Recombination hydrogen atom abstraction

The excited triplet states of quinones can be fairly readily populated by irradiation and nuclear polarization observed (Cocivera, 1968). Hydrogen atom abstraction leads to the relatively stable semiquinone radicals and, in alkaline media, radical anions. Recombination of radical pairs formed in this way can give rise to CIDNP signals, as found on irradiation of phenanthraquinone (20) in the presence of donors such as fluorene, xanthene and diphenylmethane (Maruyama et al., 1971a, c Shindo et al., 1971 see also Maruyama et al., 1972). The adducts are believed to have the 1,2-structure (21) with the methine proton appearing in absorption in the polarized spectrum, as expected for a triplet precursor. Consistently, thermal decomposition of 21 as shown in equation (61) leads to polarization of the reactant but now in emission (Maruyama... 

The most common strategy for laboratory scale hydrocarbon functionalization is hydrogen atom abstraction followed by free radical recombination. Although the conversions are often low, the simplicity of the approach can in some cases make this the method of choice for preparing a particular target molecule. A representative procedure is the cyanation of 2,3-dimethylbutane to give a 77% yield of 2,2,3-trimethylbutanenitrile1. 

The very high stability (10 years at room temperature) of R2M radicals is due to the very bulky R substituents at M hindering recombination, as well as the comparatively low values of the M—H bond energy. Therefore, hydrogen atom abstraction from the hydrocarbon solvent turns out to be unfavourable. 

Oxygen atom from Cpd I is inserted into the C-H bond of saturated hydrocarbons (Scheme la) by means of hydrogen atom abstraction followed by recombination of the transient hydroxyl with the carbon radical [the so-called oxygen rebound mechanism proposed by Groves in 1976 (8, 10)]. Another possibility can be the concerted oxygen insertion into the C-H bond. Both pathways are rationalized by the two-state mechanism developed by Shaik et al. (6, 9), which describes different reactivities... 

Photochemical C—insertion, by abstraction-recombination, can also be used to construct heterocyclic rings. In the example illustrated, l,S-hydrogen atom abstraction leads to the six-membered lactam (117 equation 41). ... 

In the case of vitamin B12, eq 19 is well investigated and its rate constant k = 4 x 109 L/mohs for primary radicals.203 Comparison of this value with the rate constant for methyl radical dimerization of 1010 L/mohs191 indicates that eq 19 proceeds at diffusion-controlled rates. Newer approaches may provide greater accuracy in the measurement of rates of recombination of free radicals, providing a better understanding of hydrogen atom abstraction (eq 18).205... 

The term Rt includes the rate of generation by heat of P" (Eqs. (a) and 3.5) and the possible ways of disappearance of P, for example, hydrogen atom abstraction and recombination. 

When pentamethyldisilanyl radicals were produced by hydrogen atom abstraction in a room-temperature solution, they also underwent disproportionation and recombination reactions238. No evidence was found for fragmentation to dimethylsilylene and a trimethylsilyl radical. 

All the known oxidative reactions catalyzed by CYP450 monooxygenase can be described in the context of a mechanistic scheme involving the ability of a high-valent iron oxenoid species to bring about the stepwise one-electron oxidation through the abstraction of hydrogen atoms, abstraction of electrons from heteroatoms, or addition to tr-bonds. A series of radical recombination reactions completes the oxidation process. 

It is not always easy to establish that an authentic insertion reaction has occurred. An alternative route to the same product is available it involves hydrogen-atom abstraction by the triplet carbene, followed by recombination of the fragments ... 

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