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(N)2 variants differing in the degree to which bond breaking is ahead of bond making including perhaps synchronous S(N)2. 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.