Palladium-catalysed carbon-carbon bond formation in supercritical carbon dioxide

Article abstract of DOI:10.1039/a802235f

Novel fluorinated phosphine palladium complexes are prepared and employed in the first examples of palladium-catalysed carbon-carbon bond formation in supercritical carbon dioxide.

Full text of DOI:10.1039/a802235f

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Palladium-catalysed carbon–carbon bond formation in supercritical carbon
dioxide
Michael A. Carroll and Andrew B. Holmes*†
Melville Laboratory for Polymer Synthesis, Department of Chemistry, University of Cambridge, Pembroke Street, Cambridge,
CB2 3RA UK
Novel fluorinated phosphine palladium complexes are
prepared and employed in the first examples of palladium-
catalysed carbon–carbon bond formation in supercritical
carbon dioxide.
having a higher degree of ionic character requiring an increased
level of fluorination (Table 1, entries 4 and 7).‡
The soluble palladium complexes§ (Table 1, entries 5 and 8)
were then used to catalyse three important carbon–carbon bond
forming reactions, namely the Heck, Suzuki and Sonogashira
couplings. The Heck reaction has emerged as one of the most
versatile members of this group.11 Table 2 (entry 1) illustrates
2
Supercritical carbon dioxide (scCO ) has recently emerged as
an environmentally benign substitute for volatile organic
1
solvents. Above the critical temperature (31.1 °C) and pressure
2
that the reaction in scCO with electron deficient alkenes occurs
(
73.8 bar) carbon dioxide possesses hybrid properties of both a
in a superior yield to that reported for conventional solvents;
intramolecular processes are also possible (entry 2). The
palladium-catalysed coupling of boronic acids with aryl or vinyl
halides, the Suzuki reaction, has certain advantages for the
liquid and a gas. Control of the solvent density by variation of
the temperature and pressure enables the solvent properties to
be ‘tuned’ to substrates and reactants. These aspects have been
2–4
2
12
used to advantage in polymer synthesis and more recently in
coupling of two sp centres. This too can be conducted in
scCO (entries 3 and 4) in yields that are comparable to those
the preparation of ‘small molecules’.4 The particular attraction
,5
2
of scCO
2
must reside in the ‘added value’ as has been
achieved in conventional solvents. Finally, the Sonogashira
coupling reaction13 affords functionalised acetylenic com-
pounds also in satisfactory yield.¶ The use of a more basic
(secondary rather than tertiary) amine in palladium-mediated
transformations typically leads to an improvement in yield.
recognised in its application to heterogeneous catalytic proc-
esses and in asymmetric catalytic hydrogenation/hydroformyla-
tion.6 In this respect synthetic transformations on small
molecules could be coupled with the advantages of processing
,7
(
separation/purification) technologies8 to provide significant
However, in scCO
the two. It is not yet clear whether this is due to carbamic acid
formation.14
Electron deficient amines, such as
2
very little difference is observed between
benefits over the use of conventional solvents.
We have chosen to study metal-catalysed bond forming
reactions, but the majority of the commercially available metal-
based catalysts, typically those bearing phosphine or amine
perfluorotrihexylamine, despite having improved solubility,
exhibit a detrimental effect.
ligands, are unsuitable for use in CO
2
owing to their low
It is of note that the work-up procedures are significantly
easier than those associated with standard reaction conditions.
Typically the use of a polar solvent such as DMF and a water
soluble base (carbonate or alkoxide) requires extraction of the
product with an organic solvent according to conventional
isolation techniques. By contrast, a simple pressure change in
solubility.9 Here we describe the synthesis of a range of
fluorinated phosphines which enable various palladium-medi-
ated reactions to be conducted in scCO
Highly fluorinated compounds have been shown to have
unusually high solubility in scCO , and their incorporation into
2
.
2
phosphine-based ligands was expected to improve the solubility
of the corresponding metal complexes. Such compounds can be
prepared by adding the Grignard reagent C F13CH CH MgI to
6 2 2
2
the scCO reactor allows the gaseous/liquid reaction medium to
be vented (possibly through a conventional solvent) thus
eliminating liquid/liquid partition.
chlorophosphines to give a range of potential ligands for the
solubilisation of palladium complexes in carbon dioxide
In summary, new fluorinated phosphine palladium com-
plexes have been prepared and used as homogeneous catalysts
in supercritical carbon dioxide. Important palladium-mediated
carbon–carbon bond forming processes are now accessible and
are expected to lead to considerable advances in synthetic
methodology in due course.15
(Scheme 1). Similar work has recently been reported regarding
the preparation of these and other, similar ligands for applica-
tions in the ‘fluorous’ phase.10 A simple but indirect means of
purification of the required phosphines was achieved by
temporary conversion to the corresponding oxide. Regeneration
of the phosphine was achieved in excellent yield by treatment
Table 1 Solubility of phosphine palladium complexes in scCO
2
with Cl
The effect of the fluorinated phosphines 1 and 2 on the
solubility of palladium complexes in scCO was then examined,
3 3
SiH and Et N.
Entry
Complex
L
Solubility
2
1
2
3
4
5
6
7
8
PdL
PdL
PdL
PdL
PdL
PdL
PdL
PdL
2
2
2
2
2
2
2
2
Cl
Cl
Cl
Cl
Cl
(OAc)
(OAc)
(OAc)
2
2
2
2
2
PPh
PCy
3
Insoluble
Insoluble
Insoluble
Partial
Soluble
and the results are summarised in Table 1. It is evident that the
solubility of the palladium complexes was dramatically im-
proved. However, the degree of ‘fluorination’ required to confer
solubility is dependent on the particular complex, those species
3
a
dppf
b
c
1
b
2
2
2
2
PPh
3
Insoluble
d
e
1
Soluble
i–iv
d
C6F13CH2CH2I
(C6F13CH2CH2)nPPh(3–n)
2
Soluble
1
2
n = 1, 67%
n = 2, 68%
a
1,1A-Bis(diphenylphosphino)ferrocene. b Prepared in CH
2
Cl
2
by the
reaction (25 °C) of the phosphine (2 equiv.) and (MeCN) PdCl
2
2
(1 equiv.).
c
Scheme 1 Reagents and conditions: i, Mg, Et
Cl PPh(32n), Et O, room temp., 3 h; iii, NaIO
iv, Et N, Cl SiH, toluene, 100 °C, 12 h
2
O, room temp., 12 h; ii,
Incomplete disappearance of solid coupled with an increase in colour
d
n
2
4
, acetone, room temp., 12 h;
intensity on warming. Prepared in situ from the phosphine (2 equiv.) and
Pd(OAc)
e
3
3
2
(1 equiv.). Decomposes at T > 45 °C.
Chem. Commun., 1998
1395

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Yield (%)b
Table 2 Palladium-catalysed carbon–carbon bond formation in scCO
2
Entry
Substrate 1 Substrate 2
Product
Catalysta
T/°C
O
CO2Me
PdL2(OAc)2c
1
PhI
100
91
Ph
OMe
H
N
H
N
PdL2(OAc)2c
2
100
37
I
3
4
PhI
PhI
B(OH)2
B(OH)2
Ph
PdL2Cl2
PdL Cl
100
100
52
49
Ph
2
2
S
S
5
6
PhI
PhI
Ph
Ph
PdL2Cl2d
PdL2Cl2d
60
60
62
18
PhO
PhOCH2
Ph
a
L = 2 (5 mol%) for 64 h with Et
3 2
N (1.1 equiv.) added. b Isolated yield, not optimised. c Prepared in situ from the phosphine 2 (2 equiv.) and Pd(OAc)
d
(1 equiv.). 5 mol% CuI added.
We thank the EPSRC for financial support and provision of
Heck procedure employed the in situ generated palladium acetate complex.
All reactions showed similar colour and physical behaviour.
the Swansea Mass Spectrometry Service, and ICI Acrylics for
financial support. We thank Dr A. I. Cooper and Mr S. A. Mang
for their interest in this work. Dedicated to the memory of
Professor Ralph Raphael.
∑
All new compounds gave spectroscopic (NMR, IR, HRMS) data in
accordance with their proposed structure.
1
2
D. A. Morgenstern, R. M. LeLacheur, D. K. Morita, S. L. Borokowsky,
S. Feng, G. H. Brown, L. Luan, M. F. Gross, M. J. Burk and W. Tumas,
ACS Symp. Ser., 1996, 626, 132.
A. I. Cooper and J. M. DeSimone, Curr. Opin. Solid State Mater. Sci.,
1996, 1, 761.
Notes and References
†
‡
E-mail: abh1@cus.cam.ac.uk
Typical procedure for determining solubility in supercritical carbon
dioxide: Pd(PPh ) Cl (7 mg, 0.01 mmol) was placed in a 10 ml stainless
3 2 2
steel cell. The cell was sealed and completely filled with liquid carbon
dioxide (ca. 1000 psi). The cell was then heated to 70 °C and the state of the
solid was monitored by viewing through a sapphire observation window.
After 10 min at the final temperature the cell was allowed to cool. When the
3 T. M. Yong, W. P. Hems, J. L. M. van Nunen, A. B. Holmes, J. H. G.
Steinke, P. L. Taylor, J. A. Segal and D. A. Griffin, Chem. Commun.,
1997, 1811.
4 A. F u¨ rstner, D. Koch, K. Langemann, W. Leitner and C. Six, Angew.
Chem., Int. Ed. Engl., 1997, 36, 2466.
5 A. R. Renslo, R. D. Weinstein, J. W. Tester and R. L. Danheiser, J. Org.
Chem., 1997, 62, 4530; M. G. Hitzler, F. R. Smail, S. K. Ross and M.
Poliakoff, Chem. Commun., 1998, 359.
6 M. G. Hitzler and M. Poliakoff, Chem. Commun., 1997, 1667; P. G.
Jessop, Y. Hsaio, T. Ikariya and R. Noyori, J. Am. Chem. Soc., 1996,
118, 344; M. J. Burk, S. Feng, M. F. Gross and W. Tumas, J. Am. Chem.
Soc., 1995, 117, 8277.
pressure had dropped below 2000 psi the cell was vented into CH
ml), the cell was opened once atmospheric pressure was reached and was
washed out with a further quantity of CH Cl (20 ml).
The determination of the crystal structure of PdL Cl (L = 2) is currently
under investigation.
Typical procedure for the Sonogashira reaction using the catalyst PdL
2) in scCO2: Bis[bis(1H, 1H, 2H, 2H-perfluorooctyl)-
2 2
Cl (100
2
2
§
2
2
¶
2 2
Cl
(L
=
7 S. Kainz, D. Koch, W. Baumann and W. Leitner, Angew. Chem., Int. Ed.
Engl., 1997, 36, 1628.
phenylphosphino]palladium dichloride∑ (89 mg, 0.05 mmol), copper(i)
iodide (9 mg, 0.05 mmol), iodobenzene (0.11 ml, 1 mmol), phenylacetylene
0.11 ml, 1 mmol) and triethylamine (0.15 ml, 1.1 mmol) were placed in a
0 ml stainless steel cell. The cell was sealed and pressurised to
approximately 1000 psi (full of carbon dioxide). The suspended reagents
were magnetically stirred and afforded a dark red-coloured medium as the
cell was heated to 60 °C. The reagents were stirred at this temperature for
8 M. A. McHugh and V. J. Krukonis, Supercritical Fluid Extraction, 2nd
edn., Butterworth-Heinman, Stoneham, MA, 1994.
9 D. R. Palo and C. Erkey, J. Chem. Eng. Data, 1998, 43, 47.
10 P. Bhattacharyya, D. Gudmunsen, E. G. Hope, R. D. W. Kemmitt, D. R.
Paige and A. M. Stuart, J. Chem. Soc., Perkin Trans. 1, 1997, 3609.
11 R. F. Heck, Palladium Reagents in Organic Synthesis. Academic Press,
Orlando, 1985.
12 N. Migaura, T. Yanagi and A. Suzuki, Synth. Commun., 1981, 11,
513.
13 K. Sonogashira, Y. Tohda and N. Hagihara, Tetrahedron. Lett., 1975,
4467.
(
1
6
4 h when a crystalline deposit formed on the sapphire window. The cell
was then cooled, and when the pressure had dropped below 2000 psi the cell
was vented into Et O (100 ml). The cell was opened once atmospheric
pressure was reached and washed out with a quantity of CH Cl (10 ml).
2
2
2
The organic fractions were combined and concentrated in vacuo to give the
crude product. The product was purified by flash column chromatography
on silica gel, eluting with hexane to give diphenylacetylene as a white
14 V. P. Savin, V. P. Talzi and N. O. Bek, Zh. Org. Khim., 1984, 20, 1842
(J. Org. Chem. USSR, 1984, 20, 1680).
crystalline solid (110 mg, 0.62 mmol, 62%), mp 58–60 °C; d
CDCl ) 7.56–7.61 (4 H, m, o-Ph), 7.33–7.43 (6 H, m, m/p-Ph); d
o-C), 128.5 (m-Ph), 128.4 (p-Ph), 123.4 (quaternary Ph), 89.5 (alkyne). A
similar procedure (excluding CuI) was used for the Suzuki reaction. The
H
(250 MHz;
15 D. K. Morita, D. R. Pesiri, S. A. David, W. H. Glaze and W. Tumas,
Chem. Commun., 1998, 1397.
3
C
131.7
(
Received in Cambridge, UK, 23rd March 1998; 8/02235F
1396
Chem. Commun., 1998