Fast and unprecedented chemoselective hydroformylation of acrylates with a fluoropolymer ligand in supercritical CO2

Article abstract of DOI:10.1039/b201166b

A fluorous polymeric phosphine, when combined with supercritical CO₂ (scCO₂) and rhodium, effects fast and highly chemoselective hydroformylation of acrylates, one of the least reactive olefins in hydroformylation reactions.

Full text of DOI:10.1039/b201166b

Fast and unprecedented chemoselective hydroformylation of acrylates
with a fluoropolymer ligand in supercritical CO2
Yulai Hu, Weiping Chen, Anna M. Banet Osuna, Jon A. Iggo and Jianliang Xiao*
Leverhulme Centre for Innovative Catalysis, Department of Chemistry, University of Liverpool, Liverpool,
UK L69 7ZD. E-mail: j.xiao@liv.ac.uk; Fax: (+44) 151 7943589; Tel: (+44) 151 7942979
Received (in Cambridge, UK) 31st January 2002, Accepted 4th March 2002
First published as an Advance Article on the web 13th March 2002
A fluorous polymeric phosphine, when combined with
supercritical CO2 (scCO2) and rhodium, effects fast and
highly chemoselective hydroformylation of acrylates, one of
the least reactive olefins in hydroformylation reactions.
lower rates in the liquid solvents cannot be ascribed to
interfacial mass transfer limitations; varying the agitation speed
from 200 to 500 to 1100 rpm resulted in no significant variations
in TOFs of the reaction in toluene (agitation speed ~ 900 rpm in
Table 1). An additional demonstration of the effectiveness of
the fluorous tags in conferring solubility on arylphosphines is
seen in the same reaction in which PPh3, a ligand electronically
similar to 1, replaced 1 leading to a TOF of only 43.
Consequently, our subsequent studies were carried out with 1 at
a total pressure of 150 bar with a P+Rh ratio of 8. Using the
conditions established, we extended the reaction to methyl,
butyl, and tert-butyl acrylates. The results are given in Table 2.
In line with the literature,2 branched aldehydes dominated the
products of these reactions, as can been seen from the high
branched to linear aldehyde ratios (B+L). The ratio of the
hydrogenated product, propionates, ranged between 1 to 2% and
was lower than that observed with the molecular ligand P(p-
C6H4C6F13)3.3 Remarkably, all the acrylates underwent the
hydroformylation with TOF values greater than 2000. These
values are far higher than those obtained in common organic
solvents and represent one of the fastest rates reported so far for
acrylate hydroformylation.
Hydroformylation of acrylates produces synthetically valuable
bifunctional compounds. Unfortunately, acrylates are one of the
least reactive olefins towards H2–CO. Using [RhH(CO)(PPh3)3]
as catalyst precursor, the hydroformylation rate of methyl
acrylate was measured to be two orders of magnitude smaller
than that of olefins such as alk-1-enes, allyl alcohol, and
styrene.1 Not surprisingly, in common organic solvents, the
reaction is in general sluggish and high temperature and/or high
pressure are often required, and although lots of studies have
been carried out in the past three decades or so, few
breakthroughs have been achieved.2 We recently reported that
when scCO2 is used as reaction medium, rapid hydro-
formylation of alkyl acrylates can readily be effected by
[Rh(acac)(CO)2] and a fluorinated ligand P(p-C6H4C6F13)3.3
We now report that the combination of the fluoropolymeric
ligand 1,4 [Rh(acac)(CO)2], and the ecologically attractive
scCO2 brings about not only fast rates but unprecedented high
chemoselectivity as well in the hydroformylation of these
olefins. The incorporation of the fluorinated side chains in the
polymer was expected to enhance the solubility of ar-
ylphosphines that are usually only sparsely soluble in scCO2,
thus providing a simple way for performing catalysis with CO2-
insoluble metal–phosphine complexes.5
For instance, in the hydroformylation of ethyl acrylate in
ethylbenzene,9 complete conversion of the olefin, correspond-
ing to a TOF value of 413 h21, was only achieved in 5 h reaction
Table 1 Hydroformylation of ethyl acrylate with Rh–1 under various
conditionsa
Solvent
P barb
P+Rh
Ald. (%)c B+Ld
TOFe
CO2
CO2
CO2
CO2
200
150
150
150
120
30i
8
8
4
0
8
8
8
8
99.4
99.0
206
155
Bf
1255
2066
1251
99.3
g
h
CO2
Hexane
Toluene
CF3C6F11
99.5
99.7
99.6
Bf
Bf
Bf
408
405
440
30i
30i
We initially examined the hydroformylation of ethyl acrylate
catalysed by [Rh(acac)(CO)2]–1 under various conditions.
Table 1 summarises the results obtained. Intriguingly, the
average turnover frequency (TOF) for the formation of ethyl
2-formylpropionate varies with the pressure of CO2. Thus, the
TOF was found to be 1255 h21 at 200 bar, but rose to a higher
value of 2066 upon lowering the pressure to 150 bar. The
change in rate with CO2 pressure could be due to 1 adopting
structures that vary with the density of CO2 (vide infra), as has
been shown for poly(vinyl acetate)-block-poly(fluoroacrylate)
copolymers at higher CO2 pressures.6 Importantly, the combi-
nation of 1 and high pressure CO2 proves to be essential for high
rates. Thus, when the quantity of 1 was reduced by half while
keeping the rhodium content constant, the TOF decreased from
2066 to 1251 and, in the absence of 1, no aldehydes could be
detected. At 120 bar total pressure, where liquid acrylate
separated from CO2, significant polymerization of the acrylate
resulted.7 Furthermore, when carried out in liquid solvents such
as toluene, hexane, and perfluoromethylcyclohexane, the latter
two resembling scCO2 in terms of solubility parameters,8 the
reaction was ca. 5 times slower. It should be pointed out that the
a Reactions were performed at olefin concentration = 0.37 mol dm23
(assuming all the olefin to be dissolved), olefin to rhodium molar ratio =
6000, 30 bar H2–CO (1+1) and 80 °C for 1 h, with the product analysed by
GC. b Total pressure. c Selectivity to aldehydes. The hydrogenation product
ethyl propionate accounts for the product balance. d Branched to linear
aldehyde molar ratio. e Average turnover frequency: mole of aldehydes
formed per mole of catalyst per hour. f Linear aldehyde was not detected.
g No reaction. h ca. 20% of the acrylate polymerised (1H NMR). i No
CO2.
a
Table 2 Hydroformylation of acrylates with Rh–1 in scCO2
Substrate
[Olefin]b Conv. (%)c Ald. (%) B+L
TOF
Methyl acrylate
Ethyl acrylate
Butyl acrylate
0.45
0.37
0.28
38.2
35.5
35.7
35.6
98.4
99.0
99.0
99.2
155
121
117
111
2196
2066
2081
2087
tert-Butyl acrylate 0.28
.
a See also Table 1. b Olefin concentration, mol dm23 c Conversion of the
acrylates.
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CHEM. COMMUN., 2002, 788–789
This journal is © The Royal Society of Chemistry 2002

time at 150 °C and ca. 100 bar syngas using [RhCl(CO)2]2 in the
presence of bis(diphenylphosphino)butane, one of the best
ligands known for this type of reaction.2 In our previous study,
several monodentate phosphines were examined for the hydro-
formylation of butyl acrylate in toluene, the best being P(p-
C6H4C6F13)3, which afforded a TOF of 122 at 50 bar H2–CO
and 80 °C.3 Much improved rates have recently been obtained
when the reaction is run in the presence of water; but the rates
are markedly affected even by a slight alteration in water
content and by the solubility of acrylates in water.2b,d
CO2 coordination to rhodium.3,11 The unique chemoselectivity
of 1 in favour of acrylates over alk-1-enes could arise from a
stronger acrylate binding to rhodium and be related to the
structure of 1 in scCO2. 1 contains CO2–philic poly(fluor-
oacrylate) as well as CO2-insoluble styryldiphenylphosphine
segments, which can prompt the formation of aggregates or
micellar structures in scCO2,6 resulting in differential partition-
ing of olefins with differing polarities in and out of the catalyst-
containing aggregates.12
To summarise, the results presented show that the combina-
tion of scCO2, 1 and rhodium brings about not only excellent
rates but also unique chemoselectivities in the hydroformylation
of usually unreactive alkyl acrylates, suggesting that engineered
macromolecular catalysts coupled with scCO2 could lead to
novel activities and selectivities in reaction chemistry.
We thank the EPSRC for postdoctoral research fellowships
(Y. H. and W. C.) and the Industrial Partners of LCIC (Synetix,
Johnson Matthey, Air Products and Syntroleum) for support.
We also thank Professor David Cole-Hamilton for advice.
Significantly more interesting is the observation that 1 is
totally chemoselective towards acrylates. Under similar reac-
tion conditions in scCO2, dec-1-ene, hex-1-ene, styrene, and
vinyl acetate all failed to react with H2–CO. The only exception
is allyl alcohol, which was converted to branched and linear
aldehydes with a TOF of 118. This is remarkable, considering
that all of these olefins have previously been shown to be much
more reactive than methyl acrylate towards syngas in common
solvents,1 and alk-1-enes usually yield high TOF values in
hydroformylations catalysed by soluble rhodium catalysts in
scCO2.10 A further demonstration of the chemoselectivity of 1
is seen in the hydroformylation of an equimolar mixture of dec-
1-ene and ethyl acrylate (olefin concentration = 0.14 mol
dm23) under conditions similar to those used for the pure
acrylate (Scheme 1). A TOF of 872 was measured for ethyl
acrylate converting into the branched aldehyde (B+L = 92).
The lower TOF in comparison with that obtained with pure
ethyl acrylate is likely to be a result of decreased olefin
concentration. In stark contrast, no product arising from dec-
1-ene could be detected by GC. A more striking example is the
reaction of but-3-enyl acrylate. The CNC bond adjacent to the
carbonyl group was hydroformylated to give but-3-enyl 2-for-
mylpropionate with a TOF of 1525, while the remote CNC bond
remained intact according to NMR and GC measurements.
It is generally believed that the low hydroformylation rates
associated with acrylates in common organic solvents stem
from the formation of thermodynamically stable five- or six-
membered rings via the coordination of the acrylate carbonyl
group to rhodium, with the reaction rate determined by opening
of the ring to give a coordinatively unsaturated intermediate.2a,d
The faster rates in scCO2 may result from CO2 facilitating the
ring opening step by CO2–carbonyl acid–base interactions and
Notes and references
1 B. Breit and W. Seiche, Synthesis, 2001, 1 and references therein.
2 (a) C. W. Lee and H. Alper, J. Org. Chem., 1995, 60, 499 and references
therein; (b) G. Fremy, E. Monflier, J. F. Carpentier, Y. Castanet and A.
Mortreux, Angew. Chem., Int. Ed. Engl., 1995, 34, 1474; (c) G. Fremy,
Y. Castanet, R. Grzybek, E. Monflier, A. Mortreux, A. M. Trzeciak and
J. J. Ziolkowski, J. Organomet. Chem., 1995, 505, 11; (d) G. Fremy, E.
Monflier, J. F. Carpentier, Y. Castanet and A. Mortreux, J. Mol. Catal.,
1998, 129, 35; (e) H. K. Reinius and A. O. I. Krause, J. Mol. Catal.,
2000, 158, 499.
3 Y. Hu, W. Chen, A. M. Banet-Osuna, A. M. Stuart, E. G. Hope and J.
Xiao, Chem. Commun., 2001, 725.
4 The phosphorus content of 1 was estimated to be 1.2%. W. Chen, L. Xu
and J. Xiao, Chem. Commun., 2000, 839; For a similar ligand, see: D.
E. Bergbreiter, J. G. Franchina and B. L. Case, Org. Lett., 2000, 2,
393.
5 For recent reviews on catalysis in scCO2, see: (a) R. Noyori, Guest Ed.,
Chem. Rev., 1999, 99, 353–633; (b) T. Ikariya and Y. Kayaki, Catal.
Surv. Jpn., 2000, 4, 39; (c) R. S. Oakes, A. A. Clifford and C. M. Rayner,
J. Chem. Soc., Perkin Trans. 1, 2001, 917.
6 S. L. Wells and J. DeSimone, Angew. Chem., Int. Ed. Engl., 2001, 40,
518.
7 Above this pressure, the reaction mixture was homogeneous, but a small
portion of 1 remained as liquid even at 200 bar, probably due to a broad
molecular weight distribution.
8 C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, VCH,
Weinheim, 1988.
9 M. Tanaka, T. Hayashi and I. Ogata, Bull. Chem. Soc. Jpn., 1977, 50,
2351.
10 For examples, see: (a) W. Koch and W. Leitner, J. Am. Chem. Soc.,
1998, 120, 13398; (b) T. Davis and C. Erkey, Ind. Eng. Chem. Res.,
2000, 39, 3671; (c) A. M. Banet-Osuna, W. Chen, E. G. Hope, R. D. W.
Kemmitt, D. R. Paige, A. M. Stuart, J. Xiao and L. Xu, J. Chem. Soc.,
Dalton Trans., 2000, 4052.
11 For references on carbonyl-CO2 interactions, see: (a) S. G. Kazarian, M.
F. Vincent, F. V. Bright, C. L. Liotta and C. A. Eckert, J. Am. Chem.
Soc., 1996, 118, 1729; (b) T. Sarbu, T. Styranec and E. J. Beckman,
Nature, 2000, 405, 165.
12 For examples of reactions conducted in micelles/microemulsions in
scCO2, see: M. A. Kane, G. A. Baker, S. Pandey and F. V. Bright,
Langmuir, 2000, 16, 4901 and references therein.
Scheme 1 Chemoselective hydroformylation of CNC bonds in scCO2
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