Radical sources

In the intermediate complexe of free radical arylation, it is necessary to oxidize the reaction intermediate to avoid dimerization and disporportio-nation (190-193, 346) In this case isomer yield and reactivity will be highest with radical sources producing very oxidative radicals or in solvents playing the role of oxidants in the reaction. The results are summarized in Tables III-29 and III-30. 

TABLE in-29. VARIATION IN THE PERCENTAGES OF PHENYL 1 SOMERS AND REACTIVITY OF THIAZOLE WITH DIFFERENT PHENYL RADICAL SOURCES (182, 184). 

VARIATION IN THE PERCENTAGE OF PHENYL ISOMERS AND REACTIVITY OF 4-METHYLTHIAZOLE WITH DIFFERENT RADICAL SOURCES (184). 

Radical source Molar concentration Temperature (°C) 2 5 Reactivitv... 

In the case of alkyl radicals [e.g., methyl radical (197, 198) and cyclohexyl radical (198)], their nucleophilic behaviour enhances the reactivity of the 2-position. Here it is necessary to have full protonation of the nitrogen atom and to use specific solvents and radical sources. 

In Table III-33 results for the methylation of thiazoles in acetic acid are given (lead tetraacetate is used as radical source), but in this case some discrepancies appear, the acidic medium being too weak, and the heterocyclic base not fully protonated. Thiazole has also been methylated by the DMSO-H2O2 method (201), and the results are in agreement with those described previously. 

Copolymerization is effected by suspension or emulsion techniques under such conditions that tetrafluoroethylene, but not ethylene, may homopolymerize. Bulk polymerization is not commercially feasible, because of heat-transfer limitations and explosion hazard of the comonomer mixture. Polymerizations typically take place below 100°C and 5 MPa (50 atm). Initiators include peroxides, redox systems (10), free-radical sources (11), and ionizing radiation (12). 

New radicals come exclusively from the decomposition of the intermediate hydroperoxide (eq. 4), provided no other radical sources, eg, peroxidic impurities, are present. Hydroperoxides have varying degrees of stabiUty, depending on their stmcture. They decompose by a variety of mechanisms and are not necessarily efficient generators of new radicals via thermolysis (19,20). 

Substitution. In free-radical substitution, the olefin reacts with a free-radical source to form the allyl free radical, which in turn reacts with available reagent to produce both the final product and a new free radical. 

The ultraviolet lamps used in the photochlorination process serve to dissociate the chlorine into free radicals and start the radical-chain reaction. Other radical sources, such as 2,2 -a2obisisobutyronitrile, have been used (63,64). Primary by-products of the photochlorination process include 1,1,2-trichloroethane (15—20%), tetrachloroethanes, and pentachloroethane. Selectivity to 1,1,1-trichloroethane is higher in vapor-phase chlorination. Various additives, most containing iodine or an aromatic ring in the molecule, have been used to increase the selectivity of the reaction to... 

Alkyl hydroperoxides give alkoxy radicals and the hydroxyl radical. r-Butyl hydroperoxide is often used as a radical source. Detailed studies on the mechanism of the decomposition indicate that it is a more complicated process than simple unimolecular decomposition. The alkyl hydroperoxides are also sometimes used in conjunction with a transition-metal salt. Under these conditions, an alkoxy radical is produced, but the hydroxyl portion appears as hydroxide ion as the result of one-electron reduction by the metal ion. ... 

Pyridine has been phenylated with the following free-radical sources benzenediazonium chloride with aluminum trichloride the Gomberg reaction " phenylhydrazine and metal oxides A -nitroso-acetanilide dibenzoyl peroxide phenylazotriphenylmethane di-phenyliodonium hydroxide and electrolysis of benzoic acid. ° Although 2-phenylpyridine usually accounts for over 50% of the total phenylated product, each of the three phenyl derivatives can be obtained from the reaction by fractional recrystallization of the... 

Unsymmetrical azo-compounds find application as initiators of polymerization in special circumstances, for example, as initiators of living radical polymerization [e.g. triphenylmethylazobenzene (30) (see 9.3.4)], as hydroxy radical sources [e.g. a-hydroperoxydiazene (31) (see 3.3.3,1)1, for enhanced solubility in organic solvents [e.g. f-butylazocyclohexanecarbonitrile (32)J, or as high temperature initiators [e.g. t-butylazoformamide (33)]. They have also been used as radical precursors in model studies of cross-termination in copolymerization (Section... 

It is believed that clay minerals promote organic reactions via an acid catalysis [2a]. They are often activated by doping with transition metals to enrich the number of Lewis-acid sites by cationic exchange [4]. Alternative radical pathways have also been proposed [5] in agreement with the observation that clay-catalyzed Diels-Alder reactions are accelerated in the presence of radical sources [6], Montmorillonite K-10 doped with Fe(III) efficiently catalyzes the Diels-Alder reaction of cyclopentadiene (1) with methyl vinyl ketone at room temperature [7] (Table 4.1). In water the diastereoselectivity is higher than in organic media in the absence of clay the cycloaddition proceeds at a much slower rate. 

The thermolysis and photolysis of azo-compounds (R.N N.R) are well documented sources of radicals (for a review, see Strausz et al., 1972). As such they have been extensively used as radical sources in CIDNP studies (Fischer and Bargon, 1969 Gloss and Trifunac, 1969, 1970b, c Iwamura and Iwamura, 1970 Iwamura et al., 1970b Kasukhin et al., 1970). The application of CIDNP to unravel some of the mechanistic complexities of the decomposition of azo-compounds has now begun. 

They are initiated or accelerated by typical free-radical sources, such as the peroxides referred to, or by light. In the latter case the concept of quantum yield applies (p. 1316). Quantum yields can be quite high, for example, 1000, if each quantum generates a long chain, or low, in the case of nonchain processes. 

Another vulcanizing agent for diene rubbers is m-phenylenebismaleimide. A catalytic free-radical source such as dicumyl peroxide or benzothiazyldisulfide (MBTS) is commonly used to initiate the reaction [61]. Phenolic curatives, benzoquinonedioxime, and m-phenylenebismaleimide are particularly useful where thermal stability is required. 

X indicates a small substituent, which may be an atom such as hydrogen (H) or chlorine (Cl) or it may he a group such as methyl (CH3), cyano (CN), carhoxyl (COOH), carbomethoxy (COOCH3), etc. The growing chain is terminated by collision with another chain or other radical source or by one of several other mechanisms. The number of monomer units in the polymer chain is the degree of polymerization, abbreviated DP. If the degree of polymerization is very low, the product is sometimes referred to as an oligomer. 

The relative product yields depend on the CHa to Cl ratio on the surface. In the studies reported here, this ratio has been adjusted to 1 1 (consistent with the CHa Cl stoichiometry in CHaCl) on the basis of a Cl(181 eV) C(272 eV) Auger peak ratio of 6.5 which is the same as that measured for physisorbed monolayers of dimethyldichlorosilane. Monolayer coverages of CHa + Cl having 1 1 stoichiometry were obtained by a 20 L exposure from the methyl radical source (approximately sahiration coverage) followed by a 9.5 L dose of CI2. 

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