Chiral-at-Iron Catalyst: Expanding the Chemical Space for Asymmetric Earth-Abundant Metal Catalysis

Article abstract of DOI:10.1021/jacs.9b01352

A new class of chiral iron catalysts is introduced that contains exclusively achiral ligands with the overall chirality being the result of a stereogenic iron center. Specifically, iron(II) is cis-coordinated to two N-(2-pyridyl)-substituted N-heterocycli

Full text of DOI:10.1021/jacs.9b01352

Communication
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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Chiral-at-Iron Catalyst: Expanding the Chemical Space for
Asymmetric Earth-Abundant Metal Catalysis
Yubiao Hong, Lucie Jarrige, Klaus Harms, and Eric Meggers*
̈
Fachbereich Chemie, Philipps-Universitat Marburg, Hans-Meerwein-Strasse 4, 35043 Marburg, Germany
S
* Supporting Information
ABSTRACT: A new class of chiral iron catalysts is
introduced that contains exclusively achiral ligands with
the overall chirality being the result of a stereogenic iron
center. Specifically, iron(II) is cis-coordinated to two N-
(2-pyridyl)-substituted N-heterocyclic carbene (PyNHC)
ligands in a bidentate fashion in addition to two
monodentate acetonitriles, and the dicationic complex is
complemented by two hexafluorophosphate ions. De-
pending on the helical twist of the PyNHC ligands, the
metal center adopts either a Λ or Δ absolute
configuration. Importantly, the two PyNHC ligands are
constitutionally and configurationally inert, while the two
acetonitriles are labile and allow asymmetric transition
metal catalysis. This is demonstrated with an enantiose-
lective Cannizzaro reaction (96% yield, 88% ee) and an
asymmetric Nazarov cyclization (89% yield, >20:1 dr, 83%
ee).
hiral transition metal complexes are an important class of
Casymmetric catalysts and typically synthesized by reacting
metal salts or organometallic precursors with carefully tailored
chiral ligands (Figure 1a).1 Recently, an alternative to this
Figure 1. Chiral transition metal catalysis. (a) Design from chiral
ligands versus achiral ligands. (b) Combining configurational stability
of a metal center with some labile ligands. (c) Chiral-at-iron catalyst
developed in this study.
modular chiral-ligand-plus-metal design has emerged in which
chiral transition metal catalysts exclusively consist of achiral
ligands.2,3 In this approach, the essential overall chirality is the
consequence of an asymmetric coordination of the achiral
ligands around the central metal, thereby implementing metal-
centered chirality (Figure 1b). Proof-of-principle for such chiral-
at-metal catalysts has been demonstrated recently for the
precious metals iridium,4 rhodium,5,6 and ruthenium7,8 by us
and others, but the design of reactive chiral-at-metal catalysts
based on earth-abundant metals, which have economical and
environmental benefits, is elusive.9 This can be pinpointed to the
much higher lability of coordinative bonds of 3d as compared to
4d and 5d transition metals, and it is an unresolved challenge to
combine a configurationally inert metal stereocenter with a
reactive metal center in a single transition metal catalyst.
The design strategy is especially appealing for its combination
of sustainability (base metals)10 and simplicity (achiral ligands).
More importantly, it is expected that without the requirement
for chiral structural motifs in the ligand sphere untapped
opportunities emerge for the design of earth-abundant metal
complexes with new electronic properties and structural
architectures that are expected to provide distinct catalytic
properties for applications in academia and industry.
Here we report a chiral transition metal catalyst scaffold that is
assembled exclusively from achiral mono- and bidentate ligands
around the metal iron, the most abundant metal on earth (Figure
1c).11−13 This work demonstrates the feasibility of designing
chiral-at-metal catalysts from earth-abundant metals and
provides a blueprint for a whole new class of earth-abundant
metal asymmetric catalysts.
The chiral-at-iron catalyst design goes back to related racemic
complexes first reported by Hahn.14 Two N-(2-pyridyl)-
substituted N-heterocyclic carbene bidentate ligands
(PyNHC) provide a helical arrangement with metal-centered
Λ (left-handed helix) or Δ configuration (right-handed helix). A
CF3 group at the 5-position of the pyridyl group was
incorporated to provide steric hindrance for an increased
asymmetric induction and to remove electron density for higher
configurational and air stability. These two cis-coordinated
Received: February 4, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.9b01352
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
bidentate ligands are complemented by two labile acetonitrile
ligands and two hexafluorophosphate counterions. The racemic
version of the complex rac-Fe1 was synthesized from elemental
iron following a procedure by Chen by converting the
imidazolium salt 1 into its silver-carbene complex followed by
electrolysis in MeCN using an iron plate as a sacrificial anode to
provide rac-Fe1 in a yield of 70% over two steps (Figure 2).15
Figure 3. Single-crystal X-ray structure of Δ-Fe1 (CCDC 1892226).
ORTEP drawing with 50% probability thermal ellipsoids. Solvent and
counterion are omitted for clarity.
Figure 4. Circular dichroism spectra of Λ- and Δ-Fe1 (MeCN at 1.0
mM).
Figure 2. Synthesis of enantiomerically pure chiral-at-iron complexes
Λ- and Δ-Fe1.
Reaction of the racemic mixture with the chiral auxiliary (S)-2 or
(R)-2 in the presence of Et3N provided the complex Λ-(S)-Fe2
or Δ-(R)-Fe2 as single enantiomers in 43% and 42% yield,
respectively. Finally, treatment of the individual complexes with
NH4PF6 in MeCN at 40 °C afforded the individual enantiomers
Λ- and Δ-Fe1 in yields of 84% and 83%, respectively.
This configurational stability, which is clearly distinguished from
typical iron(II) complexes17 with bidentate ligands, can be
rationalized with the electronic nature of the PyNHC ligand. A
strong σ-donating NHC moiety18,19 is combined with a σ-
donating and significantly π-accepting pyridyl ligand, the latter
of which is further increased by the electron-withdrawing effect
of the CF3 group.20 It is established that kinetic and
thermodynamic properties of transition metal complexes
correlate, among other parameters, with the ligand field
stabilization energy, which increases in octahedral complexes
with strong σ-donating and π-accepting ligands.21 At the same
time, the kinetic trans-effect of the σ-donating NHC ligand
assures a high lability of the two acetonitrile ligands.22 Thus, the
cis-coordinated PyNHC ligands with the two MeCN ligands in
trans-orientation to the two NHC ligands provide a structural
blueprint for combining configurational stability of the metal
stereocenter with a high reactivity of the monodentate ligands.
The same design principle has already resulted in configura-
tionally very stable chiral-at-ruthenium catalysts,8 but it was
unexpected by us that this can be even applied to the much more
labile congener iron.
A crystal structure of Δ-Fe1 is shown in Figure 3 and reveals
the relative and absolute metal-centered configuration. Note-
worthy are also the interligand π-stacking interactions of the
mesityl moieties with the pyridyl groups of the other respective
PyNHC ligand. CD spectra shown in Figure 4 furthermore
confirm the optical activity and mirror-imaged structures of the
complexes Λ- and Δ-Fe1. The enantiomeric purity of these
diamagnetic low-spin complexes was determined to be ≥99:1 er
by 1H NMR analysis after coordination to the chiral ligand (R)-
α-methoxyphenylacetic acid (see Supporting Information).16
Importantly, the chiral-at-iron complex Fe1 is constitutionally
and configurationally surprisingly robust and can be handled
under air without any decomposition. At room temperature in
solution overnight, Δ-Fe1 does not show any decomposition or
racemization as determined by 1H NMR (see Supporting
Information) and CD spectroscopy (Figure 4), respectively.
B
DOI: 10.1021/jacs.9b01352
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
a
Next, we investigated the catalytic properties of the new
chiral-at-iron complex. Inspired by recent work from Tang on
enantioselective intramolecular Cannizzaro reactions of glyoxal
monohydrates,23,24 we found that Fe1 can smoothly catalyze the
conversion of phenylglyoxal monohydrate (3) to mandelate
ester (4) (Table 1). Under optimized reaction conditions, 5 mol
Table 2. Asymmetric Nazarov Cyclization Catalyzed by Fe1
b
c
d
e
entry
conditions
c (M)
yield (%)
dr
ee (%)
Table 1. Enantioselective Intramolecular Cannizzaro
Reaction Catalyzed by Λ- or Δ-Fe1
f
a
1
2
3
4
5
6
standard
standard
HFIP
chloroform
toluene
THF
0.2
0.05
0.2
0.2
0.2
0.2
89
>20:1
>20:1
>20:1
>20:1
>20:1
83
61
71
34
89
81
28
5
g
n.d.
0
a
b
c
d
Standard conditions: 5 (0.025 mmol) with Fe cat. (0.00125 mmol)
entry cat. (mol %)
R
conditions
yield (%)
ee (%)
e
in CH2Cl2 (0.125 mL) stirred at rt for 24 h under nitrogen.
1
2
3
4
5
6
7
8
Λ-Fe1 (5.0)
Δ-Fe1 (5.0)
Λ-Fe1 (3.0)
Λ-Fe1 (3.0)
Δ-Fe1 (3.0)
Δ-Fe1 (3.0)
Λ-Fe1 (3.0)
Λ-Fe1 (3.0)
Λ-Fe1 (3.0)
Λ-Fe1 (3.0)
Λ-Fe1 (3.0)
iPr
iPr
iPr
iPr
iPr
iPr
Et2CH
tBu
nPr
Et
standard
standard
standard
99 (96)
99
99
0
98
80
50
11
98
87.5 (R)
87.0 (S)
86 (R)
b
c
Deviations from standard conditions are shown. Isolated yields.
d
Diastereoselective ratio determined by 1H NMR analysis of the
e
crude product. Enantiomeric excess determined by HPLC analysis of
purified products on a chiral stationary phase. Isolated yield from a
0.1 mmol scale reaction. Not determined.
no 4 Å MS
toluene
f
g
32 (S)
20 (S)
67 (R)
THF
standard
standard
standard
standard
standard
f
n.d.
9
10
11
68 (R)
54 (R)
10 (R)
of chiral earth-abundant metal complexes for asymmetric
catalysis.
98
91
Me
a
ASSOCIATED CONTENT
Standard conditions: 3 (0.05 mmol), ROH (0.5 mmol), 4 Å MS (25
■
S
mg powder), and Fe cat. (0.0015 or 0.0025 mmol) in CH2Cl2 (1.0
* Supporting Information
b
mL) stirred at rt for 16 h under nitrogen. Deviations from standard
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.9b01352.
c
conditions are shown. Determined by 1H NMR of the crude product
d
using Cl2CHCHCl2 as internal standard. Enantiomeric excess
determined by HPLC analysis of purified products on a chiral
stationary phase. Isolated yield in brackets (0.2 mmol scale). Not
determined.
Experimental details, NMR spectra, and HPLC traces
(PDF)
X-ray crystallographic data (CIF)
e
f
AUTHOR INFORMATION
Corresponding Author
*meggers@chemie.uni-marburg.de
ORCID
Eric Meggers: 0000-0002-8851-7623
Notes
The authors declare no competing financial interest.
■
% Λ-Fe1, 2-propanol as the alcohol of choice, and 4 Å molecular
sieves induced the 1,2-hydride shift at room temperature to
provide (R)-isopropyl mandelate with 99% NMR yield, 96%
isolated yield, and 87.5% ee (entry 1). As expected, Δ-Fe1
afforded the mirror-imaged mandelate (S)-4 instead (entry 2).
Lower catalyst loading resulted in a slightly decreased
enantioselectivity (entry 3), whereas molecular sieves are crucial
for the conversion (entry 4), and the reaction is very sensitive to
the solvent (entries 5 and 6) and the nature of the alcohol
(entries 7−11).
Finally, we also investigated an asymmetric Nazarov
cyclization and found that Fe1 can catalyze the cyclization of
5 to 6 (Table 2).25−27 Under optimized reaction conditions, Λ-
Fe1 (5 mol %) provides (1R,2S)-6 in 89% yield, >20:1 dr, and
83% ee (entry 1).28 Yields and enantiomeric excess are strongly
dependent on concentration (entry 2) and the solvent (entries
3−6). Importantly, the ee is not affected by the conversion,
which reveals configurational stability of the iron complex
throughout the catalysis (see Supporting Information).
In conclusion, we here introduced the first example of an
asymmetric iron catalyst that is exclusively composed of achiral
ligands with the overall chirality being the result of a stereogenic
iron center, implemented and retained by two configurationally
surprisingly stable bidentate N-(2-pyridyl)-substituted N-
heterocyclic carbene ligands. The chiral-at-iron complex
combines sustainability (iron as the metal) and simplicity
(easily accessible achiral ligands). Without the requirement for
chirality in the ligand sphere, new avenues emerge for the design
ACKNOWLEDGMENTS
L.J. thanks the Alexander von Humboldt Foundation for a
postdoctoral fellowship.
■
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DOI: 10.1021/jacs.9b01352
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX