Palladium-catalyzed asymmetric hydrogenation of simple ketimines using a Bronsted acid as activator

Article abstract of DOI:10.1002/adsc.201000554

Using a catalytic amount of a Bronsted acid as activator of simple imines, the highly enantioselective homogeneous palladium-catalyzed asymmetric hydrogenation of simple ketimines was successfully developed with up to 95% ee.

Full text of DOI:10.1002/adsc.201000554

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DOI: 10.1002/adsc.201000554
Palladium-Catalyzed Asymmetric Hydrogenation of Simple
Ketimines Using a Brønsted Acid as Activator
Xiao-Yu Zhou,a Ming Bao,a,* and Yong-Gui Zhoub,*
a
State Key Laboratory of Fine Chemicals, Dalian University of Technology, 158 Zhongshan Road, Dalian 116012, Peopleꢀs
Republic of China
Fax : (+86)-411-3989-3687; phone: (+86)-411-3989-3687; e-mail: mingbao@dlut.edu.cn
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan
b
Road, Dalian 116023, Peopleꢀs Republic of China
Fax : (+86)-411-8437-9220; phone: (+86)-411-8437-9220; e-mail: ygzhou@dicp.ac.cn
Received: July 14, 2010; Published online: December 15, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000554.
Abstract: Using a catalytic amount of a Brønsted
acid as activator of simple imines, the highly enan-
phosphonates with palladium catalyst systems were
reported by us[8] and Goulioukina,[9] respectively. In
comparison, there are more reports on Pd-catalyzed
asymmetric hydrogenations of activated imines. The
first hydrogenation of imino esters catalyzed by palla-
dium complexes was described by Amii[10] and co-
workers with up to 91% ee in 2001. Afterwards,
highly enantioselectively Pd-catalyzed hydrogenations
of N-diphenylphosphinyl ketimines and N-tosylimines
were reported by us[11] and the Zhang[12] group, re-
spectively [Scheme 1, Eq. (1)]. Since then, chiral pal-
ladium complexes have also been employed for the
tioselective
homogeneous
palladium-catalyzed
asymmetric hydrogenation of simple ketimines was
successfully developed with up to 95% ee.
Keywords: asymmetric hydrogenation; Brønsted
acids; ketimines; palladium catalysis
The preparation of chiral amines from the transition asymmetric hydrogenation of some other cyclic acti-
metal-catalyzed asymmetric hydrogenation or transfer vated sulfonylimines in our laboratory.[13] In 2009,
hydrogenation of prochiral imines represents one of Rubio-Pꢁrez[14] and co-workers reported the direct re-
the most direct and efficient approaches that open a ductive amination of alkyl ketones catalyzed by chiral
straightforward route to valuable building blocks for palladium complexes at 708C with up to 99% ee, but
natural products, pharmaceuticals and other fine only moderate yield and low enantiomeric excess
chemical compounds.[1] A variety of chiral metal cata- were observed in the amination of aryl ketones. Most
lysts has been successfully applied to this reaction recently, an efficient Pd-catalyzed asymmetric hydro-
over the last few decades. Among the various catalyt- genation of simple indoles was developed by us with
ic systems so far, Ru,[2] Rh,[3] Ir[4] and organocata- a Brønsted acid as activator.[15] Although much effort
lysts[5] have been successfully employed in enantiose-
lective hydrogenations and/or transfer hydrogenations
of imines. Recently, Zhang and co-workers developed
À
the first asymmetric hydrogenation of unprotected N
H iminiums with up to 95% ee using chiral iridium
complexes as catalyst.[6] Although significant progress
have been made in the enantioselective reduction of
imines, the search for new catalytic systems is still
highly desirable.
Pd-based catalytic systems have been extensively
employed for a wide range of useful synthetic organic
transformations,[7] nevertheless, very little attention
has been paid to homogeneous asymmetric hydroge-
nation with chiral Pd complexes. The asymmetric hy-
drogenations of a-phthalimide ketones and a-keto hydrogenation of simple ketimines.
Scheme 1. Activation strategy for Pd-catalyzed asymmetric
84
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Adv. Synth. Catal. 2011, 353, 84 – 88

Palladium-Catalyzed Asymmetric Hydrogenation of Simple Ketimines
has been successfully made in the asymmetric hydro- peatability was also poor. To enhance the activity of
genation of functionalized ketones and activated imines, a number of Brønsted acids was tested, the re-
imines, there is no report on the highly enantioselec- sults are summarized in Table 1. When one equivalent
tive, Pd-catalyzed hydrogenation of simple ketimines of CF3CO2H was added, both the conversion and
due to the low activity of simple ketimines.
enantioselectivity (30% ee) were drastically improved
Accordingly, Ru-, Rh- and Ir-catalyzed asymmetric (Table 1, entries 1 vs. 2). Other achiral acids also gave
hydrogenations of quinolines[17] and imines[6] revealed full conversions but still with low enantioselectivities
that the addition of a Brønsted acid is one of the key (entries 3 and 4). Next, some commercially available
factors for both high reactivity and enantioselectivity. chiral acids were also tested, (S)-mandelic acid pro-
These results encouraged us to stick to our initial hy- vided complete conversion and slightly high enantio-
pothesis, which is to employ a catalytic amount of a selectivity (50% ee, entry 5). Pleasingly, the addition
Brønsted acid as an activator in the Pd-catalyzed hy- of tartaric acid and its derivatives proceeded with
drogenation of simple ketimines [Scheme 1, Eq. (3)]. moderate enantioselectivities (46–68% ees, entries 6–
Herein, we report our preliminary results on the Pd- 10) and full conversions. Di-p-toluoyl-l-tartaric acid
catalyzed asymmetric hydrogenation of simple ket- (l-DTTA) gave the highest ee (68%, entry 10). It is
ACHTUNGTRENNUNG
Our initial study began with N-(4-methoxyphenyl) sphosphine ligands, and the configuration of the prod-
(PMP) imine 1a as the model substrate and uct is controlled by chiral palladium catalysts. The
PdACHTUNGTRENNUNG(OCOCF3)2/(S)-SynPhos as the catalyst on the effect of the amount of activator l-DTTA on the ac-
basis of our previous successful hydrogenations of tivity and ee was investigated, slightly low enantiose-
functionalized ketones[8] and activated imines.[11,13] lectivity was observed when the amount of l-DTTA
However, low conversion (49% conversion) and an was reduced to 10 mol% (entry 11), gratifyingly, the
almost racemic product were observed in the hydro- hydrogenation reaction proceed smoothly with the
genation of imine 1a (Table 1, entry 1), and the re- best 68% ee in the presence of 20 mol% l-DTTA
(entry 12). Then, different solvents were tested, and a
strong solvent-dependent phenomenon was observed.
Table 1. The effect of the Brønsted acids and solvents.[a]
MeOH and CH2Cl2 led to very low reactivity (en-
tries 13 and 14); TFE was found to be the optimal sol-
vent, which is in accordance with the Pd-catalyzed hy-
drogenation of ketones and activated imines in the lit-
erature.[8–16]
Subsequently, some commercially available bisphos-
phine ligands were examined under the above condi-
Entry
Brønsted Acids
Conversion [%][b]
ee [%][c]
tions. The results showed that axial chiral bidentate
bisphosphine ligands L1–L4 and L6–L8 (Table 2, en-
tries 1–4 and 6–8) gave full conversions with 58–75%
ee. No catalytic activity was observed for the (R,R)-
Me-DuPhos ligand. (R)-C4-TunePhos gave the the
highest ee (75%, entry 7). A decrease of the reaction
temperature does not enhance the enantioselectivity
(entry 9). Therefore, the optimized conditions are:
1
2
3
4
5
6
7
8
none
49
5
CF3CO2H
C6H5CO2H
salicylic acid
(S)-mandelic acid
l-tartaric acid
l-DBTA
l-DMTA
d-DTTA
l-DTTA
l-DTTA
l-DTTA
l-DTTA
l-DTTA
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
34
30
37
36
50
46
65
64
65
68
54
68
2
9
PdACHTNURGTENUNG(OCOCF3)2/(R)-C4-TunePhos/d-DTTA/TFE.
10
11[d]
12[e]
13[e,f]
14[e,g]
Under the optimal reaction conditions, a variety of
N-arylimines 1a–i was subjected to asymmetric hydro-
genation, as shown in Table 3. For the imines 1a–d
with different substituents on the N-atom (Table 3,
entries 1–4), full conversions and 64–75% ees were
achieved. N-(4-methoxyphenyl)imine 1a gave the
highest ee (75%). Several acyclic aryl alkyl imines 1e–
1i can be smoothly hydrogenated with 60–72% ees
(entries 5–9) regardless of the position and electronic
properties of the substituents of the aryl groups.
45
59
[a]
Reaction conditions: 2 mol% Pd-chiral ligand complex
generated from Pd(OCOCF3)2 and (S)-SynPhos in ace-
tone, H2 (600 psi), TFE (2 mL), 1.0 equiv. of additive,
room temperature, 16 h.
AHCTUNGTRENNUNG
[b]
[c]
[d]
[e]
[f]
1
Determined by H NMR.
Determined by HPLC.
10 mol% l-DTTA was added.
20 mol% l-DTTA was added.
Methanol as solvent.
Interestingly, this catalytic system is particularly ef-
fective for the asymmetric hydrogenation of cyclic
ket
ACHTUNGTRENiNGNU mines 1j–1o derived from the cyclic ketone, 1-tet-
[g]
CH2Cl2 as solvent.
ralone, 88–95% ees were achieved (Table 3, en-
Adv. Synth. Catal. 2011, 353, 84 – 88
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85

Xiao-Yu Zhou et al.
COMMUNICATIONS
Table 2. The effect of chiral ligands on the activity and ee.[a]
Table 3. Pd-catalyzed asymmetric hydrogenation of ket-
AHCTUNGTRENNUNG
imines 1.[a]
Entry
L*
Conversion [%][b]
ee [%][c]
1
2
3
4
L1
L2
L3
L4
L5
L6
L7
L8
L7
>95
>95
>95
>95
<5
>95
>95
>95
>95
68 (S)
58 (S)
71 (S)
73 (S)
N/A
54 (R)
75 (R)
72 (R)
75 (R)
Entry
Ketimines 1
Yield [%][b] (Product)
ee [%][c]
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
1q
1r
84 (2a)
95 (2b)
92 (2c)
96 (2d)
78 (2e)
98 (2f)
91 (2g)
99 (2h)
92 (2i)
95 (2j)
92 (2k)
95 (2l)
98 (2m)
92 (2n)
97 (2o)
89 (2p)
98 (2q)
84 (2r)
97 (2s)
75 (R)
71 (+)
70 (À)
64 (+)
68 (+)
72 (+)
60 (+)
64 (À)
63 (+)
94 (+)
90 (+)
95 (+)
92 (+)
95 (+)
88 (+)
93 (+)
91 (+)
93 (+)
86 (+)
5
6[d]
7[d]
8[d]
9[d,e]
[a]
Reaction conditions: 2 mol% Pd-chiral ligand complex
generated from Pd(OCOCF3)2 and chiral ligand in ace-
U
tone, H2 (600 psi), TFE (2 mL), 20 mol% l-DTTA, room
9
temperature, 16 h.
Determined by H NMR.
Determined by HPLC.
d-DTTA was used.
Carried out at 108C.
10
11
12
13
14
15
16
17
18
19
[b]
[c]
[d]
[e]
1
tries 10–15). For the cyclic imines 2p–2s derived from
4-chromanone, 86–93% ee were obtained under the
standard conditions (entries 16–19). The above ap-
proach provides a facile access to chiral cyclic amines
containing 1,2,3,4-tetrahydronanphthyl-1-amine and
3,4-dihydro-2H-chromen-4-amine frameworks, types
of privileged structural motif present in a large
number of drugs and natural compounds.[18]
1s
[a]
Reaction conditions: 2 mol% Pd-chiral ligand complex
generated from Pd(OCOCF3)2 and (R)-C4-TunePhos in
acetone, H2 (600 psi), TFE (2 mL), 20 mol% d-DTTA,
room temperature, 16 h. The conversions are >95%.
Isolated yield.
AHCTUNGTRENNUNG
[b]
[c]
Determined by HPLC.
In summary, using a catalytic amount of a Brønsted
acid as activator of simple imines, the highly enantio- Experimental Section
selective homogeneous Pd-catalyzed asymmetric hy-
drogenation of simple ketimines was successfully de- General Procedure for the Pd-Catalyzed Asymmetric
veloped with up to 95% ee. Our ongoing experiments Hydrogenation of Ketimines
are focused on the asymmetric hydrogenation of
In a flame-dried Schlenk tube equipped with septum cap
other substrates and investigations of the reaction
and stirring bar, PdACHTNUTRGNE(UGN OCOCF3)2 (2 mol%, 0.005 mmol) and
(R)-C4-TunePhos (2.4 mol%, 0.006 mmol) were dissolved in
mechanism.
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Adv. Synth. Catal. 2011, 353, 84 – 88

Palladium-Catalyzed Asymmetric Hydrogenation of Simple Ketimines
dry acetone and stirred under nitrogen at room temperature
for 1 h. The solvent was removed under vacuum to give the
catalyst. This catalyst was taken into a glove box filled with
841; c) V. I. Tararov, R. Kadyrov, T. H. Riermeire, A.
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nitrogen and dissolved in dry TFE (2 mL). To the ketimines
1 (0.25 mmol) and d-DTTA (20 mol%, 0.05 mmol) was
added this catalyst solution, and then the mixture was trans-
ferred to an autoclave. The autoclave was stirred at room
temperature for 16 h. After release of the hydrogen, the au-
toclave was opened and saturated NaHCO3 solution was
added. The organic phase was separated and the aqueous
layer was extracted with CH2Cl2. The combined organic
phases were dried over Na2SO4, concentrated under reduced
pressure, and purified by flash chromatography on silica gel
to yield the corresponding product 2. The enantiomeric
excess was determined by HPLC.
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1
0.92, CHCl3); H NMR (400 MHz, CDCl3): d=2.03–2.06 (t,
J=6.04 Hz, 2H), 3.65–3.71 (m, 4H), 4.15–4.18 (t, J=
4.60 Hz, 2H), 4.48 (s, 1H), 6.58–6.60 (d, J=8.72 Hz, 2H),
6.77–6.87 (m, 4H), 7.13–7.16 (t, J=7.88 Hz, 1H), 7.27–7.29
(d, J=7.56 Hz, 1H); 13C NMR (100 MHz, CDCl3): d=28.0,
47.7, 55.9, 62.8, 114.4, 115.2, 117.1, 120.7, 123.7, 129.1, 130.2,
140.9, 152.3, 155.1; HR-MS: m/z=256.1342, calculated for
C16H18NO2 [M+H]+: 256.1338; HPLC (IC, i-PrOH/hexane
5/95, 0.8 mLminÀ1, 254 nm): t1 =10.4 min, t2 =11.5 min.
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