Radical Initiation and Reactions

Alkyl aluminum compounds are known to be radical initiators. Kabalka and coworkers reported fhe first unequivocal example of a free-radical process in which the classical organoaluminum reagent PrsAl was used in fhe presence of O2 or under UV irradiation [203]. In modern studies, modified alkylaluminum reagents enabled more practical improvement in fhe control of diastereo- and enantioselectivity in radical initiation and reactions. 

Alfhough fhere is still controversy over whefher free radical species are in- 

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Polyester resins can be cross-linked by a free-radical initiator, usually an organic peroxide. Free radicals may be formed by thermal decomposition of an initiator such as erf-butyl perbenzoate or benzoyl peroxide, by ultraviolet (UV) or visible light decomposition, or by chemical decomposition in ambient temperature applications utilizing activators such as dimethylaniline. Inhibitors such as hydroquinone consume free radicals initially and reaction does not proceed until the inhibitor system is completely consumed. After an induction period the free... 

There is less information available in the scientific literature on the influence of forced oscillations in the control variables in polymerization reactions. A decade ago two independent theoretical studies appeared which considered the effect of periodic operation on a free radically initiated chain reaction in a well mixed isothermal reactor. Ray (11) examined a reaction mechanism with and without chain transfer to monomer. 

That the mechanism of allylic bromination is of the free-radical type was demonstrated by Dauben and McCoy, who showed that the reaction is veiy sensitive to free-radical initiators and inhibitors and indeed does not proceed at all unless at least a trace of initiator is present. Subsequent work indicated that the species that actually abstracts hydrogen from the substrate is the bromine atom. The reaction is initiated by small amounts of Br. Once it is formed, the main propagation steps are... 

The fact that these reactions are catalyzed by free-radical initiators and inhibited by galvinoxyl (a free-radical inhibitor) " indicates that free-radical mechanisms are involved. 

Photons are very clean reagents, leaving no residues. A photo-initiated process therefore has potential advantages even when compared to reactions initiated by the use of catalysts. Such processes may use fewer raw materials compared to non-photochemical alternatives, e.g. comparison of photo- and radical-initiated halogenation reactions. 

Polycarboxylated polyalkoxylates and their sulfate derivatives may be prepared by reacting an ethoxylated or propoxylated alcohol with a water-soluble, alkali or earth alkali metal salt of an unsaturated carboxylic acid [339]. The reaction occurs in aqueous solution in the presence of a free radical initiator and gives products of enhanced yield and reduced impurity levels, compared with the essentially anhydrous reactions with free carboxylic acids, which have been used otherwise. The method provides products that give solutions that are clear on neutralization, remain clear and homogeneous on dilution, and are useful as cleaning agents in drilling and other oil field operations. 

Another reagent that is extremely useful synthetically is N-bromo-succinimide (NBS, 85), which is highly selective in attacking only weak C—H bonds, i.e. at allylic, benzylic, etc., positions. It requires the presence of radical initiators, and has been shown to effect bromination through providing a constant, but very low, ambient concentration of Br2—this is maintained through reaction of the HBr produced in the reaction with NBS (c, below). TTiere is usually a trace of Br2 or HBr in the NBS that can react with the initiator to generate the initial Br to start reaction (a, below) ... 

Precipitation polymerizations dominated the early work which aimed at preparing industrially important hydrocarbon polymers in C02. In 1968, Hagiwara and coworkers explored the polymerization of ethylene in C02 using both gamma radiation and AIBN as free radical initiators [79]. Reactions were conducted at pressures of 440 bar and over the temperature range of 20-45 °C. 

Radical reduction of 1-nitro-C-glycosyl compounds. In 1983, Vasella and co-workers125 reported a stereoselective method for the synthesis of a-C-mannopyranosyl compounds by reduction of 1-nitro-C-mannopyranosyl derivatives with Bu3SnH in the presence of a,a -azoisobutyronitrile (AIBN) radical initiator. These reactions involve the formation of anomeric centered radicals. Thus, in the case of d-manno configuration as in 140, the 1,2-cts-C-pyranosyl compound 145 was obtained in 84% yield. The intermediate pyranosyl radicals 143 prefer a-attack by the tin hydride. Thus for D-glucopyranosyl derivatives, the corresponding l,2-tra x-C-pyranosyl compound 144 is obtained preferentially (Scheme 47). 

Solomon (3, h, 5.) reported that various clays inhibited or retarded free radical reactions such as thermal and peroxide-initiated polymerization of methyl methacrylate and styrene, peroxide-initiated styrene-unsaturated polyester copolymerization, as well as sulfur vulcanization of styrene-butadiene copolymer rubber. The proposed mechanism for inhibition involved deactivation of free radicals by a one-electron transfer to octahedral aluminum sites on the clay, resulting in a conversion of the free radical, i.e. catalyst radical or chain radical, to a cation which is inactive in these radical initiated and/or propagated reactions. 

Hiibner, G. and Roduner, E. 1999. EPR investigation of HO radical initiated degradation reactions of sulfonated aromatics as model compounds for fuel cell proton conducting membranes. Journal of Materials Chemistry 9 409- 18. 

Allyloxysilanes (14) undergo radical chain cyclization in the presence of di-tert-butyl hyponitrite as radical initiator and thiol as a catalyst at ca 60 °C (Reaction 6.3) [5]. The thiol promotes the overall abstraction from the Si—H moiety as shown in Scheme 6.4 and the silyl radical undergoes a rapid 5-endo-trig cyclization. Indeed, EPR studies on the reaction of t-BuO radical with silanes 14 detected only spectra from the cyclized radicals even at — 100°C, which implies that the rate constants for cyclization are > 10 s at this temperature. 

The homologous (Me3Si)2Si(H)Me does not react spontaneously with air or molecular oxygen at room temperature. However, a reaction takes place at 80 °C when air or molecular oxygen is bubbled into the pure material or its solutions to form a major product that contains the siloxane chain (Reaction 8.5) [15]. In general, yields of siloxane are about 50%. Also in this case the reaction is accelerated by radical initiation and retarded by common radical inhibitors. 

As discussed earlier (see Fig. 10.26), the contributions of 1- and 2-nitronaphthalenes and the methylni-tronaphthalene isomers to the vapor-phase mutagenicity of extracts of daytime and nighttime samples of ambient air in Redlands, California, were determined by Gupta and co-workers (1996) and shown to be major contributors to the overall vapor-phase mutagenicity of ambient air. Furthermore, the researchers demonstrated that both daytime OH radical initiated and nighttime N03 radical initiated reactions of naphthalene and the methylnaphthalenes are important in pol-... 

Due to the relative ease of carrying out the reaction and the versatility of the process, the hydrosilylation reaction has been used in a number of interesting extensions and applications. Here several of them are highlighted. In one report, Lop-inski and coworkers used the same concept of the radical-initiated hydrosilylation reaction on the Si(100)-2 x 1 surface to induce self-directed growth of molecular wires on the surface [141]. On the Si(100)-2 x 1 surface, the radical chain reaction propagates primarily along the direction of the dimer row, leading to lines of... 

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