Showing papers on "Homolysis published in 2006"
TL;DR: In this paper, the authors show that poly(methyl methacrylate) (PMMA) with a methyl trithiocarbonate end group undergoes loss of that end group at at least in part, by a mechanism believed to involve homolysis of the C-CS2SCH3 bond and subsequent depropagation.
Abstract: Thermolysis provides a simple and efficient way of eliminating thiocarbonylthio groups from RAFT-synthesized polymers. The course of thermolysis of poly(methyl methacrylate) (PMMA) prepared with dithiobenzoate and trithiocarbonate RAFT agents was followed by thermogravimetric analysis (TGA), 1H NMR spectroscopy, and gel permeation chromatography (GPC). The weight loss profile observed depends strongly on the RAFT agent used during polymer synthesis. PMMA with a methyl trithiocarbonate end group undergoes loss of that end group at ~180°C, at least in part, by a mechanism believed to involve homolysis of the C–CS2SCH3 bond and subsequent depropagation. In contrast, PMMA with a dithiobenzoate end appears more stable. Only the end group is lost at ~180°C and the dominant mechanism is proposed to be a concerted elimination process analogous to that involved in the Chugaev reaction.
110 citations
TL;DR: In this paper, all-rac-α-Selenotocopherol (6c) was synthesized in 11 steps in 6.6% total yield using electron paramagnetic resonance (EPR) equilibration techniques.
Abstract: all-rac-α-Selenotocopherol (6c) has been synthesized in 11 steps in 6.6% total yield. Key steps include chloromethylation to approach the persubstituted aromatic 9b and cyclization of alcohol precursor 10 by radical homolytic substitution at selenium to form the selenotocopherol heterocycle. Determination of the OH bond dissociation enthalpy (BDE) of 6c by electron paramagnetic resonance (EPR) equilibration techniques gave a value of 78.1 ± 0.3 kcal mol-1, approximately 1 kcal mol-1 higher than that of α-tocopherol. Kinetic studies performed by measuring oxygen uptake of the induced oxidation of styrene in the presence of an antioxidant showed that selenotocopherol (6c) was a slightly poorer inhibitor than α-tocopherol, in agreement with the BDE values. In contrast to simpler selenotocopherol analogues, 6c was not regenerable in the presence of a stoichiometric coreductant in a two-phase lipid peroxidation model.
79 citations
TL;DR: ONIOM calculations have provided novel insights into the mechanism of homolytic Co-C5' bond cleavage in the 5'-deoxyadenosylcobalamin cofactor catalyzed by methylmalonyl-CoA mutase, and it is shown that it is a stepwise process in which conformational changes in the5'-de oxygenadenosine moiety precede the actual homolysis step.
Abstract: ONIOM calculations have provided novel insights into the mechanism of homolytic Co−C5‘ bond cleavage in the 5‘-deoxyadenosylcobalamin cofactor catalyzed by methylmalonyl-CoA mutase. We have shown that it is a stepwise process in which conformational changes in the 5‘-deoxyadenosine moiety precede the actual homolysis step. In the transition state structure for homolysis, the Co−C5‘ bond elongates by ∼0.5 A from the value found in the substrate-bound reactant complex. The overall barrier to homolysis is ∼10 kcal/mol, and the radical products are ∼2.5 kcal/mol less stable than the initial ternary complex of enzyme, substrate, and cofactor. The movement of the deoxyadenosine moiety during the homolysis step positions the resulting 5‘-deoxyadenosyl radical for the subsequent hydrogen atom transfer from the substrate, methylmalonyl-CoA.
70 citations
TL;DR: In this article, a new catalytic system resulting from the combination of a persistent macrocyclic aminoxyl radical and the couple Mn(NO 3 ) 2 /Co(NO3 ) 2 has been developed for the aerobic oxidation of alcohols to aldehydes and ketones under mild conditions.
Abstract: Aminoxyl (R 2 N O ), amidoxyl (RCO N(O ) R) and imidoxyl ((RCO) 2 N O ) radicals play a key role in the aerobic oxidation catalyzed by N -hydroxyderivatives. The rationalization of the mechanisms of a variety of oxidations is based on thermochemical, kinetic and spectroscopic investigations and on solvent effects and it has suggested new selective synthetic developments. In collaboration with CIBA Speciality Chemicals a new catalytic system resulting from the combination of a persistent macrocyclic aminoxyl radical and the couple Mn(NO 3 ) 2 /Co(NO 3 ) 2 has been developed; it is particularly effective for the aerobic oxidation of alcohols to aldehydes and ketones under mild conditions (air at room temperature and atmospheric pressure); above all it presents the great advantage of an easy recovery and recycling providing the possibility of practical applications. The kinetic investigation of the substituent effect in the oxidation of benzyl alcohols has allowed identifying the rate-determining step of the oxidation. The amidoxyl radicals, generated “in situ” from N -hydroxyamide, revealed particularly effective catalysts for the aerobic peroxidation of polyunsaturated fatty acids and esters, which is involved in the origin of several important pathologies, such as tumor initiation and atherosclerosis. The kinetic investigation has contributed to explain the mechanism of the oxidation and to develop the most effective methodology for the synthesis of hydroperoxides. The importance of enthalpic, polar, captodative, solvent effects and “molecule-induced homolysis” has been emphasized in the oxidation, halogenation and acetoxylation of a variety of classes of organic compounds (hydrocarbons, alcohols, aldehydes, amines, amides, silanes) by O 2 and N -hydroxyimide catalysis. The high selectivity often observed and the very mild experimental conditions, based on the mechanistic interpretation, provide industrial potentiality to the catalytic processes.
66 citations
TL;DR: An efficient tandem radical process comprising a thermal alkoxyamine homolysis, an isonitrile trapping reaction, a 5-exo-trig cyclization, and a homolytic aromatic substitution leads to substituted dihydroquinolines in moderate to good yields.
66 citations
TL;DR: In this paper, the authors show that the reactivity and intrinsic lifetimes of metal-oxygen intermediates in aqueous solutions are a strong function of pH as the metal−oxygen interaction adds an extra dimension to the already complex pH dependence of O 2 reduction.
61 citations
59 citations
TL;DR: A correlation is developed between the number of nitrogen atoms in a heterocycle and its enthalpy, and this relationship is extrapolate to predict the enthalpies of formation of pentazole, and Trends in N-H and C-H acidities can be qualitatively explained by electrostatic effects and electron affinities.
Abstract: The nitrogen-containing heterocycles are of interest as high-energy-density materials for use as propellants and explosives, while the pyrolysis of these compounds is also important in understanding the evolution of unwanted NO and NO2 (NOx) from organic fuels such as coal and biomass. We have used ab initio and density functional methods to study the molecular structures and thermochemical properties of the five-membered nitrogen-containing heterocycles and their anions and radicals corresponding to respective heterolytic and homolytic loss of a hydrogen atom from either a nitrogen or carbon site. Many of these thermochemical properties have not previously been measured, especially for the heterocycles containing three and four nitrogen atoms. Using the theoretical methods CBS-APNO, G3, and G3B3, we calculate enthalpies of formation of 26.5, 42.4, 31.9, 63.7, 46.8, 81.0, and 79.0 kcal mol-1 for pyrrole, pyrazole, imidazole, 1,2,3-triazole, 1,2,4-triazole, 1H-tetrazole, and 2H-tetrazole. A correlation is developed between the number of nitrogen atoms in a heterocycle and its enthalpy, and we extrapolate this relationship to predict the enthalpy of formation of pentazole. N-H BDEs in the heterocycles typically increase with the number of nitrogen atoms in the molecule, while C-H BDEs are similar in all of the studied heterocycles, at around 120 kcal mol-1. In all cases the N-H BDEs are weaker than the C-H BDEs, suggesting abstraction of the N-H hydrogen atom is more likely. Deprotonation enthalpies and free energies reveal that the N-H protons become more acidic with increasing number of nitrogen atoms in the heterocycle. C-H protons are less acidic than N-H protons by ca. 49 kcal mol-1, or ca. 35 kcal mol-1 when adjacent to the NH group. Trends in N-H and C-H acidities can be qualitatively explained by electrostatic effects and electron affinities. From its use as a reference species in our calculations, we identify that the experimental enthalpy of pyrimidine (1,3-diazine) may be in error by ca. 1-3 kcal mol-1, and we recommend an enthalpy of formation of 44.8 +/- 1.0 kcal mol-1.
56 citations
TL;DR: In this paper, the authors showed that the difference in reactivity between the (RR/SS) and (RS/SR) diastereoisomers is caused by a nσσ* interaction between the nσ lone pair of the oxygen atom of the ester bond and the σ* orbital of the cleaved O-C bond.
48 citations
TL;DR: In this article, it was shown that peracids and dioxirane can be used for the aerobic free-radical epoxidation of alkenes with selectivity quite different from the well-known epoxification by peracides.
47 citations
TL;DR: It is shown that heparan sulfate N-chloro derivatives are decomposed in the presence of redox-active transition-metal ions and superoxide (O(2)(*-)).
Abstract: The highly basic heme enzyme myeloperoxidase (MPO), which is released by activated phagocytes, catalyzes the production of the potent oxidant hypochlorite (HOCl) from H2O2 and chloride ions (Cl-). Heparan sulfate proteoglycans are key components of the extracellular matrix and cell surfaces and are known to bind MPO avidly via their negatively charged heparan sulfate chains. Reaction of heparan sulfate with HOCl generates polymer-derived N-chloro derivatives (chloramines, dichloramines, N-chlorosulfonamides, and chloramides). In this study, it is shown that heparan sulfate N-chloro derivatives are decomposed in the presence of redox-active transition-metal ions and superoxide (O2•-). These processes initiate polymer modification/fragmentation. Radical intermediates in these processes have been identified by EPR spectroscopy and spin trapping. Evidence has been obtained that the N-chloro derivatives undergo reductive homolysis to nitrogen-centered (aminyl, N-chloroaminyl, sulfonamidyl, and amidyl) radicals...
TL;DR: Preliminary studies of the enzymatic activation of a coenzyme analog with a 5-fluoroimidazole axial nucleotide suggest that the catalysis of Co-C bond homolysis may indeed be significantly slowed by the decrease in basicity.
Abstract: The enzymatic “activation” of coenzyme B12 (5′-deoxyadenosylcobalamin, AdoCbl), in which homolysis of the carbon–cobalt bond of the coenzyme is catalyzed by some 109- to 1014-fold, remains one of the outstanding problems in bioinorganic chemistry. Mechanisms which feature the enzymatic manipulation of the axial Co–N bond length have been investigated by theoretical and experimental methods. Classical mechanochemical triggering, in which steric compression of the long axial Co–N bond leads to increased upward folding of the corrin ring and stretching of the Co–C bond is found to be feasible by molecular modeling, but the strain induced in the Co–C bond seems to be too small to account for the observed catalytic power. The modeling study shows that the effect is a steric one which depends on the size of the axial nucleotide base, as substitution of imidazole (Im) for the normal 5,6-dimethylbenzimidazole (Bzm) axial base decreases the Co–C bond labilization considerably. An experimental test was thus devised using the coenzyme analog with Im in place of Bzm (Ado(Im)Cbl). Studies of the enzymatic activation of this analog by the B12-dependent ribonucleoside triphosphate reductase from Lactobacillus leichmannii coupled with studies of the non-enzymatic homolytic lability of the Co–C bond of Ado(Im)Cbl show that the enzyme is only slightly less efficient (3.8-fold, 0.8 kcal mol−1) at activating Ado(Im)Cbl than at activating AdoCbl itself. This suggests, in agreement with the modeling study, that mechanochemical triggering can make only a small contribution to the enzymatic activation of AdoCbl. Another possibility, electronic stabilization of the CoII homolysis product by compression of the axial Co–N bond, requires that enzymatic activation be sensitive to the basicity of the axial nucleotide. Preliminary studies of the enzymatic activation of a coenzyme analog with a 5-fluoroimidazole axial nucleotide suggest that the catalysis of Co–C bond homolysis may indeed be significantly slowed by the decrease in basicity.
TL;DR: A series of stable 2,2-disubstituted 3-(phenylimino)indol-1-oxyls, the alkoxyamines 3, were prepared, characterized, and tested as possible candidates in controlled radical polymerization (CRP) as mentioned in this paper.
Abstract: A series of stable 2,2-disubstituted 3-(phenylimino)indol-1-oxyls, the alkoxyamines 3, were prepared, characterized, and tested as possible candidates in controlled radical polymerization (CRP). The sturctures of 3d and 10 were additionally solved by X-ray diffraction. The lability of the NO(C) and (N)OC bonds of compounds 3 were compared, and the possibility of NO vs .O C bond cleavage was evaluated by thermal degradation, ESR spin trapping, MS experiments, and DFT calculations. Alkoxyamines with a primary- or secondary-alkyl group bound to the O-atom of the nitroxide function (hexyl and i-Pr) mainly underwent (undesired) NO bond homolysis. When the O-alkyl radical was a tertiary or a benzyl group (crotonyl or styryl), OC bond cleavage occurred as the main process, thus suggesting a possible use of these compounds in CRP processes.
TL;DR: It is suggested that the high-valence Cpd I species of BLM participates in the DNA cleavage, which is an alternative mechanistic hypothesis to the exclusive reactivity scenario based on ABLM (FeIII-OOH).
Abstract: Bleomycins (BLMs) can utilize H2O2 to cleave DNA in the presence of ferric ions. DFT calculations were used to study the mechanism of O-O bond cleavage in the low-spin FeIII-hydroperoxo complex of BLM. The following alternative hypotheses were investigated using realistic structural models: (a) heterolytic cleavage of the O-O bond, generating a Compound I (Cpd I) like intermediate, formally BLM-FeV=O; (b) homolytic O-O cleavage, leading to a BLM-FeIV=O species and an OH* radical; and (c) a direct O-O cleavage/H-abstraction mechanism by ABLM. The calculations showed that (a) is a facile and viable mechanism; it involves acid-base proton reshuffle mediated by the side-chain linkers of BLM, causing thereby heterolytic cleavage of the O-O bond and generation of Cpd I. Formation of Cpd I is found to involve a barrier of 13.3 kcal/mol, which is lower than the barriers in the alternative mechanisms (b and c) that possess respective barriers of 31 and 17 kcal/mol. The so-formed Cpd I species with a radical on the side-chain linker, methylvalerate (V), adjacent to the BLM-FeIV=O complex, resembles the formation of the active species of cytochrome c peroxidase in the Poulos-Kraut proton-shuffle mechanism in heme peroxidases (Poulos, T. L.; Kraut, J. J. Biol. Chem. 1980, 255, 8199-8205). Experimental data are discussed and shown to be in accord with this proposal. It suggests that the high-valence Cpd I species of BLM participates in the DNA cleavage. This is an alternative mechanistic hypothesis to the exclusive reactivity scenario based on ABLM (FeIII-OOH).
TL;DR: In this article, the density functional theory computations have been carried out on the hydrogen adsorption mechanism on a Mo27S54 single layer cluster, which has a size (15-20 A) close to that of real catalysts.
Abstract: Density functional theory computations have been carried on the hydrogen adsorption mechanism on a Mo27S54 single layer cluster, which has a size (15–20 A) close to that of real catalysts. For one molecule of H2 adsorption, the most stable adsorption form (Eads = −27.2 kcal/mol) is the homolytic dissociation on the S edge with the hydrogen atoms keeping away from the plane consisting of all Mo atoms, followed by the heterolytic dissociation (Eads = −26.1 kcal/mol) on the intersection of S and Mo edges with the formation of Moc–H and Sc–H bonds. At high coverage with two and three H2, however, dissociated hydrogen adsorption on the Mo sites are much more favored thermodynamically than on the S sites. Moreover, the corner sites are more favored thermodynamically for hydrogen adsorption and formation of coordinatively unsaturated sites than the edge sites. In addition, the activation energy of H2 dissociation and hydrogen transfer processes have been computed to be 2.7–19.2 kcal/mol, and these rather low barriers indicate the enhanced mobility of the adsorbed hydrogen on the surface.
TL;DR: The results indicate the primary contribution to the energetics of Ti-O bond homolysis in TiCp2ClTEMPO is stabilization of the three-coordinate product by Cp.
Abstract: Ti-TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) provides a means for generating Ti(III) complexes by homolysis of the Ti-O bond. It has been determined that bis-Cp-Ti-TEMPO complexes readily undergo homolytic cleavage while the mono-Cp-Ti-TEMPO complexes do not. Here Ti K- and Cl K-edge XAS are applied to directly determine the oxidation state of TiCl3TEMPO, TiCpCl2TEMPO, and TiCp2ClTEMPO, with reference to Ti(III) and Ti(IV) complexes of known oxidation state. The Ti K-edge data show that Ti(III) complexes exhibit a pre-edge feature approximately 1 eV lower than any of the Ti(IV) complexes; while the Cl K-edges show that Ti(III) complexes have a Cl K- pre-edge feature to approximately 1 eV higher energy than any of the Ti(IV) complexes. Taken together, the Ti and Cl K-edge data indicate that the Ti-TEMPO complexes are best described as Ti(IV)-TEMPO anions (rather than Ti(III)-nitroxyl radicals). In addition, the Cl K-edges indicate that replacement of Cl by Cp weakens the bonding with the remaining ligands, with the Cl 3p covalency decreasing from 25% to 21% to 17% on going from TiCl3TEMPO to TiCpCl2TEMPO to TiCp2ClTEMPO. DFT calculations also show that the electronic structures of the Ti-TEMPO complexes are modulated by the replacement of chloride by Cp. The effect of the Cp on the ancillary ligation is one factor that contributes to facile Ti-O bond homolysis in TiCp2ClTEMPO. However, the results indicate the primary contribution to the energetics of Ti-O bond homolysis in TiCp2ClTEMPO is stabilization of the three-coordinate product by Cp.
TL;DR: Amino-functionalized MCM-41 was used to immobilize Cu/Co-Cl 16 Pc complex, i.e. for liquid phase oxidation of alkanes under mild reaction conditions.
Abstract: Amino-functionalized MCM-41 (NH 2 -MCM-41) was used to immobilize Cu/Co-Cl 16 Pc complex, i.e. Cu/Co-AM(PS) for liquid phase oxidation of alkanes under mild reaction conditions. Higher rates of reaction and better catalytic activity values were obtained for Cu/Co-AM(PS) as compared to Cu/Co-Cl 16 Pc grafted on (i) amino-functionalized SiO 2 [Cu/Co-ASiO 2 ] and (ii) non-functionalized MCM-41 [Cu/Co-M(I)] catalysts along with neat metal complex under identical conditions. The catalysts were evaluated by comparing two different oxidants: (i) TBHP and (ii) O 2 /aldehyde. The rate of conversion and percent selectivity differ for the above two oxidants due to differences in stability of radical species and in their homolytic/heterolytic pathways. The homolytic dissociation of oxygen favors a higher rate of conversion in the case of TBHP, whereas the heterolytic mechanism favors a higher selectivity for cyclohexanone in the case of O 2 /aldehyde. The catalysts were characterized by XRD, MAS NMR, N 2 -adsorption, microanalysis, UV-vis, FTIR and cyclic voltammetry. The UV-vis spectra reveal a blue shift for the metal phthalocyanine-immobilized samples, indicating unimolecular dispersion of metal complex within the channels of MCM-41. Cyclic voltammetry results suggest some coordinative interaction of the amino group of NH 2 -MCM-41 with the metal on grafting with the complex.
TL;DR: In this paper, all reaction pathways were studied by using quantum chemical calculations at the DFT/B3-LYP/TZVP/DFT/BP86/SV(P) level, by comparison of the activation parameters for the radical and the concerted rearrangement mechanisms with the experimentally obtained products.
TL;DR: The use of intramolecular homolytic substitution at the sulfur atom by aryl and vinyl radicals, as an alternative to the use of alkyl halides, and chalcogenides as radical precursors in organic synthesis is reviewed in this paper.
Abstract: The use of intramolecular homolytic substitution at the sulfur atom by aryl and vinyl radicals, as an alternative to the use of alkyl halides, and chalcogenides as radical precursors in organic synthesis is reviewed.
TL;DR: In this paper, theoretical BDEs of a large series of model N-alkoxyamines are calculated with the PM3 method, in order to provide a predictive tool, correlations between the calculated BDE and the cleavage temperature (Tc), and the dissociation rate constant (k(d)), of the n-alkoxideamines are established.
Abstract: During nitroxide-mediated polymerization (NMP) in the presence of a nitroxide R2(R1)NO*, the reversible formation of N-alkoxyamines [P-ON(R1)R2] reduces significantly the concentration of polymer radicals (P*) and their involvement in termination reactions. The control of the livingness and polydispersity of the resulting polymer depends strongly on the magnitude of the bond dissociation energy (BDE) of the C-ON(R1)R2 bond. In this study, theoretical BDEs of a large series of model N-alkoxyamines are calculated with the PM3 method. In order to provide a predictive tool, correlations between the calculated BDEs and the cleavage temperature (Tc), and the dissociation rate constant (k(d)), of the N-alkoxyamines are established. The homolytic cleavage of the N-OC bond is also investigated at the B3P86/6-311++G(d,p)//B3LYP/6-31G(d), level. Furthermore, a natural bond orbital analysis is carried out for some N-alkoxyamines with a O-C-ON(R1)R2 fragment, and the strengthening of their C-ON(R1)R2 bond is interpreted in terms of stabilizing anomeric interactions.
TL;DR: In this article, a solid phase synthesis has been used to carry out intramolecular aromatic homolytic substitution with benzoimidazole precursors, which facilitates easy separation of the cyclised products.
TL;DR: The proposed mechanism describes successfully the effects of H2O2, cyclooctene, and porphyrin concentrations, as well as the effect of alcohol concentration, a process that is favored by alcohols with a high charge density on the O atoms.
Abstract: The epoxidation of cyclooctene catalyzed by iron(III) [tetrakis(pentafluorophenyl)] porphyrin chloride [(F20TPP)FeCl] was investigated in alcohol/acetonitrile solutions in order to determine the effects of the alcohol composition on the reaction kinetics. It was observed that alcohol composition affects both the observed rate of hydrogen peroxide consumption (the limiting reagent) and the selectivity of hydrogen peroxide utilization to form cyclooctene epoxide. The catalytically active species are formed only in alcohol-containing solvents as a consequence of (F(20)TPP)FeCl dissociation into [(F20TPP)Fe(ROH)]+ cations and Cl- anions. The observed reaction kinetics are analyzed in terms of a proposed mechanism for the epoxidation of the olefin and the decomposition of H2O2. The first step in this scheme is the reversible coordination of H2O2 to [(F20TPP)Fe(ROH)]+. The O-O bond of the coordinated H2O2 then undergoes either homolytic or heterolytic cleavage. The rate of homolytic cleavage is found to be independent of alcohol composition, whereas the rate of heterolytic cleavage increases with alcohol acidity. Heterolytic cleavage is envisioned to form iron(IV) pi-radical cations, whereas homolytic cleavage forms iron(IV) hydroxo cations. The iron(IV) radical cations are active for olefin epoxidation, whereas the iron(IV) cations catalyze the decomposition of H2O2. Reaction of iron(IV) pi-radical cations with H2O2 to form iron(IV) hydroxo cations is also included in the mechanism, a process that is favored by alcohols with a high charge density on the O atoms. The proposed mechanism describes successfully the effects of H2O2, cyclooctene, and porphyrin concentrations, as well as the effects of alcohol concentration.
TL;DR: It has been found that metal binding, oxidation, and protonation do not significantly change the values of the phase angle of pseudorotation P, however, the glycosyl torsion angle chi varies considerably as a consequence of a stabilizing guanine-sugar (H8-O4') interaction due to the increase of acidity of Guanine C8-H8 upon cationization.
Abstract: The influence of oxidation, protonation, and metal cationization with Cu(+) and Cu(2+) on the strength of the N-glycosidic bond in 2'-deoxyguanosine has been studied by means of quantum chemical calculations. In all cases, the N9-C1' bond distance increases (0.03-0.06 A) upon introducing positive charge in the guanine moiety, the observed variations being more important for the dicationic systems. Binding energies show that the effect of X(n)(+) in guanine hinders the homolytic dissociation, whereas it largely favors the heterolytic process. With respect to the deoxyribose ring, it has been found that metal binding, oxidation, and protonation do not significantly change the values of the phase angle of pseudorotation P. However, the glycosyl torsion angle chi varies considerably (from 242.0 degrees to 189.8 degrees) as a consequence of a stabilizing guanine-sugar (H8-O4') interaction due to the increase of acidity of guanine C8-H8 upon cationization.
TL;DR: The gas-phase enthalpies of formation for the neutral molecules are in excellent agreement with available experimental data, supporting the estimates made for the radicals, and the latter are very important for the clarification of the thermochemistry of many beta-diketonato metal complexes previously reported in the literature.
Abstract: A computational study on the thermodynamic properties of 13 β-diketones is presented. The B3LYP//6-311+G(2d,2p)//B3LYP/6-31G(d) theoretical approach was employed to compute the O−H and C−H bond dissociation enthalpies and enthalpy of tautomerization and to estimate standard gas-phase enthalpies of formation for the radicals and for the parent molecules. The gas-phase enthalpies of formation for the neutral molecules are in excellent agreement with available experimental data, supporting the estimates made for the radicals. The latter are very important for the clarification of the thermochemistry of many β-diketonato metal complexes previously reported in the literature. Importantly, when substituents R = −CHR‘ are attached to the β-diketone's scaffold, C−H homolytic bond cleavage is always favored with respect to O−H bond scission.
TL;DR: Electron paramagnetic resonance (EPR) spectroscopy of reactive superoxo-vanadium(V) species in vanadosilicate molecular sieves generated on contact with H2O2, tert-butyl hydroperoxide (TBHP), or (H2+O2) is reported for the first time.
Abstract: Electron paramagnetic resonance (EPR) spectroscopy of reactive superoxo−vanadium(V) species in vanadosilicate molecular sieves (microporous VS-1 and mesoporous V-MCM-41) generated on contact with H2O2, tert-butyl hydroperoxide (TBHP), or (H2 + O2) is reported for the first time. By suitable choice of the silicate structure, solvent, and oxidant, we could control the vanadium−(O2-•) bond (i.e., the V−O bond) covalency, the mode of O−O cleavage (in the superoxo species), and, therefore, chemoselectivity in the oxidation of n-hexane: Oxidation by TBHP over V-MCM-41, for example, yielded 27.2% of (n-hexanol + n-hexanal + n-hexanoic acid), among the highest chemoselectivities for oxidation of the terminal −CH3 in a linear paraffin reported to date. Over these vanadosilicates, oxidation of the primary C−H bond occurs only via a homolytic O−O bond cleavage; the secondary C−H bond oxidations may proceed via both the homo- and heterolytic O−O cleavage mechanisms.
TL;DR: In this paper, it has been shown that the value of the homolysis rate constant kd is very sensitive to the stabilization of both released radical species, i.e., persistent nitroxide and persistent ammonoxyl radicals.
Abstract: Alkoxyamines and persistent nitroxide (= aminoxyl) radicals are important regulators of nitroxide-mediated radical polymerization. Since polymerization times decrease with the increasing homolysis rate constant of the CON bond homolysis between the polymer chain and the aminooxy moiety, the factors influencing the cleavage rate constant are of considerable interest. It has already been shown that the value of the homolysis rate constant kd is very sensitive to the stabilization of both released radical species. X-Ray, EPR, and kinetic data showed that the intramolecular H-bonding radical in the 1-(diethoxyphosphoryl)-2,2-dimethylpropyl 2-hydroxy-1,1-dimethylethyl nitroxide (3a) (homologue of 2-hydroxy-1,1-dimethylethyl 1-phenyl-2-methylpropyl nitroxide (2a)) did not occur with the nitroxide moiety as expected but with the phosphoryl group. However, the polymerization rate of styrene (= ethenylbenzene) was significantly enhanced.
TL;DR: In this article, it was shown that the re-formation rate constant increases with the increasing polar character of the substituents attached to the nitroxide moiety, and that the polar/stabilization effects are weaker for the reformation than for the homolysis of the CON bond.
Abstract: In nitroxide-mediated radical polymerization, the polymerization times decrease with the increasing re-formation rate constant of the CON bond ( alkoxyamine) between the growing polymer chain and the nitroxide radical. The factors influencing the re-formation rate constant are of considerable interest, but up to now, the polar/stabilization effects have not been addressed thoroughly. The combination of new data with previously reported data now showed that the re-formation rate constant kc increases with the increasing polar character of the substituents attached to the nitroxide moiety. The polar/stabilization effects are weaker for the re-formation than for the homolysis of the CON bond, and may be mainly attributed to the relocation of the odd electron onto the O-atom of the NO moiety, i.e., the stabilization of the nitroxide moiety. Hence, it is possible to predict the values of kc by combining both the polar/stabilization (σI) and steric effects (E), i.e., log(kc/M−1 s−1) = 9.86 + 0.57 ⋅ σI + 0.40 ⋅ Es.
TL;DR: A convergent strategy has allowed access to bridgehead sultam 9 and the related carboxamides 10 and 11 and the unique ability of the sulfonamide linkage to excited-state homolysis holds comparative interest.
Abstract: A convergent strategy has allowed access to bridgehead sultam 9 and the related carboxamides 10 and 11. The synthetic routing proceeds via the coupling of a suitably constructed dienamine to either o-iodobenzenesulfonyl choride or o-iodobenzoyl chloride to generate the amides. The application in sequence of ring-closing metathesis and an intramolecular Heck reaction gave rise to advanced tricyclic intermediates. The final two steps involved bromination in liquid bromine and proper 2-fold dehydrobromination. The latter maneuver was best achieved with tetrabutylammonium fluoride in DMSO at elevated temperature. While the irradiation of 9 led principally via SO2−N bond homolysis and [1,5] sigmatropic rearrangement to generate 37, 10 proceeded via disrotatory cyclization to the exo cyclobutene 39, and 11 resisted photoisomerization. The inertness of 11 may stem from its distorted structural features which force its conjugated diene double bonds to be rigidly oriented 32° out-of-plane. The unique ability of th...
TL;DR: The B3LYP results show that the primary and the secondary radicals centered on C(4), generated by homolytic break of the C(3)-C(4) bond and by 1,5 hydrogen shift, respectively, are more stable than radicalscentered on oxygen.
TL;DR: In this article, a number of spectroscopic methods were applied to obtain kinetic parameters of reactions modelling the 2,5-dihydro-1H-imidazole 1-oxide mediated living polymerization of acrylates.
Abstract: A number of spectroscopic methods were applied to obtain kinetic parameters of reactions modelling the 2,5-dihydro-1H-imidazole 1-oxide mediated living polymerization of acrylates. The homolysis rate constants of alkoxyamines based on five nitroxides were measured by EPR spectroscopy at different temperatures. The recombination rate constants kc between the corresponding alkyl radicals and the nitroxides were measured by means of laser flash photolysis. The time-resolved chemically induced dynamic nuclear polarization (TR-CIDNP) experiments revealed the negligible contribution of disproportionation in the recombination reaction. In addition, the thermodecomposition of alkoxyamines in the NMR probe showed the absence of intramolecular elimination of hydroxylamines from the corresponding alkoxyamines. Analysis of the kinetic parameters showed that the 2,5-dihydro-1H-imidazole 1-oxide type radicals are promising mediators for the living polymerization of acrylates and methacrylates.