Sp3 carbon-centered radical

As to the next step, namely, the reaction of aryl radicals with nucleophiles, we should take into account the fact that air molecular orbital, which initially accommodates the incoming electron, is available in the aryl halide. The electron is subsequently transferred in-tramolecularly from the it to the o molecular orbital of the carbon-halogen bond. Aryl radicals effectively scavenge H atoms. Therefore, an abstraction of a hydrogen atom from the solvent may occur. However, in the case of nucleophiles that can act as effective traps of aryl radicals, the addition of a nucleophile to the phenyl radical takes place. At this point, let us focus on the step of addition of the nucleophile (Y ) to the intermediate radical (Ph). When a new a bond begins to form between the sp3 carbon-centered radical (H5C6) and... 

Rate constants for the ring-closure by the sp2 carbon-centered radical are shown in Table 1.3. The rate constants are increased more than those of sp3 carbon-centered radicals, because the sp2 carbon-centered radical is much more reactive than sp3 carbon-centered radicals. This high reactivity of the sp2 carbon-centered radical is reflected by the stronger bond-dissociation energy of the sp2(carbon)-sp3(carbon) bond than that of the sp3(carbon)-sp3(carbon) bond. 

Ring-closure reaction of perfluoroalkenyl radicals, which are strong electrophilic sp3 carbon-centered radicals, is also extremely fast [25-27]. For example, ring-closure of 1,1,2,2-tetrafluoro- and l,l,2,2,3,3-hexafluoro-5-hexen-l-yl radicals is much enhanced relative to the parent radicals, as shown in Table 1.4. 

Rate constants for radical ring closure (s-1, 25 °C) (sp3 carbon-centered radicals)... 

A more interesting reaction is the shift of an sp2 carbon-centered radical to an sp3 carbon-centered radical via 1,5-H shift as shown in eq. 3.53. The formed sp2 carbon-centered radical abstracts a hydrogen atom via 1,5-H shift to generate an sp3 carbon-centered radical which then cyclizes at the olefinic position via 5-exo-trig manner to produce a cyclopentane derivative (147) [157-160]. The rate constant of this 1,5-H shift is approximately 3 X 107 s 1, and is quite fast. 

Since a vinyl radical is also an sp2 carbon-centered radical, a similar 1,5-H shift from the sp2 carbon-centered radical to the sp3 carbon-centered radical occurs, as shown in eq. 3.54, which finally produces heliotridanes (149) via the... 

Radical ring closure of an sp3 carbon-centered radical to a hydrazone group in compounds (174) can be carried out via 5-exo-trig manner. The silicon connection (Si-tethered) is removed by oxidative treatment with H202 to generate 2-amino-1,3-diols (175) (eq. 3.66) [189-191]. 

Reactivities of n-Bu (sp3 carbon-centered radical) and Ph (sp2 carbon-centered radical) to benzene, y-picoline, protonated y-picoline, protonated benzothiazole are shown in Table 5.1. As can be seen here, the reactivity is greatly increased in both radicals, as the electron-density of aromatics is decreased. Therefore, the reactivity of heteroatomatic bases such as y-picoline and benzothiazole, can be increased by their... 

Eq. 5.17 shows ipso substitution of imidazoles (30) at the 2-position by an sp3 carbon-centered radical. Here, the tosyl group plays an important role in both activation of the ipso 2-position and as an effective leaving group, Ts [40-46]. The phosphoryl group also shows the same leaving ability as the tosyl group. Preparation of [1.2-b]-fused bicyclic pyrazoles (33) from l-(phenylselenoalkyl)pyrazoles (32) also works well, as shown in eq. 5.18. 

Since the Barton reaction and the Hofmann-Lofifler-Freytag reaction generate very reactive oxygen-centered and nitrogen-centered radicals respectively, the next 1,5- and 1,6-hydrogen atom abstraction reaction readily happens. However, 1,5-H shift does not proceed effectively by carbon-centered radicals, because there is not so much energy difference between the C-H bond before and after 1,5-H shift. So the reactions are quite limited. Eq. 6.21 shows iodine transfer from reactive 1-iodoheptyl phenyl sulfone (40) to a mixture of 5-iodoheptyl phenyl sulfone (41a) and 6-iodoheptyl phenyl sulfone (41b) initiated by benzoyl peroxide, through 1,5-H shift by an sp3 carbon-centered radical [56-58]. 

However, the sp3 carbon-centered radical does not generally give rise to 1,5-H shift, although the sp2 carbon-centered radical can be used for 1,5-H shift, since it is more reactive than the sp3 carbon-centered radical and it can form a strong C(sp2)-H bond (10 15 kcal/mol stronger than that of C(sp3)-H). Eq. 6.22 shows the product distribution of a direct reduction product (43a) and an indirect reduction product (43b) via 1,5-H shift, in the reaction of arylbromide (42) with Bu3SnD. It suggests that more than half of the amount of reduction product is formed via 1,5-H shift [59]. 

Fluorine substitution enhances the reactivity of not only the sp3 carbon centered radicals, but also of the sp2 radicals such as substituted phenyl radicals toward organic hydrogen atom donors. Table 1.42 summarizes the relative reactivities of some fluorine-substituted... 

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