Hydrogen abstraction from primary amines. Substituent effects on the carbon-nitrogen bond rotation barriers in aminoalkyl radicals

Hydrogen abstraction by t-butoxyl radicals from primary amines RCH ₂NH₂ (R = H, Me, Et, Prⁱ, or Bu^t) gave mainly α-aminoalkyl radicals which were observed by e.s.r. spectroscopy. The barriers to rotation about the C-N bonds were determined from exchange broadening. For 2-fluoro- and 2-chloro-ethylamine the initial α-aminoalkyl radical lost hydrogen halide to give a mixture of syn- and anti-1-aza-allyl radicals. In contrast, the 1-amino-2,2,2-trifluoroethyl radical was stable under the same experimental conditions. The α-amino radicals formed on hydrogen abstraction from the methyl and t-butyl esters of glycine and the t-butyl ester of alanine were also examined. The stabilisation energy of the α-aminomethyl radical was estimated from the C-N bond rotation barriers of the aminoalkyl radicals. The pattern of stabilisation was found to be consistent with an extra stabilisation in radicals with captodative substitution.