Aziridines. 59. Regioselectivity in nucleophilic Ring Opening of 2‐Methylaziridines. Lag of bond making as model for the abnormal opening

The regioselectivity ratio RS = normal: abnormal opening of activated 2‐methylaziridines 2 by nucleophiles is found to range from 0.10 to unmeasurable large (only normal opening = substitution at CH₂ by strongly basic carbanions). RS is assumed to result from S_N2 variants differing in the degree to which bond breaking is ahead of bond making including perhaps synchronous S_N2. Bond breaking will be more ahead for the N‐CMe bond. High nucleophilic power pushes bond making toward a synchronous process resulting in great RS. The decrease in RS with acyl activation relative to sulfonyl activation is in accord with a flattening of the nitrogen pyramid (planarization effect). The planarization effect is retained in acidic medium by O‐protonation: Rs 0.10–0.14 for methanolysis as compared to RS 0.43 for N‐protonated sulfonylaziridine 2h. AM1 calculations support the planarization hypothesis. – No indication for SET with trityl anion was found.