Palladium-catalyzed asymmetric hydrogenation of simple ketones activated by Br?nsted acids

Article abstract of DOI:10.1016/j.tetlet.2011.03.057

Homogeneous Pd(OCOCF₃)₂/(R)-C₄-TunePhos has been successfully applied in the asymmetric hydrogenation of simple ketones activated by catalytic amount of Br?nsted acid with up to 88% ee.

Full text of DOI:10.1016/j.tetlet.2011.03.057

Tetrahedron Letters 52 (2011) 2826–2829
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Palladium-catalyzed asymmetric hydrogenation of simple ketones activated
by Brønsted acids
Xiao-Yu Zhou a, Duo-Sheng Wang b, Ming Bao a, , Yong-Gui Zhou b,
⇑
⇑
a State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China
b State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Homogeneous Pd(OCOCF3)2/(R)-C4-TunePhos has been successfully applied in the asymmetric hydroge-
nation of simple ketones activated by catalytic amount of Brønsted acid with up to 88% ee.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 21 December 2010
Revised 3 March 2011
Accepted 14 March 2011
Available online 21 March 2011
Keywords:
Palladium
Asymmetric hydrogenation
Brønsted acid
Simple ketones
Transition-metal-catalyzed asymmetric reduction of prochiral
ketones using molecular hydrogen represents the most simple
and powerful method for the preparation of a variety of enantio-
merically enriched secondary alcohols due to its low cost and com-
plete atom efficiency.1 A series of efficient systems have been
successfully developed for the asymmetric hydrogenation of ke-
tones with chiral Ru,2 Rh,3 Ir,4 Os,5 Fe6 and Cu7 complexes. Much
less attention has been devoted to other transition-metal based
systems.
by chiral palladium complexes with up to 99% ee,12 and very low
reactivity was observed for hydrogenation of ketones. Most re-
cently, we have reported Pd-catalyzed asymmetric hydrogenation
of simple fluorinated imines10g under mild conditions with up to
94% ee and unfunctionalized ketoimines10h activated by Brønsted
acid with up to 95% ee, respectively. However, there were few re-
ports on Pd-catalyzed asymmetric hydrogenation of ketones.
Raja13 and co-workers reported the heterogeneous hydrogenation
of
a-ketoesters with chiral catalyst Pd(allyl)diamino triflate an-
Recently, Pd-based catalytic systems have been developed for
asymmetric carbon–carbon and carbon-heteroatom coupling reac-
tions.8 Although many successful examples of heterogeneous
asymmetric hydrogenation reactions catalyzed by Pd(0) have been
well documented in the literature, very little attention has been
paid to Pd-catalyzed homogeneous asymmetric hydrogenation
reactions.9–14 The first Pd-catalyzed asymmetric hydrogenation of
chored to the inner wall of mesoporous with 67% ee, but in homo-
geneous form, only 55% ee was obtained. In 2005, highly
enantioselective hydrogenation of
a-phthalimide ketones was
developed by us10a using chiral Pd(OCOCF3)2/DuPhos complex,
and the asymmetric hydrogenation of simple ketone was also tried
using the above catalyst, but low reactivity and moderate 52% ee
were obtained. Goulioukina14 et al. reported asymmetric hydroge-
a-fluorinated iminoesters in 2,2,2-trifluoroethanol (TFE) was de-
nation of a-keto phosphonates using Pd(OCOCF3)2/MeO-BiPhep as
scribed by Amii and co-workers with a Pd(OCOCF3)2/BINAP com-
plex with up to 91% ee.9a,c Afterward, Pd-catalyzed asymmetric
hydrogenation of N-diphenylphosphinyl ketimines and N-tosyli-
mines were successively reported by us10b and Zhang11 group,
respectively. Then, chiral palladium complexes have been success-
fully applied to asymmetric hydrogenation of some other activated
imines in our laboratory10c–e with high reactivities and enantiose-
lectivities. In 2009, Rubio-Perez et al. reported the highly enantio-
selective one-pot reductive amination of simple ketones catalyzed
catalyst with up to 55% ee under atmospheric pressure of hydrogen
and 80 °C. Very recently, an efficient Pd-catalyzed asymmetric
hydrogenation of simple indoles was developed by us10f with the
Brønsted acid as activator and up to 96% ee was obtained. Although
much work has been devoted to the development of Pd-catalyzed
asymmetric hydrogenation of activated imines and functionalized
H
X
O
HX
H2
OH
R'
O
R
R'
[Pd], RT
R
R
R'
⇑
Corresponding authors. Tel./fax: +86 411 84379220 (Y.-G.Z).
E-mail address: ygzhou@dicp.ac.cn (Y.-G. Zhou).
Scheme 1. Activation strategy for hydrogenation of simple ketones.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.03.057

X.-Y. Zhou et al. / Tetrahedron Letters 52 (2011) 2826–2829
2827
ketones, the development of Pd-catalyzed asymmetric hydrogena-
tion of simple ketones is still a challenge.
Recently, Brønsted acid is found to be very crucial for asymmet-
ric hydrogenation of imines and heteroaromatic com-
pounds.10h,15,16 Inspired by these examples, we envisioned that
catalytic amount of Brønsted acid may play as activator, in this
case hydrogen bonding interactions, in the Pd-catalyzed asymmet-
ric hydrogenation of simple ketones (Scheme 1). Herein, we report
our preliminary results on Pd-catalyzed asymmetric hydrogena-
tion of simple ketones activated by catalytic amount of Brønsted
acid, and up to 88% ee was achieved.
Acetophenone 1a was chosen as model substrate, the initial
experiment was carried out under 1 atm of hydrogen and at room
temperature in TFE with Pd(OCOCF3)2/(S)-SynPhos system, but
only 39% conversion and 55% ee were obtained (Table 1, entry 1).
Gratifyingly, the catalytic activity was improved dramatically in
the presence of 10 mol % Brønsted acid, benzoic acid 3a, full con-
version and 59% enantioselectivity was obtained (Table 1, entry 2).
Following this encouraging result, the effect of Brønsted acid on
reactivity and enantioselectivity was investigated (Table 1, entries
2–7,). In the case of aliphatic carboxylic acids, 3b and 3c, surpris-
ingly, the catalytic activity and enantioselectivity decreased appar-
ently compared to no additive (entries 3 and 4). The stronger acids
(entries 5 and 6), such as trifluoroacetic acid and p-toluenesulfonic
acid, displayed no more positive impact. Aryl carboxylic acids are
the most effective additives for the reactivity and enantioselectiv-
ity, salicylic acid (3d) gave full conversion and the highest enanti-
oselectivity (61% ee, entry 7). Therefore, salicylic acid was chosen
as the additive for further study. The lower conversion and enanti-
oselectivity (88% conversion and 59% ee, entry 8) were observed
when 3d was reduced to 5 mol %. As the ratio increasing from 10
to 50 mol %, the conversion and enantioselectivity decreased (73%
conversion and 65% ee, entry 10). These results showed 10 mol %
amount of salicylic acid is appropriate.
Table 1
The effect of the additives and solventsa
O
OH
Pd(OCOCF3)2/L1
∗
H2
+
Additive (X mol%)
Solvent, rt, 11 h
(1 atm)
1a
2a
Entry
Solvent
Additive (X mol %)
Convn.b (%)
Eeb (%)
1
2
3
4
5
6
7
8
TFE
none
39
>95
10
22
15
23c
>95
88
92
73
<5
55
59
49
49
46
N/A
61
59
56
TFE
TFE
TFE
TFE
TFE
TFE
TFE
3a (10)
3b (10)
3c (10)
TFA (10)
TsOHÁH2O (10)
3d (10)
3d (5)
9
TFE
TFE
EtOH
Benzene
THF
CH2Cl2
3d (20)
3d (50)
3d (10)
3d (10)
3d (10)
3d (10)
10
11
12
13
14
56
N/A
N/A
N/A
N/A
<5
<5
<5
Subsequently, the effects of solvent, hydrogen pressure, reac-
tion temperature and ligand were studied; a strongly solvent-
dependent effect was observed (Table 1, entries 7, 11–14), EtOH,
benzene, THF and CH2Cl2 led to very low reactivity, only TFE was
found to be the efficient solvent, which was in accordance with
the Pd-catalyzed asymmetric hydrogenation of activated imines
Compound 3a: benzoic acid; 3b: tartaric acid; 3c: 2-hydroxy-iso-butanoic acid; 3d:
salicylic acid.
a
Conditions: 1 atm of H2, 0.25 mmol Ketone, Pd(OCOCF3)2 (2 mol %), (S)-SynPhos
(2.4 mol %), 2 mL solvent, rt, 11 h.
b
Determined by GC.
The product was ether 4: 1-(1-(2,2,2-trifluoroethoxy)ethyl)benzene.
c
Table 2
The effect of chiral ligands on the reactivity and enantioselectivitya
O
OH
H2
Pd(OCOCF3)2/Chiral Ligand
∗
+
(1 atm)
Salicylic acid (10 mol%)
TFE, rt, 11-24 h
1a
2a
Entry
Ligand
T (h)
Convn.b (%)
Eeb (%)
1
2
3
4
5
6
7
8
L1
L2
L3
L4
L5
L6
L7
L8
11
24
24
24
24
24
11
24
>95
<5
<5
8
48
>95
>95
>95
61 (S)
N/A
N/A
35 (R)
65 (S)
64 (S)
67 (R)
61 (S)
O
O
F
O
N
OMe
PPh2
O
O
F
F
PPh2
PPh2
O
O
PPh2
PPh2
Fe
PPh2
O
F
O
L2
L3 (S)-MOP
L4 (R)-Difluorphos
L1 (S)-SynPhos
O
O
O
PPh2
PPh2
MeO
MeO
PPh2
PPh2
O
O
PPh2
PPh2
PPh2
PPh2
O
L7 (R)-C4-TunePhos
L5 (S)-BINAP
L6 (S)-MeO-BiPhep
L8 (S)-SegPhos
a
Conditions: 1 atm of H2, 0.25 mmol ketone, Pd(OCOCF3)2 (2 mol %), chiral ligand (2.4 mol %), 2 mL TFE, rt.
Determined by GC.
b

2828
X.-Y. Zhou et al. / Tetrahedron Letters 52 (2011) 2826–2829
Table 3
Supplementary data
Asymmetric hydrogenation of simple ketonesa
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.03.057.
O
OH
Pd(OCOCF3)2/L7
+
H2
R1
R2
Salicylic Acid (10 mol%)
TFE, rt, 11-16 h
R1
R2
∗
(1 atm)
References and notes
1
2
1. (a) Blaser, H.-U.; Pugin, B.; Spindler, F. In Applied Homogeneous Catalysis With
Organometallic Compounds; Cornils, B., Herrmann, W. A., Eds., second ed.;
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New York, 1999; (c)The Handbook of Homogeneous Hydrogenation; Vries, J. G.,
Elsevier, C. J., Eds.; Wiley-VCH: Weinheim, Germany, 2007.
Entry
R1/R2
Yieldb (%)
Eec (%)
1
2
3
4
5
6
7
8
C6H5/CH3 (1a)
4-F-C6H4/CH3 (1b)
C6H5/Et (1c)
C6H5/n-Pr (1d)
C6H5/n-Bu (1e)
C6H5/i-Pr (1f)
C6H5/t-Bu (1g)
C6H5/C6H5CH2CH2 (1h)
C6H5/C6H5CH2C(CH3)2 (1i)
C6H5/Cyclohexyl (1j)
n-Decanyl/CH3 (1k)
93
84
94
85
99
81
93
85
93
96
83
67 (R)
59 (R)
72 (R)
71 (R)
69 (R)
71 (R)
88 (R)
59 (R)
72 (R)
70 (R)
10 (R)
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Hashiguchi, S.; Fujii, A.; Takehara, J.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc.
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M. Angew, Chem., Int. Ed. 2009, 48, 7556.
9
10
11d
a
Conditions: 1 atm of H2, 0.25 mmol ketone, Pd(OCOCF3)2 (2 mol %), (R)-C4-
TunePhos (2.4 mol %), 2 mL TFE, rt.
b
Isolated yield.
Determined by HPLC or GC.
The reaction time: 48 h and the enantioselectivity was determined by HPLC in
c
d
the form of benzoate ester.
3. Rh-catalyzed hydrogenation of ketones, see: (a) Hauptman, E.; Shapiro, R.;
Marshall, W. Organomettallics 1998, 17, 4976; (b) Kuroki, Y.; Sakamaki, Y.; Iseki,
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4. Ir-catalyzed hydrogenation of ketones, see: (a) Zhang, X.; Taketomi, T.;
Yoshizumi, T.; Kumobayashi, H.; Akutagawa, S.; Mashima, K.; Takaya, H. J.
Am. Chem. Soc. 1993, 115, 3318; (b) Ohkuma, T.; Utsumi, N.; Watanabe, M.;
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Liu, X.-Y.; Kong, W.-L.; Li, S.; Zhou, Q.-L. J. Am. Chem. Soc. 2010, 132, 4538.
5. Os-catalyzed hydrogenation of ketones, see: (a) Baratta, W.; Ballico, M.;
Chelucci, G.; Siega, K.; Rigo, P. Angew, Chem., Int. Ed. 2008, 47, 4362; (b) Baratta,
W.; Ballico, M.; Baldino, S.; Chelucci, G.; Herdtweck, E.; Siega, K.; Magnolia, S.;
Rigo, P. Chem. Eur. J. 2008, 14, 9148; (c) Baratta, W.; Barbato, C.; Magnolia, S.;
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Magnolia, S.; Siega, K.; Rigo, P. Eur. J. Inorg. Chem. 2010, 1419.
6. Fe-catalyzed hydrogenation of ketones, see: (a) Gaillard, S.; Renaud, J.-L.
ChemSusChem 2008, 1, 505; (b) Morris, R. H. Chem. Soc. Rev. 2009, 38, 2282.
7. Cu-catalyzed asymmetric hydrogenation of ketones, see: (a) Shimizu, H.;
Nagasaki, I.; Matsumura, K.; Sayo, N.; Saito, T. Acc. Chem. Res. 2007, 40, 1385;
(b) Shimizu, H.; Igarashi, D.; Kuriyama, W.; Yusa, Y.; Sayo, N.; Saito, T. Org. Lett.
2007, 9, 1655; (c) Shimizu, H.; Nagano, T.; Sayo, N.; Saito, T.; Ohshima, T.;
Mashima, K. Synlett 2009, 3143.
8. (a) Negishi, E. Handbook of Organopalladium Chemistry for Organic Synthesis;
Wiley: Hoboken, NJ, 2002; (b) Tsuji, J. Palladium Reagents and Catalysis; John
Wiley & Sons: Chichester, UK, 2004; (c) Heck, R. F. Pd Reagents in Organic
Synthesis; Academic: New York, 1985; (d) Tietze, L. F.; Ila, H.; Bell, H. P. Chem.
Rev. 2004, 104, 3453.
9. For other groups’ work on Pd-catalyzed asymmetric hydrogenation, see: (a)
Abe, H.; Amii, H.; Uneyama, K. Org. Lett. 2001, 3, 313; (b) Nanayakkara, P.;
Alper, H. Chem. Commun. 2003, 2384; (c) Suzuki, A.; Mae, M.; Amii, H.;
Uneyama, K. J. Org. Chem. 2004, 69, 5132; (d) Tsuchiya, Y.; Hamashima, Y.;
Sodeoka, M. Org. Lett. 2006, 8, 4851.
10. For our group’s work on Pd-catalyzed asymmetric hydrogenation, see: (a)
Wang, Y.-Q.; Lu, S.-M.; Zhou, Y.-G. Org. Lett. 2005, 7, 3235; (b) Wang, Y.-Q.;
Zhou, Y.-G. Synlett 2006, 1189; (c) Wang, Y.-Q.; Lu, S.-M.; Zhou, Y.-G. J. Org.
Chem. 2007, 72, 3729; (d) Wang, Y.-Q.; Yu, C.-B.; Wang, D.-W.; Wang, X.-B.;
Zhou, Y.-G. Org. Lett. 2008, 10, 2071; (e) Yu, C.-B.; Wang, D.-W.; Zhou, Y.-G. J.
Org. Chem. 2009, 74, 5633; (f) Wang, D.-S.; Chen, Q.-A.; Li, W.; Yu, C.-B.; Zhou,
Y.-G.; Zhang, X. J. Am. Chem. Soc. 2010, 132, 8909; (g) Chen, M.-W.; Duan, Y.;
Chen, Q.-A.; Wang, D.-S.; Yu, C.-B.; Zhou, Y.-G. Org. Lett. 2010, 12, 5075; (h)
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and functionalized ketones.9–12 The reaction temperature and H2
pressure had no dramatic impact on enantioselectivity of product
2a.
Then, several commercially available chiral ligands were also
examined under the above optimized conditions, as summarized
in Table 2. P,N-ligand L2, monophosphine ligand MOP L3 and elec-
tron-withdrawing bisphosphine ligand Difluorphos L4 had almost
no catalytic activity (Table 2, entries 2–4). (S)-BINAP L5 gave 48%
conversion and 65% ee. The best result was achieved with (R)-C4-
TunePhos L7, 67% ee and full conversion was observed (entry 7).
Under the optimized conditions, a variety of substituted simple
ketones 1 were hydrogenated, as shown in Table 3. In most cases,
the products were obtained in full conversions and moderate to
good enantioselectivities. Substitution of the aromatic ring in the
ketones has a detrimental effect on the selectivity and enantiose-
lectivity. Substrates bearing the electron-donating p-methyl or p-
methoxy group in the benzene ring of aromatic ketones led to a
significant amount of racemic trifluoroethyl ether as byproduct,
which may be formed via the acid catalyzed etherification of the
corresponding alcohols and solvent trifluoroethanol. Aromatic ke-
tone 1b with electron-deficient fluoro group (entry 2) could be
hydrogenated smoothly, the enantioselectivity decreased to 59%.
Alkyl phenyl ketones 1c–1i can be hydrogenated smoothly with
59–88% ee (entries 3–10), the bulky pivalophenone 1g gave the
highest 88% ee (entry 7). To probe the generality of the catalytic
system, the hydrogenation of dialkyl ketone 1k was also examined
(entry 11), the reaction was completed in 48 h, affording the corre-
sponding product 2k with full conversion but poor enantioselectiv-
ity (Determined by HPLC in the form of benzoate ester).
In summary, a homogeneous Pd-catalyzed asymmetric hydro-
genation of simple ketones activated by catalytic amount of
Brønsted acid was successfully developed using the Pd(OCOCF3)2/
(R)-C4-TunePhos as catalyst with up to 88% ee. In order to provide
the mechanism understanding to rational design of new asymmet-
ric palladium catalysts, the role of salicylic acid and TFE in the reac-
tion is under progress.
11. Yang, Q.; Shang, G.; Gao, W.; Deng, J.; Zhang, X. Angew. Chem., Int. Ed. 2006, 45,
3832.
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Acknowledgments
We are grateful to the financial support from National Science
Foundation of China (20872140), National Basic Research Program
(2010CB833300) and Chinese Academy of Sciences.

X.-Y. Zhou et al. / Tetrahedron Letters 52 (2011) 2826–2829
2829
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