C-H bond activation in dense carbon dioxide: Rhodium-catalyzed methane carbonylation and alkane dehydrogenation

Full text of DOI:10.1021/jo0102967

5262
J . Org. Chem. 2001, 66, 5262-5263
a
Ta ble 1. Ca r bon yla tion of Meth a n e in Den se CO2
C-H Bon d Activa tion in Den se Ca r bon
Dioxid e: Rh od iu m -Ca ta lyzed Meth a n e
Ca r bon yla tion a n d Alk a n e
Deh yd r ogen a tion
yield of
entry
catalyst
CH3CHO (turnover)
1
2
3
4
5
6
RhCl(CO)(PMe3)2
RhCl(CO)(PRf3)2
7.7
27.9
21.5
77.4
9.4
b
RhCl(PMe3)3
RhCl(PMe3)3
RhCl(PMe3)3
J un-Chul Choi, Yoshinobu Kobayashi, and
Toshiyasu Sakakura*
c
d
RhCl(PMe3)3
12.2e
National Institute of Advanced Industrial Science
and Technology, 1-1-1 Higashi, Tsukuba,
Ibaraki 305-8565, J apan
a
Reaction conditions: Rh complex 0.014 mmol, methane 110
atm (89 mmol), CO 3 atm, CO2 total pressure 300 atm, 4 °C, 16 h,
b
irradiated with
a
250
W
high-pressure mercury lamp. Rf:
CH2CH2(CF2)5CF3. c Benzaldehyde (18 µL) was added. Reaction
at 50 °C. e Yield of benzaldehyde: 89 mmol of benzene was used
in place of methane.
d
t-sakakura@aist.go.jp
Received March 19, 2001
The direct and selective functionalization of alkanes
under mild conditions is one of the key issues for
synthetic chemists.1 A major problem in C-H bond
activation by molecular catalysis is the lack of suitable
solvents because most organic solvents are not tolerant
under alkane activation conditions and prevent the
desired reactions. Several compounds have been proposed
for solving this problem, such as liquid xenon,2 perfluoro
organics,3 and bulky hydrocarbons.4 However, more
inexpensive reaction media with adequate solubility for
both the catalysts and alkanes are strongly desired. On
the other hand, dense carbon dioxide has recently at-
tracted increasing attention as an efficient and environ-
mentally friendly reaction medium.5 In the present paper,
we report the methane carbonylation and alkane dehy-
drogenation by molecular catalysis in dense carbon
dioxide. Although ruthenium-catalyzed alkane conversion
in supercritical carbon dioxide was attempted recently,
no C-H bond activation was observed.6
The production of fundamental chemicals based on
methane is of great importance in view of so-called “green
chemistry” because methane is a typical sustainable
resource.7 We have been investigating the direct carbo-
nylation and dehydrogenation of alkanes catalyzed by the
RhCl(L)(PMe3)2-hv system (L ) two electron-donor
ligands).8 In these reactions, the alkane itself was used
as a reaction medium to dissolve the substrates and the
catalyst. However, this methodology is hard to apply to
methane transformation due to the low boiling point of
methane. In this context, dense carbon dioxide seems
very promising as a reaction medium because it is
miscible with gaseous compounds such as methane and
possibly stable enough to survive under alkane activation
conditions. Hence, we examined the carbonylation of
methane in carbon dioxide (eq 1).
A typical experiment was carried out as follows. An
autoclave with sapphire windows (20 cm3 inner volume)
was filled with a rhodium complex (0.014 mmol), carbon
monoxide (3 atm), methane (110 atm), and carbon dioxide
(up to 300 atm total pressure) at 4 °C. The reaction
mixture was then irradiated for 16 h with a 250 W high-
pressure mercury lamp (USHIO SP3-250). The reaction
products were analyzed by GC using a capillary column,
and all the volatile products were also characterized by
GC-MS.
The results are summarized in Table 1. We first
investigated the reaction using RhCl(CO)(PMe3)2 as a
catalyst. Methane was carbonylated to afford acetalde-
hyde as expected (Table 1, entry 1); carbon dioxide did
not disturb the C-H bond activation. In addition, acetic
acid was not detected. To the best of our knowledge, this
is the first successful example of the catalytic acetalde-
hyde synthesis from methane and carbon monoxide.9 On
the basis of the previous papers about the mechanism of
the RhCl(L)(PMe3)2-hν system in organic media,10 the
carbonylation of methane in dense carbon dioxide pre-
sumably proceeds via the oxidative addition of C-H
bonds to RhCl(L)(PMe3)2 (Scheme 1).11 The resulting
(1) (a) Activation of Unreactive Bonds and Organic Synthesis; Murai,
E., Eds.; Springer: Berlin, 1999. (b) Activation and Functionalization
of Alkanes; Hill, C. L., Eds.; J ohn Wiley & Sons: New York, 1989. (c)
Shilov, A. E.; Shul′pin, G. B. Chem. Rev. 1997, 97, 2879-2932. (d)
Crabtree, R. H. Chem. Rev. 1985, 85, 245-269.
(2) Sponsler, M. B.; Weiller, B. H.; Stoutland, P. O.; Bergman, R.
G. J . Am. Chem. Soc. 1989, 111, 6841-6843.
(3) (a) Hoyano, J . K.; McMaster, A. D.; Graham, W. A. G. J . Am.
Chem. Soc. 1983, 105, 7190-7191. (b) Lin, M.; Hogan, T. E.; Sen, A.
J . Am. Chem. Soc. 1996, 118, 4574-4580. (c) Fujiwara, Y.; Takaki,
K.; Watanabe, J .; Uchida, Y.; Taniguchi, H. Chem. Lett. 1989, 1687-
1688.
(4) Sakakura, T.; Ishida, K.; Tanaka, M. Chem. Lett. 1990, 585-
588.
(8) (a) Tanaka, M.; Sakakura, T. Adv. Chem. Ser. 1992, 230, 181-
196. (b) Sakakura, T.; Sodeyama, T.; Sasaki, K.; Wada, K.; Tanaka,
M. J . Am. Chem. Soc. 1990, 112, 7221-7229. (c) Sakakura, T.;
Sodeyama, T.; Tanaka, M. New. J . Chem. 1989, 13, 737-745. (d)
Sakakura, T.; Abe, F.; Tanaka, M. Chem. Lett. 1991, 297-298. (e)
Sakakura, T.; Ishiguro, K.; Okano, M.; Sako, T. Chem. Lett. 1997,
1089-1090.
(5) (a) Chemical Synthesis Using Supercritical Fluids; J essop, P. G.,
Leitner, W., Eds.; Wiley-VCH: Weinheim, 1999. (b) J essop, P. G.;
Ikariya, T.; Noyori, R. Chem. Rev. 1999, 99, 475-493. (c) Tumas, W.;
et al. In Green Chemistry; Anastas, P. T., Williamson, T. C., Eds.; ACS
Symposium Series 626; American Chemical Society: Washington, DC,
1996; pp 132-151.
(9) Stoichiometric formation of acetaldehyde from methane and CO
under extremely high pressure was reported by a Russsian group.
Enikolopyan, N. S.; Menchikova, G. N.; Grigoryan, E. A. Dokl. Akad.
Nauk SSSR 1986, 291, 110-113.
(10) (a) Rosini, G. P.; Boese, W. T.; Goldman, A. S. J . Am. Chem.
Soc. 1994, 116, 9498-9505. (b) Boyd, S. E.; Field, L. D.; Partridge, M.
G. J . Am. Chem. Soc. 1994, 116, 9492-9497.
(6) Six, C.; Gabor, B.; Gorls, H.; Mynott, R.; Philipps, P.; Leitner,
W. Organometallics 1999, 18, 3316-3326.
(11) We have found that the reaction of RhCl(PMe3)3 with benzene
under irradiation gives the C-H bond addition product, (C6H5)(H)-
RhCl(PMe3)3. The details will be reported separately.
(7) Anastas, P. T.; Warner, J . C. Green Chemistry, Theory and
Practice; Oxford University: Oxford, 1998.
10.1021/jo0102967 CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/28/2001

Notes
J . Org. Chem., Vol. 66, No. 15, 2001 5263
a
Ta ble 2. Deh yd r ogen a tion of Cycloocta n e in Den se CO2
Sch em e 1
yield of
catalyst
precursor
cyclooctene
(turnover)
entry
reaction media
1
2
3
4
[RhCl(PMe3)2]2
[RhCl(PMe3)2]2
RhCl(PMe3)3
RhCl(PMe3)3
cyclooctane
cyclooctane + CO2
cyclooctane
23
322
58
cyclooctane + CO2
602
a
Reaction conditions: Rh complex 0.014 mmol/Rh, cyclooctane
(148 mmol) for entries 1 and 3, cyclooctane (74 mmol) and CO2
150 atm for entries 2 and 4, 50 °C, 48 h, irradiated with a 250 W
high-pressure mercury lamp.
methylrhodium complex will undergo CO insertion and
reductive elimination to afford acetaldehyde.
The catalytic activity of RhCl(CO)(PMe3)2 was not very
high probably due to the insufficient solubility in the
methane-CO2 mixture. Indeed, the use of a perfluoro-
phosphine ligand resulted in a completely homogeneous
reaction mixture and produced a higher yield (Table 1,
entry 2). The Wilkinson-type complex, RhCl(PMe3)3, was
also significantly soluble in the methane-CO2 mixture
to achieve a higher catalytic activity compared with
RhCl(CO)(PMe3)2 (Table 1, entry 3). The addition of a
small amount of benzaldehyde (18 µL) was quite effective
for promoting the reaction (Table 1, entry 4). Acetalde-
hyde was obtained in 44% yield based on carbon mon-
oxide. A possible explanation for this promoting effect is
the participation of a radical mechanism via the photo-
activation of benzaldehyde.12 Another factor to control the
catalytic activity may be the solubility of the rhodium
species. Indeed, the addition of benzaldehyde kept the
reaction mixture homogeneous throughout the reaction,
while rhodium complexes were deposited out during the
reaction in the absence of benzaldehyde. On the other
hand, the higher reaction temperature led to the lower
yield (Table 1, entry 5).8b,13 It is also noteworthy that the
carbonylation of methane takes place with a turnover
comparable to that of benzene (Table 1, entries 3 and 6).
Hence, we can conclude that molecular catalysis in dense
carbon dioxide is a powerful tool for alkane transforma-
tion.
Since it was disclosed that dense carbon dioxide is
promising as a reaction medium for the alkane carbony-
lation, we next investigated the dehydrogenation of
cyclooctane in cyclooctane-CO2 mixture catalyzed by the
RhCl(L)(PMe3)2-hv system (eq 2).8a,c,d The dehydrogena-
tion also proceeds via the oxidative addition of C-H bond
(Scheme 1). Experiments in dense carbon dioxide were
carried out in a similar manner described for the meth-
ane carbonylation and were compared with the reactions
in neat cyclooctane. These results are shown in Table 2.
In all the cases, the catalysts were soluble in the reaction
mixtures.
(Table 2, entry 1). In contrast, the reaction in the
cyclooctane-CO2 mixture proceeded much more effi-
ciently (Table 2, entry 2). It is noteworthy that nearly
the theoretical amount of hydrogen was detected in the
reaction mixture and the formation of formic acid was
negligible. Similarly, the dehydrogenation using RhCl-
(PMe3)3 proceeded smoothly in the cyclooctane-CO2 mix-
ture (entries 3 and 4). The yield of cyclooctene reached
11.4% based on the starting cyclooctane.
As for the reactions under CO2 pressure, CO2 com-
plexes such as RhCl(CO2)(PMe3)2 would be involved.
However, in the dark, we have not yet succeeded in
observing such an indication in 31P NMR spectra of
15
[RhCl(PR3)2]2 or RhCl(PR3)3 up to 60 atm of CO2 and
in high-pressure IR spectra up to 50 atm.16 To investigate
the complex structure after the photocatalysis, the reac-
tion mixture was evaporated, and the residue was
analyzed by 31P NMR in C6D6. The major species was
assigned to RhCl(CO)(PMe3)2 along with a small amount
of trimethylphosphine oxide.17 Hence, the acceleration
effect in the cyclooctane-CO2 mixture is at least partly
ascribed to the change in the complex structure.
In summary, we have revealed that dense carbon
dioxide is an efficient reaction medium for C-H bond
activation by molecular catalysis. Please note that carbon
dioxide has many advantages over previously proposed
reaction media. It is nontoxic, nonflammable, abundant,
inexpensive, highly miscible with hydrocarbons, and
easily separated from the reaction mixture. We are
currently eager to apply this methodology to various
alkane transformations.
Ack n ow led gm en t. We are indebted to Professor
Istva`n T. Horva`th for generous information about high-
pressure NMR tube assembly. This study was supported
by the Industrial Technology Research Grant Program
in 2000 from the New Energy and Industrial Technology
Development Organization (NEDO) of J apan.
J O0102967
(14) (a) Maguire, J . A.; Boese, W. T.; Goldman, A. S. J . Am. Chem.
Soc. 1989, 111, 7088-7093. (b) Maguire, J . A.; Boese, W. T.; Goldman,
M. E.; Goldman, A. S. Coord. Chem. Rev. 1990, 97, 179-192. (c) Margl,
P.; Ziegler, T.; Blo¨chl, P. E. J . Am. Chem. Soc. 1996, 118, 5412-5419.
(15) High-pressure NMR measurements were performed using
sapphire tubes designed by Horva`th et al.: Horva`th, I. T.; Millar, J .
M. Chem. Rev. 1991, 91, 1339-1351.
(16) These results are in contrast with a previous report on the facile
RhCl(CO2)(PBu3)2 formation from RhCl(PBu3)3 under atmospheric CO2.
See, Aresta, M.; Nobile, F. Inorg. Chim. Acta 1977, 24, L49-L50.
(17) [RhCl(PMe3)2]2 was converted to RhCl(CO)(PMe3)2 in 33% yield
after the reaction based on the UV and 31P NMR spectra. No rhodium
species other than RhCl(CO)(PMe3)2 were observed.
[RhCl(PMe3)2]2, an equivalent of the 14-electron spe-
cies,14 was not a very active catalyst in neat cyclooctane
(12) Boese, W. T.; Goldman, A. S. J . Am. Chem. Soc. 1992, 114, 350-
351. One of the referees addressed the doubt about the rationality of
the hydrogen abstraction from methane by photoexcited benzaldehyde
because the reaction is thermoneutral.
(13) Rosini, G. P.; Zhu, K.; Goldman, A. S. J . Organomet. Chem.
1995, 504, 115-121.