Radical Substitution Mechanism Research Articles

A new radical initiation for the ring-opening polymerization of tetrahydrofuran with a radical substitution mechanism

A new radical-Lewis acid system has been discovered to achieve the ring-opening polymerization of tetrahydrofuran. Different types of reaction mechanisms were discovered for the initiation and polymerization processes. The proposed initiation mechanism was studied by MALDI-TOF and ESI-MS as a radical substitution mechanism and polymerization was a coordination-insertion mechanism. And polytetrahydrofuran with high molecular weight (Mn = 14.6 × 104 g mol−1) and narrow molecular weight distribution (PDI = 1.44) could be obtained.

Gas-Phase Preparation of Silyl Cyanide (SiH3CN) via a Radical Substitution Mechanism.

The silyl cyanide (SiH3CN) molecule, the simplest representative of a fully saturated silacyanide, was prepared in the gas phase under single-collision conditions via a radical substitution mechanism. The chemical dynamics were direct and revealed a pronounced backward scattering as a consequence of a transition state with a pentacoordinated silicon atom and almost colinear geometry of the attacking cyano radical and leaving hydrogen. Compared to the isovalent cyano (CN)-methane (CH4) system, the CN-SiH4 system dramatically reduces the energy of the transition state to silyl cyanide by nearly 100 kJ mol-1, which reveals a profound effect on the chemical bonding and reaction mechanism. In extreme high-temperature environments including circumstellar envelopes of IRC +10216, this versatile radical substitution mechanism may synthesize organosilicon molecules via reactions of silane with doublet radicals. Overall, this study provides rare insights into the exotic reaction mechanisms of main-group XIV elements in extreme environments and affords deeper insights into fundamental molecular mass growth processes involving silicon in our universe.

A Radical Exploration of the Cobalamin-Dependent Radical SAM Enzyme CysS

The cobalamin-dependent radical SAM enzyme CysS performs methylation reactions at an unactivated carbon center using what is predicted to be an unprecedented radical substitution mechanism. Herein, we highlight significant progress made by Wang and Begley in providing support for the enzymatic relevance of this proposal.

Bimolecular Reaction Dynamics in the Phenyl-Silane System: Exploring the Prototype of a Radical Substitution Mechanism.

We present a combined experimental and theoretical investigation of the bimolecular gas-phase reaction of the phenyl radical (C6H5) with silane (SiH4) under single collision conditions to investigate the chemical dynamics of forming phenylsilane (C6H5SiH3) via a bimolecular radical substitution mechanism at a tetracoordinated silicon atom. Verified by electronic structure and quasiclassical trajectory calculations, the replacement of a single carbon atom in methane by silicon lowers the barrier to substitution, thus defying conventional wisdom that tetracoordinated hydrides undergo preferentially hydrogen abstraction. This reaction mechanism provides fundamental insights into the hitherto unexplored gas-phase chemical dynamics of radical substitution reactions of mononuclear main group hydrides under single collision conditions and highlights the distinct reactivity of silicon compared to its isovalent carbon. This mechanism might be also involved in the synthesis of cyanosilane (SiH3CN) and methylsilane (CH3SiH3) probed in the circumstellar envelope of the carbon star IRC+10216.

Influence of the nature of activated hydrogen isotope species on the isotope exchange efficiency, with preparation of labeled sodium 4-phenylbenzoate as example

The influence of the nature of activated hydrogen isotope species on the isotope exchange efficiency was studied with preparation of labeled sodium 4-phenylbenzoate as example. The effect of various factors on the deuterium labeling of this compound was examined. At temperatures lower than 200°C, deuterium is mainly incorporated into the phenyl moiety, i.e., under these conditions activated hydrogen species are incorporated by the electrophilic mechanism. In the range from 260 to 300°C, the mean number of deuterium atoms incorporated into 4-phenylbenzoic acid molecules becomes approximately constant (about 8.22 deuterium atoms per molecule). At these temperatures, deuterium is efficiently incorporated both into the phenyl fragment and into the benzoic acid residue, which suggests the prevalence of the radical substitution mechanism under these conditions. At temperatures at which the isotope substitution in sodium 4-phenylbenzoate occurs by the electrophilic mechanism, 4-cyclohexylbenzoic acid is formed concurrently, i.e., the maximal yield of 4-cyclohexylbenzoic acid can be reached at temperatures that are most favorable for the isotope exchange by the electrophilic mechanism. At 200°C, the content of 4-cyclohexylbenzoic acid in the reaction mixture reaches a maximum. A sharp increase in the contribution of the radical mechanism of the process at higher temperatures led to a decrease in the yield of 4-cyclohexylbenzoic acid. It was assumed that clusters of activated hydrogen species and electrons, solvated on the support surface, undergo rearrangement with increasing temperature. Whereas the major role in labeling by the electrophilic mechanism is played by hydrogen isotope cations, at higher temperatures hydrogen isotope cations interact with electrons to form hydrogen atoms, which become active participants of the exchange process.

Mechanism of the oxidation of ferrocene and its derivatives with hydrogen peroxide: A new effect

A new effect was recorded during the studies of the mechanism of ferrocene oxidation with hydrogen peroxide, namely, a shift of λmax of the absorption band of the ferricinium cation (ABFC) toward the long-wave region and its broadening during the reaction. The shiftΔλmax increased with the excess of the H2O2 concentration with respect to the ferrocene concentration and reached 90 nm and more at H2O2/Fc ≈ 80–100. Similar changes in ABFC took place during the oxidation of a series of ferrocene derivatives. A shift of ABFC was not revealed at comparable concentrations of the metal complex and H2O2. During the oxidation of ferrocene with other peroxides (t-C4H9OOH or (PhCOO)2), there were no changes in the spectrum of the ferricinium cation at any ratios of reagent concentrations. The observed effect is based on the formation of a {ferricinium cation + .OH} radical pair during the primary interaction of the metallocomplex with H2O2 and subsequent reaction between the radicals of the radical pair according to the radical substitution mechanism, which leads to the formation of the hydroxy derivatives of ferrocene and their cations. Sequential accumulation of OH substituents in the metallocomplex and the corresponding ferricinium cations caused a continuous shift of ABFC toward the long-wave region.

Methyl-Coenzyme M Reductase from Methanogenic Archaea: Isotope Effects on the Formation and Anaerobic Oxidation of Methane

The nickel enzyme methyl-coenzyme M reductase (MCR) catalyzes two important transformations in the global carbon cycle: methane formation and its reverse, the anaerobic oxidation of methane. MCR uses the methyl thioether methyl-coenzyme M (CH3-S-CH2CH2-SO3(-), Me-S-CoM) and the thiol coenzyme B (CoB-SH) as substrates and converts them reversibly to methane and the corresponding heterodisulfide (CoB-S-S-CoM). The catalytic mechanism is still unknown. Here, we present isotope effects for this reaction in both directions, catalyzed by the enzyme isolated from Methanothermobacter marburgensis . For methane formation, a carbon isotope effect ((12)CH3-S-CoM/(13)CH3-S-CoM) of 1.04 ± 0.01 was measured, showing that breaking of the C-S bond in the substrate Me-S-CoM is the rate-limiting step. A secondary isotope effect of 1.19 ± 0.01 per D in the methyl group of CD3-S-CoM indicates a geometric change of the methyl group from tetrahedral to trigonal planar upon going to the transition state of the rate-limiting step. This finding is consistent with an almost free methyl radical in the highest transition state. Methane activation proceeds with a primary isotope effect of 2.44 ± 0.22 for the C-H vs C-D bond breakage and a secondary isotope effect corresponding to 1.17 ± 0.05 per D. These values are consistent with isotope effects reported for oxidative cleavage/reductive coupling occurring at transition metal centers during C-H activation but are also in the range expected for the radical substitution mechanism proposed by Siegbahn et al. The isotope effects presented here constitute boundary conditions for any suggested or calculated mechanism.

Direct synthesis, characterization of Cu-SBA-15 and its high catalytic activity in hydroxylation of phenol by H 2O 2

Transition metal copper substituted mesoporous silica (Cu-SBA-15) was synthesized using triblock copolymers surfactant as template agent under acidic condition. The result Cu-SBA-15 was characterized with XRD, ICP-AES, FT-IR and N 2 adsorption–desorption measurements, which prove that Cu(II) was mainly incorporated into the framework of Cu-SBA-15. Its catalytic activity was studied for phenol hydroxylation using H 2O 2 (30%). The substituting element (Cu 2+) is incorporated into the framework position forming a new type of active site which raises the phenol conversion to 62.4% and the diphenol (the mixture of catechol (CAT) and hydroquinone (HQ)) selectivity to 97%. The Cu-SBA-15 has very high selectivity for catechol (about 71% selectivity), which is completely different from that of the microporous titanium silicalite zeolites (47.1% phenol conversion and about 50% selectivity to CAT under same reaction conditions). The results obtained indicate that the selective oxidation of phenol with H 2O 2 by a radical substitution mechanism.

Selective hydroxylation of phenol employing Cu–MCM-41 catalysts

We studied the oxidation reaction of phenol in aqueous and acetonitrile media under mild conditions, employing Cu-modified MCM-41 mesoporous catalysts. The stability of the catalysts under reaction conditions was confirmed by XRD, UV–VIS and FTIR techniques. Results obtained indicate that the selective oxidation of phenol with H2O2 by a radical substitution mechanism produces three main reaction products: catechol, hydroqinone and benzoquinone.

A novel catalyst for clean production of diphenols—ferric trisacetylacetonate/MCM-41

Ferric trisacetylacetonate has been deposited within the zeolite MCM-41 and the product characterized by XRD and IR. In water at pH 7 it catalyzes the oxidation of phenol by H2O2, giving 58% conversion in 1 h at 50 degrees C: products are catechol (66%), hydroquinone (27%) and benzoquinone (7%). Other oxidants and solvents are much less effective. UV-VIS spectra suggest a radical substitution mechanism, and a pollution-free process for phenol hydroxylation is now possible.

Iron(II)-8-quinolinol/MCM-41-Catalyzed Phenol Hydroxylation and Reaction Mechanism

Iron(II)-8-quinolinol/MCM-41 is prepared. Its catalysis is studied in phenol hydroxylation using H2O2(30%) as oxidant. The experiment shows that Iron(II)-8-quinolinol/MCM-41 has good catalytic activity and desired stability. Based on cyclic voltammetry, ESR, and UV–visible spectra studies of iron(II)-8-quinolinol complex in liquid phase, a radical substitution mechanism is proposed and used to demonstrate the experimental facts clearly.

Y‐Ba‐Cu‐O mixed oxides for production of diphenols in liquid‐solid phase

AbstractSuperconductor mixed oxides were often used as catalysts at higher temperature in gas phase oxidations, and considered not suitable for lower temperature reactions in the liquid‐solid phase; here the catalysis of YBa2Cu3O7±x and Y2BaCuO5±x, in the phenol hydroxylation at lower temperature with H2O2 as oxygen donor was studied, and found that the superconductor YBa2Cu3O7±x has no catalytic activity for phenol hydroxylation, but Y2BaCuO5±x does, even has better catalytic activity and stability than most previously reported ones. With the studies of catalysis of other simple metal oxides and perovskite‐like mixed oxides, a radical substitution mechanism is proposed and the experimental facts are explained clearly, and draw a conclusion that the perovskite‐like mixed oxides with (AO)(ABO3) and (AO)2(ABO3) structure have better catalytic activity than the simple perovskite oxides with (ABO3)3 structure alone, and (AO) structure unit is the key for the mixed oxides to have the phenol hydroxylation activity. No pollution of this process is very important for its further industrial application.

Phenol hydroxylation by iron(II) phenanthroline: The reaction mechanism

Phenol hydroxylation catalyzed by iron(II)-1,10-phenanthroline is investigated through kinetics, ESR, UV-VIS as well as cyclic voltammogram studies. The optimum reaction conditions are obtained for diphenols production. Radical substitution mechanism is first proposed to explain the effects of pH, reaction medium and other factors on the phenol hydroxylation with H 2O 2 as oxidant, and found that the coexisting of iron(II)-1,10-phenanthroline and iron(III)-1,10-phenanthroline is the key for phenol hydroxylation to occur with H 2O 2 as oxygen donor.