Triaziridines. Part V. A semiempirical MNDO study of nitrogen inversion and amide rotation in Formyltriaziridines

Since we found certain structural features of triaziridine ( 1 ) obtained by MNDO calculations to be in qualitative agreement with those derived earlier from ab initio calculations, we used the MNDO method to derive properties of formyltriaziridine ( 2 ) and 1-formyl-2,3-diisopropyltriaziridine ( 3 ) as models for the preparatively known 2,3-dialkyl-triaziridine-1-carboxylates 4 and 5 . The main results are: (a) The triaziridine N-atoms with H, alkyl, or formyl as substituents (see 2 and 3 ) are pyramidal. N(1) carrying the formyl group is flatter than N(2) and N(3) with H or alkyl substitent. Bond lengths and angles at N(2) and N(3) are almost identical with those calculated for the N-atoms of 1 . (b) The MNDO inversion barriers at the H-substituted N(2) and N(3) of 2 are higher than those at the formyl-substituted N(1), but similar to the ab initio barriers at the N-atoms of 1 . (c) The MNDO inversion barriers at N(1) of 2 and 3 are 53 to 92 kJ/mol, whereas the rotation barriers around the N(1)–C(4) bond are 7 to 23 kJ/mol; thus, the previously observed dynamic NMR phenomena in trans-2,3-diisopropyltriaziridine-carboxylates ( 5 ) can now be assigned to the slowing down of N(1) inversion rather than N(1)–C(4) rotation.