Addition vs abstraction reactions of the methyl radical with nitrones, alkenes, aldehydes, and imines

In continuation of our theoretical study of spin trapping, we have compared the abilities of nitrones, imines, aldehydes, and alkenes to add the methyl radical vs their tendency to have protons abstracted by this radical. This study confirms that for nitrones, whereas abstraction of the iminyl H is exothermic with a low activation energy, addition is even more favored, both thermodynamically and kinetically, with the addition occurring at the unsaturated carbon. For alkenes, the preferred process is also addition, but the reactions are considerably less exothermic and have higher activation energy barriers than those for nitrones. Aldehydes favor abstraction; the bulkier the group on the carbonyl carbon, the more the abstraction is preferred over addition. Imines are intermediate between alkenes and aldehydes in their tendency toward addition and abstraction. As a result, they can undergo either process, and the favored route entirely depends on the substituents present; moreover, addition can occur to either the carbon or the nitrogen with appropriate substituents.