Mixtures of chiral phosphorous acid diesters and achiral P ligands in the enantio- and diastereoselective hydrogenation of ketimines

Article abstract of DOI:10.1002/anie.200700533

Try this cocktail! Ligand systems comprising a monodentate phosphorous acid diester derived from binol (L^a) and an achiral monodentate P ligand (L^b), such as a phosphite, are surprisingly efficient in the stereoselective Ir-catalyzed hydrogenation of ketimines (see scheme).

Full text of DOI:10.1002/anie.200700533

Angewandte
Chemie
DOI: 10.1002/anie.200700533
Combinatorial Catalysis
Mixtures of Chiral Phosphorous Acid Diesters and Achiral P Ligands
in the Enantio- and Diastereoselective Hydrogenation of Ketimines**
Manfred T. Reetz* and Oleg Bondarev
In 2000, three groups independently reported that 2,2’-
dihydroxy-1,1’-binaphthyl (binol) derived monodentate P
compounds, namely phosphites of the type 1,[1] phosphonites
such as 2,[2] and phosphoramidites 3,[3] are excellent chiral
ligands in Rh-catalyzed olefin hydrogenation. Since then this
Table 1: Catalyst diversity as a result of mixing ligands La and Lb pairwise
originating from a library of n ligands.
Number of homo-combinations[a]
(MLaLa)
Number of hetero-combinations[b]
(MLaLb)
20
50
190
1225
100
200
500
4950
19950
124750
[a] Equivalent to the number of ligands (n). [b] Equivalent to nðnþ1Þꢀn.
2
tions need to be screened[11] because certain ones can be
eliminated on chemical grounds.
chemistry has expanded rapidly.[4,5] It was shown that two
monodentate phosphorous ligands bind to Rh in the tran-
sition state of the reaction.[6]
Previously we noticed a certain trend in that the optimal
combination is often composed of a relatively small ligand La
and a bulky analogue Lb (with exceptions!).[7–9] In the case of
binol-derived monodentate P ligands of the type 1, the
smallest conceivable residue at P is a hydroxy moiety. This
situation suggests the use of the binol-derived P ligand 4A
(which is in equilibrium with the corresponding tautomeric
form 4B; Scheme 1) as the small ligand La and some other
Subsequently we proposed a new tool in combinatorial
asymmetric transition-metal (M) catalysis: the use of mix-
tures of two different chiral monodentate ligands La and Lb.[7]
This approach leads to three different catalysts, two homo-
combinations MLaLa and MLbLb and one hetero-combination
MLaLb. If the latter is more reactive (and/or present in greater
amounts) and also more selective than the corresponding
homo-combinations, improved catalysts result. We first dem-
onstrated this principle in asymmetric Rh-catalyzed olefin
hydrogenation and other reactions,[7,8] and later extended the
concept to include mixtures of chiral and achiral monodentate
P ligands.[9] Other groups have also contributed to this form of
combinatorial catalysis,[10] although the method is still in its
infancy.
Scheme 1.
It has not been possible to make reliable predictions
regarding the optimal combination from a set of monodentate
ligands, making a combinatorial search necessary. The
number of possible hetero-combinations arising from a
given library of monodentate ligands increases rapidly when
increasing the number (n) of its members (Table 1). Mixing
ligands pairwise from a relatively large library leads to an
“explosion” of catalyst diversity without the need to synthe-
size any new ligands. In a given collection not all combina-
sterically more encumbered ligand Lb. Although the equilib-
rium 4AQ4B favors 4B, we have previously shown by X-ray
crystallography that transition metals, such as Pt, bind to P in
the tautomer 4A,[12] in line with numerous previous structural
and catalytic studies regarding transition-metal complexes of
phosphorous acid diesters and related secondary phosphine
oxides.[13] Moreover, we speculated that such metal complex-
ation acidifies the hydroxy moiety, making possible the
protonation and perhaps activation of such substrates as
ketimines.[12]
Herein we report that the use of ligand 4 in combination
with certain members of a fairly small library of binol-derived
phosphites 1 and phosphonites 2 and especially of achiral P
ligands 5–11 leads to remarkably efficient catalyst systems in
the Ir-catalyzed diastereo- and enantioselective hydrogena-
tion of imines.[14] A few combinations of ligands not involving
4 were also tested. All the catalyst systems were generated by
[*] Prof. Dr. M. T. Reetz, Dr. O. Bondarev
Max-Planck-Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1
45470 Mülheim/Ruhr (Germany)
Fax: (+49)208-306-2985
E-mail: reetz@mpi-muelheim.mpg.de
Homepage: http://www.mpi-muelheim.mpg.de
[**] We thank the Fonds der Chemischen Industrie for generous
support.
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Communications
results of performing a total of 60 hydrogenations of (R)- and
(S)-12, which cover only a small part of chemical space
(Table 1), are summarized in Tables 2 and 3.
The data in Tables 2 and 3 are illuminating in several
respects, and herein we focus only on the most important
points. Firstly, the stereogenic center in the substrate 12
dictates the direction of diastereoselectivity, whereas the
nature of the ligands determines the degree of diastereose-
lectivity, which varies greatly. Thus, if (R)-12 is used as the
substrate, the configuration of the newly created stereogeneic
center in 13 is always R, and by the same token (S)-12 leads to
the preferential formation of the S,S diastereomer. Secondly,
diastereoselectivity when using homo-combinations of
ligands ranges between 2.6:1 and 11.7:1 (Table 2, entries 1–
19), whereas the best hetero-combination leads to a diaste-
reoselectivity of 378.0:1 (Table 3, entry 12). Thus, the positive
treating the commercially available salt [{Ir(cod)Cl}2](cod =
Table 3: Diastereoselective Ir-catalyzed hydrogenation of imines (S)- and
(R)-12 in the presence of hetero-combinations of ligands.[a]
1,5-cyclooctadiene) with selected P ligands.
We first studied the diastereoselectivity[15] of the Ir-
catalyzed hydrogenation of (R)- and (S)-12 (Scheme 2). The
Entry
Imine
Ligand
system
Ir:La:Lb
Conv.
[%]
Diastereomer
ratio[b]
1
2
3
4
5
6
7
8
9
(R)-12
(S)-12
(R)-12
(S)-12
(R)-12
(S)-12
(R)-12
(S)-12
(R)-12
(S)-12
(S)-12
(S)-12
(R)-12
(S)-12
(R)-12
(S)-12
(R)-12
(S)-12
(R)-12
(S)-12
(R)-12
(S)-12
(S)-12
(R)-12
(S)-12
(R)-12
(S)-12
(R)-12
(S)-12
(R)-12
(S)-12
(R)-12
(S)-12
(R)-12
(S)-12
(R)-12
(S)-12
(R)-12
(S)-12
(R)-12
(R)-12
(R)-4+1b
(R)-4+1b
(R)-4+5a
(R)-4+5a
(R)-4+5b
(R)-4+5b
(R)-4+5c
(R)-4+5c
(R)-4+6
(R)-4+6
(R)-4+6
(R)-4+6
(R)-4+7
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:0.5
1:1:2
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:2
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
1:1:1
99
100
88
100
86
5.4:1
5.8:1
3.8:1
8.7:1
6.1:1
6.0:1
4.7:1
100
93
100
100
100
100
100
58
100
100
100
95
100
97
100
90
100
100
78
100
91
100
100
100
56
100
62
100
100
100
100
100
100
100
77
19.0:1
1.0:1
123.0:1
6.0:1
378.0:1
4.7:1
34.0:1
2.0:1
71.0:1
2.5:1
29.0:1
4.7:1
4.4:1
3.2:1
22.0:1
55.0:1
6.5:1
6.5:1
3.9:1
4.9:1
6.0:1
3.8:1
4.0:1
5.7:1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
(R)-4+7
(R)-4+8
(R)-4+8
(R)-4+9
Scheme 2.
Table 2: Diastereoselective Ir-catalyzed hydrogenation of imines (S)- and
(R)-12 in the presence of homo-combinations of ligands.[a]
(R)-4+9
(R)-4+10
(R)-4+10
(R)-4+11
(R)-4+11
(R)-4+11
(S)-1a+5c
(S)-1a+5c
(S)-2b+(S)-2a
(S)-2b+(S)-2a
(R)-2b+(S)-2a
(R)-2b+(S)-2a
(S)-2b+5a
(S)-2b+5a
(S)-2b+5c
(S)-2b+5c
(S)-2b+6
(S)-2b+6
(S)-2b+8
(S)-2b+8
(S)-2b+10
(S)-2b+10
5a+5c
Entry
Imine
Ligand
system
Ir:La
Conv.
[%]
Diastereomer
ratio[b]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
(R)-12
(S)-12
(R)-12
(S)-12
(R)-12
(S)-12
(R)-12
(S)-12
(R)-12
(S)-12
(R)-12
(R)-12
(R)-12
(R)-12
(R)-12
(R)-12
(R)-12
(S)-12
(R)-12
(S)-1a
(S)-1a
(S)-1b
(S)-1b
(S)-2a
(S)-2a
(S)-2b
(S)-2b
(R)-4
(R)-4
5a
5b
5c
6
7
8
9
10
11
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
81
100
76
100
92
100
100
100
47
100
70
3.8:1
3.8:1
4.8:1
4.7:1
3.6:1
4.5:1
7.0:1
7.9:1
5.1:1
5.5:1
6.4:1
5.9:1
5.6:1
5.3:1
11.7:1
3.9:1
2.6:1
5.2:1
5.1:1
7.7:1
83
94
7.9:1
5.4:1
1.5:1
6.0:1
3.7:1
5.4:1
16.5:1
4.7:1
100
100
100
100
100
100
5c+8
100
8.5:1
[a] Solvent: CH2Cl2; 50 bar H2; Ir:12 (1:100); 48 h. [b] (R,R)-13:(S,R)-13
or (S,S)-13:(R,S)-13.
[a] Conditions as in Table 2. [b] (R,R)-13:(S,R)-13 or (S,S)-13:(R,S)-13.
4524
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Angewandte
Chemie
Table 4: Enantioselective Ir-catalyzed hydrogenation of imines 14a–d.[a]
effect arising from mixing is enormous. The best catalyst
results from a mixture of the Ir salt [{Ir(cod)Cl}2](one part),
phosphoric acid diester 4A (one part) and the sterically
hindered achiral phosphite 6 (two parts; Table 3, entry 12). If
the usual 1:1:1 ratio (Ir:La:Lb) is employed, diastereoselec-
tivity is lower, but still considerable (123.0:1; Table 3,
entry 10). Not only are the enhanced diastereoselectivities
relative to those of the respective homo-combinations
pronounced, so is the matched/mismatched effect[16] in several
cases. When using the same Ir catalyst derived from (R)-4/6 to
hydrogenate the enantiomeric substrate (R)-12, the reaction
turned out to be stereorandom (1.0:1 diastereomer ratio;
Table 3, entry 9). Of course, this reaction is stereochemically
equivalent to the hydrogenation of (S)-12 using enantiomeric
ligand (S)-4. Another example of a notable matched/mis-
matched effect is the system involving the phosphite 7 which
is actually a fluxional mixture of the R and S enantiomers
(Table 3, entries 13 and 14). In other cases much smaller
matched/mismatched effects occur, notably when using ligand
4 alone (Table 2, entries 9 and 10).
Finally, several further hits resulting from other hetero-
combinations deserve attention, for example, when using two
achiral ligands La and Lb. The respective homo-combinations
derived from the phosphite 5c and triphenylphosphine 8
provide low diastereoselectivities of 5.6:1 and 3.9:1, respec-
tively, whereas the corresponding mixture raises the ratio to
8.5:1 (Table 2, entries 13 and 16 versus Table 3, entry 41).
Finally, the possible effect of amine additives, such as NEt3 or
pyridine, on the diastereoselectivity when using ligand 4 was
studied, but the effect proved to be very small.
Entry
Imine
Ligand
system
Ir:La
Conv.
[%]
ee
[%]
or Ir:La:Lb
Homo-combinations
1
2
14a
14a
(R)-4
(R)-4
1:2
1:4
31
21
18
45
Hetero-combinations
3
4
5
6
7
8
9
10
11
12
13
14
15
14a
14a
14a
14a
14a
14a
14a
14a
14a
14a
14a
14a
14a
(R)-4+6
(R)-4+6
(R)-4+6
(R)-4+6
(R)-4+6
(R)-4+6
(R)-4+8
(R)-4+8
(R)-4+8
(R)-4+8
(R)-4+8
(R)-4+8
(R)-4+8
1:1:1
1:1:2
1:2:1
1:2:2
1:0.5:0.5
1:1.5:0.5
1:1:1
1:1:2
1:2:2
1:2:1
1:3:1
1:3:2
1:1.5:0.5
94
100
100
100
100
100
90
100
100
100
100
100
100
65
43
76
80
15
28
60
83
82
84
82
87
88
Homo-combinations
16
17
18
14b
14b
14b
(R)-4
(R)-4
(R)-4
1:2
1:3
1:4
36
33
36
8
30
36
Hetero-combinations
19
20
21
14b
14b
14b
(R)-4+8
(R)-4+8
(R)-4+8
1:1:1
1:2:1
1:2:2
78
99
99
77
88
90
Homo-combinations
22
23
14c
14c
(R)-4
(R)-4
1:2
1:3
39
20
33
35
Hetero-combinations
24
25
14c
14c
(R)-4+8
(R)-4+8
1:2:1
1:2:2
99
99
87
92
We then turned to the enantioselective hydrogenation of
prochiral imines 14 (Scheme 3). In this case the size of the
Homo-combinations
26
27
14d
14d
(R)-4
(R)-4
1:2
1:4
100
91
7
2
Hetero-combinations
28
29
30
31
32
33
34
35
14d
14d
14d
14d
14d
14d
14d
14d
(R)-4+6
(R)-4+6
(R)-4+6
(R)-4+8
(R)-4+8
(R)-4+8
(R)-4+8
(R)-4+8
1:1:1
1:2:2
1:2:1
1:1:1
1:2:2
1:2:1
1:3:1
1:3:2
34
71
82
100
100
96
100
100
47
72
58
13
91
81
89
92
Scheme 3.
[a] Conditions as in Table 2; all products 15 have the S configuration.
ligand library was restricted considerably by choosing only
those ligands which had previously displayed the largest
effects in enhancing diastereoselectivity, specifically 4 and
two simple achiral P ligands 6 and 8 (Table 4).
of up to 92% ee (Table 4, entry 35). Thus the effect of mixing
a chiral and an achiral ligand is dramatic. Overall, the best
enantioselectivities achieved when hydrogenating ketimines
14a–d (88–92% ee) compare well with those of recently
published catalyst systems.[14] This study demonstrates once
more the power of the combinatorial approach of using
mixtures of monodentate ligands.
In these cases again several notable effects were observed.
Most importantly, the concept of using hetero-combinations is
once more highly successful. For example, in the hydro-
genation of imine 14a, ligand 4 at several Ir:ligand ratios leads
at best to an ee value of only 45% (Table 4, entry 2), whereas
in combination with triphenylphosphine 8 an enantioselec-
tivity of ee = 88% was achieved (Table 4, entry 15). The p-
chloro and p-methoxy derivatives 14b and 14c behave
similarly (Table 4, entries 16–25). Furthermore, in the hydro-
genation of 14d, ligand 4 alone results in almost racemic
product 15d (Table 4, entries 26 and 27), in contrast to the
hetero-combination 4/8 which leads to an enantioselectivity
Experimental Section
General procedure for the hydrogenation using ligand mixtures (ratio
Ir:La:Lb = 1:1:1): A dry 10-mL flask under argon atmosphere was
charged with a mixture of a 0.01m solution of the first ligand (0.5 mL)
and of a 0.01m solution of the second ligand (0.5 mL) in dry
dichloromethane. The solution was treated with a 0.005m solution of
Angew. Chem. Int. Ed. 2007, 46, 4523 –4526
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www.angewandte.org
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Communications
[{Ir(cod)Cl}2](0.5 mL) in dichloromethane and stirred for 15 min at
2005, 1, 3; e) M. T. Reetz, Y. Fu, A. Meiswinkel, Angew. Chem.
2006, 118, 1440 – 1443; Angew. Chem. Int. Ed. 2006, 45, 1412 –
1415; f) M. T. Reetz, M. Surowiec, Heterocycles 2006, 67, 567 –
574.
room temperature. Then a 0.5m solution of the imine in dichloro-
methane (1 mL) was added. The hydrogenation experiments were
carried out in a parallel manner using 8 flasks. A dry autoclave under
argon atmosphere was charged with 8 flasks. H2 was blown through,
and H2 at 50 bar was introduced. Hydrogenation was carried out for
48 h. Following dilution, conversion and the diastereomeric ratio
were determined by gas chromatography (GC) [HP-5 (Crosslinked
5% PHMESiloxane) 30 m, 0.32-mm ID. 12: tR 8.34 min, (S,R)-13: tR
10.42 min, (R,R)-13: tR 9.86 min]. To determine the ee-values, about
1.5 mL of the respective reaction solution was passed through a small
amount of silica gel prior to HPLC analysis [15a: Chiralcel OD-H,
250 mm, 4.6-mm ID, n-heptane/2-propanol (99:1), 0.50 mLminꢀ1, tR1
11.94 min, tR2 13.22 min; 15b: Chiralpak AD-RH, 125 mm, 4.6-mm
ID, acetonitrile/10 mmol triethylammonium acetate (TEAA)
pH 7.0 = 60:40, 0.50 mLminꢀ1, tR1 15.32 min, tR2 17.18 min; 15c:
Chiralcel OD-H, 250 mm, 4.6 mm ID, n-heptane + 0.1% triethyl-
amine (TEA)/2-propanol (98:2), 0.50 mLminꢀ1, tR1 12.71 min, tR2
16.43 min; 15d: Chiralcel OD-H, 250 mm, 4.6-mm ID, n-heptane (+
[9]a) M. T. Reetz, G. Mehler, Tetrahedron Lett. 2003, 44, 4593 –
4596; b) M. T. Reetz, X. Li, Angew. Chem. 2005, 117, 3019 –
3021; Angew. Chem. Int. Ed. 2005, 44, 2959 – 2962; c) M. T.
Reetz, X. Li, Angew. Chem. 2005, 117, 3022 – 3024; Angew.
Chem. Int. Ed. 2005, 44, 2962 – 2964.
[10]a) D. Peæa, A. J. Minnaard, J. A. F. Boogers, A. H. M. de Vries,
J. G. de Vries, B. L. Feringa, Org. Biomol. Chem. 2003, 1, 1087 –
1089; b) R. Hoen, J. A. F. Boogers, H. Bernsmann, A. J.
Minnaard, A. Meetsma, T. D. Tiemersma-Wegman, A. H. M.
de Vries, J. G. de Vries, B. L. Feringa, Angew. Chem. 2005, 117,
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Duursma, D. Peæa, A. J. Minnaard, B. L. Feringa, Tetrahedron:
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[11]Screening methods: a) M. T. Reetz, Angew. Chem. 2001, 113,
292 – 320; Angew. Chem. Int. Ed. 2001, 40, 284 – 310; b) M. T.
Reetz, Angew. Chem. 2002, 114, 1391 – 1394; Angew. Chem. Int.
Ed. 2002, 41, 1335 – 1338; c) K. Ding, H. Du, Y. Yuan, J. Long,
Chem. Eur. J. 2004, 10, 2872 – 2884; d) L. Lefort, A. F. Boogers,
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[12]a) M. T. Reetz, T. Sell, R. Goddard, Chimia 2003, 57, 290 – 292;
For the taddol ((R,R)-4,5-bis(diphenylhydroxymethyl)-2,2-
dimethyl-1,3-dioxolane) analogue of 4, see: b) D. Enders, L.
Tedeschi, J. W. Bats, Angew. Chem. 2000, 112, 4774 – 4776;
Angew. Chem. Int. Ed. 2000, 39, 4605 – 4607; c) A. Alexakis, J.
Burton, J. Vastra, C. Benhaim, X. Fournioux, A. van den Heuvel,
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Feringa, A. L. L. Duchateau, J. G. O. Andrien, J. A. F. Boogers,
J. G. de Vries, Org. Lett. 2003, 5, 1503 – 1506.
0.1%TEA)/2-propanol (98:2), 0.50 mLminꢀ1
13.60 min].
, tR1 12.23 min, tR2
Received: February 6, 2007
Published online: May 8, 2007
Keywords: amines · asymmetric catalysis ·
.
combinatorial chemistry · hydrogenation · iridium
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Mehler, A. Meiswinkel (SGK), patent application DE-A
10027505.2, June 6, 2000.
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