1010 Bull. Chem. Soc. Jpn. Vol. 82, No. 8 (2009)
© 2009 The Chemical Society of Japan
Table 1. Carbonylation Experimentsa)
CH3CO2CH3 CH3CO2H
Run
Rh cat. T/°C t/h
/mmol
/mmol
1
2
3
4
5
6
7b)
8c)
7
8
9
7
8
9
7
7
120
120
120
180
180
180
120
120
1.0
1.0
1.0
0.5
0.5
0.5
1.0
1.0
2.48
6.24
2.81
5.15
5.00
6.24
1.88
2.48
0.300
0.891
0.491
4.77
5.10
3.80
0.276
0.362
a) Experiments were carried out in a 50-cm3 autoclave
equipped with a vessel. Reaction conditions after 15 min at
room temperature: Rh catalyst (0.0280 mmol), MeOH
(10.5 mL), H2O (2.28 mL), MeI (1.74 mL, 28.0 mmol), and
CO 2.5 MPa at rt (3.2 MPa at 120 °C, 5.2 MPa at 180 °C). Yield
was determined by NMR spectra with internal standard
(CHCl2CHCl2). b) [PBu3Me]+I¹ (0.0280 mmol) was added.
c) O=POct3 (0.0280 mmol) was added.
Figure 1. ORTEP drawing of 8 (all H atoms are omitted for
clarity).
spectra of Rh-P bonds with ligands 5 and 6,14 ligands 5 and 6
did not possess enough stability under the industrial conditions
(180 °C, CO 5.2 MPa), while other thioether-bearing phosphine
ligands such as Ph2P(2-MeSC6H4) and Ph2PC2H4SMe can
increase catalytic activity under the harsh conditions (185 °C,
CO 70 bar).3 It should be noted that the ratio of CH3CO2H
to CH3CO2CH3 was greatly enhanced at 180 °C, compared
to 120 °C. In order to confirm that the enhanced activity
was achieved by not phosphine oxide and/or phosphonium
iodide derived from 5 but ligand 5, phosphine oxide and
phosphonium iodide were added to 7 (Runs 7 and 8). Addition
of [PBu3Me]+I¹ to [RhCl(CO)2]2 (7) inhibited catalytic activity
(Run 7), compared with the result of [RhCl(CO)2]2 (7) (Run 1).
The dissociation and degradation of phosphine ligand into
phosphonium iodide would retard the catalytic cycle.4 Finally,
addition of phosphine oxide O=POct3 to [RhCl(CO)2]2 (7)
showed no influence on catalytic activity (Run 8). Results of
Runs 7 and 8 indicate that enhanced catalytic activity (Run 2)
results from coordination of ligand 5 on the Rh center.
In conclusion, new mixed phosphine-thioether ligands 5 and
6 and their rhodium complexes 8 and 9 were synthesized and
applied to the carbonylation of methanol. Rhodium complex 8
demonstrated higher yield of CH3CO2CH3 and CH3CO2H
at 120 °C than [RhCl(CO)2]2 (7), while rhodium complex 9
gave slightly higher catalytic activity. On the other hand,
at 180 °C, all the complexes 7-9 provided nearly the same
amount of CH3CO2CH3 and CH3CO2H. This may be due to the
dissociation of ligands which were converted into phosphine
oxide and phosphonium iodide.
Figure 2. ORTEP drawing of 9 (all H atoms are omitted for
clarity).
difference of ¯(CO) absorption between 8 and 9 indicates little
difference of electron-donating ability between phenyl sulfide
and alkyl sulfide. Naturally, both rhodium centers of 8 and
9 are more electron-rich than the rhodium complex with
triarylphosphine-thioether, Ph2P(o-MeSC6H4): ¯(CO) 1998
cm¹1.3 Furthermore, no difference in stability between 8 and
9 was seen, judging from decomposition points: 8 (177.2 to
179.3 °C) and 9 (178.2 to 180.2 °C).
The application of the rhodium complexes 8 and 9 to the
carbonylation of methanol was investigated as shown in
Table 1. At 120 °C for 1 h,11 catalyst precursor [RhCl(CO)2]2
(7) provided 2.78 mmol of CH3CO2CH3 and CH3CO2H
(Run 1). Under the same conditions, catalyst precursor 8
catalyzed the carbonylation of MeOH to yield 7.13 mmol of
CH3CO2CH3 and CH3CO2H which is more than double the rate
(Run 2), of [RhCl(CO)2]2 (7). However, the 31P NMR spectrum
of the inorganic residue indicated that some amount of ligand 5
remained bound to the Rh center after 1 h at 120 °C in the
presence of large excess of MeI, while the dissociation and
degradation of ligand 5 occurred.12 The phosphine ligand
would result in phosphine oxide and/or phosphonium iodide.13
On the other hand, another precursor 9 showed slightly higher
catalytic activity than [RhCl(CO)2]2 (7) (Run 3). Thus, sig-
nificant difference in catalytic activity between 8 and 9 was
seen although complexes 8 and 9 have nearly the same v(CO)
absorption values indicating negligible difference of the
electron center on the Rh centers in 8 and 9. At 180 °C,
all the complexes 7-9 provided nearly the same amount
of CH3CO2CH3 and CH3CO2H (Runs 4-6). At 180 °C, the
dissociation of ligands would be promoted to give phosphine
oxide and phosphonium iodide. Judging from no 31P NMR
Experimental
Carbonylation of MeOH.
A solution of Rh catalyst
(28.0 ¯mol) and MeI (28.0 mmol) in MeOH (10.5 mL) and H2O
(2.28 mL) was added to a 50-cm3 autoclave equipped with a vessel.
All reactor valves were closed. The solution was stirred at room
temperature for 15 min under CO pressure (2.5 MPa at room
temperature). The solution in the autoclave was stirred at an
assigned temperature. After the reactor was rapidly cooled to room