Enantioselective synthesis of amines: General, efficient iron-catalyzed asymmetric transfer hydrogenation of imines

Article abstract of DOI:10.1002/anie.201002456

In the iron age: A readily accessible, active iron catalyst serves in the straightforward, catalytic transfer hydrogenation of imines (see scheme). A series of imines are converted into chiral amines in high yields and very good enantioselectivities. This

Full text of DOI:10.1002/anie.201002456

Angewandte
Chemie
DOI: 10.1002/anie.201002456
Iron Catalysis
Enantioselective Synthesis of Amines: General, Efficient Iron-
Catalyzed Asymmetric Transfer Hydrogenation of Imines**
Shaolin Zhou, Steffen Fleischer, Kathrin Junge, Shoubhik Das, Daniele Addis, and
Matthias Beller*
Chiral amines find numerous applications in the pharmaceu-
tical and agrochemical industries. Notable examples of
established drugs and their areas of application include
Zoloft (depression), Cinacalcet (secondary hyperparathyr-
oidism), Flomax (prostate), and Rivastigmine (Alzheimerꢀs
and Parkinsonꢀs disease) as well as the chiral herbicide
Metolachlor (Scheme 1). The most direct and efficient syn-
hydrogenations[14] of ketones to give the corresponding chiral
alcohols. However, related asymmetric reductions of imines
have remained an unexplored area so far. Herein, we report
the first highly efficient iron-catalyzed asymmetric reduction
of imines.
Morris and co-workers recently reported the synthesis of
the well-defined catalyst [Fe(CO)(NCMe){(S,S)-cyP2N2}]-
(BF4)2 (11; Scheme 2) and similar chiral iron complexes.[14c]
Remarkably, these complexes give excellent results in the
transfer hydrogenation of ketones and could be also success-
fully applied in the transfer hydrogenation of N-benzyl-
À
=
ideneaniline (Ph CH NPh). Unfortunately, no desired reac-
tion occurred with prochiral N-(1-phenylethylidene)aniline
(Ph CMe NPh).[14c] Inspired by this seminal work and based
on our own experience in transfer hydrogenations, we started
À
=
Scheme 1. Selected chiral amines that are used as pharmaceuticals
and agrochemicals.
thesis of chiral amines is the asymmetric reduction of
ketimines, an apparently simple but still challenging trans-
formation.[1] Although in the last decades impressive advan-
ces have been made based on various transition-metal
catalysts[2–9] and also organocatalysts,[10] the development of
cost-efficient, environmentally benign catalysts for this trans-
formation is still desirable. In this respect, iron-based catalysts
are highly attractive candidates[11,12] owing to the abundant
availability of the central metal and the relatively few
investigations in the past.
Recently, significant progress has been achieved in iron-
catalyzed enantioselective hydrosilylations[13] and transfer
[*] S. Zhou, S. Fleischer, Dr. K. Junge, S. Das, D. Addis,
Prof. Dr. M. Beller
Leibniz-Institut fꢀr Katalyse an der Universitꢁt Rostock
Albert-Einstein-Straße 29a, 18059 Rostock (Germany)
Fax: (+49)381-1281-51113
E-mail: matthias.beller@catalysis.de
Homepage: http://www.catalysis.de
[**] This work was funded by the State of Mecklenburg-Western
Pomerania, the BMBF, and the DFG (Leibniz prize). We thank Dr. W.
Baumann, Dr. C. Fischer, S. Buchholz, S. Schareina, and A. Kammer,
A. Koch, S. Rossmeisl, and K. Mevius for their excellent technical
and analytical support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002456.
Scheme 2. Structures of chiral ligands and iron complexes for asymmetric
transfer hydrogenations.
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Table 1: Iron-catalyzed asymmetric transfer hydrogenation of imines 1.[a]
a program to develop iron catalysts for the enantioselective
reduction of imines.
In exploratory studies different N-(1-phenylethylidene)-
amines were tested with respect to their activity. Among the
different substrates, (diphenylphosphinyl)(1-phenylethylide-
ne)amine (1a) showed best reactivity. This class of imines is
easily obtained as single isomers (syn/anti) from the corre-
sponding oximes.[15] Advantageously, they are tolerant to air
and moisture, and the removal of the phosphinyl group is
conveniently achieved under acidic conditions. Selected
experiments for the catalytic transfer hydrogenation of 1a
are outlined in Table 1. Here, several chiral ligands
(Scheme 2) were tested with the iron carbonyl hydride cluster
complex [Et3NH][HFe3(CO)11] as the catalyst precursor in the
presence of KOH. Privileged chiral nitrogen and phosphorus
ligands such as (S,S)-Ts-dpen (L1), (R)-pybox-ip (L2), (S,S)-
Jacobsen ligand (L3), (S)-binap (L4), (S)-segphos (L5), (S,S)-
Me-duphos (L6), (2S,4S)-ppm (L7), (S,S)-taniaphos (L8), and
(S,S)-dach-phenyl Trost ligand (L9) did not show any
appreciable activity. However, to our delight ligand L10,
which was also used by Gaoꢀs group for the transfer hydro-
genation of ketones,[16] showed excellent activity and enan-
tioselectivity: 99% yield with 96% ee within 30 min (Table 1,
entry 1)!
Entry
Imine
Ligand[b]
Base
Yield[c] [%]
ee[d] [%]
(config.)[e]
1
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
L10
L10
L10
L10
L10
L10
L10
KOH
KOH
–
99
99
90
99
99
96 (R)
96 (R)
42 (R)
96 (R)
96 (R)
96 (R)
–
2[f]
3[g]
4
NaOtBu
NaOiPr
KOH
KOH
KOH
KOH
KOH
KOH
KOH
5
6[h]
7[i]
8
95
n.r.
n.r.
93
99
n.r.
n.r.
–
9[j]
10[k]
11
12
L10
L11
L10
L10
55 (R)
91 (R)
–
–
[a] Unless otherwise noted, the reaction was carried out with
[Et3NH][HFe3(CO)11] (0.33 mol%), ligand (1 mol%), base (5 mol%),
imine (0.5 mmol), iPrOH (10 mL) at 458C for 30 min. [b] For ligand
structures see Scheme 2. [c] Determined by 1H NMR analysis using
dibromomethane as an internal standard. [d] Determined by HPLC on a
chiral stationary phase. [e] Determined by comparison with reported
data. [f] RT, 5.5 h. [g] 44 h. [h] 0.17 mol% catalyst. [i] No iron source. [j] 1
mol% [Fe3(CO)12], 24 h. [k] 1 mol% 11, 10 min. n.r.=no reaction.
Next, the influence of critical reaction parameters, for
example, temperature, base, and iron sources, was investi-
gated. The reaction proceeded smoothly even at room
temperature within 6 h giving the
Table 2: Scope of the iron-catalyzed asymmetric transfer hydrogenation of N-(diphenylphosphinyl)-
imines.
corresponding amine 2a in 99%
yield again with excellent enantio-
selectivity (96% ee) (Table 1,
entry 2). In the absence of base,
the reaction rate decreased and the
ee value dropped to 42% (Table 1,
entry 3). The type of base had no
apparent effect on conversion
or enantioselectivity (Table 1,
entries 4 and 5). It is worth men-
tioning that even at lower catalyst
loading (0.17 mol% Fe), excellent
yield (95%), and enantioselectivity
(96% ee) were obtained within 1 h
(Table 1, entry 6). Control experi-
ments confirmed that no amine 2a
was obtained in the absence of
either iron source or ligand
(Table 1, entries 7 and 8). Other
iron complexes, for example
Fe3(CO)12, led to only moderate
Entry
1
Imine
Catalyst [mol%]
0.33
Yield[a] [%]
ee[b] [%] (config.)[c]
96 (+)-(R)
1a
1d
1e
2a 87
2
3
0.33
0.33
2d 87
2e 95
96 (+)-(R)
97 (+)-(R)
4
5
6
7
1 f
1g
1h
1i
0.33
1.33
0.67
0.67
2 f 94
2g 94
2h 85
2i 94
96 (+)-(R)
96 (+)-(R)
95 (+)-(R)
96 (+)-(R)
enantioselectivities
(55% ee;
Table 1, entry 9). With respect to
the active catalyst species, it is
interesting to note that Morrisꢀ
chiral iron complex 11 gave some-
what similar results. While the
activity of this complex is some-
what higher, which can be
explained by the faster preforma-
tion of the active species, the
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Table 2: (Continued)
an effective conversion of 1g, which
Entry
Imine
Catalyst [mol%]
0.67
Yield[a] [%]
ee[b] [%] (config.)[c]
94 (+)-(R)
bears a methoxy group at the ortho
position, and no reduction was
observed in the case of the corre-
sponding isopropylphenylimine. In
the latter case, decomposition to
isobutyrophenone and diphenyl-
phosphinamide was observed.
8
1j
1k
1l
2j 87
9
0.67
0.33
2k 85
2l 98
95 (+)-(R)
98 (+)-(R)
Gratifyingly,
heteroaromatic
imines were also reduced with
excellent enantioselectivities (92–
95% ee; Table 2, entries 11–14).
Similarly, reduction of cyclic
imines 1r and 1s, derived from 1-
indanone and a-tetralone, was
achieved with excellent enantiose-
lectivities (90% and 94% ee) to
afford a-aminoindane 2r and
a-aminotetralin 2s, respectively
(Table 2, entries 16 and 17). Both
are important structural motifs in a
number of biologically active com-
pounds. Apart from acetophenone-
type imines, also alkylidene(diphe-
nylphosphinyl)amines can be re-
duced with our protocol, albeit
lower activity and ee values were
observed in preliminary studies
(Table 2, entries 18 and 19).
10
11
12
13
1m
1n
1o
1.0
2m 67
2n 72
2o 68
92 (+)-(R)
94 (+)-(R)
94 (+)-(R)
0.67
1.0
14
1p
0.67
2p 78
95 (+)-(R)
15
16
17
1q
1r
0.67
1.33
1.33
2q 90
2r 95
2s 95
89 (+)-(R)
90 (+)-(R)
94 (+)-(R)
In conclusion, we have devel-
oped the first iron-catalyzed asym-
metric transfer hydrogenation of
imines. Excellent enantioselectivi-
ties and high yields were observed
for a variety of N-(diphenylphos-
phinyl)ketimines. Notable features
of our protocol are the convenient
formation of the active iron cata-
lyst, the operational simplicity, and
the safe and mild reaction condi-
tions. This makes the new protocol
attractive for various laboratory-
scale applications. Further studies
1s
18[d]
19[d]
1t
0.67
0.67
2t 62
39 (À)-(R)
29 (À)-(R)
1u
2u 35
[a] Yield of isolated product. [b] Determined by HPLC on a chiral stationary phase. [c] Determined by
comparison with reported data or by analogy. [d] Partial decomposition of imine to corresponding
ketone and diphenylphosphinamide.
enantioselectivity was reproducibly lower (91% ee; Table 1,
entry 10).
to improve the activity and enantioselectivity of aliphatic
imines are in progress in our laboratory.
Once a suitable catalyst system was identified, the scope
and limitations of our novel iron-catalyzed asymmetric trans-
fer hydrogenation of N-(diphenylphosphinyl)ketimines were
explored. A variety of ketimines, including aromatic, hetero-
aromatic, and cyclic imines were hydrogenated smoothly with
high yields up to 98% and good to excellent enantioselectiv-
ities up to 98% ee (Table 2, entries 1–17). Both electron-
donating and electron-withdrawing substituents on the aro-
matic ring at the ortho, meta, or para position had little impact
on the enantioselectivity (94–97% ee; Table 2, entries 2–9).
On the other hand, the steric hindrance of the substituent on
Experimental Section
General procedure for transfer hydrogenation imine 1a: Ligand L10
(3.3 mg, 0.005 mmol) and catalyst precursor [Et3NH][HFe3(CO)11]
(0.96 mg, 0.0017 mmol) were dissolved in degassed 2-propanol in a
25 mL Schlenk tube under argon. After the solution had been stirred
at room temperature for 20 min, KOH (0.5 mL, 0.05m in iPrOH) was
added and the solution was stirred for another 10 min. Then imine 1a
(160 mg, 0.5 mmol) was added to the catalyst solution and the
reaction mixture was stirred at 458C for 30 min. After completion of
the reaction, the solvent was evaporated under reduced pressure and
the residue was purified by column chromatography to give the
=
the C N bond had a significant effect on the activity, but only
a little effect on the enantioselectivity (Table 2, entries 5, 15,
and 16). For example, higher catalyst loading was needed for
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corresponding amine 2a, which was then analyzed by HPLC to
determine the ee value.
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Received: April 25, 2010
Published online: July 19, 2010
Keywords: asymmetric catalysis · enantioselectivity · imines ·
.
iron catalysis · transfer hydrogenation
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