Homolytic Dissociation Research Articles

Alkane als initiatoren zur radikalischen polymerization

AbstractAlkanes with highly substituted carbon‐carbon bonds can be cleaved in free radicals by homolytic dissociation under the influence of thermal energy. The energy of dissociation is mainly determined by the type and size of the respective substituents. For tetraarylbutane dinitriles the temperatures of decomposition are in the range of other technically used initiators. But nevertheless, these compounds are up to now rather seldom used to initiate free radical polymerizations. This is connected with the special mechanism of initiation: the formed highly substituted alkyl radicals are relatively stable and show only a rather low reactivity against the usually used monomers. Therefore, in the beginning of the polymerization the radical concentration is rather high which leads not only to addition to monomers but also to a pronounced primary radical termination. In some cases this termination process is reversible which results in an unusual polymerization kinetics. Depending on the type of monomer and the degree of reversibility of the primary radical termination, either a period of “dead‐end” polymerization follows or the cleavage of the oligomers at the chain end results in a re‐initiation process with a “normal” polymerization with increased rate. The reaction mechanism, kinetics and some possible applications of such initiating alkanes are reported.

Demonstration of the synthetic utility of the generation of alkoxy radicals by the photo-induced homolytic dissociation of alkyl 4-nitrobenzenesulfenates

The synthetic utility of the generation and study of the reactions of alkoxy radicals generated by the photo-induced, homolytic dissociation of alkyl 4-nitrobenzenesulfenates is demonstrated. The efficient formation of remotely substituted alcohol derivatives, and the addition of the alkoxy radical to a remotely situated CC is illustrated.

Homolytic bond-dissociation in peroxides, peroxyacids, peroxyesters and related radicals: ab-initio MO calculations.

Homolytic dissociation energies for different cleavage paths in a number of peroxides peroxyacids and peroxyesters and in the radicals formed from these molecules, were calculated from the total MO ab-initio molecular energies of the chemical species involved in the bond cleavage reactions and compared with experimental values, where known.

Thermochemical properties and homolytic bond cleavage of organic peroxyacids and peroxyesters: an empirical approach based on ab initio MO Calculations

Heats of formation Δ H o fof a number of peroxyacids RC(O)OOH, peroxyesters RC(O)OOR' and of the radicals RC(O), RC(O)O, RC(O)OO and C(O)OOR', related to possible routes of their thermal decomposition, were estimated by employing empirical approaches. R and R' are a wide set of alkyl and aryl groups. The approaches were based on isodesmic reactions, atom equivalent, group additivity and molecular mechanics (MM2) calculations. Isodesmic reactions and atom equivalent schemes were employed with HF/6-31G ∗ ab initio total molecular energies. The MM2 calculations were applied to peroxyacids and peroxyesters and the values of Δ H o fobtained were corrected by a constant quantity because they appeared systematically in excess with respect to those obtained by the other approaches. In atom equivalent and group additivity methods, new parameters were evaluated in order to be able to calculate the heats of formation of these molecules. Values of Δ Hdg fobtained from the different approaches turn out to be consistent within ±2 kcal mol −1. For the RC(O) radicals with R = H, CH 3 C 6H 5 and CF 3, a comparison with values reported in the literature is possible and agreement can be considered satisfactory. For the RC(O)O radicals with R = H, CH 3, C 6H 5, the Δ H o f values estimated in this work from isodesmic reactions and atom equivalent approaches are 7–10 kcal mor −1 higher than those reported in the literature. This difference appears to be due to improper evaluation of the total molecular energy with HF/6-31G ∗ ab initio calculations for this radical, owing to symmetry breaking in SCF methods. With the heats of formation calculated for peroxyacids, peroxyesters and for the radicals structurally related to these molecules, homolytic dissociation energies for the various bonds were derived and discussed as a function of the structural features of the R and R' groups. Possible routes for the thermal decomposition of the peroxyesters were examined.

Chemisorption of H2and H2—O2on polymorphic zirconia

Powders of polymorphic zirconia contain crystallites incorporating monoclinic and tetragonal domains. Thermal activation in high vacuum results in a significant amount of highly reactive radical surface centres (O–, Zr3+) mainly located in tetragonal surface sites. Their exposure to molecular H2 leads uniquely to homolytic dissociation processes clearly evidenced by IR spectroscopy. These processes give rise to an irreversible formation of multiple-coordinated surface OH groups on tetragonal surface sites (3668 cm–1) and to a reversible formation of simple (ZrH; 1565 cm–1) and bridged hydrides (Zr2H; 1360 cm–1). Simple hydrides are very reactive towards any kind of oxygen species (O2, O). Their reaction with physisorbed O2 leads to irreversible formation of multiple-coordinated OH groups and O2.– radical anions [ν(O—O).–; 1118 cm–1] evidenced by 18O2 isotopic experiments. The appearance of these OH groups necessarily implies surface mobility of intermediately existing H atoms on both monoclinic and tetragonal surface domains.

Preparation and ESR‐Spectroscopical Investigation of Remarkably Persistent Oxoisobenzofuranyl Radicals

AbstractThe synthesis of the diastereoisomeric 1,1′‐diaryl‐1,1′‐bi(isobenzofuran)‐3,3′(1H,1′H)‐diones 3a–d starting from the readily available 2‐aroylbenzoic acids 1a–d is described (Scheme 1). Of the colourless dimers 3a–d, only the sterically congested 3a and 3b dissociate at ambient temperature in solution to the deep red free 3‐oxoisobenzofuran‐1‐yl radicals 4a and 4b, respectively. The radicals 4a, b are extremely persistent in the absence of O2. The structures of these radicals are confirmed and the coupling constants assigned by ESR and ENDOR spectroscopy and computer simulation of their ESR spectra. The dissociation equilibrium constant at 20° in toluene for 3a is determined to be 1.18 · 10−5 M. By studying the steady‐state radical concentration as a function of temperature, the enthalpy and entropy changes for the homolytic dissociation of 3a are determined.

Self-induced, photochemical, singlet-oxygen oxidation of 4-nitrobenzenesulfenates to 4-nitrobenzenesulfinates

It has been previously reported that the photo-induced homolytic dissociation of homoallyl and alkyl 4-nitrobenzenesulfenates, followed by β-scission of the alkoxy radicals to allyl and alkyl radicals, leads to the formation of sulfides. In the presence of dioxygen the alkyl sulfenates undergo oxidation to the corresponding alkyl sulfinates. The results of theoretical studies suggest that the absorption of the photon results in the excitation of an electron from the π-type sulfur nonbonded-pair MO (the HOMO) to a π * MO of the aromatic ring (the LUMO). Quenching studies indicate that a triplet excited state is ultimately formed which then reacts with triplet dioxygen to form singlet dioxygen, which can be trapped with 1,3-cyclohexadiene, which then reacts with ground-state sulfenate

Study on Coenzyme B_(12) Model Compounds by Electrochemistry and Quantum Chemistry

It is now widely recognized that coenzyme B_(12)-dependent rearrangements are caused by homolytic dissociation of the Co-C bond giving rise to a 5'-deoxyadenosyl radical. The Co-C bonding strength is related to the electronic and steric effect of the 5'-deoxyadenosyl radical and the axial organic base, playing a key role in the catalytic cycle. To clarify the mechanism of Co-C bond breaking in coenzyme B_(12), many experimental and theoretic researches on B_(12) model compounds have been conducted, especially on the effect

Conformational properties and homolytic bond cleavage of organic peroxides. I. An empirical approach based upon molecular mechanics and ab initio calculations

AbstractThe conformational features of a large number of hydroperoxides ROOH and peroxides ROOR′, where R and R′ are alkyl groups of different and increasing size and phenyl rings, including ortho substituted derivatives, were obtained from molecular mechanics calculations by employing a standard package. For the molecules of small molecular size, comparison was carried out with the results of ab initio calculations. Heats of formation were also obtained from molecular mechanics for hydroperoxides and peroxides: The values are, in general, overestimated. For the molecules containing the CF3 group, the calculated values are subject to large errors and heats of formation were obtained from ab initio total energies in the “atom equivalents” scheme. To estimate the homolytic dissociation energies of the different bonds in the peroxide molecules, heats of formation of R·, ·OR, and ·OOR radicals were employed and several of them had to be calculated. Different approaches were employed—molecular mechanics calculations, ab initio energies within the atom equivalent and isodesmic reaction schemes, and Benson's group additivity rule; values consistent within the different calculation methods were chosen for estimating dissociation energies. The bond dissociation energies indicate different trends in these molecules as a function of the nature of the R and R′ groups and the possible electronic effects operating in these molecules are discussed. © 1993 John Wiley & Sons, Inc.

Oxidatively induced nucleophilic capture vs. degradation of cyclopentadienyl iron derivatives of simple carboxylic acids and of α-amino acids. A comparative study

The electrochemical behaviour and the chemical oxidation of various cyclopentadienyl iron complexes of simple carboxylic acids and of amino-protected α-amino acids have been studied in relation to several factors including the nature of the ligands on the metal (Cp, Cp★, CO, PPh 3), the nature of the oxidizing agent (one-electron oxidants such as Cu(OTf) 2, FcOTf and CAN or a two-electron oxidant such as NBS) and the medium effects (especially the presence or absence of nucleophilic species). With simple carboxylic acid derivatives either homolytic dissociation (leading to alkyl radicals) or nucleophilic capture of the first-formed iron radical cations is observed, depending on the reaction conditions. With α-amino acid derivatives, an oxidative degradation to aldehyde is observed invariably, which is likely to proceed through the successive and transient formation of N-acyl α-amino radicals and N-acyl iminium ions.

ESR STUDY OF THE STABLE PHOSPHORANYL-IDENEAMINO(DICYANO)METHYL RADICALS, RESULTING FROM REVERSIBLE THERMAL HOMOLYSIS OF 1,2-BIS(PHOSPHORANYLIDENE-AMINO)TETRACYANOETHANES

Abstract Thermally-induced equilibrium homolytic dissociation of 1,2-bis(phosphoranylideneamino)tetracyanoethanes, resulting in the formation of the stable phosphoranylideneamino(dicyano)methyl radicals has been studied by ESR spectroscopy. Enthalpies and entropies of the dissociation have been estimated. Magnetic resonance parameters, found from ESR spectra, are characteristic of π-radicals. Temperature-dependent phosphorus hyperfine splittings have been studied.

Charge oscillation in the homolysis of MeX derivatives

Ab initio electronic structure methods are used to show that, in the (barrierless) homolytic dissociation of the CH 3 X system (X=an atom or group more electronegative than CH 3 ) going from equilibrium geometry to products (CH 3 . +X . ), the charge on CH 3 first increases and then decreases to zero. This result, which can be rationalized on the basis of simple models, violates the common assumption that the character of intermediates along a reaction path is bound by the properties of reactants and products

Homolytic dissociation energies from GVB-LSDC calculations

GVB-LSDC provides a reasonable account of correlation effects needed for the calculation of bond dissociation energies. 27 homolytic dissociation reactions have been calculated with GVB-LSDC/6-31G(d). GVB-LSDC bond dissociation energies are better than both HF, HF-LSDC, and GVB results. The average error is 4 kcal/mol.

An investigation of the homolytic dissociation of [.eta.5-C5Me5Cr(CO)3]2 and related complexes. The role of ligand substitution on the solution thermochemistry of metal-metal bond cleavage

An investigation of the homolytic dissociation of [.eta.5-C5Me5Cr(CO)3]2 and related complexes. The role of ligand substitution on the solution thermochemistry of metal-metal bond cleavage

Comparison of the cleavage reactions of αnitrocumene radical anion and α,p–dinitrocumene radical anion

AbstractThe rates of loss of NO2− from the radical anions of 2‐(4‐nitrophenyl)‐2‐nitropropane (α,p‐dinitrocumene) and 2‐phenyl‐2‐nitropropane (α‐nitrocumene) were compared by three different methods. The rate constant for the apparently heterolytic process (DN) undergone by the radical anion of the dinitrocumene was measured by pulse radiolysis to be 2 × 103 s−1 in dimethylformide at 25°C. This value is in reasonable agreement with values estimated from an indirect chemical method and from cyclic voltammetry at low temperature. The rate for the formally homolytic dissociation process (DR) undergone by the radical anion of the mononitrocumene could not be determined by pulse radiolysis but could be shown to be between 50 and 1000 times faster than the dinitro case using the other two methods. Possible reasons for the faster rate for the DR process are discussed with reference to related systems for which the DR process had been found to be unexpectedly slow.

Homolytic dissociation of the carbon-element ? bonds by Grignard reagents

The reaction of 2-ethyl-3-tetrahydrofuryl thiocyanate (I) and triphenylacetonitrile (VIII) with methylmagnesium iodide in ether was studied using GC/MS, PMR and13C NMR spectroscopy. The reaction of (I) with CH3MgI gives the product of the homolysis of the C-S σ bond, namely, bis(2-ethyl-3-tetrahydrofuryl) disulfide (II), in 81% yield. This reaction is assumed to be general in nature since triphenylacetonitrile (VIII) is also homolytically cleaved by the action of CH3MgI to give triphenylmethane.

Allyltris(trimethylsilyl)silane as Radical Allylating Agent for Organic Halides

Abstract Reaction of allyltris(trimethylsilyl)silane with organic halides under the free radical conditions resulted in allylation of the halides in modrate to good yields. The presence of three trimethylsilyl groups on the silicon could enhance the homolytic dissociation of a Si–C bond in the β-silylalkyl radical.

Metal-assisted reactions. Part 22. Synthesis of perhalogenated prophyrins and their use as oxidation catalysts

Meso-tetrakisarylporphyrins have been perchlorinated and perbrominated at peripheral (β)-positions of the pyrrole rings by high-yielding procedures. These halogenated porphyrins show enhanced catalytic activity towards oxidation of alkanes and alkenes but are not stable when hydrogen peroxide is used as the oxygen donor. The reason for this instability appears to lie in excessive homolytic dissociation of H 2 O 2 with consequent rapid attack at the porphyrin β-position. Improved methods for chlorinating or brominating the β-positions in meso-substituted porphyrins are desdribed. The reactivity and stability of such porphyrins under oxidising conditions are discussed.

Oxidatively induced radical chemistry of dicarbonyl η5-cyclopentadienyl iron derivatives of carboxylic acids (Fp–acyl complexes)

The 17-electron cationic [Fp–acyl] intermediates generated by cerium ammonium nitrate (CAN) oxidation of Fp–acyl complexes in dry acetonitrile undergo a fast homolytic dissociation leading to alkyl radical species whose subsequent fate is dependent on the structural features and reaction conditions.

MNDO MO theoretical study of electronic structure and homolytic dissociation of perfluoroalkanoyl peroxides

The MNDO molecular orbital method was applied to alkanoyl peroxides and the effect of fluorination on the electronic structure and the rate of the homolytic 0-0 cleavage of alkanoyl peroxides were analyzed. The fluorine atom introduced at the α carbon atom makes the peroxy 0-0 bond long and the dihedral angle between two COO planes large. The analysis of two-atom energy and its components energies has shown that the rate of the 0-0 cleavage correlates well with the destabilization of the resonance energy of the 0-0 bond and not with the electrostatic energy. The effect of elongation of the perfluoroalkyl group is also well elucidated by the two-atom energy. The perfluoroalkyl group lowers the σ*(0-0) anti-bonding orbital considerably, and this fully explains the observed rate of induced decomposition of perfluoroalkanoyl peroxides with benzene.