Cooperative transition-metal and chiral Bronsted acid catalysis: Enantioselective hydrogenation of imines to form amines

Article abstract of DOI:10.1002/anie.201100878

Control with an iron hand: A broad range of ketimines underwent enantioselective hydrogenation in the presence of a chiral Bronsted catalyst and a well-defined nonchiral iron catalyst (see scheme). This procedure constitutes an attractive and environmentally favorable alternative to well-established asymmetric hydrogenation reactions with precious-metal catalysts. Copyright

Full text of DOI:10.1002/anie.201100878

Communications
DOI: 10.1002/anie.201100878
Iron Catalysis
Cooperative Transition-Metal and Chiral Brønsted Acid Catalysis:
Enantioselective Hydrogenation of Imines To Form Amines**
Shaolin Zhou, Steffen Fleischer, Kathrin Junge, and Matthias Beller*
Chiral amines are an integral part of numerous important
bioactive compounds. This privileged structural motif is found
in naturally occurring alkaloids and amino acid derivatives as
well as in pharmaceuticals, herbicides, and insecticides.[1]
Typically, enantiomerically pure amines are key building
blocks in the drug-discovery process, but they are also sold on
a multi-hundred-ton-scale as kinetic-resolution reagents.[2]
With the growing importance of chiral compounds in the
life-science industries, the development of efficient method-
ologies continues to be a topic of interest in organic chemistry
and catalysis. Clearly, asymmetric hydrogenation has become
a prime technology for the synthesis of chiral compounds.[3–12]
In this respect, the atom-efficient reaction of imines with
inexpensive hydrogen to form amines has received much
attention in recent years. The largest metal-catalyzed asym-
metric process in industry today, the iridium-catalyzed hydro-
genation in the production of metolachlor,[3d] is a prime
example of this methodology. Virtually all of the known
catalyst systems employed for asymmetric hydrogenation are
derived from precious late transition metals in combination
with specific chiral phosphine ligands for the control of
enantioselectivity.
Herein, we describe a different approach involving the
combination of a molecularly defined iron hydrogenation
catalyst with a chiral Brønsted acid for the reduction of
various imines with hydrogen to form the corresponding
amines with high selectivity (Scheme 1).[15] Conceptually, the
Scheme 1. Cooperative hydrogenation of imines with an iron/Brønsted
acid catalyst system.
Brønsted acid catalyst and the organometallic center work
together in a cooperative manner, in analogy with catalysis by
iron-based hydrogenases.[16] Notably, the levels of enantiose-
lectivity observed with a simple iron catalyst rival those of the
most efficient precious-metal-based systems.
To date, no catalytic hydrogenation of imines in the
presence of nonchiral metal complexes has been reported to
give the corresponding amines with high enantioselectivity.[12-
n,o,w] Potential problems with the envisioned reaction were the
possible inadequate reactivity of the two components of the
catalytic system: the organometallic complex and the
Brønsted acid. The latter has to specifically activate the
original substrate, whereas the former has to react with the
activated intermediate. Furthermore, nonspecific deactiva-
tion reactions between the two components might compro-
mise the desired overall process.
Owing to the industrial importance of 1-aryl ethylamines,
our initial catalytic investigations were carried out on the
hydrogenation of N-(1-phenylethylidene)aniline (2a) as a
benchmark reaction (Table 1). According to our concept, we
intended to use commercially available chiral 3,3’-bisaryl 1,1’-
binaphthyl-2,2’-diyl hydrogen phosphates 1 for the induction
of stereoselectivity.
An interesting alternative organocatalytic approach for
the reduction of imines in the presence of chiral Brønsted
acids was reported by the research groups of Rueping,[13a–c]
List,[13d–g] MacMillan,[13h] and Antilla.[13i] Unfortunately, the
requirement of stoichiometric amounts of expensive
Hantzsch dihydropyridines as the hydrogen source has
limited the application of this methodology. Over the past
decade, the field of asymmetric Brønsted acid catalysis has
grown at a dramatic pace. This form of catalysis provides new
ways for the enantioselective preparation of various kinds of
C C, C O, and C N bonds.[14] However, the majority of these
methods are restricted to the generation of a stabilized
carbocation and its direct reaction with active carbon- or
heteroatom-based nucleophiles.
À
À
À
[*] S. Zhou,[+] S. Fleischer,[+] Dr. K. Junge, 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
Well-known homogeneous and heterogeneous hydroge-
nation catalysts were added to the reaction mixture to
promote the desired catalytic reduction with hydrogen.
Selected results are shown in Table 1. First experiments
revealed that the desired N-(1-phenylethyl)aniline was
formed in the presence of various Ru, Rh, Ir, and Pd catalysts
(Table 1, entries 2–7). However, none of these established
precious-metal hydrogenation catalysts in combination with
chiral Brønsted acids, for example, 3,3’-bis(2,4,6-triisopropyl-
Homepage: http://www.catalysis.de
[+] These authors contributed equally to this work.
[**] This research has been 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, A. Kammer, A.
Koch, and S. Rossmeisl (all at the LIKAT) 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.201100878.
5120
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 5120 –5124

Table 1: Initial investigations of the proposed asymmetric hydrogena-
A number of active iron hydrogenation and transfer-
hydrogenation catalysts have been developed in the last
decade.[17,18] We thus turned our interest to the performance
of these biorelevant complexes.[19] To our surprise, even
simple triiron dodecacarbonyl generated an active catalyst
with 1e and provided the corresponding amine in 93% yield.
Unfortunately, again no stereoselectivity was observed. The
use of other simple iron salts or the Morris complex 6 led to
no amine at all. However, dicarbonylhydro[2,5-di(trimethyl-
silyl)-3,4-butylene-1-hydroxy(h5-cyclopentadienyl)]iron (5),
which was first synthesized by Knꢀlker et al.,[20] yielded N-
phenyl-1-phenylethylamine in 81% yield with 94% ee
(Table 1, entry 11)! The combination of 5 with other chiral
phosphates also created active catalysts; however, the reac-
tion yields and enantioselectivities were lower (Table 1,
entries 14–18). Excellent enantioselectivities were also
observed at lower pressure (20 bar) and with a smaller
amount of the iron catalyst 5 (3 mol%; Table 1, entries 19 and
20).
tion.[a]
Entry
Brønsted
catalyst
(mol%)
Transition-metal
catalyst (mol%)
Conv.[b]
[%]
Yield[b]
[%]
ee[c]
[%]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20[d]
1e (5)
1e (1)
1e (1)
1e (1)
1e (1)
1e (1)
1e (1)
1e (1)
1e (1)
1e (1)
1e (1)
1e (1)
1e (1)
1a (1)
1b (1)
1c (1)
1d (1)
1 f (1)
1e (1)
1e (1)
–
80
>99
>99
97
>99
>99
>99
>99
>99
85
>99
98
98
76
54
0
71
45
3
62
16
35
93
0
–
8
0
0
0
0
0
0
–
–
94
–
–
0
47
41
30
4
94
94
4 (0.5)
[(Ph3P)3RhCl] (1)
[Rh(CO)H(PPh3)3] (1)
[{IrCl(C8H12)}2] (0.5)
Rh/C (1)
Pd/C (1)
[Fe3(CO)12] (1.67)
[(C8H8)Fe(CO)3] (5)
6 (5)
5 (5)
FeCl3 (5)
Fe(OAc)2 (5)
5 (5)
5 (5)
5 (5)
5 (5)
0
81
0
In situ NMR spectroscopic investigations of a 1:1 mixture
of TRIP and the Knꢀlker iron complex at the reaction
temperature (658C) showed immediate formation of hydro-
gen and the coordinated species 7 (Scheme 2, Figure 1d). This
0
35
40
12
6
8
63
71
23
25
15
>99
98
5 (5)
5 (3)
5 (5)
[a] Reaction conditions: 2a (0.5 mmol), Brønsted catalyst (1–5 mol%),
transition-metal catalyst (1–5 mol%), toluene (0.2 mL), H2 (50 bar),
658C, 24 h. [b] The conversion and the yield were determined by GC
methods with hexadecane as an internal standard. [c] The ee value was
determined by HPLC on a chiral stationary phase. [d] H2 (20 bar). TMS=
trimethylsilyl.
Scheme 2. Proposed reaction intermediates. R=2,4,6-iPr3C6H2.
reaction also proceeded slowly at room temperature and
could be reversed by the addition of hydrogen. Upon the
addition of N-(1-phenylethylidene)aniline (2a) to a 1:1
mixture of TRIP and the Knꢀlker iron complex 5, the
corresponding iron–amine complex 8 was observed as the
major reaction product alongside 7 (Figure 1e). The TRIP–
amine adduct and the Knꢀlker iron complex 5 were observed
upon hydrogenation (Figure 1 f). The ee value measured for
the amine was 98% both before and after hydrogenation.
Encouraged by these results, we studied the general use of
this cooperative catalyst system in more detail. Various
aromatic ketimines were hydrogenated smoothly in high
yields and with excellent enantioselectivity (Table 2). Both
electron-donating and electron-withdrawing substituents on
the aromatic rings at meta or para positions had little impact
phenyl)-1,1’-binaphthyl-2,2’-diyl hydrogen phosphate (TRIP,
1e), induced any significant enantioselectivity. This behavior
is easily explained: the corresponding Ru-, Rh-, Ir-, and Pd-
catalyzed hydrogenations of imine 2a also took place without
any Brønsted acid present (see Table S1 in the Supporting
Information). Notably, the differences between conversion
and product yield resulted from aldol condensation reactions,
which are observed as unwanted side reactions in the
presence of Rh, Ir, and Pd catalysts (Table 1, entries 3–7).
Angew. Chem. Int. Ed. 2011, 50, 5120 –5124
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5121

Communications
Table 2: (Continued)
Entry
Imine
Yield[b] [%]
ee[c] (S) [%]
5
6
7
8
9
2e
2 f
2g
2h
2i
3e: 87 (70)
93
3 f: 99 (90)
3g: 92 (68)
3h: 70 (67)
3i: 96 (86)
92
91
93
90
10
2j
3j: 94 (89)
88
11
12
2k
2l
3k: 96 (86)
3l: 80 (71)
93
91
Figure 1. In situ 31P NMR spectra of (S)-TRIP, iron complex 5, imine
2a, and amine 3a in different ratios: a) pure (S)-TRIP; b) 1:1 mixture
of (S)-TRIP and imine 2a (spectrum shows the generation of an
iminium phosphate salt); c) 1:1 mixture of (S)-TRIP and amine (S)-3a
(spectrum shows the generation of an ammonium phosphate salt);
d) 1:1 mixture of (S)-TRIP and iron complex 5 at room temperature
(spectrum shows the generation of a small amount of intermediate 7
(3%)); e) 1:1:1 mixture of (S)-TRIP, imine 2a, and complex 5
(spectrum shows the generation of intermediates 7 and 8); f) reaction
mixture after the hydrogenation reaction (spectrum shows the gener-
ation of an ammonium phosphate salt).
13
14
2m
2n
3m: 91 (91)
3n: 80 (71)
93
91
15
2o
3o: 99 (93)
80
Table 2: Scope of the iron-catalyzed asymmetric hydrogenation of N-aryl
imines.[a]
16
2p
2q
3p: 87 (79)
3q: 99 (91)
98
83
17[d]
Entry
1
Imine
Yield[b] [%]
ee[c] (S) [%]
18[d]
19[d]
20[d]
2r
2s
2t
3r: 99 (93)
3s: 99 (94)
3r: 89 (69)
81
67
70
2a
2b
2c
2d
3a: 82 (80)
94
2
3
4
3b: 84 (82)
3c: 84 (82)
3d: 88 (60)
96
95
93
[a] General reaction conditions: imine (0.5 mmol), (S)-TRIP (1 mol%), 5
(5 mol%), H2 (50 bar), 658C, 24 h. [b] The yield was determined by
1H NMR spectroscopic analysis with dibromomethane as an internal
standard. The yield of the pure isolated product is given in brackets.
[c] The ee value was determined by HPLC on a chiral stationary phase.
[d] (S)-TRIP: 2 mol%, 5: 2.5 mol%. PMP=p-methoxyphenyl.
5122
www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 5120 –5124

3029 – 3069; f) S.-L. You, Chem. Asian J. 2007, 2, 820 – 827;
g) T. C. Nugent, M. El-Shazly, Adv. Synth. Catal. 2010, 352, 753 –
819; h) M. Rueping, E. Sugiono, F. R. Schoepke, Synlett 2010,
852 – 865; i) N. Fleury-Brꢁgeot, V. de la Fuente, S. Castillꢂn, C.
Claver, ChemCatChem 2010, 2, 1346 – 1371; j) J.-H. Xie, S.-F.
Zhu, Q.-L. Zhou, Chem. Rev. 2011, 111, 1713 – 1760.
on the hydrogenation activity; high enantioselectivity was
observed for unsubstituted as well as meta- and para-
substituted 2-aryl ethylamines (88–96% ee; Table 2,
entries 1–12). Gratifyingly, heteroaromatic imines and N-
heteroaryl-substituted imines were also reduced with excel-
lent enantioselectivity (91–98% ee; Table 2, entries 13, 14,
and 16).
Acetophenonimines are typical substrates for asymmetric
hydrogenation. We were also interested in the behavior of our
system with less common, more challenging substrates. The
reduction of aliphatic imines occurred with high enantiose-
lectivity (67–83% ee; Table 2, entries 17–20). Notably, in case
of the a,b-unsaturated imine 2t, no reduction of the
conjugated double bond was observed.
In conclusion, we have demonstrated the possibility of
performing enantioselective reduction reactions with hydro-
gen without the use of precious-metal catalysts and chiral
ligands. Instead, the combination of an iron complex with
chiral acids enabled smooth hydrogenation. It was shown that
both components of the catalyst system act in a cooperative
manner reminiscent of biocatalysis.
[4] For the use of Ti and Zr catalysts, see: a) C. A. Willoughby, S. L.
Buchwald, J. Am. Chem. Soc. 1992, 114, 7562 – 7564; b) C. A.
Willoughby, S. L. Buchwald, J. Am. Chem. Soc. 1994, 116, 8952 –
8965; c) X. Verdaguer, U. E. W. Langer, S. L. Buchwald, Angew.
Chem. 1998, 110, 1174 – 1178; Angew. Chem. Int. Ed. 1998, 37,
1103 – 1107; d) M. Ringwald, R. Stꢃrmer, H. H. Brintzinger, J.
Am. Chem. Soc. 1999, 121, 1524 – 1527; e) M. C. Hansen, S. L.
Buchwald, Org. Lett. 2000, 2, 713 – 715; f) J. Yun, S. L. Buchwald,
J. Org. Chem. 2000, 65, 767 – 774.
[5] For the use of Cu catalysts, see: B. H. Lipshutz, H. Shimizu,
Angew. Chem. 2004, 116, 2278 – 2280; Angew. Chem. Int. Ed.
2004, 43, 2228 – 2230.
[6] For the use of Zn catalysts, see: B.-M. Park, S. Mun, J. Yun, Adv.
Synth. Catal. 2006, 348, 1029 – 1032.
[7] For the use of Fe catalysts, see: S. Zhou, S. Fleischer, K. Junge, S.
Das, D. Addis, M. Beller, Angew. Chem. 2010, 122, 8298 – 8302;
Angew. Chem. Int. Ed. 2010, 49, 8121 – 8125.
[8] For the use of Ru catalysts, see: a) W. Oppolzer, M. Wills, M.
Starkemann, G. Bernardinelli, Tetrahedron Lett. 1990, 31, 4117 –
4120; b) N. Uematsu, A. Fujii, S. Hashiguchi, T. Ikariya, R.
Noyori, J. Am. Chem. Soc. 1996, 118, 4916 – 4917; c) K. Abdur-
Rashid, A. J. Lough, R. H. Morris, Organometallics 2001, 20,
1047 – 1049; d) C. J. Cobley, J. P. Henschke, Adv. Synth. Catal.
2003, 345, 195 – 201; e) D. Guijarro, ꢄ. Pablo, M. Yus, Tetrahe-
dron Lett. 2009, 50, 5386 – 5388.
[9] For the use of Rh catalysts, see: a) A. G. Becalski, W. R. Cullen,
M. D. Fryzuk, B. R. James, G.-J. Kang, S. R. Rettig, Inorg. Chem.
1991, 30, 5002 – 5008; b) M. J. Burk, J. E. Feaster, J. Am. Chem.
Soc. 1992, 114, 6266 – 6267; c) J. Mao, D. C. Baker, Org. Lett.
1999, 1, 841 – 843; d) F. Spindler, H.-U. Blaser, Adv. Synth. Catal.
2001, 343, 68 – 70; e) R. Kadyrov, T. H. Riermeier, Angew.
Chem. 2003, 115, 5630 – 5632; Angew. Chem. Int. Ed. 2003, 42,
5472 – 5474; f) G. Shang, Q. Yang, X. Zhang, Angew. Chem.
2006, 118, 6508 – 6510; Angew. Chem. Int. Ed. 2006, 45, 6360 –
6362.
[10] For the use of Pd catalysts, see: a) H. Abe, H. Amii, K.
Uneyama, Org. Lett. 2001, 3, 313 – 315; b) Q. Yang, G. Shang, W.
Gao, J. Deng, X. Zhang, Angew. Chem. 2006, 118, 3916 – 3919;
Angew. Chem. Int. Ed. 2006, 45, 3832 – 3835; c) Y.-Q. Wang, S.-
M. Lu, Y.-G. Zhou, J. Org. Chem. 2007, 72, 3729 – 3734.
[11] For the use of Re catalysts, see: a) K. A. Nolin, R. W. Ahn, F. D.
Toste, J. Am. Chem. Soc. 2005, 127, 12462 – 12463; b) K. A.
Nolin, R. W. Ahn, Y. Kobayashi, J. J. Kennedy-Smith, F. Toste,
Chem. Eur. J. 2010, 16, 9555 – 9562.
[12] For applications of Ir catalysts, see: a) F. Spindler, B. Pugin, H.-
U. Blaser, Angew. Chem. 1990, 102, 561 – 562; Angew. Chem. Int.
Ed. Engl. 1990, 29, 558 – 559; b) A. Togni, Angew. Chem. 1996,
108, 1581 – 1583; Angew. Chem. Int. Ed. Engl. 1996, 35, 1475 –
1477; c) R. Sablong, J. A. Osborn, Tetrahedron: Asymmetry
1996, 7, 3059 – 3062; d) G. Zhu, X. Zhang, Tetrahedron: Asym-
metry 1998, 9, 2415 – 2418; e) C. Bianchini, P. Barbaro, G.
Scapacchi, E. Farnetti, M. Graziani, Organometallics 1998, 17,
3308 – 3310; f) D. Xiao, X. Zhang, Angew. Chem. 2001, 113,
3533 – 3536; Angew. Chem. Int. Ed. 2001, 40, 3425 – 3428; g) X.-
b. Jiang, A. J. Minnaard, B. Hessen, B. L. Feringa, A. L. L.
Duchateau, J. G. O. Andrien, J. A. Boogers, J. G. de Vries, Org.
Lett. 2003, 5, 1503 – 1506; h) E. Guiu, B. Muꢅoz, S. Castillꢂn, C.
Claver, Adv. Synth. Catal. 2003, 345, 169 – 171; i) C. Moessner, C.
Bolm, Angew. Chem. 2005, 117, 7736 – 7739; Angew. Chem. Int.
Ed. 2005, 44, 7564 – 7567; j) T. Imamoto, N. Iwadate, K. Yoshida,
Org. Lett. 2006, 8, 2289 – 2292; k) S.-F. Zhu, J.-B. Xie, Y.-Z.
Experimental Section
All hydrogenation experiments were carried out in a Parr Instruments
4560 series autoclave (300 mL) containing an alloy plate with wells for
seven 4 mL glass vials.
Typical procedure: In a glovebox under an argon atmosphere,
imine 2a (98 mg, 0.5 mmol), (S)-TRIP (3.7 mg, 0.005 mol), complex 5
(9.8 mg, 0.025 mmol), toluene (0.2 mL), and a magnetic stir bar were
placed in a vial, which was then capped with a septum equipped with a
syringe. The vial was placed in the alloy plate, which was then placed
in the predried autoclave. Once sealed, the autoclave was purged
three times with hydrogen, then pressurized to 50 bar and heated at
658C for 1 day. The autoclave was then cooled to 58C and
depressurized, and the reaction mixture was transferred to a flask.
A 1m solution of KOH in methanol (0.1 mL) was added to the
reaction mixture, and the solvent was evaporated under reduced
pressure. The residue was purified by column chromatography on
silica gel (eluant: hexane/ethyl acetate 10:1) to give the corresponding
amine 3a (79 mg, 80%), which was analyzed by HPLC to determine
the ee value.
Received: February 3, 2011
Published online: April 15, 2011
Keywords: asymmetric catalysis · hydrogenation · imines · iron ·
.
organocatalysis
[1] Modern Amination Methods (Ed.: A. Ricci), Wiley-VCH,
Weinheim, 2000.
[2] See, for example: http://www.basf.de/en/intermed/products/chi-
pros/amines/.
[3] For reviews on this topic, see: a) F. Spindler, H.-U. Blaser in
Transition Metals for Organic Synthesis, 2nd ed., Vol. 2 (Eds.: M.
Beller, C. Bolm), Wiley-VCH, Weinheim, 2004, p. 113; b) H.-U.
Blaser, F. Spindler in Handbook of Homogeneous Hydrogena-
tion, Vol. 3 (Eds.: J. G. de Vires, C. J. Elsevier), Wiley-VCH,
Weinheim, 2007, p. 1193; c) M. J. Palmer, M. Wills, Tetrahedron:
Asymmetry 1999, 10, 2045 – 2061; d) H.-U. Blaser, C. Malan, B.
Pugin, F. Spinder, H. Steiner, M. Studer, Adv. Synth. Catal. 2003,
345, 103 – 151; e) W. Tang, X. Zhang, Chem. Rev. 2003, 103,
Angew. Chem. Int. Ed. 2011, 50, 5120 –5124
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5123

Communications
Zhang, S. Li, Q.-L. Zhou, J. Am. Chem. Soc. 2006, 128, 12886 –
d) B. D. Sherry, A. Fꢃrstner, Acc. Chem. Res. 2008, 41, 1500 –
1511; e) R. H. Morris, Chem. Soc. Rev. 2009, 38, 2282 – 2291;
f) Junge, K. Schrꢀder, M. Beller, Chem. Commun. 2011,
DOI:10.1039/C0CC05733A.
12891; l) M. T. Reetz, O. Bondarev, Angew. Chem. 2007, 119,
4607 – 4610; Angew. Chem. Int. Ed. 2007, 46, 4523 – 4526; m) C.
Li, C. Wang, B. Villa-Marcos, J. Xiao, J. Am. Chem. Soc. 2008,
130, 14450 – 14451; n) C. Li, B. Villa-Marcos, J. Xiao, J. Am.
Chem. Soc. 2009, 131, 6967 – 6969; o) C. Li, C. Wang, B. Villa-
Marcos, J. Xiao, J. Am. Chem. Soc. 2009, 130, 14450 – 14451;
p) S. Shirai, H. Nara, Y. Kayaki, T. Ikariya, Organometallics
2009, 28, 802 – 809; q) Z. Han, Z. Wang, X. Zhang, K. Ding,
Angew. Chem. 2009, 121, 5449 – 5453; Angew. Chem. Int. Ed.
2009, 48, 5345 – 5349; r) W. Li, G. Hou, M. Chang, X. Zhang,
[18] a) J.-S. Chen, L.-L. Chen, Y. Xing, G. Chen, W.-Y. Shen, Z.-R.
Dong, Y.-Y. Dong, Y.-Y. Li, J.-X. Gao, Acta Chim. Sin. 2004, 62,
1745 – 1750; b) C. P. Casey, H. Guan, J. Am. Chem. Soc. 2007,
129, 5816 – 5817; c) H. Nishiyama, A. Furuta, Chem. Commun.
2007, 760 – 762; d) N. S. Shaikh, S. Enthaler, K. Junge, M. Beller,
Angew. Chem. 2008, 120, 2531 – 2535; Angew. Chem. Int. Ed.
2008, 47, 2497 – 2501; e) B. K. Langlotz, H. Wadepohl, L. H.
Gade, Angew. Chem. 2008, 120, 4748 – 4752; Angew. Chem. Int.
Ed. 2008, 47, 4670 – 4674; f) A. M. Tondreau, J. M. Darmon,
B. M. Wile, S. K. Floyd, E. Lobkovsky, P. J. Chrik, Organo-
metallics 2009, 28, 3928 – 3940; g) C. Sui-Seng, F. Freutel, A. J.
Lough, R. H. Morris, Angew. Chem. 2008, 120, 954 – 957; Angew.
Chem. Int. Ed. 2008, 47, 940 – 943; h) N. Meyer, A. J. Lough,
R. H. Morris, Chem. Eur. J. 2009, 15, 5605 – 5610; i) A. Mikhai-
line, A. J. Lough, R. H. Morris, J. Am. Chem. Soc. 2009, 131,
1394 – 1395; j) C. Sui-Seng, F. N. Haque, A. Hadzovic, A.-M.
Pꢃtz, V. Reuss, N. Meyer, A. J. Lough, M. Zimmer-De Iuliis,
R. H. Morris, Inorg. Chem. 2009, 48, 735 – 743; k) C. P. Casey, H.
Guan, J. Am. Chem. Soc. 2009, 131, 2499 – 2507; l) D. Addis, N.
Shaikh, S. Zhou, S. Das, K. Junge, M. Beller, Chem. Asian J.
2010, 5, 1687 – 1691; m) J. Yang, T. D. Tilley, Angew. Chem. 2010,
122, 10384 – 10386; Angew. Chem. Int. Ed. 2010, 49, 10186 –
10188; n) A. Naik, T. Maji, O. Reiser, Chem. Commun. 2010,
46, 4475 – 4477; o) R. Langer, G. Leitus, Y. Ben-David, D.
Milstein, Angew. Chem. 2011, 123, 2168 – 2172; Angew. Chem.
Int. Ed. 2011, 50, 2120 – 2124.
[19] For selected examples of iron-catalyzed reactions from our
research group, see: a) I. Iovel, K. Mertins, J. Kischel, A. Zapf,
M. Beller, Angew. Chem. 2005, 117, 3981 – 3985; Angew. Chem.
Int. Ed. 2005, 44, 3913 – 3917; b) J. Kischel, I. Jovel, K. Mertins,
A. Zapf, M. Beller, Org. Lett. 2006, 8, 19 – 22; c) S. Enthaler, G.
Erre, M. K. Tse, K. Junge, M. Beller, Tetrahedron Lett. 2006, 47,
8095 – 8099; d) S. Enthaler, B. Hagemann, G. Erre, K. Junge, M.
Beller, Chem. Asian J. 2006, 1, 598 – 604; e) G. Anilkumar, B.
Bitterlich, F. G. Gelalcha, M. K. Tse, M. Beller, Chem. Commun.
2007, 289 – 291; f) F. G. Gelalcha, B. Bitterlich, G. Anilkumar,
M. K. Tse, M. Beller, Angew. Chem. 2007, 119, 7431 – 7435;
Angew. Chem. Int. Ed. 2007, 46, 7293 – 7296; g) F. Shi, M. K. Tse,
M.-M. Pohl, A. Brꢃckner, S. Zhang, M. Beller, Angew. Chem.
2007, 119, 9022 – 9024; Angew. Chem. Int. Ed. 2007, 46, 8866 –
8868; h) S. Zhou, K. Junge, D. Addis, S. Das, M. Beller, Angew.
Chem. 2009, 121, 9671 – 9674; Angew. Chem. Int. Ed. 2009, 48,
9507 – 9510; i) S. Zhou, D. Addis, S. Das, K. Junge, M. Beller,
Chem. Commun. 2009, 4883 – 4885; j) K. Junge, B. Wendt, N.
Shaikh, M. Beller, Chem. Commun. 2010, 46, 1769 – 1771.
[20] H.-J. Knꢀlker, E. Baum, H. Goesmann, R. Klauss, Angew. Chem.
1999, 111, 2196 – 2199; Angew. Chem. Int. Ed. 1999, 38, 2064 –
2066.
ˇ ´
Adv. Synth. Catal. 2009, 351, 3123 – 3127; s) N. Mrsic, A. J.
Minnaard, B. L. Feringa, J. G. de Vries, J. Am. Chem. Soc. 2009,
131, 8358 – 8359; t) G. Hou, F. Gosselin, W. Li, J. C. McWilliams,
Y. Sun, M. Weisel, P. D. OꢆShea, C.-Y. Chen, I. W. Davies, X.
Zhang, J. Am. Chem. Soc. 2009, 131, 9882 – 9883; u) A. Beaza, A.
Pfaltz, Chem. Eur. J. 2010, 16, 4003 – 4009; v) G. Hou, R. Tao, Y.
Sun, X. Zhang, F. Gosselin, J. Am. Chem. Soc. 2010, 132, 2124 –
2125; w) B. Villa-Marcos, C. Li, K. Mulholland, P. J. Hogan, J.
Xiao, Molecules 2010, 15, 2453 – 2472.
[13] a) M. Rueping, E. Sugiono, C. Azap, T. Theissmann, M. Bolte,
Org. Lett. 2005, 7, 3781 – 3783; b) M. Rueping, A. P. Antonchik,
T. Theissmann, Angew. Chem. 2006, 118, 3765 – 3768; Angew.
Chem. Int. Ed. 2006, 45, 3683 – 3686; c) M. Rueping, A. P.
Antonchik, T. Theissmann, Angew. Chem. 2006, 118, 6903 – 6907;
Angew. Chem. Int. Ed. 2006, 45, 6751 – 6755; d) S. Hoffmann,
A. M. Seayad, B. List, Angew. Chem. 2005, 117, 7590 – 7593;
Angew. Chem. Int. Ed. 2005, 44, 7424 – 7427; e) S. Hoffmann, M.
Nicoletti, B. List, J. Am. Chem. Soc. 2006, 128, 13074 – 13075;
f) V. N. Wakchaure, J. Zhou, S. Hoffmann, B. List, Angew. Chem.
2010, 122, 4716 – 4718; Angew. Chem. Int. Ed. 2010, 49, 4612 –
4614; g) V. N. Wakchaure, M. Nicoletti, L. Batjen, B. List, Synlett
2010, 2708 – 2710; h) R. I. Storer, D. E. Carrera, Y. Ni, D. W. C.
MacMillan, J. Am. Chem. Soc. 2006, 128, 84 – 86; i) G. Li, Y.
Liang, J. C. Antilla, J. Am. Chem. Soc. 2007, 129, 5830 – 5831.
[14] For selected reviews on chiral Brønsted acid catalysis, see:
a) A. G. Doyle, E. N. Jacobsen, Chem. Rev. 2007, 107, 5713 –
5743; b) T. Akiyama, J. Itoh, K. Fuchibe, Adv. Synth. Catal. 2006,
348, 999 – 1010.
[15] For an example of and a review on the combination of chiral
Brønsted acid catalysis with transition-metal catalysis, see:
a) G. L. Hamilton, E. J. Kang, M. Mba, F. D. Toste, Science
2007, 317, 496 – 499; b) C. Zhong, X. Shi, Eur. J. Org. Chem.
2010, 2999 – 3025.
[16] S. Shima, O. Pilak, S. Vogt, M. Schick, M. S. Stagni, W. Meyer-
Klaucke, E. Warkentin, R. K. Thauer, U. Ermler, Science 2008,
321, 572 – 575.
[17] For selected reviews on iron catalysis, see: a) C. Bolm, J. Legros,
J. L. Paith, L. Zani, Chem. Rev. 2004, 104, 6217 – 6254; b) S.
Enthaler, K. Junge, M. Beller, Angew. Chem. 2008, 120, 3363 –
3367; Angew. Chem. Int. Ed. 2008, 47, 3317 – 3321; c) A. Correa,
O. G. Mancheꢅo, C. Bolm, Chem. Soc. Rev. 2008, 37, 1108 – 1117;
5124
www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 5120 –5124