POSSphites - Monophosphites derived from incompletely condensed silsesquioxanes

Article abstract of DOI:10.1016/j.tetlet.2003.09.064

A straightforward three-step synthetic route is followed to obtain nanostructured silsesquioxane based monophosphite compounds 1-3, which we named POSSphites. These compounds, derived from (c-C₅H₉) ₇Si₇O₉

Full text of DOI:10.1016/j.tetlet.2003.09.064

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 8301–8305
POSSphites—monophosphites derived from incompletely
condensed silsesquioxanes
Jarl Ivar van der Vlugt, Michiel M. P. Grutters, Jens Ackerstaff, Rob W. J. M. Hanssen,
Hendrikus C. L. Abbenhuis and Dieter Vogt*
Schuit Institute of Catalysis, Laboratory of Homogeneous Catalysis, Eindhoven University of Technology, PO Box 513,
5600 MB Eindhoven, The Netherlands
Received 18 June 2003; revised 28 August 2003; accepted 5 September 2003
Abstract—A straightforward three-step synthetic route is followed to obtain nanostructured silsesquioxane based monophosphite
compounds 1–3, which we named POSSphites. These compounds, derived from (c-C5H9)7Si7O9(OH)3 and commercially available
phenol derivatives, differ mainly in the steric bulk around the phosphorus atom. Preliminary results in the rhodium-catalyzed
hydroformylation of 1-octene exemplify the use of these ligands for homogeneous catalysis. High activities, with a turnover
frequency of up to 5000 h−1 were obtained. Ligands 1–3 are the first silsesquioxane derived monophosphite ligands to be
successfully applied in homogeneous catalysis.
© 2003 Elsevier Ltd. All rights reserved.
The hydroformylation of alkenes using rhodium com-
plexes is the most prominent example of a homoge-
neously catalyzed reaction performed on a large
industrial scale.1 Diphosphite ligands have been studied
intensively for this reaction,2 but monophosphites have
also received attention.3 Rhodium complexes with
bulky ligands of this latter type have been shown to be
highly active catalysts for hydroformylation which in
some cases can also convert internal double bonds to
the desired linear aldehyde.4
monodentate phosphorus p-acceptor ligands. We could
thereby make use of the fact that silsesquioxanes
exhibit electron-withdrawing character, similar to sil-
ica.9 To illustrate the applicability of these ligands in
homogeneous catalysis, we have chosen the Rh-cata-
lyzed hydroformylation of 1-octene. We pursued the
synthesis of monophosphite compounds 1–3, which we
term POSSphites. In these compounds, the phosphorus
atom is built into the silsesquioxane framework and the
degree of steric bulk can be varied. To establish an
initial structure–activity relationship we included com-
pound 4 in the study which was previously reported but
without use in catalysis (Fig. 1).10
Polyhedral oligosilsesquioxanes (POSS) have been stud-
ied extensively for a number of years, either as homoge-
neous models for silica supports,5 as building blocks for
inorganic or hybrid materials6 or as ligand backbones
for (transition) metal complexes.7 However limited
attention has been paid to the synthesis and application
of silsesquioxane based phosphorus containing ligands.
The few systems reported so far deal with peripheral
functionalization of (dendritic) silsesquioxanes with
phosphorus moieties, tethered by various (alkyl)
spacers.8
Starting from the incompletely condensed silsesquiox-
11
ane trisilanol (R)7Si7O9(OH)3
(R=c-C5H9), the
We were interested to see whether silsesquioxanes could
be valuable as a backbone for the synthesis of
Keywords: silsesquioxanes; phosphite; rhodium; phosphorus ligand;
homogeneous catalysis; hydroformylation.
* Corresponding author. Tel.: +31 40 2472483; fax: +31 40 2455054;
e-mail: d.vogt@tue.nl
Figure 1. Schematic representation of the silsesquioxane com-
pounds used. 1–3: R=cyclopentyl; Ar=phenyl (1), 3,5-di-
tert-butylphenyl (2), 1-naphthyl (3). 4: R=cyclohexyl.
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.09.064

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monophosphite compounds 1–3 were synthesized in
three steps and obtained in good yield. After initial
monosilylation of the trisilanol,12 the resulting disilanol
was reacted with PCl3 to generate a phosphorochlorid-
ite functionalized silsesquioxane which was converted in
situ into the desired monophosphite by reaction with
the appropriate phenol under basic conditions (Scheme
1).13
low reactivity.14 Therefore, the commercially available
tris-(tert-butyldimethylsilyl)phosphite 5 was chosen as a
reference compound.
In order to get a first impression on the behavior of
these novel ligands under catalytic conditions we stud-
ied the rhodium-catalyzed hydroformylation of 1-
octene (Scheme 3). The catalysis was carried out under
standard conditions, at 20 bar of syngas (1:1) and a
reaction temperature of 80°C.
This is different from the synthesis of compound 4,
where the silsesquioxane trisilanol (R)7Si7O9(OH)3 (R=
c-C6H11), is corner-capped with the phosphorus atom
by reaction with PCl3. Remarkably, attempts to prepare
an analogue of the monophosphites 1–3 with R=c-
C6H11 or of 4 with R=c-C5H9 met with complications.
With monophosphite 1 the catalytic system showed
very high activity with a turnover frequency of around
5000 h−1 and a conversion of 30% after 15 min (entry
1). Even when increasing the substrate to catalyst ratio,
the activity remained nearly constant, indicative of a
stable catalytic complex (entry 2). The selectivity to the
linear aldehyde nonanal was moderate with a linear to
branched ratio of 2.1, as expected for a monophosphite.
Both ligands 2 and 3 were applied, to study the effect of
steric bulk around the phosphorus center on the cata-
lytic activity. The Rh-complexes of these ligands were
less active, as is evidenced by the lower turnover fre-
quencies of 2500 and 2200 h−1, respectively (entries 3
and 4). The regioselectivity however did not change.
This led us to conclude that no mono-ligated complexes
are formed with these ligands. Interestingly, under these
conditions the reference material 4 showed no catalytic
activity at all, even after 1 h (entry 5). At the moment
we have no decisive explanation why this reference
compound shows virtually no activity. It might origi-
nate from unfavorable coordination to the metal center,
arising from constraints in the ligand structure, such as
a large Tolman angle.10 We are currently investigating
this in more detail. Also the simple silylphosphite 5
showed little activity under these conditions (entry 6),
probably due to the electronic character of the ligand
which prevents favourable coordination to the rhodium
atom (Table 1).
The appropriate regions of the 13C and 29Si NMR
spectra of 1–3 clearly demonstrate the incorporation of
the phosphorus atom into the silsesquioxane skeleton
for each of the novel monophosphites. The 2:2:1:1:1
ratio of the signals for the ipso-carbons of the organic
side group on the silicon corner-atoms remains but with
chemical shifts of 0.1–0.5 ppm compared to the
monosilylated starting material while the 29Si NMR
spectrum shows the same pattern, indicative of a struc-
ture where the two remaining silanols are attached to a
phosphorus atom (Fig. 2).
In an attempt to compare reactivity and electron-with-
drawing character of phosphites derived from
silsesquioxanes with phosphites based on simple silox-
anes, disilanol (HO)Si(Ph)2OSi(Ph)2(OH) 5 was selected
as the backbone for the preparation of such a com-
pound. Synthesis of the corresponding phosphite, fol-
lowing the same procedure as for 2, however, failed
(Scheme 2). Further investigations and comparison
with literature showed that starting compound A had a
completely different electronic character, resulting in
Scheme 1. Synthetic route to compounds 1–3: Reagents and conditions: (i) ClSiMePh2, NEt3, THF, 1 h; (ii) PCl3, NEt3, THF,
−15°C, 30 min; (iii) desired phenol, THF, −78°C, 15 min, then rt, 16 h.

J. I. 6an der Vlugt et al. / Tetrahedron Letters 44 (2003) 8301–8305
8303
Figure 2. 13C NMR spectra of (c-C5H9)7Si7O9(OH)2(OSiMePh2) (top) and compound 1 (middle); 29Si NMR spectrum of 1
(bottom).
Scheme 2.

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J. I. 6an der Vlugt et al. / Tetrahedron Letters 44 (2003) 8301–8305
Scheme 3.
Table 1. Rhodium catalyzed hydroformylation of 1-octene using monophosphite ligands 1–5a
Entry
Ligand
Time (min)
S/Rh ratio
Conversion [%]b
Selectivity aldehydes [%]b
l/b ratiob
TOFc
1
2
3
4
5
6
1
1
2
3
4
5
15
15
20
20
60
20
4000
8000d
4000
4000e
4000
4000
31.5
14.5
22.9
17.8
<1%f
2.2
94.5
95.5
94.4
94.8
–
2.2
2.1
2.1
2.1
–
5000
4500
2500
2200
–
31.9
2.2
260
a Reaction conditions: 1-octene (31.0 mmol), decane (12.5 mmol), toluene (12.7 mL); p=20 bar CO/H2 (1:1); T=80°C; [Rh(acac)(CO)2]=0.39
mM; ligand:Rh=10:1.
b Determined by GC analysis.
c Turnover frequency, defined as (mol octene converted) (mol rhodium)−1 hour−1
.
d [Rh(acac)(CO)2]=0.20 mM.
e Ligand:Rh=5:1.
f Below detection limit.
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In summary, we have described the synthesis of three
new monophosphites 1–3 based on an incompletely
condensed silsesquioxane framework. Rhodium cata-
lysts containing these ligands show high activities in the
hydroformylation of 1-octene, with turnover frequen-
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Acknowledgements
We are indebted to Drs. Tessa Dijkstra for measuring
the 29Si NMR spectra. We thank the National Research
School Combination for Catalysis (NRSCC) for the
financial support. Jens Ackerstaff thanks the Erasmus
Program of the European Union for a travel grant. The
OMG Group is acknowledged for a generous loan of
Rh(acac)(CO)2.
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1
solidified upon standing, of the pure product. H NMR
(400 MHz, CDCl3, ppm) l 7.63 (dd, J1=7.3 Hz, J2=2.2
Hz), 7.31 (m), 7.11 (t), 6.95 (t), 6.80 (d, J=8.8 Hz), 1.77
(br m C5H9), 1.58 (br m C5H9), 1.28 (br m C5H9), 0.71 (s,
CH3) 13C NMR (100 MHz, CDCl3, ppm) l 137.9, 134.2,
129.1, 127.4, 123.0, 121.0, 120.9, 31.7, 27.6, 27.5, 27.4,
27.3, 27.2, 27.1, 27.0, 26.9, 24.1, 23.0, 22.6, 22.4, 21.9
(1:2:2:1:1 for CipsoH), −0.6 (SiMePh2) 31P NMR (162
MHz, CDCl3, ppm) l 122.6 (s) 29Si NMR (99 MHz,
CDCl3, ppm) −10.58 (SiMePh2), −64.01, −65.02, −65.06,
−66.60, −67.65 (1:2:1:1:2).
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13. Synthesis of compounds 1–3, illustrated for compound 1:
To a solution of (c-C5H9)7Si7O9(OH)2(OSiMePh2) (1.76
g, 1.64 mmol) in 25 mL THF was added NEt3 (0.83 g,
8.20 mmol). At −15°C PCl3 (236 mg, 1.72 mmol) was
added dropwise by syringe. After stirring for 1 h at rt, the
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