Stereochemistry in Trivalent Nitrogen Compounds. 30. Torsional and Inversional Barriers in Sulfenylaziridines

The barriers to nitrogen inversion in a series of N-benzenesulfenyl-2,2-dimethylaziridines substituted in the sulfenyl phenyl ring have been measured using nuclear magnetic resonance spectroscopy. Substituents, coalescence temperatures (°C), and free energies of activation (kcal/mol) were: p-OCH₃, −20, 12.6; p-CH₃, −21, 12.5; H, −23, 12.4; p-Cl −21, 12.5; p-Br, -24, 12.4; p-NO₂, −17, 12.8, o,p-(NO₂)₂, −10, 13.3. Hammett analysis afforded a reaction constant (at 300 °C) of −0.3 ± 0.1. The small magnitude and negative sign argue against the possibility that d-orbital conjugation has a significant effect on nitrogen inversion barriers in sulfenylaziridines. On the other hand, low coalescence temperatures and free energies of activation were obtained for N-trifluoromethanesulfenyl-2,2-dimethylaziridine (−61 °C, 10.4 kcal/mol) and N-trichloromethanesulfenyl-2,2-dimethylaziridine (−86 °C, 9.2 kcal/mol). An analysis of steric effects indicated that the barriers in these compounds were 2-3 kcal/mol lower than could be accounted for on the basis of steric factors. This additional lowering was ascribed to an electronegativity effect involving overlap between the nitrogen lone pair orbital with the antibonding orbital associated with the S-C σ-bond, i.e., π-conjugation or negative hyperconjugation. The experimental data obtained also serve to provide upper limits to the barriers to torsion about N-S bonds in sulfenylaziridines. These limits indicate that torsional barriers are considerably lower in some sulfenylaziridines than in their acyclic analogues. Possible reasons for this lowering are discussed.