Fine-tuning ligands for catalysis using supramolecular strategies

Article abstract of DOI:10.1002/ejic.200700550

Coordinative bonds have been used to prepare supramolecular ligands leading to well-defined catalysts formed by assembly. The construction of these ligands is based on selective metal-ligand interactions between nitrogen donor atoms of phosphorus-nitrogen building blocks and various zinc(II) porphyrins. The major advantage of this supramolecular approach of catalyst preparation is the simplification of ligand variation enabling straightforward modification of steric, electronic and chiral properties of the supramolecular ligand. A large number of new ligands becomes accessible by this modular variation of the building blocks. The ligand assembly based on pyridyl phosphites and zinc(II) porphyrin with electron-withdrawing substituents led to a twelve-fold increase in activity and an increase in enantioselectjvity from 17 to 50% in the rhodium-catalyzed hydrogenation of dimethyl itaconate. The first examples of assemblies based on non-chiral ligands and chiral zinc(II) porphyrin template molecules show, as proof of principle, an enantiomeric excess up to 18 % in the asymmetric palladium-catalyzed allylic alkylation. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200700550

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DOI: 10.1002/ejic.200700550
Fine-Tuning Ligands for Catalysis Using Supramolecular Strategies
Vincent F. Slagt,[a][‡] Patrick Kaiser,[b] Albrecht Berkessel,[b] Mark Kuil,[a]
Alexander M. Kluwer,[a] Piet W. N. M. van Leeuwen,[a] and Joost N. H. Reek*[a]
Keywords: Supramolecular catalysts / Self-assembly / Ligand effects / Phosphorus ligands / Porphyrinoids
Coordinative bonds have been used to prepare supramolec-
ular ligands leading to well-defined catalysts formed by as-
sembly. The construction of these ligands is based on selec-
tive metal–ligand interactions between nitrogen donor atoms
of phosphorus-nitrogen building blocks and various zinc(II)
porphyrins. The major advantage of this supramolecular ap-
proach of catalyst preparation is the simplification of ligand
variation enabling straightforward modification of steric,
electronic and chiral properties of the supramolecular ligand.
A large number of new ligands becomes accessible by this
modular variation of the building blocks. The ligand as-
sembly based on pyridyl phosphites and zinc(II) porphyrin
with electron-withdrawing substituents led to a twelve-fold
increase in activity and an increase in enantioselectivity from
17 to 50% in the rhodium-catalyzed hydrogenation of di-
methyl itaconate. The first examples of assemblies based on
non-chiral ligands and chiral zinc(II) porphyrin template
molecules show, as proof of principle, an enantiomeric excess
up to 18% in the asymmetric palladium-catalyzed allylic al-
kylation.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
Introduction
Recently, we[17,18] and others[19] have introduced a new
class of bidentate ligands that are formed by a self-assembly
process of two monodentate ligands. We demonstrated that
the SUPRAphos[18b,18e,18f] concept – bringing two mono-
dentate ligands together via zinc(II)–porphyrin–pyridyl in-
teractions to form bidentate ligands – provides easy access
to large bidentate ligand libraries, and that these ligands
can successfully be applied to various reactions including
the rhodium-catalyzed asymmetric hydrogenation. High-
throughput screening of a part of the library revealed a cat-
alyst that hydrogenates a trisubstituted cyclic enamide, a
notoriously difficult substrate, with the highest enantio-
selectivity known to date.[18f] This unambiguously shows
that the supramolecular approach to create bidentate li-
gands is a very powerful tool that brings about new cata-
lysts with properties that surpass those already known. In
addition, from the library of 64 SUPRAphos ligands that
was investigated, only one provided a catalyst that induced
high ee values, stressing the point that for challenging asym-
metric conversion such as the hydrogenation of the trisub-
stituted cyclic enamide, large ligand libraries are required.
One of the advantages typical of supramolecular ligands is
the easy access to these large libraries. After the successful
exploration of our SUPRAphos[18b,18e,18f] library based on
porphyrin-appended ligands we have extended our ap-
proach to urea-based homo-bidentate ligands[20] and chiral
analogues thereof for the use in asymmetric conversions.[21]
This class of supramolecular bidentate ligands are easily ac-
cessible, in large variety and at an industrially interesting
scale.
In the past three decades, the exploration of ligand ef-
fects in transition-metal catalysis has proven to be a very
powerful tool in homogeneous catalysis. Variation of ste-
ric[1] and electronic properties,[2] as well as the “natural bite
angle”[3] for bidentate ligands, has been used to optimize
many catalytic reactions. These ligand parameters have
proven to control key features of catalysts, like activity,
selectivity and stability, and systematic variation has re-
sulted in better understanding of ligand effects in cataly-
sis.[1–14] For example, electron-withdrawing substituents on
the aryl ring of the phosphorus donor atom of mono- and
bidentate phosphane ligands increases the activity and
selectivity of the catalyst in the rhodium-catalyzed hydro-
formylation of 1-octene.[15] In the palladium-catalyzed aryl-
ation of alkenes, bulky phosphorus amidite ligands yielded
very fast catalysts.[16] Both electronic and steric parameters
of these ligands were found to be extremely important. The
ligand has to be sufficiently bulky to impose formation of
monoligated palladium complexes as active catalysts, but
ligands that are too large retard the reaction rate.
[a] Van’t Hoff Institute for Molecular Sciences, University of Amster-
dam,
Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Nether-
lands
Fax: +31-20-525-6422
E-mail: reek@science.uva.nl
[b] Institut für Organische Chemie der Universität zu Köln,
Greinstrasse 4, 50939 Köln, Germany
[‡] Current address: Syncom BV,
Kadijk 3, 9747 AT Groningen, The Netherlands
Eur. J. Inorg. Chem. 2007, 4653–4662
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Although the preparation of monodentate ligands[22] is
much easier than bidentate ligands,[23] a novel supramolec-
ular strategy could also facilitate the preparation of large
libraries of monodentate ligands. Here we report such a
supramolecular strategy to change the electronic and steric
properties of ligands that are equipped with pyridine and
amine functional groups for the assembly of phosphorus
ligands on zinc(II) porphyrin template molecules (Figure 1).
The typical binding motif was previously used in the SUP-
RAphos concept and for the encapsulation of transition
metal catalysts[24] by assembly of zinc(II) porphyrin build-
ing blocks.[25] By using the same binding motif we now
show that we can vary the steric and electronic properties
of phosphorus ligands by the association of the zinc(II)
porphyrin templates. In this way, a series of new supramo-
lecular ligands is prepared by just mixing the components.
NMR, IR, and UV/Vis spectroscopy have been used to
study the selective metal–ligand interactions and to quan-
tify the steric and electronic properties of these supramolec-
ular ligands. The supramolecular ligands control the ac-
tivity and (enantio)selectivity of the transition metal cata-
lysts as observed in rhodium-catalyzed hydroformylation,
asymmetric hydrogenation, and asymmetric palladium-cat-
alyzed allylic alkylation. The approach is extended to the
assembly of a chiral environment around a non-chiral cata-
lyst.
Figure 2. Various building blocks used to construct supramolecular
ligands.
The coordination behavior of the pyridine phosphorus
ligands 2 to 1a has been studied using NMR and UV/Vis
spectroscopy.[29,30] The addition of 1a to a solution of 2
resulted in large upfield shifts of the protons on the pyridyl
1
ring in H NMR spectroscopy (∆δH1 = 5.7 ppm, ∆δH2
=
5.8 ppm), which is typical of axial binding of pyridine li-
gands to zinc(II) porphyrins (Scheme 1).[24,25] Also in 31P
Figure 1. Schematic representation of the assembly of a phospho-
rus ligand on a zinc(II) porphyrin template molecule.
Results and Discussion
The Assembly of Phosphorus Ligands and Zinc(II)
Porphyrin Building Blocks
For the preparation of supramolecular ligands with dif-
ferent steric and electronic properties, we synthesized sev-
eral phosphorus ligands functionalized with a nitrogen do-
nor atom and zinc(II) porphyrins (Figure 2). The phosphite
ligands 2–4 were obtained in high yields by a reaction of
the phosphor chloridite[26] with 3-hydroxypyridine. Zinc(II)
meso,meso,meso,meso-tetraphenylporphyrin (1a) was pre-
pared according to standard procedures.[27,28] All new com-
pounds were fully characterized with H NMR, 31P-NMR
1
Scheme 1. Assembly of (S)-(1,1Ј-binaphthyl-2,2Ј-diyl) (3-pyridyl)
phosphite (2) on zinc(II) meso,meso,meso,meso-tetraphenylporphy-
rin (1a).
and 13C-NMR spectroscopy, elemental analysis and mass
spectrometry.
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Fine-Tuning Ligands for Catalysis Using Supramolecular Strategies
NMR spectroscopy, a large shift to higher field (∆δP
=
isomerization side reaction resulted in 25% internal alkenes.
4.6 ppm) was observed on binding of 2 to the complex 1a. The assembly based on the zinc(II) porphyrin 1a and 2 did
In the UV/Vis spectrum of 1a the typical red-shift of the Q- not influence the catalytic behavior and similar activity,
bands of the porphyrin on the addition of 2 was observed, selectivity and isomerization were observed.[32] The more
indicating axial coordination of the pyridine to the zinc, bulky ligand 4 yielded a catalyst with different properties;
and the associated binding constant determined by UV ti- an increase in activity and decrease in selectivity (linear-
trations was found to be high (K1a.2 = 1.3ϫ103 –1).
branched ratio, l/b = 2.1) was observed for this system com-
The binding of zinc(II) porphyrin building blocks pared to ligand 2. These differences are explained by the
through the nitrogen donor atom is very selective and there- formation of monoligated rhodium phosphite complexes, as
fore the phosphorus atom is available for coordination to previously observed for other bulky phosphite ligands.[33]
transition metals that potentially can act as catalysts. Mix- Upon using assembly 4·1a an increase in activity and
ing two equivalents 1a (with respect to the complex and one isomerization was observed, showing that the assembly of
equivalent with respect to the ligand) with in situ formed zinc(II) porphyrin templates to ligands with nitrogen donor
[Rh(acac)(2)2] yielded an assembly in which the transition atoms influences the performance of the transition metal
metal is embedded between the two porphyrin building catalyst based on these ligands, which stimulated further
blocks [Rh(acac)(2·1a)2], as was evident from the shifts in studies on these assembled ligand systems.
NMR spectroscopy. In the 31P NMR spectra the addition
of the porphyrin 1a resulted in an interestingly large upfield
shift of the phosphite signals, from δ = 151.3 ppm (doublet,
JP–Rh = 303 Hz) to δ = 144.5 ppm (doublet, JP–Rh
=
304 Hz) (Figure 3). The large shift of ∆δP = 6.8 ppm is at-
tributed to a combination of effects: changes in the elec-
tronic and steric properties of the phosphite and the ring-
current effect of the nearby complexed zinc(II) porphyrin
1a.
Scheme 2. The rhodium-catalyzed hydroformylation leading to the
linear and branched aldehyde.
Table 1. Rhodium-catalyzed hydroformylation of 1-octene using as-
semblies of pyridyl phosphite and zinc(II) porphyrins template as
ligands.[a]
Ligand[b]
Temp. [°C]
T.O.F.[c]
l/b[d]
% 2-Octene[e]
2
120
120
80
1.2ϫ103
1.3ϫ103
2.3ϫ103
3.0ϫ103
6.0
7.0
2.1
2.0
24.6
19.8
21.6
26.0
2·1a
4
4·1a
80
[a] [Rh(acac)(CO)2] = 0.084 mmol/L in toluene, pressure: 20 bar
(CO/H2 = 1:1), 1-octene/rhodium = 5160. [b] [Phosphorus ligand
2 or 4] = 2.1 mmol/L, [porphyrin] = 2.1 mmol/L. [c] T.O.F. =
average turn over frequency: (mol aldehyde)(molRh)–1 h–1, the re-
action was stopped after 1 h. [d] l/b = linear/branched. [e] Percent
isomerization to 2-octene (and some 3- and 4-octene).
Figure 3. 31P NMR spectra before (above) and after (below) ad-
dition of two equivalents of zinc(II) meso,meso,meso,meso-tet-
raphenylporphyrin (1a) to the complex [Rh(acac)(2)2].
Changing the Electronic Properties of Phosphorus Ligands
To study these new self-assembled ligands based on selec-
tive nitrogen–zinc interactions in more detail, several zinc(II)
porphyrins 1a–d were synthesized with different electron-
withdrawing and -donating substituents.[27,28] The binding
Assemblies of Zinc(II) Porphyrin and Phosphorus Ligands
in Transition-Metal Catalysis
The supramolecular complexes based on the zinc(II) por- constants of zinc(II) porphyrins 1a–d to diphenyl(4-pyridyl)-
phyrin 1a and pyridyl phosphite ligands 2 and 4 were phosphane (7) (Figure 4) in toluene were determined using
studied in the rhodium-catalyzed hydroformylation of 1-oc- UV/Vis spectroscopy. It was found that the porphyrin with
tene (Scheme 2).[9,31] The experiments were carried out in the most electron-withdrawing substituents yielded the
toluene under 20 bar of syngas (H2/CO = 1:1) and the re- highest binding constant (Table 2) and a linear relation be-
sults are summarized in Table 1. The catalyst formed by tween binding constants and the Hammett values[34] was
ligand 2 yielded a moderate activity and reasonable selectiv- observed, with a corresponding electronic dependence value
ity in the hydroformylation of 1-octene at 120 °C, and the of ρ = 0.285 (Figure 5).
Eur. J. Inorg. Chem. 2007, 4653–4662
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phenyl)(4-pyridyl)phosphane (7) and [Rh(CO)2(Cl)]2 were
mixed in a 4:1 ratio to form in situ [Rh(7)2(CO)Cl] and
subsequently a stoichiometric amount of zinc(II) porphyrin
1a–d was added.[36] The IR spectra of these assembled rho-
dium complexes show that addition of zinc(II) porphyrin
changes the shift of the carbonyl (ν
= 1982 cm–1) of
˜
CO
[Rh(7) (CO)Cl] to higher wave numbers (up to ν
=
˜
2
CO
1986 cm–1). The ν
band shows a gradual shift with
˜
CO
decreasing phosphane basicity (Table 2). The lower electron
density on the rhodium leads to a decrease in back bonding
from the rhodium to the carbonyl ligand and hence higher
CO stretching frequencies are observed. For comparison,
when the strongly electron-withdrawing tris(4-α,α,α-trifluo-
rotoluyl)phosphane is used, the ν band is at 1990 cm–1,
˜
CO
which is a shift of 12 cm–1 compared to triphenylphos-
phane, (ν = 1978 cm–1).[36,37] These results show that the
˜
CO
effects induced by the zinc(II) porphyrin templates 1a–d are
significant. For phosphane 7, the pyridine is directly at-
tached to the phosphorus, and therefore changes on the ni-
trogen donor atom were expected to change the basicity of
the phosphorus. Upon using a different spacer between the
phosphorus and the nitrogen one can envisage that elec-
tronic effects on the phosphorus caused by the assembly of
zinc(II) porphyrin templates 1a–d will be different. Indeed,
the use of diphenyl(3-pyridylmethyl)phosphane (8), a sim-
ilar building block as 7 but with a spacer that has changed
from 4-pyridine to 3-methylpyridine, shows that the car-
bonyl peak in the infrared spectrum of complex [Rh-
(8)2(CO)Cl] hardly changes upon assembling the zinc(II)
Figure 4. Various ligand building blocks used to construct supra-
molecular ligands.
Table 2. Binding constants observed for diphenyl(4-pyridyl)phos-
phane (7) to various zinc(II) porphyrin templates measured by UV/
Vis titrations and the IR-carbonyl frequencies of the corresponding
metal complexes [Rh(7)2(CO)Cl] with the attached zinc(II) porphy-
rin.
Porphyrin
K [–1]
log K
4σ
ν[a] [cm–1]
˜
1c (OCH3-porphyrin) 2.9ϫ103
1b (CH3-porphyrin)
1a (H-porphyrin)
1d (CF3-porphyrin)
3.469 –1.08
3.564 –0.68
1983
1984
1985
3.7ϫ103
6.1ϫ103
24.4ϫ103
3.785
0
4.387 2.16
1986/87
porphyrin templates 1a–d (ν
= 1979–1980 cm–1). Thus,
˜
[a] Measured in dichloromethane, carbonyl region.
CO
these experiments show that electron-withdrawing substitu-
ents on the zinc(II) porphyrin that is used as a template can
modify the electronic properties of the phosphane ligands,
depending on the spacer between the nitrogen and phos-
phorus donor atom of the ligand building block.
The electronic effect of zinc(II) porphyrin templates 1a–
d on diphenyl(4-pyridyl)phosphane (7) and diphenyl(3-pyr-
idylmethyl)phosphane (8) were studied in the rhodium-cata-
lyzed hydroformylation of 1-octene (Table 3). The assembl-
ies of zinc(II) porphyrin 1a–d and 7 show a small but dis-
tinct trend in activity (7·1cϽ7·1bϽ7·1aϽ7·1d). The as-
semblies based on more electron-withdrawing zinc(II) por-
phyrin templates and 7 show a small increase in conversion
in the rhodium-catalyzed hydroformylation of 1-octene.
From the infrared spectra we know that this effect is caused
by the lower basicity of the phosphane donor atoms. In the
literature a similar trend has been reported for systems in
which the electronic properties of the ligands were varied in
a covalent manner.[15] For the various ligand assemblies
Figure 5. Plot logK vs. 4σ (sigma para) for the assembly of zinc(II)
porphyrin complexes 1a–d and diphenyl(4-pyridyl)phosphane (7).
The substituents on the zinc(II) porphyrin template 1a– based on phosphane 8 and zinc(II) porphyrin 1a–d the dif-
d change the π back bonding of the zinc(II) metal ion to ferences in activity are negligible. This is in line with the
the porphyrin macrocycle, hence influencing the electron almost identical infrared CO stretching frequencies ob-
density of the pyridine moiety ring fragment.[35] Therefore, served for the rhodium complexes. The catalytic results ob-
also an effect in electron density on the phosphorus donor tained for the assemblies based on zinc(II) porphyrin 1a–d
atom is expected. Zinc(II) porphyrins containing more elec- and ligands 7 and 8 show that the spacer between the nitro-
tron-withdrawing substituents will increase the electron- gen and phosphorus donor atoms is of great importance for
withdrawing capacity of the pyridine ring, which will result the small but distinct electronic influences on the phospho-
in a lower basicity of the phosphorus donor atom. (Di- rus induced by the assembly process.
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