An efficient carbonylation of aryl halides catalysed by palladium complexes with phosphite ligands in supercritical carbon dioxide

Article abstract of DOI:10.1039/a902942g

The carbonylation of aryl halides catalysed by CO₂ soluble Pd complexes with trialkyl or triaryl phosphite ligands proceeds rapidly in scCO₂, in which the rate of the reaction is higher than those attained in solution phase reactions.

Full text of DOI:10.1039/a902942g

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An efficient carbonylation of aryl halides catalysed by palladium complexes
with phosphite ligands in supercritical carbon dioxide
Yoshihito Kayaki,a Yushi Noguchi,a Seiji Iwasa,a Takao Ikariya*a and Ryoji Noyorib
a
Department of Chemical Technology, Tokyo Institute of Technology and CREST, Japan Science and Technology
Corporation, O-okayama, Meguro-ku, Tokyo 152-8552, Japan. E-mail: tikariya@o.cc.titech.ac.jp
Department of Chemistry and Research Center for Materials Science, Nagoya University, Chikusa, Nagoya
b
464-8602, Japan
Received (in Cambridge, UK) 13th April 1999, Accepted 24th May 1999
The carbonylation of aryl halides catalysed by CO2 soluble
Pd complexes with trialkyl or triaryl phosphite ligands
proceeds rapidly in scCO2, in which the rate of the reaction
is higher than those attained in solution phase reactions.
clearly demonstrated that 1 3 1022 mmol (5.1 mg) of
[PdCl2{P(OEt)3}2] 3b were dissolved in scCO2 (300 atm) at
130 °C in a 50 mL reactor vessel. The success of this strategy is
shown by the greater TON attained in the reaction catalysed by
3b under the standard conditions mentioned above compared to
that with 3a. The reaction of 1 with 3b proceeded efficiently at
130 °C in a homogeneous single phase containing all the
reactants and the catalyst to provide quantitatively the product 2
(Table 1).
The phosphite compounds are the best choice of ligand for an
increase in the solubility of the Pd complex. The compound’s
structure and electronic properties strongly affect the outcome
of the reactions, as listed in Table 1. The scCO2 soluble Pd
complexes with trialkyl or triphenyl phosphite ligands (3b and
3c) are highly efficient catalyst precursors. The complex 3g
with PMe3 ligands was less reactive because the strongly
coordinated ligand retards slightly the reaction in scCO2 as
observed in conventional liquid solvents.8 Replacement of the
phenyl groups in PPh3 with methoxy groups (3d and 3e)
facilitates the carbonylation due to an increase in the solubility
of the Pd catalyst precursor. On the other hand, the reaction with
the CO2 insoluble PPh3 complex 3f proceeded rapidly almost to
completion accompanied with metal deposition. One possible
explanation is that the phosphorous ligand-free, CO2 soluble Pd
carbonyl species9,10 generated might be extracted into the
Supercritical fluids (SCFs) are attractive alternatives to organic
solvents as reaction media for chemical reactions for a number
of reasons. These reasons include their pressure tunable
physical properties and solvent powers as well as the high
miscibility of the reactant gases, efficient mass transfer, local
clustering, and possible weakening of the solvation of the
reactants.1 In particular, homogeneous transition metal-cata-
lysed reactions involving gaseous reactants in SCFs have been
intensively investigated, e.g. hydrogenation of olefins,1a,2
hydroformylation,3 hydrogenation of carbon dioxide,4 and
oxidation by O2.2e,5 However, no discernible benefit in terms of
rate was obtained by performing the reactions under super-
critical conditions except for a rapid hydrogenation of super-
critical carbon dioxide (scCO2) with Ru complexes4 and an
olefin hydroformylation catalysed by Rh complexes.3b,c This
report describes the first carbonylation reactions of aryl halides
catalysed by CO2 soluble Pd complexes which proceed rapidly
in scCO2. The rate of the reaction is higher than those attained
in solution phase reactions.
We first examined the intramolecular carbonylation of
2-iodobenzyl alcohol 16 catalysed by palladium complex
[PdCl2(MeCN)2] 3a in a supercritical mixture of CO2 and CO
Table 1 Pd complex catalysed carbonylation of 2-iodobenzyl alcohol 1 in
(PCO = 200 atm and PCO = 10 atm)† at 130 °C (Table 1). The
a
2
reaction proceeds smoothl2y in the presence of 2.2 equiv. of Et3N
(1+Pd+Et3N = 5000+1+11000) to give phthalide, 2 in a
moderate yield, with the TON (turnover number, mol product
per mol catalyst) = 2190 after 2 h. A visual inspection of the
reaction mixture in the window-equipped reactor vessel con-
firms that all the reactants as well as the product 2 are soluble in
scCO2, although the catalyst 3a is only partly soluble. The
reaction was started by introducing CO2 and then CO into the
reactor containing the reactants and the catalyst at the reaction
temperature (Method A). In this procedure, the reaction
proceeded rapidly in the liquid phase which formed before
reaching the supercritical homogeneous state. Therefore, the
outcome of the reaction is at least partly due to the reaction in
the liquid phase. In fact, the reaction performed in a liquid phase
consisting of 1 and Et3N without CO2 under otherwise identical
conditions (neat condition) (substrate+catalyst = 5000+1, PCO
= 10 atm, 130 °C, 2 h) produced 2 with a TON of 3570. Such
reactions in the liquid phase can be prevented by separating the
catalyst from the reactants using a small glass container
(Method B, standard procedure).‡ Reactions using this standard
procedure gave a TON of 120 after 2 h and 1880 after 18 h as
shown in Table 1, which clearly indicates that the carbonylation
proceeds in the homogeneous scCO2 phase.
scCO2
O
PdCl2L2
I
Et3N (2.2 equiv.)
+
O
CO
OH
scCO2 (200 atm)
130 °C, 18 h
1
2
Run
L (Pd complex) Medium (atm)
CO/atm t/h
TON
1b
2c
3
4
5
6
7
8
9
10
11
12
13
14
a
MeCN 3a
MeCN 3a
MeCN 3a
MeCN 3a
P(OEt)3 3b
P(OPh)3 3c
CO2 (200)
—
CO2 (200)
CO2 (200)
CO2 (200)
CO2 (200)
10
10
10
10
10
10
10
10
10
10
1
2
2
2
2190
3570
120
18
18
18
18
18
18
18
18
18
18
18
1880
5000
4510
4470
3880
4800
3180
4650
5000
3100
3800
PPh(OMe)2 3d CO2 (200)
PPh2(OMe) 3e
PPh3 3f
PMe3 3g
P(OEt)3 3b
P(OEt)3 3b
P(OEt)3 3b
P(OEt)3 3b
CO2 (200)
CO2 (200)
CO2 (200)
CO2 (200)
CO2 (200)
toluened
5
1
5
toluened
Reaction was conducted at 130 °C in a 50 mL reaction vessel
containing 0.1 mmol of Pd catalyst (10 ml DMF solution), substrate and Et3N
Encouraged by the marked catalytic activity of the less
soluble catalyst precursor 3a in scCO2, we then tried to employ
Pd complexes with trialkyl phosphite ligands, which are highly
soluble even in nonpolar hydrocarbons.7 The solubility test
(Method B). Substrate+Pd catalyst+Et3N
= 5000+1+11000. TON =
b
product mol/catalyst mol. The catalyst was not separated from the
substrate (Method A). c Neat. d 50 ml.
Chem. Commun., 1999, 1235–1236
1235

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‡ Standard procedure for the carbonylation: The reactor equipped with a
small vial was charged with argon gas in a desiccator. Then, a DMF (10 ml)
solution of Pd catalyst (0.1 mmol) was charged in this vial, and the reactor
was placed in an oven. After reaching the desired temperature of 130 °C (for
ca. 1.5 h), a mixture of the substrate (0.5 mmol) and Et3N (1.1 mmol) was
added into the reactor (outside the vial) with a syringe through an opening
against a flow of CO2. Subsequently, CO (1–10 atm) was introduced, and
then CO2 (200 atm) was added with an HPLC pump. After stirring for 18 h,
the reactor was cooled in a bath of MeOH and dry ice. The mixture of CO
and CO2 was vented, and the reactor was slowly warmed to room
temperature. The yield of 2 was determined by 1H NMR and GC analyses
of CDCl3 solutions of the products. On-line analysis was conducted by
using a reactor vessel connected to the FT-IR equipment (JASCO FT/IR-
610). The reaction was monitored by collecting spectra at intervals over the
period of the reaction.
Fig. 1 Time vs. TON plots for the carbonylation of 1 catalysed by
[PdCl2{P(OEt)3}2] 3b, in (a) scCO2 (standard procedure) or (b) toluene
(substrate+catalyst = 5000+1, catalyst = 0.1 mmol).
1 Recent reviews; (a) P. G. Jessop, T. Ikariya and R. Noyori, Science,
1995, 269, 1065; (b) P. G. Jessop, T. Ikariya and R. Noyori, Chem. Rev.,
1999, 99, 475.
2 (a) P. G. Jessop, T. Ikariya and R. Noyori, Organometallics, 1995, 14,
1510; (b) M. J. Burk, S. Feng, M. F. Gross and W. Tumas, J. Am. Chem.
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Birnbaum, S. Borkowsky, S. Feng, L. Luan, D. Morgenstern and W.
Tumas, Green Chemistry, ed. O. T. Anastas and T. C. Williamson, OUP,
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scCO2 phase, where the carbonyl complex effects the reaction
highly efficiently and is deactivated during the later stages of
the reaction because of the lack of a stabilizing ligand in the
scCO2 phase. In order to increase the solubility of the transition
metal complexes in scCO2, ligand modifications such as
replacement of ligands containing aryl substituents to ligands
bearing alkyl or perfluoroalkyl groups or introduction of CO2-
philic ponytails as substituents on the aryl groups have been
developed previously.4b,c,11 The use of phosphonate ligands has
proved to be a practical alternative ligand modification for
increasing the solubility of transition metal complexes in
supercritical fluids.
The advantage of performing the carbonylation in super-
critical fluids is that there is no marked effect of the CO pressure
on the TON values obtained in scCO2. Because of the higher
diffusivity of scCO2 compared to organic solvents as well as the
high solubility of CO in this medium, the carbonylation even at
1 atm of CO pressure (CO+substrate = ca. 1.5+1) proceeded to
near completion (TON = 4650). The yield of the product 2
(TON ~ 5000) is independent of the CO pressure over a range
of 1–5 atm in scCO2 (200 atm), whereas the reaction in toluene
under otherwise identical conditions provided lower TON
values (3100–3800) over a range of 1–5 atm of CO. The time
conversion curve obtained by batch-wise experiments, illus-
trated in Fig. 1, as well as direct monitoring of the reaction by
on-line FTIR revealed that the reaction proceeds rapidly to
completion within 3–4 h after a ca. 2 h induction period.
The phosphonate–Pd catalyst can be applied to the inter-
molecular carbonylation of iodobenzene 4 and methanol
(substrate+catalyst = 500+1) under the standard conditions in
scCO2 to yield methyl benzoate (TON = 260). Further
optimization of the reaction is now in progress.
4 (a) P. G. Jessop, T. Ikariya and R. Noyori, Nature, 1994, 368, 231;
(b) P. G. Jessop, Y. Hsiao, T. Ikariya and R. Noyori, J. Am. Chem. Soc.,
1996, 118, 344.
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70, 1141.
8 The TON values obtained from 18 h reactions in toluene under
otherwise identical conditions decrease in the order: P(OEt)3
PMe3.
>
9 [PdCl2{PPh3}2] complex is known to react with CO in in Et3N
containing alcohols to give several kinds of Pd carbonyls and PPh3. M.
Hidai, M. Kokura and Y. Uchida, J. Organomet. Chem., 1973, 52,
431.
10 [Pd(CO)4] is known to be thermally unstable; J. H. Darling and J. S.
Ogden, J. Chem. Soc., Dalton Trans., 1973, 1079.
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W. H. Glaze and W. Tumas, Chem. Commun., 1998, 1397.
In conclusion, the carbonylation of aryl halides catalysed by
Pd complexes with phosphonate ligands proceeds rapidly in
scCO2. The rate of the reaction is higher than that in the liquid
phase.
Notes and references
† SAFETY WARNING: Operators of high-pressure equipment should
take proper precautions to minimize the risk of personal injury [ref. 4(b)].
Communication 9/02942G
1236
Chem. Commun., 1999, 1235–1236