Insights on the Origin of Regiodivergence in the Parallel
Kinetic Resolution of <i>rac</i>-Aziridines Using a Chiral
Lanthanum–Yttrium Bimetallic Catalyst
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Abstract
Parallel
kinetic resolution of racemic mixtures is an important
method used in asymmetric synthesis of chiral compounds. In a recent
example, a <i>rac</i>-<i>cis</i>-2,3-substituted
chiral <i>N</i>-benzoyl aziridine was reacted with dimethyl
malonate, in the presence of a La–Y heterobimetallic chiral
BINAM Schiff base (L) catalyst, to form enantiomerically pure (ee
> 98%) γ-amino acid derivatives through a ring-opening reaction
in near-quantitative yields from both the enantiomers (∼48%).
High regio- and enantioselectivities even with a <i>rac</i>-aziridine, having C2 and C3 substituents as similar as ethyl and <i>n</i>-propyl. Through a comprehensive computational investigation,
we delineate the origin of regio-divergent and enantioselective formation
of γ-amino ester derivatives. The Gibbs free energy of the transition
state for the ring-opening at the propyl substituted C2 carbon leading
to 3-benzamidoheptan-4-yl malonate is found to be 7.2 kcal/mol lower
than that at the ethyl substituted C3 carbon in the case of (2<i>R</i>,3<i>S</i>)-aziridine. A reversal of the regio-chemical
preference for its enantiomeric (2<i>S</i>,3<i>R</i>)-aziridine is noted where the ring-opening occurs at the ethyl substituted
C3 carbon. The La–Y catalyst is found to initially “recognize”
both the enantiomers of the <i>rac</i>-aziridine rather
indiscriminately. The activation barriers for the most-preferred ring-opening
for each enantiomer are found to be closely similar, suggesting that
both enantiomers would react. The high regio-selectivity in the addition
of lanthanum-bound malonate to the aziridine anchored onto the yttrium
center is due to a unique geometric disposition of the aziridine in
the stereocontrolling ring-opening transition state. The lowest-energy
ring-opening transition state for each enantiomer of aziridine exhibited
very similar geometries, while notable geometric distortions is identified
in the malonate addition to less-preferred site of the same enantiomer