Communications
DOI: 10.1002/anie.200905332
Asymmetric Epoxidation
Asymmetric Counteranion-Directed Transition-Metal Catalysis:
Enantioselective Epoxidation of Alkenes with Manganese(III) Salen
Phosphate Complexes**
Saihu Liao and Benjamin List*
Dedicated to Professor Eric N. Jacobsen on the occasion of his 50th birthday
Inspired by pioneering contributions on chiral Brønsted acid
mediated reactions[1] and our own studies in aminocatalysis,[2]
we are currently exploring asymmetric counteranion-directed
catalysis (ACDC) as a general strategy for asymmetric
synthesis.[3] According to this concept, catalytic reactions
that proceed via cationic intermediates can be performed
highly enantioselectively by the incorporation of a chiral
counteranion into the catalyst. After our initial proof of
concept with organocatalytic transfer hydrogenations and
epoxidations,[3,4] ACDC has recently been extended to
transition-metal catalysis with Tosteꢀs gold-catalyzed allene
cyclizations,[5] and our palladium-catalyzed Tsuji–Trost-type
a-allylation of aldehydes.[6] The further exploration of the
potential of ACDC in transition-metal catalysis, especially
applied to redox reactions, is of great interest and signifi-
cance.[7] Here we report a highly enantioselective epoxidation
of olefins that is catalyzed by a chiral ion pair consisting of an
achiral MnIII–salen cation and a chiral phosphate counter-
anion.
and chiral N-oxides have been used in combination with an
achiral Mn–salen complex and shown to shift the equilibrium
of enantiomeric conformational isomers of the cationic metal
complex to one side. Reasonably good enantioselectivities
have been achieved in the corresponding epoxidations.[11]
We hypothesized that a chiral counteranion should also be
able to induce a preference for one of the two enantiomorphic
conformations. Specifically, chiral binol-derived phosphate
anions are ideally suited for our purposes, because in addition
to possibly inducing one enantiomorphic conformation of the
cationic complex, these ions may also amplify the chiral
microenvironment around the metal center with suitable
substituents at the 3,3’-positions (Figure 1). Overall, this may
lead to a new type of chiral Mn–salen catalyst with unique
properties.
Stimulated by an important contribution from Kochi
et al., Jacobsen and Katsuki have significantly advanced the
catalytic asymmetric epoxidation of unfunctionalized alkenes
by introducing chiral MnIII–salen catalysts.[8,9] These com-
plexes display a broad substrate scope although certain olefin
classes still fail to be converted with high enantioselectivity.
Interestingly, cationic Mn–salen complexes are C2-symmet-
rical and inherently chiral—even when the salen ligand itself
is achiral. In case of the Jacobsen–Kastuki epoxidation, the
chiral backbone of the salen ligand fixes the complex in one of
the two enantiomorphic confirmations. The neutral donor
ligands typically added increase reactivity and enantioselec-
tivity by displacing the apically coordinated anion of the Mn
complex.[9e,10] Chiral neutral donor ligands such as sparteine
Figure 1. Design principle and modeling of a chiral ion-pair epoxida-
tion catalyst (R=CH3 in 3D model).
We initially combined the achiral Mn–salen cation 1a with
different binol-derived phosphates 2. The ion-pair catalysts
form quickly when the corresponding phosphoric acids and
the Mn–salen chloride complex are mixed in the presence of
1 equivalent of aq. NaOH, and can be used directly. Chro-
mene 3 was chosen as a model substrate because it is
particularly suited for Jacobsen–Katsuki-type epoxidations
and its product is of potential value as a pharmaceutical
intermediate.[12] In the presence of 10 mol% of the ion-pair
catalyst and 1.2 equivalents of iodosobenzene (PhIO) as the
oxidant, the epoxidation of chromene 3 proceeded highly
effectively in benzene at room temperature (see Table 1).
[*] S. Liao, Prof. Dr. B. List
Max-Planck-Institut fꢀr Kohlenforschung
Kaiser Wilhelm-Platz 1, 5470 Mꢀlheim an der Ruhr (Germany)
Fax: (+49)208-306-2982
E-mail: list@mpi-muelheim.mpg.de
[**] Generous funding by the China Scholarship Council (fellowship to
S.L.), the Max-Planck-Society, the Deutsche Forschungsgemein-
schaft (SPP 1179, Organokatalyse), and the Fonds der Chemischen
Industrie (award to B.L.) is gratefully acknowledged. We thank Dr.
Artur Pinto and our GC, HPLC, and X-ray departments for their
support.
Supporting information for this article is available on the WWW
628
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 628 –631