nitroxides with organic peracids were studied, and were found to give substitution products at the aromatic nucleus, i.e. 5-aroyloxy-and 7-ARoyloxynitroxides.
18 citations
18 citations
01 Jan 1977
TL;DR: A free-radical reaction is a chemical process in which molecules having unpaired electrons are involved as discussed by the authors, and the radical species could be a starting compound or a product, but in organic chemistry, the most common cases are reactions that involve radicals as intermediates.
Abstract: A free-radical reaction is a chemical process in which molecules having unpaired electrons are involved. The radical species could be a starting compound or a product, but in organic chemistry, the most common cases are reactions that involve radicals as intermediates. Most of the reactions discussed to this point have been heterolytic processes involving polar intermediates or transition states in which all electrons remain paired throughout the course of the reaction. In radical reactions, homolytic bond cleavages occur.
17 citations
01 Jan 1977
TL;DR: In this paper, a review is given of the four main types of reaction which are important in determining the distribution of products in the oxidation of hydrocarbons: free-radical attack on the hydrocarbon RH, decomposition of the radical R by C-C homolysis, reaction of R radical with O 2 to form the conjugate alkene, and reaction of r radical with o 2 to give O-heterocycles and other oxygenated products.
Abstract: A review is given of the four main types of reaction which are important in determining the distribution of products in the oxidation of hydrocarbons: o (a) free-radical attack on the hydrocarbon RH; (b) decomposition of the radical R by C-C homolysis; (c) reaction of R radical with O 2 to form the conjugate alkene; (d) reaction of R radical with O 2 to give O-heterocycles and other oxygenated products. Rate constants for these types of reaction, obtained from studies of the oxidation of C 2 -C 4 hydrocarbons and neopentane, are applied to predict the yields of product in the oxidation of pentane.
16 citations
TL;DR: In this article, photolysis of some carbonyl halides of Mn and cyclopentadienylmetal carbonyls halide derivatives of Fe, Mo and W in a 2-methyltetrahydrofuran (MTHF) matrix at about 77 leads to extrusion of CO, which becomes matrix-trapped, and formation of an unsaturated derivative characterised by i.r. spectroscopy.
TL;DR: In this paper, 1-Adamantyl radicals react with several 2-substituted benzothiazoles to afford 2-(1-adamantyl)-benzothiazole as a result of a homolytic substitution occurring at the ipso position.
Abstract: 1-Adamantyl radicals react with several 2-substituted benzothiazoles to afford 2-(1-adamantyl)-benzothiazole as a result of a homolytic substitution occurring at the ipso position; the yields are greater when the groups to be displaced are electron-withdrawing.
TL;DR: In this article, the reaction of 1-adamantyl, 3,3-dimethylbicyclo[2.2] octan-1-yl and 7,7-dimethyltimethylbicyl[2] heptan- 1-yl radicals with protonated pyridines and quinoline was investigated and the relative rates of substitution were determined.
Abstract: The reactions of 1-adamantyl (1), 3,3-dimethylbicyclo[2.2.2]octan-1-yl (2), and 7,7-dimethylbicyclo[2.2.1]heptan-1-yl (3) radicals with protonated pyridines and quinoline have been investigated and the relative rates of substitution have been determined. The results obtained showed that the three bridgehead radicals have nucleophilic properties which decrease in the order: (1) > (2) > (3). This behaviour is explained on the basis of a different degree of charge development in the transition state of the addition step of the radicals to the protonated substrates, as a consequence of different ring strain in the three polycyclic systems. A comparison of the present results with those already available for other alkyl radicals is also reported, in order to evaluate the effect of electronic configuration on the reactivity of carbon radicals in homolytic aromatic substitution.
TL;DR: In this article, the standard enthalpy of formation of bis(tropolonato)copper (II) has been determined at 298.15 K by solution calorimetry: ΔHf°[Cu(trop)2] (c)=−102.47±0.42 kcal mol−1.
Abstract: The standard enthalpy of formation of crystalline bis(tropolonato)copper(II) has been determined at 298.15 K by solution calorimetry: ΔHf°[Cu(trop)2] (c)=−102.47±0.42 kcal mol−1. Enthalpy changes at 298.15 K for the following hypothetical gaseous reactions have been subsequently derived:2& Cu(g)+2 trop^.(g)=[Cu(trop)_2](c) & & ΔH=-150.4±14.1 kcal mol^-1& Cu^2+(g)+2 trop^-(g)=[Cu(trop)_2](c) & & ΔH=-731.4±11.4 kcal mol^-1The corresponding homolytic (E) and heterolytic(E′) copper(II)-oxygen bond energy parameters were calculated asE_Cu–o=38±4 kcal mol^-1 and E’_Cu–o=183±3 kcal mol^-1respectively.
TL;DR: In this paper, a free-radical chain process involving amine radical cations is suggested to explain the clean reaction of a protonated quinoxaline base with N-chlorodialkylamine as a radical source.
Abstract: Selective homolytic alkylation of protonated quinoxaline has been achieved by using an alkyl derivative with N-chlorodialkylamine as radical source. The method shows great synthetic potential and can also be used for other heteroaromatic bases. A free-radical chain process involving amine radical cations is suggested to explain the clean reaction. Positional selectivity is strongly affected by the medium acidity, and a correlation between the π-electron density of mono- and di-protonated quinoxaline and the substitution orientation is found, in agreement with the nucleophilic character of the alkyl radicals.
TL;DR: A quantity of negative evidence has accumulated suggesting that group X migration does not involve alkylation of the cobalt of cobalamin by the substrate, but in the absence of firm data supporting an alternative mechanism, even this weak conclusion must be regarded as provisional.
Abstract: Adenosylcobalamin-dependent rearrangements are enzyme catalyzed reactions in which a hydrogen atom is transferred from one carbon atom to an adjacent one in exchange for a group X which migrates in the opposite direction. In the hydrogen transfer step, the mechanism of which is reasonably well understood, the cofactor serves as an intermediate hydrogen carrier. The transfer of hydrogen to the cofactor involves homolysis of the carbon-cobalt bond to generate cob(II)alamin and the 5′-deoxyadenos-5′-yl radical, followed by abstraction of a hydrogen atom from the substrate to form 5′-deoxyadenosine and the substrate radical. After migration of group X, the hydrogen atom is returned to the product radical by the reverse of the above reactions to generate the final product and reconstitute the cofactor.
TL;DR: The photolysis of bis aryl-1,3 triazenes carried out in non-aromatic solvents gives products whose structures are consistent with a cage recombination process of homolytically formed radicals and the subsequent abstraction of hydrogen from the solvent molecules by these arylamino radicals as discussed by the authors.
TL;DR: In this paper, 1-adamantyl and other alkyl radicals, produced by the AgI catalysed decarboxylation of carboxylic acids by ammonium peroxydisulphate, react with 2-acylbenzothiazoles, effecting the displacement of the acyl group to afford 2-alkyl benzothiazole in good yield.
Abstract: 1-Adamantyl and other alkyl radicals, produced by the AgI catalysed decarboxylation of carboxylic acids by ammonium peroxydisulphate, react with 2-acylbenzothiazoles effecting the displacement of the acyl group to afford 2-alkylbenzothiazoles in good yield Thereaction with 1-adamantyl radicals is facilitated by the presence of electronwithdrawing substituents in the 5- and 6-positions of the benzothiazole nucleus The displacement process is regarded as an aromatic SR reaction occurring at the ipso-position
TL;DR: In this article nucleophilic acetyl radicals react with 2-substituted benzothiazoles effecting the displacement of several groups to afford 2-acetylbenzothiazole; the reactions are proposed to occur through a homolytic aromatic substitution at the ipso position.
Abstract: Nucleophilic acetyl radicals react with 2-substituted benzothiazoles effecting the displacement of several groups to afford 2-acetylbenzothiazole; the reactions are proposed to occur through a homolytic aromatic substitution at the ipso position.
TL;DR: The alkylation reactions of co-ordinated N2 in [M(N2)2(dppe)2]-M = Mo or W; dppe = Ph2PCH2CH2PPh2) as discussed by the authors proceed via a rate-controlling loss of one dinitrogen ligand.
Abstract: The alkylation reactions of co-ordinated N2 in [M(N2)2(dppe)2](M = Mo or W; dppe = Ph2PCH2CH2PPh2) proceed via a rate-controlling loss of one dinitrogen ligand, co-ordination of alkyl halide, homolytic splitting of the carbon–halogen bond, and attack of the carboncentred free radical upon the remaining dinitrogen ligand.
TL;DR: The first evidence of β-hydrogen activation in metal alkyls to homolytic abstraction was shown in this article. But the origin of the activation was discussed relative to the formation of reactive intermediates including β-mercuraalkyl radicals and alkylmercury(I) species.
TL;DR: In this paper, a free radical chain process involving a homolytic substitution at the boron center is described, providing a new route from organoboranes to the corresponding alkyl chlorides.
TL;DR: In this article, a study of the reaction of WCl6 with LiCH2ZMe3 (Z Si or Ge) evidence for metal-carbene intermediates emerged.
TL;DR: In this paper, it was suggested that γ-and δ-cyclisation reactions occur with amidyls in the Σ- and Π-electronic states.
Abstract: Intramolecular cyclisations of amido-radicals generated by homolysis of N-iodo-amides onto aromatic systems gives γ- and δ-lactams, in cases where the aromatic rings can rotate, and only δ-lactams in rigid planar systems. Kinetic studies of the cyclisation of N-methylbiphenyl-2-carboxamide with t-butyl hypoiodite under irradiation have shown that γ- and δ-cyclisation products are formed in parallel and that the reactions have different activation energies and entropies. Reactions of N-methyl derivatives of 1,2,3,4-tetrahydro- and 1,2-dihydro-phenanthrene-4-carboxamides with t-butyl hypoiodite under irradiation have provided evidence that Π-conjugation is a prerequisite for δ-lactam formation. From a consideration of the molecular orbitals involved it is suggested that γ- and δ-cyclisation reactions occur with amidyls in the Σ- and Π-electronic states.
TL;DR: In this article, a series of dialkylmercury compounds are consistent with a charge transfer interaction between carbon tetrachloride as the rate-limiting step leading to the spontaneous initiation of homolytic processes.
TL;DR: In this paper, a unified mechanism is presented for reductive elimination and various substitution processes observed concurrently in dialkylmercury compounds, in which the radical intermediates are shared in common.
TL;DR: In this paper, the rate constants of the reaction of t-butoxyl radicals with 1,1-dimethyl- and 1, 1-dibutyl-stannacyclopentane have been measured to be 1.0 × 106 and 5.5 × 105 I mol-1 s-1, respectively, at 213 K.
Abstract: t-Butoxyl, trimethylsiloxyl, benzoyloxyl, and phenylthiyl radicals, X·, were generated by photolysis of the appropriate precursor, X2, and caused to react with the acyclic butyltin compounds BunSnCl4–n(n= 1–4) and the cyclic 1,1-dialkylstannacycloalkanes R2[graphic omitted]H2]n(R = Me, Et, Bun, or But, n= 4, 5, or 6). The resulting radicals were observed by e.s.r. spectroscopy.t-Butoxyl and, particularly, trimethylsiloxyl radicals react with tetra-n-butyltin at hydrogen in the α-methylene group to give the radical Bu3SnĊHPr. The stannacycloalkanes, on the other hand, react also at the tin centre to yield the ring-opened radicals XSnR2[CH2]n–1CH2·, and this tendency increases as the size of the ring is reduced. The stannacyclopentanes R2[graphic omitted]H2]4 react with the radicals Me3CO·, PhCO2·, and PhS· exclusively by ring opening, and the rate constants of the reaction of t-butoxyl radicals with 1,1-dimethyl- and 1,1 -dibutyl-stannacyclopentane have been measured to be 1.0 × 106 and 5.5 × 105 I mol–1 s–1, respectively, at 213 K. This reactivity is ascribed mainly to the relief of angle strain in the formation of a five-co-ordinate transition state or intermediate.Abstraction of hydrogen from an α-methylene group in organotin compounds is accompanied by abstraction from the β-methylene group to give the radical R3SnCH2ĊHR′, but this radical apparently rap'idly undergoes fragmentation to give the trialkylstannyl radical R3Sn· and the alkene CH2CHR′.
TL;DR: In this paper, a 2-benzyl-2-hydroxybenzo[b]furan-3(2H)-one was reconstructed using a benzilic acid rearrangement of the α-diketone formed from homolysis of the heterocycle.
Abstract: Irradiation of a 2-benzyl-2-hydroxybenzo[b]furan-3(2H)-one in acetone–water results in the equivalent of a benzilic acid rearrangement of the aryl benzyl α-diketone formed from homolysis of the heterocycle, to give the 3-benzyl-3-hydroxybenzo[b]furan-2(3H)-one analogue.