Tert-butoxyl radicals

The relative reactivities of alkyltin compounds towards tert-butoxyl radicals, ketone triplets, and succinimidyl radicals are dominated by the steric effect of the alkyl ligands (R" > R"), but that towards bromine atoms follows the reverse sequence (R" < R ). 

If the initiator is a peroxide compound capable of generating very active but nonselective alkoxyl radicals, the concentration of the inhibitor must be sufficiently high to provide the above conditions. For instance, the ratio krJk2i=50 (408 K) for the reaction of the tert-butoxyl radical with ionol and cyclohexane [46]. In this case, alkoxyl radicals will react with InH more readily than with RH, provided [InH] > 0.2 mol L-1. 

Alkoxyl radicals react with phenols extremely rapidly. The rate constants of the reactions of tert-butoxyl radicals with some phenols in benzene are given below (Y is the /(-substituent in the phenol molecule) [43,44],... 

One of the methods used in PV to produce the free radicals is similar to the one illustrated in chapter 13, that is, when a substance like di-tert-butylperoxide is irradiated with energy of a suitable wavelength, the 0-0 bond is cleaved and two tert-butoxyl radicals are generated. These radicals can then abstract a hydrogen atom from another molecule RH, yielding the radical R. 

The reaction of thermally and photochemically generated tert-butoxyl radicals with trisubstituted silanes [Eqs. (6) and (7)] has been used extensively for the generation of silyl radicals in ESR studies, in time-resolved optical techniques, and in organic synthesis. Absolute rate constants for reaction (7) were measured directly by LFP techniques,56,62,63 whereas the gas phase kinetic values for reactions of Me3SiH were obtained by competition with decomposition of the tert-butoxyl radical.64,65... 

Clearly there must be a price to pay for determining rate constants by a method where all the participants (reagents and products) are invisible to the technique. The reaction of Figure 18.8 provides an excellent example. In principle, 1,7-octadiene can react with tert-butoxyl radicals by all the mechanisms of Scheme 18.5. The probe method cannot distinguish them, the rate constant obtained (2.3 X 10 A/ s ) includes all possible modes and sites of reaction. It is simply the rate constant with which 1,7-octadiene removes tert-butoxyl from the system. In this case the first reaction in Scheme 18.5 accounts for the reactivity of tert-butoxyl, perhaps with a minor contribution from the second reaction. In all cases, the site and form of reaction must be known independently. Chemical intuition, product... 

Why Are the Cubyl Hydrogens of Methylcubane More Reactive than the Methyl Hydrogens Toward Abstraction by tert-Butoxyl Radicals The experimental puzzle, which our calculations on 4 helped to solve, was posed by Della s finding that terf-butoxyl radicals preferentially abstract hydrogens from the carbons of the cube in methylcubane (6), rather than from the methyl... 

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

An intermediacy of electrophilic alkoxyl radicals (e.g., tert-butoxyl radical) possibly generated in the reaction mixture, which exhibit polarity matching with ethereal a-C-H bonds, may also be considered. Studies on the mechanism are in progress. 

Rate constants for hydrogen abstraction by tert-butoxyl radical... 

Results are selected from Reference 191. RO is tert-butoxyl radical with a-Toc, and cumyloxyl radical with phenol. 

Hydrogen abstraction from bicyclo[4.1.0]heptan-2-ols 30 by tert-butoxyl radicals took place at the a-position, adjacent to the hydroxyl group and cyclopropane ring, to generate substituted bicyclo[4.1.0]hept-2-yl radicals which rearranged to give 3-methylcyclohexanones 31 as the main products. ... 

In practice, tert-butoxyl radicals were generated from di-rert-butyl peroxide as reaction partner, but this led to unrearranged ketones 11 as the major products. The reaction has found some use in spectroscopic studies of the rearrangements. ... 

The relative homolytic reactivities of 2- and 3-methylthiophenes toward hydrogen abstraction from the side chain by tert-butoxyl radicals to form 98 and 99 have been investigated by the spin-trapping method. The... 

It has been proposed that the isotope elFects for the P450-catalyzed oxidations of hydrocarbons and alkylamines are similar to those observed in the reactions of the same substrates with the tert-butoxyl radical The finding that the measured kinetic isotope effects for the hydrogen abstraction from benzylic methyl groups fall on the same line as those for the iV-demethylation of... 

Choi, S.-Y., RE. Eaton, P.F. Hollenberg, K.E. Liiu, S.J. Lippard, M. Newcomb et at. (1996). Regiochemical variations in reactions of methyl-cubane with tert-butoxyl radical, cytochrome P-450 enzymes and a methane monooxygenase system. J. Am. Chem. Soc. 118, 6547-6555. 

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