Ferf-Butyl radical

Of the two extremes, experimental studies indicate that the planar sp model describes the bonding in alkyl radicals better than the pyramidal sp model. Methyl radical is planar, and more highly substituted radicals such as ferf-butyl radical are flattened pyramids closer in shape to that expected for 5/) -hybridized carbon than for sp. ... 

The cumulative effects of multiple substituents have been studied at length in search of particularly stable radicals. It is generally found that the repetitive addition of identical substituents leads to a stepwise decrease in RSE values. This is well illustrated by the comparison of the methyl, ethyl, isopropyl, and ferf-butyl radicals with RSE values of 0.0, - 13.8, - 23.3, and - 28.3 kj/mol. Thus, while the stability of the alkyl radicals clearly increases with the number of alkyl substituents attached to the radical center, the substituent ef-... 

Interconversions of acychc carbon-centered radicals between n and a types are low-energy processes. The methyl radical is planar, but increasing alkyl substitution at the radical center results in an increasing preference for pyramidalization. The ferf-butyl radical is pyramidalized with the methyl groups 10° from planarity (the deviation from planarity for a tetrahedral atom is 19°) and a barrier to inversion of 0.5 kcal/mol. When a radical center is in a carbocycle, a planar radical is favored for all cases except the cyclopropyl radical, and the barrier for inversion in cyclopropyl is only 3 kcal/mol. ... 

The utility of ESR spectra for determining the structures of radicals is demonstrated by considering some examples. Methyl group substitution for hydrogen in the methyl radical ultimately results in slight deviation from planarity with a low inversion barrier. The a values for methyl, ethyl, isopropyl, and terf-butyl are 38.3, 39.1, 41.3, and 45.2 G, respectively. The ferf-butyl radical is indicated to have 10° deviation from planarity, which is confirmed by infrared (IR) and Raman spectroscopy. ... 

In recent years, direct, time-resolved methods have been extensively employed to obtain absolute kinetic data for a wide variety of alkyl radical reactions in the liquid phase, and there is presently a considerable body of data available for alkene addition reactions of a wide variety of radical types [104]. For example, rates of alkene addition reactions of the nucleophilic ferf-butyl radical (with its high-lying SOMO) have been found to correlate with alkene electron affinities (EAs), which provide a measure of the alkene s LUMO energies [105,106]. The data indicate that the reactivity of such nucleophilic radicals is best understood as deriving from a dominant SOMO-LUMO interaction, leading to charge transfer interactions which stabilize the early transition state and lower both the enthalpic and entropic barriers to reaction, with consequent rate increase. A similar recent study of the methyl radical indicated that it also had nucleophilic character, but its nucleophilic behavior is weaker than that expressed by other alkyl radicals [107]. 

Table 8 provides the absolute rates of addition of (CF3)3C, (CF3)2CF-, CF3CF2-, and CF3 to a group of alkenes of variable reactivity. It can be seen from the Table that both the perfluoro-iso-propyl and perfluoro-ferf-butyl radicals give evidence of much greater electrophilicity in their alkene addition reactions. For example, the latter radical reacts significantly (6.8 times) faster than CF3- with the nucleophilic a-methylstyrene (IP = 8.9 eV), while reacting somewhat (1.6 times) slower than CF3- with the more electrophilic pentafluorostyrene (IP = 9.2 eV). Comparative plots of all of the available rate data for alkene additions of CF3- and (CF3)3C- vs alkene IPs, as seen in Fig. 4, leave no doubt as to the relative electrophilicity of the two species. Moreover, the rates of addition of the very electrophilic, but non-o, planar perfluoro-ferf-butyl radical to the more nucleophilic... 

Table 8. Absolute rate constants for the addition of trifluoromethyl, pentafluoroethyl, heptafluoro-iso-propyl, and nonafluoro-ferf-butyl radicals to various olefins at 290 °K in FI 13 [117,118]...

Something that comes as a surprise at first glance is that the ferf-butyl radical is not planar, while the methyl radical is. Deviation from planarity implies a narrowing of the bond angles and thus a mutual convergence of the substituents at the radical center. Nevertheless, the iert-butyl radical with its 40% pyramidalization of an ideal tetrahedral center is 1.2 kcal/ mol more stable than a planar ferf-butyl radical. 

In hydrogen atom abstractions, alkyl radicals change, as the degree of substitution increases, from being mildly electrophilic (the methyl radical) to being mildly nucleophilic (the ferf-butyl radical). In addition reactions to pyridinium cations, the Minisci reaction, they are all relatively nucleophilic, as shown by their... 

The more substituted radicals continue to be measurably the more nucleophilic. The relative rates with which the various alkyl radicals react with the 4-cyan-opyridinium cation (7.33, Y = CN) and the 4-methoxypyridinium cation (7.33, Y = OMe) are given in Table 7.2. The LUMO of the former will obviously be lower than that of the latter. The most selective radical is the ferf-butyl, which reacts 350000 times more rapidly with the cyano compound than with the methoxy. This is because the ferf-butyl radical has the highest-energy SOMO, which interacts (B in Fig. 7.4) very well with the LUMO of the 4-cyanopyridi-nium ion, and not nearly so well (A) with the LUMO of the 4-methoxypyridinium ion. At the other end of the scale, the methyl radical has the lowest-energy SOMO, and hence the difference between the interactions C and D in Fig. 7.4 is not so great as for the corresponding interactions (A and B) of the ferf-butyl radical. Therefore, it is the least selective radical, reacting only 50 times more rapidly with the cyano compound than with the methoxy. 

We can use the data in Table 10.1 to make a similar comparison of the ferf-butyl radical (a 3° radicai) and the isobutyi radicai (a 1 ° radicai) reiative to isobutane ... 

The HAS reaction proceeds via a sigma (a) complex (1) with substitution being completed by the loss of the leaving group Y, which is usually hydrogen (Scheme 9.1, Y = H). Examples where the cyclohexadienyl radicals become trapped by fast reductants to form cyclohexadiene [2] and the detection of radical intermediates by ESR or CIDNP provide evidence that the cyclohexadienyl radicals are intermediates in this reaction [3]. In some systems, the addition of a radical onto the arene is the rate-determining step, because of the loss of aromaticity. For example, the rate constant for the addition of the ferf-butyl radical to benzene at 79°C is 3.8 x 10 M s [4], which is clearly at the lower end of a useful radical reaction. The arene needs to be used at high concentration, or as the solvent, in order to compensate for poor rates. On the other hand, as the rate of addition of the phenyl radical to benzene is 4.5 x 10 s" [5], it is more useful in these kind of reactions. 

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