Free-radical abstraction reactions with concerted fragmentation and NO formation

The enthalpy and activation energy of reactions involving attack by MeO₂^? and MeO₂^? on CH₂ groups of 2-butyl nitrite and 2-nitrosobutane have been calculated by quantum chemical methods. The abstraction of a hydrogen atom is accompanied, in the former case, by concerted N–O bond breaking and, in the latter case, by concerted C–N bond breaking, resulting in NO? formation. On the basis of the results obtained, an algorithm has been developed within the intersecting parabolas model for calculating the enthalpies, activation energies, and rate constants of these types of reactions involving alkyl, alkoxyl, aminyl, peroxyl, phenoxyl, thiyl, and hydroxyl radicals.     

The liquid-phase free radical isomerization of cyclic dimethylformamide acetals

In the presence of tert-butyl peroxide, 2-dimethylamino-1,3-dioxacyclanes are converted to esters of dimethylcarbamic acid. The reaction is described by a kinetic equation for an unbranched chain reaction with quadratic chain termination. The five-membered heterocycle is more reactive than the six-membered heterocycle. The predominant site for free radical attack is the methine group adjacent to the three heteroatoms.     

Novel concerted fragmentation upon alcoholysis of a urazole

[reaction: see text] Instead of yielding the expected hydrazine, alcoholysis of the above heterocycle results in fragmentation via a highly unusual pathway.     

Selection rule in free radical reactions

The insertion and abstraction reactions are investigated on the basis of the Heisenberg model. These reactions are characterized as spin-symmetry forbidden and allowed reactions, respectively. It is pointed out that free radical reactions occur readily when there is no spin degeneracy in the course of these reactions. The rule obtained is discussed in relation to the orbital symmetry conservation rule for concerted reactions.     

A novel polyurethane macroinitiator for free radical polymerization

Polyurethane macroinitiator (PUMI) has been prepared from toluene diisocyanate and benzopinacol and successfully used to polymerize acrylonitrile (AN). The effects of PUMI and AN concentrations and polymerization time on the yield, number average molecular weight and intrinsic viscosity of the resulting Polyacrylonitrile have been studied.     

The CNO free radical and its isomerization in inert-gas matrices

The CNO free radical is generated in solid Ne by in situ photolysis of fulminic acid, HCNO. Its electronic spectra and photoisomerization to NCO are examined.     

Tin-free radical chemistry using the persistent radical effect: alkoxyamine isomerization, addition reactions and polymerizations

In this tutorial review applications of alkoxyamines as C-radical precursors for the conduction of tin-free radical reactions are presented. These processes are controlled by the Persistent Radical Effect. A brief introduction on the Persistent Radical Effect is provided. In addition, the use of microwave irradiation to conduct thermal radical reactions is discussed. Finally, the use of alkoxyamines as initiators/mediators for the controlled/living radical polymerization is highlighted.     

Oxidative free radical reactions of enamino esters

Oxidative free radical reactions of enamino esters are described. Electrophilic carbon-centered radicals produced by the cerium(IV) ammonium nitrate (CAN) oxidation of β-dicarbonyl compounds undergo efficient addition to the C-C double bond of enamino esters. This CAN mediated free radical reaction between enamino esters and β-dicarbonyl compounds provides a novel method for the synthesis of highly substituted pyrroles. The direct CAN oxidation of β-enaminocinnamates gave the dimerization products effectively.     

A recipe for the computation of the free energy barrier and the lowest free energy path of concerted reactions

The recently introduced hills method (Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12562) is a powerful tool to compute the multidimensional free energy surface of intrinsically concerted reactions. We have extended this method by focusing our attention on localizing the lowest free energy path that connects the stable reactant and product states. This path represents the most probable reaction mechanism, similar to the zero temperature intrinsic reaction coordinate, but also includes finite temperature effects. The transformation of the multidimensional problem to a one-dimensional reaction coordinate allows for accurate convergence of the free energy profile along the lowest free energy path using standard free energy methods. Here we apply the hills method, our lowest free energy path search algorithm, and umbrella sampling to the prototype S(N)2 reaction. The hills method replaces the in many cases difficult problem of finding a good reaction coordinate with choosing relatively simple collective variables, such as the bond lengths of the broken and formed chemical bonds. The second part of the paper presents a guide to using the hills method, in which we test and fine-tune the method for optimal accuracy and efficiency using the umbrella sampling results as a reference.     

Hypochlorite-mediated fragmentation of hyaluronan, chondroitin sulfates, and related N-acetyl glycosamines: evidence for chloramide intermediates, free radical transfer reactions, and site-specific fragmentation

Myeloperoxidase released from activated phagocytes reacts with H(2)O(2) in the presence of chloride ions to give hypochlorous acid. This oxidant has been implicated in the fragmentation of glycosaminoglycans, such as hyaluronan and chondroitin sulfates. In this study it is shown that reaction of HOCl with glycosaminoglycans and model compounds yields chloramides derived from the N-acetyl function of the glycosamine rings. The results of EPR spin trapping and product studies are consistent with the formation of amidyl radicals from these chloramides via both metal ion-dependent and -independent processes. In the case of glycosaminoglycan-derived amidyl radicals, evidence has been obtained in studies with model glycosides that these radicals undergo rapid intramolecular abstraction reactions to give carbon-centered radicals at C-2 on the N-acetyl glycosamine rings (via a 1,2-hydrogen atom shift) and at C-4 on the neighboring uronic acid residues (via 1,5-hydrogen atom shifts). The C-4 carbon-centered radicals, and analogous species derived from model glycosides, undergo pH-independent beta-scission reactions that result in glycosidic bond cleavage. With N-acetyl glucosamine C-1 alkyl glycosides, product formation via this mechanism is near quantitative with respect to chloramide loss. Analogous reactions with the glycosaminoglycans result in selective fragmentation at disaccharide intervals, as evidenced by the formation of "ladders" on gels; this selectivity is less marked under atmospheric oxygen concentrations than under anoxic conditions, due to competing peroxyl radical reactions. As the extracellular matrix plays a key role in mediating cell adhesion, growth, activation, and signaling, such HOCl-mediated glycosaminoglycan fragmentation may play a key role in disease progression and resolution, with the resulting fragments modulating the magnitude and quality of the immune response in inflammatory conditions.     

OH radical reactions with phenylalanine in free and peptide forms

Density functional theory has been used to model the reaction of OH with l-phenylalanine, as a free molecule and in the Gly-Phe-Gly peptide. The influence of the environment has been investigated using water and benzene as models for polar and non-polar surroundings, in addition to gas phase calculations. Different paths of reaction have been considered, involving H abstractions and addition reactions, with global contributions to the overall reaction around 10% and 90% respectively when Phe is in its free form. The ortho-adducts (o-tyrosine) were found to be the major products of the Phe+OH reaction, for all the modeled environments and especially in water solutions. The reactivity of phenylalanine towards OH radical attacks is predicted to be higher in its peptidic form, compared to the free molecule. The peptidic environment also changes the sites' reactivity, and for the Gly-Phe-Gly+OH reaction H abstraction becomes the major path of reaction. The good agreement found between the calculated and the available experimental data supports the methodology used in this work, as well as the data reported here for the first time.     

An expedient synthesis of murrayaquinone A via a novel oxidative free radical reaction

Murrayaquinones A–D is a group of four bioactive carbazole-1,4-dione natural products isolated from the root bark of the plant Murraya eucrestifolia hayata. Murrayaquinone is synthesized in five steps starting from the commercially available 2,4,5-trimethoxybenzaldehyde with an overall yield of 45%. The novelty of this murrayaquinone synthesis is in the use of a Mn(OAc)₃ mediated oxidative radical reaction of a N-benzylaminoquinone derivative with 2-cyclohexen-1-one for the late-stage indole ring construction.     

Chemiluminescence of organometallic compounds in reactions with the free radical galvinoxyl

Chemiluminescence (CL) is observed in reactions of the free radical galvinoxyl (RO^•) with PhMgBr, PhMnCl, PhTi(BuⁱO)₃, and sodium naphthalenide or sodium anthracenide during their oxidation with air and in reactions of RO^• with the initial organometallic compounds. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 611–612, March, 1997.     

High-pressure rate rules for alkyl + O2 reactions. 1. The dissociation, concerted elimination, and isomerization channels of the alkyl peroxy radical

The reactions of alkyl peroxy radicals (RO(2)) play a central role in the low-temperature oxidation of hydrocarbons. In this work, we present high-pressure rate estimation rules for the dissociation, concerted elimination, and isomerization reactions of RO(2). These rate rules are derived from a systematic investigation of sets of reactions within a given reaction class using electronic structure calculations performed at the CBS-QB3 level of theory. The rate constants for the dissociation reactions are obtained from calculated equilibrium constants and a literature review of experimental rate constants for the reverse association reactions. For the concerted elimination and isomerization channels, rate constants are calculated using canonical transition state theory. To determine if the high-pressure rate expressions from this work can directly be used in ignition models, we use the QRRK/MSC method to calculate apparent pressure and temperature dependent rate constants for representative reactions of small, medium, and large alkyl radicals with O(2). A comparison of concentration versus time profiles obtained using either the pressure dependent rate constants or the corresponding high-pressure values reveals that under most conditions relevant to combustion/ignition problems, the high-pressure rate rules can be used directly to describe the reactions of RO(2).     

Oligomeric cadmium-phthalocyanine complexes: novel supramolecular free radical structures

Certain cadmium-metallated phthalocyanines give rise to EPR active triple-decker sandwich complexes containing two Cd ions and three phthalocyanine (Pc) ligands. These have been shown to form when the ligands bear either eight non- peripheral alkyl or alkenyl substituents or eight peripheral 2-ethylhexyl groups. They can be derived either from three equivalents of a cadmium phthalocyanine precursor or from a 2:1 mixture of a cadmium phthalocyanine (CdPc) and a metal-free phthalocyanine (H(2)Pc). The mode of their formation has been investigated by a series of "cross" experiments. The results indicate that the triple-decker structures are formed by a self-assembly process. This is deduced from results that show that they can disassemble and reassemble with incorporation of differently substituted ligands derived from either an H(2)Pc or CdPc. The reassembled structures in these cross experiments can contain more than one ligand that originated from either the added CdPc or, and more surprisingly, the H(2)Pc compound. Mass spectrometry has also established that higher order oligomers can be formed when steric requirements between the alkyl substituents on adjacent rings in the stack are reduced. Thus an isotopic cluster for a Cd(5)Pc(6) complex has been observed when the eight peripheral substituents are hexyl chains and tetrameric complexes are formed when two different ligands are incorporated within a stack, with one carrying substituents at the peripheral sites and the other bearing substituents at the non-peripheral sites.     

Tin-free radical alkoxyamine addition and isomerization reactions by using the persistent radical effect: variation of the alkoxyamine structure

Various C-centered radicals can efficiently be generated through thermal C-O-bond homolysis of alkoxyamines. This method is used to perform environmentally benign radical cyclization and intermolecular addition reactions. These alkoxyamine isomerizations and intermolecular carboaminoxylations are mediated by the persistent radical effect (PRE). In the paper, the effect of the variation of the alkoxyamine structure--in particular steric effects in the nitroxide moiety--on the outcome of the PRE mediated radical reactions will be discussed. Fourteen different nitroxides were used in the studies. It will be shown that reaction times can be shortened about 100 times upon careful tuning of the alkoxyamine structure. Activation energies for the C-O-bond homolysis of the various alkoxyamines are provided. The kinetic data are used to explain the reaction outcome of the PRE-mediated processes.     

Photochemical isomerization reactions of cyanopyrroles: a theoretical study

The mechanisms of the photochemical isomerization reactions were investigated theoretically using a model system of 2-cyanopyrrole and 2-cyano-5-methylpyrrole with the CASSCF (eight-electron/seven-orbital active space) and MP2-CAS methods and the 6-311(d,p) basis set. The structures of the conical intersections, which play a decisive role in such phototranspositions, were obtained. The intermediates and transition structures of the ground state were also calculated to assist in providing a qualitative explanation of the reaction pathways. Our model investigations suggest that the preferred reaction route for the cyanopyrroles is as follows: reactant --> Franck-Condon region --> conical intersection --> photoproduct. In particular, the conical intersection mechanism found in this work gives a better explanation than the previously proposed internal cyclization-isomerization mechanism and supports the experimental observations. In addition, we suggest a simple p-pi orbital topology model, which can be used as a guide tool to predict the location at which conical intersections are likely to occur, as well as the conformations of the phototransposition products of various heterocycles.