Iridium-catalyzed highly enantioselective hydrogenation of the C=C bond of α, β-unsaturated ketones

Article abstract of DOI:10.1002/anie.200803872

(Chemical Equation Presented) A fine selection: The installation of chiral centers at the α position of ketones is achieved with excellent enantioselectivity by means of iridium-catalyzed asymmetric hydrogenation of α,β-unsaturated ketones under mild conditions. The catalyst incorporates a phosphinooxazoline (PHOX)-based ligand, which exhibits high levels of asymmetric induction (see scheme; BAr_F^-= tetrakis[3,5-bis(trifluoromethyl)phenyl]borate; cod=cyclooctadiene).

Full text of DOI:10.1002/anie.200803872

Angewandte
Chemie
DOI: 10.1002/anie.200803872
Asymmetric Catalysis
=
Iridium-Catalyzed Highly Enantioselective Hydrogenation of the C C
Bond of a, b-Unsaturated Ketones**
Wei-Jing Lu, Yun-Wei Chen, and Xue-Long Hou*
Ketones with a-chiral carbon centers are an important group
of compounds in organic synthesis,[1] and many efforts have
been focused on their preparation. However, high enantiose-
lectivity has seldom been reported for the synthesis of a-
substituted ketones,[2–5] because the products undergo race-
mization of the chiral carbon center through enolization.
Installing chiral centers at the a-position of ketones in high
enantioselectivity remains a great challenge. Conversely,
asymmetric hydrogenation is a very practical method and
has attracted much attention. Many transition metals have
been used in catalysts for such conversions, and a variety of
unsaturated compounds have been hydrogenated in the
presence of chiral transition-metal complexes.[6] Among
them, chiral iridium complexes have unique properties, both
in asymmetric induction and catalytic activity.[7] Not only
alkenes but also imines and ketones are suitable substrates for
the hydrogenation reaction. Seemingly, ketones with a-chiral
carbon centers are accessible from asymmetric hydrogenation
of corresponding a-substituted a,b-unsaturated ketones.
However, few examples were found to establish chiral centers
at the a position of ketones using the hydrogenation
strategy.[8,9] Although asymmetric hydrogenation of a,b-
unsaturated ketones has been well documented, in most
cases, the carbonyl group is reduced preferentially to afford
the allyl alcohol.[10] In studying the effectiveness of chiral
ligands in asymmetric catalysis, we investigated whether or
not the carbon–carbon double bond of a,b-unsaturated
ketones is hydrogenated selectively to provide ketones with
chiral center at the a position of carbonyl group. Herein, we
address this important issue and disclose preliminary results
on the asymmetric hydrogenation of a,b-unsaturated ketones,
using chiral iridium complexes to provide ketones with a-
carbon chiral center in high enantioselectivity.[7]
Initially, asymmetric hydrogenation of a,b-unsaturated
ketones 1a and 2 were investigated by using 1 mol% of the Ir
catalyst 4a in dichloromethane under 50 bar of H2 pressure at
room temperature [Eq. (1)].[7d] Hydrogenation of 1a pro-
vided the ketone 3a in 91% ee, whereas that of 2d afforded a
mixture of the alkyl ketone, the alkyl alcohol, and residual
substrate. No reaction took place for 2a–c. These results
demonstrated that the reaction, under the aforementioned
conditions, preferentially reduces carbon–carbon double
bonds of acyclic a-substituted a,b-unsaturated ketones, such
as 1a.
Based upon these results, screening of solvents and
catalyst counterions was carried out (Table 1). Only the
reactions in weakly polar solvents toluene (Table 1, entry 2)
and CH2Cl2 (Table 1, entry 7) afforded high conversion and
high enantioselectivity, with the reaction in CH2Cl2 furnishing
Table 1: The effect of various solvents and counterions on the asym-
metric hydrogenation of a,b-unsaturated ketone 1a.[a]
[*] W.-J. Lu, Y.-W. Chen, Prof. X.-L. Hou
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
Entry
Solvent
Counterion
Conv. [%][b]
ee [%][c]
À
1
2
3
4
5
6
7
8
9
MeOH
toluene
n-hexane
THF
ethyl acetate
acetone
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
BArFÀ
0
100
6
20
58
0
100
9
16
13
–
86
–
BArFÀ
BArFÀ
BArFÀ
BArFÀ
BArFÀ
BArF
354 Fenglin Road, Shanghai 200032 (P.R. China)
Fax: (+86)21-54925100
E-mail: xlhou@mail.sioc.ac.cn
n.d.[d]
n.d.[d]
–
[**] This work was financially supported by the Major Basic Research
Development Program (2006CB806106), National Natural Science
Foundation of China (20532050, 20672130), Chinese Academy of
Sciences, Croucher Foundation of Hong Kong, and Science and
Technology Commission of Shanghai Municipality. Y.-W. C. is a
participant of the undergraduate program of Shanghai Normal
University.
91
ClÀ
n.d.[d]
n.d.[d]
n.d.[d]
À
BF4À
10
PF6
[a] 1a (0.2 mmol), 4a (1 mol%, counterions vary), H2 (50 bar) in 2 mL of
solvent at room temperature for 24 h. [b] Determined by 1H NMR
spectroscopy. [c] Determined by chiral HPLC. [d] n.d.=not determined.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803872.
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10133

Communications
better results. Reactions in other nonpolar and polar solvents,
PHOX gave better enantioselectivity than the analogous
benzylic substituted catalyst 4d (Table 2, entry 8 vs. entry 4).
Under the above reaction conditions, different substrates
were subjected to hydrogenation using 4h as a catalyst,
(Table 3). All reactions of ketones incorporating aryl b-
substituents, including cyclic ketones 1e, 1 f, and 1h–j
(Table 3, entries 5, 6, and 8–10) and acyclic ketones 1a–d
such as hexane, THF, ethyl acetate, MeOH, and acetone, gave
either product in low conversion or no product (Table 1,
entries 1,3–6). Among the counterions tested, only tetrakis-
[3,5-bis(trifluoromethyl)phenyl]borate (BArF) resulted in a
high-yielding, enantioselective reaction (Table 1, entries 7–
10). These results reflect the same tendency as repor-
ted.[7d,11,12]
The impact of changes in the phosphane ligands devel-
oped by our group,[13] having different substituents on the
oxazoline ring and different aryl groups on P , as well as
phosphinooxazoline (PHOX) ligands, was investigated.[7d,11]
(Table 2). Catalysts 4a–f, from benzylic-substituted ligands,
and catalyst 4h, from the tBu-substituted PHOX ligand,
Table 3: Asymmetric hydrogenation of a,b-unsaturated ketone 1 using
catalyst 4h.[a]
Table 2: The effect of the catalyst structures on the asymmetric hydro-
genation of a,b-unsaturated ketone 1a.[a]
Entry
Catalyst
Conv. [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
4a
4b
4c
4d
4e
4 f
100
100
100
100
100
100
93
91
34
89
86
92
75
79
98
Under 50 bar of H2 Under ambient pressure of H2
Entry Substrate Conv. [%][b] ee [%][c] Conv. [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1 f
1g
1h
1i
100
100
100
100
100
100
100
100
100
100
78
98
98
97
98
98
98
97
100
100
100
100
100
100
95
98[f]
98
4g
4h
97[f]
98
100
[a] 1a (0.2 mmol), catalyst (1 mol%), H2 (50 bar) in 2 mL of CH2Cl2 at
room temperature for 24 h. BArFÀ =tetrakis[3,5-bis(trifluoromethyl)phe-
nyl]borate; cod=cyclooctadiene. [b] Determined by 1H NMR spectros-
copy. [c] Determined by chiral HPLC.
99
97[f]
97
>99[d] 100
>99[f,g]
>99[h,i]
98[f]
–
95[e,f] 100
98
40
–
10
11
12[j]
1j
1k
1l
100
–
–
achieved complete conversion of a,b-unsaturated ketones 1a
(Table 2, entries 1–6 and 8), whereas the conversion was only
93% using catalyst 4g, from iPr-substituted PHOX (Table 2,
entry 7). Using catalyst 4h led to formation of product 3a
with 98% ee, the highest enantioselectivity among the
catalysts screened, (Table 2, entry 8), whereas catalyst 4g
gave 3a in only 79% ee (Table 2, entry 7). For benzylic
ligands, catalyst 4a, with iPr-substituted oxazoline, and
catalyst 4e, with iPr-substituted oxazoline and two p-anisyl
groups on P, provided 3a in 91% and 92% ee respectively
(Table 2, entries 1 and 5), whereas catalyst 4b, with Me-
substituted oxazoline, provided 3a in 34% ee (Table 2,
entry 2). The other catalysts 4c and 4d, incorporating iBu
and tBu oxazoline substituents, afforded 3a with 89% and
86% ee (Table 2, entries 3 and 4). Notably, the two types of
ligands showed different tendencies towards enantioselectiv-
ity. Catalyst 4a, derived from benzylic substituted ligands with
iPr-substituted oxazoline, gave better enantioselectivity than
catalyst 4g, derived from iPr-substituted PHOX (Table 2,
entry 1 vs. entry 7), whereas catalyst 4h, derived from tBu
0
–
[a] Conditions unless otherwise stated: substrate (0.2 mmol), catalyst
4h (1 mol%), CH2Cl2 (2 mL), room temperature for 24 h. [b] Determined
1
by H NMR spectroscopy. [c] Determined by chiral HPLC. [d] cis/trans=
1:49, determined by 1H NMR spectroscopy. [e] cis/trans=1:10.
[f] 3 mol% of catalyst was used. [g] cis/trans=1:>50. [h] 5 mol% of
catalyst was used. [i] cis/trans=1:38. [j] Catalyst 4a was used.
and 1g (Table 3, entries 1–4 and 7), afforded chiral ketones in
95–99% ee. Heteroaryl substituted 1j was also a suitable
substrate (Table 3,entry 10), affording 3j in 98% ee. How-
ever, hydrogenation of ketone 1k, with iPr at the b position,
gave 3k in 40% ee with 78% conversion using catalyst 4h
(Table 3, entry 11), and with 100% conversion but no
enantioselectivity using catalyst 4a (not shown in Table),
No reaction took place for ketone substrate 1l, with nPr at the
b position (Table 3, entry 12). Interestingly, a,a’-bis(benzyl-
idene) ketones 1h and 1i were smoothly hydrogenated to
provide corresponding ketones in high diastereo- and enan-
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Chemie
Tetrahedron Lett. 2007, 48, 7591; k) D. J. Weix, J. F. Hartwig, J.
tioselectivities; > 99% ee with a ratio of 1:49 in favor of the
trans product for 3h (Table 3, entry 8), and 95% ee with 1:10
diastereoselectivity in favor of the trans product for 3i
(Table 3, entry 9). Even under 50 bar pressure of H2, no
Am. Chem. Soc. 2007, 129, 7720; l) H. He, X.-J. Zheng, Y. Li, L.-
X. Dai, S.-L. You, Org. Lett. 2007, 9, 4339; m) M. Braun, T.
Meier, F. Laicher, P. Meletis, M. Fidan, Adv. Synth. Catal. 2008,
350, 303.
1
reduction of the carbonyl group was detected by H NMR
[4] a) S.-L. You, X.-L. Hou, L.-X. Dai, X.-Z. Zhu, Org. Lett. 2001, 3,
149; b) X. X. Yan, C. G. Liang, Y. Zhang, W. Hong, B. X. Cao,
L. X. Dai, X. L. Hou, Angew. Chem. 2005, 117, 6702; Angew.
Chem. Int. Ed. 2005, 44, 6544; Angew. Chem. 2005, 117, 6702;
c) W.-H. Zheng, B.-H. Zheng, Y. Zhang, X. L. Hou, J. Am.
Chem. Soc. 2007, 129, 7718.
[5] Asymmetric alkylation of ketones by using imines was also
reported: a) for a review: A. Job, C. F. Janeck, W. Bettray, R.
Peters, D. Enders, Tetrahedron 2002, 58, 2253; b) A. I. Meyers,
D. R. Williams, M. Druelinger, J. Am. Chem. Soc. 1976, 98, 3032;
c) A. I. Meyers, D. R. Williams, G. W. Erickson, S. White, M.
Druelinger, J. Am. Chem. Soc. 1981, 103, 3081; d) S.-I. Hashi-
moto, K. Koga, Tetrahedron Lett. 1978, 19, 573; e) D. Lim, D. M.
Coltart, Angew. Chem. 2008, 120, 5285; Angew. Chem. Int. Ed.
2008, 47, 5207.
[6] For reviews, see: a) The Handbook of Homogeneous Hydro-
genation (Eds.: J. G. de Vries, C. J. Elsevier), Wiley-VCH,
Weinheim, 2007; b) S. Kobayashi, H. Ishitani, Chem. Rev. 1999,
99, 1069 – 1094; c) R. Noyori, T. Ohkuma, Angew. Chem. 2001,
113, 40; Angew. Chem. Int. Ed. 2001, 40, 40; d) H. U. Blaser, C.
Malan, B. Pugin, F. Spindler, H. Steiner, M. Studer, Adv. Synth.
Catal. 2003, 345, 103; e) W. Tang, X. Zhang, Chem. Rev. 2003,
103, 3029; f) T. Jerphagnon, J. L. Renaud, C. Bruneau, Tetrahe-
dron: Asymmetry 2004, 15, 2101; g) W. Zhang, Y. Chi, X. Zhang,
Acc. Chem. Res. 2007, 40, 1278; h) A. J. Minnaard, B. L. Feringa,
L. Lefort, J. G. de Vries, Acc. Chem. Res. 2007, 40, 1267; i) W. P.
Hems, M. Groarke, A. Zanotti-Gerosa, G. A. Grasa, Acc. Chem.
Res. 2007, 40, 1340; j) F. D. Klingler, Acc. Chem. Res. 2007, 40,
1367; k) Y. G. Zhou, Acc. Chem. Res. 2007, 40, 1357.
spectroscopy for any substrate. These reaction conditions are
chemospecific for reduction of the carbon–carbon double
bond.
Hydrogenation under mild conditions, especially under
ambient pressure of hydrogen, is extremely desirable in
industry. Thus, the reaction was investigated at this pressure
(Table 3). Most substrates delivered the products in compa-
rable ee values or, in the cases of 1e and 1i (Table 3, entries 5
and 9), even higher ee values than under 50 bar H2, although
higher catalyst loading was required in the reactions of 1c, 1 f,
and 1h–j (Table 3, entries 3, 6 and 8–10).
The high activity of the catalytic system is also evident in
the reaction of 1a under 20 bar of H2 using 0.3 mol% of 4h,
giving the product in 97% ee with 84% conversion.
In summary, a method has been developed for the
installation of chiral centers at the a-position of ketones, by
Ir-catalyzed asymmetric hydrogenation of a, b-unsaturated
ketones. High enantioselectivity was achieved, even under
ambient pressure of hydrogen. The PHOX ligand with a tBu-
substituted oxazoline moiety afforded the system excellent
asymmetric induction ability. Further investigations on the
understanding and applications of these ligands in asymmetric
hydrogenation are currently in progress.
Received: August 6, 2008
Revised: October 13, 2008
[7] For reviews on Ir-catalyzed asymmetric hydrogenation, see:
a) A. Pfaltz, J. Blankenstein, R. Hilgraf, E. Hꢀrmann, S.
Mclntyre, F. Menges, M. Schꢀnleber, S. P. Smidt, B. Wꢁstenberg,
N. Zimmermann, Adv. Synth. Catal. 2003, 345, 33; b) X. Cui, K.
Burgess, Chem. Rev. 2005, 105, 3272; c) K. Kꢂllstrꢀm, I.
Munslow, P. G. Andersson, Chem. Eur. J. 2006, 12, 3194; For
examples of Ir-catalyzed asymmetric hydrogenation, see: d) A.
Lightfoot, P. Schnider, A. Pfaltz, Angew. Chem. 1998, 110, 3047;
Angew. Chem. Int. Ed. 1998, 37, 2897; e) S. Kainz, A. Brink-
mann, W. Leitner, A. Pfaltz, J. Am. Chem. Soc. 1999, 121, 6421;
f) D. Xiao, X. Zhang, Angew. Chem. 2001, 113, 3533; Angew.
Chem. Int. Ed. 2001, 40, 3425; g) W. B. Wang, S. M. Lu, P. Y.
Yang, X. W. Han, Y. G. Zhou, J. Am. Chem. Soc. 2003, 125,
10536; h) Y. Fan, X. Cui, K. Burgess, M. B. Hall, J. Am. Chem.
Soc. 2004, 126, 16688; i) S. M. Lu, X. W. Han, Y. G. Zhou, Adv.
Synth. Catal. 2004, 346, 909; j) K. H. Lam, L. Xu, L. Feng, Q. H.
Fan, F. L. Lam, W. Lo, A. S. C. Chan, Adv. Synth. Catal. 2005,
347, 1755; k) C. Moessner, C. Bolm, Angew. Chem. 2005, 117,
7736; Angew. Chem. Int. Ed. 2005, 44, 7564; l) L. Xu, K. H. Lam,
J. Ji, J. Wu, Q. H. Fan, W. H. Lo, A. S. C. Chan, Chem. Commun.
2005, 1390; m) M. T. Reetz, X. Li, Chem. Commun. 2006, 2159;
n) S. Bell, B. Wꢁstenberg, S. Kaiser, F. Menges, T. Netscher, A.
Pfaltz, Science 2006, 311, 642; o) S. F. Zhu, J. B. Xie, Y. Z. Zhang,
S. Li, Q. L. Zhou, J. Am. Chem. Soc. 2006, 128, 12886; p) C.
Hedberg, K. Kꢂllstrꢀm , P. Brandt, L. K. Hansen, P. G. Ander-
sson, J. Am. Chem. Soc. 2006, 128, 2995; q) S. M. Lu, Y. Q. Wang,
X. W. Han, Y. G. Zhou, Angew. Chem. 2006, 118, 2318; Angew.
Chem. Int. Ed. 2006, 45, 2260; r) G. H. Hou, J. H. Xie, L. X.
Wang, Q. L. Zhou, J. Am. Chem. Soc. 2006, 128, 11774; s) M. N.
Cheemala, P. Knochel, Org. Lett. 2007, 9, 3089; t) M. Engman,
J. S. Diesen, A. Paptchikhine, P. G. Andersson, J. Am. Chem.
Soc. 2007, 129, 4536; u) Z. J. Wang, G. J. Deng, Y. Li, Y. M. He,
W. J. Tang, Q. H. Fan, Org. Lett. 2007, 9, 1243; v) F. Giacomina,
Published online: November 19, 2008
Keywords: enantioselectivity · enones · hydrogenation · iridium ·
.
N,P-ligands
[1] a) K. C. Nicolaou, D. J. Edmonds, A. Li, G. S. Tria, Angew.
Chem. 2007, 119, 4016; Angew. Chem. Int. Ed. 2007, 46, 3942;
b) S. H. Lim, M. D. Curtis, P. Beak, Org. Lett. 2001, 3, 711; c) U.
Schneider, X. Pannecoucke, J. C. Quirion, Synlett 2005, 1853.
[2] a) W. Oppolzer, C. Darcel, P. Rochet, S. Rosset, J. de Brabander,
Helv. Chim. Acta 1997, 80, 1319; b) M. Kossenjans, J. Martens,
Tetrahedron: Asymmetry 1999, 10, 3409; c) A. G. Myers, B. H.
Yang, H. Chen, J. L. Gleason, J. Am. Chem. Soc. 1994, 116, 9361;
d) D. A. Evans, M. D. Ennis, D. J. Mathre, J. Am. Chem. Soc.
1982, 104, 1737; e) P. E. Sonnet, R. R. Heath, J. Org. Chem. 1980,
45, 3137; f) H. Kaku, S. Takaoka, T. Tsunoda, Tetrahedron 2002,
58, 3401; g) P. C. Hutchison, T. D. Heightman, D. J. Procter, Org.
Lett. 2002, 4, 4583; h) A. M. McGhee, J. C. Kizirian, D. J.
Procter, Org. Biomol. Chem. 2007, 5, 1021.
[3] Some examples of alkylation of ketones by transition-metal-
catalyzed asymmetric allylic alkylation: a) B. M. Trost, G. M.
Schroeder, J. Am. Chem. Soc. 1999, 121, 6759; b) M. Braun, F.
Laicher, T. Meier, Angew. Chem. 2000, 112, 3637; Angew. Chem.
Int. Ed. 2000, 39, 3494; c) U. Kazmaier, Curr. Org. Chem. 2003, 7,
317; d) E. C. Burger, J. A. Tunge, Org. Lett. 2004, 6, 4113;
e) D. C. Behenna, B. M. Stoltz, J. Am. Chem. Soc. 2004, 126,
15044; f) B. M. Trost, J. Xu, J. Am. Chem. Soc. 2005, 127, 2846;
g) B. M. Trost, J. Xu, J. Am. Chem. Soc. 2005, 127, 17180; h) M.
Braun, T. Meier, Synlett 2005, 2968; i) T. Graening, J. F. Hartwig,
J. Am. Chem. Soc. 2005, 127, 17192; j) D. Liu, F. Xie, W. Zhang,
Angew. Chem. Int. Ed. 2008, 47, 10133 –10136
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 10135

Communications
A. Meetsma, L. Panella, L. Lefort, A. H. M. de Vries, J. G.
118, 1281; Angew. Chem. Int. Ed. 2006, 45, 1259; i) Y. Tsuchiya,
Y. Hamashima, M. Sodeoka, Org. Lett. 2006, 8, 4851; j) T.
Ikariya, A. J. Blacker, Acc. Chem. Res. 2007, 40, 1300; k) T.
Ohshima, H. Tadaoka, K. Hori, N. Sayo, K. Mashima, Chem.
Eur. J. 2008, 14, 2060; l) S. M. Lu, C. Bolm, Chem. Eur. J. 2008,
14, 7513.
de Vries, Angew. Chem. 2007, 119, 1519; Angew. Chem. Int. Ed.
2007, 46, 1497; w) M. G. Schrems, E. Neumann, A. Pfaltz,
Angew. Chem. 2007, 119, 8422; Angew. Chem. Int. Ed. 2007, 46,
8274; x) S. Li, S. F. Zhu, C. M. Zhang, S. Song, Q. L. Zhou, J. Am.
Chem. Soc. 2008, 130, 8584; y) Y. Wang, T. Sturm, M. Steurer,
V. B. Arion, K. Mereiter, F. Spindler, W. Weissensteiner,
Organometallics 2008, 27, 1119; z) X. B. Wang, W. Zeng, Y. G.
Zhou, Tetrahedron Lett. 2008, 49, 4922.
[10] a) K. Mashima, T. Akutagawa, X. Zhang, H. Takaya, T. Take-
tomi, H. Kumobayashi, S. Akutagawa, J. Organomet. Chem.
1992, 428, 213; b) T. Ohkuma, H. Ooka, T. Ikariya, R. Noyori, J.
Am. Chem. Soc. 1995, 117, 10417; c) T. Ohkuma, M. Koizumi, H.
Doucet, T. Pham, M. Kozawa, K. Murata, E. Katayama, T.
Yokozawa, T. Ikariya, R. Noyori, J. Am. Chem. Soc. 1998, 120,
13529; d) M. J. Burk, W. Hems, D. Herzberg, C. Malan, A.
Zanotti-Gerosa, Org. Lett. 2000, 2, 4173; e) J. Xie, L. Wang, Y.
Fu, S. Zhu, B. Fan, H. Duan, Q. Zhou, J. Am. Chem. Soc. 2003,
125, 4404.
[11] a) G. Helmchen, A. Pfaltz, Acc. Chem. Res. 2000, 33, 336; b) S. J.
Roseblade, A. Pfaltz, Acc. Chem. Res. 2007, 40, 1402.
[12] S. P. Smidt, N. Zimmermann, M. Studer, A. Pfaltz, Chem. Eur. J.
2004, 10, 4685.
[13] a) X. L. Hou, D. X. Dong, K. Yuan, Tetrahedron: Asymmetry
2004, 15, 2189; b) W. Q. Wu, Q. Peng, D. X. Dong, X. L. Hou,
Y. D. Wu, J. Am. Chem. Soc. 2008, 130, 9717.
[8] For examples of organocatalyzed asymmetric hydrogenation,
see: a) J. B. Tuttle, S. G. Ouellet, D. W. C. MacMillan, J. Am.
Chem. Soc. 2006, 128, 12662; b) N. J. A. Martin, B. List, J. Am.
Chem. Soc. 2006, 128, 13368.
[9] a) A. Tungler, T. Mꢃthꢄ, J. Petrꢅ, T. Tarnai, J. Mol. Catal. 1990,
61, 259; b) T. Ohta, T. Miyake, N. Seido, H. Kumobayashi, H.
Takaya, J. Org. Chem. 1995, 60, 357; c) G. Fogassy, A. Tungler, A.
Lꢄvai, G. Tꢅth, J. Mol. Catal. A 2002, 179, 101; d) C. Thorey, S.
Bouquillon, A. Helimi, F. Hꢄnin, J. Muzart, Eur. J. Org. Chem.
2002, 2151; e) R. Hilgraf, A. Pfaltz, Adv. Synth. Catal. 2005, 347,
61; f) A. I. McIntosh, D. J. Watson, J. W. Burton, R. M. Lambert,
J. Am. Chem. Soc. 2006, 128, 7329; g) L. Qiu, F. Y. Kwong, J. Wu,
W. H. Lam, S. Chan, W. Y. Yu, Y. M. Li, R. Guo, Z. Zhou,
A. S. C. Chan, J. Am. Chem. Soc. 2006, 128, 5955; h) B. H.
Lipshutz, B. A. Frieman, A. E. Tomaso Jr. , Angew. Chem. 2006,
10136 www.angewandte.org
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