Hafnium(IV)-Salen-Catalyzed Highly Reactive and Enantioselective Epoxidation Directed by Amides

Substrate-directed
enantioselective olefin epoxidation was limited
to a few types of directing groups and relatively low catalyst activities.
Herein, an expeditious asymmetric epoxidation protocol is reported
for allylic amides and sulfonamides using a hafnium-BINAM-salen catalyst
with high enantioselectivity and turnover frequency. Aryl carboxamides
of allylic amines with a trisubstituted olefin motif were suitable
substrates. Sequential intramolecular ring-opening in a one-pot reactor
furnished highly enantiopure oxazoline alcohols. Crystallographic
studies suggested the catalyst possessed a mononuclear cis-α configuration. Computational studies of the transition state
indicated that this conversion underwent a substrate-directed pathway.