Solvent derived peroxides

Such solvent-derived radicals can induce the decomposition of the hydroperoxide or react with oxygen in the system to form peroxidic solvent molecules. They may also react with other radicals either by coupling or disproportionation. 

In a broad sense radical-induced decomposition may be considered as the reaction of any substrate promoted by a free radical, but the more common usage is to restrict consideration to those cases in which the molecule undergoing reaction is a free radical initiator (1, ). Thus radical-induced decomposition may be de-fTned as reaction of a free radical with the primary source of the radical. This phenomenon is particularly noticeable when a free radical initiator does not react with first-order kinetics, but is consumed by the radicals it generates in chain processes. This case was originally elucidated for the kinetics of benzoyl peroxide decomposition in solvents such as ethyl ether ( - ), and was found to occur with radical displacement (the SH2 reaction) (S) by solvent derived radicals on the peroxidic oxygen of benzoyl peroxide as shown in Fig. 1. 

Another classic study of benzoyl peroxide involved the addition of solvent derived radicals to the para position of the initiator. For example, decomposition of benzoyl peroxide in cyclohexane led to the formation of -cyclohexylbenzoic acid <2) The absence of meta product led to the suggestion (7, 8 ) that para substitu-tion was enhanced by o-lactone formation concerted with addition. An alternative route would involve addition followed by subsequent a-lactone formation ( ) or transannular hydrogen transfer (Fig. 2) (] ). In this case the absence of meta substitution product could be ascribed to reversibility of the addition, and this is known to occur ( ). A preference for para substitution leading to formation of a radical intermediate stabilized by an adjacent carboxylate function would also be expected. 

The reaaions of the radicals (whether primary, secondary, solvent-derived, etc.) with monomer may not be entirely regio-or chemoselective. Reactions, such as head addition, abstraction, or aromatic substitution, often compete with tail addition. In the sections that follow, the complexities of the initiation process will be illustrated by examining the initiation of polymerization of two commercially important monomers, S and methyl methacrylate (MMA), with each of three commonly used initiators, azobisisobutyronitrile (AIBN), dibenzoyl peroxide (BPO), and di-t-butyl peroxyoxalate (DBPOX). The primary radicals formed from these three initiators are cyanoisopropyl, benzoyloxy, and t-butoxy radicals, respectively (Scheme 7). BPO and DBPOX may also afford phenyl and methyl radicals, respectively, as secondary radicals. 

The effect of activators like FeS04 [11,12] for emulsion polymerization and ferric stearate [13] for bulk polymerization of vinyl monomers in combination with acyl peroxide has been studied. The ferrous ion catalyzed decomposition of BZ2O2 in ethanol has been studied in some detail by Has-egawa and co-workers [14,15]. The cycle, which requires reduction of Fe " by solvent-derived radicals, yields a steady-state concentration of Fe after a few minutes, shown spectroscopically to be proportional to the initial concentration of the ferrous ion [14]. The second-order rate o)nstant for the following reaction was found to be 8.4 L moP sec at 25 C, with an activation energy of 14.2 kcal moP ... 

Peracid Processes. Peracids, derived from hydrogen peroxide reaction with the corresponding carboxyUc acids in the presence of sulfuric acid and water, react with propylene in the presence of a chlorinated organic solvent to yield propylene oxide and carboxyUc acid (194—196). 

The amount of induced decomposition that occurs depends on the concentration and reactivity of the radical intermediates and the susceptibility of the substrate to radical attack. The radical X- may be formed from the peroxide, but it can also be derived from subsequent reactions with the solvent. For this reason, both the structure of the peroxide and the nature of the reaction medium are important in determining the extent of induced decomposition, relative to unimolecular homolysis. 

A strong acceptor TCNE undergoes [2+2] rather than [4+2] cycloaddition reactions even with dienes. 1,1-Diphenylbutadiene [20] and 2,5-dimethyl-2,4-hexadiene (Scheme 5) [21] afford mainly and exclusively vinyl cyclobutane derivatives, respectively. In the reactions of 2,5-dimethyl-2,4-hexadiene (1) the observed rate constant, is greater for chloroform solvent than for a more polar solvent, acetonitrile (2) the trapping of a zwitterion intermediate by either methanol or p-toluenethiol was unsuccessful (3) radical initiators such as benzyl peroxide, or radical inhibitors like hydroquinone, have no effect on the rate (4) the entropies of activation are of... 

Analyses for the Saxitoxins. Early methods for analysis of the saxitoxins evolved from those used for toxin isolation and purification. The principal landmarks in the development of preparative separation techniques for the saxitoxins were 1) the employment of carboxylate cation exchange resins by Schantz et al. (82) 2) the use of the polyacrylamide gel Bio-Gel P2 by Buckley and by Shimizu (5,78) and 3) the development by Buckley of an effective TLC system, including a new solvent mixture and a new visualization technique (83). The solvent mixture, designated by Buckley as "E", remains the best for general resolution of the saxitoxins. The visualization method, oxidation of the saxitoxins on silica gel TLC plates to fluorescent degradation products with hydrogen peroxide and heat, is an adaptation of the Bates and Rapoport fluorescence assay for saxitoxin in solution. Curiously, while peroxide oxidation in solution provides little or no response for the N-l-hydroxy saxitoxins, peroxide spray on TLC plates is a sensitive test for all saxitoxin derivatives with the C-12 gemdiol intact. 

Basically, three reactions were evoked to support the occurrence of 5a-C-centered radicals 10 in tocopherol chemistry. The first one is the formation of 5a-substituted derivatives (8) in the reaction of a-tocopherol (1) with radicals and radical initiators. The most prominent example here is the reaction of 1 with dibenzoyl peroxide leading to 5a-a-tocopheryl benzoate (11) in fair yields,12 so that a typical radical recombination mechanism was postulated (Fig. 6.6). Similarly, low yields of 5a-alkoxy-a-tocopherols were obtained by oxidation of a-tocopherol with tert-butyl hydroperoxide or other peroxides in inert solvents containing various alcohols,23 24 although the involvement of 5 a-C-centered radicals in the formation mechanism was not evoked for explanation in these cases. 

The addition of hydrogen halide to alkene is another classical electrophilic addition of alkene. Although normally such reactions are carried out under anhydrous conditions, occasionally aqueous conditions have been used.25 However, some difference in regioselectivity (Markovnikov and anti-Markovnikov addition) was observed. The addition product formed in an organic solvent with dry HBr gives exclusively the 1-Br derivative whereas with aq. HBr, 2-Br derivative is formed. The difference in the products formed by the two methods is believed to be due primarily to the difference in the solvents and not to the presence of any peroxide in the olefin.26... 

The luminol dianion Lum2< > does not exist in appreciable quantities in aqueous solvents hydrogen peroxide and a catalyst such as hemin are required. Thus another mechanism seems to be at work here. Perhaps a hydrogen atom is abstracted from the luminol monoanion Lum( > to yield a luminol radical anion 55 which then reacts with oxygen or a radical ion derived from hydrogen peroxide according to 3,4,109)... 

Several N-methyl-9-acridinecarboxylic acid derivatives (e.g., 10-methyl-9-acridinecarboxylic chloride and esters derived therefrom [39]) are chemiluminescent in alkaline aqueous solutions (but not in aprotic solvents). The emission is similar to that seen in the CL of lucigenin and the ultimate product of the reaction is N-methylacridone, leading to the conclusion that the lowest excited singlet state of N-methylacridone is the emitting species [40], In the case of the N-methyl-9-acridinecarboxylates the critical intermediate is believed to be either a linear peroxide [41, 42] or a dioxetanone [43, 44], Reduced acridines (acridanes) such as N-methyl-9-bis (alkoxy) methylacridan [45] also emit N-methylacridone-like CL when oxidized in alkaline, aqueous solutions. Presumably an early step in the oxidation process aromatizes the acridan ring. 

Aminoanthracene forms a Schiff base with dimethylacetaldehyde (isobutyral-dehyde). This compound can be oxidized by peroxide under basic conditions to form 9-formamidoanthracene and acetone in dimethylformamide as a solvent [54, 55], CL from this system can be observed in other aprotic solvents as well. A limited amount of work has been done with the CLs of Schiff bases or anthracene derivatives. Presumably, this will change in the future. 

A very different route to soluble PPP derivatives was demonstrated by Yoshino and coworkers [586], who introduced perfluorinated alkyl substituents into PPP 471 by reaction with perfluorobutanoyl peroxide. The resulting modified polymer 475 was soluble in common organic solvents and a solution-fabricated PLED ITO/475/Mg In emitted blue to green light (depending on voltage) with band half-width of over 200 nm. 

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