Trichloromethyl radical proton abstraction reaction

This information is, of course, only of a qualitative nature. To obtain a better picture of alkane reactivity in radical abstraction reactions, the activation barrier was computed for the reaction between alkanes and radical reactant. The example used was the reaction between 2-methylpropane, propane, ethane, and methane as alkanes and the trichloromethyl radical as a radical reactant (Table 16). The B3LYP computed activation barriers were not corrected of zero point energy, which is usually 1-2 kcal/mol. With this correction computed, experimental [119] values should be in excellent agreement. As expected, 2-methylpropane was the most susceptible in the hydrogen radical abstraction reaction. With the activation barrier around 8 kcal/mol, it was possible to perform the reaction at a 

Reaction AEcomo. AEexD I como. Scomo. Scorr. 

2 Intramolecular radical addition to carbon-carbon double bond 

The cause for the regio-and stereoselectivity has been traced to stereoelectronic effects. In order for a bonding interaction to occur, the radical center must interact with the 7i orbital of the alkane (LUMO). According to semiempirical and ab initio calculations, the preferred direction of attack was from an angle of about 70° with respect to the plane of the double bond [124-126]. The 

The most interesting studies included the evaluation of the reactivity of the same radicals for two different reaction paths, 16 and 17. The computational studies correctly selected path 16 as the most reactive one. However, the question remains is there sufficient selectivity to accomplish only transformation 16. The computed selectivity index was 0.0336 (0.1216-0.0980). These values assured the formation of products only through a five-memebered ring formation. This was in full agreement with experimenal results [128]. It is well known that a secondary radical is more stable than a primary radical. The calculations supported this by favoring the cyclohexyl radical by 7.7 kcal/mol over the cyclopentylmethyl radical. Therefore, formation of the six-membered ring product was thermodynamically controlled. 

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Thermolysis of 59 in chloroform (17) led to formation of carbonyl ylide 64, subsequent proton abstraction from chloroform (p fa = 24.1), and recombination with the trichloromethyl anion gave acetal 65. The intermediacy of radicals was discounted since conducting the reaction in neat BuaSnH did not change the product distribution. 

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