Ruthenium-catalyzed hydroformylation of alkenes by using carbon dioxide as the carbon monoxide source in the presence of ionic liquids

Article abstract of DOI:10.1002/cctc.201402226

The reaction of [BMI·Cl] (BMI=1-butyl-3-methylimidazolium) or [BMMI·Cl] (BMMI=3-butyl-1,2-dimethylimidazolium) with Ru ₃(CO)₁₂ generates Ru-hydride-carbonyl-carbene species in situ that are efficient catalysts for a reverse water gas shift/ hydroformylation/hydrogenation cascade reaction. The addition of H ₃PO₄ increased the catalytic activity of the first step (i.e., the hydrogenation of CO₂ to CO). Under the optimized reaction conditions [120°C and 6.0 MPa CO₂/H₂ (1:1) for 17 h], cyclohexene and 2,2-disubstituted alkenes were easily functionalized to alcohols through sequential hydroformylation/carbonyl reduction.

Full text of DOI:10.1002/cctc.201402226

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DOI: 10.1002/cctc.201402226
Ruthenium-Catalyzed Hydroformylation of Alkenes by
using Carbon Dioxide as the Carbon Monoxide Source in
the Presence of Ionic Liquids
Meher Ali, Aitor Gual, Gꢀnter Ebeling, and Jairton Dupont*[a]
Dedicated to the memory of Prof. Roberto F. Souza, one of the pioneers of ionic liquid phase organometallic catalysis
The reaction of [BMI·Cl] (BMI=1-butyl-3-methylimidazolium) or
[BMMI·Cl] (BMMI=3-butyl-1,2-dimethylimidazolium) with
quences.[21–25] The differences in selectivity could be easily un-
derstood if we look at the hydroformylation and hydrogena-
tion mechanisms catalyzed by RhI and Ru0 systems, as the
active catalytic species under hydroformylation conditions are
the RhÀH and the RuÀ(H)2 species.[7–9,26]
Ru3(CO)12 generates Ru–hydride–carbonyl–carbene species
in situ that are efficient catalysts for a reverse water gas shift/
hydroformylation/hydrogenation cascade reaction. The addi-
tion of H3PO4 increased the catalytic activity of the first step
(i.e., the hydrogenation of CO2 to CO). Under the optimized re-
action conditions [1208C and 6.0 MPa CO2/H2 (1:1) for 17 h],
cyclohexene and 2,2-disubstituted alkenes were easily func-
tionalized to alcohols through sequential hydroformylation/car-
bonyl reduction.
The accepted mechanism for the formation of MÀ(H)2 is the
homolytic cleavage of the HÀH bond, as the formation of MÀH
occurs either by intramolecular (ligands) or intermolecular (ex-
ternal base) heterolysis of the HÀH bond, which favors the for-
mation of MÀH species.[12,27,28] For instance, cationic [RuÀH]+
species species are described as the catallytically active species
in the Ru-catalyzed hydrogenation of acids, cyclic carbonates,
and CO2 to alcohol.[29–34] The effect of the solvent is important
because it was proposed that for Rh-catalyzed hydroaminome-
thylation reactions with the use of imidazolium ionic liquids
(ILs) as the reaction media, the acidic protons of the imidazoli-
um ring could shift the equilibrium between RhÀH and [RhÀ
(H)2]+ to the formation of the hydrogenation catalytically
active [RhÀ(H)2]+ species.[28] This hypothesis was confirmed by
the observation that the presence of 3-alkyl-1,2-dimethylimida-
zolium salts slowed the hydrogenation rates.
Carbon dioxide sequestration and its use as a feedstock in in-
dustrial processes are major challenges in the development of
alternative greener and sustainable processes.[1] Indeed, several
catalytic processes are under investigation, and they include
the incorporation of CO2 through the addition to epoxides to
generate organic carbonates, diols, and polycarbonates by
using either organic or metal-based catalysts.[2] However, the
substitution of carbon monoxide (CO) by carbon dioxide (CO2)
in carbonylation reactions[3–6] may allow for new applications
with broader use for this important and abundant but poorly
reactive substrate. Indeed, at the industrial scale, the hydrofor-
mylation[7,8] of alkenes is one of the most important applica-
tions of homogeneous catalysis, and over 9 million tons of so-
called oxo products, such as aldehydes and alcohols, are pro-
duced each year.[9]
The potential use of Ru as a hydroformylation catalyst is of
interest, as Rh is much less abundant,[7–9] and several Ru-cata-
lyzed processes involving hydroformylation/hydrogenation[21–25]
and hydroaminomethylation[17] were recently developed. How-
ever, a much more interesting system is that which can cata-
lyze cascade reactions (i.e., CO2 reduction to CO, hydroformyla-
tion, and hydrogenation sequence to produce alcohols of high
added value).There are reports that this process is viable if
either a Ru-based system with LiCl salts dissolved in N-methyl-
pyrrolidone[35–37] or a biphasic imidazolium ILs/toluene[38–41]
(Figure 1) system is used. Advances in this area revealed that if
the Ru-catalyzed hydroformylation of 1-hexene (using CO2 as
the CO source) was performed by using 1-butyl-3-methylimida-
zolium chloride [BMI·Cl] salts[42] (30 equiv. IL/equiv. Ru) as the
solvent and a 1:1 mixture of [BMI·Cl] (15 equiv. IL/equiv. Ru)
and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)i-
Rh-modified systems are the catalysts of choice for this reac-
tion, as they are highly active and selective for the formation
of aldehydes,[7,8] as well as for cascade reactions involving hy-
droformylation/Witting,[10] hydroaminomethylation sequen-
ces,[11,12] and hydroformylation/cyclization reactions.[13–16] Other
metals, such as Pt and Ru, have been much less studied owing
to their high hydrogenation activity, which results in the for-
mation of large amounts of alkanes as byproducts.[17–20] How-
ever, the use of Rh–Ru bimetallic systems is of interest for the
production of alcohols by hydroformylation/hydrogenation se-
[a] M. Ali, Dr. A. Gual, Dr. G. Ebeling, Prof. Dr. J. Dupont
Laboratory of Molecular Catalysis, Institute of Chemistry
Universidade Federal do Rio Grande do Sul
Avenida Bento GonÅalves, 9500, Porto Alegre 91501-970 RS (Brazil)
Fax: (+55)51-3308-7304
E-mail: jairton.dupont@ufrgs.br
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402226.
Figure 1. Organic solvents and additives used in the hydroformylation
reactions.
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mide [BMI·NTf2] (15 equiv. IL/equiv. Ru), alcohol yields of up to
51 and 82%, respectively, were achieved.[39] Recent reports also
describe the use of LiCl–benzyltriethylammonium chloride
(BTAC; Figure 1) salts in the Ru-catalyzed hydroaminomethyla-
tion of cyclopentene with morpholine, which resulted in prod-
uct yields up to 98% after 5 days of reaction at 1608C.[43]
Herein, we report the Ru-catalyzed hydroformylation of alkenes
by using CO2 as a CO source in the presence of [BMI·Cl] and 3-
butyl-1,2-dimethylimidazolium chloride [BMMI·Cl] salts. The in-
volvement of Ru–carbene complexes as the catalytically active
species was revealed by NMR experiments by using labeled 13C
compounds (CO, CO2, and 13C-2-BMI·Cl) in reactions promoted
by Ru3(CO)12 in the presence of [BMI·Cl]. Finally, the use of acid
additives, such as H3PO4, improved both the catalytic activity
and the selectivity.
sions and much lower oxo-product selectivities (less than 31%)
were obtained, which thus confirms the key role of the chlor-
ides (Table 1, entry 1 vs. entries 4 and 5). Interestingly, an im-
provement in the oxo-product selectivity (up to 98%) with
similar alcohol selectivities in the oxo-product fraction, but
with much lower conversions (lower than 51%), was achieved
by using [BMMI·Cl] (Table 1, entry 3 vs. 6). This behavior can be
explained if we assume the presence of an equilibrium be-
tween the MÀ(H)2 and MÀH species, as was proposed for the
hydroaminomethylation reaction.[28] Note that similar catalytic
activity and selectivity was obtained in the hydroformylation of
cyclohexene with the use of CO instead of CO2 catalyzed by
[BMI·Cl]/Ru3(CO)12 (Table 1, entry 7). The hydrogenation
(3.0 MPa) of cyclohexene by the catalytic mixture [BMI·Cl]/
Ru3(CO)12 was only marginal (less than 1%) in the absence of
CO2 (Table 1, entry 8). Notably, in this hydrogenation reaction
the amount of oxo products was almost the same expect for
the total consumption of CO present on the catalyst precursor.
Therefore, the presence of CO2 is necessary to induce the for-
mation of the catalytically active species for the C=C hydroge-
nation and/or to increase the solubility of the alkene into the
IL catalytic phase. The effect of the molar ratio of [BMI·Cl]/Ru
was studied by performing the Ru-catalyzed hydroformylation
of 1-hexene (a substrate with high hydrogenating tendency)
by using CO2 as a CO source (Figure S1, Supporting Informa-
tion). It was found that the yield of the oxo product drastically
increased if the molar ratio of [BMI·Cl]/Ru was raised from 0.3
to 3.0, whereas a [BMI·Cl]/Ru molar ratio of 16.3 was optimal
for reducing the undesired hydrogenation of the alkene.
First, the effect of the metallic precursor on the catalytic per-
formance was investigated by performing the hydroformyla-
tion of cyclohexene with the use of RuCl3·nH2O, [RuCl2(cod)]n
(cod=1,5-cyclooctadiene), and Ru3(CO)12 as catalytic precursors
in the presence of [BMI·Cl] (16.3 equiv. of IL/equiv. of Ru) under
6.0 MPa CO2/H2 (1:1) at 1208C for 17 h (Table 1, entries 1–3).
Under these reaction conditions, the ruthenium RuCl3·nH2O
and [RuCl2(cod)]n precursors exclusively formed hexane, with
conversions up to 99 and 90%, respectively (Table 1, entries 1
and 2). In contrast, Ru3(CO)12 produced conversions up to 96%
with oxo-product selectivity up to 83% and alcohol selectivity
in the oxo-product fraction up to 94% (Table 1, entry 3). The
effect of the counterion of the IL was studied by performing
the reaction in the presence of 3-butyl-1-methylimidazolium
bromide [BMI·Br] and iodide [BMI·I]. They yielded similar alco-
hol selectivities in the oxo-product fraction, but lower conver-
It is known that carbenes are formed by heat treatment of
Ru3(CO)12 in the presence of imidazolium chloride salts.[44] To
check the possible involvement of carbenes, the species
formed by treating Ru3(CO)12with [BMI·Cl] were investigated by
1
NMR spectroscopy (Figures S2 and S3). The H NMR spectra re-
Table 1. Ru-catalyzed hydroformylation of cyclohexene by using CO2 as
the CO source.[a]
vealed the presence of hydride signals at d=À12.2, À12.3,
À13.2, À17.6, and À19.8 ppm (Figure S2), whereas the pres-
ence of signals at d=160.7 and 194.1 ppm in the 13C NMR
spectra (Figure S3) suggested the formation of Ru–hydride–car-
bonyl–carbene complexes under the hydroformylation reaction
conditions. Experiments performed with the use of 13C-labeled
compounds were performed to confirm the assignment of
these signals in the 13C NMR spectra (see Figure 2).
Entry
Precursor
IL
Additive[b] Conv.[c] Sel.[c] Sel.[c]
[%]
[%]
[%]
Notably, the signals for the hydride at very high field (d=
À17.6 and À19.8 ppm) in the 1H NMR spectrum (Figure S2),
which is typical for a hydride, and the characteristic carbene
signal at d=160.7 ppm in the 13C NMR spectrum for the reac-
tion without CO2/H2 indicate the formation of a RuII–hydride–
NHC–carbene species (NHC=N-heterocyclic carbene).[45–47] This
complex is probably a result of the oxidative addition of a CÀH
imidazolium bond to the Ru0 center, similar to that reported
for Ni, Pd, and Ir.[48,49] However, heat treatment under CO2/H2
could also favor the formation of carbene complexes by reac-
tion of the imidazolium carboxylate with the ruthenium com-
plexes.[50–52] Notably, similar ruthenium biscarbene complexes
promote the reverse water gas shift, and thus, these species
could be involved in our reaction mechanism.[53] In fact, gas
chromatography thermal conductivity detector (GC–TCD) anal-
1
2
3
4
5
6
7
8
RuCl3·nH2O
[RuCl2(cod)]n [BMI·Cl]
[BMI·Cl]
–
–
–
–
–
–
–
–
>99
90
96
78
71
51
86
6.8
92
93
–
–
–
–
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
[BMI·Cl]
[BMI·Br]
[BMI·I]
[BMMI·Cl]
[BMI·Cl]
[BMI·Cl]
[BMI·Cl]
[BMMI·Cl] P(OEt)3
[BMI·Cl] H3PO4
[BMMI·Cl] H3PO4
83
31
31
98
98
91
82
93
95
97
94
96
94
93
96
85
93
93
99
98
[d]
[e]
9
P(OEt)3
10
11
12
85
>99
[a] Reaction conditions: Cyclohexene (20.0 mmol), ionic liquid (5.1 mmol),
cyclohexene/Ru=64 (1.6 mol%), IL/Ru=16.3, 6.0 MPa CO2/H2 (1:1),
1208C, 17 h. [b] Additive/Ru=3.0. [c] Conversion and selectivity as deter-
mined by GC–MS and GC. [d] Using CO instead of CO2. [e] Using only H2
(3.0 MPa).
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to 93%) with similar product selectivities were obtained
(Table 1, entry 6 vs. 10). However, the 31P NMR spectrum of the
crude reaction mixture revealed the formation of H3PO4 by oxi-
dation/hydrolysis under the reaction conditions. Therefore, ex-
periments were performed by adding H3PO4 instead of P(OEt)3,
which resulted in an improvement in the selectivity of the oxo
product and the alcohol selectivity in the oxo-product fraction
(Table 1, entries 9 and 10 vs. 11 and 12) in both cases. In partic-
ular, the conversion to the oxo products increased from 51%
without acid to 99% in the presence of H3PO4 (Table 1, entry 6
vs. 12). The acid likely facilitates hydride transfer and protonol-
ysis, which are key steps for the addition of hydrogen to car-
boxylate groups.[29] Therefore, the process occurs through
known mechanisms: CO2 hydrogenation to CO, hydroformyla-
tion, and aldehyde reduction by Ru–carbene complexes.
Under the optimized reaction conditions, cyclic disubstituted
alkenes (cyclooctene and norbornene), cyclic disubstituted
dienes (1,5-cyclooctadiene), cyclic disubstituted conjugated
dienes (1,3-cyclooctadiene), and 2,2’-disubstituted alkenes
[1-methylsytrene, (R)-(+)-limonene, and carvone] were easily
hydroformylated, which thus led to the alcohols with moder-
ate-to-high conversions (74 to >99%) and selectivities (69 to
93%, Figure 3).
Alkenes such as 2-methyl-2-butene, which are hindered, dis-
played low conversions (25%) with selectivities up to 90%. Ter-
minal alkenes, such as 1-hexene, gave full conversions, with
the production of internal hexenes (selectivity up to 43%), hex-
anes (selectivity up to 19%), and heptanols (selectivity up to
38%). Substrates bearing alkenes conjugated with aromatic
groups (styrene and indene) gave full conversions with high
hydrogenated product selectivity. This behavior was ascribed
to the faster hydrogenation rate of the C=C bond of these
Figure 2. 13C{1H} NMR (100 MHz, CDCl3) spectra of Ru3(CO)12 dissolved in
BMI·Cl and warmed at 1208C for 16 h: a) under 6.0 MPa CO2/H2 (1:1),
b) under 6.0 MPa 13CO/CO2/H2 (1:30:30), c) under 6.0 MPa 13CO2/H2 (1:1),
d) BMI·Cl marked with 13C at the C-2 position and under 6.0 MPa CO2/H2
(1:1), and e) BMI·Cl marked with 13C at the C-2 position and under an argon
atmosphere.
ysis of the gas phase during the reaction showed the presence
of CO together with CO2 and H2. Moreover, the 13C NMR spec-
trum of the reaction by using 13CO2 shows the signals of IL-dis-
solved CO2 (d=121.7 ppm)[54] and, more importantly, the
signal at d=160.1 ppm (Figure 2) corresponding to formic
acid, which is the primary product of CO2 hydrogenation. In
the case of [BMMI·Cl], it is probable that reaction with the Ru0
precursor also produced carbenes by C-4/C-5 of the imidazoli-
um ring (“abnormal type”), which have been observed and iso-
1
lated with other metals.[55,56] Indeed, the H NMR and 13C NMR
spectra of the reaction between [BMMI·Cl] and Ru3(CO)12
showed similar signals corresponding to hydrido, carbonyl, and
carbene ligands (see Figure S3).
The effect of using P ligands as additives on the catalytic
performance of Ru was also explored. The use of monodentate
and bidentate phosphine ligands as additives resulted in much
lower conversions and higher hydrogenation product yields
(see Table S1). In particular, no appreciable effect on the con-
version or product selectivity was observed if P(OEt)3 was used
as the additive in the presence of [BMI·Cl] (Table 1, entry 3 vs.
9). In the presence of [BMMI·Cl], much higher conversions (up
Figure 3. Alkene conversion and alcohol selectivity obtained from the Ru-
catalyzed hydroformylation of alkenes by using CO2 as the CO source. Reac-
tion conditions: Alkene (20.0 mmol), [BMMI·Cl] (5.1 mmol), 2,2’-disubstituted
alkene/Ru=64 (1.6 mol%), IL/Ru=16.3, H3PO4/Ru=3.0, 6.0 MPa CO2/H2 (1:1),
1208C, 17 h.
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that of the other two processes (reverse water gas shift and
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The hydrogenation of CO2 to CO was facilitated by the addi-
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Experimental Section
We used a common procedure for the hydroformylation of alkenes
by using CO2 as the CO source. In a typical experiment, the sub-
strate (20.0 mmol) was added to a 100 mL reactor vessel (Parr
Micro-reactor 4590) containing the Ru precursor (1.6 mol%) and
the imidazolium IL. The reactor was pressurized with CO2/H2 and
warmed to the desired reaction temperature for 17 h. The reactor
was cooled, and the products were extracted with Et2O (3ꢂ15 mL).
The products were determined by GC–MS analysis (DB-17; Tinjector
=
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Keywords: carbon dioxide · hydroformylation · ionic liquids ·
reverse water gas shift · ruthenium
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Received: April 10, 2014
Published online on July 7, 2014
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