Iron-Catalyzed Highly Enantioselective cis-Dihydroxylation of Trisubstituted Alkenes with Aqueous H2O2

Abstract

© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Reliable methods for enantioselective cis-dihydroxylation of trisubstituted alkenes are scarce. The iron(II) complex cis-α-[FeII(2-Me2-BQPN)(OTf)2], which bears a tetradentate N4 ligand (Me2-BQPN=(R,R)-N,N′-dimethyl-N,N′-bis(2-methylquinolin-8-yl)-1,2-diphenylethane-1,2-diamine), was prepared and characterized. With this complex as the catalyst, a broad range of trisubstituted electron-deficient alkenes were efficiently oxidized to chiral cis-diols in yields of up to 98 % and up to 99.9 % ee when using hydrogen peroxide (H2O2) as oxidant under mild conditions. Experimental studies (including 18O-labeling, ESI-MS, NMR, EPR, and UV/Vis analyses) and DFT calculations were performed to gain mechanistic insight, which suggested possible involvement of a chiral cis-FeV(O)2 reaction intermediate as an active oxidant. This cis-[FeII(chiral N4 ligand)]2+/H2O2 method could be a viable green alternative/complement to the existing OsO4-based methods for asymmetric alkene dihydroxylation reactions.