A Ferrocene-Based NH-Free Phosphine-Oxazoline Ligand for Iridium-Catalyzed Asymmetric Hydrogenation of Ketones

Article abstract of DOI:10.1021/acs.orglett.8b02591

A new type of ferrocene-based phosphine-oxazoline ligand has been prepared over a few simple steps. An iridium complex of this ligand is air stable and exhibits excellent performance for the asymmetric hydrogenation of simple ketones (up to 98% yield, up to 99% ee, and 20?000 S/C). Exo-α,β-unsaturated cyclic ketones could be regiospecifically hydrogenated to give chiral allylic alcohols with good results. This study indicates that P,N-ligands can also efficiently promote Ir-catalyzed asymmetric hydrogenation without NH-hydrogen-bonding assistance.

Full text of DOI:10.1021/acs.orglett.8b02591

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
A Ferrocene-Based NH-Free Phosphine-Oxazoline Ligand for
Iridium-Catalyzed Asymmetric Hydrogenation of Ketones
Yanzhao Wang, Guoqiang Yang, Fang Xie,* and Wanbin Zhang*
Shanghai Key Laboratory for Molecular Engineering of Chiral Drugs, School of Chemistry and Chemical Engineering, Shanghai Jiao
Tong University, 800 Dongchuan Road, Shanghai 200240, P. R. China
S
* Supporting Information
ABSTRACT: A new type of ferrocene-based phosphine-oxazoline ligand
has been prepared over a few simple steps. An iridium complex of this
ligand is air stable and exhibits excellent performance for the asymmetric
hydrogenation of simple ketones (up to 98% yield, up to 99% ee, and
20 000 S/C). Exo-α,β-unsaturated cyclic ketones could be regiospecifically
hydrogenated to give chiral allylic alcohols with good results. This study
indicates that P,N-ligands can also efficiently promote Ir-catalyzed
asymmetric hydrogenation without NH-hydrogen-bonding assistance.
symmetric hydrogenation employing chiral metal com-
plexes is an efficient, economical, and environmentally
A
benign methodology for the preparation of chiral compounds.1
Chiral alcohols, being a class of versatile building block and
important skeletons for pharmaceutical compounds, have
attracted much attention from chemists.2 The asymmetric
hydrogenation of ketones is one of the most efficient methods
for the preparation of chiral alcohols.3 Since the pioneering
work of Noyori’s [RuCl2(diphosphine)-(diamine)] catalytic
system,4 many chiral Ru-catalysts have been developed.5 Over
the past two decades, asymmetric hydrogenation using iridium
catalysts has received much attention due to its high efficiency
for the asymmetric hydrogenation of imines and unfunction-
alized and weakly functionalized olefins.6 However, iridium
catalysis has not been widely employed in the asymmetric
hydrogenation of ketones. In 2011, Kempe developed a
phosphine-free amido-iridium complex for the asymmetric
hydrogenation of ketones.7 In the same year, the Zhou group
divulged a SpiroPNP-Ir catalyst for such reactions, showing
excellent enantioselectivities (up to 99.9% ee) and an
extremely high TON for the hydrogenation of simple ketones.8
Recently, several groups have developed ferrocene-based
tridentate amino-phosphine ligands (such as f-Ampha) which
exhibited excellent performance in the Ir-catalyzed asymmetric
hydrogenation of simple ketones, affording chiral alcohols (full
conversions, up to >99% ee) (Figure 1).9 The high efficiency
of these Ir catalysts relies on the assistance of the amino NH
group of the chiral ligands (possibly via hydrogen bonding).
Chiral iridium complexes with phosphine-oxazoline ligands
are commonly used for the asymmetric hydrogenation of
olefins and imines,6 but rarely for the asymmetric hydro-
genation of ketones.5g We were interested in determining if
phosphine-oxazoline is capable of promoting the asymmetric
hydrogenation of simple ketones without the assistance of an
amino group; thus, we tested several well-known phosphine-
oxazoline ligands L1 and related ligands L2 that have been
developed by our group. The results show that these
Figure 1. Examples of efficient and practical chiral ligands.
complexes are able to catalyze the asymmetric hydrogenation
of ketones smoothly, albeit with somewhat low enantioselec-
tivity (Figure 2, up to 60% ee, see below). We therefore
Figure 2. Examples of our group’s phosphine-oxazoline ligands.
decided to design new phosphine-oxazoline ligands to enable
this reaction. Since the ferrocene-based phosphine-oxazoline
ligands showed the best catalytic activity, we chose ferrocene as
the backbone to design our new ligand. Taking into account
the design concept of axially unfixed ligands,10 ease of
preparation, and different coordinating ring size, we designed
ligand L3. This ligand bears two flexible axes and, thus,
possesses several potential configurations, allowing it to
coordinate with a transition metal to form different complexes
with different configurations. We propose that one of the
possible Ir-complexes of L3 can be formed predominantly as a
Received: August 12, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02591
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
result of the chiral-inducing effect from the already present
chiral elements. Herein, we report the design and preparation
of a ferrocene-based phosphine-oxazoline ligand and its
application in the iridium-catalyzed asymmetric hydrogenation
of ketones.
With the new phosphine-oxazoline ligands in hand, we
began our study by employing acetophenone 4a as the model
substrate with 0.5 mol % Ir-catalyst in MeOH under 5 bar of
hydrogen for 24 h. Detailed hydrogenation conditions are
listed in Table 1. First, different catalysts consisting of different
Phosphine-oxazoline ligands L3 were easily prepared via a
simple synthetic route (Scheme 1). Starting from commercially
Table 1. Optimizing the Reaction Conditions for the
Asymmetric Hydrogenation of Acetophenone
a
Scheme 1. Synthetic Route to the Phosphine-Oxazoline
Ligands
b
c
entry
ligand
solvent
conv [%]
ee [%]
1
2
3
4
5
6
7
8
L1a
L1b
L1c
L1d
L2a
L2b
L3a
L3c
L4
L3a
L3a
L3a
L3a
L3a
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
91
67
28
57
25
64
>99
>99
67
>99
>99
trace
17
−31
5
12
60
23
52
99
97
45
95
90
−
9
10
11
12
13
14
iPrOH
THF
toluene
DCM
51
−
NR
a
Reaction conditions: 4a (0.40 mmol), ratio of substrate/catalyst (S/
C) = 200, Na2CO3 (10 mol %), H2 (5 bar), solvent (2.0 mL).
b
c
Determined by 1H NMR spectroscopy. Enantioselectivity was
determined by HPLC using a chiral Daicel column.
available ferrocenecarboxylic acid, key intermediate 1 was
obtained according to the reported literature.11 After lithiation
of 1 followed by treatment with 2-(diphenylphosphino)-
benzaldehyde, 2 and 3 were obtained in more than 80% overall
yields. The alternative diastereomers, corresponding to
lithiation at the other ortho position of oxazoline group, were
not observed. Finally, 2 and 3 were reacted with iodomethane
or iodoethane to generate L3.
chiral phosphine-oxazoline ligands were screened (entries 1−
9).12 The planar chiral Fe/Ru-metalocene-based phosphine-
oxazoline ligands (L1a−L1d) were examined. However, most
of them gave poor enantioselectivity and/or conversion
(entries 1−4). The well-known center chiral ligand (s)-tBu-
PHOX (L4) and axially flexible ligands iPr-BiphPHOX (L2a)
and In-BiphPHOX (L2b) also gave poor conversion and
enantioselectivity (entries 5, 6, and 9). Finally, our new
phosphine-oxazoline ligands L3 proved to be the best with
regards to both conversion and enantioselectivity (>99% conv,
up to 99% ee, entries 7 and 8). This reaction cannot proceed in
the absence of base. The effect of solvent on the reaction was
also examined. The hydrogenation is best conducted in protic
The coordination of ligands L3a to [Ir(COD)Cl]2 was
verified by analysis of the single crystal structure of the
complex [Ir(COD)(L3a)]BArF (CCDC 1858033) (Figure 3)
i
solvents with MeOH, EtOH, and PrOH all providing
comparable conversions and selectivities (entries 7, 10, and
11, respectively). However, the reactivity and enantioselectivity
decreased sharply when the solvent was changed to THF,
toluene, or DCM (entries 12−14). Therefore, the optimal
reaction conditions were found to include using [Ir(L3a)-
(COD)]BArF as a catalyst and Na2CO3 as a base and carrying
out the reaction under 5 bar of hydrogen in MeOH at room
temperature for 24 h.
Figure 3. Single crystal structure of [Ir(COD)(L3a)]BArF.
With the optimized conditions in hand, a variety of aryl alkyl
ketones were examined, as shown in Scheme 2. It was found
that electron-neutral, electron-deficient, and electron-rich,
monosubstituted and dual substituted aryl groups gave the
corresponding products with excellent yields and good to
excellent enantioselectivities (up to 98% yield and 99% ee, 5a−
5n). However, increasing the electron-withdrawing ability of
the substituents on the phenyl group led to a concomitant
decrease in ee (5j vs 5n, 5k vs 5m). An exception to these
results was observed when the Me substituent was present at
the meta position (4c), giving the corresponding product with
relatively low ee; the reason for this is unclear. It is worth
obtained from the reaction of L3a with [Ir(COD)Cl]2 and
NaBArF in DCM under nitrogen. In the crystal structure,
ligand L3a is coordinated to the iridium atom by means of one
phosphorus atom and one nitrogen atom, forming a conforma-
tionally restricted nine-membered ring. It appears that the
configuration of the MeO-linked carbon is essential for
coordination because L3b cannot form its corresponding Ir-
complex under the same conditions. The complex [Ir(COD)-
(L3a)]BArF exhibits a stable conformation in which steric
interactions between the MeO group and the groups around
the Ir center are avoided.
B
DOI: 10.1021/acs.orglett.8b02591
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 2. Substrate Scope of Simples Ketones
Table 2. Substrate Scope of Exo-α,β-Unsaturated Cyclic
a
Ketones
b
c
entry
Ar
n
product
yield [%]
ee [%]
1
2
3
4
5
6
Ph
1
1
1
1
1
1
2
7a
7b
7c
7d
7e
7f
97
97
95
95
96
97
95
91
91
88
88
92
95
87
4-MeC6H4
2-ClC6H4
3-ClC6H4
4-ClC6H4
2-naphthyl
Ph
7
7g
a
Reaction conditions: substrate (0.30 mmol), ratio of substrate/
catalyst (S/C) = 100, Na2CO3 (10 mol %), H2 (5 bar), MeOH (2.0
b
c
mL). Isolated yield. Ee was determined by HPLC using a chiral
column.
The Ir-L3a complex was very stable and exhibited high
catalytic activity. When the catalyst loading was reduced to
0.005 mol % (S/C = 20 000), the asymmetric hydrogenation of
acetophenone 4a on a 4.80 g scale proceeded well providing
product 5a with 95% yield and 96% ee (Scheme 3, eq 1).
Scheme 3. Asymmetric Hydrogenation with High S/C and
Deuteration Experiment
a
Reaction conditions: substrate (0.40 mmol), ratio of substrate/
catalyst (S/C) = 200, Na2CO3 (10 mol %), H2 (5 bar), MeOH (2.0
b
c
mL), room temperature for 24 h. H2 (10 bar). S/C = 100, 50 °C.
mentioning that fused-ring aryl and heterocyclic substrates (4o
and 4p) also gave their corresponding products with excellent
yields and enantioselectivities (up to 98% yield and 98% ee).
When R was changed from Me to Et, nPr, and nBu, they also
could be successfully converted to the corresponding adducts
with excellent yields and enantioselectivities (up to 97% yield
and 98% ee, 5q−5s) Finally, bisaryl ketones with different
substituents at the ortho position of the benzene ring were also
used for this hydrogenation; to our delight, the desired product
was obtained with up to 94% ee.
Pleasingly, this catalytic asymmetric hydrogenation system is
also amenable to α,β-unsaturated cyclic ketones 6, of which the
corresponding hydrogenation products are important skeletons
present in numerous bioactive compounds. Zhou et al.
reported the asymmetric hydrogenation of (E)-α-arylmethy-
lene cyclohexanones to afford chiral allylic alcohols with
excellent enantioselectivities.13 With our catalytic system, this
type of substrate can also be successfully converted to the
corresponding adducts with full conversions and good to
excellent enantioselectivities. As summarized in Table 2, a
number of (E)-α-arylmethylene cyclopentanones 6 can be
hydrogenated to the chiral β-arylmethylene cyclopentanols 7 in
high yields and with good to excellent enantioselectivities
(entries 1−6). Electron-donating and -withdrawing substitu-
ents on the phenyl ring of the substrate had a slight effect on
both the reactivity and enantioselectivity of the reaction. Lower
ee was also observed when the substituent Cl was present at
the ortho or meta position of the phenyl ring (entries 3−4). It
is worth mentioning that a naphthyl-substituted substrate gave
its corresponding product 7f with the best results (entry 6,
97% yield and 95% ee). When a substrate bearing a six-
membered ring was subjected to the hydrogenation, the
corresponding β-arylmethylene cyclic alcohol 7g was obtained
in 95% yield and with 87% enantioselectivity (entry 7).
A deuteration experiment was performed to determine if
transfer hydrogenation occurs in the reaction. Under a D2
atomsphere, the hydrogenation gave the desired product with
95% D incorporation at the α-position. Based on these
experiments and the fact that there was no reaction in the
absence of hydrogen gas, a transfer hydrogenation pathway can
be ruled out (Scheme 3, eq 2).
In conclusion, we have developed a novel planar chiral
ferrocene phosphine-oxazoline ligand, which has been
successfully applied to the iridium-catalyzed asymmetric
hydrogenation of simple ketones and exo-α, β-unsaturated
cyclic ketones. Under the optimal reaction conditions, the
desired products can be obtained in up to 98% yield and 99%
ee. The reaction can be conducted on a gram scale and with a
low catalyst loading (S/C = 20 000) with little loss in
enantioselectivity. Deuterium labeling experiments revealed
that the reaction proceeds with hydrogen gas rather than via
transfer hydrogenation with the alcohol solvent.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02591.
Details on experimental procedures, characterization
1
data, copies of H NMR, 13C NMR spectra and HPLC
charts of new compounds (PDF)
C
DOI: 10.1021/acs.orglett.8b02591
Org. Lett. XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: xiefang@sjtu.edu.cn.
*E-mail: wanbin@sjtu.edu.cn.
ORCID
Guoqiang Yang: 0000-0002-8356-6653
Wanbin Zhang: 0000-0002-4788-4195
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We thank the NSFC (Nos. 21620102003 and 21772119),
STCSM (No. 15JC1402200), and SHMEC (No.
201701070002E00030) for financial support. We also thank
the Instrumental Analysis Center of SJTU.
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DOI: 10.1021/acs.orglett.8b02591
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Organic Letters
Letter
Synth. Catal. 2010, 352, 1841. (c) Liu, Y.; Yao, D.; Li, K.; Tian, F.;
Xie, F.; Zhang, W. Tetrahedron 2011, 67, 8445. (d) Liu, Y.; Zhang, W.
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L.; Shen, J.; Yang, G.; Zhang, W. Chem. Commun. 2017, 53, 609.
(g) Meng, K.; Xia, J.; Wang, Y.; Zhang, X.; Yang, G.; Zhang, W. Org.
Chem. Front. 2017, 4, 1601. (h) Guo, H.; Li, J.; Liu, D.; Zhang, W.
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(11) Park, J.; Lee, S.; Ahn, K. H.; Cho, C.-H. Tetrahedron Lett. 1995,
36, 7263.
(12) The absolute configuration is consistent with previous reports.
See refs 8 and 9.
(13) Xie, J.-B.; Xie, J.-H.; Liu, X.-Y.; Kong, W.-L.; Li, S.; Zhou, Q.-L.
J. Am. Chem. Soc. 2010, 132, 4538.
E
DOI: 10.1021/acs.orglett.8b02591
Org. Lett. XXXX, XXX, XXX−XXX