Primary alkyl radical spin trapping

The tri-t-butylnitrosobenzene, TNB, is monomeric even in the solid state, but the principal advantage of this scavenger, exemplified in the mechanistic studies described in Section 3 (p. 47), is that it functions as an ambident spin trap (Terabe and Konaka, 1973). Thus, primary alkyl radicals add to form nitroxides in the normal way, but with t-alkyl radicals, addition occurs at oxygen, alkoxyaminyl radicals (ArNOR) being formed. Secondary alkyl radicals give mixtures of both species (Fig. 5). The alkoxyaminyl radicals have a lower g-value than the nitroxides (ca. 2.004 vs. 2.006) and their spectra are therefore centred at slightly higher field positions than those of the nitroxides. 

A more recent example is found in the work of Schmid and Ingold (1978), who used the rate of rearrangement (17) of 5-hexenyl radicals into cyclopentylmethyl radicals (R- and R - in Scheme 5) to time the spin trapping of primary alkyl radicals. In this system, both R and R are primary alkyl, and their spin adducts with several traps therefore have virtually indistinguishable spectra. This difficulty was circumvented by labelling C-l in the hex-5-enyl radical with 13C the unrearranged radical then gives spin... 

Xenon difluoride reacted with various carboxylic acids, and the type of transformation depends on the structure of the organic molecules35-39. The reaction with primary carboxylic acids involves free-radical intermediates. 6-Hexenoic acid was used as a free-radical clock device in which a A abs of 1.1 x 106 M-1s-1 at 25 °C was determined, while the alkyl radical was also spin-trapped to give an ESR signal37. The primary free radical was trapped by internal cyclization, and (fluoromethyl) cyclopentane in 25% yield was formed, while 6-fluoro-l-hexene could be formed from a radical or ionic intermediate, but 1-fluo-rocycloclohexane was not observed as a product (Scheme 42). 

Our initial kinetic E.S.R. study of this reaction (T.) has now led us into a quantitative investigation of the "spin-trapping (2U) of JU and other primary alkyl radicals. 

Table II. Some Rate Constants for the Spin-Trapping of Primary Alkyl Radicals in Benzene at i+0°C...

A simple extension of the competition technique is to the comparison of scavenger efficiencies. Thus pairs of spin traps have been allowed to compete for a variety of radicals, including t-butoxyl, phenyl, and primary alkyl. Much more revealing, however, is the type of experiment in which the bimolecular trapping of a radical is allowed to compete with some other reaction of that radical whose absolute rate constant is known. In this way, the rate constant for the trapping reaction itself is accessible. 

In a reducing environment, conditions may allow for the same type of mechanism to occur, but with the radical anion of the spin trap as the intermediate. Actually, the possibility of radical ion-mediated spin trapping was first discussed in a study of a reductive system, namely in the search for radical intermediates in the reaction between alkanethiolates and alkyl halides conducted in the presence of TBN [2] (Crozet et al., 1975). TBN is known to trap primary radicals with formation of nitroxides (attack of R at N), and it was therefore anomalous to find alkoxyaminyl radicals (attack of R at O) in the above reaction. It was suggested that the alkanethiolate or some other reductant reduces TBN to its radical anion, which attacks the alkyl halide via oxygen in an SN2 fashion, as in equations (8) and (9) (see p. 129). 

The nucleophilicity of O2 - toward primary alkyl halides (Scheme 7-2) results in an Sn2 displacement of halide ion from the carbon center. The normal reactivity order, benzyl>primary>secondary>tertiary, and leaving-group order, I>Br>OTs>Cl, are observed, as are the expected stereoselectivity and inversion at the carbon center. In dimethylformamide the final product is the dialkyl peroxide. The peroxy radical (ROO), which is produced in the primary step and has been detected by spin trapping,25 is an oxidant that is readily reduced by O2 -to form the peroxy anion (ROO ). Because the latter can oxygenate Me2SO to its sulphone, the main product in this solvent is the alcohol (ROH) rather than the dialkyl peroxide. 

Since the rate constants for this, and for related ( ), cyclizations are known these reactions can be used to determine the rates of reaction of primary alkyls with such diverse species as cupric ion (32, 37) and tert-butyl hypochlorite (38). However, there can be no doubt that some of the most interesting radical-molecule reactions are those involved in spin-trapping . This is a technique which has been used qualitatively for several years to detect and identify transient free-radicals (2U). Its quantitative use in mechanistic studies has been hampered by the paucity of data available for the rate constants, k, for the addition of radicals to the spin traps, T. 

What little rate data is available rests on competitive experiments with reactions having known rate-constants but, unfortunately, even these "known rate constants are somewhat uncertain. In view of the great potential of spin-trapping we have begun a program to determine accurate rate constants for the trapping of some commonly encountered radicals, starting with primary alkyls (39). 

X 10 sec i for the reaction ChIctIZ HCH2 — 6H2CH2CH=CH2. These isomerizations provide useful probes in mechanistic studies and can be used to determine the rates with which primary alkyls react with a variety of molecules. The latter use is illustrated by the measurement of the rates of spin trapping of the 5-hexenyl radical by some common traps. Aryl radical isomerizations that proceed by quantum-mechanical tunneling are reviewed briefly. 

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