Nickel-catalysed enantioselective hydrovinylation of styrenes in liquid or supercritical carbon dioxide

Article abstract of DOI:10.1039/a904799i

Compressed (liquid or supercritical) CO₂ is an environmentally benign reaction medium for the highly efficient regio-, chemo- and enantio-selective nickel-catalysed hydrovinylation of styrenes and it allows for catalyst recycling and selective

Full text of DOI:10.1039/a904799i

Nickel-catalysed enantioselective hydrovinylation of styrenes in liquid or
supercritical carbon dioxide
Andreas Wegner and Walter Leitner*
Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mu¨lheim an der Ruhr, Germany.
E-mail: leitner@mpi-muelheim.mpg.de
Received (in Cambridge, UK) 16th June 1999, Accepted 14th July 1999
Compressed (liquid or supercritical) CO2 is an environmen-
tally benign reaction medium for the highly efficient regio-,
chemo- and enantio-selective nickel-catalysed hydrovinyla-
tion of styrenes and it allows for catalyst recycling and
selective removal of the product from the reaction mix-
ture.
activity was observed using TfO2 or [Al(O(CH2)2C6F13)4]2 as
co-catalysts. The use of [Al(OC(Ph)(CF3)2)4]2 led to an active
catalytic system but reactivity and stereoselectivity were low.
Finally, we were pleased to find that the use of tetrakis[3,5-
bis(trifluoromethyl)phenyl]borate (BARF) yields a catalytic
system that is comparable to Et3Al2Cl3 in activity and
chemoselectivity but gives far better ees.† The use of BARF
activates 3, increases the thermal stability of the catalyst, and
provides sufficient solubility of the active species in liquid and
supercritical CO2,5 thus allowing the reaction to proceed close
to and beyond the critical point of pure CO2 (Tc = 31 °C, Pc =
73 atm, dc = 0.468 g cm23). The 4-substituted styrenes 1b,c
were also successfully reacted with ethene in liquid and
supercritical CO2 to yield 2b,c in high yield and with high ee.
We note that 2c is a key intermediate in a potential route to the
non-steroidal anti-inflammatory drug Ibuprofen™.1
The formation of carbon–carbon bonds is a fundamental
operation in synthetic organic chemistry. There is a great
demand for catalytic C–C coupling reactions that allow a high
level of chemo-, regio- and stereo-selectivity and can be carried
out under environmentally friendly conditions. A very efficient
catalytic carbon–carbon bond forming reaction is the transition
metal catalysed co-dimerisation of alkenes with ethene yielding
3-substituted but-1-enes (hydrovinylation).1 The hydrovinyla-
tion reaction is particularly intriguing because both starting
materials are incorporated entirely in the product and the
resulting double bond can be used for further transformations to
highly functionalised molecules. Wilke et al. introduced the
3
(h -allyl)nickel(ii) complex 3 (containing a unique chiral
1,2-substituted 1-azaphospholene ligand) as a catalyst precursor
for the asymmetric hydrovinylation of aromatic olefins
(Scheme 1).1,2 Complex 3 catalyses the enantioselective
hydrovinylation of styrenes at extremely high rates and with
unrivalled chemo-, regio- and enantio-selectivity, but it needs to
be activated by an excess of highly flammable Et3Al2Cl3.
Furthermore, the reaction requires temperatures below 260 °C
and the use of toxicologically disreputable CH2Cl2 as a
solvent.
In order to overcome these drawbacks, we investigated the
hydrovinylation with complex 3 in liquid and supercritical CO2
as an alternative solvent.3,4 The reaction proceeded smoothly
using Et3Al2Cl3 as the co-catalyst, but ees were disappointingly
low (Table 1). We also observed some corrosion of the stainless
steel high pressure reactor under these conditions, and therefore
tried to replaced the aggressive Lewis acid by weakly co-
ordinating anions which are easier to handle and non-corrosive
under the reaction conditions. Unfortunately, no catalytic
Scheme 1
a
Table 1 Enantioselective hydrovinylation of styrenes 1a–c in CO2
Substrate/
Conversionb Selectivityb,c
Ee of 2b
product
Co-catalyst
T/°C
(%)
(%)
(%)
1a/2a
1a/2a
1a/2a
1a/2a
1a/2a
1a/2a
1a/2a
1a/2a
1b/2b
1b/2b
1c/2c
1c/2c
Cl3Al2Et3
1
40
22
22
1
23
40
35d
40
24
35e
20e
> 99
< 1.0
0
92.9
—
—
70.2 (R)
—
—
TfO2
[Al(O(CH2)2C6F13)4]2
[Al(OC(Ph)(CF3)2)4]2
35
94.3
88.9
71.4
74.9
38.9
80.5
87.0
44.4
71.4
76.0 (R)
86.2 (R)
85.6 (R)
83.6 (R)
91.6 (R)
81.0 (2)
81.3 (2)
89.4 (R)
76.6 (R)
BARF
BARF
BARF
BARF
BARF
BARF
BARF
BARF
b
> 99
> 99
> 99
> 99
> 99
> 99
> 99
75.8
a
c
For standard reaction conditions see note †. Determined by GC analysis. Fraction of 2 in product mixture. Typical by-products were 2-phenylbut-
2-enes and hydrovinylation products of 2. d Reaction time 30 min. e Reaction time 50 min.
Chem. Commun., 1999, 1583–1584
1583

Another intriguing feature of our new procedure for hydro-
vinylation is the difference in product distribution of the
condensate collected from the cold trap and the residue in the
high pressure vessel. If the reactor was vented at temperatures
above Tc of pure CO2, the trap content was considerably
enriched with 2a (e.g. 81.0 vs. 60.5% in the residue) and
contained less of the by-products formed from isomerisation
and hydrovinylation of 2a. This finding indicates that it is
possible to remove 2a selectively from the reaction mixture
with CO2. Additionally, the catalyst can be easily recycled after
venting and removal of all volatiles in vacuo. At a reaction
temperature of 22 °C, the ee was minimally reduced (from
83.4% at the beginning down to 79.8%), but some loss in
activity was observed (from > 99% conversion down to 33%
conversion of 1a) in four subsequent runs.
In summary, we obtained highly promising results perform-
ing enantioselective nickel-catalysed hydrovinylation in liquid
and supercritical CO2. The attractive prospects for catalyst
recycling and selective removal of the product encourage our
ongoing efforts to explore CO2 as a solvent for enantioselective
catalysis.
This work was supported by the Max-Planck-Gesellschaft,
the Deutsche Forschungsgemeinschaft (Gerhard-Hess-Pro-
gramm) and the Fonds der Chemischen Industrie. We thank
Professor G. Wilke for his encouragement in this project and for
stimulating discussions.
Fig. 1 Enantiomeric excess of 2a obtained from hydrovinylation using the
3–NaBARF catalyst in (5) CO2 and (-) CH2Cl2 at different tem-
peratures.
In hydrovinylation, the positive ‘BARF effect’ on the thermal
stability of catalyst is not limited to scCO2 as a solvent.6 This
anion can also be used as co-catalyst with 3 for the hydro-
vinylation of 1a in CH2Cl2 and, at temperatures above 0 °C,
proves to be superior to Et3Al2Cl3 in terms of enantioselectivity
(84.8 vs. 70.2% at 1 °C) with similar chemo- and regio-
selectivities. However, a comparison of the solvents CO2 and
CH2Cl2 reveals that enantioselectivity is generally better in CO2
(Fig. 1). Chemoselectivity for 2a is also considerably higher in
CO2 at elevated temperatures; 57.2% selectivity for 2a are
observed in CH2Cl2 at 35 °C compared to 74.9% in scCO2 at
40 °C, with complete conversion in both cases.
In scCO2, the catalyst formed from 3 and NaBARF exhibits
remarkable stability as demonstrated in an experiment at 40 °C
where a 1a+Ni ratio of 10 000+1 was used (Fig. 2). With an
initial turnover frequency of greater than 1300 h21, a total
turnover number of 5000 (corresponding to 50% conversion)
was achieved. This result is quite remarkable because pre-
viously reported turnover numbers for other nickel-catalysed
reactions in the presence of scCO2 were below 22.7 A catalyst
formed from [Ni(cod)2] and PEt3 was found to be highly
susceptible to ligand oxidation in scCO2, resulting in the
formation of nickel carbonyls and the corresponding phosphin-
oxides.7c In the experiment shown in Fig. 2, the desired product
2a was formed with extremely high chemoselectivity and no
other products could be detected by NMR or GC analyses. The
ee of 79.8% was only slightly lower than in experiments leading
to complete conversion of 1a.
Notes and references
† In a typical experiment, a stainless steel high pressure reactor (V = 27
cm3) equipped with thick-wall glass windows, a PTFE stirring bar, a bore
hole for a thermocouple, and needle and ball valves was charged with
NaBARF (33.4 mg, 35.5 3 1023 mmol) under argon. A dosing unit
containing a solution of 3 (5.4 mg, 6.3 3 1023 mmol) in 1a (450 mg, 4.37
mmol) was connected to the reactor through the closed ball valve and
pressurised with CO2 (7.1 g). The reactor was filled through the needle
valve with ethene (1.1 g, 39.3 mmol) and CO2 (14.6 g) using a compressor.
The reaction mixture was then warmed to 40 °C, while the dosing unit was
heated to 60 °C. Opening the ball valve started the reaction, which was
allowed to proceed for 15 min. The reactor contents were then vented
through a trap cooled to 255 °C. The products were collected separately
from the trap and the reactor by extraction with Et2O or acetone and
analysed by GC and NMR.
1 For a review, see P. W. Jolly and G. Wilke, Applied Homogenous
Catalysis with Organic Compounds 2, ed. B. Cornils and W. A. Herrman,
Wiley-VCH, Weinheim, 1996, p. 1024.
2 G. Wilke and J. Monkiewicz, DOS 3 618 169, Priority 30.05.86; Chem.
Abstr. 1988, 109, P6735.
3 Chemical Synthesis Using Supercritical Fluids, ed. P. G. Jessop and W.
Leitner, Wiley-VCH, Weinheim, 1999.
4 For recent reviews, see P. G. Jessop, T. Ikariya and R. Noyori, Science,
1995, 269, 1065; D. A. Morgenstern, R. M. LeLacheur, D. K. Morita, S.
L. Borkowsky, S. Feng, G. H. Brown, L. Luan, M. F. Gross, M. J. Burk
and W. Tumas, Green Chemistry, ed. P. T. Anastas and T. C. Williamson,
ACS Symp. Ser. 626, American Chemical Society, Washington DC,
1996, p. 132; P.G. Jessop, T. Ikariya and R. Noyori, Chem. Rev., 1999,
99, 475; M. Poliakoff, S. M. Howdle and S. G. Kazarian, Angew. Chem.,
Int. Ed. Engl., 1995, 34, 1275.
5 For the use of BARF in other enantioselective metal-catalysed reactions
in scCO2, see: J. Burk, S. Feng, M. F. Gross and W. Tumas, J. Am. Chem.
Soc., 1995, 117, 8277; S. Kainz, A. Brinkmann, W. Leitner and A. Pfaltz,
J. Am. Chem. Soc., 1999, 121, 6421.
6 For the use of BARF to activate less selective hydrovinylation catalysts
in conventional solvents, see: N. Nomura, J. Jin, H. Park and T. V.
RajanBabu, J. Am. Chem. Soc., 1998, 120, 459.
7 (a) M. T. Reetz, W. Ko¨nen and T. Strack, Chimia, 1993, 47, 493; (b) E.
Dinjus, R. Fornika and M. Scholz, Chemistry Under Extreme or Non-
Classical Conditions, ed. R. van Eldik and C. D. Hubbard, Wiley, New
York, 1996, p. 219; (c) U. Kreher, S. Schebesta and D. Walther, Z. Anorg.
Allg. Chem., 1998, 624, 602.
Fig. 2 (5) Turnover number of hydrovinylation of 1a in the presence of
3–NaBARF (1a+Ni = 10.000) in CO2 at 40 °C. (:) Turnover frequencies,
derived from the first derivative of the least-squares fit of the turnover
number data.
Communication 9/04799I
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Chem. Commun., 1999, 1583–1584