Radical tert-butoxy

Separate experiments in which tert.-butoxy radicals were produced thermally in benzene from di-tert.-butyl peroxyoxalate failed to reveal any direct reaction of these radicals with amine II. Even at higher temperatures (A/ 150°C, dichlorobenzene, +00+ decomposition), the +0 radicals attacked neither amine II nor nitroxide I. The earlier described experiments of ketone photooxidation showed additionally that amine II displays no specially marked reactivity towards peroxy radicals. 

TABLE 2. Values of A//(4 5 eq. 6) for the reaction between di-tert-butoxy radicals and organogermanes and bond dissociation energies, BDE(Ge—H)... 

The laser flash photolysis of aromatic diisocyanate based polyurethanes in solution provides evidence for a dual mechanism for photodegradation. One of the processes, an N-C bond cleavage, is common to both TDI (toluene diisocyanate) and MDI (methylene 4,4 -diphenyldiisocyanate) based polyurethanes. The second process, exclusive to MDI based polyurethanes, involves formation of a substituted diphenylmethyl radical. The diphenylmethyl radical, which readily reacts with oxygen, is generated either by direct excitation (248 nm) or indirectly by reaction with a tert-butoxy radical produced upon excitation of tert-butyl peroxide at 351 nm. 

Laser Flash Photolysis at 351 nm of tert-Butyl Peroxide/Benzene Solutions Containing MDI-PUE and Model Compounds. Photolysis of tert-butyl peroxide (TBP) results in a highly efficient production of tert-butoxy radicals. It has recently been shown (15) that tert-butoxy radicals generated by the laser flash photolysis of TBP can rapidly extract hydrogen atoms from appropriate substrates such as aniline and diphenylamine (Scheme III). 

The laser flash photolysis (Xex=351 nm) of a TBP/BP-MDI solution in benzene (Figure 7) yields a transient spectra with distinct maximum at 370 nm which can most likely be attributed to a substituted diphenylmethyl radical. (Similar results are obtained in other solvents such as DMF). No detectable transient species were generated above 350 nm by the laser flash photolysis (Xex=351 nm) of the 60/40 mixture of TBP and benzene alone. Results for the TBP/MDI-PU (7.0 X 10 2 g/dL) system in Figure 8 show, as in the case of the model BP-MDI (Figure 7), that the transient spectrum of MDI-PU obtained indirectly through tert-butoxy radicals has a maximum at 370 nm. This provides additional support for assignment of the transient species responsible for the 370 nm absorbance to a diphenylmethyl radical. 

For MDI based polyurethanes we have provided evidence for formation of a diphenylmethyl radical by direct excitation (248 nm) of the carbamate moiety as well as hydrogen abstraction by a tert-butoxy radical which is produced by excitation (351 nm) of tert-butyl peroxide. The diphenylmethyl radical readily reacts with oxygen. A proposed mechanism which accounts for the production (direct or indirect) and subsequent reaction with oxygen of the diphenylmethyl radical is shown in Scheme IV. The hydrogen peroxide product depicted in Scheme IV has been previously identified by FT-IR (7) we have simply provided a plausible mechanism for its formation. 

Finally, the use of TBP to produce photolytically the tert-butoxy radical has not only proven to be an excellent mechanistic... 

C. F. Correia, R. M. Borges dos Santos, S. G. Estacio, J. P. Telo, B. J. Costa Cabral, J. A. Martinho Simoes. Reaction of para-Hydroxyl-substituted Diphenylmethanes with tert-Butoxy Radical. Chem. Phys. Chem. 2004, 5, 1217-1221. 

Ethane is formed by combination of methyl radicals formed from tert-butoxy radicals. 

The energy of activation for the addition of trifluoroinethyl radical to the C=0 double bond of hexafluoroaeetone was calculated to be 9.7 0.26 kcal. mol.-1 and that for the decomposition of the perfluoro tert-butoxy radical was found to be 30.6 1.3 kcal. mol.-1, so that AH for the formation of the perfluoro ferf-butoxy radical is —20.9 kcal. mol.-1. Thus at high light intensities and elevated temperatures, the contributions of these reactions cannot be neglected. 

The mechanisms for ambient temperature oxidations become more complex as the alkyl radical R becomes more complex and as the reactions proceed. Thus, with azo-2-methylpropane (52) the pyrolysis of the tert-butoxy radical must be included, and in all but the initial stages, reactions of alkoxy radicals with products containing weak C—H bonds must be included. Numerous tertiary reactions can then occur. As with most free radical systems, useful information can be obtained only if the degree of conversion of the starting material is kept low. 

The rate constant for the decomposition of tert-butoxy radicals... 

Hoare and Wellington (22) produced CH3O radicals from the photochemical (50° and 100°C.) and thermal (135°C.) decompositions of di-terf-butyl peroxide in the presence of 02. The initially formed tert-butoxy radicals decomposed to acetone plus methyl radicals, and the methyl radicals oxidized to methoxy radicals. Formaldehyde and CH3OH were products of the reaction the formation of the former was inhibited, and the latter was enhanced as the reaction proceeded. If the sole fate of CH3O were either... 

Distonic cation radicals with onium ethereal oxygens are also known. For instance, addition of tert-butoxy radicals to ethylenic or acetylenic compounds was described (Bloodworth et al. 1988), Scheme 3-36 (below). The butoxy radicals are added in their pro-tonated forms (CF3COOH is the proton source) ... 

Das PK, Encinas MV, Steenken S, Scaiano JC (1981) Reactions of tert-butoxy radicals with phenols. 

The transient spectrum of the phenoxy radical can be readily generated by reaction of phenol with tert-butoxy radicals, formed by the photodecomposition of di-jm-butyl peroxide. This is a very efficient reaction which takes place with a quantum yield of approximately 0.7-0.9 (13, 14). The transient spectra of foe phenoxy and guaiacoxy radicals are shown in Figure 4. 

Aliphatic alcohols and benzyl alcohol are oxidized by Ph3Bi(OOtBu)2 to the corresponding carbonyl compounds (Equation (141)).233 It has been suggested that the oxidation occurs via dehydrogenation of alcohols by phenyl or tert-butoxy radical. When treated with diethyl ether, Ph3Bi(OOtBu)2 oxidizes the methyl group to afford ethoxyacetic... 

Large-ring heterocyclic radicals are not particularly well known as a class. Their behavior often resembles that of their acyclic counterparts, except for transannular reactions, such as the intramolecular hydrogen transfer in 1-azacyclononan-1-yl 1 (Scheme 1). As is the case with acyclic ethers, oxepane in the reaction with tert-butoxy radical suffers abstraction of a hydrogen atom from the 2-position in the first reaction step (Scheme 2). 

Compared with chlorination, hydrogen abstraction reactions of alkoxy radicals are relatively insensitive to solvent effects [160, 222, 223]. The results of the AIBN-initiated radical chain chlorination of 2,3-dimethylbutane with tert-butyl hypochlorite indicate a solvent effect on tert-butoxy radical reactions of much smaller magnitude, but greater selectivity in aromatic solvents [222, 223], The reduced solvent effect for this hydrogen abstraction reaction has been attributed to steric effects. Due to the bulky... 

According to Scheme 14, terf-butoxy and polymer carboxylate primary radicals are formed by photodissociation. However, the polymer-bound carboxylate radical may firstly lose CO2 giving rise to a polymer-bound phenyl radical which is also able to initiate the polymerization. The tert-butoxy radical may evolve to give acetone and a very reactive methyl radical. 

The experimental conditions concerning the kinetics of interaction of tert-butoxy radical with some model compounds were reported elsewhere ... 

This mechanism, involving a free radical R , is compatible with the allylic rearrangements found The finding that ferf-butyl peroxyesters labeled with in the carbonyl oxygen gave ester with 50% of the label in each oxygen is in accord with a combination of R. with the intermediate 32, in which the copper is ionically bound, so that the oxygens are essentially equivalent. Other evidence is that tert-butoxy radicals have been trapped with dienes.Much less is known about the mechanisms of the reactions with metal acetates. 

By ESR spectroscopy Janzen measured the rate of formation of aminyloxides from benzoyloxy radicals and N-aryliden-tert-butylamin-N-oxides 81 and so determined the rate of thermal decomposition of benzoylperoxide90cl The value estimated in this manner is in accordance with those determined by other methods. In the same way the rate constants for spin trapping of tert-butoxy radicals by different spin traps were determined901. The relative values listed in the following table show how effectively the spin traps operate. 

Alkyl radicals can be obtained by abstraction of a hydrogen atom from an alkyl group by another radical. This method was utilized for the generation of benzyl radicals from toluene with tert-butoxy radical obtained on heating di-terf-butyl peroxide.38 Benzoyl92 and carboxymethyl88 radicals have also been obtained by this method. The reaction gives rise to a complex mixture of products and therefore is of rather limited use. 

HO shows little selectivity in H abstractions from pyranose rings, but shows enhanced reactivity towards the 4 -positions of aldofuranosides and the 5 -positions of ketofuranosides such as sucrose. This appears concordant with studies on H abstractions by (CH3)3C-0, where an enhanced reactivity of C-H bonds antiperiplanar to a p-type lone pair on oxygen was noted. As discussed in Section 7.5.1, this could be a least-motion effect as much as a manifestation of ALPH. The labilisation of hydrogens a to an ether linkage -the cause of the well-known peroxidation of ethers - was apparent even in hydrogen abstraction by the normally avid and unselective tert-butoxy radical, with reactivities, relative to the C-H bonds in cyclohexane, of 2.7,0.14 and 0.21 for the ot, p and y hydrogens of tetrahydropyran. ... 

Li believed that a cyclic alkane radical was generated from the hydrogen abstraction by a tert-butoxy radical that arose from the iron-catalyzed decomposition of the tert-butyl peroxide. The cyclic radical 62 was then believed to react with the iron enolate 63 to form the alkylated product 61 (Scheme 34). 

Even less obvious in this particular case is evidence that the less-substituted radical is actually more stabilised, presumably by negative hyperconjugation with the f3 C—F bonds, than the more-substituted radical is by the attached F atoms.997 Furthermore, in the other striking reversal of expectations, the tert-butoxy radical, which is certainly more electrophilic than methyl, adds to 1,1-difluoroethylene 7.45 at the substituted carbon with a selectivity of 80 20,998 whereas the methyl radical is normal in this case. Thus the coefficients in the LUMO are hardly likely to be the explanation, and one suggestion in this particular case is that there is a growing anomeric effect between the oxygen atom and the two fluorine substituents in the transition structure for the formation of the major intermediate 7.46.988... 

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