Novel arsine ligands for selective hydroformylation of alk-1-enes employing platinum/tin catalysts

Article abstract of DOI:10.1039/a906903h

The synthesis and application of new wide bite angle arsine based ligands in the platinum/tin-catalysed hydroformylation of oct-1-ene is reported; an unprecedented high activity and selectivity is obtained employing a mixed phosphine/arsine ligand.

Full text of DOI:10.1039/a906903h

Novel arsine ligands for selective hydroformylation of alk-1-enes employing
platinum/tin catalysts†
Lars A. van der Veen, Peter K. Keeven, Paul C. J. Kamer and Piet W. N. M. van Leeuwen*
Institute of Molecular Chemistry, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The
Netherlands. E-mail: pwnm@anorg.chem.uva.nl
Received (in Basel, Switzerland) 25th August 1999, Accepted 20th January 2000
The synthesis and application of new wide bite angle arsine
based ligands in the platinum/tin-catalysed hydroformyla-
tion of oct-1-ene is reported; an unprecedented high activity
and selectivity is obtained employing a mixed phosphine/
arsine ligand.
Hydroformylation of alkenes is one of the world’s largest
homogeneously catalysed reactions in industry (Scheme 1),
producing more than six million tons of aldehydes and alcohols
annually.1 The commercial hydroformylation processes are run
exclusively on cobalt or rhodium complexes as catalysts.
Platinum complexes also give active hydroformylation cata-
lysts, but are mainly of academic interest. Both terminal and
internal alkenes can be hydroformylated selectively employing
platinum–diphosphine complexes activated by tin chloride as
the co-catalyst.2 Tin-free catalyst systems have been reported as
well.3 Despite the very high linear over branched (l+b) aldehyde
ratios induced by the platinum/tin–diphosphine catalysts, these
systems have mainly been applied to asymmetric hydro-
formylation so far.1,4 The major drawbacks of these catalysts
are extensive isomerisation and hydrogenation of the substrate
alkenes.
Both in the rhodium- and in the platinum/tin-catalysed
hydroformylation of alk-1-enes, widening of the natural bite
angle of the diphosphine ligands has proven to be favourable for
the catalytic performance.2a,5 Recently, it was also demon-
strated that in the selective hydroformylation of internal alkenes
toward linear aldehydes wide bite angle diphosphine ligands
can be very efficient.6 Based on these results we wondered
whether wide natural bite angles could also improve the
catalytic performance of ligands having donor atoms other than
phosphorus. Since the xanthene backbone is an excellent
scaffold for the construction of ligands with wide natural bite
angles, we set out to synthesise the (mixed) group 15 derivatives
of the xantphos ligand 1 (Fig. 1). Here, we report the synthesis
of the arsine analogues of xantphos 1 and their excellent
performance in the platinum/tin-catalysed hydroformylation
reaction. To our knowledge, xantarsine and xantphosarsine
ligands 2 and 3 constitute the first efficient arsine modified
platinum/tin catalysts for selective hydroformylation of termi-
nal alkenes.
Fig. 1
2 in 83% yield. Xantphosarsine ligand 3 was obtained in 49%
yield by monolithiation of bromoxantphos 6 and subsequent
reaction with chlorodiphenylarsine. Bromoxantphos 6 was
synthesised by monolithiation of compound 5, followed by
reaction with chorodiphenylphosphine. The calculated natural
bite angles of ligands 1, 2, 3 and 4 are 110, 113, 111 and 102°,
respectively.7
Ligands 1–4 were tested in the platinum/tin-catalysed
hydroformylation of oct-1-ene (Table 1). In the hydro-
Scheme 2 Synthesis of ligands 1–3 and 6: (i) BunLi, THF, 265 °C; (ii)
Ph2PCl, THF, 265 to 25 °C, 81% (1), 48% (6); (iii) Ph2AsCl, THF, 265 to
25 °C, 83% (2), 49% (3).
The xantarsine and xantphosarsine ligands 2 and 3 were
synthesised via procedures similar to the synthesis of xantphos
1 (Scheme 2).6a Dilithiation of 4,5-dibromo-2,7-di-tert-butyl-
9,9-dimethylxanthene 5 with n-butyllithium in THF at 265 °C,
followed by reaction with chlorodiphenylarsine gave xantarsine
Table 1 Platinum/tin-catalysed hydroformylation of oct-1-ene at 60 °Ca
Ligand
l+b Ratiob
% n-Nonanalb
% Isomerizationb TOFbc
1
2
3
4
230
> 250
200
95
92
96
88
4.5
8.0
3.1
18
210
350
720
> 250
12
a Reactions were carried out in a 180 mL stainless steel autoclave in
dichloromethane at 60 °C under 40 bar of CO–H2 (1+1), catalyst precursor
Scheme 1 The hydroformylation reaction.
[Pt(cod)Cl2], [Pt]
= 2.5 mM, Pt+SnCl2+P+1-octene = 1+2+4+255.
b Determined by GC with decane as the internal standard. c Averaged
turnover frequencies (TOF) were calculated as (mol of aldehyde) (mol Pt)21
h21 at 20–30% conversion.
† Electronic supplementary information (ESI) available: full character-
isation data for the new compounds. See http://www.rsc.org/suppdata/cc/
a9/a906903h/
DOI: 10.1039/a906903h
Chem. Commun., 2000, 333–334
This journal is © The Royal Society of Chemistry 2000
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formylation of oct-1-ene the arsine based ligands 2 and 3 proved
to give more efficient catalysts than the parent xantphos ligand
1. The xantarsine ligand 2 is only slightly less selective than
xantphos 1, but more than 10 times as active. The xantphos-
arsine ligand 3 is even 20 times as active as xantphos 1, while
displaying the same excellent selectivity for linear aldehyde
formation. This is remarkable, since up to now, without
exception, arsine ligands have performed worse than phosphine
ligands in platinum/tin-catalysed hydroformylation.2c,3a,8 To
our knowledge the high activity and selectivity displayed by the
mixed xantphosarsine ligand 3 under these mild conditions is
unprecedented.
Comparison of the activities of the xantphos ligands 1 and 4
reveals a dramatic effect of the natural bite angle. Narrowing of
the natural bite angle from 110 to 102°, results in a 40 fold
higher hydroformylation rate. This is accompanied, however,
by a considerable increase in isomerization activity. As a result,
the selectivity for linear aldehyde obtained for xantphos 4 is
lower than that for xantphos 1. It is striking that the selectivities
of the xantphos ligands 1 and 4 observed in the platinum/tin-
catalysed hydroformylation are virtually identical to those
obtained before for rhodium.5c,6a
The high selectivities of ligands 1, 2 and 3 compared to
xantphos 4 can be ascribed to the wider natural bite angles of the
former ligands.‡ Widening of the bite angle of the ligand will
increase the steric congestion around the platinum centre
resulting in more selective formation of the sterically less
hindered linear aldehydes. An explanation for the higher
activities of ligands 2, 3 and 4 compared to xantphos 1 is still
lacking, but we speculate that it is caused by the coordination
behaviour of the ligands in the platinum/tin complexes.
Increasing the natural bite angle of bidentate ligands favours the
formation of trans complexes. Compared to xantphos 1,
xantphos 4 and the arsine ligands 2 and 3 probably give more or
easier formation of the cis-platinum complexes, a prerequisite
for efficient hydroformylation.10
In conclusion, wide bite angle arsine based ligands can give
very efficient catalysts for selective hydroformylation of
terminal alkenes. The catalytic performances of the arsine
modified platinum/tin systems can compete with the best results
obtained using rhodium catalysts.1,5c,11 Especially in applica-
tions where very high l:b ratios are a necessity these systems
could be interesting alternatives for rhodium–diphosphine
catalysts.
Financial support from the Technology Foundation (STW) of
the Netherlands Organization for Scientific research (NWO) is
gratefully acknowledged.
Notes and references
‡ When comparing the activities and selectivities of ligands 1, 2, and 3, it
should be noted that in general the s-donor ability and the steric effects of
substituents on the donor atom decrease in the order of P > As.9
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