Pd-catalyzed asymmetric hydrogenation of C=C bond of α,β- unsaturated ketones

Article abstract of DOI:10.1055/s-0030-1259725

Homogenous palladium-catalyzed asymmetric hydrogenation of C=C double bond of ,-unsaturated ketones has been -developed by using palladium(II) trifluoroacetate/(S)-7,7-bis-[di(4-methoxyphenyl]phosphino)-1,1-spirobiindane complex [Pd(OCOCF₃)₂-(S)-An-SDP] as the catalyst under ambient hydrogen pressure and room temperature with up to 89% ee. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0030-1259725

LETTER
947
Pd-Catalyzed Asymmetric Hydrogenation of C=C Bond of a,b-Unsaturated
Ketones
A
a
symmetric
Hydro
u
genation of
o
C=C Bond
o
-
f
a
,
b-
U
n
S
saturated Ket
h
ones eng Wang,a Da-Wei Wang,a Yong-Gui Zhou*a,b
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
457 Zhongshan Road, Dalian 116023, P. R. of China
Fax +86(411)84379220; E-mail: ygzhou@dicp.ac.cn
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
345 Lingling Lu, Shanghai 200032, P. R. of China
b
Received 26 November 2010
Dedicated to Professor Xi-Yan Lu and Professor Li-Xin Dai for their lifelong contributions to the development of organic chemistry
which are important groups of compounds in organic syn-
thesis.4 Nevertheless, it has long been a challenge issue.5
For the successful examples, it was mainly focused on cy-
clic substrates,6 and the corresponding linear analogues
Abstract: Homogenous palladium-catalyzed asymmetric hydroge-
nation of C=C double bond of a,b-unsaturated ketones has been
developed by using palladium(II) trifluoroacetate/(S)-7,7¢-bis-
[di(4-methoxyphenyl]phosphino)-1,1¢-spirobiindane
complex
[Pd(OCOCF3)2–(S)-An-SDP] as the catalyst under ambient hydro- were usually overlooked. In 2008, the asymmetric hydro-
gen pressure and room temperature with up to 89% ee.
genation of linear unsaturated ketones achieved a break-
through with Ir complexes by the groups of Bolm and
Hou, respectively (Scheme 2).7 As our ongoing work on
Pd-catalyzed asymmetric hydrogenation reactions, we
tried this kind of substrates.
Key words: palladium, hydrogenation, unsaturated ketones, SDP
ligand
Transition-metal-catalyzed asymmetric hydrogenation is
one of the most powerful tools for the synthesis of optical-
ly active compounds and occupied a venerable position in
organic synthesis.1 Although chiral Ru, Rh, and Ir com-
plexes have been extensively employed in asymmetric hy-
drogenation reactions, it was not until the past few years
that homogenous palladium catalysts were successfully
introduced for this process.2,3 In 2001, Uneyama and co-
workers reported the first homogenous Pd-catalyzed
asymmetric hydrogenation of a-fluorinated iminoesters
with up to 91% ee.2a Subsequently, Pd catalysts were ap-
plied to the asymmetric hydrogenation of a series of acti-
vated imines, such as N-diphenylphosphinyl and N-
tosylimines, and cyclic N-sulfonylimines by us and other
groups, respectively.2d,3b–e In 2005, this catalytic system
was extended to the asymmetric hydrogenation of func-
tionalized ketones by our group with high enantioselectiv-
ity.3a Very recently, Pd-catalyzed asymmetric hydro-
genation of simple indoles and ketimines was developed
by our group successively.3f,h It was also found to be
effective in the asymmetric hydrogenation of N-PMP-
protected fluorinated imines for the synthesis of the corre-
sponding chiral fluorinated amines.3g Despite the progress
achieved, to the best of our knowledge, homogenous Pd-
catalyzed asymmetric hydrogenation of carbon–carbon
double bond has never been realized and attracted our at-
tention (Scheme 1).
R1
O
R1
*
O
Pd, P-P*
H2
R2
R3
R2
R3
1
2
Scheme 1 Pd-catalyzed asymmetric hydrogenation of C=C bond
Ir complex
H2
Bolm and Hou group
R1
*
O
R1
O
homogenous Pd catalyst
R2
R3
R2
R3
H2
1
2
this work
Scheme 2 Asymmetric hydrogenation of linear a,b-unsaturated
ketones
In 2006, Sodeoka and co-workers reported the asymmet-
ric conjugate reduction of unsaturated ketones catalyzed
by palladium complex with EtOH as hydride source.8 This
result promoted us to envision that Pd hydride species
generated under hydrogen atmosphere may provide an al-
ternative approach to reduce unsaturated ketones to satu-
rated ones. Herein, we depicted the first example of
homogenous Pd-catalyzed enantioselective hydrogena-
tion of unsaturated ketones with specific chemoselectivity
using Pd(OCOCF3)2–(S)-An-SDP as catalyst.
The selective asymmetric hydrogenation of C=C bond of
a,b-unsaturated ketones is one of the most straight access
to ketones with a chiral center at the a- or b-position,
In our primary exploratory studies, different types of un-
saturated ketones were tested, and the linear b,b-disubsti-
tuted 1,3-diphenyl-2-butenone [(E)-1a] showed the best
result in terms of enantioselectivity. Therefore, it was se-
lected as a model substrate for further studies.
SYNLETT 2011, No. 7, pp 0947–0950
x
x.
x
x.
2
0
1
1
Advanced online publication: 10.03.2011
DOI: 10.1055/s-0030-1259725; Art ID: W34410ST
© Georg Thieme Verlag Stuttgart · New York

948
D.-S. Wang et al.
LETTER
Table 1 Optimization of Reaction Conditionsa
hydrogenated product saturated alcohol appeared during
this process. Subsequently, a control experiment was car-
ried out to distinguish our catalytic system from Sodeo-
ka’s.8 No conversion occurred when the reaction was
performed in TFE without hydrogen (Table 1, entry 2),
suggesting that hydrogen rather than TFE itself acts as hy-
dride source; thus, this reaction is a hydrogenation pro-
cess. Subsequently, the effect of solvent was investigated
(Table 1, entries 1 and 3–6). It was found that this reaction
was strongly solvent-dependent. As shown in our previ-
ous work, TFE is most effective in terms of both activity
and enantioselectivity.3 Other alcoholic solvents such as
ethanol and isopropyl alcohol were excluded as they could
serve as hydride sources.
Me
O
Me
*
O
Pd(OCOCF3)2, ligand
solvent, H2 (1 atm), r.t.
Ph
Ph
Ph
Ph
1a
2a
Entry
1
Ligand
L1
Solvent
TFE
Time (h) Conv. (%)b ee (%)c
38
8
>95
<5
61
–
2d
3
L1
TFE
L1
THF
16
14
13
14
33
24
33
24
21
31
12
39
31
68
4
4
L1
acetone
toluene
CH2Cl2
TFE
52
5
5
L1
<5
–
6
L1
<5
–
Various commercially available ligands were then
screened for high enantioselectivity. With axially chiral
bisphosphine ligands L1–5 (Figure 1), the reactions oc-
curred smoothly, 58–70% ee were obtained (Table 1, en-
tries 7–10). Finally, a series of chiral spiro bisphosphine
ligands SDP (L6a–e), which was developed by Zhou
group and successfully applied in asymmetric transforma-
tions,9 were examined (entries 11–15). Ligand L6c (An-
SDP) gave high activity and best enantioselectivity
(Table 1, entry 13, 86% ee). Therefore, the optimal condi-
tions were established as: Pd(OCOCF3)2, An-SDP, H2 (1
atm), TFE, room temperature.
7
L2
>95
>95
>95
>95
>95
>95
>95
>95
>95
58
70
61
60
74
83
86
30
13
8
L3
TFE
9
L4
TFE
10
11
12
13
14
15
L5
TFE
L6a
L6b
L6c
L6d
L6e
TFE
TFE
TFE
TFE
With the optimized reaction conditions in hand, the scope
of substrate was explored (Table 2). Generally, the reac-
tions completed within 12 hours with good to excellent
enantioselectivity. The phenyl group R2 was necessary for
high activity and ee values, and if both R1 and R2 are alkyl
groups the reaction proceeded slowly and with poor enan-
tioselectivity. The length and steric property of R1 dis-
played little effect to the enantioselectivity (Table 2,
entries 1, 2, 4, and 9, 86–88% ee). Changing the substitu-
tion pattern on the ketonic phenyl group affected the ac-
tivity and enantioselectivity variationally within a small
range. For the isopropyl-substituted unsaturated ketones
(E)-1d–h, the enantioselectivity varied from 85% to 89%
(Table 2, entries 4–8). The highest ee was obtained for 2h
with methoxy substituent at the 4-position (Table 2, entry
8, 89% ee). Gratifyingly, this catalytic system was also ef-
TFE
a Conditions: 1a (0.25 mmol), Pd(OCOCF3)2 (0.005 mmol), ligand
(0.006 mmol), solvent (2 mL).
b Determined by GC or 1H NMR.
c Determined by HPLC.
d Without H2.
Initially, the reaction was carried out in 2,2,2-trifluoro-
ethanol (TFE) with Pd(OCOCF3)2–(S)-SynPhos as cata-
lytic system. The reaction occurred smoothly with
saturated ketone 2a as the sole product with 61% ee after
38 hours at room temperature under ambient pressure of
hydrogen (Table 1, entry 1). The reaction was monitored
by GC and 1H NMR analysis, no allylic alcohol or excess
O
O
O
O
O
O
PPh2
PPh2
PPh2
PPh2
PPh2
PPh2
MeO
MeO
PPh2
PPh2
O
O
L1 (S)-SynPhos
L2 (S)-SegPhos
L3 (R)-BINAP
L4 (R)-MeO-BiPhep
O
O
PPh2
PPh2
L6a (R)-SDP, Ar = Ph
PAr2
PAr2
L6b (R)-Tol-SDP, Ar = 4-MeC6H4
L6c (R)-An-SDP, Ar = 4-MeOC6H4
L6d (R)-Xyl-SDP, Ar = 3,5-Me2C6H3
L6e (R)-DMM-SDP, Ar = 4-MeO-3,5-Me2C6H2
L5 (R)-C4-TunePhos
L6 SDP
Figure 1
Synlett 2011, No. 7, 947–950 © Thieme Stuttgart · New York

LETTER
Asymmetric Hydrogenation of C=C Bond of a,b-Unsaturated Ketones
949
fective for alkyl ketones though the enantioselectivity was the benzene ring afforded the corresponding product with
a little lower than the corresponding aryl ketones. In this full conversion and 88% ee (Table 2, entry 5). No other
case, the steric effect of the alkyl group (R1) at the b-posi- byproduct was observed, suggesting that no Pd(0) species
tion on the enantioselectivity was more obvious. For the is involed in the reaction. It is noteworthy that this is the
methyl ketone 1j, the reaction complete within 26 hours first example with a Pd/bisphosphine complex as a ho-
and with only 78% ee (Table 2, entry 10). In contrast, the mogenous asymmetric hydrogenation catalyst of unsatur-
reaction was completed within 8 hours when the substrate ated ketones.
bearing i-Pr or c-Hex, affording the products 2k and 2l in
higher enantioselective manner (83% ee for both). In or-
der to test the geometry of the enone whether it will have
significant effect on the activity and enantioselectivity of
the reaction, (Z)-1d with inverse geometry of the enone
(E)-1d, gave 2d with the same stereochemical outcome
but much lower ee value (Table 2, entry 13, 34% ee) un-
der otherwise identical reaction conditions. Notably, the
hydrogenation of enone 2e with 4-chloro substituent on
In conclusion, we have developed the asymmetric hydro-
genation of C=C bond of linear b,b-disubstituted unsatur-
ated ketones with specific chemoselectivity and high
enantioselectivity by using Pd(OCOCF3)2–(S)-An-SDP
complex as the catalyst under mild conditions.10 Our on-
going experiments are focused on the asymmetric hydro-
genation of other types of substrates with C=C and on
disclosure of the mechanism of Pd-catalyzed asymmetric
hydrogenation reaction.
Table 2 Asymmetric Hydrogenation of Unsaturated Ketone 1 by
Pd(OCOCF3)2–(S)-An-SDP Systema
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
R1
O
R1
*
O
Pd(OCOCF3)2, (S)-An-SDP
TFE, H2 (1 atm), r.t.
R2
R3
R2
R3
Acknowledgment
1
2
We are grateful to the financial support from National Science
Foundation of China (20921092 & 21032003), National Basic Re-
search Program (2010CB833300), and Chinese Academy of Sci-
ences. Thanks for the generous gift of SDP ligands L6a–e from
Prof. Qi-Lin Zhou of Nankai University.
PAr2
PAr2
References and Notes
Ar = 4-MeOC6H4
Entry R1
R2
R3
Time Yield
ee
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(h)
12
36
8
(%)b
(%)c
1d
2
3
4
5
6
7
8
9
Me
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
i-Pr
Ph
2a 95
2b 93
2c 88
2d 97
2e 89
2f 89
2g 86
2h 89
2i 86
2j 70
2k 71
2l 90
2d 99
86 (S)
86 (R)
86 (+)
88 (–)
88 (+)
89 (+)
86 (+)
85 (–)
87 (–)
78 (R)
83 (S)
83 (–)
34 (–)
Et
Ph
Et
4-MeC6H4
Ph
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
Cy
8
4-ClC6H4
4-MeOC6H4
2-MeOC6H4
3-MeOC6H4
Ph
8
10
6
70
4
10
Me
i-Pr
Cy
Me
26
8
11
12
13
Me
Me
5
Ph
Ph
12
a Conditions: 1 (0.125 mmol), Pd(OCOCF3)2 (0.0025 mmol), (S)-An-
SDP (0.003 mmol), TFE (2 mL).
b Isolated yields.
c Determined by HPLC.
d Conditions: 1a (0.25 mmol), Pd(OCOCF3)2 (0.005 mmol), (R)-An-
SDP (0.006 mmol), TFE (2 mL).
Synlett 2011, No. 7, 947–950 © Thieme Stuttgart · New York

950
D.-S. Wang et al.
LETTER
(7) For homogenous hydrogenations of linear enones, see:
(4) Importance of compounds with stereogenic centers at the a-
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(5) Catalytic asymmetric hydrogenation of C=O over C=C:
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(10) Typical Procedure for the Asymmetric Hydrogenation of
b,b-Disubstituted Unsaturated Ketones
(S)-An-SDP (4.2 mg, 0.006 mmol) and Pd(OCOCF3)2 (1.7
mg, 0.005 mmol) were placed in a dried Schlenk tube under
nitrogen atmosphere, and degassed anhyd acetone was
added. The mixture was stirred at r.t. for 1 h. The solvent was
removed under vacuum to give the catalyst. To this catalyst
was added substrate 1a (0.25 mmol) and dry TFE (2 mL)
under a hydrogen atmosphere, then the mixture was stirred
at r.t. After confirmation of consumption of substrate by GC,
the hydrogen gas was slowly released from the reaction
vessel. The ee was determined by HPLC after purification on
silica gel using PE and EtOAc.
(S)-1,3-Diphenylbutan-1-one (2a)
Yield 95%, 86% ee; [a]Dr.t. +0.70 (c 1.87, CHCl3). 1H NMR
(400 MHz, CDCl3): d =1.36 (d, J = 6.9 Hz, 3 H), 3.21 (dd,
J = 16.4, 8.2 Hz, 1 H), 3.32 (dd, J = 16.5, 5.8 Hz, 1 H), 3.49–
3.58 (m, 1 H), 7.20–7.24 (m, 1 H), 7.28–7.36 (m, 4 H), 7.42–
7.50 (m, 2 H), 7.53–7.56 (m, 1 H), 7.93–7.96 (m, 2 H). 13
C
NMR (100 MHz, CDCl3): d = 22.1, 35.7, 47.2, 126.5, 127.0,
128.2, 128.5, 128.7, 133.1, 137.4, 146.7, 199.2. HPLC (AD-
H, eluent: hexanes–i-PrOH = 95:5, detector: 230 nm, flow
rate: 0.8 mL/min), tR1 (S) = 7.6 min (major); tR2 (R) = 8.7
min.
(k) Scheuermann née Taylor, C. J.; Jaekel, C. Adv. Synth.
Catal. 2008, 350, 2708. (l) Shimizu, H.; Nagano, T.; Sayo,
N.; Saito, T.; Ohshima, T.; Mashima, K. Synlett 2009, 3143.
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