Hydrogen-atom abstraction from dimethylamine in solution: EPR spectroscopic studies and ab initio molecular orbital calculations

Abstract

The reactions of photochemically-generated tert-butoxyl and of bis(trimethylsilyl)aminyl radicals with dimethylamine in cyclopropane solution have been studied using EPR spectroscopy. Both radicals abstract hydrogen competitively from the NH and CH groups to produce the radicals Me₂N˙ and MeNHĊH₂, respectively, but the silylaminyl radical shows a greater propensity to attack the less sterically hindered methyl groups. Analogous trends are shown in hydrogen-atom abstraction from propane, the isoelectronic hydrocarbon analogue of dimethylamine. The regioselectivity of hydrogen abstraction from dimethylamine depends upon the concentration of the amine, because association by hydrogen-bonding reduces the number of reactive NH groups relative to the CH groups. The activation energy for hydrogen-atom transfer to Bu^tO˙ from the NH group of monomeric dimethylamine is 4.6 kJ mol^–1 less than that for transfer from a CH group, while the Arrhenius A-factor for abstraction from the methyl groups is 3.4 times greater than that for abstraction from the NH group, presumably mainly for statistical reasons. Absolute rate constants for hydrogen abstraction have been determined in competition experiments with tetrahydrofuran. Hydrogen-bonding to Me₂N˙ produces changes in its EPR spectrum in the same direction as, but of smaller magnitude than, does full protonation. Ab initio molecular orbital calculations have been carried out for the reactants, products and transition states involved in hydrogen-atom abstraction from dimethylamine by the methoxyl radical, as well as for the hydrogen-bonded complexes formed between Me₂N˙ and methanol and between MeO˙ and dimethylamine. The dissociation enthalpies of the NH and CH bonds in dimethylamine are computed to be larger than the currently accepted experimental values and the activation energies for hydrogen-atom transfer from the amine to the methoxyl radical are in good agreement with the experimental values for transfer to the tert-butoxyl radical. The experimental and theoretical results are compared with the predictions of a previously-published empirical algorithm for the estimation of activation energies for hydrogen-atom transfer processes.