Increased selectivity in hydroformylation reactions using dendrimer based catalysts; a positive dendrimer effect

Article abstract of DOI:10.1039/b009574p

Dendrimers based on polyhedral oligomeric silsesquioxanes (POSS) cores with 16 PPh₂ arms give much higher linear selectivities (14:1) than their small molecule analogues (3-4:1) in the hydroformylation of oct-1-ene.

Full text of DOI:10.1039/b009574p

Increased selectivity in hydroformylation reactions using dendrimer based
catalysts; a positive dendrimer effect
Loïc Ropartz,a Russell E. Morris,a Douglas F. Fosterb and David J. Cole-Hamilton*a
a School of Chemistry, University of St. Andrews, St. Andrews, Fife, Scotland, UK KY16 9ST.
E-mail: djc@st.andrews.ac.uk
b Catalyst Evaluation and Optimistaion Service (CATS), School of Chemistry, University of St. Andrews, St. Andrews,
Fife, Scotland, UK KY16 9ST
Received (in Cambridge, UK) 29th November 2000, Accepted 11th January 2001
First published as an Advance Article on the web 8th February 2001
Dendrimers based on polyhedral oligomeric silsesquioxanes
(POSS) cores with 16 PPh2 arms give much higher linear
selectivities (14+1) than their small molecule analogues
(3–4+1) in the hydroformylation of oct-1-ene.
Dendrimers are globular molecules in which the groups on the
periphery are all the same. They have been used for a variety of
applications, but interesting opportunities are offered if the
dendrimer is used as a ligand in homogeneous catalysis. Most
research in this area is aimed at using the dendrimer to
overcome the main problem of homogeneous catalysts, that the
reaction products can be hard to separate from the catalyst and
the solvent. Because of their persistent shape and size
(1–10 nm), there is interest in attempting to remove the catalyst
by nanofiltration through a suitable membrane. Filtration of this
kind has been demonstrated.1–3 Another possible advantage of
dendrimers is that crowding in the periphery might lead to
constrained geometries in the metal binding, which might affect
the activity or selectivity of the catalyst. In many cases, reaction
rates are slowed in the presence of a dendrimer, but there are
also examples of reactions being totally or partially inhibited
because the dendrimer is present. In one case it is believed that
Scheme 1 Synthesis of 16-arm dendrimer phosphine 1. Reagents and
conditions: i, MeSiHCl2, H2PtCl6 (catalyst); ii, (vinyl)MgBr; iii, Ph2PH,
azodi(isobutyryl)nitrile. 1 has an average of 15 PPh2P groups, 5 has an
an unpaired electron is passed from metal to metal on the
dendrimer, thus inhibiting its use in the Kharasch reaction.4
Examples of enhanced reactivity in C–C coupling reactions
average of 12 PPh2 groups. In the POSS core, each edge of the cube
have been reported3,5 and very recently it has been shown that
represents Si–O–Si.
epoxidation reactions can be accelerated if the catalyst is bound
to the dendrimer and this is thought to be because the oxidising
exactly the same conditions, catalysts prepared in situ from 1
and [Rh(acac)(CO)2] (acacH = pentane-2,4-dione) give l+b
peroxide binds to one metal, whilst the substrate is activated by
another. Having the two metals held in close proximity
ratios of up to 14. A ratio this high would be very unusual for an
increases the reaction rate.6 To our knowledge, examples of
unconstrained ligand of this size and certainly shows that a
positive dendrimer effect is operating. The reaction rate and l+b
ratio increase with increasing temperature or decreasing gas
pressure (Table 1) but are only slightly affected by the Rh+P
ratio (3.6–10.8) at a given Rh concentration. The generally high
l+b ratios suggest that strong bidentate coordination occurs or
that the high local concentration of P atoms on the surface of the
positive dendrimer effects on the selectivity of catalytic
reactions have not been reported, but we now report such an
effect in the hydroformylation of oct-1-ene. Hydroformylation
using metals bound to dendrimers with phosphines on the
periphery have been reported,7,8 but generally the selectivity to
the desired linear product is similar to that using model small
molecule ligands, although we noted a slightly enhanced linear
dendrimer increases the concentration of complexes containing
selectivity using dendrimer-bound trialkylphosphines.8
three P donors. Further confirmation that the metal is strongly
Dendrimers based on polyhedral oligomeric silsesquioxane
bound to the dendrimer comes from 31P NMR studies of
(POSS) cores were prepared as shown in Scheme 1 such that
solutions prepared from [Rh(acac)(CO)2] and 1 (1+3) under CO
they had up to 16 PPh2 groups on the periphery (1). Attempts to
and H2. Two resonances are observed at room temperature, one
prepare the dendrimer with 24 PPh2 arms leads to incomplete
(d 37) for P bound to Rh and the other (d 2 9.5) from the free
conversion to phosphines, presumably because of excessive
P atoms (1 resonates at d 2 9.5). Both resonances are very broad
(width at half maximum = 470 and 88 Hz respectively),
possibly because of different binding environments for the
rhodium since the spectrum is the same in different solvents
(CH2Cl2, tetrahydrofuran) and the linewidth at half maximum
of the signal from the unbound P atoms only changes slightly
steric crowding.8 Analysis of 1 by NMR and by MALDI-TOF
mass spectrometry shows that they have 14, 15 or 16 PPh2
groups per dendrimer, with the average number being ca 15.
Within one arm of the dendrimer, the P atoms are separated by
five atoms. Simple diphosphines separated by five or six atoms
are only expected to favour bidentate binding when constrained,
as in the case of Xantphos9 and bis(diphenylphosphino-
methyl)biphenyl (BISBI)10 ligands. For such constrained li-
gands, the linear+branched ratios can be very high. Indeed,
using the unconstrained small molecules, 2 and 3, l+b ratios are
only of the order of 3-4 (Table 1 and Fig. 1). In contrast, under
DOI: 10.1039/b009574p
Chem. Commun., 2001, 361–362
This journal is © The Royal Society of Chemistry 2001
361

Table 1 Results of hydroformylation reactions using various ligandsa
Ligand
T/°C
p/bar
kb/1023 s21
Conversion (%)
l+b
Nonanal (%)
1
120
120
100
80
100
120
120
120
120
120
10
10
10
10
20
10
10
10
10
10
1.2
1.1
> 99.9
> 99.9
> 99.9
> 99.9
> 99.9
> 99.9
> 99.9
> 99.9
> 99.9
> 99.9
13.9
12.0
10.8
8.8
86
86
86
84
85
85
84
70
73
77
5c
5c,d
5c,e
5c,f
5g
5h
2
0.42
0.08
0.33
1.5
0.95
3.6
7.5
12.2
11.9
3.4
3.8
5.2
3
4
3.0
2.1
a [Rh(acac)(CO)2] (2.0 3 1025 mol); P+Rh = 6+1, toluene (4 cm3), heated under CO/H2 (6 bar) for 1 h. Oct-1-ene (8.3 3 1023 mol) injected and pressure
increased. Pressure kept constant through mass flow controller and fed from a ballast vessel. Pressure drop in ballast vessel monitored every 5 s. Total reaction
time, 2 h. b First-order rate constant measured from gas uptake plots at constant pressure. c P+Rh = 5.4+1. d t = 4 h. e t = 19 h. f t = 6 h. g P+Rh = 10.8.
h P+Rh = 3.6.
tetravinylsilane containing 16 Ph2P arms, but with only one
CH2 spacer between Si and P, i.e. with a potential ring size of six
atoms for bidentate bonding, shows no special enhancement
over the small molecule analogue.7
We conclude that the dendrimers synthesised here have
sufficient steric crowding to make eight-membered ring
bidentate coordination favourable and that these rings enhance
the linear selectivity in hydroformylation reactions. Whether the
binding in five-coordinate intermediates is equatorial–equato-
rial, which has been shown to give enhanced selectivity,9,10
whether the P atoms are mutually trans throughout the reaction,
which may also give enhanced selectivity, or whether the reason
for the enhanced selectivity is the locally high Ph2P concentra-
tion is not clear at present, although the low-temperature 31P
NMR spectrum of the rhodium complex under CO/H2 suggests
that the P atoms bound to rhodium are not all equivalent.
We thank the University of St. Andrews for a studentship
(L. R.), the Royal Society for a University Research Fellowship
(R. E. M.) and Professor Dieter Vogt and Dr Joost Reek for
helpful discussions.
Fig. 1 l+b Ratio of product C9 aldehydes at 120 °C and 10 bar. P+Rh = 6+1,
except for 5 (P+Rh = 5.4+1).
between 240 (63 Hz) and +60 °C (150 Hz). This shows that the
rhodium is not migrating rapidly around the surface of the
dendrimer, nor dissociating on the NMR timescale. The
resonance from the Rh-bound phosphines appears as two broad
overlapping doublets (d 37 and 36) at 240 °C, but as a single
broad doublet (d 37 JP–H ≈ 130 Hz) at +60 °C, suggesting
fluxionality within the bound complex. A broad hydride signal
is observed at d 210.5 (width at half maximum = 80 Hz) in the
1H NMR spectrum at room temperature.
Notes and references
† This dendrimer was prepared for 1, but the reaction was not taken to
completion. It was shown to have an average of 12 PPh2 groups by MALDI-
TOF mass spectrometry.
In order to try to understand the higher l+b ratios observed
with the dendrimer bound catalysts, we have carried out some
molecular modelling of the dendrimer using the Discovery
programme contained in the Insight(II) Molecular Modelling
Suite of Molecular Simulations Inc.11 This shows that within an
arm, the P atoms are separated by 4–7 Å, whilst between arms
there are always some distances in the 5–10 Å region. Rh–P
distances are of the order of 2.5 Å, so very little disruption of the
ground state structure of the dendrimer is required to facilitate
bidentate binding. Presumably the orientation of the PPh2
groups relative to one another is determined by steric repulsions
on the surface of the dendrimer whilst models of the small
molecules show that the lowest energy structures have the
phosphine groups far away from one another. If this is the case,
one might expect that the compound prepared from addition of
Ph2PH across the double bonds of tetravinylsilane 4, for which
the X-ray crystal structure shows that the P atoms are 6.94 and
8.33 Å apart,12 might show intermediate behaviour between 1
and 3. Indeed this is the case, with the l+b ratio being 6 (Table 1
and Fig. 1). Reducing the PPh2 loading on the dendrimer to an
average value of 12 PPh2 groups (5)† gives a slightly lower l+b
ratio (12.0, Table 1, Fig. 1) than the more completely substituted
dendrimer, again suggesting that steric crowding on the
dendrimer periphery is important.
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Chem. Commun., 2001, 361–362