Showing papers on "Homolysis published in 1985"

Mechanisms of coenzyme B12-dependent rearrangements.

Abstract: Coenzyme B12 serves as a cofactor in various enzymatic reactions in which a hydrogen atom is interchanged with a substituent on an adjacent carbon atom. Measurement of the dissociation energy of the coenzyme's cobalt-carbon bond and studies of the rearrangement of model free radicals related to those derived from methylmalonyl-coenzyme A suggest that these enzymatic reactions occur through homolytic dissociation of the coenzyme's cobalt-carbon bond, abstraction of a hydrogen atom from the substrate by the coenzyme-derived 5'-deoxyadenosyl radical, and rearrangement of the resulting substrate radical. The only role thus far identified for coenzyme B12 in these reactions--namely, that of a free radical precursor--reflects the weakness, and facile dissociation, of the cobalt-carbon bond.

Mechanism of decomposition of nitroaromatics. Laser-powered homogeneous pyrolysis of substituted nitrobenzenes

Abstract: Laser-powered homogeneous pyrolysis (LPHP) has been used to study the gas-phase thermal decomposition of nitrobenzene (NB), m-dinitrobenzene (m-DNB), p-nitrotoluene (p-NT), o-nitrotoluene (o-NT), and 2,4-dinitrotoluene (2,4-DNT) under conditions where surface-catalyzed reactions are precluded. The Arrhenius parameters have been determined by comparative rate measurements relative to cyclohexene decomposition and the reaction mechanisms have been established. In all cases, the rate-limiting step is the homolysis of the Ar-NO2 bond. The measured Arrhenius parameters for homolysis range from log A = 14.5 to 16.4 and E/sub a/ from 67.0 to 70.0 kcal/mol. The c-NO2 bond dissociation energies (kcal/mol) and values of log k (s ) for NB, m-DNB, p-NT, o-NT, and 2,4-DNT have been derived from these results and are as follows: 71.4 +/- 2.0, (15.5 +/- 0.5) - (68.2 +/- 1.7)/2.3RT; 73.2 +/- 2.4, (14.5 +/- 0.5) - (70.0 +/- 2.1)/2.3RT; 71.4 +/- 2.3, (14.9 +/- 0.5) - (68.2 +/- 2.0)/2.3RT; 70.2 +/- 2.5, (16.4 +/- 0.6) - (67.0 +/- 2.2)/2.3RT; 70.6 +/- 2.0, (15.3 +/- 0.4) - (67.4 +/- 1.7)/2.3RT. 15 references, 6 figures, 4 tables.

Secondary isotope effects and structure-reactivity correlations in the dopamine beta-monooxygenase reaction: evidence for a chemical mechanism.

Abstract: The chemical mechanism of hydroxylation, catalyzed by dopamine beta-monooxygenase, has been explored with a combination of secondary kinetic isotope effects and structure-reactivity correlations. Measurement of primary and secondary isotope effects on Vmax/Km under conditions where the intrinsic primary hydrogen isotope effect is known allows calculation of the corresponding intrinsic secondary isotope effect. By this method we have obtained an alpha-deuterium isotope effect, Dk alpha = 1.19 +/- 0.06, with dopamine as substrate. The beta-deuterium isotope effect is indistinguishable from one. The large magnitude of Dk alpha, together with our previous determination of a near maximal primary deuterium isotope effect of 9.4-11, clearly indicates the occurrence of a stepwise process for C-H bond cleavage and C-O bond formation and hence the presence of a substrate-derived intermediate. To probe the nature of this intermediate, a structure-reactivity study was performed by using a series of para-substituted phenylethylamines. Deuterium isotope effects on Vmax and Vmax/Km parameters were determined for all of the substrates, allowing calculation of the rate constants for C-H bond cleavage and product dissociation and dissociation constants for amine and O2 loss from the enzyme-substrate ternary complex. Multiple regression analysis yielded an electronic effect of p = -1.5 for the C-H bond cleavage step, eliminating the possibility of a carbanion intermediate. A negative p value is consistent with formation of either a radical or a carbocation; however, a significantly better correlation is obtained with sigma p rather than sigma p+, implying formation of a radical intermediate via a polarized transition state. Additional effects determined from the regression analyses include steric effects on rate constants for substrate hydroxylation and product release and on KDamine, consistent with a sterically restricted binding site, and a positive electronic effect of p = 1.4 on product dissociation, ascribed to a loss of product from an enzyme-bound Cu(II)-alkoxide complex. These results lead us to propose a mechanism in which O-O homolysis [from a putative Cu(II)-OOH species] and C-H homolysis (from substrate) occur in a concerted fashion, circumventing the formation of a discrete, high energy oxygen species such as hydroxyl radical. The substrate and peroxide-derived radical intermediates thus formed undergo a recombination, kinetically limited by displacement of an intervening water molecule, to give the postulated Cu(II)-alkoxide product complex.

Ferrocenium/alpha-cleavage photoinitiator systems for free radical polymerizable compositions

Abstract: A free radical polymerizable composition comprising a free radical polymerizable material and a photoinitiator system therefor comprising appropriate amounts of an alpha-cleavage or homolytic bond cleavage, photoinitiator and a ferrocenium salt, said composition being suitable for use in the preparation of protective layers and photographic images.

Homolysis and electron-transfer reactions of benzylcobalamin

Abstract: Reaction de la benzylcobalamine avec l'oxygene et son rapport avec l'homolyse. Clivage oxydatif de la benzylcobalamine par reaction avec les ions Fe(III). Activation de O 2 pour l'oxydation de l'acide ascorbique

Homolytic versus heterolytic cleavage for the photochemistry of 1-naphthylmethyl derivatives

Abstract: The photochemical cleavage of the 1-naphthylmethyl derivatives, 1–7, has been examined in methanol solvent under both direct and sensitized conditions The competitition between homolytic and heterolytic cleavage as a function of multiplicity and leaving group has been studied in detail Only substrates 1, 2, 3, and 7 react on sensitization with xanthone but evidence is presented that the resulting reactivity of 1, 2, and 3 may not be triplet energy transfer but rather exciplex formation A semi-quantitative scale for photofugacities of the leaving groups from the excited singlet states has been established

The photolysis of N,O-diacyl-N-phenylhydroxylamines

Abstract: The photochemical decomposition of the title compounds yielded the rearrangement products derived from 1,3- and 1,5-aroyloxyl migrations in addition to the fragmentation products typical of the aroyloxyl and amido radicals, while only 1,3-aroyloxyl migration was observed on thermolysis. The results of crossover experiments indicate an intramolecular rearrangement, probably involving homolysis of the N–O bond in the excited states. Triplet quenching studies demonstrate that the rearrangement and fragmentation proceed exclusively via the first excited singlet state. No formation of the rearrangement products on triplet sensitization was explained on the basis of the spin multiplicity effects on a radical–radical recombination process within a solvent cage.

The chemistry of vitamin B12. Part 25. Mechanism of the β-elimination of olefins from alkylcorrinoids; evidence for an initial homolytic fission of the Co–C bond

Abstract: Equilibrium constants (log10K/dm3 mol–1) have been determined for the co-ordination of imidazole by the five-co-ordinate alkylcobinamides with R (= alkyl)= Me (0.9), Et(–0.5), neopentyl (np)(–1.4), Pri( < –1.9), and cyclohexyl (C6)( < –1.9) in aqueous solution at 25 °C; this confirms that contact between dbzm (the heterocyclic base, 5,6-dimethylbenzimidazole, in the cobalamin side-chain) and the corrin ring can play only a minor part in the similar effect of varying R on the co-ordination of dbzm. The maximum (pH- and po2-independent) rates of decomposition of np- and C6-cobalamin (at 36 °C) and of the corresponding cobinamides (at 80 °C) have been determined. Comparison of present and published kinetic data show that changing R (from Et to np and C6) and changing from six- to five-co-ordination (for both R = np and C6) produce very similar and large changes in rate, irrespective of whether the overall reaction corresponds to homolytic fission (R = np) or β-elimination (R = C6); this provides indirect evidence that both reactions involve a common first step, viz. homolytic fission of the Co–C bond to produce a caged (CoII+ radical) pair. The decomposition of C6-cobinamide in air at pH 1 and 25 °C is catalysed by vanadyl ions in solution and by cobalt boride in suspension. Me- and C6-cobinamide are both diamagnetic.

Nucleophilic photoreaction of chlorobenzene in methanol as studied by emission spectroscopy.

Abstract: The photoreaction of chlorobenzene in methanol has been studied by means of emission spectroscopy. Upon irradiation with UV light, chlorobenzene reacts with methanol to form anisole with a quantum yield of 0.049 at 18 °C. Contrary to the case of chlorobenzene, the photoreaction of p-dichlorobenzene with methanol is not observed. By combining these results with those of our recent studies on the dual phosphorescence from low-lying triplet states of halogenated benzenes, it is suggested that the photoreaction of chlorobenzene with methanol, which is a nucleophilic substitution reaction, occurs in the 3(π,σ*) state. From the temperature dependence of the quantum yield, the activation energy for the nucleophilic photoreaction is estimated to be larger by about 3–4 kcal mol−1 than the apparent activation energy for a combination of the nonradiative 3(π,σ*)→S0 process and the homolytic dissociative process concerning the C–Cl bond in the 3(π,σ*) state. It is suggested that the nucleophilic photoreaction contras...

Substituent effects and isokinetic relations in the homolysis of hyponitrite esters. The question of free-radical homoconjugation

Abstract: L'etude est faite sur 14 hyponitrites trans (alkyl, benzyl et benzhydryl) dans le t-butylbenzene

Zinc–sulphur bond enthalpy: its determination in the compound bis(diethyldithiocarbamato)zinc(II)

Abstract: The standard enthalpy of formation of bis(diethyldithiocarbamato)zinc(II), as well as that of the salt diethylammonium chloride, has been determined by solution calorimetry at 298.15 K, giving –(289.65 ± 4.9) and –(359.63 ± 1.22) kJ mol–1, respectively. From the standard enthalpy of formation of the chelare, the enthalpy of formation in the gaseous phase, ΔHf⊖[Zn(S2CNEt2)2,g]=–(147.0 ± 5.0) kJ mol–1 was derived. The homolytic and heterolytic zinc–sulphur bond enthalpy parameters were calculated to be 177.4 ± 3.5 and 740.2 ± 3.5 kJ mol–1, respectively. These values are discussed and appear to correlate with the zinc–sulphur bond distance.

The theoretical study of chemical intermediates

Abstract: We show in this paper that the thermochemistry and kinetics of some typical reactions involving free radicals or ions can be rationalized in terms of stabilization energies of these species. Homolytic and heterolytic bond cleavage reactions, hydrogen abstraction reactions, ionic and radical additions to alkenes, direct coordination of carbocations with carbanions and dimerization of free radicals have been particularly studied in this work.

Flow ESR and static studies of the decomposition of phenyl-substituted ethanes

Abstract: The thermal decomposition of 1,1,2,2-tetraphenylethane (DD) was studied in high-temperature fluids by using both sealed tube and flow ESR techniques. In the sealed tube experiments its central C-C bond homolysis rate constant in tetralin solvent was measured. Concentrations of diphenylmethyl radicals in equilibrium with DD were measured by ESR between 648 and 683. K. The equilibrium constant for the dissociation reaction was determined. A recombination rate constant for diphenylmethyl radicals was calculated. Bond homolysis of 1,1,2-triphenylethane (BD) was also studied in tetralin. An analysis of the above data shows that the rate of BD dissociation is not strongly affected by release of steric energy. In contrast, the DD homolysis rate was increased by more than an order of magnitude through release of strain. 30 references, 3 figures, 6 tables.