Isonitrile Iron(II) complexes with chiral N2P2 macrocycles in the enantioselective transfer hydrogenation of ketones

Article abstract of DOI:10.1021/ol503295c

Bis(isonitrile) iron(II) complexes bearing a C₂-symmetric N₂P₂ macrocyclic ligand, which are easily prepared from the corresponding bis(acetonitrile) analogue, catalyze the asymmetric transfer hydrogenation (ATH) of a broad scope of ketones in excellent yields (up to 98%) and with high enantioselectivity (up to 91% ee).

Full text of DOI:10.1021/ol503295c

Letter
pubs.acs.org/OrgLett
Isonitrile Iron(II) Complexes with Chiral N2P2 Macrocycles in the
Enantioselective Transfer Hydrogenation of Ketones
Raphael Bigler and Antonio Mezzetti*
Department of Chemistry and Applied Biosciences, ETH Zurich, CH-8093 Zurich, Switzerland
̈
̈
S
* Supporting Information
ABSTRACT: Bis(isonitrile) iron(II) complexes bearing a C2-
symmetric N2P2 macrocyclic ligand, which are easily prepared
from the corresponding bis(acetonitrile) analogue, catalyze the
asymmetric transfer hydrogenation (ATH) of a broad scope of
ketones in excellent yields (up to 98%) and with high
enantioselectivity (up to 91% ee).
nantiopure alcohols are important in pharmaceutical and
Efragrance chemistry and are easily accessed by asymmetric
reduction of the corresponding ketones using a plethora of
chiral ruthenium, iridium, or rhodium catalysts.1 However, the
high cost and toxicity of the involved transition metals have
hampered their use in industry.2 Recently, nontoxic and cheap,
but achiral, iron catalysts3 have been mainly used in
hydrogenation, hydrosilylation, and cross-coupling reactions.4
In contrast, only a handful of chiral catalysts have been
developed for enantioselective transformations such as hydro-
silylation,5 direct H2-hydrogenation6 (AH), and transfer
hydrogenation7−11 of ketones (ATH). Despite spectacular
advances, the stability of iron catalysts is still a major issue. For
example, Morris has shown that the iron catalysts with an open-
chain tetradentate PNNP ligand A (Figure 1) decompose to
iron(0) nanoparticles during catalysis,7c whereas the less bulky
ligand B gives robust, highly active, and enantioselective
catalysts when combined with a strong π-acceptor (such as
carbon monoxide).8
has been found to be heterogeneous.6a,10 As large rings are
unsuitable to unleash a strong macrocyclic effect,12 we decided
to prepare N2P2 macrocycles with a ring size between 14 and
16. It should be noted that most previously reported
macrocycles of this kind are achiral and have not been used
in catalysis13 or have a pseudo meso configuration.14 We have
recently reported the first examples of enantiopure C2-
symmetric N2P2 macrocycles (1a−c, Figure 1).15 Ligands
1a−c form the mononuclear, stable, diamagnetic bis-
(acetonitrile) complexes [Fe(MeCN)2(N2P2)](BF4)2 (2a−c)
and the bromocarbonyl derivatives [FeBr(CO)(N2P2)]BPh4
(3a, 3c) (Figure 2).15 In this paper, we report the application
of these complexes in the ATH of acetophenone in basic
isopropanol.16
However, in a preliminary screening, the bis(acetonitrile)
complex 2a gave irreproducible results; its diamino analogue 2b
was essentially inactive, and 2c gave low enantioselectivity
(Table S1). The 31P{1H} NMR spectrum of the reaction
In an alternative approach, Gao has combined a chiral N4P2
macrocycle with [Fe3(CO)12] in situ, but the resulting catalyst
Figure 2. Bis(acetonitrile) complexes 2a−c and bromocarbonyl
complexes 3a and 3c.
Figure 1. Open-chain PNNP ligands A and B and macrocyclic
analogues 1a−c.
Received: November 13, 2014
© XXXX American Chemical Society
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Letter
solution with complex 2a showed the presence of free ligand,
which we attribute to partial complex decomposition under
catalysis conditions. Therefore, we sought to further stabilize
the system by using strongly binding ancillary ligands in
combination with the best performing cyclohexane-based ligand
1a. As carbonyl ligands were not effective (Table S1,
Supporting Information),17 we focused on isonitriles, which
are sterically and electronically tunable ligands and have
previously been applied in the iron-catalyzed (asymmetric)
transfer hydrogenation of ketones.7a,11,18
(cis-β vs trans), the compression of the Fe−P and Fe−N
distances in 4e with respect to the open-chain analogue7a
[Fe(MeCN)2(A)]2+ can be taken as an indication of a
significant macrocylic effect.
A screening of the bis(isonitrile) complexes 4a−g in the
ATH of acetophenone in basic isopropanol showed that the
tert-butyl isocyanide derivative 4a is the catalyst of choice
(Table S2). Complex 4a (1 mol %) reduced a broad scope of
aromatic ketones with high to excellent enantioselectivity in the
presence of NaOtBu (4 mol %) (Scheme 2).
The acetonitrile ligands of [Fe(MeCN)2(1a)](BF4)2 (2a)
were smoothly substituted by tert-butyl isocyanide (4 equiv) to
give the bright yellow/orange complex Λ-cis-β-[Fe-
(CNtBu)2(1a)](BF4)2 (4a),19 which was isolated as a stable,
diamagnetic solid in 95% yield (Scheme 1). Using the same
protocol, a small library of bis(isonitrile) complexes 4a−g was
prepared with different isonitriles, all of which are commercially
available.
a
Scheme 2. ATH of Aromatic Ketones with Catalyst 4a
Scheme 1. Synthesis of Bis(isonitrile) Complexes 4a−g
The bis(isonitrile) complexes 4a−g preferentially adopt a Λ-
cis-β configuration (Λ-cis-β:trans ≈ 93:7). The 31P{1H} NMR
AX spin systems (4a: δ 91.7 and 84.1 (2JP,P′ = 42.0 Hz))
indicates that one phosphine is trans to isonitrile and one trans
to imine. The IR spectrum of 4a shows two new bands at 2165
and 2142 cm−1, in agreement with the mutual cis arrangement
of the isonitrile ligands. Complex 4e bearing two phenyl
isocyanide ligands was structurally characterized by X-ray
diffraction (Figure 3). Despite the different overall structure
a
i
Reactions were performed on a 1.0 mmol scale in PrOH (0.25 M).
Yields and ee’s were determined by GC.
Substitution by methyl, methoxy, and chloro groups showed
a trend toward higher selectivity for ortho-substituted
acetophenones. Thus, 91% ee was obtained for 1-(2-
methoxyphenyl)ethan-1-ol (6g). Similarly, 1-acetonaphthone
gave superior results as compared to 2-acetonaphthone (5k vs
5l), and the corresponding alcohol 6k was obtained with 91%
ee. Overall, the hydrogenation of standard acetophenone
derivatives is efficient, reproducible, and highly enantioselective.
More hindered ketones 5m-5p are also hydrogenated with high
enantioselectivity albeit, not surprisingly, the corresponding
alcohols 6m−p were obtained in lower yields. Thus, the tert-
butyl-substituted ketone 5p gave 2,2-dimethyl-1-phenylpropan-
1-ol (6p) in 48% yield and with 90% ee (Scheme 2). This is a
remarkable result because few well-defined chiral catalysts give
high enantioselectivity with phenyl alkyl ketones PhC(O)R that
bear bulky R groups, and with the notable exception of
[FeBr(CO)(B)]+,8 most of them are based on precious
metals.20 We also tested cyclic aromatic ketones, but the
corresponding alcohols 6q and 6r were obtained with modest
enantioselectivity (46% and 44% ee, respectively).
Figure 3. ORTEP plot of the dication of 4e (thermal ellipsoids at 30%
probability). Selected bond lengths (Å) and angles (deg): P(1)−Fe
2.1710(5), P(2)−Fe 2.2280(4), N(1)−Fe 1.9797(14), N(2)−Fe
1.9987(13), C(35)−Fe 1.8856(17), C(42)−Fe 1.8433(17), C(35)−
Fe−C(42) 79.95(7), P(2)−Fe−N(1) 101.95(4), Fe−C(35)−N(3)
173.14(15), Fe−C(42)−N(4) 168.01(16), C(35)−N(3)−C(36)
176.05(17), C(42)−N(4)−C(43) 164.00(19).
B
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Overall, complex 4a is less active but considerably more
enantioselective in the ATH of acetophenone than Morris’ first-
generation catalysts (PNNP = A, Figure 1).7 In addition, the
observed enantioselectivity is in the range of the second-
generation complexes with ligand B.8 The moderate activity of
4a is in line with analogous observations made with open-chain
PNNP derivatives bearing isonitrile ligands.7a
A series of homogeneity tests were performed by adding
mercury, 1,10-phenanthroline, or triphenylphosphine6a to the
reaction solution with catalyst 4a and acetophenone. No effect
on the outcome of the reaction was observed, which supports a
mechanism involving a homogeneous catalyst.
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38.6 Hz, Figure S60, Supporting Information). In 2-propanol-
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In this paper, we have reported an iron(II) catalyst for the
predictable and reliable transfer hydrogenation of a broad scope
of ketones with high yield and high enantioselectivity that is
based on well-defined, fully characterized iron(II) complexes of
a N2P2 macrocycle in combination with isonitriles as ancillary
ligands. As standard tests indicate a homogeneous mechanism,
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promising strategy to prepare well-defined, robust, and highly
enantioselective Fe(II) catalysts that withstand harsh reaction
conditions.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and characterization of all new
products. This material is available free of charge via the
Internet at http://pubs.acs.org.
(g) Fluckiger, M.; Togni, A. Eur. J. Org. Chem. 2011, 4353.
̈
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hydrogenation reactions, see: (a) Li, Y.; Yu, S.; Wu, X.; Xiao, J.;
Shen, W.; Dong, Z.; Gao, J. J. Am. Chem. Soc. 2014, 136, 4031.
(b) Lagaditis, P. O.; Sues, P. E.; Sonnenberg, J. F.; Wan, K. Y.; Lough,
A. J.; Morris, R. H. J. Am. Chem. Soc. 2014, 136, 1367. (c) Sui-Seng, C.;
AUTHOR INFORMATION
Corresponding Author
■
Haque, F. N.; Hadzovic, A.; Putz, A.-M.; Reuss, V.; Meyer, N.; Lough,
̈
*E-mail: mezzetti@inorg.chem.ethz.ch.
Notes
A. J.; Zimmer-De Iuliis, M.; Morris, R. H. Inorg. Chem. 2009, 48, 735.
(d) Zhou, S.; Fleischer, S.; Junge, K.; Beller, M. Angew. Chem., Int. Ed.
2011, 50, 5120. (e) Fleischer, S.; Zhou, S.; Werkmeister, S.; Junge, K.;
Beller, M. Chem.Eur. J. 2013, 19, 4997. (f) Berkessel, A.; Reichau, S.;
The authors declare no competing financial interest.
von der Hoh, A.; Leconte, N.; Neudorfl, J.-M. Organometallics 2011,
̈
̈
ACKNOWLEDGMENTS
■
30, 3880.
We are grateful to the Swiss National Science Foundation for
financial support.
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