Fluorinated alcohols as solvents for enantioselective hydrognation with chiral self-assembling rhodium catalysts

Article abstract of DOI:10.1002/adsc.200700177

We herein describe the beneficial effect of fluorinated alcohols on asymmetric hydrogenation using chiral self-assembling rhodium complexes. Previously, the application of these catalysts has been hampered by low reaction rates in non-polar solvents, which are essential for establishing the self-assembling architecture via hydrogen bonding. Excellent reaction rates are usually observed in alcohols as solvents, but the characteristic hydrogen bonds are cleaved in those media, resulting in poor ee values. We now show for the first time that the disadvantageous properties of both solvent classes on the catalytic reaction can be overcome by using fluorinated alcohols. Due to this key finding, homogeneous catalysis with self-assembling catalysts is much closer to practical application.

Full text of DOI:10.1002/adsc.200700177

UPDATES
DOI: 10.1002/adsc.200700177
Fluorinated Alcohols as Solvents for Enantioselective Hydrogena-
tion with Chiral Self-AssemblingRhodium Catalysts
a,
a
a
Natalia V. Dubrovina, * Ivan A. Shuklov, Mandy-Nicole Birkholz,
a
b
a,c,
Dirk Michalik, Rocco Paciello, and Armin Bçrner *
a
Leibniz-Institut für Katalyse an der Universität Rostock e.V., A.-Einstein-Str. 29a, 18059 Rostock, Germany
Fax : (+49)-381-1281-51202; e-mail: armin.boerner@catalysis.de or natalia.dubrovina@catalysis.de
BASF AG, Basic Chemicals Research, GCB/O – M 313, 67056 Ludwigshafen, Germany
Institut für Chemie der Universität Rostock, A.-Einstein-Str. 3a, 18059 Rostock, Germany
b
c
Received: April 10, 2007; Published online: September 11, 2007
Abstract: We herein describe the beneficial effect
of fluorinated alcohols on asymmetric hydrogena-
tion using chiral self-assembling rhodium com-
plexes. Previously, the application of these catalysts
has been hampered by low reaction rates in non-
polar solvents, which are essential for establishing
the self-assembling architecture via hydrogen bond-
ing. Excellent reaction rates are usually observed in
alcohols as solvents, but the characteristic hydrogen
bonds are cleaved in those media, resulting in poor
ee values. We now show for the first time that the
disadvantageous properties of both solvent classes
on the catalytic reaction can be overcome by using
fluorinated alcohols. Due to this key finding, homo-
geneous catalysis with self-assembling catalysts is
much closer to practical application.
Scheme 1. Self-assembly of 6-phosphanyl-2-pyridone in the
presence of a transition metal salt in dependence on the sol-
vent.
Rhodium(I) complexes of IIA derived fromthis
systemturned out to be highly efficient in the regiose-
[
4a–c]
lective hydroformylation.
Moreover, P ligands
Keywords: asymmetric catalysis; hydrogenation;
phosphane ligands; rhodium; self-assembly; solvent
effects
equipped with complementary H-bonding motifs, on
the basis of an A–T base pair analogy, have been pre-
pared and advantageously employed in a combinato-
rial approach. The concept was extended to the Ru-
catalyzed hydration of nitriles
[
5]
[
4d]
and anti-Markovni-
[4e]
kov hydration of terminal alkynes. Furthermore, in
The application of self-assembling catalysts has been an HTS approach, a library of self-assembling chiral
established in recent years as a valued alternative to phosphorus donor ligands has been studied in the
the employment of metal complexes based on con- course of Rh-catalyzed enantioselective hydrogena-
[
6]
ventional monodentate or bidentate ligands within tions. Hydrogen bonding has been also identified as
[
1,2]
the field of metal-catalyzed homogeneous catalysis.
Amongst other aspects, such conformationally flexible dentate phosphoramidites by Ding and co-workers.
chelates can better adapt to steric requirements of a In a recent systematic study on the catalyzed enan-
a critical issue of chiral Rh catalysts bearing mono-
[7]
[
3]
substrate (“induced fit”). Additionally, a reduction tioselective hydrogenation of dimethyl itaconate and
in the synthetic work necessary to build libraries for methyl a-(Z)-N-acetamidocinnamate with cationic Rh
screening is expected.
complexes, we reported evidence that the efficiency
Impressive results have been documented by Breit of the reaction is strongly dependent on the bulk of
et al. using the 6-phosphanyl-2-pyridone system IB, the chiral phosphine moiety as well as on the solvent
[
8]
which self-assembles through hydrogen bonding with used. Superior enantioselectivities (99% ee) were
its tautomer IA in the presence of a late transition observed with phosphepine 2 as ligand in comparison
[
4]
metal salt (Scheme 1).
to phospholanes 1a, b in the non-polar solvent
CH Cl . Unfortunately, slow conversion was noted in
2
2
Adv. Synth. Catal. 2007, 349, 2183 – 2187
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2183

Natalia V. Dubrovina et al.
UPDATES
most cases under these conditions. This was particu- should be suitable solvents in terms of polarity and
larly pronounced for complexes bearing phospholane hydrogen bonding tolerance.
ligands 1a, b. In contrast, in most cases enantioselec-
Indeed, NMR investigations of a cationic Rh-com-
plex of 1a in CF CD OD (Figure 2) showed a similar
3
2
[
8]
signal pattern as found in CDCl . Two doublets of
doublets d=52.0 [dd, J( P, Rh)=147 Hz, J( P, P)=
6 Hz] and d=50.4 [J( P, Rh)=140 Hz, J
3
3
1
103
31 31
ACHTREUNG
3
1
103
31 31
ACHTREUNG
3
3
[
10]
and conclusions of the preceding paper these signals
can be attributed to two different P groups: one at-
tached to a pyridone and the other to a pyridinol scaf-
fold. This is clear evidence that the self-assembly is
completely maintained in the fluorinated alcohol.
Moreover, signals at lower field usually observed in
tive hydrogenations proceeded faster in polar solvents CDCl and probably characterizing polymeric spe-
3
[
11]
[12]
like MeOH, but in this case low ee values in the cies or P,N complexes are not visible.
range of 0 to 47% were observed. Detailed NMR Results of the asymmetric hydrogenation of the
spectroscopic investigation showed that self-assembly benchmark substrates methyl Z-a-N-acetamidocinna-
and therefore chelate formation only took place in mate and dimethyl itaconate are detailed in Table 1.
non-polar solvents, whereas in CD OD the hydrogen For comparison, results measured in CH Cl and
3
2
2
[
8]
bonds were broken and metal complexes bearing MeOH are also listed.
monodentate ligands formed (Scheme 1, IIB).
Remarkably, the application of CF CH OH as sol-
3 2
Obviously unique polar solvents are required which vent yielded similarly high ee values as those obtained
[
13]
do not disturb the self-assembling architecture of the in CH Cl . Moreover, with the exception of run 15,
2
2
hydrogenation catalyst in order to maintain the bene- reactions proceeded much faster and the rates ob-
ficial effect of hydrogen bonding on the enantioselec- served even exceed those achievable in the “favour-
tivity. At the same time the low reaction rates must ite” solvent CH OH. Interestingly, other fluorinated
3
be overcome. As shown in Figure 1, fluorinated alco- solvents are also highly useful for this catalytic reac-
hols with low hydrogen bond acceptor properties tion. Thus, the use of CH FCH OH yielded similar su-
2
2
perior results (runs 10 and 23). With CF CH(OH)CF ,
3
3
the reaction rate was lowered. However, in general,
high enantioselectivities were obtained (runs 11 and
2
2). Remarkably, employment of CCl CH OH gave
3 2
poor results (runs 12 and 24), which clearly shows the
unique properties of the fluorinated alcohols.
In order to provide additional proof for the benefi-
cial effect of self-assembling, the “ordinary” non-as-
sembling monodentate ligands 3a, b and 4 were tested
T
Figure 1. Correlation of the polarity (EN ) with hydrogen
bond acceptor properties (b). HFIP=1,1,1,3,3,3-hexafluoro-
2
-propanol [CF CH(OH)CF ], TFE=2,2,2-trifluoroethanol
3
3
[
CF CH OH],
MFE=2-fluoroethanol
[CH FCH OH],
3
2
2 2
TCE=2,2,2-trichloroethanol [CCl CH OH], DCM=di-
3
2
3
1
chloromethane [CH Cl ], THF=tetrahydrofuran [C H O],
Figure 2. P NMR spectrumof [Rh
CF CD OD at 08C.
A
H
R
U
G
2
2
4
8
[9]
MeOH=methanol [CH OH].
3
3
2
2184
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ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2007, 349, 2183 – 2187

Fluorinated Alcohols as Solvents for Enantioselective Hydrogenation
UPDATES
[a]
Table 1. Comparison of results of the enantioselective hydrogenation in different solvents.
Entry
Ligand
Substrate
R
Solvent
CH Cl
t [min]
Conversion [%]
ee [%]
1
2
R
[
b]
b]
b]
[d,e]
1
2
3
4
5
6
7
8
9
1a[
NHAc
NHAc
NHAc
NHAc
NHAc
NHAc
NHAc
NHAc
NHAc
NHAc
NHAc
NHAc
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
H
H
H
H
H
H
H
H
H
1400
1300
1200
3900
2760
1800
10
50
10
6
800
1380
1300
400
1400
2880
2880
300
20
60
10
30
25
100
20
100
92
100
100
100
100
100
100
100
2
69 (R)
13 (R)
68 (R)
83 (R)
27 (R)
84 (R)
94 (R)
64 (R)
95 (R)
96 (R)
94 (R)
n.d.
2
2
[d,e]
[d]
1a
CH OH
3
[
1a
CF CH OH
3
2
[
b]
b]
b]
[d,e]
[d,e]
[d]
1b[
CH Cl
2
2
1b
1b
CH OH
3
[
CF CH OH
3
2
[
[
[
[
[
[
c]
c]
c]
b]
b]
b]
[d,e]
[d,e]
[d]
2
2
2
2
2
2
CH Cl
2
2
CH OH
3
CF CH OH
3
2
[d]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
CH FCH OH
2
2
[d]
CF CH(OH)CF
3
3
CCl CH OH
3
2
[
b]
b]
b]
[e,f]
1a[
CH COOMe
CH Cl
88
100
93
83 (S)
2
2
2
[f]
1a
1a
CH COOMe
CH OH
rac
2
3
[
e
CH COOMe
CF CH OH
68 (S)
51 (S)
47 (S)
40 (S)
99 (S)
64 (S)
97 (S)
96 (S)
89 (S)
n.d.
2
3
2
[
b]
b]
b]
[e,f]
[e,f]
[f]
1b[
CH COOMe
CH Cl
53
2
2
2
1b
1b
CH COOMe
CH OH
100
100
100
100
100
100
100
1
2
3
[
CH COOMe
CF CH OH
2
3
2
[
[
[
[
[
[
c]
c]
c]
b]
b]
b]
[e,f]
[e,f]
[f]
2
2
2
2
2
2
CH COOMe
CH Cl
2
2
2
CH COOMe
CH OH
2
3
CH COOMe
CF CH OH
2
3
2
[f]
CH COOMe
H
H
H
CF CH(OH)CF
2
3
3
[f]
CH COOMe
CH FCH OH
2
2
2
CH COOMe
CCl CH OH
1050
2
3
2
[
[
[
[
[
[
a]
Conditions: Precatalyst generated in situ by reaction of [Rh
Substrate:catalyst=50:1, 7.5 mL solvent.
Substrate:catalyst 100:1, 7.5 mL solvent.
A
H
R
U
b]
c]
d]
e]
f]
Determined by GC, 25 m g-cyclodextrin, Lipodex E (Machery and Nagel), fused silica, 1308C.
[8]
See Ref.
Determined by GC, 25 m g-cyclodextrin, Lipodex E (Machery and Nagel), fused silica; 708C.
lytic effect of complexes with self-assembling proper-
ties is strongly dependent on the solvent used.
In conclusion, the beneficial effect of fluorinated al-
cohols on the asymmetric hydrogenation with self-as-
sembling catalyst has been documented for the first
time. Due the broad availability of fluorinated sol-
vents our findings are also of great practical impor-
[
13]
tance. Moreover, our observation should also be of
great importance for other catalysts bearing two mon-
in the enantioselective hydrogenation in CF CH OH odentate ligands where intramolecular interactions
3
2
[
7,13g]
as a solvent.
via hydrogen bondings have been assumed.
As it can be seen in Table 2 catalysts with non-as-
sembling ligands induce much lower enantioselectivi-
ty. In accordance to results of the reaction in polar
Experimental Section
(
CH OH) or non-polar solvents (CH Cl ), where only
3
2
2
[
8]
small differences in ee values were noted, also only
General Remarks
small deviations in CF CH OH are visible. These re-
3
2
sults give evidence that these catalysts work rather in- All reactions involving phosphanes were performed under
dependent of the solvent used. In contrast, the cata- an inert atmosphere of argon by using standard Schlenk
Adv. Synth. Catal. 2007, 349, 2183 – 2187
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2185

Natalia V. Dubrovina et al.
UPDATES
[a]
Table 2. Comparison of results of the enantioselective hydrogenation with monodentate ligands 3a, b and 4.
Entry
Ligand
Substrate
R
Solvent
t [min]
Conversion [%]
ee [%]
1
2
R
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
3a
3a
3a
3a
3a
3a
3b
3b
3b
3b
3b
3b
4
NHAc
NHAc
NHAc
Ph
Ph
Ph
H
H
H
Ph
Ph
Ph
H
H
H
Ph
Ph
Ph
H
H
H
CF CH OH
40
30
400
10
1400
300
200
60
70
60
40
45
25
7
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
27 (R)
23 (R)
22 (R)
18 (S)
11 (S)
12 (S)
31 (R)
71 (R)
77 (R)
57 (R)
49 (R)
76 (R)
13 (R)
56 (R)
90 (R)
61 (S)
87 (S)
58 (S)
3
2
CH Cl
2
2
CH OH
3
CH COOMe
CF CH OH
2
3
2
CH COOMe
CH Cl
2
2
2
CH COOMe
CH OH
2
3
NHAc
NHAc
NHAc
CF CH OH
3
2
CH Cl
2
2
CH OH
3
1
1
1
1
1
1
1
1
1
CH COOMe
CF CH OH
2
3
2
CH COOMe
CH Cl
2
2
2
CH COOMe
CH OH
2
3
NHAc
NHAc
NHAc
CF CH OH
3
2
4
4
4
4
CH Cl
2
2
CH OH
2
4
8
2
3
CH COOMe
CF CH OH
2
3
2
CH COOMe
CH Cl
2
2
2
4
CH COOMe
CH OH
2
3
[
a]
For reaction conditions and analysis of products, see Table 1.
techniques. Solvents were dried and freshly distilled under
argon before use. The 1,1,1,3,3,3-hexafluoro-2-propanol and
Acknowledgements
2
-fluoroethanol were purchased fromABCR and Alfa
The authors are grateful to BASF AG(Ludwigshafen) for fi-
nancial support of this work. We acknowledge valuable dis-
cussions with Prof. Dr. B. Breit (Freiburg). We thank Dr. C.
Fischer and Mrs. S. Buchholz for the analysis of the chiral
hydrogenation products.
Aesar, respectively. Syntheses of the phospholane ligands 1,
[8] 1
3
and phosphepine ligands 2, 4 were reported recently. H,
13
31
C and P NMR spectra were recorded on Bruker Spec-
1
trometer AVANCE 500 and 300 ( H: 500.13 MHz and
3
1
1
3
31
00.13 MHz;
C: 125.8 MHz and 75.5 MHz;
P:
1
13
31
62.0 MHz). Chemical shifts of H, C and P NMR spectra
1
13
are reported in ppm. The calibration of H and C spectra
was carried out on solvent signals [d (CDCl )=7.25 and
3
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31
7
7.0]. The P chemical shifts are referenced to 85% H PO .
3 4
The optical rotations were measured on a “gyromat-HP” in-
strument (Fa. Dr. Kernchen). Mass spectra were recorded
on a MAT 95XP (Finnigan).
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A
C
H
T
R
E
U
N
G
(COD) ]BF (1 equiv.) and chiral ligands (2 equivs.) under
2 4
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2186
asc.wiley-vch.de
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Adv. Synth. Catal. 2007, 349, 2183 – 2187

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UPDATES
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5
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31
[
10] P NMR data measured for the same complex in
3
1
103
31 31
ACHTREUNG
CDCl : d=54.5 [dd, J( P, Rh)=146 Hz, J( P, P)=
3
3
1
103
31 31
ACHTREUNG
3
3
6 Hz]; d=50.7 [dd, J( P, Rh)=137 Hz, J( P, P)=
6 Hz].
Adv. Synth. Catal. 2007, 349, 2183 – 2187
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2187