Immobilisation of the BINAP ligand on dendrimers and hyper-branched polymers: Dependence of the catalytic properties on the linker unit

Article abstract of DOI:10.1002/adsc.200800741

A series of immobilised Carbo-BINAP ligands has been synthesised using poly(propylene imine) (PPI) dendrimers as soluble supports. They contain up to 64 BINAP ligands at their periphery without an additional linking unit. Despite the high steric requirements of the ligand, all dendrimers could be completely functionalised, resulting in the immobilised systems in good yields. Furthermore, the immobilisation strategy that worked out for the fixation of AMINAP ligands with additional linking units as well as of Carbo-BINAP ligands without additional linking units on dendrimers has thus been extended to less regularly hyperbranched poly (ethylene imines) (PEI) as soluble supports. In that way it has been possible to attach on average 9, 26, and 138 Glutaroyl-AMINAP or Carbo-BINAP ligands to PEIs of different molecular weights. The catalytic properties of these systems in the copper-catalysed hydrosilylation of acetophenone were investigated. The dendritic PPI-bound Carbo-BINAP ligands displayed a strong dependence of enantioselectivity and activity on the generation of the dendrimer. For the Carbo-BINAP and Glutaroyl-AMINAP ligands immobilised on the hyperbranched polymers, however, activities and enantioselectivities comparable to those of the mononuclear catalysts were found. The macromolecular, immobilised BINAP ligands could be recycled several times without any observable loss of activity or enantioselectivity.

Full text of DOI:10.1002/adsc.200800741

FULL PAPERS
DOI: 10.1002/adsc.200800741
Immobilisation of the BINAP Ligand on Dendrimers and Hyper-
branched Polymers: Dependence of the Catalytic Properties on
the Linker Unit
a
a
a,
Jutta K. Kassube, Hubert Wadepohl, and Lutz H. Gade *
a
Anorganisch-Chemisches Institut der Universitꢀt Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
Fax : (+49)-6221-54-5609; e-mail: lutz.gade@uni-hd.de
Received: November 28, 2008; Revised: February 3, 2009; Published online: March 4, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800741.
Abstract: A series of immobilised Carbo-BINAP li- properties of these systems in the copper-catalysed
gands has been synthesised using poly(propylene hydrosilylation of acetophenone were investigated.
imine) (PPI) dendrimers as soluble supports. They The dendritic PPI-bound Carbo-BINAP ligands dis-
contain up to 64 BINAP ligands at their periphery played a strong dependence of enantioselectivity and
without an additional linking unit. Despite the high activity on the generation of the dendrimer. For the
steric requirements of the ligand, all dendrimers Carbo-BINAP and Glutaroyl-AMINAP ligands im-
could be completely functionalised, resulting in the mobilised on the hyperbranched polymers, however,
immobilised systems in good yields. Furthermore, activities and enantioselectivities comparable to
the immobilisation strategy that worked out for the those of the mononuclear catalysts were found. The
fixation of AMINAP ligands with additional linking macromolecular, immobilised BINAP ligands could
units as well as of Carbo-BINAP ligands without ad- be recycled several times without any observable loss
ditional linking units on dendrimers has thus been of activity or enantioselectivity.
extended to less regularly hyperbranched poly(ethy-
lene imines) (PEI) as soluble supports. In that way it
has been possible to attach on average 9, 26, and 138 Keywords: asymmetric catalysis; BINAP; catalyst
Glutaroyl-AMINAP or Carbo-BINAP ligands to immobilization; dendrimers; hydrosilylation; hyper-
PEIs of different molecular weights. The catalytic branched polymers
Introduction
obtained in the reaction of 2-acetyl-4,4-dimethylcyclo-
hexanone with cinnamyl acetate.
The BINAP-copper(I) complexes have also found
[
6]
In 1980 Noyori et al. synthesised the chiral diphos-
[1]
phine BINAP. Up to now this ligand has found vari- use as catalysts. A very recent example is the conju-
ous applications in asymmetric catalysis, resulting in gate reduction of 2-pyridyl vinyl sulfones, affording
the desired chiral products in good to excellent enan- the chiral sulfones with yields and enantioselectivities
[
7]
tioselectivities of up to 99% ee. Apart from the most of 90% and higher. A more established reaction cat-
popular BINAP-ruthenium and BINAP-rhodium alysed by diphosphine-Cu(I) complexes is the hydrosi-
complexes that catalyse the hydrogenation of olefins lylation of aryl ketones. Using acetophenone as sub-
[2–4]
and ketones,
many other transition metal-BINAP strate the BINAP-Cu catalyst was found to give mod-
complexes were discovered to be active as catalysts in erate enantioselectivity (75% ee) compared with
[8]
miscellaneous chemical transformations. The Pt(II) other diphosphine ligands tested by Lipshutz et al.
complexes, for example, show enantioselectivities of By variation of the reaction conditions the enantiose-
[
9]
up to 99% ee in the asymmetric Diels–Alder reaction lectivity could be improved to over 90% ee.
of acryloyl-N-oxazolidinone and cyclopentadiene,
[
5]
Fan et al. synthesised a variety of functionalised
whereas the Pd(II) complexes are good catalysts in BINAP-ligands bearing dendritic wedges.
[
10–15]
To
the asymmetric allylation of unsymmetrical 1,3-dike- attach the polyaryl ether dendrons to the ligand they
tones, where enantioselectivities of 89% ee have been introduced amino groups in the 5- or 5,5’-positions.
Those dendritic ligands have been tested as catalysts
Adv. Synth. Catal. 2009, 351, 607 – 616
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Jutta K. Kassube et al.
FULL PAPERS
Scheme 1. Attachment of the BINAP ligand on dendrimers with (left) and without (right) an additional linking unit.
in various hydrogenation reactions, showing similar ligand was attached to the periphery of PPI- and
enantioselectivities as the BINAP ligand itself but PAMAM-dendrimers using glutaric acid as a linking
[
16]
with improved recycling properties.
unit.
This linker is relatively long and enables a
A methylene amino-functionalised BINAP ligand high mobility of the fixed ligands. On the other hand
“H-AMINAP”) has been attached to the periphery this flexibility may also lead to a backfolding of the
(
of poly(propylene imine) (PPI) and poly(amido catalytic active sites into the interior of the dendritic
amine) (PAMAM) dendrimers using glutaric acid as a structure. Because of this, we have now synthesised a
[
16]
linking unit.
dendritic systems can be used as “ligands” in the ligand-support linking units.
copper-catalysed hydrosilylation of acetophenone The previously reported AMINAP ligand was pre-
It has been demonstrated that these series of dendritic BINAP ligands with much shorter
[
22]
without any loss of enantioselectivity resulting from pared from (R)-6-Cyano-BINAP as a starting mate-
their immobilisation onto the dendrimer compared to rial. By hydrolysis of the cyano function it is also pos-
[
16]
the mononuclear ligand Benzoyl-AMINAP.
sible to create a carboxyl group directly on the bi-
[
22]
In addition to dendrimers that are characterised by naphthyl backbone of the ligand that should allow
their monodispersity and high regularity,
[
17]
hyper- a direct attachment of the ligand to the amino end
branched polymers have also been used as soluble groups in PPI and – in principle – also PAMAM den-
[
18]
supports for catalysts.
In contrast to dendrimers, drimers without an additional linking unit
these supports are polydisperse and randomly (Scheme 1).
branched. Their production is much cheaper since
they are generated in one-step processes. Neverthe-
less it has been shown that catalysts immobilised on Synthesis and Characterisation of the Mononuclear
hyperbranched polymers may possess similar proper- Carbo-BINAP Reference Systems
[
19]
ties as regular dendritic systems.
It is because of
this relationship that dendrimer catalysts may serve as The direct attachment of the Carbo-BINAP ligand to
model systems for catalysts attached to hyper- the periphery of dendrimers is expected to lead to a
branched polymers.
stronger influence of the supporting material on the
ligated catalysts than has been the case with glutaric
acid previously employed as linker. Therefore, four
mononuclear reference ligands were synthesised,
which were thought to represent a range of steric en-
[
16]
Results and Discussion
The function of the linking unit in immobilised cata- vironments relevant to the functionalised dendritic
lysts is an important factor. Depending on the size of target compounds. To this end, the carboxyl group of
the catalyst, the linking unit can determine the densi- HO-Carbo-BINAP was coupled with the primary
ty of the catalytically active centres in the immobi- amines methylamine, n-propylamine, tert-butylamine,
[20]
lised system.
Additionally, the linker acts as a and benzylamine (Scheme 2) using ethyl-N,N-dime-
spacer between the catalyst and the support and will thylcarbodiimide (EDC) and 1-hydroxybenzotriazole
influence the catalytic properties, owing to an en- as coupling reagents, a strategy well known in poly-
[
21]
[23]
hanced mobility of the active centres.
Therefore, we decided to investigate the influence
peptide synthesis.
Crystals of 1, 2 and 3 suitable for a single crystal X-
of the linking unit on the immobilised BINAP ligands ray structure analysis were obtained from toluene.
in more detail. In our previous work, the AMINAP The molecular structure of 1 is depicted in Figure 1.
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Immobilisation of the BINAP Ligand on Dendrimers and Hyperbranched Polymers
FULL PAPERS
The structures of 2 and 3 are quite similar (allowing
for the different substituents R) and are displayed in
the Supporting Information. Table 1 provides a com-
parative listing of the principal bond lengths and
angles of compounds 1–3.
The principal structural features are similar to the
crystallographically determined molecular structure of
Scheme 2. Synthesis of the mononuclear Carbo-BINAP ref-
[
24]
erence ligands (R=methyl 1, n-propyl 2, tert-butyl 3, benzyl the unfunctionalised BINAP.
This also holds true
4
).
for the torsion angles between the two naphthyl frag-
[24]
ments . The values are 87.88 in BINAP, 93.38 in 1,
5.58 in 2 and 86.68 in 3. The greater angle observed
8
in 1 is tentatively attributed to crystal packaging ef-
fects. The corresponding torsion angle in Benzoyl-
AMINAP differs with 78.88 more considerably from
[16]
the angle in the unfunctionalised BINAP ligand.
This shows that in this case the functionalisation of
the ligand backbone has a larger influence on the tor-
sion angle than in the Carbo-BINAP ligands.
In addition to these mononuclear reference ligands,
a bitopic ligand, Bis-BINAP 5, was synthesised by
combination of HO-Carbo-BINAP and H-AMINAP,
thus further increasing the steric demand of the “pro-
tecting group” (Scheme 3).
3
1
The P NMR spectrum of 5 clearly indicates the
connection of these two differently functionalised
BINAP ligands. The signals of the diphosphine halves
are observed as AB spin systems at À15.13 and
À15.25 ppm for the Carbo-BINAP part (J
=
=
P, P
P, P
1
1
3.5 Hz) and at À15.44 and À15.54 ppm (J
1.3 Hz) for the AMINAP moiety.
Figure 1. Molecular structure of Carbomethyl-BINAP 1, hy-
drogen atoms are omitted for clarity.
Synthesis and Characterisation of the Carbo-BINAP
Functionalised PPI Dendrimers
Table 1. Selected bond lengths [ꢂ] and angles [8] of Carbo-
methyl-BINAP 1, Carbopropyl-BINAP 2 and Carbo-tert- The essentially complete functionalisation of all
butyl-BINAP 3.
amino end groups of PPI dendrimers of generations 1
to 5 was achieved following the same strategy that
was developed for the dendritic AMINAP ligands,
1
2
3
[16]
ÀC(1’)
1.497(3)
1.841(2)
1.839(2)
1.385(3)
1.386(3)
117.9(2)
118.6(2)
116.4(2)
93.3(3)
1.504(4)
1.844(3)
1.845(3)
1.382(4)
1.382(4)
118.4(2)
119.2(2)
116.7(3)
85.5(3)
1.498(2)
1.841(2)
1.836(2)
1.384(2)
1.388(2)
118.76(13)
119.42(12)
116.40(16)
86.6(2)
using ethyl-N,N-dimethylcarbodiimide (EDC) and 1-
hydroxybenzotriazole as coupling reagents and an
excess of HO-Carbo-BINAP (Scheme 4).
C(1)
C(2)ÀP(1)
C(2’)ÀP(2)
C(1)ÀC(2)
By this method, Carbo-BINAP functionalised PPI
dendrimers have been synthesised, bearing 4 to 64 di-
phosphines at their periphery without an additional
linking unit. All dendritic ligands were characterised
C(1’)ÀC(2’)
C(1)ÀC(2)ÀP(1)
C(1’)ÀC(2’)ÀP(2)
NÀC(11)ÀC(6)
C(2’)ÀC(1’)ÀC(1)-C(9)
1
13
31
by H, C, and P NMR spectroscopy, and elemental
analysis. The lower generations up to {G3} have also
Scheme 3. Synthesis of Bis-BINAP 5.
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Jutta K. Kassube et al.
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Scheme 4. Synthesis of the dendritic Carbo-BINAP ligands 6–10.
been characterised by MALDI-TOF mass spectrome- Synthesis and Characterisation of the AMINAP and
try, indicating a complete functionalisation of all den- Carbo-BINAP Functionalised Hyperbranched
dritic end groups. In Figure 2 {G2}-DAB-dendr- Poly(ethylene imines)
(
Carbo-BINAP) 7 is depicted as a representative ex-
8
ample, illustrating the high topological symmetry of We then investigated whether the immobilisation
the dendritic ligands.
In contrast,
strategy of both Carbo-BINAP and AMINAP ligands
complete functionalisation of developed for dendrimers as supporting materials was
a
PAMAM dendrimers was not achieved with the also applicable to the less regularly structured hyper-
31
Carbo-BINAP ligand as indicated by the P NMR branched poly(ethylene imines) (PEIs). These were
spectra. This negative result was obtained even for chosen as supports because they exhibit a chemical
the lower dendrimer generations and upon employing structure similar to the poly(propylene imine) den-
a greater excess of functionalised ligand. Likewise, drimers used so far. The hyperbranched polymers are
longer reaction times or higher reaction temperatures commercially available with different average molecu-
did not lead to complete conversion. This contrasts lar weights. PEIs with average MW of 800 (PEI ),
0.8
À1
with the previously reported facile immobilisation of 2000 (PEI ) and 25000 gmol (PEI ) have been used
2
25
ligand-linker units of AMINAP on PAMAM den- in this study (for further information about the prop-
[16]
drimers.
erties of the hyperbranched PEIs see the Supporting
Information and ref. ). In Figure 3 a section of a
PEI is depicted.
[19]
In contrast to the PPI dendrimers that essentially
possess only primary amino groups (due to the almost
Figure 2. {G2}-DAB-dendr-(Carbo-BINAP) 7.
8
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Immobilisation of the BINAP Ligand on Dendrimers and Hyperbranched Polymers
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Table 2. Degree of functionalisation (DF) of the BINAP
functionalised PEIs 11–16, along with the number of at-
tached ligands.
DF [%]
BINAP
PEI -(Carbo-BINAP) 11
100
100
91
100
96
9
0
.8
PEI -(Carbo-BINAP) 12
26
138
9
25
138
2
PEI -(Carbo-BINAP) 13
Figure 3. Section of a hyperbranched PEI.
25
PEI -(Glutaroyl-AMINAP) 14
0
.8
PEI -(Glutaroyl-AMINAP) 15
2
PEI -(Glutaroyl-AMINAP) 16
91
2
5
perfect branching of these molecules) there are also
secondary amino groups in the hyperbranched PEIs
that result from incomplete conversion at some
branching points in their one-step synthesis. It was
1
In Figure 5 the H NMR spectra of {G2}-DAB-
therefore important to functionalise both primary and dendr-(Carbo-BINAP) 7 and PEI -(Carbo-BINAP)
8
0.8
secondary amino groups in the PEIs because free NH 11 are depicted for comparison. Both molecules are
functionalities were thought to act as potential (unde- comparable in size and number of fixed ligands (7:
À1
À1
sired) ligands in the catalyst assembly. The coupling 5963 gmol , 8 ligands; 11: 6400 gmol , 9 ligands).
reagents used for dendrimer functionalisation HOBT Nevertheless the resolution of the spectrum corre-
and EDC were found to be appropriate for both sponding to the hyperbranched system is much lower
types of amino functionalities. Using a greater excess than that of the regular dendrimer. This is a result of
of ligand (1.3 equiv. of HO-Carbo-BINAP or HO- the polydispersity of the hyperbranched support and
Glutaroyl-AMINAP, respectively, per primary and the irregularity of the structure.
secondary amino group) we were able to synthesise
the Carbo-BINAP (11–13) and Glutaroyl-AMINAP trate why it has been proven advantageous in a first
14–16) functionalised hyperbranched PEIs depicted step to develop an immobilisation strategy using regu-
in Figure 4. larly structured monodisperse dendrimers as supports.
The two spectra depicted in Figure 5 clearly illus-
(
The degree of functionalisation of these new These allow a facile characterisation by NMR and
BINAP-functionalised hyperbranched poly(ethylene mass spectrometry and therefore the determination of
imines) was calculated from the elemental analyses “optimal” reaction conditions and work-up proce-
obtained for these materials. The results are listed in dures for an essentially complete functionalisation
Table 2, along with the average number of attached and a complete removal of excess ligand as well as
BINAP ligands.
coupling regents.
According to the elemental analysis, PEI0.8 and
PEI were functionalised with both BINAP deriva-
2
tives almost quantitatively. Using the highly polymeric The Enantioselective Cu-Catalysed Hydrosilylation of
PEI as support, a degree of functionalisation of 91% Acetophenone
25
has been achieved in both cases yielding catalysts
bearing on average 138 BINAPs per molecule!
In order to investigate the catalytic properties of the
new immobilised BINAP ligands and to compare
them with those of the dendritic AMINAP ligands
published previously, the copper-catalysed hydosily-
lation of acetophenone was chosen as a test reaction
employing the reaction conditions optimised previous-
ly (Scheme 5).
[
16]
In a first run, the mononuclear reference ligands 1–
were tested. After 24 h reaction time all (isolated)
4
yields were excellent (>95%) and the observed enan-
tioselectivities were between 94 and 95% ee. This in-
dicates that the different protecting groups do not
induce different torsion angles in the copper com-
plexes. Additionally, the nature of the functional
group in the 6-position of the ligand backbone does
not change the electronic properties of the ligand
since these results are similar to those observed using
Figure 4. Carbo-BINAP (top) and Glutaroyl-AMINAP Benzoyl-AMINAP as stereodirecting ligand (95%
(
bottom) functionalised hyperbranched PEIs (x=0.8, 2, 25). yield, 94% ee). Bis-BINAP 5 was also used as ligand
Adv. Synth. Catal. 2009, 351, 607 – 616
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Jutta K. Kassube et al.
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1
Figure 5. H NMR spectra (600 MHz) of {G2}-DAB-dendr-(Carbo-BINAP) 7 and PEI -(Carbo-BINAP) 11.
8
0.8
Scheme 5. Copper-catalysed hydrosilylation of acetophenone.
in the hydrosilylation of acetophenone giving rise to a
In order to further investigate this behaviour the
yield of 98% at 93% ee. This demonstrates that even catalytic hydrosilylations were performed using only
the attachment of a very large “protecting group” (a 0.5 equivalents of CuCl per diphosphine unit. The
second BINAP ligand) does not lead to a change of enantioselectivities obtained under these reaction
the biaryl conformation in the ligand backbone and conditions are summarised in Figure 7.
thus the stereoinduction of the system is not influ-
enced.
To account for this increase in selectivity of all den-
dritic Carbo-BINAP ligands 6–10 when using only
These initial tests were followed by an investigation half an equivalent of copper to values (that are simi-
of the catalytic properties of the dendritic Carbo- lar to those achieved when using the mononuclear
BINAP ligands 6–10. The results of this study are reference ligands as well as the dendritic AMINAP li-
shown in Figure 6.
gands), we propose a partial dismutation and the for-
In contrast to the results obtained for the dendritic mation of copper(I) complexes ligated by two BINAP
AMINAP copper catalysts (incorporating glutarate ligands when one equivalent of copper is used. This
linking units!), for which the enantioselectivities and dismutation appears to be favoured in the small den-
+
activities remained almost unchanged in the immobili- dritic Carbo-BINAP derivatives. The [Cu ACHTUNGTERNNUN(G BINAP*) ]
2
sation and were found to be independent of the den- species (BINAP*=immobilised BINAP) formed in
[
16]
drimer generation, a considerable variation of ste- the process are probably catalytically inactive. This
reoinduction was observed for 6–10. Whereas the first explains the reduced activity of the dendrimers com-
generation system catalysed the reaction with a selec- pared with the catalytic reference systems with the
tivity of only 34% ee, the fifth generation dendrimer monotopic BINAP ligands. Upon formation of the
+
reached 90% ee. On the other hand, the reaction time [Cu
A
H
U
G
R
N
U
G
2
required for complete conversion increased from 24 h acts as competing achiral catalyst for this reaction,
to 96 h upon going from the mononuclear catalysts to leading to an overall reduction of the stereoselectivity
the lower generation dendritic catalysts whilst de- observed for the smaller dendrimer generations. The
creasing again for the higher generation dendrimers.
formation of the homoleptic copper complexes seems
to be suppressed with increasing size of the dendrimer
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Immobilisation of the BINAP Ligand on Dendrimers and Hyperbranched Polymers
FULL PAPERS
Figure 6. Enantioselectivity of the asymmetric hydrosilylation of acetophenone for the different dendrimer generations (6–
0).
1
Figure 7. Enantioselectivity of the asymmetric hydrosilylation of acetophenone for the different dendrimer generations (6–
0) using only 0.5 equiv. CuCl per diphosphine unit.
1
due to the increasing density of ligands in the dendri- tation of the catalyst and then reused in three succes-
mer periphery. The conformation of the ligating units sive catalytic runs without loss of activity (TOFs at
À1
required for the complex dismutation does not seem 50% conversion of 4.7Æ0.3 h ) or enantioselectivity.
to be favourable under these conditions. This would
In a next step, the catalysts immobilised on the hy-
explain the high level of stereoinduction and activity perbrached PEIs were investigated. Table 3 summaris-
of the fifth generation Carbo-BINAP ligand 10 even es the enantioselectivities induced by the hyper-
if one equivalent of CuCl is used. All BINAP-cop- branched Carbo-BINAP 11–13 and AMINAP ligands
per(I) complexes (mononuclear or dendritic) proved 14–16 for the Cu-catalysed hydrosilylation of aceto-
to be substitutionally highly labile which has preclud- phenone as reference reaction.
ed the direct observation of the proposed redistribu-
For the Carbo-BINAP ligands attached to the hy-
31
tion equilibria. The P NMR spectra of all metallated perbranched poly(ethylene imines) 11–13 uniform
phosphines only displayed broad dynamic spectral high enantioselectivities of 93–94% ee and good activ-
features at temperatures between 0 and À808C. Cata- ities were observed in contrast to the dendritic Carbo-
lyst 10 was shown to be recyclable by simple precipi- BINAP systems 6–10 discussed above. This illustrates
Adv. Synth. Catal. 2009, 351, 607 – 616
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Jutta K. Kassube et al.
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Table 3. Enantioselectivity of the asymmetric hydrosilylation 9, 26, and 138 Glutaroyl-AMINAP or Carbo-BINAP
of acetophenone for the different hyperbranched Carbo-
BINAP 11–13 and AMINAP ligands 14–16. For 13 and 16
the results of several runs involving the recycling of the cat-
alysts are given.
ligands onto PEIs of different molecular weights.
The catalytic properties of these systems in the
copper-catalysed hydrosilylation of acetophenone
were investigated. The dendritic PPI-bound Carbo-
BINAP ligands showed a strong dependence of enan-
tioselectivity and activity on the generation of the
dendrimer. The Carbo-BINAP and Glutaroyl-
AMINAP ligands immobilised on the hyperbranched
polymers, however, displayed activities and enantiose-
lectivities comparable to those of the mononuclear
catalysts. The highest generation dendrimers contain-
ing the immobilised BINAP ligands could be recycled
and re-used several times without any observable loss
of activity or selectivity.
Ligand
Run Yield [%] ee [%]
PEI -(Carbo-BINAP) 11
–
–
1
2
3
–
–
1
2
3
96
97
100
100
99
98
98
95
95
94
94
93
93
93
93
92
93
93
92
92
0.8
PEI -(Carbo-BINAP) 12
2
PEI -(Carbo-BINAP) 13
25
PEI -(Carbo-BINAP) 13
25
PEI -(Carbo-BINAP) 13
25
PEI -(Glutaroyl-AMINAP) 14
0.8
PEI -(Glutaroyl-AMINAP) 15
2
PEI -(Glutaroyl-AMINAP) 16
25
PEI -(Glutaroyl-AMINAP) 16
25
PEI -(Glutaroyl-AMINAP) 16
Extending the use of these new dendrimer catalysts
and those based on hyperbranched polymers to other
catalytic reactions is the objective of current and
25
the limits of the use of dendrimer catalysts as model future research in our laboratory.
systems for the less regularly structured hyper-
branched PEI derivatives. In fact the less densely
structured hyperbranched systems 11–13 display the Experimental Section
“
ideal” behaviour of the mononuclear reference li-
gands 1–4 not attainable with the dendrimers under The general information about equipment and methods em-
ployed in this work along with the complete spectroscopic
standard reaction conditions (1 equivalent of Cu). We
note that both PEI -(Carbo-BINAP) (13) and
PEI (Glutaroyl-AMINAP) (16) could be precipitated
and analytical data of all new compounds is provided in the
Supporting Information.
25
25
and recycled without loss of activity and selectivity
(
Table 3).
General Procedure for the Synthesis of the Protected
The use of the hyperbranched AMINAP ligands Carbo-BINAP ligands (1–4) and Bis-BINAP (5)
1
4–16 also led to high enantioselectivities of 92–93%
A mixture of (R)-6-hydroxycarbonyl-2,2’-bis(diphenylphos-
ee, that are in this case comparable to the results ob-
tained when using the AMINAP-PPI dendrimers as
phino)-1,1’-binaphthyl
(HO-Carbo-BINAP,
1.0 equiv.),
EDC·HCl (1.1 equiv.), 1-HOBT (1.5 equiv.) and triethyl-
catalysts. This means that employing glutarate as a amine (1.7 equiv.) was stirred in DMF at 08C for 40 min. To
linking unit neutralises the effects discussed above, this suspension was added the respective primary amine dis-
and renders the functionalised dendrimers and hyper- solved in DMF. The resulting solution was warmed to room
temperature and stirred for 48 h. All volatiles were com-
branched polymers very similar in their behaviour in
pletely removed under vacuum. The residue was taken up in
enantioselective catalysis.
2
2
1
0 mL CH Cl and thoroughly extracted with KOH (0.2M,
2 2
ꢃ15 mL), H O (2ꢃ15 mL), hydrochloric acid (0.2M, 2ꢃ
2
5 mL), KOH (0.2M, 2ꢃ15 mL) and again H O (2ꢃ15 mL).
2
Conclusions
Finally, the solvent was removed under vacuum. The result-
ing solid was washed several times with n-pentane and dried
under vacuum yielding the protected Carbo-BINAP ligands
In summary, a series of immobilised Carbo-BINAP li-
gands has been synthesised using PPI dendrimers as as pale yellow powders. Crystals suitable for X-ray analysis
soluble supports. They contain up to 64 BINAP li- of 1, 2 and 3 were obtained by recrystallisation from tolu-
gands at their periphery without an additional linking ene.
unit. Despite the high steric requirements of the
ligand, all dendrimers could be completely functional-
ised (within the accuracy of the analytical methods)
giving the immobilised systems in good yields.
Furthermore, the immobilisation strategy worked
out for the fixation of AMINAP ligands with addi-
tional linking units as well as of Carbo-BINAP li-
gands without an additional linking unit on dendrim-
ers has been extended to less regularly built hyper-
General Procedure for the Synthesis of the Dendritic
Carbo-BINAP ligands (6–10)
A mixture of (R)-6-hydroxycarbonyl-2,2’-bis(diphenylphos-
phino)-1,1’-binaphthyl (HO-Carbo-BINAP, 1.10 equiv.),
EDC·HCl (1.21 equiv.), 1-HOBT (1.65 equiv.) and triethyl-
amine (1.85 equiv.) was stirred in DMF at 08C for 40 min.
To this suspension was added the respective amino-terminat-
ed poly(propylene imine) (PPI) dendrimer dissolved in
branched poly(ethylene imines) as soluble supports. DMF. The resulting solution was warmed to room tempera-
In this way, it has been possible to attach (on average) ture and stirred for 48 to 90 h. All volatiles were completely
614
asc.wiley-vch.de
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 607 – 616

Immobilisation of the BINAP Ligand on Dendrimers and Hyperbranched Polymers
FULL PAPERS
3
removed under vacuum. The residue was taken up in 20 mL
99.697(2)8, V=2087.1(3) ꢂ , Z=2, space group P2 , D
=
1
calcd
À3
À1
CH Cl2 and thoroughly extracted with KOH (0.2M, 2ꢃ
1.228 Mg·m , m=0.145 mm . Reflections measd.: 34484,
indep.: 7364 [Rint =0.0552], obsvd. [I> 2 s(I)]: 6253. R(F)
[F > 4 s(F )]=0.0332, wR(F ) [all unique data]=0.0720,
2
1
1
5 mL), H O (2ꢃ15 mL), hydrochloric acid (0.2M, 2ꢃ
2
2
5 mL), KOH (0.2M, 2ꢃ15 mL) and again H O (2ꢃ15 mL).
2
o
o
Finally, the solvent was removed under vacuum. The result-
ing solid was washed several times with n-pentane and dried
under vacuum yielding the Carbo-BINAP-functionalised
dendrimers as off-white powders.
Flack x=0.02(6).
Crystallographic data for 2: C H NOP ·C H , M=799.88,
4
8
39
2
7
8
a=13.289(2), b=9.7042(15), c=16.908(3) ꢂ, b=94.496(3)8,
3
V=2173.7(6) ꢂ , Z=2, space group P21, Dcalcd =1.222
À3
À1
Mg·m , m=0.141 mm . Reflections measd.: 55703, indep.:
1
4
4586 [Rint =0.0416], obsvd. [I> 2 s(I)]: 12567. R(F) [F >
General Procedure for the Sythesis of the
Hyperbranched Carbo-BINAP (11–13) and AMINAP
Ligands (14–16)
o
2
s(F )]=0.0448, wR(F ) [all unique data]=0.1127, Flack x=
o
À0.07(8).
Crystallographic data for 3: C H NOP ·C H , M=813.90,
4
9
41
2
7
8
A mixture of (R)-6-hydroxycarbonyl-2,2’-bis(diphenyl-phos-
phino)-1,1’-binaphthyl (HO-Carbo-BINAP, 1.30 equiv.) or
Glutaroyl-AMINAP (1.30 equiv.), EDC·HCl (1.43 equiv.), 1-
HOBT (1.95 equiv.) and triethylamine (2.09 equiv.) was
stirred in DMF at 08C for 40 min. To this suspension was
added the respective amino-terminated hyperbranched
poly(ethylene imine) dissolved in DMF. The resulting solu-
tion was warmed to room temperature and stirred for 48 to
a=13.1004(14), b=10.1545(11), c=16.7341(18) ꢂ, b=
3
95.595(2)8, V=2215.5(4) ꢂ , Z=2, space group P2 , D
=
1
calcd
À3
À1.
1.220 Mg·m , m=0.140 mm Reflections measd.: 44422,
indep.: 2430 [Rint =0.0929], obsvd. [I> 2 s(I)]: 1979. R(F)
2
[F > 4 s(F )]=0.0467, wR(F ) [all unique data]=0.1209,
o
o
Flack x=0.00(6).
Intensity data were collected at 100 K (Bruker AXS
Smart 1000 CCD diffractometer, Mo-K radiation, graphite
a
90 h. All volatiles were completely removed under vacuum.
monochromator, l=0.71073 ꢂ) and corrected for Lorentz,
The residue was taken up in 20 mL CH Cl and thoroughly
polarisation and absorption effects (semiempirical,
2
2
[25]
[26,27]
extracted with KOH (0.2M, 2ꢃ15 mL), H O (2ꢃ15 mL),
SADABS).
Structure solution: direct methods.
Re-
2
2
[27,28]
hydrochloric acid (0.2M, 2ꢃ15 mL), KOH (0.2M, 2ꢃ
finement: full-matrix least squares methods based on F ;
1
5 mL) and again H O (2ꢃ15 mL). Finally, the solvent was
all non-hydrogen atoms anisotropic. Adp and/or distance
similarity restraints were applied to the solvent of crystalli-
sation (toluene) in some cases. Hydrogen atoms (except
those of the methyl groups and some of the toluene solvent)
were located and refined.
2
removed under vacuum. The resulting solid was washed sev-
eral times with n-pentane and dried in vacuo yielding the
Carbo-BINAP- and Glutaroyl-AMINAP-functionalised hy-
perbranched poly(ethylene imines) as off-white powders.
Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with
the Cambridge Crystallographic Data Center: CCDC
General Procedure for the Hydrosilylation
Experiments
7
10932 (1), 710933 (2) and 710934 (3). These data can be ob-
The appropriate amount of the dendritic ligand correspond-
ing to 0.03 mmol of diphosphine, 3.0 mg (0.03 mmol) of
CuCl and 2.9 mg (0.03 mmol) of t-BuONa were dissolved in
tained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif or
from CCDC, 12 Union Road, Cambridge CB2 1EZ, UK;
Phone: +44 1223 336408, Fax: +44 1223 336033; E-mail:
deposit@ccdc.cam.ac.uk].
2
mL of toluene/THF 8/3 and the resulting mixture was
stirred for 30 min at room temperature. Diphenylsilane
210 mL, 1.1 mmol) was then added and the solution cooled
(
to À788C. After addition of 117 mL (1.0 mmol) of acetophe-
none dissolved in 1 mL toluene/THF 8/3, the reaction mix-
ture was stirred for 24 h at this temperature. A solution of
K CO (3 mL, 1% in methanol) was added to hydrolyse the
Acknowledgements
2
3
reaction mixture, then the solvent was removed under
vacuum and the residue purified by column chromatography
on silica (pentane/diethylether 85/15). Enantioselectivities
We thank the Deutsche Forschungsgemeinschaft (SFB 623,
C8) and the Fonds der Chemischen Industrie (Ph.D. scholar-
ship to J. K. K.) for support. We are also grateful to Dr.
Bernd Bruchmann (BASF, SE) for providing the hyper-
branched poly(ethylene imines).
were determined by GC (Chiraldex b-PM (50 mꢃ0.25 mm);
À1
4
08C, 58Cmin to 1208C, 14 min; tR [(R)-sec-phenylethyl
alcohol]=25.59 min, tR [(S)-sec-phenylethyl alcohol]=
6.34 min.). For the recycling experiments the catalyst was
2
precipitated at the end of the reaction by addition of diethyl
ether and separated by centrifugation. The centrifugate was
subsequently hydrolysed as described above and purified.
The recycled catalyst was dried under vacuum. Then new
solvent and silane were added, the mixture was cooled to
À788C and the reaction was started again by addition of
acetophenone.
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Adv. Synth. Catal. 2009, 351, 607 – 616