Continuous asymmetric hydrogenation in supercritical carbon dioxide using an immobilised homogeneous catalyst

Article abstract of DOI:10.1002/adsc.200606172

Continuous flow supercritical carbon dioxide (scCO₂) has previously been shown (P. Stephenson, P. Licence, S. K. Ross, M. Poliakoff, Green Chem. 2004, 6, 521) to be a viable medium for conducting continuous asymmetric hydrogenation when it is combined with an appropriate enantioselective catalyst. Here we examine the use of a composite catalyst immobilisation system modified with several different types of asymmetric bisphosphine ligands in continuous flow ScCO₂. In particular, proprietary ligands from Solvias AG were found to be the most successful, with Josiphos 001 improving the enantiomeric excess (ee) to > 80 % in the asymmetric hydrogenation of dimethyl itaconate (DMIT); this ee is higher than that reported for the batch hydrogenation of DMIT using a homogeneous catalyst fully dissolved in scCO₂ (S. Lange, A. Brinkmann, P. Trautner, K. Woelk, J. Bargon, W. Leitner, Chirality 2000, 12, 450).

Full text of DOI:10.1002/adsc.200606172

FULL PAPERS
DOI: 10.1002/adsc.200606172
Continuous Asymmetric Hydrogenation in Supercritical Carbon
Dioxide using an Immobilised Homogeneous Catalyst
Phil Stephenson,a Betti Kondor,a Peter Licence,a Katherine Scovell,a
Stephen K. Ross,b and Martyn Poliakoffa,*
a
School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, UK
Phone: (+44)-115-951-3520; Fax: (+44)-115-951-3058; e-mail: martyn.poliakoff@nottingham.ac.uk
Thomas Swan & Co., Ltd., Crookhall, Consett, DH8 7ND, UK
b
Phone: (+44)-1207-505131; Fax: (+44)-1207-590467; e-mail: sross@thomas-swan.co.uk
Received: April 10, 2006; Accepted: July 4, 2006
Abstract: Continuous flow supercritical carbon diox- meric excess (ee) to>80% in the asymmetric hydro-
ide (scCO2) has previously been shown (P. Stephen- genation of dimethyl itaconate (DMIT); this ee is
son, P. Licence, S. K. Ross, M. Poliakoff, Green higher than that reported for the batch hydrogena-
Chem. 2004, 6, 521) to be a viable medium for con- tion of DMIT using a homogeneous catalyst fully dis-
ducting continuous asymmetric hydrogenation when solved in scCO2 (S. Lange, A. Brinkmann, P. Traut-
it is combined with an appropriate enantioselective ner, K. Woelk, J. Bargon, W. Leitner, Chirality 2000,
catalyst. Here we examine the use of a composite 12, 450).
catalyst immobilisation system modified with several
different types of asymmetric bisphosphine ligands in
continuous flow scCO2. In particular, proprietary li- Keywords: asymmetric hydrogenation; continuous
gands from Solvias AG were found to be the most reactions; immobilized homogeneous catalyst; multi-
successful, with Josiphos 001 improving the enantio- phase catalysis; supercritical carbon dioxide
Introduction
metal-based catalysis.[10] However, there are surpris-
ingly few examples of the combination of asymmetric
Increasing environmental awareness and regulation catalysis and SCFs. The reactions that have been suc-
have placed constraints on the use of ꢀtraditionalꢁ cessfully demonstrated include hydrogenations,[11–14]
chlorinated solvents and volatile organic compounds hydroformylations,[15,16] oxidations,[17,18] cyclopropa-
(VOCs). Alternative solvents such as ionic liquids and nation,[19] hydrovinylation[20] and addition reactions
supercritical fluids (SCFs) are becoming increasingly of organozinc[21] and a single example of an aldol re-
accepted as viable alternative reaction media for con- action.[22] The vast majority of these reactions have
ducting synthetically valuable chemical transforma- been conducted under batch conditions. Exceptions to
tions.[1–4] SCFs, particularly supercritical carbon diox- this include the hydrovinylation of styrene[23] using a
ide (scCO2), are attractive alternative solvents be- nickel catalyst immobilised in an ionic liquid by Leit-
cause of their unique combination of gas-like and ner et al. that produced enantiomeric excess (ee)
liquid-like physical properties. The solubility of many values up to 95%. Recently, Leitner has also demon-
reagents, including gases and high diffusivity can lead strated outstanding enantioselectivities (ee>95%) in
to increased reaction rates.[5] Also, product recovery an inverted CO2/aqueous biphasic semi-continuous
is facile by simple depressurisation of the reaction flow media.[24] However, the first example of continu-
mixture. In addition, when combined with a fixed cat- ous asymmetric hydrogenation in SCFs was reported
alyst bed SCFs can also be used in continuous flow re- by Baiker and co-workers[25] who used cinchonidine-
gimes which are highly efficient[6] and often favoured modified Pt in the continuous asymmetric hydrogena-
by industry.[7]
tion of ethyl pyruvate, to achieve an ee of up to 75%
Catalytic asymmetric hydrogenation reactions for in supercritical C2H6. However, enantioselectivities in
the production of enantio-enriched and enantio-pure scCO2 were not as encouraging.
compounds have also received significant interest
Perhaps the main reason for the lack of asymmetric
over the past two decades.[8,9] A wide variety of chiral transformations in supercritical continuous flow re-
ligands has been developed for use in transition gimes is the fact that reliable immobilisation of a ho-
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Phil Stephenson et al.
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mogeneous catalyst is difficult to achieve. There are
several criteria that have to be satisfied if a catalyst
immobilisation method is to be considered a success;
the method of support has to be strong enough to
withstand the reaction conditions but subtle enough
not to interfere with the catalytic cycle, similarly the
immobilisation system must not interfere with the
asymmetric modifier otherwise the stereoselectivity of
the system could be reduced. This ꢀbalancing actꢁ be-
tween immobilisation and catalytic effect is all the
more crucial in any continuous flow system. If the im-
Scheme 1. The catalytic asymmetric hydrogenation of DMIT
using the immobilised homogeneous catalyst.
mobilisation method is too weak, either the entire tems.[28] In this particular case,[26] we used the catalyst
catalyst or just the asymmetric ligand will be flushed [Rh(S,S-Skewphos)
(nbd)]+[BF4]À immobilized[27] on
away in the passing reaction mixture. Detachment is g-alumina via phosphotungstic acid linker
A
ACHTREUNG
a
less of a problem in batch reactions because, in princi- H3O40PW12. Although respectable conversions (66%)
ple at least, the catalyst has the opportunity to reat- and ee (63%) were achieved[26] the true power of this
tach itself to the support. As no single ligand is suita- composite catalyst system lies in the fact that it can
ble for all asymmetric transformations, a further crite- be used with a wide range of chiral bisphosphine li-
rion must be added, namely that the immobilised cat- gands, Scheme 2.
alyst should be usable with a variety of ligands and,
In this paper, we expand the scope of this investiga-
indeed, be able to be screened for use with specific tion by screening other chiral bisphosphine ligands, to
substrates. Finally, smaller continuous flow reactors improve the hydrogenation of dimethyl itaconate,
are commercially more viable;[2b] therefore the reac- with the ultimate goal of increasing enantioselectivity
tor temperature is usually elevated to increase pro- and improving ee. Achieving high conversions was not
ductivity, placing additional demands on the thermal of such a high priority. Provided that the reaction rate
stability of both catalyst and support.
is sufficient for a reaction to occur, low conversions
Previously we have reported the asymmetric hydro- can be improved simply by increasing the residence
genation of dimethyl itaconate[26] (DMIT), using an time of the reactor. In principle, any bisphosphine
established immobilised homogeneous asymmetric ligand can be incorporated into this composite system
catalyst,[27] in continuous flow scCO2, Scheme 1.
DMIT is often used as a prototypical molecule
when evaluating new asymmetric hydrogenation sys-
by the ligand substitution reaction in Scheme 2
Scheme 2. The ligand substitution of a diene with the bisphosphine ligands shown. The diene ligand used in this paper was
cycloctadiene (COD); PTA=phosphotungstic acid, H3O40PW12.
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Continuous Asymmetric Hydrogenation in Supercritical Carbon Dioxide
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Results and Discussion
asymmetric hydrogenation of DMIT in scCO2, pre-
sumably because under these conditions the system is
The hydrogenation of DMIT was used to investigate not limited by mass transport of H2. (iii) Skewphos
the performance of the ligands shown in Scheme 2. (Entries 1 and 2) gave a maximum ee, 68% at
The results are presented in Table 1. The goal of this 17MPa, this is comparable to the ee (72%) reported
by Leitner and co-workers in their batch hydrogena-
tion with a fully homogeneous catalyst.[32] (iv) Pro-
Table 1. Hydrogenation of DMIT with various bisphosphine
phos has a structure closely related to that of Skew-
phos so it is quite surprising that Prophos gave essen-
tially racemic product (Entries 3 and 4); it is unlikely
that the Prophos was leaching from the catalyst since
it gave less conversion than the control experiment
without phosphines (Entry 9). This was supported by
ICP-MS data which indicated that at 608C very little
Rh leaching occurs (i.e., [Rh]<1 ppm) but that at
1008C leaching became significant ([Rh]=8.6 ppm).
Although clearly less sensitive, 31P NMR of selected
samples collected at 608C showed no traces of phos-
phine present. (v) Neither DIOP nor BINAP were
particularly successful ligands in terms of asymmetric
hydrogenation (Entries 5–8), although DIOP gave
much higher conversions than BINAP; this result is
somewhat unexpected because both ligands perform
successfully in conventional solvents.[29] Possibly, the
difference in behaviour in scCO2 may be due to the
low dielectric constant of this solvent.[33] However, it
is known that hydrogenation under these conditions
in scCO2 is usually a multi-phase process with both a
liquid and gas phase.[7b] The bulk of the substrate is
found in the condensed phase but the high CO2 con-
centration in this phase could still lead to a reduction
in the dielectric constant of this phase when com-
pared to pure organic. Also it is possible that, in our
experiments, DIOP and BINAP could have been oxi-
dised by trace impurities (in this case<0.01% wt) in
the flowing scCO2.
ligands combined with the CATAXAꢃ immobilised catalyst
system.[a]
Entry Ligand
Temp.
[8C]
Pressure
[MPa]
Conv.[b]
[%]
ee
[%][c]
1
2
3
4
5
6
7
8
9
Skewphos 40–80
Skewphos 30–70
Prophos 40–70
Prophos 50–70
10
17
10
20
10
16
10
20
10
10–66
20–65
30–34
14–37
42–61
45–67
5–19
5–63
9–68[e]
0–1
0–1
DIOP
35–55
35–55
30–50
30–50
40
10–18
10–12
0–11
9
DIOP
BINAP
BINAP
COD[d]
20–26
58
0
[a]
Reaction conditions: CO2 flow rate 0.5 mLminÀ1 (lique-
fied at À108C), substrate flow rate 0.15 mLminÀ1 (2.5M
solution in 2-propanol), H2:substrate ratio 2.5:1, dosed
with a 5MPa overpressure.
Low temperatures correspond to low conversions, with
increasing reactor temperature the conversions increased
accordingly. Reactants and products were quantified
using a Shimadzu GC-2010 GLC equipped with a b-
DEX 110 chiral column supplied by Supelcoꢃ.
[b]
[c]
The % enantiomeric excess (ee) was calculated from GC
peak areas = 100 ” [Area(A)ÀArea (B)]/
The unmodified CATAXAꢃ (Entry 9) is included as a
control reaction.
A
Area(B)].
[d]
[e]
Maximum ee was observed at 308C with a corresponding
conversion of 20%.
The second part of our investigation involved bi-
study was to obtain enantioselectivities as high as pos- sphosphine ligands, Figure 1, based on a highly stable
sible, therefore for conciseness, the results are pre- ferrocenyl backbone, which have been highly success-
sented as a range. For each point in the range, the ful in asymmetric hydrogenation in conventional sol-
value was confirmed by maintaining the experiment vents.[30] The results obtained using these ligands in
under steady state conditions over several hours and scCO2 are summarised in Table 2.
taking repeat samples every 20 min. Importantly nei-
It can immediately be seen from Table 2 that the
ther conversion nor ee were observed to degrade over overall activity of these ligands was generally lower
time, under steady state conditions. This resulted in than those listed in Table 1. However, like those li-
minimum catalyst turnover numbers (TONs) of 250 gands there was a range of asymmetric induction. The
for all samples, the maximum recorded is 4600 [moles ligands incorporating nitrogen moieties, BoPhoz, Ta-
product]
corresponds to a turnover frequency (TOF) of 560 clear why, although CO2 can react with primary
[moles product] amines to form carbamates and, if wet, could in prin-
[moles Rh]À1 hÀ1 for an 8-hour run.
[moles Rh]À1 at 608C and 10MPa and this niaphos and Mandyphos all gave very low ee. It is not
ACHTREUNG
A number of points are clear from Table 1. (i) ciple form carbonates with any amine. BoPhoz is in-
Skewphos (Entries 1 and 2) gives the best ee values teresting, as it is essentially inactive until tempera-
yet with conversions that can be as high as that of the tures of ca. 1008C and then the product formed is rac-
unmodified CATAXAꢃ (Entry 9). (ii) The system emic (Entry 2). The lack of enantioselectivity is prob-
pressure does not have a significant effect on either ably to be expected because of the high temperature;
ee or conversion (Entries 1 and 2); a similar lack of indeed, it is possible that the ligand decomplexes. The
pressure effect has been reported[32] for the batch inactivity of BoPhoz is surprising since there are only
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Phil Stephenson et al.
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Figure 1. Structures of the ferrocenyl-based ligands used in Table 2.
Table 2. Results for the continuous hydrogenation of DMIT using ferrocenyl-type catalysts.[a]
Entry
Ligand
Temperature [8C]
Pressure [MPa]
Conversion [%][b]
ee [%]
1
2
3
4
5
6
7
8
BoPhoz
BoPhoz
30–100
30–100
35–55
35–55
35–55
35–55
35–55
35–55
35–55
35–55
35–55
35–55
35–55
35–55
10
20
10
17
10
17
10
17
10
17
10
17
10
16
0–12
0–15
6–12
1–3
46–59
22–42
12–14
11–17
14–56
41–57
20–55
3–28
0
0
Josiphos 005
Josiphos 005
Walphos 002
Walphos 002
Mandyphos 001
Mandyphos 001
Taniaphos 001
Taniaphos 001
Taniaphos 002
Taniaphos 002
Josiphos 001
Josiphos 001
5–37
1–9
43–57
23–44
1–2
1–3
6–10
7–8
9–15
3–15
0–51
58–83[c]
9
10
11
12
13
14
0–19
24–35
[a]
Using the same reaction conditions and analytical method as in Table 1.
Low temperatures correspond to low conversions, with increasing reactor temperature the conversions increased accord-
[b]
ingly.
[c]
Maximum ee was observed at 358C with a corresponding conversion of 24%.
relatively small structural differences between greater effect on the metal centre. This effect was ex-
BoPhoz and Josiphos, and both ligands give good ee pected to reduce conversion but lead to an increase in
in the asymmetric hydrogenation of DMIT in conven- ee. However, this is not necessarily the case; e.g.,
tional solvents such as ethanol.[31] It is therefore quite Mandyphos 001 (Entries 7/8) gives low conversion
possible that the polarity of the solvent has an effect combined with low ee. The best results were obtained
on the ee produced in the reactions.
with Walphos 002 (Entries 5/6) and Josiphos 001 (En-
Throughout this investigation it was hoped that li- tries 13/14). There was some effect of pressure; for
gands with a stronger chiral motif would exert a Walphos 002, higher pressures appear to give slightly
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Continuous Asymmetric Hydrogenation in Supercritical Carbon Dioxide
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higher ee but the effect may not be significant. The
most important observation is that Josiphos gave an
ee of 83% at 558C (Entry 14). This is a higher ee than
reported[32] for the fully homogeneous hydrogenation
of DMIT in scCO2, albeit with a different chiral bis-
phosphine.
Conclusions
This study has highlighted the fact that immobilising
an asymmetric catalyst and retaining high enantiose-
lectivity is by no means a trivial task, especially when
the catalyst is to be used in a solvent as unusual as
scCO2. Combining this with the elevated tempera-
tures typically employed in a continuous flow scCO2
reactor these problems are compounded. We have
shown that the CATAXAꢃ approach can be used
with a wide range of different chiral bisphosphine li-
gands and is therefore a powerful tool for immobi-
lised asymmetric catalysis.
From the results presented in this paper, it can be
seen that the delicate balancing act of catalytic activi-
ty and enantioselectivity is further strained by being
placed in a continuous flow scCO2 solvent. Although
the majority of ligands were unsuccessful, Josiphos
001 has produced an ee of 83% at 558C. This clearly
Figure 2. Schematic of the reactor and components; hydro-
gen gas is pressurised to a constant pressure using a gas
booster, GB-1 [NWA GmbH, CU-105], and regulator, R-1,
before being dosed into the reactor by a gas dosage unit,
GD-1 [Rheodyne 7000 L]; the supply is isolable using the
ball valve (BV-1). The CO2 is pressurised and delivered by
SCF-1 [Jasco PU-1580-CO2]. The organic substrate is deliv-
ered via a standard HPLC pump, SP-1 [Jasco PU-980], all
components are connected by 1/16 inch 316 stainless steel
tubing. The tubular reactor holds approximately 0.1 g of cat-
alyst.
demonstrates that a good ee is achievable in a contin- Catalyst Preparation
uous flow scCO2 hydrogenation reactor. With further
optimisation, this value can most probably be in-
creased.
All the catalyst preparations were performed under an inert
atmosphere using appropriate Schlenk techniques.[27] [Rh-
(COD)2]+[BF4]À/PTA/alumina (0.25 g, 8.75”10À3 mmol Rh;
A
ACHTREUNG
supplied until recently by Johnson Matthey under the trade
name, CATAXAꢃ) and 13”10À2 mmol of ligand (supplied
by Johnson Matthey or Solvias AG) were added to 30 mL of
ethanol and the solution was left to stir overnight. The etha-
nol was removed and the catalyst washed with 3”15 mL of
ethanol. The wash solution was removed by cannula and the
remaining solid was dried under vacuum to yield a free
flowing powder, usually with a pale yellow colouration.
Experimental Section
Safety note: All supercritical experiments involve the use of
relatively high pressures and require equipment with the ap-
propriate pressure rating. It is the responsibility of individu-
al researchers to verify that their particular apparatus meets
the necessary safety requirements. The individual compo-
nents that we describe work well, but they are not necessari-
ly the only equipment of this type available nor the most
suitable for the purpose.
Acknowledgements
Continuous Flow Hydrogenation Apparatus
We would like to thank our colleagues in the Clean Technolo-
gy Group at the University of Nottingham, in particular to
Messrs M. Guyler, M. Dellar, P. Fields. R. Wilson for their in-
valuable technical assistance. We are grateful to Drs. J. R.
Hyde, B. Walsh, E. Rowsell, J. Rouzoud and H.-U. Blaser for
helpful discussions. We are also grateful for materials sup-
plied by Johnson Matthey, Solvias AC and Eastman Chemi-
cals. We thank Thomas Swan & Co. Ltd., the CRYSTAL
Faraday Partnership, the EPSRC (Grant No. GR/R02863)
and the EU Marie Curie EST network, SubClean Probiomat
for financial support. PL thanks the Leverhulme Trust for an
Early Career Research Fellowship.
The apparatus for small-scale continuous flow reactions has
been described previously.[26] The system is essentially mod-
ular, made from commercially available components. A
schematic diagram of the reactor and its components can be
seen in Figure 2.
Adv. Synth. Catal. 2006, 348, 1605 – 1610
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