Inhibiting deactivation of iridium catalysts with bulky substituents on coordination atoms

Article abstract of DOI:10.1016/j.tetlet.2009.11.075

Introducing bulky groups on the coordination phosphorus atoms can effectively block the formation of inactive dimer species and improve the activity of the iridium catalysts. Results of ESI-MS analysis gave strong evidence. This strategy was successfully

Full text of DOI:10.1016/j.tetlet.2009.11.075

Tetrahedron Letters 51 (2010) 525–528
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Tetrahedron Letters
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Inhibiting deactivation of iridium catalysts with bulky substituents
on coordination atoms
Duo-Sheng Wang , Juan Zhou , Da-Wei Wang , Yin-Long Guo b,*, Yong-Gui Zhou
a
b
a
a,b,*
a
State Key of Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China
Shanghai Mass Spectrometry Center, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Introducing bulky groups on the coordination phosphorus atoms can effectively block the formation of
inactive dimer species and improve the activity of the iridium catalysts. Results of ESI-MS analysis gave
strong evidence. This strategy was successfully applied to the asymmetric hydrogenation of quinolines
with up to 93% ee on S/C ratio of 25,000.
Received 19 October 2009
Revised 16 November 2009
Accepted 17 November 2009
Available online 20 November 2009
Crown Copyright Ó 2009 Published by Elsevier Ltd. All rights reserved.
7
The chiral iridium catalysts with P,P or N,P ligands have been
extensively investigated and successfully applied to the asymmet-
ric hydrogenation of imines and unfunctionalized olefins. How-
catalyst can efficiently inhibit the formation of trimers. Although
great success has been obtained, searching for new strategy for
inhibiting deactivation of chiral iridium catalysts is highly desirable.
1
7
ever, a general drawback of these Ir/P,P and Ir/N,P catalysts is the
deactivation by the irreversible formation of inactive dimers and
trimers through hydrid-bridged bonds in the presence of hydro-
Encouraged by Brown’s findings, we envisioned that the introduc-
tion of the steric bulkiness of the substituents at the coordination
phosphorus atoms should inhibit the formation of inactive dimers
or trimers, and improve the catalytic activity of the chiral Ir/P,P
and Ir/N,P catalysts (as shown in Scheme 1).
2
gen. Recently, some successful efforts have been devoted to effec-
tively inhibiting deactivation (as shown in Scheme 1).
Pfaltz and coworkers investigated the effect of counteranions in
iridium-catalyzed enantioselectively hydrogenation of alkenes.
Keeping this in mind, some ligands bearing substituents at both
3 and 5-position of the phenyl of phosphorous atoms were inves-
3
They found that the catalytic activity of the Ir/N,P catalysts increased
dramatically by introducing the bulky counteranion BArF (tetra-
tigated in the Ir-catalyzed asymmetric hydrogenation of quino-
ꢀ
5,6b,8–11
lines.
2-Methylquinoline was chosen as a model substrate,
kis[3,5-bis-(trifluoromethyl)phenyl]borate), which is
a
break-
the catalysts were prepared in situ from the ligands and
[Ir(COD)Cl] with I as an additive. The hydrogenation reaction
was run on a substrate-to-catalyst (S/C) molar ratio of 500. For
the simple MeO-BiPhep ligand, 61% conversion and 84% ee were
through in asymmetric hydrogenation. A straight method to
prevent dimerization/trimerization is the immobilization of the chi-
ral Ir catalysts on a solid support to achieve the site-isolation effect
and inhibiting the formation of dimers/trimers.2 Pugin and Blaser
et al. studied the immobilization of Ir-diphosphine catalysts on silica
gel, which exhibited high turnover number in the asymmetric
hydrogenation of imines.4a Fan and coworkers introduced BINAP-
cored dendrimers to the iridium-catalyzed hydrogenation of quino-
2
2
c,4
F
3
C
CF
3
-
Ir
Ir
Ir
5
F
F
3
3
C
C
CF
3
3
lines, excellent enantioselectivities and activities were obtained.
Ir
B
With the encapsulation of the iridium complex into the dendrimer
framework which reduced dimerization, therefore enhanced the
productivity of the catalyst. Zhou and coworkers introduced the
rigid spirobiindane scaffold to the ligand skeleton, successfully
improving both reactivity and enantioselectivity of the Ir/N,P and
Ir/P,P catalytic systems in the asymmetric hydrogenation of imines
CF
Inorganic Support
F
3
C
CF
3
Bulky Counteranion
Pfaltz, A. ref. 3
The Effect of Site-isolation
Pugin, B. ref. 4a
Dendrimer
The Effect of Site-isolation
Fan, Q.-H. ref. 5
O
N
6
and quinolines. Very recently, Brown and coworkers observed that
R
Inhibit
P
by increasing the steric bulkiness of the ligand of achiral Crabtree
Deactivation
PPh
2
this work:
Ir X = P or N
Improve
X
Reactivity
*
Corresponding authors. Tel./fax: +86 411 84379220 (Y.-G.Z.); Tel./fax: +86 21
Novel Backbone
Zhou, Q.-L. ref. 6
5
4925300 (Y.-L.G.).
E-mail addresses: ygzhou@dicp.ac.cn (Y.-G. Zhou), ylguo@mail.sioc.ac.cn
(
Y.-L. Guo).
Scheme 1. Some strategies for inhibiting deactivation.
0
040-4039/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.11.075

5
26
D.-S. Wang et al. / Tetrahedron Letters 51 (2010) 525–528
Table 1
With these promising results in hand, the effect of solvents,
Asymmetric hydrogenation of 2-methylquinolinea
hydrogenation pressure, and temperature on conversion and
enantioselectivity was further studied using iridium complexes of
L2 and L3b as the catalysts, respectively (Table 2). For the ligand
L2, both benzene and toluene gave complete conversions, and
the former showed slightly higher enantioselectivity (entries 1
and 2, 78% ee vs 74% ee). For L3b, toluene showed better enantiose-
lectivity and higher activity. Both by increasing the temperature or
reducing pressure gave lower conversion and enantioselectivity.
Thus, the optimized reaction conditions for L2 and L3b were: [Ir(-
[
Ir(COD)Cl]
2
2
/ L* / I , benzene
7
00 psi H
2
, rt, 16 h
N
H
N
1
a
2a
Convn.b (%)
Eec (%)
Entry
Ligand
1
2
3
4
5
6
7
8
L1a
L1b
L1c
L1d
L2
L3a
L3b
L4
61
66
>95
>95
>95
76
84
73
55
55
78
79
83
28
2 2 2
COD)Cl] /L2/benzene/700 psi H and [Ir(COD)Cl] /L3b/toluene/700
psi H , respectively.
2
Having established the optimal conditions, the scope of sub-
strates of these catalytic systems was explored. As summarized
in Table 3, 2-alkyl-substituted quinolines were hydrogenated with
moderate enantioselectivities and high yields with L2 (entries 1–4,
61–78% ee). In contrast, moderate to excellent enantioselectivities
were obtained with L3b (51–93% ee). For substrates with bulky 2-
substituent, the hydrogenation were not fully completed. With
L3b, the best results were obtained for 2-methylquinoline and
90
86
O
O
O
O
tBu MeO
MeO
MeO
MeO
PAr
PAr
2
PAr
PAr
2
PPh
PAr
2
N
Fe PAr2
2
2
2
O
L1a: Ar = C
6
H
5
L2: Ar = DTBM L3a: Ar = C
2 6 3
L1b: Ar = 3,5-Me C H L3b: Ar = DTBM
6
H
5
L4: Ar = DTBM
2
,6-dimethylquinoline, the ee values were 89% and 93%, respec-
tively (entries 5 and 8).
L1c: Ar = 3,5-t-Bu
L1d: Ar = DTBM
2
C H
6 3
2 6 2
DTBM = 4-MeO-3,5-t-Bu C H
To further unclose the efficiency of our catalytic system, the S/C
ratio increased to 5000. As shown in Table 4, for most of the sub-
strates, L2 displayed full conversions with moderate to good
enantioselectivities. For the 2,6-dimethylquinoline, only 61% yield
but 82% ee was obtained (Table 4, entry 7). As to L3b, for 2-meth-
ylquinoline, 94% yield and 89% ee were obtained (entry 10), for 2,6-
dimethylquinoline, 93% ee and 67% yield were obtained (entry 11).
Ligand L2 exhibited very high efficiency, even when the S/C ratio
increased to 25,000, the reaction could also proceed almost com-
pletely with lower ee value of 64% (entry 12). It is noteworthy that
the ee values with L2 dropped dramatically as S/C increased from
500 to 25,000 (78–64%). In contrast, for L3b, the S/C ratio has no
effect on the enantioselectivities. These results encouraged us that
an effective strategy for inhibiting deactivation of chiral iridium
catalysts had been established.
a
2
Conditions: 2-methylquinoline (2.5 mmol), [Ir(COD)Cl] (0.0025 mmol), ligand
(
0.0055 mmol), I
2
(0.0125 mmol), 10 mL benzene.
b
c
Determined by 1H NMR.
Determined by HPLC.
obtained, which were lower than that on a ratio of 100 (conver-
9a
sion: 61% vs >95%, ee: 84% vs 94%). Slightly higher conversion
was obtained for the ligand L1b with 3,5-dimethyl group. Full con-
versions were obtained for ligands L1c, L1d and L2 (entries 3–5),
which bear bulky tert-butyl at 3 and 5-position of the phenyl,
but lower enantioselectivities were obtained. Similar conversions
were also obtained for the ferrocene derived P,N ligands L3a and
L3b (conversion: 76% vs 90%, ee: 79% vs 83%), but the slightly high-
er enantioselectivity was obtained. Not full conversion and poor
enantioselectivity were observed for the unsymmetric diphosphine
ligand L4 with a simple diphenylphosphino and bulky DTBM group
Recently, ESI-MS has been used as an effective method for the
characterization of catalytic species, thus providing useful infor-
mation for acquisition of the existed form of the active cata-
2
d,f,6a,12
lyst.
To confirm our hypothesis and further understand
(Table 1, entry 8, 86% conversion with 28% ee). It was obvious that
the unique nature of catalytic species in our reaction system, ESI-
MS analysis was performed. The spectra were collected in posi-
the ligands with bulky aryl groups showed higher activity in the Ir-
catalyzed asymmetric hydrogenation of quinolines.
Table 3
a
Hydrogenation of 1 on S/C ratio of 500
Table 2
R1
R
1
Asymmetric hydrogenation of 2-methylquinolinea
[Ir(COD)Cl]
2
/ L2 or L3b / I
2
R2
R2
N
H
N
Benzene or Toluene, H
2
(700 psi)
[
Ir(COD)Cl]
2
/ L2 or L3b / I
2
1
rt, 36 h
2
Solvent, H
2
, 16 h
N
H
N
1
a
2a
R1/R2
Yieldb (%)
Eec (%)
Entry
Ligand
1d
2
3
L2
L2
L2
L2
L3b
L3b
L3b
L3b
L3b
H/Me
H/n-Bu
H/Me CH(OH)CH –
2 2
H/3,4-(MeO)
H/Me
H/Me CH(OH)CH
2 2
H/3,4-(MeO)
Me/Me
MeO/Me
98 (2a)
90 (2c)
94 (2e)
91 (2g)
97 (2a)
96 (2e)
89 (2g)
96 (2h)
84 (2i)
78 (R)
68 (R)
62 (S)
61 (R)
89 (R)
57 (S)
51 (R)
93 (R)
79 (R)
Entry
Ligand
H
2
(psi); T (°C)
Solvent
Convn.b (%)
Eec (%)
1
2
3
4
5
6
7
8
9
L2
L2
700; 25
700; 25
700; 25
700; 25
700; 25
700; 25
700; 25
700; 50
300; 25
Benzene
Toluene
>95
>95
15
>95
>95
90
>95
94
75
78
74
63
61
35
83
89
84
78
4
5
2
C
6
H
3
(CH
2
)
2
–
–
e
L3b
L3b
L3b
L3b
L3b
L3b
L3b
CH
2
Cl
2
i-PrOH
THF
Benzene
Toluene
Toluene
Toluene
6
7
8
9
–
2 6 3
C H (CH
)
2 2
a
Conditions: quinoline (2.5 mmol), [Ir(COD)Cl]
0.0055 mmol), I (0.0125 mmol), 10 mL solvent.
Isolated yields.
Determined by HPLC.
Benzene for L2.
2
(0.0025 mmol), ligand
(
2
a
b
Conditions: 2-methylquinoline (2.5 mmol), [Ir(COD)Cl]
2
(0.0025 mmol), ligand
c
d
e
(
0.0055 mmol), I
2
(0.0125 mmol), 10 mL solvent.
b
Determined by 1H NMR.
Determined by HPLC.
c
Toluene for L3b.

D.-S. Wang et al. / Tetrahedron Letters 51 (2010) 525–528
527
2
Figure 1. ESI-MS spectrum for the catalyst systems after suffering to H for 16 h: (a) (R)-SegPhos as the ligand, (b) (R)-DTBM-SegPhos as ligand, (c) experimental isotopic
distribution of the peak at m/z = 1645, (d) theoretical simulation of isotopic distribution of the peak at m/z = 1645.
was formed and no dimer or trimer was found (Fig. 1b). These re-
Table 4
a
Hydrogenation of 1 on S/C ratio of 5000
sults were consistent with our expectation and indicated that the
catalyst with more hindrance group at the coordination atoms
could effectively inhibit the formation of dimers and trimers and
showed higher activity. For more detailed information about the
ESI-MS spectrum please see the Supplementary data.
R1
R1
[
Ir(COD)Cl]2 / L2 or L3b / I2
N
H
R2
R2 Benzene or Toluene, H2 (700 psi)
rt, 36 h
N
1
2
In conclusion, a new strategy about the inhibition of deactiva-
tion of Ir catalysts, with bulky aryl groups on the coordination
phosphorous atoms of P,P and N,P ligands, has been developed.
This method was successfully applied to the asymmetric hydroge-
nation of quinolines, up to 93% ee were obtained and the S/C ratio
reached 25,000. Our ongoing work is focusing on ligands design in
other new catalytic system and to extend our catalytic system to
other hydrogenation reactions.
Entry
Ligand
R1/R2
Yieldb (%)
Eec (%)
1d
L2
L2
L2
L2
L2
L2
L2
L2
L2
L3b
L3b
L2
H/Me
H/n-Pr
H/3-Butenyl
H/n-Pentyl
2 2
H/Me CH(OH)CH -
H/Phenethyl
Me/Me
MeO/Me
F/Me
H/Me
Me/Me
H/Me
97 (2a)
94 (2b)
93 (2c)
93 (2d)
91 (2e)
93 (2f)
61 (2h)
92 (2i)
95 (2j)
94 (2a)
67 (2h)
94 (2a)
70 (R)
49 (R)
50 (R)
52 (R)
57 (S)
49 (R)
82 (R)
79 (R)
68 (R)
89 (R)
93 (R)
64 (R)
2
3
4
5
6
7
8
9
0e
Acknowledgements
1
1
1
1
2
f
Financial support from National Science Foundation of China
(20621063 and 20872140). Thanks for the generous gift of ligands
L1–L2 from Solvias Company.
a
Conditions: quinoline (25 mmol), [Ir(COD)Cl]
0.0055 mmol), I (0.0125 mmol), 30 mL solvent.
Isolated yields.
Determined by HPLC.
Benzene for L2.
Toluene for L3b.
S/C = 25,000/1.
2
(0.0025 mmol), ligand
(
2
b
c
d
e
f
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.11.075.
tive-ion mode by diluting the toluene solutions of the catalytic sys-
tem in acetonitrile after suffered to 700 psi of H for 16 h. The spec-
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2
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