Supramolecular hybrid bidentate ligands in asymmetric hydrogenation

Article abstract of DOI:10.1002/ejic.201200549

In this study we introduce a novel class of supramolecular bidentate hybrid ligands and their application in the rhodium-catalysed asymmetric hydrogenation of prochiral olefins. A new supramolecular strategy is reported in which the two nonequivalent phosphorus atoms are linked covalently to a chiral scaffold, and the supramolecular interactions are used to control the second coordination sphere of the transition-metal catalyst. The supramolecular assembly is formed in situ by selective interaction between the nitrogen-donor atoms and the zinc(II) template, which is essential for obtaining high activity and selectivity. The investigations of the different zinc(II) and ruthenium(II) templates on the reaction parameters revealed a dependence of the activity and selectivity on the association constant between the supramolecular template and the pyridyl ligands. The scope of the supramolecular assemblies was explored in the Rh-catalysed asymmetric hydrogenation of α-dehydroamino acid esters and Roche ester derivatives. High activities and good to excellent enantioselectivities up to 99 % ee were obtained with the supramolecular ligands. Application of the supramolecular strategy on the basis of variations of the steric and electronic properties of zinc(II) templates demonstrate that these changes can influence key reaction parameters such as activity and selectivity. Copyright

Full text of DOI:10.1002/ejic.201200549

Job/Unit: I20549
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Date: 13-08-12 11:39:00
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FULL PAPER
DOI: 10.1002/ejic.201200549
Supramolecular Hybrid Bidentate Ligands in Asymmetric Hydrogenation
Rosalba Bellini[a] and Joost N. H. Reek*[a]
Keywords: Supramolecular chemistry / Self-assembly / Hybrid bidentate ligands / Hydrogenation / Asymmetric catalysis
In this study we introduce a novel class of supramolecular
bidentate hybrid ligands and their application in the rho-
dium-catalysed asymmetric hydrogenation of prochiral ole-
fins. A new supramolecular strategy is reported in which the
two nonequivalent phosphorus atoms are linked covalently
to a chiral scaffold, and the supramolecular interactions are
used to control the second coordination sphere of the transi-
tion-metal catalyst. The supramolecular assembly is formed
in situ by selective interaction between the nitrogen-donor
atoms and the zinc(II) template, which is essential for ob-
taining high activity and selectivity. The investigations of the
different zinc(II) and ruthenium(II) templates on the reaction
parameters revealed a dependence of the activity and selec-
tivity on the association constant between the supramolec-
ular template and the pyridyl ligands. The scope of the sup-
ramolecular assemblies was explored in the Rh-catalysed
asymmetric hydrogenation of α-dehydroamino acid esters
and Roche ester derivatives. High activities and good to ex-
cellent enantioselectivities up to 99% ee were obtained with
the supramolecular ligands. Application of the supramolec-
ular strategy on the basis of variations of the steric and elec-
tronic properties of zinc(II) templates demonstrate that these
changes can influence key reaction parameters such as ac-
tivity and selectivity.
different phosphorus atoms (e.g., phosphane–phosphite or
phosphane–phosphoramidite ligands). Application of phos-
phane–phosphite ligands in asymmetric hydrogenation have
been reported by Pizzano and co-workers.[14] Noteworthy
examples of hybrid phosphane–phosphoramidite ligands
have been reported by the groups of Leitner (Quina-
phos),[15] Reek (IndolPhos),[16] Zhang (triphosphorus
phosphane–phosphoramidite ligand),[17] Zhuo Zhang
(THNAPhos and HY-Phos)[18] and Kostas and Börner
(Me-AnilaPhos).[19]
An emerging class of hybrid ligands are the supramolec-
ular bidentate ligands, which are formed by self-assembly
of complementary monodentate units.[20] According to this
approach, several groups have successfully developed het-
erobidentate ligands on the basis of dynamic metal–ligand
interactions[21] as well as hydrogen-bond[22] and ionic inter-
actions.[23] In most of these examples, the supramolecular
interactions are used to bring two monodentate ligand
building blocks together to form a heterobidentate ligand.
In this contribution, we report an alternative supramolec-
ular strategy in which the two non-equivalent phosphorus
atoms are covalently linked together, and the supramolec-
ular interactions are used as an additional handle to control
the second coordination sphere of the transition metal atom
(Figure 1). These ligands were successfully applied in rho-
dium-catalysed asymmetric hydrogenation of prochiral ole-
fins, in which a significant effect was observed in the cata-
lytic behaviour upon variation of zinc(II) templates. In ad-
dition, we report the investigations of the substrate scope
for these supramolecular bidentate ligands, and high-pres-
sure NMR spectroscopy is used to characterise the coordi-
nation complexes.
Introduction
For both academia and industry, asymmetric hydrogena-
tion is one of the most important industrial reactions for
the preparation of optically active compounds.[1] A power-
ful tool for the optimisation of catalytic systems is based
on ligand variation.[2] Since the disclosure of P-chirogenic
monodentate phosphane ligands by Horner[3] and
Knowles,[4] and of chelating C2-symmetric ligands such as
diisooctyl phthalate (DIOP) by Kagan,[5] ethane-1,2-di-
ylbis[(2-methoxyphenyl)(phenyl)phosphane] (DIPAMP) by
Knowles,[6] 2,2Ј-bis(diphenylphosphanyl)-1,1Ј-binaphthyl
(BINAP) by Noyori and Takaya[7] and DuPhos by Burk,[8]
many new systems have been reported, thus enabling a vari-
ety of metal-catalysed asymmetric transformations.[1,2] The
next important development was the introduction of C1-
nonsymmetrical bidentate ligands.[9] In these ligands, the
presence of two different P-donor atoms causes an asym-
metric distribution of charges and steric control of the coor-
dination sphere of the transition metal, characteristics
which were determined to be beneficial for improving the
stereoface-discrimination capacity of the catalysts. Unsym-
metrical ligands can be composed of two different donor
atoms (P,N or P,S). Examples of P,N ligands have been re-
ported by Pfaltz,[10] Helmchen[11] and Andersson.[12] Com-
plementary P,S ligands have been developed by Evans and
co-workers.[13] Hybrid bidentate ligands can also bear two
[a] Van’t Hoff Institute for Molecular Science,
University of Amsterdam
Science Park 904, 1098 XH, The Netherlands
Fax: +31-205255604
E-mail: j.n.h.reek@uva.nl
Homepage: http://www.science.uva.nl/research/imc/Homkat/
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R. Bellini, J. N. H. Reek
FULL PAPER
Figure 1. General structure of supramolecular hybrid bidentate ligands by using pyridyl-based bidentate ligands and zinc(II) or rutheni-
um(II) templates.
Coordination Studies
Results and Discussion
Assembly of Ligands 1a,b with Template 8
Ligand Synthesis
Supramolecular ligands (S)-1a(8)2 and (S)-1b(8)2 were
formed in situ by selective coordination of the nitrogen-do-
nor atom to the zinc(II) template 8 (Scheme 2). The coordi-
nation behaviour of pyridine-based ligands 1a and 1b to
zinc(II)–salphen compound 8 was studied by UV/Vis spec-
troscopy. Identical and independent association constants
for the first and second binding of 8 to ligands 1a were
obtained [1a(8)2: K1(1a-8) = K2(1a-8) = 2.1ϫ104 m–1], as well
The novel bidentate phosphane–phosphoramidite li-
gands (S)-1a–c were prepared according to the procedure
displayed in Scheme 1. The synthetic procedure started with
the conversion of N-methylaniline (2) into the lithium carb-
amate compound, followed by o-lithiation with tBuLi and
treatment with Ph2PCl and subsequent hydrolysis to give
compound 3.[24] Phosphane 3 was transformed into 4 by
treatment with PCl3 at low temperature and under basic
conditions. Quantitative yields were achieved, and the phos-
phorodichloride compound was used in the next step with-
out any further purification. Compounds (S)-7a–c were pre-
pared by starting from the commercially available (S)-Binol,
which was reduced in the presence of Pd/C.[25] Diol 5 was
selectively brominated at the 3,3Ј-positions to give product
6.[26] Suzuki coupling of 6 with the appropriate boronic ac-
as for the assembly 1b(8)2 (K1(1b-8)
= K2(1b-8) =
3.3ϫ104 m–1).
Application of Supramolecular Bidentate Hybrid Ligands in
Asymmetric Hydrogenation
To evaluate the new supramolecular hybrid ligands in
ids provided compounds 7a–c.[27] Condensation of com- asymmetric hydrogenation, we performed a series of cata-
pound 4 with diols 7a–c produced the hybrid ligands 1a–c lytic experiments using dimethyl itaconate (9) as a bench-
in good yields, up to 86% after flash column chromatog- mark substrate with the supramolecular ligand (S)-1a(8)2.
1
raphy. All new compounds were fully characterised by H, Initial hydrogenation experiments were performed by using
13C and 31P NMR spectroscopy and high-resolution mass different metal precursors based on iridium or rhodium in
spectrometry.
combination with the supramolecular ligand (S)-1a(8)2 and
2
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Supramolecular Hybrid Bidentate Ligands
Scheme 1. Synthesis of the chiral bidentate hybrid ligands (S)-1a–c. PEPPSI = [1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene](3-chloro-
pyridyl)palladium(II) dichloride.
the nontemplated ligand (S)-1a, which was used as the con- Table 1. Asymmetric hydrogenation of dimethyl itaconate: varia-
tion of the metal precursor.[a]
trol ligand. Catalysts based on ligand (S)-1a(8)2 in the pres-
ence of rhodium precursors gave excellent results in terms
of activity and selectivity (Table 1, Entries 1 and 3). No dif-
ference could be observed when using cationic Rh precur-
sors based on norbornadiene (nbd) or 1,5-cyclooctadiene
(cod; Table 1, Entries 1–4). In the absence of template 8,
Entry
M
Ligand Substrate Conv. [%][b] ee [%][c]
ligand (S)-1a showed extremely poor activity and no selec-
tivity (Table 1, Entries 2 and 4). This might be attributed to
the formation of unreactive rhodium complexes, in which
the nitrogen atom of the pyridine is coordinated to the tran-
sition-metal centre that blocks a vacant site required for
the coordination of the substrate. When we used iridium
catalysts, neutral or cationic, no conversion was observed
(Table 1, Entries 5–8). In line with this, no reports could be
found in the literature of an iridium catalyst that was active
in the hydrogenation of dimethyl itaconate (9). In contrast,
in a recent publication a neutral iridium catalyst based on
an anionic bidentate ligand was reported to be an efficient
system for the asymmetric hydrogenation of tetrasubsti-
tuted cyclic enamides, such as substrate 10, to give the cor-
responding amides with high enantioselectivitities.[28] Thus,
we tested the current iridium catalysts in the asymmetric
1
2
3
4
5
6
7
8
9
[Rh(nbd)2]BF4 1a(8)2
[Rh(nbd)2]BF4 1a
[Rh(cod)2]BF4 1a(8)2
9
9
9
9
9
9
9
9
10
10
Ͼ99
Ͼ99 (R)
2
Ͼ99
2
4
0
0
0
0
0
0
Ͼ99 (R)
[Rh(cod)2]BF4
[Ir(cod)2Cl]2
[Ir(cod)2Cl]2
[Ir(cod)2]BF4
[Ir(cod)2]BF4
1a
1a(8)2
1a
1a(8)2
1a
0
19 (R)
0
0
0
0
0
[{Ir(cod)2Cl}2] 1a(8)2
[{Ir(cod)2Cl}2] 1a
10
[a] [M] = 1 mm in CH2Cl2, ligand/M = 1.1, substrate/M = 100,
25 °C, 10 bar of H2, 16 h. [b] Percentage conversion calculated on
the basis of GC analysis. [c] Enantiomeric ratio determined by chi-
ral GC (Supelco BETA DEX 225). All the catalysis experiments
were performed in duplicate.
Next, a series of control experiments were performed to
hydrogenation of substrate 10, but also in this case no con- evaluate possible influences of the zinc(II) template on the
version was observed (Table 1, Entries 9 and 10). activity and selectivity of the reaction. From the results dis-
As it emerged from these first experiments that the Rh played in Table 2, it is clear that the reaction in the presence
precursors formed far more active catalysts, the rest of the of only template 8 and [Rh(nbd)2]BF4 results in poor con-
catalytic reactions were conducted by using [Rh(nbd)2]BF4 version (Table 2, Entry 3). Interestingly, the catalysis in the
as the metal precursor.
absence of both the ligand and the template provided 66%
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R. Bellini, J. N. H. Reek
FULL PAPER
Scheme 2. Assembly of ligands 1a–b on zinc(II)–salphen 8.
Table 2. Asymmetric hydrogenation of dimethyl itaconate.[a]
conversion; clearly the product was formed in a racemic
fashion (Table 2, Entry 4). We were surprised to see that in
the presence of template 8 this conversion was reduced to
2% (Table 2, Entry 3). This could be due to a transmet-
alation process in which the metal atom of the zinc–salphen
is replaced by the rhodium atom, thus leading to catalyst
Entry
M
Ligand Template Conv. [%][b] ee [%][c]
1
2
3
4
5
[Rh(nbd)2]BF4
[Rh(nbd)2]BF4
[Rh(nbd)2]BF4
[Rh(nbd)2]BF4
–
1a
1a
–
–
–
8
–
8
–
8
Ͼ99
2
2
66
0
Ͼ99 (R)
0
0
0
0
1
deactivation. Analysis by H NMR spectroscopy of the re-
action mixture after catalysis shows a small peak in the im-
ine region (δ = 8.80 ppm in [D6]acetone), whereas the imine
peak of the zinc(II)–salphen resides further downfield (δ =
9.05 ppm). The catalysis in the absence of Rh, when em-
[a] [[Rh(nbd)2]BF4] = 1 mm in CH2Cl2, ligand/rhodium = 1.1, sub-
strate/Rh = 100, 25 °C, 10 bar of H2, 16 h. [b] Percentage conver-
sion on the basis of GC analysis. [c] Enantiomeric ratio determined
by chiral GC (Supelco BETA DEX 225). All the catalysis experi-
ploying only template 8, did not produce any product ments were performed in duplicate.
4
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Supramolecular Hybrid Bidentate Ligands
(Table 2, Entry 5). These results indicate that template 8 can try 5). Also, zinc(II)–salphen complexes are known to bind
interfere in the rhodium-catalysed asymmetric hydrogena- pyridines stronger than zinc(II)–porphyrins (104 versus
tion, but only in the absence of pyridine-based ligands. In- 103 m–1, respectively).[30] These results suggest that forma-
deed, the presence of the latter leads to the coordination of tion of a more stable supramolecular system is required to
Zn to the nitrogen-donor atom, thereby leaving the rho- achieve high conversion and enantioselectivity in catalytic
dium centre free for the coordination of the two phos- systems, likely to effectively block pyridine coordination to
phorus atoms.
the rhodium atom. On the basis of the results, we decided
to use template 8 for further catalytic experiments.
Asymmetric Hydrogenation of Dimethyl Itaconate
Substrate Scope
Template Variation
Further optimisation of the catalytic system was ex-
plored by using a small series of different zinc(II) and ruth-
enium(II) templates (Figure 2) in combination with ligand
(S)-1a. Good activities and selectivities were achieved by
using zinc(II)–salphen building blocks (Table 3, Entries 1–
3). Interestingly, the conversion varied slightly with the dif-
ferent templates, with template 8 giving the highest conver-
sion. Zinc(II)–porphyrin 13a gave low conversion and selec-
tivity (Table 3, Entry 4). In contrast, the ruthenium(II)–por-
phyrin 14a, which is known to bind pyridines with higher
binding constants (105 m–1),[29] performed much better in
terms of conversion and enantioselectivity (Table 3, En-
After optimising the reaction conditions, we decided to
evaluate the performance of supramolecular ligands (S)-
1a(8)2 and (S)-1b(8)2, nontemplated ligands (S)-1a and (S)-
1b and control ligand (S)-1c in the rhodium-catalysed
asymmetric hydrogenation of α-dehydroamino acid esters
15a–c and methyl 2-hydroxymethylacrylate 16. Good con-
versions (up to 99%) and high enantioselectivities (up to
99%) were obtained with the supramolecular ligands for
substrates 15a–c (Table 4, Entries 1, 3, 6, 8, 11 and 13). In
contrast, poor or no activity was achieved with the nontem-
plated ligands (Table 4, Entries 2, 4, 7, 9, 12 and 14). In
line with the results obtained for dimethyl itaconate, control
ligand 1c, which lacks the pyridyl groups, also gave good
conversions and enantioselectivities for these substrates
(Table 4, Entries 5, 10 and 15). The supramolecular ligands
displayed high conversion (up to 99%) and moderate selec-
tivity (up to 55%) for the hydrogenation of the more chal-
lenging Roche ester derivative 16 (Table 4, Entries 16 and
18). No difference in activity and selectivity could be ob-
served between the two supramolecular ligands (S)-1a(8)2
and (S)-1b(8)2. Again, control ligand 1c displays similar re-
sults in terms of activity and selectivity as the supramolec-
ular ligands (Table 4, Entry 20). Also for this substrate, the
catalyst generated from the nontemplated ligands gave no
conversion (Table 4, Entries 17 and 19).
Table 3. Asymmetric hydrogenation of dimethyl itaconate: template
variation.[a]
Entry
Template
Conv. [%][b]
ee [%][c]
1
2
3
4
5
8
Ͼ99
90
87
5
Ͼ99
Ͼ99 (R)
99 (R)
99 (R)
57 (R)
Ͼ99 (R)
11
12
13a
14a
[a] [[Rh(nbd)2]BF4] = 1 mm in CH2Cl2, ligand/rhodium = 1.1, sub-
strate/Rh = 100, 25 °C, 10 bar of H2, 16 h. [b] Percentage conver-
sion on the basis of GC analysis. [c] Enantiomeric ratio determined
by chiral GC (Supelco BETA DEX 225). All the catalysis experi-
ments were performed in duplicate.
Figure 2. Zinc(II) and ruthenium(II) templates used for the current study.
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R. Bellini, J. N. H. Reek
FULL PAPER
Table 4. Asymmetric hydrogenation: substrate scope.[a]
and (S)-1b(8)2 assemblies, we decide to investigate a small
series of zinc(II) and ruthenium(II) templates with the aim
of steering the catalytic properties of these supramolecular
systems by using different templates (Figure 3). The cataly-
sis results are depicted in Table 5. In line with our previous
observations, supramolecular catalysts based on zinc(II)–
salphens and ruthenium(II)–porphyrins displayed higher
activity and selectivity than the one based on zinc(II)–por-
phyrins (Table 5, Entries 1, 2, 6 and 7 versus Entries 3, 4
and 5; Table 5, Entries 9, 10, 14 and 15 versus Entries 11–
13). An increase in conversion and enantioselectivity was
Entry Ligand Template Substrate Conv. [%][b] ee [%][c]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
1a
1a
1b
1b
1c
1a
1a
1b
1b
1c
1a
1a
1b
1b
1c
1a
1a
1b
1b
1c
8
–
8
–
–
8
–
8
–
–
8
–
8
–
–
8
–
8
–
–
15a
15a
15a
15a
15a
15b
15b
15b
15b
15b
15c
15c
15c
15c
15c
16
Ͼ99
4
96
98 (R)
0
96 (R)
0
97 (R)
Ͼ99 (R)
0
Ͼ99 (R)
0
Ͼ99 (R)
91 (R)
0
91 (R)
0
90 (R)
55 (S)
0
53 (S)
0
3
Ͼ99
Ͼ99
0
Ͼ99
0
Ͼ99
70
3
73
5
Ͼ99
Ͼ99
0
Ͼ99
0
Table 5. Asymmetric hydrogenation of methyl 2-hydroxymethyl-
acrylate: template screening.[a]
Entry
Ligand
Template
Conv. [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1b
1b
1b
1b
1b
11
12
88
87
59
13
16
Ͼ99
Ͼ99
17
80
84
29
17
13
91
90
9
52 (S)
53 (S)
39 (S)
21 (S)
32 (S)
57 (S)
55 (S)
3 (S)
51 (S)
52 (S)
23 (S)
26 (S)
15 (S)
55 (S)
53 (S)
0
13a
13b
13c
14a
14b
17
16
11
12
16
16
13a
13b
13c
14a
14b
17
16
Ͼ99
55 (S)
[a] [[Rh(nbd)2]BF4] = 1 mm in CH2Cl2, ligand/Rh = 1.1, substrate/
Rh = 100, 25 °C, 10 bar of H2, 16 h. [b] Percentage conversion on
the basis of GC analysis. [c] Enantiomeric ratio determined by chi-
ral GC and HPLC. All the catalysis experiments were performed
in duplicate.
[a] [[Rh(nbd)2]BF4] = 1 mm in CH2Cl2, ligand/Rh = 1.1, substrate/
Rh = 100, 25 °C, 10 bar of H2, 16 h. [b] Percentage conversion on
the basis of GC analysis. [c] Enantiomeric ratio determined by
In light of these results in which only moderate enantio-
selectivity was achieved for substrate 16 with the (S)-1a(8)2 HPLC. All the catalysis experiments were performed in duplicate.
Figure 3. Zinc(II) and ruthenium(II) templates used in this study.

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Supramolecular Hybrid Bidentate Ligands
observed with the change in electronics of the zinc(II)–por- BF4.[32] However, no signals that corresponded to the cata-
1
phyrins 13a–c for both ligands 1a and 1b (Table 5, Entries lyst–substrate complexes were identified in the H and 31P
3–5 and Entries 11–13). Changing the steric properties of NMR spectra. Moreover, exposing this mixture to 5 bar of
the templates, for example, with template 14b, did not have H2 yielded the hydrogenation products with identical
influence on the catalysis (Table 5, Entries 7 and 15). In ad- enantioselectivity to that obtained in catalytic experiments
dition, the catalysis does not seem to be influenced by the [99% ee, (R) enantiomer]. Next, we performed high-pres-
chirality of this template. Further increase in the steric bulk sure NMR spectroscopic experiments to identify possible
of the template when using template 17 led to a decrease in dihydride species by applying the protocol reported by
both conversion and enantioselectivity (Table 5, Entries 8 Gridnev, Imamoto and co-workers.[33] However, under
and 16).
these conditions, the solvate complex [Rh{1a(8)2}(MeCN)]-
BF4 at –90 °C was not converted to a complex that displays
1
hydride signals in the H NMR spectrum.
On the basis of these NMR spectroscopic studies in
which neither catalyst–substrate complexes nor dihydride
In Situ High-Pressure NMR Spectroscopic Studies
To gain structural information about the catalytic species species were observed, as with IndolPhos ligand, it is likely
involved in the asymmetric hydrogenation, we performed that our supramolecular catalysts follow a similar mechan-
high-pressure NMR spectroscopic experiments. The forma- istic pathway. However, detailed kinetic experiments will be
tion of the rhodium hydrogenation complexes was studied necessary to verify this hypothesis.
for the supramolecular ligands (S)-1a(8)2 and (S)-1b(8)2 as
well as for control ligands (S)-1a–c. All of the complexes
were generated in situ by mixing the cationic metal precur-
sor [Rh(nbd)2]BF4 with 1 equiv. of ligand in CD2Cl2. Mix-
ing the rhodium precursor [Rh(nbd)2]BF4 with ligand (S)-
Conclusion
1a(8)2 led to the formation of the complex [Rh{1a(8)2}-
A new class of supramolecular phosphane–phosphor-
(nbd)]BF4, as indicated by the two doublets of doublets in amidite ligands was evaluated in the asymmetric hydrogena-
the 31P NMR spectrum at δ = 19.9 ppm (JP,Rh = 148.7 Hz, tion of prochiral olefins. Whereas the supramolecular li-
JP,P = 66.0 Hz) and δ = 131.7 ppm (JP,Rh = 260.1 Hz, JP,P gands reported so far are formed by assembly of mono-
= 66.0 Hz), respectively. The addition of MeCN to this dentate building blocks, in the current studies we used bi-
complex partially yielded the solvate complex, [Rh{1a(8)2}- dentate ligands, the catalyst environment of which changed
(MeCN)]BF4, with a new set of doublets of doublets at δ = by association of different templates. After finding the opti-
34.2 ppm (JP,Rh = 160.7 Hz, JP,P = 73.5 Hz) and δ = mal reaction conditions by using dimethyl itaconate as sub-
142.3 ppm (JP,Rh = 290.7 Hz, JP,P = 73.5 Hz), respectively. strate, we used these ligands in the rhodium-catalysed asym-
Exposing this complex to 5 bar of H2 pressure resulted in metric hydrogenation of α-dehydroamino acid esters and
full conversion to the MeCN complex, as was clear from Roche ester derivatives. The supramolecular ligands exhibit
the 31P NMR spectrum. No hydride signals could be de- high activity and enantioselectivity for these substrates, al-
tected in the 1H NMR spectra. Similar results were ob- though similar results were also obtained for control ligand
tained for the complexes that were based on the supra- 1c. In the absence of zinc(II) template, the pyridine-based
molecular ligand (S)-1b(8)2 and reference ligand (S)-1c. Co- ligands showed no activity for any of the substrates studied.
ordination studies by using ligands (S)-1a and (S)-1b in the The presence of the template is necessary to achieve activity,
absence of template show broad signals in the 31P NMR otherwise the pyridines coordinate to the rhodium atom.
spectrum, thus indicating possible coordination of the Rh Investigations of the influence of different zinc(II) and ru-
atom to the pyridine groups.
thenium(II) templates on the reaction parameters showed a
Recently, our group has reported on IndolPhos ligands, dependence of the activity and selectivity on the association
hybrid ligands that feature, similarly to 1a–c, phosphane– constant between the supramolecular templates and the
phosphoramidite as phosphorus-donor atoms.[31] By using pyridine group of the ligand. In particular, strong associa-
these ligands, kinetic experiments performed for the asym- tion constants are necessary to achieve high conversion and
metric hydrogenation of substrate 15a were in agreement enantioselectivity. In further studies we investigated the ef-
with the unsaturated/dihydride mechanism. Under catalytic fect of sterically and electronically different templates in the
conditions, only stable solvate complexes were observed by hydrogenation of Roche ester 16. These experiments showed
high-pressure NMR spectroscopy, and neither the catalyst– that the fine-tuning of the catalyst properties could be easily
substrate complexes nor the dihydride species were de- achieved by using a supramolecular strategy on the basis of
tected, thereby indicating that the solvate complex repre- template modifications. Coordination studies by high-pres-
sents the resting state in the mechanism.
sure NMR spectroscopy revealed the formation of stable
For the current study, we performed high-pressure NMR solvate complexes. As no catalyst–substrate species or dihy-
spectroscopic studies to probe the formation of these inter- dride species could be detected under the conditions in
mediates. First, we investigated the formation of catalyst– which the substrate was converted, the solvate complex is
substrate complexes by adding a fourfold excess amount of most likely the resting state of the catalytic cycle, as was
substrate 15a to the solvate complex [Rh{1a(8)2}(MeCN)]- also found for similar hybrid ligands such as IndolPhos.
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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R. Bellini, J. N. H. Reek
FULL PAPER
8.1 (m, 2 H), 7.3 (m, 2 H), 7.1 (m, 6 H), 6.9 (m, 8 H), 6.7 (m, 2
H), 2.9 (m, 4 H), 2.7 (m, 2 H), 2.4 (m, 2 H), 1.7 (m, 8 H), 1.6 (s, 3
H) ppm. 13C-ATP (101 MHz): δ = 150.3 (CH), 150.0 (CH), 148.3
(C), 147.9 (CH), 147.8 (CH), 145.7 (C), 138.2 (C), 137.8 (C), 137.9
(CH), 137.4 (CH), 134.8 (CH), 134.6 (C), 134.4 (C), 134.0 (CH),
133.8 (CH), 133.6 (C), 133.5 (C), 133.4 (CH), 133.2 (CH), 130.7
(CH), 130.3 (C), 129.9 (CH), 129.6 (C), 128.9 (C), 128.2 (CH),
128.1 (CH), 128.0 (CH), 127.9 (CH), 127.8 (CH), 126.6 (CH), 123.4
(CH), 123.2 (CH), 35.6 (CH3), 29.2 (CH2), 29.2 (CH2), 28.0 (CH2),
27.7 (CH2) 22.8 (CH2), 22.7 (CH2), 22.6 (CH2), 22.5 (CH2) ppm.
31P NMR (162 MHz): δ = 138.5 (d, J = 57 Hz), –14.5 (d, J = 57 Hz)
ppm. HRMS (FAB+): calcd. for C49H43N3O2P2 [MH+] 768.2909;
found 768.2906.
Experimental Section
General: Unless stated otherwise, all reactions were performed un-
der nitrogen by using standard Schlenk techniques. THF was dis-
tilled from sodium/benzophenone ketyl; CH2Cl2 was distilled from
CaH2, and toluene was distilled from sodium under nitrogen. With
the exception of the compounds given below, all reagents were pur-
chased from commercial suppliers and used without further purifi-
cation. NMR spectra (1H, 31P and 13C) were measured with Bruker
DRX 400 MHz and Inova 500 MHz spectrometers. CDCl3 was
used as solvent, if not otherwise specified. High-resolution mass
spectra were recorded with a JEOL JMS SX/SX102A four-sector
mass spectrometer; for FAB-MS, 3-nitrobenzyl alcohol was used as
matrix. UV/Vis spectroscopy experiments were performed with a
Cary UV/Vis System. Chiral GC separations were conducted with
an Interscience HR GC (FID detector) apparatus equipped with a
Supelco β-dex 225 column (0.25 mmϫ30 m) and with an Intersci-
ence Focus GC (FID detector) equipped with a Chirasil Dex CB
column (internal diameter 0.1 mm, 5 m column, film thickness
0.1 μm). The following compounds were synthesised according to
published procedures: 2-(diphenylphosphanyl)-N-methylaniline
(3),[24] (S)-5,6,7,8,5,6,7,8-octahydro-(1,1Ј-binaphthalene)-2,2Ј-diol
(5)[25] and (S)-3,3Ј-dibromo-5,6,7,8,5Ј,6Ј,7Ј,8Ј-octahydro-(1,1Ј-bi-
naphthalene)-2,2Ј-diol (6).[26] The detailed synthesis for obtaining
(S)-7a–c was reported in a previous work.[27]
Ligand (S)-1c: White foam, yield 80%. 1H NMR (400 MHz,
CDCl3): δ = 7.7 (m, 4 H), 7.4 (m, 4 H), 7.3 (m, 2 H), 7.2 (m, 6 H),
7.1 (m, 4 H), 6.9 (m, 4 H), 6.8 (m, 2 H), 2.9 (m, 4 H), 2.7 (m, 2 H),
2.4 (m, 2 H), 1.8 (m, 8 H), 1.6 (s, 3 H) ppm. 13C-ATP (101 MHz): δ
= 138.8 (C), 137.9 (C), 137.0 (C), 134.5 (CH), 134.1 (CH), 133.8
(CH), 133.7 (C), 133.6 (CH), 133.5 (CH), 132.9 (C), 130.8 (CH),
130.7 (C), 130.2 (CH), 130.0 (CH), 129.6 (CH), 128.4 (CH), 128.2
(CH), 128.1 (CH), 128.0 (CH), 127.9 (CH), 127.8 (CH), 126.8
(CH), 126.6 (CH), 30.3 (CH3), 29.3 (CH2), 29.2 (CH2), 27.9(CH2),
27.7 (CH2) 23.0 (CH2), 22.9 (CH2), 22.8 (CH2), 22.5 (CH2) ppm.
31P NMR (162 MHz): δ = 133.2 (d, J = 47 Hz), –20.5 (d, J = 47 Hz)
ppm. HRMS (FAB+): calcd. for C51H45NO2P2 [MH+] 766.2926;
found 766.3012.
Synthesis
of
[2-(Diphenylphosphanyl)phenyl](methyl)phosphor-
amidous Dichloride (4): In a flame-dried Schlenk flask, distilled
PCl3 (0.22 mL, 2.6 mmol) was added dropwise to a solution of 2-
(diphenylphosphanyl)-N-methylaniline (500 mg, 1.7 mmol) and
Et3N (0.24 mL, 1.7 mmol) in toluene (8.5 mL) at 0 °C. The pale
yellow slurry was warmed to room temperature and then to 75 °C
for 16 h. The reaction mixture was cooled to room temperature,
and the formation of product was checked by 31P NMR spec-
troscopy. The solvent and the residual PCl3 were removed under
vacuum, and the resulting solid was used for the next step without
any further purification.
General Procedure for the Rhodium-Catalysed Asymmetric Hydro-
genation Reactions: A typical experiment was carried out in a stain-
less steel autoclave (150 mL) charged with an insert suitable for
14 reaction vessels (equipped with Teflon mini stirring bars) for
performing parallel reactions. [Rh(nbd)2]BF4 (1 μmol), ligand (S)-
1a–b (1.1 μmol, 1.1 equiv.) and 8 (2.2 μmol, 2.2 equiv.) were dis-
solved in CH2Cl2, and the reaction mixture was stirred at room
temperature for 1 h. After this time, the mixture was concentrated
under vacuum to dryness to give the desired compound. In the
autoclave, each vial was charged with the rhodium complex pre-
viously prepared (1 μmol), substrate (100 μmol) and dry CH2Cl2
(1 mL). The substrate was filtered through basic alumina prior its
use to remove possible peroxide impurities. Before starting the ca-
talysis, the charged autoclave was purged three times with 10 bar
of H2, then pressurised to 10 bar and the mixtures stirred at 25 °C
for 16 h. After catalysis, the pressure was reduced to 1.0 bar, and
the conversion and enantiomeric purity of the product was deter-
mined by chiral GC and HPLC.
General Procedure for the Preparation of Ligands (S)-1a–c: In a
flame-dried Schlenk flask, compound (S)-7a–c (1.2 mmol), 4-(di-
methylamino)pyridine (DMAP; 10 mol-%) and pyridine (0.1 mL,
1.3 mmol) were dissolved in dry THF (4 mL). The resulting solu-
tion was cooled to 0 °C and phosphoramidous dichloride 4
(467 mg, 1.2 mmol) in THF (2 mL) was added. The reaction mix-
ture was stirred overnight and allowed to warm to room tempera-
ture. The resulting pale yellow solution was concentrated to dry-
ness, and the resulting residue was purified by flash column
chromatography [silica gel; hexanes/CH2Cl2 (1:1) to hexanes/
CH2Cl2/Et3N (1:1:0.2)].
Ligand (S)-1a: White foam, yield 85%. 1H NMR (400 MHz,
CDCl3): δ = 8.7 (d, J = 5.7 Hz, 4 H), 8.5 (d, J = 5.7 Hz, 2 H), 7.7
(m, 7 H), 7.0 (m, 7 H), 6.8 (s, 2 H), 2.9 (m, 4 H), 2.7 (m, 2 H), 2.4
(m, 2 H), 1.7 (m, 8 H), 1.6 (s, 3 H) ppm. 13C-ATP (101 MHz): δ =
150.0 (CH), 149.6 (CH), 148.5 (C), 146.2 (C), 145.4 (C), 138.9 (C),
138.6 (C), 135.1 (CH), 134.5 (C), 133.7 (C), 133.7 (CH), 133.6
(CH), 133.4 (CH), 133.2 (CH), 130.4 (CH), 130.2 (C), 129.9 (CH),
129.4 (C), 129.0 (CH), 128.9 (CH), 128.7 (CH), 128.6 (CH), 128.5
(CH), 128.4 (CH), 128.3 (CH), 128.2 (CH), 128.1 (CH), 127.9 (C),
126.9 (CH), 124.9 (CH), 124.5 (CH), 35.6 (CH3), 29.2 (CH2), 29.1
(CH2), 28.0 (CH2), 27.7 (CH2) 22.7 (CH2), 22.7 (CH2), 22.6 (CH2),
22.5 (CH2) ppm. 31P NMR (162 MHz): δ = 139.5 (d, J = 55 Hz),
Chiral GC and HPLC Data for Hydrogenation Products: Dimethyl
itaconate (9). The conversion and enantiomeric excess were calcu-
lated by chiral GC analysis (Supelco β-dex 225) isothermal at 75 °C
for 2.0 min, 4 °Cmin–1 to 120 °C for 2.0 min, 50 °Cmin–1 to 220 °C;
tR(S) = 12.71 min, tR(R) = 12.85 min, tR(9) = 14.57 min.
N-(3,4-Dihydronaphthalen-2-yl)acetamide (10): The conversion and
enantiomeric excess were calculated by chiral GC analysis (Supelco
β-dex 225) isothermal at 70 °C, 10 °Cmin–1 to 160 °C for 5.0 min,
4 °Cmin–1 to 220 C; tR(R) = 27.78 min, tR(S) = 27.92 min, tR(10)
= 32.63 min.
Methyl 2-Acetamidoacrylate (15a): The conversion and enantio-
meric excess were calculated by chiral GC analysis (Supelco β-dex
225) isothermal at 140 °C for 14.0 min, 50 °Cmin–1 to 220 °C; tR(S)
= 11.56 min, tR(R) = 12.14 min, tR(15a) = 10.77 min.
–14.5 (d,
J = 55 Hz) ppm. HRMS (FAB+): calcd. for
C49H43N3O2P2 [MH+] 768.2909; found 768.2906.
Ligand (S)-1b: White foam, yield 86%. 1H NMR (400 MHz,
Methyl (Z)-2-Acetamidobut-2-enoate (15b): The conversion and en-
CDCl3): δ = 8.9 (s, 1 H), 8.8 (s, 1 H), 8.6 (m, 1 H), 8.2 (m, 1 H), antiomeric excess were calculated by chiral GC analysis (Supelco
8
www.eurjic.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 0000, 0–0

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Pages: 11
Supramolecular Hybrid Bidentate Ligands
β-dex 225) isothermal at 140 °C for 14.0 min, 50 °Cmin–1 to
220 °C; tR(S) = 15.22 min, tR(R) = 15.36 min, tR(15b) = 12.70 min.
[9]
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and enantiomeric excess were calculated by chiral GC analysis
(Chirasil DEX-CB) isothermal at 90 °C, 5 °Cmin–1 to 220 °C;
tR(R) = 15.35 min, tR(S) = 15.49 min, tR(15c) = 16.50 min.
[10]
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Methyl 2-Hydroxymethylacrylate (16): The conversion and enantio-
meric excess were calculated by chiral HPLC analysis (Chiralcel
AD-H) flow rate: 1.0 mLmin–1, eluent: heptane/2-propanol (90:10),
detection at 210 nm; tR(R) = 7.51 min, tR(S) = 9.51 min, tR(16) =
17.52 min.
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[Rh(1a)(8)2(nbd)]BF4: [Rh(nbd)2]BF4 (7.5 mg, 20.0 μmol), ligand 1a
(15.3 mg, 20.0 μmol) and template 8 (24.2 mg, 40.0 μmol) were dis-
solved in CH2Cl2 (0.6 mL) and stirred at room temperature for
3 h to allow complex formation. 31P NMR (CD2Cl2, 202.3 MHz,
298 K): δ = 131.7 (dd, JP,Rh = 260.1 Hz, JP,P = 66.0 Hz), 19.9 (dd,
JP,Rh = 148.7 Hz, JP,P = 66.0 Hz) ppm.
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Acknowledgments
We thank InCatT B.V. for providing substrate 16. Vladica Bocokic
is kindly acknowledged for the synthesis of template 17. This work
was financially supported by the National Research School Combi-
nation Chemistry (NRSC-C).
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R. Bellini, J. N. H. Reek
FULL PAPER
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Received: May 22, 2012
Published Online: ᭿
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 0000, 0–0

Job/Unit: I20549
/KAP1
Date: 13-08-12 11:39:00
Pages: 11
Supramolecular Hybrid Bidentate Ligands
Supramolecular Catalysis
The performance of a new class of supra-
molecular hybrid bidentate ligands was in-
vestigated in rhodium-catalysed asymmet-
ric hydrogenation. After optimisation of
the reaction conditions, the scope of these
ligands was explored in the asymmetric
hydrogenation of various prochiral olefins.
Higher activity and selectivity was ob-
tained with the supramolecular catalysts
than the nontemplated analogues.
R. Bellini, J. N. H. Reek* ............... 1–11
Supramolecular Hybrid Bidentate Ligands
in Asymmetric Hydrogenation
Keywords: Supramolecular chemistry / Self-
assembly
/
Hybrid bidentate ligands
/
Hydrogenation / Asymmetric catalysis
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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