Combination of radicals

Substituted 5, 8-cyclopurine and 5, 6-cyclopyrimidine nucleosides have been synthesized by combination of radical translocation/cyclization... 

If reactions other than those considered so far did not take place to an appreciable extent, the average degree of polymerization would be directly related to the kinetic chain length v. Assuming termination by combination of radicals, as is indicated by experiments previously cited, each polymer molecule formed in a monoradical-initiated polymerization would consist of two kinetic chains grown from two other-... 

In the more general case of joint control of molecular weight by both transfer and radical termination, it is appropriate to consider that two distributions are formed simultaneously. One of these distributions consists of molecules terminated by chain transfer the other of pairs of chains joined by the combination of radicals. For any conversion increment, the two coexisting distributions will depend on the same parameter p representing the probability of continuation of the growth of any chain, i.e. 

The pinacol formation reaction follows a radical mechanism. Benzopinacol, benzophenone and the mixed pinacol are formed jointly with many radical species [72, 74]. In the course of the reaction, first a high-energy excited state is generated with the aid of photons. Thereafter, this excited-state species reacts with a solvent molecule 2-propanol to give two respective radicals. The 2-propanol radical reacts with one molecule of benzophenone (in the ground state, without photon aid) to lengthen the radical chain. By combination of radicals, adducts are formed, including the desired product benzopinacol. Chain termination reactions quench the radicals by other paths. 

Another new example using titanocene as catalyst has been revealed by Malacria and coworkers. Here, a previously unknown combination of radical cyclizahon involving an epoxide-opening of 3-154 and a 3-phosphinoyl-elimination takes place to furnish various pyrrolidines 3-155, bearing a tetrasubstituted exo-double bond, in good yields (Scheme 3.41) [66]. 

Note 1 Examples of reactive blending are (a) blending accompanied by the formation of a polymer-polymer complex, (b) the formation of block or graft copolymers by a combination of radicals formed by the mechanochemical scission of polymers during blending. 

Thus, if a mixture of these hydrocarbons reacts with a singlet state, then the addition product will consist entirely of undeuterated (do) and deuterated(i i2) compounds. If some of the reaction occurs by the hydrogen abstraction-recombination route, then some crossed products (di and/or du compounds) will be formed. By using this technique, it has been shown that, while some of the DPC-cyclohexane adduct (34) is formed by combination of radical pairs, there is no evidence of crossover product present in the FL-cyclohexane adduct (37). The results are interpreted by assuming rapid spin state equilibration relative to the reaction of either spin state with solvent. The larger amount of singlet chemistry of FL relative to DPC can then be explained if the S-T gap is smaller in FL than in DPC. 

A final possibility for a type of reaction resulting in acetylene formation which must be considered is that of combination of radicals. This is not likely to be significant in most systems, but in the case of methane reactions it becomes of particular interest. The production of ethylene, and acetylene in the thermal decomposition of methane requires some sort of combination reaction (40, 65). Kassel suggested a mechanism involving methylene radicals ... 

If the rates of combination of radicals or atoms are so fast, you might well wonder how chain propagation ever could compete. Of course, competition will be possible if the propagation reactions themselves are fast, but another important consideration is the fact that the atom or radical concentrations are very low. Suppose that the concentration of Cl - is 10 UM and the CH4 concentration 1M. The probability of encounters between two Cl atoms will be proportional to 10-11 x 10 u, and between CH4 and Cl - atoms it will... 

The most important processes that remove radicals are (1) combination of radicals with each other, either by direct bond formation (recombination) or by hydrogen atom transfer from one to the other (disproportionation), and (2) electron transfer between a radical and an oxidizing or reducing agent. 

Bryant has calculated the changes in free energy for various reaction steps of the polymerization of tetrafluoroethylene. He concluded (1) that the initiation and propagation are about twice as favorable for tetrafluoroethylene as the analogous reactions for ethylene, (2) that termination by combination is more favorable than disproportionation, and (3) that chain-transfer to monomer and to polymer are less likely than the combination of radicals. 

Radical chemistry has witnessed remarkable progress since the mid 1980s [1], In addition to common radical C2 synthons such as alkenes and alkynes [2], several radical Cl synthons are also available, including carbon monoxide, isonitriles, and sulfonyl oxime ethers [3, 4]. As featured in Scheme 6.1, a variety of combinations of radical Cl and C2 synthons are now possible, which makes radical methodologies more attractive and permits the design and implementation of a wide range of multicomponent processes. 

A metal-induced one-electron reduction is frequently used to generate radical species. Termination of the radical reactions is due to a one-electron reduction process to give anions and therefore constitutes a non-chain process. As featured in Scheme 6.32, in many cases the multicomponent processes described here are a combination of radical and anionic bond-forming reactions. 

Polystyrene. The polymerization of styrene is most commonly done under free radical conditions. Peroxides are used to initiate the reaction at low temperatures. At 100°C styrene acts as its own initiator. Below 80°C the termination mechanism primarily involves combination of radicals. Above 80°C both disproportionation and chain transfer with the Diels-Alder dimer are important. 

The concentrations of reactants are represented by I = initiator, R- = initiator radicals, M = monomer, Pn- = macroradicals of degree of polymerization n, and Pn+m = polymer formed by combination of radicals of length n and m. 

CIDEP (Chemically Induced Dynamic Electron Polarization) Non-Boltzmann electron spin state population produced in thermal or photochemical reactions, either from a combination of radical pairs (called radical-pair mechanism), or directly from the triplet state (called triplet mechanism), and detected by ESR spectroscope... 

A special case of donor-acceptor interactions occurs when two monomers come into contact such that one exhibits a signicant donor and the other an acceptor character. The donor-acceptor complex then has the structure of a zwitterion resulting from the combination of radical ions after electron transfer from donor to acceptor... 

The radical mechanism generates block copolymers mainly by means of multifunctional or polymeric initiators [270] or by the combination of radical chain ends produced by the separation of two propagating monomers into an aqueous and a micellar phase (in emulsion) [271],... 

A combination of radical and electron transfer steps mediated by manganese triacetate has been used in the synthesis of 5-acetoxyfuranones 21 through carbox-ymethyl radical addition to mono- and disubstituted alkynes 20, followed by oxidative cyclization of the resulting vinyl radicals 22 (Scheme 2.4). The cyclic intermediate 24 is transformed into the furanone 21 through stepwise one-electron oxidation and capture of the resulting aUyl cation 26 by acetate. 

The kinetics of combination of radical pairs formed by quenching of the triplet excited state of B by 2,4,6-trimethylphenol (HR) in films of rigid and plasticized polyvinylchloride (PVC) were studied by the method of laser flash photolysis by Levin et al. At high concentrations of the phenol quencher ( 15 wt%), most of the triplet excited state of the ketone is quenched and the formation and disappearance of the ketyl and phenoxyl radicals can be followed from changes in their characteristic transient absorptions at 540 and 390 nm, respectively. 

The chain length of these two radical-reaction steps is about 100. When the radical concentration has reached a certain limit, the chain reaction is gradually stopped by mutual combination of radicals, the termination step. 

The formation of propane was attributed to combination of methyl and ethyl radicals. Obi and Tanaka have reexamined the formation of propane in the photolysis of deuterated ethane at 1470 A, and have confirmed on the basis of the isotopic distribution of the propanes formed that it is the combination of radicals that produces propane. The alternative suggestion would be the formation of propane via insertion of CH2 into ethane. Both studies have ruled this out on the basis that the propane/methane ratio is greater than unity, a possibility which is not allowed if one methane is formed for each methylene as in reaction (3). The fact that propane/methane is greater than unity forces one to conclude that at 1470 A a primary process giving methyl radicals, reaction (14), must represent a small but significant fraction of the total primary processes, in order that methyl radicals are present to combine with the ethyl radicals. 

Fe(III) salts are known to oxidise electron-rich centres to foster the formation of radical species. They are particularly efficient in the oxidation of aromatic systems or a carbanion to the corresponding carbon-centred radical which undergoes C-C bond formation to yield the coupled products. For a successful synthesis, it is important to work in the absence of reactive synthetic molecules other than those which form the combination of radicals. Barton et al. used a simple water-soluble diselenide derivative that shows radical scavenger properties towards alkyl and hydroxyl radicals in Fenton-type chemistry (Fe2+-H202)4 The reaction rate between the produced alkyl radical and the diselenide overwhelms self-termination and halogen transfer reactions. The ability of diselenide to scavenge alkyl and hydroxyl radicals [ 3(0 °C) = 6.1 x 108 M-1 s-1] could be exploited as a new tool in both synthetic and mechanistic work conducted in aqueous media (Scheme 8.5).4... 

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