A Theoretical and Experimental Study on Acid-Catalyzed Isomerization of 1-Acylaziridines to the Oxazolines. Reexamination of a Possible S(N)i Mechanism by Using ab Initio Molecular Orbital Calculations.

The S(N)i mechanism, which was previously proposed for the isomerization of 1-acylaziridines to the oxazolines, was reexamined theoretically by performing molecular orbital (MO) calculations of 1-formylaziridine and its derivatives as model compounds and experimentally by using 1(R)-[alpha-methoxy-alpha-(trifluoromethyl)phenylacetyl]-2(S)-methylaziridine (5). At the MP2/6-31G//RHF/6-31G level, the activation energy was estimated to be 38.9 kcal mol(-1) for the S(N)i mechanism in which N-protonated 1-formylaziridine 8a(NH(+)) isomerizes to the N-protonated oxazoline 9a(NH(+)).