Showing papers on "Homolysis published in 1980"

The kinetics and mechanism of ring opening of radicals containing the cyclobutylcarbinyl system

Abstract: The kinetic parameters for β-fission of radicals containing the cyclobutylcarbinyl system have been determined by analysis of the mixtures obtained when suitable chloro-compounds are treated with tributylstannane. Under these conditions ring opening is irreversible and in the rigid bicyclic system (4) is under stereoelectronic control. For ring opening of cyclobutylcarbinyl radical (8)kf= 4.3 × 103 s–1 at 60°, and the best values of the activation parameters appear to be ΔH‡= 12.2 kcal mol–1 and ΔS‡=–7.4 cal mol–1 K1. Monocyclic systems undergo preferential fission of the more substituted βγ-bond. Methyl substituents at the α-, β-, or δ-positions have little effect but γ-substitution strongly enhances the rate of ring opening. The transition state is reactant-like and has a similar disposition of centres to that (1) for homolytic addition.

The chemistry of vitamin B12. Part 19. Labilisation of the cobalt–carbon bond in organocobalamins by steric distortions; neopentyl-cobalamin as a model for labilisation of the vitamin B12 coenzymes

Abstract: Neopentylcobalamin in neutral solution at 25 °C is stable under nitrogen but is decomposed by O2 and by imidazole to give a cob(III)alamin and by hydrogen-atom donors such as thiols and PriOH to give the cob(II) alamin and neopentane. The reaction is ascribed to reversible homolytic fission of the Co–C bond to give a low steady-state concentration of the cob(II)alamin and neopentyl radicals, which can then form neopentane by the abstraction of a hydrogen atom. Neopentylcobinamide is also decomposed by high concentrations of imidazole, while the purely five-co-ordinate neopentylcobalamin in acid and neopentylcobinamide in neutral solution are stable both in the presence and absence of O2. Labilisation of the Co–C bond is ascribed to steric distortion around the co-ordinated C atom in the six-co-ordinate neopentyl complexes. It is suggested that neopentylcobalamin provides a model for the labilisation of the Co–C bond in the vitamin B12 coenzymes, and possible mechanisms for the isomerase reactions are discussed.

Photochemical Rearrangements and Fragmentations of Benzene Derivatives and Annelated Arenes

Abstract: In this article the numerous intramolecular reactions of electronically excited benezene derivatives, which in many instances are only mentioned in original papers, are systematically analyzed and arranged according to reaction types. All known reaction types can be classified, and subdivided into reactions of the benzene ring (ionization, ring opening, ring alteration), reactions with participation of side chains (α-, β-, γ-cleavage, homolysis, heterolysis), reactions of substituents with side chains (cyclization, dealkylation, cleavage of protective groups), and reactions of side chains with the aromatic ring (substitution, addition, dearomatization, cyclization). The selectivity of the energetically feasible competing reactions is primarily determined by geometric factors. Applications of the empirical effects are numerous and varied in preparative organic chemistry. Many of the reactions under discussion are already utilized industrially (e.g. in photochromism, UV stabilization, photography, information storage, printing, coating and polymer technology, and pharmacy).

Electron spin resonance studies of radicals formed during the thermolysis and photolysis of sulphoxides and thiolsulphonates

Abstract: E.s.r. spectroscopy has been employed to characterize radical pathways in the thermal and photolytic decomposition of a variety of diaryl sulphoxides and aryl arenethiolsulphonates (ArSO2SAr). For the sulphoxides, S–C bond cleavage leads to the formation of the delocalized and relatively unreactive sulphinyl radicals (ArSO·); their subsequent reaction evidently involves disproportionation to sulphonyl (ArSO2·) and thiyl (ArS·) radicals. This pair of radicals is also formed directly in the homolysis of the thiolsulphonates, and the subsequent formation of sulphinyl radicals has been investigated. The results are analysed in terms of the products of the reactions and the efficacy of some of the parent substrates as antioxidants.

Photocyclization of aryl halides. Part I. 5-(2-Halogenophenyl)-1,3-diphenylpyrazoles; homolytic fission assisted by radical complexation

Abstract: The photocyclization of 5-(2-halogenophenyl)-1,3-diphenyl pyrazoles proceeds efficiently with the exclusion of solvent participation hydrogen abstraction reactions. In a given solvent, the quantum ...

Studies on Pyrimidine Derivatives. XVI. Site Selectivity in the Homolytic Substitution of Simple Pyrimidines

Abstract: Pyrimidine derivatives in which both the 2- and 4-positions are free exhibited site selectivity in their reactions with radicals generated in redox systems. Namely, 6-phenyl-(I), 6-methylpyrimidine (XV), and 5, 6, 7, 8-tetrahydroquinazoline (XVII) reacted with radicals such as RCO, R2NCO, EtOCO, and CH2OH to give predominantly the 4-substituted products. Except in the reaction of I with the N, N-dimethylcarbamoyl radical, the formation of the corresponding 2-substituted isomers was not observed.

Free-radical reductions of arenediazonium ions in aqueous solution. IV. Kinetics of reactions of para-substituted diazonium ions with benzyl alcohol, isopropyl alcohol and methanol

Abstract: Electron-withdrawing substituents are shown to increase the chain length of free-radical hydro-dediazoniation reactions, but the actual reaction step causing the substituent effect depends on the relative rates of propagation and termination reactions. With benzyl alcohol as reducing agent the rate of the slow propagation step is increased, while with isopropyl alcohol the rate of the termination step is decreased. Rate constants for some reactions of radicals with diazonium ions are reported, and the nature of some of these reactions and their implication for an understanding of the homolysis of aromatic diazo compounds are discussed.

N-oxides and related compounds. Part 60. Novel thermal and photochemical rearrangements of N-substituted 2-pyridones

Abstract: Examples of four novel rearrangements of derivatives of 1-hydroxy-4,6-diphenyl-2-pyridone are reported: all involve N–O fission and formation of 3-substituted or both 3- and 5-substituted-4,6-diphenyl-2-pyridones. (a) 1-OCH2CH2R (R = vinyl or phenyl) compounds give 3-CH2R (R = vinyl or phenyl) derivatives with elimination of CH2O. (b) The 1-octyloxy-compound gives the 3-octyloxy-derivative by simple transposition. (c) 1-Acyloxy-compounds form the corresponding 3- and 5-acyloxy-2-pyridones. (d) 1-Imidoyloxy-compounds yield the rearranged 3-and 5-amido-2-pyridones. The mechanisms probably all involve homolytic N–O fission.

Etude du Comportement Photochimique de quelques Diaryl‐1,3‐triazènes

Abstract: Study of the Photochemical Behaviour of Some Diaryl-1,3-triazenes The photolysis of Diaryl-1,3-triazenes gives products whose structures are consistent with a cage recombination process of homolytically formed radicals and subsequent abstraction of hydrogen from the solvent molecules by arylamino radicals In aromatic solvents, a free-radical chain process leads to the formation of products resulting from the homolytic substitution on the solvent Quenching experiments show that singlet and triplet excited states are reactive but that intersystem crossing efficiency is low

Photocyclisation of 2-halogenobenzanilides: an extreme example of halogen atom, solvent, and isomer dependence. A practical phenanthridine synthesis

Abstract: 2-Chlorobenzanilides photocyclise to phenanthridones by assisted homolysis of the carbon–chlorine bond.

Factors controlling the selectivity of ipso-attack in homolytic aromatic substitutions. Reactions of alkyl radicals with nitrothiophen derivatives

Abstract: The reactions of the nearly electroneutral methyl and of the nucleophilic 1-adamantyl radical with some selected nitrothiophen derivatives have been investigated in order to elucidate the factors which control the positional selectivity of radical addition to an aromatic substrate. With 5-nitro-2-X-thiophens (II), (V), and (VI) and 3,5-dinitro-2-methoxycarbonylthiophen (III) the adamantyl radical gave exclusively the products of ipso-attack, whereas the methyl radical selectively added at the unsubstituted 4-position. On the other hand, with 4-nitro-2-methoxy-carbonylthiophen (I) both radicals added at the 5-position and with 4,5-dinitro-2-methoxycarbonylthiophen (IV) both radicals gave the products of ipso-substitution by displacing the nitro-group from the 5-position. These changes in positional selectivity are explained by assuming that the nature of the transition state of the addition step changes as a function of the polar character of the radical and of the electron deficiency of the aromatic substrates.

Studies on allylic peroxides. II. Allyl-t-butyl peroxide

Abstract: Allyl-tert-butyl peroxide decomposes thermally in toluene with kd = 1013.8 exp (−33200/θ) (s−1). Product studies show the reaction to be a mixture of homolysis and radical-induced decomposition, the latter involving both radical addition to C=C and allyl hydrogen abstraction by alkoxy radicals. Computer modelling of the system gives good agreement with the experimental results and suggests that the overall decomposition is about 50% homolysis.

Phosphite radicals and their reactions. Examples of redox, substitution, and addition reactions

Abstract: Phosphite radicals HPO/sub 3/- and PO/sub 3//sup 2/-, which exist in an acid-base equilibrium with pK = 5.75, are shown to take part in various types of reactions. In the absence of scavengers, they disappear mainly by second-order disproportionation and combination; a first-order contribution to the decay is also indicated. HPO/sub 3/- and PO/sub 3//sup 2/- are good reductants toward electron acceptors such as tetranitromethane. In this reaction phosphate and C(NO/sub 2/)/sub 3/- are formed. Phosphite radicals can, however, also act as good oxidants, e.g., toward thiols and thiolate ions. These reactions lead to the formation of RS. radicals which were identified either directly, as in the case of penicillamine, through the optical absorption of PenS. or more indirectly through equilibration of RS. with RS- to the optically absorbing RSSR-. disulfide radical anion. A homolytic substitution reaction (S/sub H/2) occurs in the reaction of the phosphite radicals with aliphatic disulfides, yielding RS. radicals and phosphate thioester RSPO/sub 3//sup 2/-. Lipoic acid, as an example of a cyclic disulfide, is reduced to the corresponding RSSR-. radical anion and also undergoes the S/sub H/2 reaction with about equal probability. An addition reaction is observed between phosphite radicals and molecular oxygen. The resultingmore » peroxo phosphate radicals establish an acid-base equilibrium HPO/sub 5//sup -/. reversible PO/sub 5//sup 2 -/. + H+ with a pK = 3.4. Absolute rate constants were determined for all reactions discussed.« less

Preparation, reactions, and reaction mechanisms of a family of difunctional complexes of bis(benzylchromium) cations

Abstract: Six new dimetallic organochromium cations, each containing two benzylchromium-like units per molecule, were prepared from selected dibromides and chromium(II). These tetrapositive complexes (and certain related monochromium analogues) were isolated and purified by ion-exchange chromatography in aqueous solution. The complexes were characterized by their uv-visible absorption spectra and by the stoichiometry and composition of the products obtained from reactions with chloropentaamminecobalt(III) ion, with sodium carbonate, and with mercury(II) chloride. These complexes react with oxidizing agents such as iron(III) ion, copper(II) ion, and halopentaamminecobalt(III) ion in reactions that are first order in the concentration of organochromium ion, but independent of the identity as well as the concentration of oxidizing agent (except for iron(III) ion which shows, for some complexes, a small contribution of an Fe/sup 3 +/-dependent pathway). These data, and comparisons with mononuclear, substituted benzylchromium cations, suggest that the rate-limiting step is unimolecular homolysis of a chromium-carbon bond. The dichromium cations produce only polymer. The monochromium cation (p-CrCh/sub 2/C/sub 6/H/sub 4/CH/sub 2/OH)/sup 2 +/ also polymerizes upon oxidation and has k/sub h/ = 1.66 x 10/sup -3/ s/sup -1/ at 25/sup 0/C, consistent with other rates and leading to a Hammett substituent constant for p-CH/sub 2/OH of sigma/sub p/ = +0.19.more » The mechanism of homolysis and of the polymerization initiation step are discussed in light of the kinetic data which give no evidence of a biphasic or stepwise loss of the benzylchromium absorption bands used to monitor the reaction rate. 3 tables.« less

Model experiments relevant to the mechanism of adenosylcobalamin-dependent diol dehydrase: further investigations of isomerisations of dihydroxyalkyl radicals

Abstract: Anaerobic photolysis of 4,5-dihydroxypentyl(pyridine)cobaloxime in 0.1M acetic acid (pH 3) gives ca. 10% of pentanal, whereas 1,1,5,5-tetradeuterio-4,5-dihydroxypentyl(pyridine)cobaloxime affords 0.6% of 1,5,5,5-tetradeuteriopentanal. Under similar conditions, 5-deuterio-5,6-dihydroxyhexyl(pyridine)cobaloxime gives ca. 1% of 6-deuteriohexan-2-one and 4% of 2-deuteriohexanal, in contrast to 5,6-dihydroxyhexyl(pyridine)cobaloxime which affords 16% of hexan-2-one and 4% of hexanal. These results support a mechanism for the photocon-version of dihydroxyalkylcobaloximes into aldehydes or ketones at pH 3, in which light-induced homolysis of the Co–C σ-bond to give a dihydroxyalkyl radical is followed by a 1,5-hydrogen shift. The resulting isomeric dihydroxyalkyl radical, possessing a hydroxy-group at the radical centre, undergoes an acid-catalysed transformation to aldehyde or ketone. With the deuteriated cobaloximes the 1,5-H shift is impeded by a primary isotope effect (kH/kDca. 20). Anaerobic photolysis of 4,5-dihydroxycyclo-octyl(pyridine)cobaloxime at pH 3 yields ca. 40% of cyclo-octanone. This reaction owes its efficiency to a favourable transannular 1,5-H shift which converts the 4,5-dihydroxycyclo-octyl radical into the 1,2-dihydroxycyclo-octyl radical. The relevance of the above reactions to adenosylcobalamin-dependent reactions catalysed by diol dehydrase is discussed.

Radical reactions in the co-ordination field of transition metals. VIII—σ-phenoxy and σ-cyclohexadienoneoxy radicals co-ordinated to cobalt (III) and their generation by tert-butylperoxy radicals or by molecular oxygen complexed to cobalt†

Abstract: The formation of a ternary complex between phenols, Co (II)-acetylacetonate and molecular oxygen has been established at ambient temperature and in non-polar solvents. After intramolecular electron transfer the transient radical enhances the homolytic scission of the OH bond of the complexed phenol. The low g value of the observed ESR spectra, the net spin delocalization to the cobalt nucleus and the interaction of the unpaired electron with only a limited number of benzene ring protons, indicate the presence of co-ordinated s-phenoxy radicals generated from unhindered phenols (g = 2.0006 ∼ 1.9997) and of co-ordinated s-cyclohexadienoneoxy radicals generated from hindered phenols (g=1.9990 ∼ 1.9980). Without the cobalt-chelating acetylacetonate ligands neither the co-ordinated s-phenoxy nor the co-ordinated s-cyclohexadienoneoxy radicals can be prepared, whereas co-ordinated π-tert-butylperoxy radicals [Co(III)BuO2·] are generated in a high concentration during the reaction of tert-butyl hydroperoxide with CoCl2 in acetone.

Rearrangement of radical ipso-intermediates by 1,2-shift of a formyl group. Reactions of adamantyl radicals with thiophencarbaldehydes

Abstract: The reaction of 1-adamantyl radicals with some 5-substituted thiophen-2-carbaldehydes gives rise to the products of adamantyldeformylation; this process is a new example of homolytic aromatic ipso-substitution. Moreover, the reaction with thiophen-2,5-dicarbaldehyde affords, as the major product, 2-(1-adamantyl)thiophen-3,5-dicarbaldehyde in which a formyl group has migrated from the 2- to the 3-position. This result is explained by assuming that the initially formed radical ipso-intermediate rearranges by 1,2-shift of the ipso-substituent.

Intramolecular homolytic displacement of bromine atoms from carbon in the reaction of but-3-enyl bromides with radicals

Abstract: The homolytic displacement of bromine atoms from 1-bromo-1-cyano-2,2-dimethylbut-3-ene by trichloromethyl radicals takes place by a stepwise mechanism in which a trichloromethyl radical attacks carbon-4 to give an intermediate radical which can undergo an intramolecular cyclisation with loss of a bromine atom or a radical transfer reaction with the addend.