Singly hydrogen bonded supramolecular ligands for highly selective rhodium-catalyzed hydrogenation reactions

Article abstract of DOI:10.1002/anie.200806177

(Chemical Presented) H bonds make the catalyst! A single hydrogen bond between ligands coordinated to a rhodium center is critical for the formation of pure supramolecular catalyst for asymmmetric hydrogenation reactions. The ester group of the amidite ligand (see scheme) also forms a hydrogen bond with the coordinated substrate. Use of the herecomplex afforded the highest enantioselectivity reported to date for the hydrogenation of several ester substrates.

Full text of DOI:10.1002/anie.200806177

Communications
DOI: 10.1002/anie.200806177
Asymmetric Catalysis
Singly Hydrogen Bonded Supramolecular Ligands for Highly Selective
Rhodium-Catalyzed Hydrogenation Reactions**
Pierre-Alain R. Breuil, Frederic W. Patureau, and Joost N. H. Reek*
Ligand variation is a key tool for the optimization of
transition metal catalysts. Ligand effects are sufficiently
well-understood to facilitate ligand design in several reac-
tions. For asymmetric catalysis, however, catalyst optimiza-
tion relies to a large extent on trial and error, hence the
combinatorial screening of libraries of chiral catalysts is a
frequently applied strategy.[1] Besides catalyst screening,
attempts toward the rational design of chiral catalysts have
also been made, which generally lead to strategies for ligand
development.[2] The asymmetric hydrogenation reaction is
among the classic success stories in this respect since it has
resulted in several scientific breakthroughs,[3] as well as the
development of commercial processes.[4] Importantly, in the
rhodium-catalyzed asymmetric hydrogenation of functional-
ized substrates, the substrate coordinates in a bidentate
fashion to square-planar rhodium, which gives rise to the
formation of four substrate–metal coordination modes. The
use of C2-symmetric[5] bidentate ligands reduces the number
of coordination modes to only two (Re, Si), and this has
therefore been a successful strategy.[2] Another approach to
reduce the number of coordination modes is the design of
strongly unsymmetrical ligands.[2] Strong donor/strong p-
acceptor bidentate ligands[6] provide sufficient differences in
electronic properties to direct the coordination of the
chelating substrate. The disadvantage associated with this
approach is the often tedious synthesis of unsymmetrical
bidentate ligands. Interesting breakthroughs in this respect
are the use of mixtures of monodentate ligands as reported by
Reetz et al.[7] and Feringa and co-workers,[8] and the supra-
molecular approach to make heterobidentate ligands.[9] In the
mixture approach, the presence of homocomplexes (metal
complexes with two identical ligands) can significantly alter
the outcomes of the reaction. By optimization of the ratio of
the two monodentate ligands, the composition of the catalyst
mixture can be tuned and, with this, the selectivity can be
optimized. However, a proportion of the precious metal will
be kept in an inactive state. Intrigued by this problem, we
decided to study the effect of electronic and steric effects as
well as hydrogen bonds on the formation and catalytic
properties of the heteroligand complex. The rhodium com-
plexes were evaluated in the asymmetric hydrogenation of the
methyl 2-hydroxymethylacrylate (10a), which afforded
methyl 3-hydroxy-2-methylpropionate (11a), also known as
the Roche ester, which represents an important synthon for
the synthesis of the antitumor agents tedanolide and disco-
dermolide.[10] Importantly, the product is liquid at room
temperature and consequently a very high enantiopure
synthesis is required as further purification by crystallization
is not possible. We demonstrate herein that a single hydrogen
bond between LEUPhos (1) and urea–phosphine 8 is
sufficient to form pure supramolecular heterobidentate com-
plexes. We also show that a hydrogen bond between the
ligand 1 and the substrate is important to produce an
intermediate complex in a hydrogenation reaction from
which the product is obtained with 99% ee, which is the
highest enantioselectivity reported to date.[11]
The new ligand LEUPhos (1, Scheme 1) was synthesized
by a simple condensation reaction between enantiopure (S)-
2,2’-dihydroxy-1,1’-binaphthyl ((S)-binol) and PCl3 , followed
by the addition of l-leucine methyl ester (see the Supporting
Information). This chiral ligand was studied in combination
with achiral aromatic phosphines 3–7 to evaluate the effect of
electronic and steric properties of the phosphine ligand on the
formation of heterocomplexes under stoichiometric condi-
tions.
We anticipated that hydrogen bonds could be formed
between the urea NH group in 8 and the ester functionality of
[*] P.-A. R. Breuil, F. W. Patureau, Prof. Dr. J. N. H. Reek
Van’t Hoff Institute for Molecular Sciences
University of Amsterdam
Nieuwe Achtergracht 166 1018 WV, Amsterdam (The Netherlands)
Fax: (+31)20-525-5604
E-mail: j.n.h.reek@uva.nl
Homepage: http://www.science.uva.nl/research/imc/HomKat/
[**] We acknowledge the NWO and the University of Amsterdam (VICI
grant) for financial support and J. Wassenaar for providing methyl
2-hydroxymethylacrylate.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200806177.
Scheme 1. Chiral phosphoramidite and achiral aromatic phosphine
ligands used in this study.
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Chemie
Table 1: Rhodium-catalyzed asymmetric hydrogenation of methyl
2-hydroxymethylacrylate (10a).[a]
Entry
L
L’ ee [%]
R, S Effect
Heterocomplex [%][b]
1
2
3
4
5
6
7
8
9
1
2
1
2
1
1
1
1
1
1
2
3
3
4
5
6
7
8
31
13
S
S
R
S
R
R
R
R
R
–
–
–
–
91
85
94
97
86
70
>99
94 (94)[c]
34
electronic
electronic
electronic
electronic
steric
94
94
94
95
steric
H bond
>99
[a] Ratio L/L’/[Rh(cod)2]BF4/substrate=1.1:1:1:100; solvent: CH2Cl2.
Reaction performed at 10 bar H2 pressure at 298 K for 16h. Full
conversions were obtained in all cases. [b] The amount of heterocomplex
present in solution was determined by integration of the phosphine
signals in the 31P NMR spectrum (20 mm in CD2Cl2, 298 K). [c] Deter-
mined in the presence of phenylurea (1 equiv with respect to L’).
Figure 1. Supramolecular bidentate complexes [Rh(cod)(1)(8)]BF4 (cod
omitted for clarity) and DFT calculations: a) Hydrogen bonds between
the ester and urea units. Relative energy=+7.5 kcalmolÀ1, d(H-bond)
=
phosphine ligands have, on the other hand, a dramatic effect
on heterocomplex formation, as is evident from experiments
with 6 and 7 (Table 1, 31P NMR spectra are shown in the
Supporting Information). Importantly, the complex that
1.9 and 2.6 ꢀ. b) Single hydrogen bond between the NH group of the
phosphoramidite unit and the urea unit. Relative energy=0 kcalmolÀ1
,
d(H-bond) =2.0 ꢀ.
forms
a hydrogen bond between the ligands, [Rh-
(cod)(1)(8)BF4], is the only complex that was formed in
more than 99% purity (see the Supporting Information).
Consistent with this observation, the combination of ligands 8
and 2 did not lead to pure heterocomplex formation, but a
mixture of different species which are difficult to assign.
We next studied the performance of these complexes in
the asymmetric hydrogenation of methyl 2-hydroxymethyl-
acrylate 10a. Ligand 2 was used for comparison with 1. Under
mild conditions (catalyst (1 mol%), H2 (10 bar), 298 K, 16 h),
full conversion was obtained in all experiments. Both
homocomplexes [Rh(cod)(1)2]BF4 and [Rh(cod)(2)2]BF4
gave low selectivities of 31% and 13%, respectively
(Table 1 entries 1 and 2). An excellent enantioselectivity
(94% ee) was obtained with 1 in combination with PPh3 (3;
Table 1 entry 3) while 2 with PPh3 afforded only a moderate
ee values of 34% (Table 1 entry 4). Contrary to our expect-
ations, the amount of heterocomplex present in solution
hardly affected the enantiopurity of the product that is
formed; products were obtained in 94-95% ee in all cases
where this mixed ligand approach was used with 1 (Table 1,
entries 5–8). This result suggests that the heterocomplexes are
much more active than the unselective homocomplexes.
Although the formation of heterocomplexes can be dramat-
ically enhanced (50% for a statistical mixture, 97% for the
combination of ligands 1 and 5) by fine-tuning the electronic
and steric properties of a series of ligands, it does not translate
to higher selectivity in the reaction studied here. In contrast to
these experiments, the supramolecular complex [Rh-
(cod)(1)(8)]BF4 did convert the substrate with the highest
selectivity reported to date (Table 1 entry 9).[11] In a control
experiment in which phenylurea was used as an additive for
the complex [Rh(cod)(1)(3)]BF4 , the selectivity did not
change (Table 1 entry 3), which indicates that the urea
group of the ligand 8 plays a crucial role in the selectivity of
the reaction. Since it is unlikely that the purity of the complex
1 (Figure 1a). Alternatively, a hydrogen bond may be formed
between the NH group of the phosphoramidite 1, which is
known to be a good hydrogen-bond donor[12] and the urea
carbonyl group (Figure 1b). The structures (calculated by
using DFT, BLYP) show that the single hydrogen-bond
interaction (d(H-bond) = 2.0 ꢀ) is more favorable (7.5 kcal
molÀ1) than the double hydrogen bond between the urea–
NH group and the ester carbonyl group. Some other
structures have been calculated in which no hydrogen bonds
were formed; these were all higher in energy (see the
Supporting Information). IR studies on the [Rh-
(cod)(1)(8)]BF4 complex (cod = cyclooctadiene) also con-
firmed the formation of the hydrogen bond between the PNH
unit of 1 and the urea carbonyl group (see below); the effect
of this hydrogen bond on the selectivity of heterocomplex
formation was next studied.
We first studied the complexes that were formed by
mixing [Rh(cod)2]BF4 , 1, and one of the phosphines 3–8 in a
1:1:1 ratio. Interestingly, heterocomplex [Rh(cod)(1)(3)]BF4
was formed in 91% yield, according to the 31P NMR spectrum
of the mixture in CD2Cl2. This value is far above the
statistically expected value and the remaining signals in the
NMR spectrum correspond to the homocombinations. A
similar experiment with the archetypical phosphoramidite
ligand (S)-(+)-(3,5-dioxa-4-phosphacyclohepta[2,1-a;3,4-a’]-
dinaphthalen-4-yl)dimethylamine
((S)-MonoPhos,
2)
showed that only 85% of the heterocomplex [Rh-
(cod)(2)(3)]BF4 was formed. By varying the electronic
properties of the aromatic phosphines, the formation of the
heterocomplex occurred in up to 97% yield for [Rh-
(cod)(1)(5)]BF4 (Table 1, entries 5 and 6). This result is
interesting in itself as it provides a simple tool to make
relatively pure heterocomplexes without using an excess of
one of the ligands. Small changes in the size of the aromatic
Angew. Chem. Int. Ed. 2009, 48, 2162 –2165
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Communications
Table 2: Asymmetric hydrogenation of methyl 2-hydroxymethylacrylate
10a (10e for entry 4) catalyzed by supramolecular heterocomplexes.[a]
is the cause of these observations, we studied the complex in
more detail.
We first calculated several structures of complex
Entry
L
L’
ee [%]
R,S
Effect
Heterocomplex [%][b]
[Rh(1)(8)]BF4–(substrate), which is one of the important
intermediates of the catalytic cycle, by using DFT (BLYP).
The minimum-energy structure of the catalyst shows that
1
2
3
4
1
2
9
1
8
8
8
8
>99
38
R
R
R
R
H bond
electronic
H bond
H bond
>99
mixture[c]
>99
88
52[d]
>99
1) the urea–carbonyl hydrogen bond is still present (dH-bond
=
2.0 ꢀ), 2) there is a hydrogen bond between the alcohol of
the substrate and the carbonyl unit of the ester group of 1
(dH-bond=2.1 ꢀ, Figure 2). These calculations suggest that the
high ee value obtained by the application of [Rh-
(cod)(1)(8)]BF4 is caused by the substrate orientation from
[a] Ratio L/L’/[Rh(cod)2BF4]/Substrate=1.1:1:1:100; solvent: CH2Cl2.
Reaction performed at 10 bar H2 pressure at 298 K for 16h. Full
conversions were obtained in all cases. [b] The amount of hetero-
complex present in solution was evaluated by integration of the
phosphine signals in the 31P NMR spectrum (20 mm in CD2Cl2, 298 K).
[c] The heterocomplex was observed among a mixture of (dynamic)
species. [d] Trimethylsilyl-protected substrate 10e was used as control.
ee value of 99% obtained with the heterocomplex [Rh-
(cod)(1)(8)]BF4 (Table 2 entries 1 and 3). As a control
experiment, we studied the hydrogenation of trimethylsilyl-
protected substrate 10e, which also is unable to form the
critical hydrogen bond. As expected, only moderate ee values
(52% and 48% ee; Table 2, entry 4 and the Supporting
Information) were obtained with [Rh(cod)(1)(8)]BF4 and
[Rh(cod)(9)(8)]BF4 , respectively. These results support our
hypothesis that the hydrogen bond between the substrate and
the ligand plays a crucial role in the hydrogenation reaction.
We next explored the scope of our new concept by
extending our hydrogenation experiments to several deriva-
tives of methyl 2-hydroxymethylacrylate (10b–d, Scheme 2).
The high enantioselectivity induced by [Rh(cod)(1)(8)]BF4
appears to be relatively insensitive to modifications of the
ester group. The product was obtained with 99% ee for the
substrate with the bulky tert-butyl ester group (Table 3
entry 1) and 92% ee for the substrate with a benzyl moiety
(Table 3 entry 2). More interestingly, the scope of the reaction
can be extended to the hydrogenation of more hindered
Figure 2. Substrate orientation through hydrogen bonding between the
hydroxy group of the substrate and the ester function of the phosphor-
amidite unit (binol backbone omitted for clarity).
a hydrogen bond between the ligand and the substrate, an
effect similar to that observed for other selective trans-
formations.[13,14]
We expected that, if this substrate-orientation effect were
to play a role in the hydrogenation reaction, the use of ligands
that would be unable to form this hydrogen bond would result
in lower ee values. For this reason, ligand 9, which has a PNH
unit similar to 1 that can form a hydrogen bond with 8, but
lacks the ester moiety of 1, was prepared. As observed for
[Rh(cod)(1)(8)]BF4, the complex [Rh(cod)(9)(8)]BF4 was
formed in more than 99% purity (calculated from the
31P NMR spectrum) by using stoichiometric amounts of the
ligands. IR spectroscopy showed that the vibration of the
carbonyl group of the ester moiety of 1 does not change in the
complex compared to the free ligand (1737 cmÀ1). The IR
band of the carbonyl group of the urea moiety in 8 is
significantly shifted to lower wavenumbers (from 1703 cmÀ1
to 1687 cmÀ1) compared to the free ligand in both complexes,
which confirms its participation in hydrogen bonding. Impor-
tantly, in a mixture with 2, the carbonyl group of 8 is found at
the original position (1700 cmÀ1). These experiments also
show that ligand 9 with 8 gives rise to pure heterocomplex by
formation of a single hydrogen bond between the two simple
monodentate ligands.
Scheme 2. Asymmetric hydrogenation of 10a–e with various (supra-
molecular) rhodium complexes.
We next evaluated the effect of the hydrogen bond
between the substrate and the ester functional group of the
ligand, as calculated for the [Rh(1)(8)]BF4–(substrate) com-
plex by comparing the properties of the various complexes in
the rhodium-catalyzed asymmetric hydrogenation of methyl
2-hydroxymethylacrylate 10a with [Rh(cod)(1)(8)]BF4, [Rh-
(cod)(2)(8)]BF4, and [Rh(cod)(9)(8)]BF4. As expected, the
MonoPhos-based complex [Rh(cod)(2)(8)]BF4 produced the
product with low selectivity (38% ee, Table 2 entry 2).
Although the complex [Rh(cod)(9)(8)]BF4 gave a reasonable
selectivity (88% ee), the selectivity is much lower than the
Table 3: Asymmetric hydrogenation of 2-hydroxymethylacrylate esters
10b–d catalyzed by supramolecular complex [Rh(1)(8)(cod)]BF4.
Entry
Substrate
Conversion [%]
ee [%]
R,S
1
2
3
10b
10c
10d
100
100
83
>99
92
96
R
R
R
[a] Ratio L/L’/[Rh(cod)2BF4]/substrate=1.1:1:1:100; solvent: CH2Cl2.
Reaction performed at 10 bar H2 pressure at 298 K for 16h.
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Chemie
X. G. Li, Angew. Chem. 2005, 117, 3019 – 3021; Angew. Chem.
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of [Rh(cod)(1)(8)]BF4 with the highest selectivity reported to
date (96% ee, Table 3, entry 3). The more sterically hindered
alkenes are generally more difficult to hydrogenate, which is
reflected in the slightly lower yield of the product (83% for
unoptimized conversion).
In summary, we have introduced LEUPhos (1) as a new
supramolecular ligand by forming a pure heterocomplex
through a single hydrogen bond between the NH group of the
phosphoramidite and the urea carbonyl group of a function-
alized phosphine. This heterocomplex afforded the highest
enantioselectivity (> 99% ee) reported to date for the hydro-
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Received: December 18, 2008
Published online: February 10, 2009
Keywords: asymmetric catalysis · hydrogen bonds · P ligands ·
.
rhodium · supramolecular chemistry
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