Showing papers on "Homolysis published in 1995"

Alkoxyamine-Initiated Living Radical Polymerization: Factors Affecting Alkoxyamine Homolysis Rates

Abstract: Previous work from these laboratories has shown that the success of alkoxyamine-initiated living radical polymerization (polydispersity of product, rate of polymerization) is critically dependent on the rate of homolysis of the C-O bond of the alkoxyamine initiator. Half-lives for a range of alkoxyamines based on initiator-derived radicals or low molecular weight propagating species have been measured experimentally. The values show a marked dependence on the structure of both the nitroxide and radical components. In this work, we demonstrate that semiempirical molecular orbital calculations provide a reliable, though qualitative, prediction of the experimentally observed trends in alkoxyamine homolysis rates. For example, for a series of alkoxyamines based on nitroxides (R(CH 3 ) 2 C) 2 NO . , C-O bond dissociation energies are predicted to decrease with an increase in ring size or the C-N-C angle (i.e., 5-membered > 6-membered > open chain > 7-membered), which is in accord with experimental results (5-membered > 6-membered > open chain). More importantly, calculations allow an assessment of the relative importance of steric and polar factors and radical stability in determining the order of alkoxyamine homolysis rates. In the case of secondary and tertiary alkoxyamines, steric factors appear to be the dominant influence. The calculations have application in the design of new initiators for living radical polymerization.

CH bond activation in hydrocarbon oxidation on solid catalysts

Abstract: The activation of CH bonds in saturated hydrocarbons is the crucial step in several different types of oxidation reaction on a variety of catalysts and could occur through homolytic or heterolytic mechanisms. From the available evidence it appears that the heterolytic mechanism, with the extraction of a proton, is the most likely process on oxide catalysts and this may also apply to metallic catalysts under typical (oxidising) reaction conditions. However, the state of oxidation of a metallic surface under reaction conditions is complex and the degree of oxidation will depend on the metal, the temperature, the oxygen partial pressure, the metal particle size, the support and the choice of hydrocarbon. The importance of coadsorbates in facilitating the dissociative adsorption of saturated hydrocarbons seems to be well established and could explain the unusual enhancement of activity observed for the oxidation of some saturated hydrocarbons when inorganic gases, such as SO2, are added to the reaction mixture. The inhibition by products of the oxidation reaction (CO2 and H2O) can be quite severe, but H2O has by far the greatest effect. This is interpreted in terms of an equilibrium involving the formation of surface hydroxide ions which are considered to be inactive for activation of CH bonds. As a consequence, it is possible, especially at low temperatures, that the rate determining step in CH bond activation could be the regeneration of the active sites, through desorption of H2O, rather than CH bond activation as is commonly assumed. The activation of CH bonds by NO2 is addressed in the context of selective reduction of NOx by hydrocarbons on various types of oxide catalyst and possible similarities with the promotion of the CH bond activation process by SO2 are discussed.

Accurate Structural Data Demystify B-12 - High-Resolution Solid-State Structure of Aquocobalamin Perchlorate and Structure-Analysis of the Aquocobalamin Ion in Solution

Abstract: Experiments are described to elucidate the structure and solvation of aquocobalamin (vitamin Biza, 1+) in the crystal and in aqueous solution. Aquocobalamin (1+) is the B12 derivative in which a water molecule replaces the axially coordinating organic substituent (methyl or 5'-desoxyadenosyl) at the P-side of the cobalt of the B12 coenzymes. (1) A single-crystal structure analysis of aquocobalamin perchlorate (l'ClO,), using synchrotron radiation in combination with an imaging plate detector, yielded the most accurate structural data ever determined for a B12 molecule. l'C10, crystallizes in the orthorhombic space group P212121, a = 15.042(11) A, b = 23.715(14) A, c = 25.104(12) A, with four 1'CIOi moieties plus about 100 solvent water molecules per unit cell; 22 867 independent and 20 942 significant intensity data were recorded to a nominal resolution of 0.8 A, and refinement against quantities led to a conventional R-value of 0.050 for all 22 867 observations and to a structural model with an average ESD for all carbon-carbon bonds of 0.003 A. In the crystal, the aquocobalamin ion has a very short axial bond between cobalt and the dimethylbenzimidazole (DMB) base of 1.925 (0.002) A, which is rationalized as resulting from the very weak trans axial donor (water). Steric repulsion between the DMB base and the corrin ring induced by this short Co-DMB bond leads to a relatively large "butteffly" deformation with an "upward" fold angle of 18.7 l(0.07)". The relevance of this observation for the "upward conformational deformation" hypothesis for the initiation of Co-C bond homolysis in coenzyme B12 dependent enzymes is discussed. (2) EXAFS spectra were taken from an authentic sample of the aquocobalamin perchlorate crystals used for the X-ray structure analysis, as well as from 1+C104 dissolved in a 1:l mixture of watedethylene glycol. Absorption spectra were recorded at 20 K between 7300 and 8700 eV, and the k3-weighted EXAFS was extracted for a k-value between 2.7 and 14.4 A-l. EXAFS spectra for solid and dissolved l'C10, agree closely, establishing identical cobalt coordination for the aquocobalamin ion in solution and in the solid state. A curve-fitting analysis on the Fourier filtered first-shell data yields a coordination number of 6 and an average distance of 1.90 A for both samples. There is no evidence for a longer Co-N distance. This refutes data published by Sagi and Chance (J. Am. Chem. Soc. 1992, 114, 8061). (3) NMR experiments are described, constituting the first detailed NMR investigations on a Bl2 derivative in H20, including 2D homo- and heteronuclear studies on aquocobalamin chloride (l+Cl-) and assignment of signals due to the exchangeable amide protons of all nitrogens as well a measurements of amide proton exchange rates. The NMR data confirm the occupation of the axial coordination site at the Co(II1) center by water, as well as the occurrence in solution of an intramolecular hydrogen bond to the axially coordinating water molecule, as observed in the crystal structure of l'C10,. However, significant differences of the structure of 1+ in crystals of l'C10, and of 1+ in aqueous solution are indicated from NOE data concerning the time-averaged conformation of the hydrogen-bonding c-acetamide side chain.

Liquid-phase reaction rate constants

Abstract: one - Reactions of Molecules.- I. Monomolecular reactions.- 1. Methods for Measuring Rate Constants of Monomolecular Reactions.- 2. Rate Constants for Decomposition of Peroxide Compounds.- 3. Decomposition of Azo Compounds.- 4. Decomposition at C - C, N-N, N-C,N-O, and C-Metal Bonds.- 5. Decomposition of Iodobenzene Dichloride.- 6. Correlation Equations.- 7. Effect of Pressure and Solvent on Monomolecular Reactions.- II. Bimolecular and Trimolecular Reactions.- 1. Methods for Measuring Rate Constants of Bimoleeular Reactions.- 2. Diels-Alder Reaction.- 3. Bimolecular Reactions with Peroxide Participation.- 4. Oxidation-Reduction Reactions with Participation of CIO2, O2, and CI2.- 5. Reactions with Rupture of Metal-Carbon Bond.- 6. Trimolecular Reactions.- III. The cage effect.- 1. Questions of Theory.- 2. Methods for Measuring Initiator Efficiency.- 3. Initiator Efficiency.- Literature Cited (Part One).- Two - Reactions of free Atoms and Radicals.- IV. Methods for Measuring Rate Constants of Radical Reactions.- 1. Measurement of Relative Rate Constants of Radical Reactions.- 2. Measurement of Absolute Rate Constants of Reactions of Atoms and Radicals.- V. Isomerization and Decomposition of Free Radicals.- 1. Isomerization of Free Radicals.- 2. Decomposition of Free Radicals.- VI. Radical Substitution Reactions.- 1. Reactions of Atoms.- 2. Reactions of Radicals Having Free Valence on Oxygen.- 3. Reactions of Radicals Having Free Valence on Carbon.- 4. Chain Transfer in Radical Polymerization.- 5. Reactions of Radicals Having Free Valence on Nitrogen.- 6. Correlation Equations for Radical Substitution Reactions.- VII. Addition Reactions of Atoms and Radicals.- 1. Addition of Atoms and Radicals to Molecular Oxygen.- 2. Addition at C = C Bond.- 3. Addition to Aromatic Compounds.- 4. Addition to Quinones, Carbonyl Compounds, Nitriles, and Nitro Compounds.- VIII. Recombination and Disproportionate of Free Atoms and Radicals.- 1. Recombination of Atoms.- 2. Disproportionation and Recombination of Alkyl and RO? Radicals.- 3. Reactions between Peroxy Radicals.- 4. Reactions of Radicals Having Free Valence on Nitrogen or Tin.- 5. Reactions between Free Radicals of Different Types.- IX. Effect of Solvent on Free Radical Reactions.- 1. Solvent Viscosity.- 2. Internal Pressure of Liquid.- 3. Nonspecific Solvation.- 4. Hydrogen Bond between Molecules.- 5. Radical Hydrogen Bond.- 6. Formation of ? -Complexes.- Literature Cited (Part Two).- Three - Ionic Homolytic Reactions.- X. Oxidation-Reduction Reactions of Ions With Molecules.- Oxidation of Organic Compounds by Variable-Valence Metal Ions.- Reaction of Variable-Valence Metal Ions with Oxygen, Peroxides, and Quinones.- XI. Reactions of Atoms and Radicals with Ions.- 1. Reactions of Hydrogen Atom.- 2. Reactions of Free Radicals.- XII. Reactions of Ion-Radicals and Solvated Electrons.- 1. Reactions of Ion-Radicals.- 2. Solvated Electron.- XIII. Ionic Oxidation-Reduction Reactions.- 1. Methods for Measuring Reaction Rates for Electron Transfer from Ion to Ion.- 2. Electron Exchange Reactions.- 3. Oxidation-Reduction Reactions between Ions.- Literature Cited (Part Three).

Tandem radical translocation and homolytic aromatic substitution: a convenient and efficient route to oxindoles

Abstract: Suitable o-bromo-N-methylanilides are efficiently converted into oxindoles by treatment with tributylstannane at 160 °C tanden translocation of the initially formed aryl radical and intramolecular homolytic substitution.

Organic nitrates, thionitrates, peroxynitrites, and nitric oxide: a molecular orbital study of the 2 RXNO, - RXONO (X = 0, S) rearrangement, a reaction of potential biological significance

Abstract: The rearrangement of organic thionitrate to sulfenyl nitrite potentially mediates the release of nitric oxide from organic nitrates, such as nitroglycerin, in the presence of thiol. The biological activity of these nitrovasodilators is proposed to result from release of nitric oxide in vivo. The thionitrate rearrangement bears analogy to the rearrangement of peroxynitrous acid to nitric acid, which has been proposed to mediate the biological toxicity of nitric oxide and superoxide. In this paper, the two concerted rearrangement processes and competing homolytic reactions are explored using molecular orbital calculations at levels up to MP4SDQl6- 31G*lIMP216-31G*. Examination of structure and energy for all conformers and isomers of RSONO, (R = H, Me), models for organic thionitrates and their isomers, demonstrates that structures corresponding to thionitrates and sulfenyl nitrates are of similar energy. Free energies of reaction for homolysis of these compounds are low (AGO < 19 kcallmol), whereas the bamer for concerted rearrangement is large (AGf (aq.) = 56 kcaVmo1). The larger bamer for concerted rearrangement of peroxynitrous acid to nitric acid (aGz(aq.) = 60 kcallmol) again compares unfavourably with homolysis (AGO c 11 kcallmol for homolysis to NO2 or 'NO). The transition state structures, confirmed by normal mode and intrinsic reaction coordinate analysis, indicate that considerable structural reorganization is required for concerted rearrangement of the ground state species. These results suggest that concerted rearrangement is not likely to be a viable step in either biological process. However, rearrangement via homolysis and radical recombination may provide an energetically accessible pathway for peroxynitrous acid rearrangement to nitric acid and rearrangement of thionitrate to sulfenyl nitrite. In this case, NO, will be a primary product of both reactions.

Role of Environmental Factors in the Dynamics of Intramolecular Dissociative Electron Transfer. Effect of Solvation and Ion-Pairing on Cleavage Rates of Anion Radicals

Abstract: Solvation and ion-pairing effects on the cleavage reactivity of anion radicals containing frangible bonds strongly depend upon the localization of the negative charge. When this is spread out over the entire molecular framework, as in haloanthracenes, the addition of water results in a small accelerating effect caused by specific solvation of the leaving anion. In anion radicals where the negative charge is concentrated on a small portion of the molecule, as on the oxygen atom of carbonyl or nitro groups, there is a considerable decrease of the cleavage reactivity upon addition of water. Stabilization of the leaving anion is still present, but it is largely overcompensated by a lowering of the energy of the orbital where the unpaired electron is located as revealed by a strong positive shift of the standard potential for the generation of the anion radical. The intramolecular electron transfer to the o* of the breaking bond thus possesses a lesser driving force. A change of mechanism ensues where cleavage is eventually replaced by 2e+ 2H+ hydrogenation. For similar reasons, ion pairing agents, such as Li+ or Mg2+ ions, have practically no effect on the cleavage rates and standard potentials of anion radicals with a largely delocalized negative charge, whereas strong effects are observed with anion radicals bearing a charge-localizing group, such as CO or NO2. The strong decrease in cleavage reactivity is again caused by the lowering of the energy well where the transferring electron sits. A change in mechanism also ensues where cleavage is eventually overrun by dimerization of the ion-paired radicals. Coupling between electron transfer and bond dissociation (or bond formation) is one of the key problems in the comprehension of chemistry triggered by single electron transfer. This question indeed attracts continuous attention whatever the mode of electron injection involving direct or indirect electrochemistry,' pulse radiolysis,2 and photochemi~try.~.~ The distinction between concerted and sequential processes has now been fixmly established on convergent experimental and theoretical grounds. The main molecular structure factors that govern the occurrence of one or the other mechanism have been identified.'*5 With concerted processes, application of a dissociative electron transfer Morse curve model has allowed the connection between @Abstract published in Advance ACS Abstracrs, September 1, 1995. (1) (a) Savtant, J.-M. Single Electron Transfer and Nucleophilic Substitution. In Advances in Physical Organic Chemistry; Bethel, D., Ed.; Academic Press: New York, 1990; Vol. 26, pp 1-130. (b) SavCant, J.-M. Acc. Chem. Res. 1993, 26, 455. (c) SavCant, J.-M. Dissociative Electron Transfer in Advances in Electron Transfer Chemistry: Mariano, P. S . , Ed.; JAI Press: New York, 1994; Vol. 4, pp 53-116. (d) SavCant, J.-M. Tetrahedron 1994, 50, 10117. (2) (a) Neta, P.; Behar, D. J. Am. Chem. SOC. 1980,102,4798. (b) Behar, D.; Neta, P. J. Phys. Chem. 1981, 85, 690. (c) Behar, D.; Neta, P. J. Am. Chem. Suc. 1981, 103, 103. (d) Behar, D.; Neta, P. J . Am. Chem. Suc. 1981, 103, 2280. (e) Bays, J. P.; Blumer, S. T.; Baral-Tosh, S.; Behar, D.; Neta, P. J. Am. Chem. SOC. 1983, 105. 320. (0 Nonis, R. K.; Barker, S . D.; Neta, P. J . Am. Chem. SOC. 1984, 106, 3140. (g) Meot-Ner, M.; Neta, P.; Noms, R. K.; Wilson, K. J. Phys. Chem. 1986, 90, 168. (3) (a) Saeva, F. D. Topics Curr. Chem. 1990, 156,61. (b) Saeva, F. D. Intramolecular Photochemical Electron Transfer (PET)-Induced Bond Cleavage Reactions in some Sulfonium Salts Derivatives. In Advances in Electron Transfer Chemistry; Mariano, P. S . , Ed.; JAI Press: New York, (4) (a) Amold, B. R.; Scaino, J. C.; McGimmev, W. G. J. Am. Chem. 1994; V O ~ . 4, pp 1-25. SOC. 1992, 114, 9978. (b) Chen, L.; Farahat, M . S . ; Gan, H.; Farid, S.; Whitten, D. G. J. Am. Chem. Soc., in press. ( 5 ) (a) Andrieux, C. P.; Le Gorande, A,; SavCant, J.-M. J . Am. Chem. SOC. 1992, 114, 6892. (b) Andrieux, C. P.; Differding, E.; Robert, M.; SavCant, J.-M. J. Am. Chem. SOC. 1993, 115, 6592. (c) Andrieux, C. P.; Robert, M.; Saeva, F. D.; SavCant, J.-M. J. Am. Chem. SOC. 1994, 116, 7864. 0002-7863195115 17-9340$09.00/0 reaction dynamics and molecular structure factors such as homolytic bond dissociation energy and oxidation potential of the leaving gr0up.I5-~ Within sequential processes, the electron transfer step is of the outersphere type, and its dynamics may therefore be described by the Marcus-Hush model.* The bond breaking step may be viewed as an intramolecular dissociative electron transfer and the reverse bond forming step as an intramolecular associative electron transfer. They may therefore be described by an extension of the dissociative electron transfer t h e ~ r y . ' ~ , ~ . ~ Here too, the predicted relationships between reactivity and molecular structure have received support from the available kinetic and reactivity trends data for bond cleavage in anion radicals as well as for coupling of radicals with nucleophiles, the two key steps of electron transfer stimulafed reactions as, for example, S R N ~ substitution^.'^.^*^ Solvation and ion-pairing effects on ion radical cleavage reactivity have so far received less attention than molecular structure effects. For dissociative electron tranbfer, the exact (6) (a) SavCant, J.-M. J . Am. Chem. SOC. 1987, 109,6788. (b) Savtant, J.-M. J . Am. Chem. SOC. 1992, 114, 10595. (c) Bertran, J.; Gallardo, I.; Moreno, M.; SavCant, J.-M. J. Am. Chem. Suc. 1992,114,9576. (d) Lexa, D.; SavCant, J.-M.; Su, K. B.; Wang, D. L. J . Am. Chem. SOC. 1987, 109, 6464. (e) Lexa, D.; SavCant, J.-M.; Schafer, H. J.; Su, K. B.; Vering, B.; Wang, D. L. J. Am. Chem. SOC. 1990, 112, 271. (0 Adcock, W.; Clark, C.; Houmam, A.; Krstic, A. R.; Pinson, J.; SavCant, J.-M.; Taylor, D. K.; Taylor, J. F. J. Am. Chem. SOC. 1994, 116, 4653. (7) (a) Clark, K. B.; Wayner, D. D. M. J . Am. Chem. SOC. 1991, 113, 9363. (b) Huang, Y.; Wayner, D. D. M. J. Am. Chem. SOC. 1993, 115, 2157. (c) Workentin, M. S.; Maran, F.; Wayner, D. D. M. J . Am. Chem. Suc. 1995, 117, 2120. (8) (a) Marcus, R. A. J . Chem. Phys. 1956, 24, 4966. (b) Hush, N. S. J . Chem. Phys. 1958,28, 962. (c) Marcus, R. A. Theory and Applications of Electron Transfers at Electrodes and in Solution. In Special Topics in Elecrrochemistry; Rock, P . A., Ed.; Elsevier: New York, 1977; pp 161179. (d) Marcus, R. A. Faraday Discuss. Chem. SOC. 1982, 74, 7. (e) Marcus, R. A.; Sutin, N. Biophys. Biochim. Acta 1985, 811, 265. (9) SavCant, J.-M. J. Phys. Chem. 1994, 98, 3716.

Generation and Reactions of Trifluoroacetimidoyl Radicals

Abstract: N-Aryltrifluoroacetimidoyl radicals have been generated by three different methods: the tin-radical promoted deiodination and photochemical homolysis of imidoyl iodides and the thermal homolysis of imidoyl azo-compounds. N-[2-(1-alkynyl)phenyl]trifluoroacetimidoyl iodides have been converted to trifluoromethylated indoles by the tin-radical promoted homolysis of the carbon–iodine bond followed by intramolecular cyclization. The photolysis of trifluoroacetimidoyl iodides enabled both intra- and intermolecular cyclization to indoles and quinolines. Likewise, thermal reactions of N-(2,2,2-trifluoro-1-tritylazoethylidene)anilines provided trifluoromethylated quinolines and indoles.

Transfer Hydrogenation of Nitro‐, Nitroso‐ and Azoarenes by Homolytic Retrodisproportionation

Abstract: Nitro-, nitroso- and azobenzene are reduced almost quantitatively to aniline when heated to 230–300°C with 9,10-dihydroanthracene (DHA), xanthene or tetralin. From the effect of polar substituents and polar solvents on the reactivity and from the isotope effect kH/kD ≈ 2.4 (280°C), a hydrogen-atom transfer from the H donor to the acceptor (retrodisproportionation) is proposed as the rate-determining step. The lower reactivity of xanthene compared with 9,10-dihydroanthracene eliminates the possibility of a rate-determining hydride transfer. The observation of an intense ESR signal of 9-xanthyl radicals during the reaction in diphenyl ether and the typical products support the proposed homolytic mechanism.

Side Chain Entropy and the Activation of Organocobalamins for Carbon-Cobalt Bond Homolysis: Thermolysis of Neopentylcobalamin-c-monocarboxylate, -c-N-methylamide, -c-N,N-dimethylamide, and -c-N-isopropylamide

Abstract: The contribution of c side chain rotational motions to the energetics of thermal carbon-cobalt bond homolysis in neopentylcobalamin (NpCbl) has been investigated by studies of NpCbl analogs including the c-monocarboxylate, and the c-N-methyl, c-N,N-dimethyl, and c-N-isopropyl derivatives. Spectrophotometric kinetic studies of the thermolysis of these NpCbl analogs in neutral aerobic aqueous solution, after correction for the measured amount of base-off species present under these conditions, showed that the enthalpy of activation was essentially constant (28.4 f 1.1 kcal mol-') but that the entropy of activation increased with increasing size of the c-COX moiety (X = 0-, 16.4 f 0.4, X = NH2, 19.3 4~ 0.6, X = NHMe, 21.1 f 0.7, X = NMe2, 24.8 f 0.6, and X = NHiPr, 24.9 f 0.3 cal mol-' K-I). Molecular mechanics calculations showed that the Co-C bond length and the Co-C-C and Co-C-H bond angles were not altered by rotation of the c side chain through 360" about the C7-C37 bond, nor were they significantly altered by the increasing steric bulk of the c side chain across the series of compounds. However, the net steric strain experienced by the compound upon rotation of the c side chain increased monotonically with the steric bulk of the c-COX moiety. Thus, the increase in the rate of thermolysis of the NpCbl analogs as the c side chain steric bulk is increased is not due to ground state enthalpic destabilization of the carbon-cobalt bond, but to increasing activation entropy, interpreted as being due to increasing restriction of c side chain rotation in the ground state which is relieved (or partially relieved) in the transition state for Co-C bond homolysis. These results may be used to estimate that approximately 30-40% of the decrease in the free energy of thermolysis of 5'-deoxyadenosylcobalamin (AdoCbl, coenzyme B 12) brought about by enzyme catalysis could possibly be due to enzymatically induced restriction of the ground state rotational freedom of the three upward projecting acetamide side chains. Thus, such a mechanism could be an important contributor to the catalysis of AdoCbl homolysis although it is almost surely not the sole mechanism employed.

Singlet Oxygen-Trapping Reaction as a Method of 1O2 Detection: Role of Some Reducing Agents

Abstract: The production of singlet oxygen by H2O2 disproportionation and via the oxidation of H2O2 by NaOCl in a neutral medium was monitored by spin trapping with 2,2,6,6 tetramethyl-4-piperidone (TMPone). The singlet oxygen formed in both reactions oxidized 2,2,6,6 tetramethyl-4-piperidone to give nitroxide radicals. However the production of nitroxide radicals was relatively small considering the concentrations of H2O2 and NaOCl used in the reaction systems. Addition of electron donating agents: ascorbate, Fe2+ and desferrioxamine leads to an increase in the production of nitroxide radicals. We assumed that a very slow step of the reaction sequence, the homolytic breaking of the O-O bond of N-hydroperoxide (formed as an intermediate product during the reaction of 1O2 with TMPone) could be responsible for the relatively small production of nitroxide radicals. Electron donating agents added to the reaction system probably raise the rate of the hydroperoxide decomposition by allowing a more rapid heterolytic cleav...

Organotransition-Metal Chemistry and Homogeneous Catalysis in Aqueous Solution

Abstract: Publisher Summary This chapter reviews both the organo-metallic compounds that can be obtained in aqueous solution and the catalytic systems that can be used in this solvent. In general for transition-metal systems, water-soluble organometallics are known for the kinetically inert metal ions. Alkylcobalt (III) complexes can also be synthesized in aqueous solution. Two of the best-known systems are methylcobalamin and a group of related cobaloximes, and alkylcobalt (III) complexes having ancillary cyanide ligands. As with the chromium (III) system, alkyl cobalt (III) complexes having dimethylglyoxime (DMG) or cyanide ligands can be synthesized by reaction of the cobalt (II) precursor with alkyl halides. Pulse radiolysis has been used to study the transient formation and decomposition of cobalt–alkyl bonds in aqueous solution in the same manner as it has been used for chromium alkyls. In general for all of these metal–alkyl bond homolysis reactions of the aquo complexes, steric hindrance facilitates the reaction. Since many organometallic compounds are stabilized by tertiary phosphines and phosphites, then if organometallic chemistry is to find major applications in aqueous solution it is necessary that these complexes be made compatible with such a solvent system.

Homolytic cleavage of the β-ether bond in phenolic β-O-4 structures in wood lignin and in guaiacylglycerol-β-guaiacyl ether

Abstract: The degradation of guaiacylglycerol-β-guaiacyl ether and lignin in spruce wood was studied in order to investigate the extent of homolytic reactions on heating with dioxane: water media (“mild hydrolysis”). The degradation reactions were studied at different temperatures, reaction times and pH-values. The results clearly show that extensive cleavage of phenolic β-ethers occur under the conditions prevailing during heating in dioxane: water media. Homolytic cleavage of phenolic β-ethers should therefore be considered in interpretations of results from “mild hydrolysis” of lignin.

Induced homolysis of dimethyldioxirane by alkanes and alkyl radicals in oxidation processes. The dramatic role of molecular oxygen and radical inhibitors

Abstract: The oxidation of alkanes by dimethyldioxirane is dramatically affected by the presence of molecular oxygen and radical inhibitors, supporting the hypothesis of a free-radical mechanism for these extraordinarily selective reactions; the factors affecting the free-radical selectivity are suggested.