Palladium-catalyzed cross-coupling reactions in supercritical carbon dioxide

Article abstract of DOI:10.1039/a802621a

Palladium-catalyzed carbon-carbon bond coupling reactions, the Heck and Stille reactions, proceed in supercritical carbon dioxide with a number of phosphine ligands including tris[3,5-bis(trifluoromethyl)phenyl]phosphine, which affords high conversions an

Full text of DOI:10.1039/a802621a

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Palladium-catalyzed cross-coupling reactions in supercritical carbon dioxide
a
b
a
c
a
David K. Morita, David R. Pesiri, Scott A. David, William H. Glaze and William Tumas* †
a
Los Alamos Catalysis Initiative, Chemical Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM
8
7545, USA
b
Department of Environmental Sciences and Engineering, University of North Carolina, Chapel Hill, NC 27599, USA
Carolina Environmental Program, University of North Carolina, Chapel Hill, NC 27599, USA
c
Palladium-catalyzed carbon–carbon bond coupling reac-
tions, the Heck and Stille reactions, proceed in supercritical
carbon dioxide with a number of phosphine ligands includ-
ing tris[3,5-bis(trifluoromethyl)phenyl]phosphine, which
affords high conversions and selectivities.
o-tolylphosphine 1 were characterized by significantly lower
turnover numbers, with PPh 2 resulting in conversions only
slightly higher than the reaction in the absence of added ligand.
We also ran the Stille coupling reaction of PhI with vinyl-
3
2
(tributyl)tin with the same ligands in CO and THF at 50 °C and
found that the conversions were considerably higher in THF.
For example, in THF at 50 °C the tri-2-furylphosphine and
tris[3,5-bis(trifluoromethyl)phenyl]phosphine ligands both re-
sulted in conversions above 95% while slightly lower conver-
1
Elevated pressures have been reported to offer benefits for
2
3
homogeneous catalyst lifetime and selectivity. Such pressure-
related advantages, in combination with the properties of
supercritical carbon dioxide (scCO
neous catalytic reactions in this medium. A number of recent
reports describe examples of homogeneous catalytic reactions
2
), led us to explore homoge-
sions (81%) were measured with PPh
scCO at 50 °C for the Stille reaction was only 28% after 5 h
using the tri-2-furylphosphine ligand.
2
The much higher conversions in scCO for tris[3,5-
3
. The conversion in
2
4
5
in scCO
2
including hydrogenations, hydroformylations, epox-
6
7
idations and polymerizations. We report the first demonstra-
tion that palladium-catalyzed carbon–carbon coupling reactions
bis(trifluoromethyl)phenyl] phosphine appear to arise from the
enhanced solubilization of the catalytically active palladium–
phosphine complexes. A number of reports have demonstrated
the ability of fluorine substitution on organic polymers and
can be carried out in supercritical CO
catalyst activity, selectivity and ligand effects in scCO
2
. We have examined
in a first
2
7
a,7b,12,13
step toward capitalizing on the benefits of high pressures for
homogeneous palladium-catalyzed reactions. The reactivity and
ligands to enhance solubility in scCO
2
.
In particular,
for
Leitner has recently reported catalytic activity in scCO
2
8
9
selectivity of Heck and Stille reactions using several different
rhodium catalysts containing perfluoroalkyl-substituted
1
0
12
phosphine ligands in scCO
organic solvents (THF and toluene).
All reactions in scCO were carried out batchwise in
2
were compared to those in
arylphosphines. Although we have not carried out any
quantitative solubility studies, we observed no visible precipita-
2
tion for the Stille coupling reactions in CO
bis(trifluoromethyl)phenyl]phosphine ligand. We did, however,
observe significant precipitation in CO (presumably pallad-
ium–phosphine complexes) under identical conditions with
PPh . The perfluorinated ligand, tris(pentafluorophenyl)-
2
using the tris[3,5-
magnetically stirred, custom built 316 stainless steel reactors
with sapphire windows to allow viewing of the contents at high
pressure.11 Stille cross-coupling reactions of PhI and vinyl-
2
(
tributyl)tin in CO
number of different ligands (Table 1). The highest conversion in
scCO was achieved with the tris[3,5-bis(trifluoro-
2
(345 bar) were run at 90 °C for 5 h using a
3
phosphine, also conferred solubility indicated by no observed
precipitation, although lower conversions (@ 75%) were
obtained.
2
methyl)phenyl]phosphine ligand 4 ( > 99%) which was compa-
rable to the results using tri-2-furylphosphine 3 (86%), a ligand
In order to further probe these ligand effects, and to
noted for high palladium-coupling activity.9c PPh
2 and tri-
benchmark the chemical reactivity in CO
solvents, we carried out preliminary kinetic studies of the Stille
coupling of PhI and vinyl(tributyl)tin at 75 °C in scCO and
toluene. Reaction rates were determined as initial rates using
3
2
with organic
2
2 3
Table 1 Stille cross-coupling of PhI with vinyl(tributyl)tin using Pd (dba)
1
4
a
in supercritical CO
2
2
a high pressure reactor with a six-port sampling valve for CO ,
which has been described previously,11 and standard Schlenk-
ware for toluene. As shown in Table 2, the initial rate of reaction
Pd2(dba)3, ligand
I +
+ ISnBu3
SnBu3
5
h
using tris[3,5-bis(trifluoromethyl)phenyl]phosphine
4 was
twice that of PPh 2 in both CO and toluene. Moreover, the
3
2
Conversion
(%)
Selectivityc
(%)
d
21
Ligand
TON /h
Tri-o-tolylphosphine 1
No ligand
9
38
49
86
> 99
99
96
96
97
99
0.5
1.9
2.5
4.3
> 5
2
Table 2 Rates of Stille coupling reactions in supercritical CO and
toluenea
PPh
3
2
k/min2
1
Tri-2-furylphosphine 3
P[3,5-(CF
3
)
2
C
6
3
H ]
3
4
Ligand
CO
2
Toluene
a
2
90 °C, 5 h, 345 bar CO , 2 equiv. of phosphine ligand for each equiv. of
palladium(0) in all coupling reactions, [2.7 mmol (0.790 ml) vinyl-
tributyl)tin, 2.7 mmol (0.3 ml) PhI, 0.054 mmol (0.048 g) Pd (dba) , 0.216
PPh
3
2
0.016
0.036
0.034
0.057
(
2
3
3 2 6 3 3
P[3,5-(CF ) C H ] 4
mmol phosphine ligand], reactions were quenched in acetone and analyzed
by GC using known standards and confirmed by GC–MS and/or H NMR
1
a
75 °C, 0.024 mmol (0.022 g) Pd
mmol PhI, 33 ml toluene or 33 ml CO
measured by GC with internal standard (octafluoronaphthalene). Initial
2
(dba)
3
, 2.4 mmol vinyl(tributyl)tin, 2.4
b
c
spectroscopy. Conversion = iodobenzene reacted (%). Selectivity =
2
at 310 bar used. Initial rates
d
21
styrene (%) with respect to all products formed. TON/h = [(moles of
iodobenzene) 3 (% conversion)]/[(moles of catalyst added) 3 (hours of
reaction)].
o
rates (30 min) were obtained from a ln [c/c ] vs. time plot and are accurate
to within 10%.
Chem. Commun., 1998
1397

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a
Table 3 Heck cross-coupling reaction in supercritical CO
2
R
Pd(OAc)2, Et3N, L
I
+
R
90 °C, 345 bar
R = CO
2
Me
R = Ph
Conversion Selectivity
Conversion Selectivity
(%)
L
(%)
TON/h2
1
(%)
(%)
TON/h21
Tri-o-tolylphosphine 1
No ligand
35
17
—
96
94
—
80
82
—
78
91
—
1
—
37
20
—
99
99
—
99
99
—
99
99
—
1
0.6
—
2.8
2.8
0.5
—
2.7
2.6
—
PPh
Tri-2-furylphosphine 3
P[3,5-(CF
P(C
3
2
3 2 6 3 3
) C H ] 4
6 5 3
F ) 5
a
9
0 °C, 24 h (R = CO
2
2 3
Me), 12 h, 0.3 mmol ligand, 0.15 mmol (0.034 g) Pd(OAc) , 15 mmol (2.25 ml) Et N, 5 mmol (0.560 ml) PhI, 25 mmol (2.25 ml)
methyl acrylate, octafluoronaphthalene internal standard. Reactions were analyzed as in Table 1.
observed rates in scCO
observed in toluene for both phosphine ligands (Table 2). The
initial rates of the PPh 2 and tris[3,5-bis(trifluoromethyl)phe-
2
were within a factor of two of those
Notes and References
†
E-mail: tumas@lanl.gov
3
nyl]phosphine 4 ligands are similar but the conversions are so
different, suggesting that the enhanced solubility of the
trifluoromethyl substituted system is probably more important
than any increased activity through electronic effects. We found
that the much less basic fluorinated ligand, tris(pentafluoro-
phenyl)phosphine 5, also enhanced solubility relative to PPh 2
3
but the initial rate was two orders of magnitude less than the
other ligands, presumably due to electronic effects.
We have also examined the palladium-catalyzed Heck
coupling of PhI with simple olefins using several phosphines in
2
scCO at 90 °C. As Table 3 illustrates, we observe that the
fluorinated phosphines result in high conversions for both
styrene and methyl acrylate ( > 94%), similar to the proven tri-
1
Organic Synthesis at High Pressures, ed. K. Matsumoto and R. M.
Acheson, Wiley, New York, 1990; J. H. Espenson, Chemical Kinetics
and Reaction Mechanisms, McGraw-Hill, New York, 1981.
2 S. Hillers, S. Sartori and O. Reiser, J. Am. Chem. Soc., 1996, 118,
2087.
3
4
Y. Sun, R. N. Landau, J. Wang, C. LeBland and D. G. Blackmond,
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8
277.
5
J. W. Rathke, R. J. Klingler and T. B. Krause, Organometallics, 1991,
10, 1350.
6 D. R. Pesiri, D. K. Morita, W. H. Glaze and W. Tumas, Chem.
Commum., 1998, 1015.
2
-furylphosphine ligand. The non-fluorinated triarylphosphines
7
8
(a) J. B. McClain, D. E. Betts, D. A. Canelas, E. T. Samulski, J. M.
DeSimone, J. D. London, H. D. Cochran, G. D. Wignall, D. Chillura-
Martino and R. Triolo, Science, 1996, 274, 2049; (b) C. A. Mertdogan,
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London, 1985; R. F. Heck and J. P. Nolley Jr., J. Org. Chem., 1972, 37,
led to lower conversions (20–35%), which were comparable to
reactions run in the absence of added ligand. Unlike the Stille
couplings, the reaction solution for all the Heck couplings was
dark and opaque and prevented even qualitative observation of
precipitation; however, we believe the higher turnovers for the
fluorinated phosphines probably arise from enhanced sol-
ubilization of the palladium complexes.
2
320.
9 (a) J. K. Stille, Angew. Chem., Int. Ed. Engl., 1986, 25, 508; (b) I. P.
Beletskaya, J. Organomet. Chem., 1983, 250, 551; (c) V. Farina and
G. P. Roth, in Advances in Metal-Organic Chemistry, ed. L. S.
Liebeskind, JAI Press, Greenwich, CT, 1996, vol. 5, p. 1.
0 For similar reactions and conditions please see: M. A. Carroll and A. B.
Holmes, Chem. Commun., 1998, 1395.
1 S. Buelow, P. Dell’Orco, D. K. Morita, D. R. Pesiri, E. Birnbaum, S.
Borkowsky, G. Brown, S. Feng, L. Luan, D. A. Morgenstern and W.
Tumas, Recent Advances in Chemical Processing in Dense Phase
Carbon Dioxide at Los Alamos, in Frontiers in Benign Chemical
Synthesis and Processing, ed. P. T. Anastas and T. C. Williamson,
Oxford University Press, in the press.
In summary, we have observed that both Heck and Stille
couplings can proceed in supercritical CO with rates and
2
selectivites comparable to those in toluene. Of particular note,
we find that fluorinated phosphines, particularly tris[3,5-
bis(trifluoromethyl)phenyl]phosphine, result in high conver-
sions due to the ability of these ligands to enhance the solubility
1
1
of the metal complexes in scCO
fluorinated phosphines should expand the utility of scCO
2
. ‘CO
2
-philic’ ligands such as
for
2
homogeneous catalysis. Another logical extension of our work
2
would be to explore supercritical fluids at pressures reported to
be high enough ( > 1 kbar) to enhance some catalytic reactions.
12 S. Kainz, D. Koch, W. Baumann and W. Leitner, Angew. Chem., Int. Ed.
Engl., 1997, 36, 1628.
1
3 B. Betzemeier and P. Knochel, Angew. Chem., Int. Ed. Engl., 1997, 36,
623; I. T. Horvarth and J. R a´ bai, Science, 1994, 266, 72; K. Laintz, C.
Wai, C. Yonker and R. Smith, J. Supercritical Fluids, 1991, 4, 194; A.
Yazdi, C. Lepilleur, E. Singley, W. Liu, F. Adamsky, R. Encik and E.
Beckman, Fluid Phase Equilib., 1996, 117, 297.
This work was supported as part of the Los Alamos Catalysis
Initiative by The Department of Energy through Laboratory
Directed Research and Development (LDRD) funding, and a
grant from the US EPA, Office of Pollution Prevention and
Toxics (Grant Number 1877-97-1). We would like to acknowl-
edge many helpful discussions with Dr Tom Baker and Dr Steve
Buelow.
2
1
4 V. Farina and B. Krishnan, J. Am. Chem. Soc., 1991, 113, 9585.
Received in Cambridge, UK, 6th April 1998; 8/02621A
1398
Chem. Commun., 1998