Discovery of an iridacycle catalyst with improved reactivity and enantioselectivity in the hydrogenation of dialkyl ketimines

Article abstract of DOI:10.1039/c3sc50587a

Catalytically active iridacycles are formed by cyclometalation of acetophenone imines with Ir-PHOX complexes under hydrogen atmosphere. These complexes show unusually high reactivity and enantioselectivity in the hydrogenation of alkyl methyl ketimines. T

Full text of DOI:10.1039/c3sc50587a

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Discovery of an iridacycle catalyst with improved
reactivity and enantioselectivity in the hydrogenation of
Cite this: Chem. Sci., 2013, 4, 2760
dialkyl ketimines†
*
York Schramm, Fabiola Barrios-Landeros‡ and Andreas Pfaltz
Received 1st March 2013
Accepted 9th April 2013
Catalytically active iridacycles are formed by cyclometalation of acetophenone imines with Ir–PHOX
complexes under hydrogen atmosphere. These complexes show unusually high reactivity and
enantioselectivity in the hydrogenation of alkyl methyl ketimines. The structure of the cyclometalated
imine has a strong effect on the conversion and enantiomeric excess.
DOI: 10.1039/c3sc50587a
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from lower yields and very long reaction times compared to
transition-metal catalyzed hydrogenations. Furthermore, they
Introduction
Chiral amines play an important role as building blocks for the
synthesis of pharmaceuticals and agrochemicals. They are also
of great importance as chiral auxiliaries, catalysts and resolving
agents. Therefore, asymmetric hydrogenation of ketimines has
received much attention as an attractive, very direct route to
enantiomerically enriched amines.1 High yields, perfect atom
economy and mild conditions make this approach ideal for
industrial applications. This is impressively demonstrated by
the multi-ton scale production of the herbicide metolachlor,
based on an extremely active and productive Ir-diphosphine
catalyst.2
During the last two decades a wide range of chiral Ti, Rh, Ir,
Pd, Ru,1 and most recently Fe1,3 complexes have been developed
that catalyze the hydrogenation of various imines with high
enantioselectivity. However, the scope of most catalysts is rather
narrow and there are still important classes of imines that give
unsatisfactory results with the available catalysts. Especially the
hydrogenation of imines derived from dialkyl ketones remains a
challenging problem. With the exception of the dual catalyst
system reported by Xiao and co-workers,4 consisting of a chiral
Ir(Cp*)–diamine complex and an elaborate chiral binaphthol-
derived phosphoric acid (TRIP), most catalysts give very low
enantioselectivities with these substrates. Organocatalytic
asymmetric imine reduction and reductive amination has also
been developed to afford chiral aliphatic amines with high
enantioselectivities.5 However, these reactions generally suffer
require hydride donors such as dihydropyridines that generate
stoichiometric waste products. So a practical readily accessible
catalyst for the asymmetric hydrogenation of dialkyl ketimines
remains elusive.
We have recently begun to reinvestigate Ir–phosphinoox-
azoline complexes that we originally introduced as catalysts for
imine hydrogenation in 1997.6a Aer evaluation of a wide range
of phosphinooxazoline (PHOX) derivatives and careful optimi-
zation of the reaction conditions, excellent enantioselectivities
and high turnover numbers have been achieved in the hydro-
genation of aryl alkyl N-arylketimines such as I1 (Scheme 1).6b
However, analogous dialkyl ketimines still gave disap-
pointing results. We therefore decided to conduct a mechanistic
study, in the hope that it would guide the development of
improved catalysts with broader substrate scope.
Results and discussion
Here we report the outcome of this study that led to surprising
insights into the structure of the catalytic intermediates and,
ultimately, to a new catalyst system that gave promising results
in the hydrogenation of dialkyl ketimines.
Experimental mechanistic studies of imine hydrogenation
with Ir-PHOX complexes have not been reported. However,
University of Basel, Department of Organic Chemistry, St. Johanns-Ring 19, 4056
Basel, Switzerland. E-mail: andreas.pfaltz@unibas.ch; Fax: +41 61 2671103; Tel:
+41 61 2671108
† Electronic supplementary information (ESI) available: Protocols for procedures
and experimental data. CCDC 916564. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c3sc50587a
‡ Current address: Yeshiva University, Department of Chemistry, New York, NY,
10033, USA.
Scheme 1 Asymmetric hydrogenation of acetophenone N-phenylimine I1.
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Hopmann and Bayer7 carried out DFT calculations with catalyst
1a on nine different catalytic cycles based on proposed mech-
anisms for imine hydrogenations with other types of Ir
complexes. The computed energies of the turnover-limiting
transition states indicated a pathway involving proton transfer
from an Ir–hydride complex to a free, uncoordinated imine,
followed by hydride transfer to the resulting iminium ion. As all
proposed mechanisms included Ir–dihydride complexes as
intermediates, we decided to prepare a dihydride complex from
the Ir(PHOX) precursor 1a following the procedure of Mazet
et al.8 and to study its reactivity with I1 as a typical substrate
(Scheme 2).
When the dihydride complex, generated by reaction of 1a with
H2 in THF at ꢀ25 ꢁC, was treated with I1 at 0 ꢁC, rapid formation
of an iridacycle 2 was observed. This complex proved to be very
sensitive and rapidly decomposed upon exposure to air. Attempts
to obtain suitable crystals for X-ray analysis failed and, therefore,
the BArF (tetrakis[(3,5-triuoromethyl)phenyl]borate) counterion
that, in our experience, oen impedes crystallization, was
exchanged with chloride or hexauorophosphate. The chloride
complex 3 was readily obtained by treatment with LiCl and silica
gel in ethyl acetate and puried by ash chromatography on
silica gel. Ion exchange with NaBArF in THF led back to the BArF
complex 2.
Scheme 3 Preparation of iridacycle 5.
What are the possible conclusions that can be drawn from
these results? The iridacycle formed under hydrogenation
conditions could be an inactive species outside the catalytic
cycle that is in equilibrium with an active catalytic intermediate
through reversible cyclometalation/reductive elimination,
similar to the reaction scheme proposed by Marcazzan and
James.12
Alternatively, it could be directly involved in the catalytic
cycle. In this case, it could either react via reduction of the
cyclometalated imine, followed by reductive elimination
releasing the saturated amine, or the cyclometalated imine
could serve as a stable ligand that remains bound throughout
the catalytic cycle.
To distinguish between these possibilities we carried out the
cross-over experiment shown in Scheme 6. If a catalytic inter-
mediate derived from 3 would release the free imine I1, the
cyclometalation product of substrate I2 along with amine A1
would be formed. However, we did not observe even traces of I1
or A1 in the course of the hydrogenation reaction by GC anal-
ysis.13,14 These results are consistent with the hypothesis that
cyclometalation is irreversible and the imine remains bound to
iridium throughout the catalytic reaction. The enantiomeric
excess of A2 remained constant throughout the reaction.13
We speculated that the cyclometalated complex formed
under hydrogenation conditions could be a superior catalyst
compared to complex 1a and that the poor results in the
hydrogenation of aliphatic ketimines could be a consequence of
the inability of these substrates to form cyclometalated
complexes.
While crystallization attempts of 3 were unsuccessful, the
hexauorophosphate salt 5 furnished suitable crystals for X-ray
analysis (Scheme 3 and Fig. 1).9 Furthermore, a preliminary
crystal structure of the analogous iridacycle 7 prepared from
SimplePHOX complex 6 (Scheme 4 and Fig. 2) could also be
determined.10
An analogous cyclometalation reaction of [Ir(H)2(PPh3)2-
(acetone)2]PF6 with benzaldehyde N-benzylimine has been repor-
ted by James and co-workers.11 The resulting iridacycle was tested
in the hydrogenation of imines but showed no catalytic activity.11a
When complex 3 was tested as catalyst for the hydrogenation
of I1, no reaction was observed. However, when the chloride was
replaced with the non-coordinating anion BArF by addition of
an equimolar amount of NaBArF, an active catalyst was gener-
ated that furnished the same enantiomer of amine A1 with
identical enantioselectivity as the reduction catalyzed by the
parent complex 1a (Scheme 5).
Scheme 2 Formation of cyclometalated imine complexes.
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Fig. 1 Crystal structure of 5 (PF6 counterion omitted for clarity).
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Indeed, the catalyst generated in situ from the iridacycle 3 by
treatment with NaBArF gave much higher conversion and
enantioselectivity (Table 1, entry 1) in the hydrogenation of
cyclohexyl methyl ketimine I3 than the parent Ir–PHOX complex
1a (entry 2) or iridacycle 3 alone (entry 3). Identical ee and
higher conversion was obtained when the catalyst was gener-
ated by treating complex 1a with H2 and an equimolar amount
of acetophenone imine I1 (entry 4). At 50 bar H2 pressure 71%
ee and full conversion were observed aer activation with imine
I1 (entry 5), compared to 27% conversion and 35% ee with
complex 1a alone (entry 6).
Scheme 4 Preparation of iridacycle 7.
These ndings clearly indicate that the new more efficient
catalyst is a cationic cyclometalated complex that arises by
chloride abstraction from 3 or by reaction of precatalyst 1a with
H2 and imine I1 (cf. structure 2 in Scheme 2). Because cationic
complexes such as 2 or 5 with very weakly coordinating anions
proved to be too unstable and impractical to be used as catalysts,
in situ activation of precatalyst 1a with acetophenone imine I1 or
derivatives thereof was the method of choice for further
experiments.
If the imine remains bound to the catalyst throughout the
catalytic cycle, it is also involved in the enantiodiscriminating
step. Cyclometalation of a chiral additive to an achiral complex
would thus provide a chiral imine hydrogenation catalyst. We
therefore prepared chiral complex (S)-11 derived from an achi-
ral iridium–PHOX complex 8 and a chiral imine 9 (Scheme 7).
NMR analysis indicated the presence of two diastereomers of
complex (S)-10. However, upon treatment with an equimolar
amount of NaBArF in CD2Cl2, a single hydride species (S)-11 was
observed.
Fig. 2 Preliminary crystal structure of 7.
Both enantiomers of complex 11, formed in situ from the
diastereomeric mixture of (S)-10ab or (R)-10ab, were tested as
catalysts for the hydrogenation of I1 (Table 2). While low
conversion was observed at atmospheric hydrogen pressure,
hydrogenation at 5 bar afforded A1 with 54% conversion (entry 3).
The resulting ee values showed a weak but reproducible
inuence of the chiral imine 9. Analogous catalysts derived
from 1,3-benzoxazines were tested as well (Table 3) and gave
reproducible enantioselectivities of up to 23% ee.
Scheme 5 Comparison of complexes 1a and 3 as precatalysts for the hydro-
genation of imine I1.
Table 1 Hydrogenation of imine I3 using catalyst 1a in the presence or absence
of imine I1 or using iridacycle 3 in the presence or absence of NaBArF
Entry
Catalyst
Additive
p(H2)/bar
Conv.a (%)
eeb (%)
1
2
3
4
5
6
3
1a
3
1a
1a
1a
NaBArF
—
—
I1 (2 mol%)
I1 (2 mol%)
—
1
1
1
1
50
50
40
5
0
50
>99
27
73 (R)
69 (R)
—
73 (R)
71 (R)
35 (R)
a
b
Scheme 6 Hydrogenation of imine I2 with iridacycle 3 in combination with
Determined by GC analysis. Determined by HPLC analysis on a
chiral stationary phase.
NaBArF.
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Table 3 Hydrogenation of imine I1 using iridacycle 12–14
Entry
Catalyst
H2/bar
Conv.a (%)
eeb (%)
1
2
3
4
5
6
(S)-12
(S)-13
(S)-13
(R)-13
(R)-13
(R)-14
1
1
5
1
5
1
63
31
97
20
99
40
15 (S)
23 (R)
14 (R)
23 (S)
14 (S)
17 (S)
a
b
Determined by GC analysis. Determined by HPLC analysis on a
chiral stationary phase.
Scheme 7 Preparation of chiral iridium complexes (S)-10 derived from achiral Ir-
PHOX complex 8 and chiral imine 9. Upon addition of NaBArF, chloride abstrac-
tion results in the formation of one single hydride complex (S)-11 in solution.
ligand is replaced by imine I1 resulting in a complex with two
achiral ligands, which produces racemic product. However, the
high initial enantioselectivity of 44% ee clearly demonstrates
that the cyclometalated ligand is involved in the enantiodis-
criminating step of the reaction.
Table 2 Hydrogenation of imine I1 using iridacycle 10
Attempts to improve the enantioselectivity by using catalysts
prepared from a combination of chiral PHOX complexes with
chiral benzoxazines gave disappointing results. However,
systematic evaluation of various achiral N-aryl acetophenone
imines as additives was more successful (Table 4).
Entry
Catalyst
H2/bar
Conv.a (%)
eeb (%)
The inuence of substituents in the acetophenone phenyl ring
showed no apparent trends that could be correlated with steric or
electronic effects. Surprisingly, meta-substituents distinctly low-
ered the enantioselectivities (entries 4–8),15 while an ortho-methyl
or ortho-uoro substituent had essentially no effect (entries 2 and
3). Overall, introduction of substituents in the acetophenone
phenyl ring did not improve the enantioselectivity. In contrast,
ortho-alkyl groups in the N-aryl ring resulted in enhanced
1
2
3
4
(S)-10
(R)-10
(S)-10
(R)-10
1
1
5
5
16
18
54
44
4 (S)
6 (R)
4 (S)
6 (R)
a
b
Determined by GC analysis. Determined by HPLC analysis on a
chiral stationary phase.
Erosion of enantioselectivity was observed when hydroge-
nations were conducted at 5 bar (entries 2 vs. 3 and 4 vs. 5).
Furthermore, complex 14 gave higher conversion but lower
enantioselectivity than complex 13 (entries 2, 4 and 6). To get a
more accurate correlation of the ee with conversion, the reac-
tion was followed by GC and HPLC analysis (Fig. 3).
While complex 13 reacted with almost constant enantiose-
lectivity, 14 showed a strong erosion of the enantioselectivity
with increasing conversion. Furthermore, an overall higher
conversion and faster reaction was observed for 14.
The following conclusions were drawn from these observa-
tions: while in 13 the benzoxazine remains bound to the iridium
center throughout the reaction, complex 14 is not stable under
Fig. 3 Conversion (dotted) and enantioselectivity (dashed) of A1 using iridacycle
the reaction conditions. As a consequence the benzoxazine 13 (blue triangles) and 14 (red diamonds) as the catalyst.
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Table 4 Screening of different imines as additives
Table 5 Asymmetric hydrogenation of N-phenyl aliphatic imines
Entry Substrate
Catalyst Additive T/ꢁC Conv.a (%) eeb (%)
Entry
Additive
R
R0
Conv.a (%)
eeb (%)
1
2
3
4
1a
1a
1a
1a
I1
I1
I24
I24
23
ꢀ5
23
ꢀ5
>99
>99
>99
>99
71 (R)
73 (R)
85 (R)
92 (R)
1
2
3
4
5
6
7
8
I1
I4
I5
I6
I7
I8
I9
I10
I2
I11
I12
I13
I14
I15
I16
I17
I18
I19
I20
I21
I22
I23
I24
H
2-Me
2-F
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
>99
>99
90–95
75–99
10
>99
34
11
>99
>99
>99
>99
>99
95
71 (R)
69 (R)
71 (R)
48 (R)
15 (R)
3 (S)
3-NO2
3,5-(NO2)2
3,5-Me2
3,5-iPr2
3,5-tBu2
4-MeO
4-tBu
4-Me
4-Cl
42 (S)
10 (R)
67 (R)
64 (R)
63 (R)
56 (R)
66 (R)
50 (R)
45 (R)
68 (R)
78 (R)
71 (R)
68 (R)
69 (R)
59 (R)
81 (R)
85 (R)
5
1a
I24
ꢀ5
33
84 (R)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
6
7
8
1a
1a
1b
I1
I24
I24
ꢀ5
ꢀ5
ꢀ5
>99
>99
>99
49 (ꢀ)
70 (ꢀ)
80 (ꢀ)
4-F
4-CF3
4-NO2
4-Me
H
H
H
H
H
H
H
40
2-Br
2-Me
2-MeO
3-MeO
4-MeO
4-CF3
2-iPr
2,6-Me2
>99
>99
>99
>99
>99
>99
>99
>99
9
10
1a
1b
I24
I24
ꢀ5
ꢀ5
>99
94
62 (+)
72 (+)
11
12
1b
1b
I24
I24
ꢀ5
ꢀ5
>99
52 (ꢀ)
56 (ꢀ)
a
b
Determined by GC analysis. Determined by HPLC analysis on a
chiral stationary phase.
>99c
enantioselectivity (entries 17, 22 and 23). Other iridium
complexes were screened as well, but none of them reached the
enantioselectivities achieved with PHOX complex 1a.13
13
14
1b
1a
I24
I24
ꢀ5
ꢀ5
>99
8
57 (R)
63 (ꢀ)
With an optimized catalyst system in hand we studied the
scope for the hydrogenation of aliphatic ketimines (Table 5).
Higher conversions and enantioselectivities were obtained in all
cases compared to reactions using complex 1a alone. The high
reactivity of this catalyst system allowed lowering the reaction
temperature to ꢀ5 ꢁC. Under these conditions imine I3 fur-
nished the product A3 with an improved ee of 92% and full
conversion (entry 4). Isopropyl methyl ketimine I25 gave 84% ee
but only 33% conversion (entry 5). The ee was further improved
when the more bulky complex 1b was used as precatalyst for
isobutyl methyl ketimine I26 and benzyl methyl ketimine I27
(entries 8 and 10). However, for a-branched alkyl methyl keti-
mines such as I3, I25, I31 and I32, no reaction was observed
with complex 1b. The sterically less demanding n-alkyl methyl
ketimines I28, I29 and I30 gave markedly lower enantiose-
lectivity (entries 11–13). Chemoselective hydrogenation of an
15
1a
I1
ꢀ5
8
46 (R)
a
b
Determined by GC analysis. Determined by HPLC analysis on a
chiral stationary phase aer purication by ash chromatography.
c
Contained <3% of fully saturated product.
To see whether the N-phenyl group was essential for achieving
imine double bond in the presence of a trisubstituted olen can high enantioselectivity, we investigated N-alkyl imines I33–I36 as
be achieved as shown in I29 (entry 12).
substrates (Table 6). Asymmetric hydrogenation of imines of this
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type has not been reported yet apart from I33. As these substrates in an autoclave (60 mL) and purged with argon for 5 min.
proved to be less reactive, hydrogenations were conducted at Anhydrous CH2Cl2 (1 mL) was added by syringe under a stream
room temperature. Using imine I1 as additive the N-benzylimine of argon and the autoclave was closed. For reactions at low
I33 furnished a moderate ee of 44% (entry 1). An even lower temperature the autoclave was immersed in a cooling bath for
enantioselectivity is observed for the N-n-butylimine I34 (entry 2). 60 min before starting the reaction. The autoclave was pres-
On the other hand almost the same ee as for corresponding surized with hydrogen gas, hydrogen was released and the
N-phenylimine I3 was observed in the hydrogenation of I35 (entry autoclave pressurized again. It was then placed on a stirring
3). The N-cyclohexyl analogue I34 reacted with even higher plate for the time indicated. Aer releasing the pressure, the
enantioselectivity of 77% ee (entry 4), demonstrating that purely solvent was evaporated under a stream of nitrogen. The residue
alkyl-substituted imines are suitable substrates for this catalyst was suspended in pentane–diethyl ether (5 : 1) and ltered
system. The more bulky complex 1b and the sterically demanding through a short elution plug (cotton bottom, 40 ꢂ 5 mm silica
N-(2,6-dimethylphenyl)imine I24 afforded lower yields and gel). The crude ltrate was analysed by GC for conversion before
enantioselectivities with these substrates.
being puried by ash chromatography (SiO2, pentane–diethyl
ether (20 : 1), 15 ꢂ 2 cm) and analysed by HPLC on a chiral
stationary phase for determination of the enantiomeric excess.
Conclusions
We have found that the active catalyst in the hydrogenation of
acetophenone-derived imines with Ir–PHOX precatalysts is an
iridacycle generated under hydrogenation conditions by cyclo-
metalation of the substrate. Cyclometalated complexes of this
type, formed in situ by addition of an equimolar equivalent of
acetophenone imine, show higher reactivity and better enan-
tioselectivity in the hydrogenation of N-phenyl and N-alkyl
aliphatic ketimines than the corresponding Ir-PHOX complex
alone. Obviously, the reaction proceeds through a pathway that
differs from the catalytic cycles proposed in the literature.7
Although at present the scope is still limited, our ndings
indicate many opportunities for further improvement of this
catalyst system by structural variation of both the chiral P,N
ligand and the cyclometalated imine.
Preparative reaction
Imine I3 (1.005 g, 5 mmol), 1a (0.1 mmol), I24 (0.1 mmol), and a
stir bar were added to a 25 mL Pyrex oven-dried glass vial that
had been placed in an autoclave (60 mL) and purged with argon
for 5 min. Anhydrous CH2Cl2 (5 mL) was added by syringe under
a stream of argon and the autoclave was closed. The autoclave
ꢁ
was immersed in a cooling bath for 60 min at ꢀ5 C before it
was pressurized with hydrogen gas. Hydrogen was released and
the autoclave pressurized again before being placed on a stir-
ring plate for 18 h. Aer pressure release the reaction mixture
was transferred to a 50 mL round-bottom ask and solvents
removed under reduced pressure. The residue was suspended
in pentane–diethyl ether (20 : 1) and puried by ash chroma-
tography (SiO2, pentane–diethyl ether (10 : 1), 21 ꢂ 3 cm).
Solvents were removed under reduced pressure and the residue
was dried in vacuo to afford A3 (998 mg, 4.92 mmol, 98%).
Experimental section
Screening
Acknowledgements
Imine (0.1 mmol), catalyst (2 mmol), additive (2 mmol), and a stir
bar were added to an oven-dried glass vial that had been placed
Support of this work by the Swiss National Science Foundation
(SNF) and the Federal Commission for Technology and Innovation
(KTI) is gratefully acknowledged. We thank Dr Markus
Neuburger for the crystal structure analysis, Robin Wehlauch
for synthetic contributions and Prof. Dr Klaus Dittrich from
BASF for generous gis of chemicals.
Table 6 Asymmetric hydrogenation of N-alkyl aliphatic imines
Notes and references
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Entry
Substrate (R)
Conv.a (%)
eeb (%)
1
2
3
4
I33 (CH2Ph)
I34 (nBu)
I35 (iPr)
>99
>99
>99
>99
44 (R)c
33 (R)
73 (R)d
77 (R)c
I36 (c-C6H11
)
a
Determined by GC analysis. b Determined by GC or HPLC analysis on a
c
chiral stationary phase aer derivatisation.
derivatisation to the 1-naphthoyl amide.
derivatisation to the acetamide.
Determined aer
Determined aer
d
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2766 | Chem. Sci., 2013, 4, 2760–2766
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