Radical ferf-butoxyl

FIGURE 2. Relative reactivities toward the ferf-butoxyl radical of individual positions (per hydrogen atom) relative to a hydrogen atom in a methyl group in pentane. Reprinted with permission from Reference 23. Copyright (1985) American Chemical Society... 

Figure 13.7 (a) Results of calibration using a solution of ferrocene in benzene (correlation coefficient 0.9998). (b) Results of an experiment involving the abstraction of the hydrox-ylic hydrogen in phenol by ferf-butoxyl radicals, in benzene (correlation coefficient 0.99998). Adapted from [285]. 

In a typical experiment, the sample is a solution (e.g., in benzene) of both the ferf-butoxyl radical precursor (di-tert-butylperoxide) and the substrate (phenol). The phenol concentration is defined by the time constraint referred to before. The net reaction must be complete much faster than the intrinsic response of the microphone. Because reaction 13.23 is, in practical terms, instantaneous, that requirement will fall only on reaction 13.24. The time scale of this reaction can be quantified by its lifetime rr, which is related to its pseudo-first-order rate constant k [PhOH] and can be set by choosing an adequate concentration of phenol, according to equation 13.25 ... 

Numerous examples of hydrogen abstraction by rcrt-butoxyl radicals have been reported. The unusual choice of melatonin is not accidental note that the radical has simply been identified as (melatonin) and seems to emphasize that the site of reaction (in fact, rather well established in this case) may not necessarily be derived from the laser experiment. The laser technique observes all sites and forms of reaction regardless of which species is monitored and how well characterized it may be (see below). The trace of Figure 18.7 shows the formation of the melatonin radical following hydrogen transfer to ferf-butoxyl. ... 

The reaction can be generalized, as illustrated in Scheme 18.4, for the case of ferf-butoxyl radicals. 

In the examples of Figures 18.8 and 18.9 the probe molecule is diphenyl-methanol, and it reacts with ferf-butoxyl radicals as shown in Scheme 18.3. Usual probe concentrations were between 50 and 200 mM. Figure 18.8 shows a representative trace for the formation of the ketyl radical from diphenylmethanol (i.e., the same formed by photoreduction of benzophenone), the only detectable species in the system. Figure 18.8 shows how the value of growth, given by the slope of the plots, changes with substrate (1,7-octadiene) concentration, as predicted by Eq. 18. 

The underlying reason for all these observations is that the growth of any product of reaction always reflects the lifetime of the precursor species, ferf-butoxyl radical in our examples. Interestingly, this is the same concept that applies when one measures properties, such as fluorescence that is, the observable fluorescence lifetime reflects the lifetime of the singlet state and not its radiative lifetime. 

Similarly, the p-fragmentation of tertiary alkoxyl radicals [reaction (2)] is a well-known process. Interestingly, this unimolecular decay is speeded up in a polar environment. For example, the decay of the ferf-butoxyl radical into acetone and a methyl radical proceeds in the gas phase at a rate of 103 s 1 (for kinetic details and quantum-mechanical calculations see Fittschen et al. 2000), increases with increasing solvent polarity (Walling and Wagner 1964), and in water it is faster than 106 s 1 (Gilbert et al. 1981 Table 7.2). 

Table 13. Comparison of rates of H-abstraction by perfluoro-n-alkyl and ferf-butoxyl radicals...

The EPR spectra of the NHC boryl radicals that were generated through HAT to the ferf-butoxyl radical clearly show the delocalized 7i-type nature of these intermediates postulated to be essential by calculations [10, 12]. It was also demonstrated that the decay of the EPR signals could be fitted to a second-order decay having 2kt = 9 x 106 M-1 s-1. In agreement with this kinetic analysis, the NHC boryl radicals ultimately dimerize to give bis-NHC diborane derivatives. With the aid of EPR spectroscopy it was also established that the NHC boryl radicals readily abstract bromine atoms from primary, secondary, and tertiary alkyl bromides. However, chlorine atom abstraction is much slower and useful only for benzyl chloride. 

Li and coworkers published addition reactions of ethers, sulfides, or tertiary amines 40 to p-dicarbonyl compounds 39 (Fig. 8) [96]. Fe2(CO)9 proved to be the catalyst of choice and di-tert-butyl peroxide the optimal oxidant. a-Functionalized p-dicarbonyl compounds 41 were isolated in 52-98% yield. Although the details of the catalytic cycle remain unclear, it seems to be likely that the peroxide is reductively cleaved by the Fe(0) catalyst leading to an Fe(I) complex and a ferf-butoxyl radical, which abstracts the a-hydrogen atom of 40. Addition of the resulting radical to the free enol form of 39 or the corresponding iron enolate of 39 may subsequently occur. It remains unclear, however, whether the main catalytic reaction proceeds on an Fe(0)-Fe(I) oxidation stage or whether further oxidation of initially formed Fe(I) rather leads to an Fe(II) catalyst. This cannot be excluded,... 

Recently, an interesting report appeared on the antioxidant profiles of some 2,3-dihydrobenzo[fc]furan-5-ol and 1-thio, 1-seleno and 1-telluro analogs. Redox properties and rate constants with the ferf-butoxyl radical were measured in acetonitrile. The values found for the oxygen and sulfur analogs were the same, 2 x 10 s , less than the... 

A reaction between Craq2+ and ferf-BuOOH generates tert-butoxyl radicals that either eliminate CH3 or react with Craq2 + ... 

The first step is the generation of a A /T-butoxyl radical through the photochemical decomposition of ferf-butylperoxide the second step corresponds to a a(C-H) hydrogen abstraction reaction from triethylamine as proposed many years ago [291]. The originality of the present procedure lies in the third step that corresponds to the addition of the aminoalkyl radical to a (meth) acrylate double bond. Indeed, upon addition of, for example, methyl acrylate MA or methyl methacrylate MMA, new RM transients are observed with absorption maxima at 470 and 425 nm for MA and MMA, respectively, as shown in Fig. 10.8. 

Incubation of rat embryonic mesencephalic tissue, rich in dopaminergic neurones, with 0.2 mM tert-butyl hydroperoxide in the presence of the spin trap 5,5-dimethyl-1-pyrroline N-oxide for 20 min resulted in the trapping of radicals (Karlsson et al. 2000). The main radicals detected in cell suspensions were the tert-butoxyl radical and the methyl radical, indicating the one-electron reduction of the peroxide followed by a P-scission reaction. The appearance of electron paramagnetic resonance signals from the trapped radicals preceded the onset of cytotoxicity, which was almost exclusively necrotic in nature. The inclusion of resveratrol (3,5,4 -trihydroxy-fra s-stilbene) in incubations resulted in the marked protection of cells from ferf-butyl hydroperoxide. 

X 10 for cyclohexene, 2.3 x 10 for 1,3-cyclooctadiene, 2.0 x 10 for cyclohexane all in M- s" from Ref. [288]. Rate constant for 2,5-dimethyl-2,4-hexadiene refers to cumyloxyl in benzene. From Table 3.9. From Ref. [215] in acetonitrile. From Ref. [256] in acetonitrile for cyclohexene see also Refs. [258,259]. From Ref. [33] see also Ref. [72] in isooctane. Calculated from fluorescence lifetime in neat ethanol based on a lifetime in gas phase of 1030 ns. "From Ref. [230] in benzene/di-tert-butoxyperoxide (1/2). "From Ref. [289] in benzene. °From Ref. [288] in chlorobenzene Ref. [290] in benzene/di-ferf-butoxyperoxide (1/2). Values for cumyloxyl radicals can be found in Refs. [288,291,292]. PFrom Ref. [74] in benzene. From Ref. [291] in benzene see also Refs. [74,293]. From Ref. [232] in benzene at 37°C. Value for diethylsulfide. From Ref. [294] in benzene." From Ref. [70] in neat acetone. For quenching of triplet acetone by n-propylamine and triethylamine in acetonitrile see also Refs. [172,271]. In the case of alkoxyl radicals the values for cumyloxyl radicals are reported. From Refs. [29,231] reaction with fert-butoxyl radicals in benzene/di-tert-butoxyperoxide (1/2). Values not reported in table n-propylamine 1.6 x 10 M s, diethylamine ... 

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