R-Butyl radical

In contrast, one need not consult an Aldrich catalog to see if r-butyl radical is commercially available. Nor can one generate a significant quantity of r-butyl radical in solution prior to the addition of a reagent with which it is destined to react. This is because r-butyl radical, like nearly every other radical that is employed in synthesis, is transient. The lifetime of a transient radical rarely exceeds 1 ps and is limited by the rate of self reaction. This self reaction, which can be either a disproportionation or a recombination (Scheme 2), typically occurs at the diffusion-controlled limit the enthalpy of activation of most radical-radical reactions = 0 kcal mol-1. 

The medium has no specific oxidative action towards the alkyl radicals, and in the presence of protonated base the major reaction is a substitution. Thus, imidazoles and 1-substituted imidazoles are alkylated exclusively at C-2, albeit in rather low yields. The use of isopropyl and r-butyl radicals gives improved yields (80-90%) but benzyl and allyl radicals tend to dimerize in preference. Benzimidazoles are also alkylated at C-2, and with isopropyl and t-butyl radicals yields of 50-80% can be achieved (80AHC(27)24l, 74AHC(16)123). 

Kinetic measurements of reactions of t-BuMgCl with 19 different substituted benzophenones led the authors to the conclusion that, in all of these reactions, the most probable common rate-limiting step is the electron transfer from the Grignard reagent to the ketone to form the ketyl radical anion and the r-butyl radical. This brought them to the statement that This would indicate that the radical path is not a side reaction, but rather constitutes the main, if not the only mechanism [44]. 

The mechanism of this reaction was studied 11011 and was found to involve the production of a r-butyl radical and a magnesium ester hcniikctal radical, which could recombine to form the normal... 

We start with the extensive and rigorous study of Schuh and Fischer on r-butyl radical self-termination (6). Earlier work on this reaction, reviewed in Ref. 6, had yielded k values in the 10 -10 ° s range with poor agreement among... 

Figure 9. Experimental vs. calculated termination rate constants for r-butyl radicals in ( ) n-heptane, (O) n-octane, ( ) n-decane, ( ) n-dodecane, ( ) n-tetradecane, (S/) n-hexadecane, ( ) acetonitrile, and C /) benzene see text. The line has unit slope. From Ref. 6. 

Figure 10. 2kf vs. for r-butyl radical self-termination in ( ) n-heptane and (O) acetonitrile(6) seetext.ThelineisbasedonEqs.6and61 withp = 5.6 Aando-= A. 

An example of the synthetic application of this reaction is the introduction of a benzoate substituent into the 7-position of norbornadiene. Decomposition of t-butyl peroxybenzoate is effected by Cu(I). The r-butyl radical then abstracts hydrogen from norbornadiene. The Cu(II) is regenerated by oxidation of the resulting radical. The cation then captures benzoate ion, giving the product. 

The heat of formation of r-butyl cation is critical to the anchoring of the PA scale, but its value has recently shifted by about 21 kJ mol" to what appears, with guidance from G2 level theoretical calculations, to be an accurate number. The problem has come from some inaccurate appearance energies, difficulties in the determination of the heat of formation of r-butyl radical, inconsistencies between different PA scales, and a lack of accurate entropy data for conversion of GBs to PAs. Each of these experimental problems now appears to have been resolved with improvements in experimental methodology and accuracy, and the agreement between theory and experiment in Table 3 is now very good. 

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