Bidentate ligands formed by self-assembly

Article abstract of DOI:10.1039/b306683e

A novel supramolecular strategy to prepare bidentate ligands via the assembly of functionalised monomeric ligands on a dimeric zinc(n) porphyrin template is presented; the assembled bidentate ligands show chelating behaviour and their rhodium complexes display enhanced selectivity in the hydroformylation compared to the non-template analogue.

Full text of DOI:10.1039/b306683e

Bidentate ligands formed by self-assembly†
Vincent F. Slagt, Piet W. N. M. van Leeuwen and Joost N. H. Reek*
Institute of molecular chemistry, University of Amsterdam, Nieuwe Achtergracht 166, Amsterdam, The
Netherlands. E-mail: reek@science.uva.nl; Fax: (+31) 20-5256422; Tel: (+31) 20-5256960
Received (in Cambridge, UK) 12th June 2003, Accepted 5th August 2003
First published as an Advance Article on the web 2nd September 2003
3
21
= 1.4 3 10 M21. This shows that bidentate
3
A novel supramolecular strategy to prepare bidentate
ligands via the assembly of functionalised monomeric
ligands on a dimeric zinc(II) porphyrin template is pre-
sented; the assembled bidentate ligands show chelating
behaviour and their rhodium complexes display enhanced
selectivity in the hydroformylation compared to the non-
template analogue.
10 M and K
ligands such as 2(b)
2
2
can be prepared by simple mixing of the
two monomeric compounds with the dimeric template.
The coordination behaviour of these novel ligand systems to
transition metals was studied using high-pressure NMR spec-
8
troscopy in toluene-d under 20 bars of syn-gas (H
2
/CO = 1/1).
Rh(acac)(CO)
idyldiphenylphosphine b as the ligand, which in the absence of
a template resulted in the formation of HRh(b) (CO) as was
evident from the typical rhodium hydride signal at 29.5 ppm.
This hydride signal was shifted upfield (211.1 ppm) upon
addition of template 2, caused by the shielding effect of the
porphyrins that embrace the rhodium catalyst, indicating that
2
was used as a metal precursor and 4-pyr-
Ligand variation has proven to be a very powerful tool in
transition metal catalysis. Key features of transition metal
catalysts such as activity, selectivity and stability are dictated by
the steric and electronic properties of ligands that are co-
2
2
1
ordinated to the metal. Initially, the focus has been mainly on
monodentate phosphorus ligands, leading to a variety of
transition metal complexes among which are active catalysts for
various reactions including hydrocyanation, hydrogenation and
2 2
complex [HRh(2(b) )(CO) ] has formed (Scheme 2). Addition
of triphenylphosphine a to this mixture did not change the
1
hydride signal in the H NMR, showing the chelating effect of
2
hydroformylation. In the early seventies chelating bidentate
the bidentate ligand assembly in complex [HRh(2(b)
2 2
)(CO) ].
ligands were found to yield selective catalysts for the asym-
In contrast, mixing ligand a with HRh(b) (CO) in the presence
2
2
3
metric hydrogenation and many examples of active catalysts
of monomeric zinc(II) porphyrin 1 resulted in a mixture of
rhodium–hydride signals demonstrating that ligand exchange
takes place in the non-templated complex.
The self-assembled ligands were tested in the rhodium-
catalysed hydroformylation of 1-octene under 20 bar of syn-gas
(H /CO = 1/1) in toluene. The rhodium complex based on
2
monodentate 4-pyridyldiphenylphosphine b showed high activ-
based on bidentate ligands have appeared ever since.4
Recently, there is renewed interest in the use of monodentate
ligands in catalysis6 triggered by their simple synthetic
procedures compared to bidentates, especially if sophisticated
chiral entities are involved. However, for several reactions
including hydroformylation, chelating bidentate ligands yield
,5
4,7
more active or selective catalyst systems. Here we report a
new strategy to prepare bidentate chelating ligands that involves
the assembly of monodentate ligands L on a bisporphyrin
template (Fig. 1). This strategy combines the easy access of
monodentate ligands with the properties of chelating systems.
For the assembly of the bidentate ligands we used monomeric
pyridine phosphorus compounds b–d and bis-zinc(II) porphyrin
template 2 (Scheme 1). The bidentate phosphorus ligand is
formed in situ by selective coordination of the nitrogen donor
atom of building block b–d to the zinc atoms of the porphyrin.8
We recently reported that such a pyridine–zinc interaction is
9
indeed selective and, in addition, that after complexation of the
zinc(II) porphyrin the phosphorus donor atom is still available
for complexation to transition metals such as palladium and
rhodium. UV-vis spectroscopy titrations in toluene confirmed
that bis-zinc(II) porphyrin 2 coordinates two pyridylphosphine
units b, with corresponding binding constants of K = 5.1 3
1
Scheme 1 Zinc(II) porphyrins and pyridylphosphorus ligands used for the
catalyst assemblies.
Fig. 1 Schematic representation of a self-assembled chelating ligand, L =
monodentate ligand (b–d) and M = transition metal.
†
Electronic supplementary information (ESI) available: ligand synthesis
2 2
Scheme 2 Transition metal catalyst [HRh(2(b) )(CO) ] formed by self-
and detailed experimental data. See http://www.rsc.org/suppdata/cc/b3/
b306683e/
assembly of 4-pyridyldiphenylphosphine b on dimeric zinc(II) porphyrin 2
and in the presence of a rhodium precursor.
2
474
CHEM. COMMUN., 2003, 2474–2475
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ity and moderate selectivity for the linear aldehyde at 80 °C
with an increase in activity. Although so far only moderate
Table 1), as is usually found for monodentate ligands.1
0,11
The
enantioselectivities and activities in the rhodium-catalysed
hydroformylation of styrene have been observed, these results
are very promising considering the challenge that is involved.
Initial experiments showed that the chiral ligand assemblies
gave also active palladium catalysts for the allylic alkylation
reaction.14 Under the conditions applied (2 mol% palladium, 25
°C, see ESI†) all reactions went to full conversion and the
palladium complexes based on monomeric pyridyl phosphite
ligands c and d yielded low enantioselectivity (18%). The
monodentate assemblies 1(c) and 1(d) resulted in palladium
complexes that gave 32% ee. Again the templated ligand
(
rhodium complex based on the assembled bidentate ligand 2(b)
2
gave a lower reaction rate along with a slight increase in
selectivity for the linear product and smaller amount of
isomerised side-product. This catalytic behaviour is typical of
bidentate phosphine ligands in the rhodium-catalysed hydro-
formylation of 1-octene7 and shows that the assembled
bidentates are stable under catalytic conditions. In contrast, the
ligand assembly based on monomeric zinc(II) porphyrin 1 and b
resulted in typical monodentate ligand behaviour as only a small
change in activity and the same selectivity was observed.
At 25 °C a similar difference between the rhodium catalysts
2 2
assemblies 2(c) and 2(d) resulted in the highest selectivities
based on the templated ligand 2(b)
(b) was observed; the bidentate assembly resulted in lower
activity and higher selectivity for the linear aldehyde (l/b =
.3). Related experiments were performed with the more bulky
phosphite ligand c, forming a bidentate phosphite ligand in the
presence of template 2. The bidentate assembled ligand 2(c)
resulted in a rhodium catalyst with a slightly lower activity than
that based on monodentate 1(c), but the selectivity significantly
increased in favour of the linear adduct (94%, compared to the
2
and the monodentate ligand
leading to 45% ee, showing that bidentate ligand assemblies can
be used under these conditions.
1
In conclusion, a new strategy to prepare bidentate ligands
using supramolecular interactions is presented. The bidentate
ligands are prepared by just mixing monodentate ligands with a
template, forming novel chelating ligands by selective coor-
dination to the template as proven by NMR spectroscopy. For
the current assemblies pyridine phosphorus ligands b–d and
zinc(II) porphyrin dimer 2 were used and in rhodium-catalysed
hydroformylation typical bidentate behaviour was observed.
Also in palladium catalysed allylic alkylation the selectivity
improved significantly upon using the assembled bidentate
ligand systems. So far, we only used a limited set of building
blocks, but is is anticipated that the extension to other building
blocks will lead to a large library of new assembled catalyst
systems that can be tested using high throughput screening
techniques. As such, we are currently extending the library
including assembled systems based on other binding motifs
such as hydrogen bonds.
3
2
8
3%). Although the activities of the catalysts are low at these
low temperatures (25 °C), these results demonstrate that
assembled bidentate ligand systems show catalytic properties
that are typical of bidentate ligands.
We were also interested in asymmetric catalysis using
bidentate assemblies based on the chiral building blocks c and
d and template bisporphyrin 2, and we expected a large template
effect in the rhodium-catalysed hydroformylation of styrene
,12
(
Table 2).7 The rhodium complexes based on monodentates c
and d resulted both in a low enantiomeric excess (approx-
imately 7%), which is in line with previous results of
monodentate ligands.13 Also the rhodium complexes based on
monodentate assemblies 1(c) and 1(d) gave low ee. Inter-
Notes and references
estingly, the templated ligand assemblies 2(c)
resulted in significantly higher enantioselectivity (33%), along
2
and 2(d)
2
1 C. A. Tolman, Chem. Rev., 1977, 77, 313; C. A. Tolman, J. Am. Chem.
Soc., 1970, 92, 2953.
2
W. Reppe and W. J. Schweckendiek, Annalen, 1948, 104, 560; L. H.
Slaugh and R. D. Mullineaux, J. Organomet. Chem., 1968, 13, 469; R.
D. Cramer, E. L. Jenner and U. G. Stolberg, J. Am. Chem. Soc., 1963, 85,
Table 1 Rhodium catalysed hydroformylation of 1-octenea
1
691; J. F. Young, J. A. Osborn, F. A. Jardine and G. Wilkinson, J.
Conversionc
Ligandb T/°C (%)
2-Octenef Linearg
Chem Soc., Chem Commun., 1965, 131.
J. C. Poulin, T. P. Dang and H. B. Kagan, J. Organomet. Chem., 1972,
9
P. W. N. M. van Leeuwen, P. C. J. Kamer, J. N. H. Reek and P. Dierkes,
Chem. Rev., 2000, 100, 2741.
Some reviews: S. Mecking, Coord. Chem. Rev., 2000, 203, 325; C. J.
Elsevier, Coord. Chem. Rev., 1999, 185–186, 809; A. Börner, Chirality,
T.O.F.d
l/be
2.7
2.7
3.0
2.9
3.3
5.0
16.4
(%)
(%)
3
4
5
2; R. Selke and H. Pracejus, J. Mol. Catal., 1986, 37, 213.
b
80
80
80
25
25
25
25
93
89
33
2250
2100
727
7.4
1.8
1.8
0.5
0.1
0.1
0.0
0.0
71.7
71.5
74.5
74.1
76.5
83.3
94.3
1
2
1
2
1
2
a
(b)
(b)
(b)
(b)
(c)
2
6.2
2
4.7
1.1
0.8
5.6
1.3
0.9
2
001, 13, 625.
6
B. L. Feringa, Acc. Chem. Res., 2000, 33, 346; M. T. Reetz and G.
Mehler, Angew. Chem., Int. Ed., 2000, 39, 3889; T. Hayashi, Acc. Chem.
Res., 2000, 33, 354; I. V. Komarov and A. Borner, Angew. Chem., Int.
Ed., 2001, 40, 1197.
7 Rhodium-catalysed Hydroformylation, ed. P. W. N. M. van Leeuwen
and C. Claver, Kluwer Academic Publishers, Dordrecht, 2000.
8 Other supramolecular catalysts based on porphyrin assemblies: C. J.
Walter, H. L. Anderson and J. K. M. Sanders, J. Chem. Soc., Chem
Commun., 1993, 458; L. G. Mackay, R. S. Wylie and J. K. M. Sanders,
J. Am. Chem. Soc., 1994, 116, 3141; G. A. Morris, S. T. Nguyen and J.
T. Hupp, J. Mol. Catal. A, 2001, 174, 15; M. L. Merlau, M. del Pilar
Mejia, S. T. Nguyen and J. T. Hupp, Angew. Chem., Int. Ed., 2001, 40,
(c)
2
[
Rh] = 0.084 mmol l21 in toluene, pressure = 20 bar (CO/H
2
= 1/1),
b
21
1
l
-octene/rhodium = 5200. [Phosphorus] = 2.1 mmol l , [1] = 2.1 mmol
2
1
21 c
, [2] = 1.1 mmol l
.
Percent total conversion of 1-octene to aldehydes
d
and 2-octene. T.O.F. = average turn over frequency = (mol aldehyde)
2
1
(
(
mol Rh) h21, the reaction was stopped after 2 hours (80 °C) or 43 hours
25 °C). l/b = linear/branched. Percent 2-octene formed. Percent linear
e
f
g
aldehyde formed based on 1-octene conversion.
Table 2 Rhodium catalysed hydroformylation of styrenea
4
239; J. A. A. W. Elemans, E. J. A. Bijsterveld, A. E. Rowan and R. J.
Ligandb
T/°C
T.O.F.c
b/ld
e.e. (%)
e
M. Nolte, Chem. Commun., 2000, 2443.
9
V. F. Slagt, J. N. H. Reek, P. C. J. Kamer and P. W. M. N. van Leeuwen,
Angew. Chem., Int. Ed., 2001, 40, 4271.
c
1
2
d
1
2
25
25
25
25
25
25
0.01
0.02
0.15
0.01
0.02
0.14
> 100
> 100
> 100
> 100
> 100
> 100
7.2 (S)
6.0 (S)
33.2 (S)
7.0 (R)
6.3 (R)
32.6 (R)
(c)
(c)
10 A. Buhling, P. C. J. Kamer and P. W. N. M. van Leeuwen, J. Mol. Catal.
A, 1995, 98, 69.
11 A. M. Trzreciak and J. J. Ziólkowski, Coord. Chem. Rev., 1999,
190–192, 883.
2
(d)
(d)
2
12 F. Agbossou, J-F. Carpentier and A. Mortreux, Chem. Rev., 1995, 95,
2485; S. Gladiali, J. C. Bayón and C. Claver, Tetrahedron Asymmetry,
1995, 6, 1453.
13 S. Gladiali, A. Dore, D. Fabbri, O. De Lucchi and M. Manassero,
Tetrahedron Asymmetry, 1994, 5, 511; M. Tanaka, Bull. Chem. Soc.
Jpn., 1972, 47, 1698.
a
b
] = 0.084 mmol l21, pressure = 20 bar (CO/H
[
[
Rh(acac)(CO)
2
2
= 1/1).
Phosphite] = 2.1 mmol l21, [1] = 2.1 mmol l , [2] = 1.1 mmol l
21
21
,
c
styrene/rhodium = 5200. T.O.F. = turn over frequency = (mol aldehyde)
mol Rh)2
1
21
d
(
h
, the reaction was stopped after 64 hours (25 °C). b/l =
e
branched/linear. e.e. = enantiomeric excess (%).
1
4 B. M. Trost and D. L. Van Vranken, Chem. Rev., 1996, 96, 395.
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