Possible Reason for the Unusual Regioselectivity in Nucleophilic Ring Opening of Trisubstituted Aziridines under Mildly Basic Conditions

2,2,3-Trisubstituted
aziridines are known to undergo ring opening
at the more substituted carbon under mildly basic conditions. However,
the reason for the formation of the more sterically encumbered product
has never been examined. Several trisubstituted aziridines, with different
substitution patterns at the C-2 and C-3 carbons, were synthesized
to change the electronics of the aziridine ring system. These changes
had no effect on the regioselectivity of the ring-opening reaction.
Using the B3LYP/6-31G* DFT basis set it was determined that the transition
state for opening at the more substituted carbon proceeds at a lower
energy than the transition state at the less substituted carbon.