A general and efficient iridium-catalyzed hydroformylation of olefins

Article abstract of DOI:10.1002/anie.201001972

Breaking with conventional wisdom: Hydroformylation catalysts are generally based on rhodium; earlier, cobalt was used. Iridium, which is less expensive than rhodium, was considered too unreactive. However, iridium/phosphine complexes have now been shown to form active catalysts for the hydroformylation of olefins under mild conditions (see scheme; R¹, R²=H, alkyl, aryl; R³=H, alkyl). Competing hydrogenation side reactions can be suppressed. Copyright

Full text of DOI:10.1002/anie.201001972

Communications
DOI: 10.1002/anie.201001972
Hydroformylation
A General and Efficient Iridium-Catalyzed Hydroformylation of
Olefins**
Irene Piras, Reiko Jennerjahn, Ralf Jackstell, Anke Spannenberg, Robert Franke, and
Matthias Beller*
Hydroformylation is defined as the catalytic addition of a
formyl group to olefins or alkynes to give linear and branched
aldehydes with complete atom efficiency (Scheme 1).[1]
Today, it is the most widely applied homogeneously catalyzed
process in industry.[2] Annually more than 8 million tons of so-
called oxo products are formed. These compounds are mainly
converted into plasticizers and detergent alcohols.
hydroformylation reactions,[8] we were recently attracted to
the study of less common hydroformylation catalysts.[9]
Herein, we present a general and efficient iridium-
catalyzed hydroformylation reaction of olefins that proceeds
under mild conditions. Notably, competing hydrogenation
side reactions can be suppressed.
Despite industrial applications, such as the Cativa process
for the synthesis of acetic acid from methanol,[10] and
economical advantages over rhodium, iridium catalysts have
remained undeveloped in carbonylation reactions.[11,12] Owing
to their significantly lower reactivity, iridium complexes have
so far mostly been used as models in kinetic and mechanistic
studies of hydroformylation reactions.[13] The major problem
concerning the synthetic application of iridium-catalyzed
hydroformylation reactions has been the competing hydro-
genation activity of iridium complexes under hydroformyla-
tion conditions. Thus, large amounts of unwanted alkanes
were produced, and low chemoselectivity was observed.
Interestingly, Haukka and co-workers[12] reported that the
addition of inorganic salts suppressed hydrogenation side
reactions; however, only low catalyst activities (turnover
frequency < 0.5 hÀ1) were observed for aldehydes.
In our initial studies, we tested different solvents and
iridium precursors under a given set of conditions with 1-
octene as the model olefin. All high-pressure experiments
were performed in new uncontaminated autoclaves. In
general, we used 0.2 mol% of the respective iridium pre-
cursor and 0.4 mol% of triphenylphosphane as the added
ligand. The solvent has a significant influence on the chemo-
selectivity of the reaction (Table 1). In particular, polar
solvents, such as THF, diglyme, and N-methylpyrrolidone
Scheme 1. Formation of linear and branched aldehydes by the hydro-
formylation of terminal olefins.
This reaction of olefins with synthesis gas was discovered
by Roelen as early as 1938 during investigations on the origin
of oxygenated products obtained in cobalt-catalyzed Fisher–
Tropsch reactions.[3] Until the early 1970s, when the first
rhodium-based processes were commercialized, cobalt cata-
lysts dominated academic and industrial hydroformylation
chemistry. Since then, both in industry and in academic
laboratories, the majority of studies have focused on rhodium-
based catalysts.[4,5] Furthermore, comparably few investiga-
tions on asymmetric hydroformylation reactions with plati-
num catalysts have been carried out.[6] This limited interest in
alternative hydroformylation catalysts is partially explained
by the generally accepted order of activity of unmodified
metal carbonyl complexes.[7] Thus, it has been established that
rhodium is by far the most active system, followed by cobalt.
Other metals, such as Ru, Ir, Os, Pt, Pd, Fe, and Ni, display
significantly lower activities. However, such activities have
been established under a fixed set of reaction conditions and
might be changed by the use of other ligands and different
reactions conditions. Owing to our interest in biphasic
Table 1: Effect of the solvent and catalyst precursor on the iridium-
catalyzed hydroformylation of 1-octene.[a]
Entry Ir source
Solvent Yield[b] n/iso
H[c] Octene
[%]
[%] isomers[d] [%]
1
2
3
4
5
6
7
8
[Ir(cod)acac] THF
58
[Ir(cod)acac] o-xylene 61
[Ir(cod)acac] heptane 27
[Ir(cod)acac] diglyme 57
[Ir(cod)acac] toluene 50
76:24 14
75:25 23
68:32 61
75:25 17
75:25 33
2
[*] I. Piras, R. Jennerjahn, Dr. R. Jackstell, Dr. A. Spannenberg,
Prof. M. Beller
2
10
2
5
1
Leibniz-Institut fꢀr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
Fax: (+49)381-1281-51113
E-mail: matthias.beller@catalysis.de
Homepage: http://www.catalysis.de
[Ir(cod)acac] NMP
[{Ir(cod)Cl}2] THF
[Ir(cod)2]BF4 THF
74
30
42
74:26
9
71:28 47
74:26 12
10
12
Dr. R. Franke
Evonik Oxeno GmbH, Marl (Germany)
[a] Reaction conditions: 1-octene (5.1 mmol), 0.2 mol% Ir, PPh3
(2.2 equiv), syngas (40 bar) introduced at room temperature, solvent
(2 mL), 1008C, 16 h. [b] The yield was determined by GC. [c] Yield of
octane formed through hydrogenation. [d] Yield of isomers of 1-octene:
2-octene@3-octene. acac=acetylacetonate, cod=1,5-cyclooctadiene.
[**] This research was funded by Evonik Oxeno GmbH and the Deutsche
Forschungsgemeinschaft (Leibniz Prize). We thank Dr. C. Fisher, S.
Buchholz, and S. Schareina for their excellent technical and
analytical support.
280
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 280 –284

Table 3: Effect of the phosphine ligand on the iridium-catalyzed hydro-
(NMP), favored the desired hydroformylation over hydro-
genation of the olefin. [Ir(cod)acac] was found to be the best
catalyst precursor in terms of chemoselectivity. As a result of
its low cost and ease of handling, triphenylphosphane was
chosen as the ligand for further investigations on the influence
of pressure and temperature on the benchmark reaction
(Table 2). Lowering of the synthesis-gas pressure generally
formylation of 1-octene.[a]
Entry
Ligand (or complex)
L/Ir
Yield[b]
[%]
n/iso
H[c]
[%]
1[d]
2[d]
3[d]
4
–
PPh3
PPh3
PPh3
PPh3
[HIrCO(PPh3)3]
((MeO)3C6H2)3P
L1
dppb
L2
PCy3
L3
–
1
4
3
30
44
61
81
83
80
43
38
41
85
26
7
52:48
72:28
76:24
76:24
76:24
76:24
86:14
71:29
74:26
72:28
67:33
74:26
77:23
74:26
65
43
13
12
12
13
37
52
9
12
55
3
16
41
5
2.2
–
2.2
2.2
1
2.2
2.2
2.2
2.2
–
Table 2: Optimization of reaction conditions for the iridium-catalyzed
6[e]
7
hydroformylation of 1-octene.[a]
Entry
T
Pressure
Yield[b] n/iso
[%]
H[c] Octene
8
9
[8C] [bar]
[%] isomers[d] [%]
1[e]
2[e]
3[e]
4[e]
5[e]
6[e]
7[f]
8[g]
9[h]
120 40 (CO/H2 1:1) 70
100 40 (CO/H2 1:1) 69
80 40 (CO/H2 1:1) 13
100 50 (CO/H2 1:1) 62
100 20 (CO/H2 1:1) 74
100 10 (CO/H2 1:1) 72
100 20 (CO/H2 2:1) 73
100 20 (CO/H2 2:1) 77
100 20 (CO/H2 3:1) 71
71:29 29
74:26 20
75:25
74:26 17
75:25 20
75:25 23
75:25 14
76:24 12
1
2
–
1
3
5
3
1
1
10
11
12
13
14[f]
3
L4
58
46
[Ir2(CO)6(PPh3)2]
[a] Reaction
conditions:
1-octene
(10.2 mmol),
[Ir(cod)acac]
(0.2 mol%), CO/H2 (2:1, 20 bar), THF (6 mL), 1008C, 20 h: CO
(7 bar) was introduced at room temperature, and syngas was (13 bar)
introduced at 1008C. [b] The yield was determined by GC analysis.
[c] Yield of octane. [d] Reaction time: 16 h. [e] [HIrCO(PPh3)3]
(0.2 mol%) was used as the catalyst. [f] [Ir2(CO)6(PPh3)2] (0.2 mol%)
was used as the catalyst. Cy=cyclohexyl, dppb=1,4-bis(diphenylphos-
phanyl)butane.
76:24
8
[a] Reaction
conditions:
1-octene
(10.2 mmol),
[Ir(cod)acac]
(0.2 mol%), PPh3 (2.2 equiv), THF (6 mL), 16 h. [b] The yield was
determined by GC analysis. [c] Yield of octane. [d] Yield of isomers of 1-
octene: 2-octene@3-octene. [e] Syngas was introduced when the
reaction temperature had been reached. [f] CO (7 bar) and syngas
(13 bar) were introduced at 1008C. [g] CO (7 bar) was introduced at
room temperature, and syngas (13 bar) was introduced at 1008C. [h] CO
(10 bar) was introduced at room temperature, and syngas (10 bar) was
introduced at 1008C.
resulted in higher reaction rates. Thus, when the reaction was
carried out at 20 bar, the yield of aldehydes rose to 74%
(Table 2, entry 5). An increase in the partial pressure of
carbon monoxide improved the chemoselectivity further for
formation of the aldehydes (Table 2, entries 7 and 8). While
studying the effects of different partial pressures, we observed
that the introduction of CO at room temperature and
subsequent pressurization of the synthesis gas at 1008C led
to a significant increase in the desired chemoselectivity and
promoted the carbonylation reaction over hydrogenation.
Since it is well-known that the ligand constitutes a key
element in hydroformylation catalysts, we studied the influ-
ence of different ligands and the ligand concentration. The
use of a higher concentration of the phosphine did not lead to
an increase in the reaction rate (Table 3). Indeed, NMR
spectroscopic investigations showed that only two phosphine
ligands are bound to the iridium center under catalytic
conditions. Among the various mono- and bidentate phos-
phorus ligands tested, PPh3 was found to be optimal in terms
of yield and selectivity (Table 3, entry 5). We also tested the
Scheme 2. Ligands L1–L4.
sterically hindered ligand ((MeO)3C6H2)3P showed improved
regioselectivity of 86:14 in favor of the linear aldehyde;
however, an increased amount of octane was obtained with
this ligand (Table 3, entry 7). Nevertheless, this regioselectiv-
ity is the highest reported so far for hydroformylation with
any iridium catalyst. No significant amounts of isomerized
octene products were detected in any of these reactions.
When the reaction mixture was allowed to stand at 0–
108C for several hours, precipitation of [Ir2(CO)6(PPh3)2] was
observed (Figure 1).[14] The use of this complex as a catalyst in
the hydroformylation of 1-octene gave n-nonanal in 46%
yield with a regioselectivity of 74:26 (Table 3, entry 14).
Apparently, this complex readily forms the “real” active
species. Clearly, the performance of the presented iridium
catalyst system in the benchmark reaction under optimized
conditions is superior to that of all previously reported
defined
hydridocarbonyltris(triphenylphosphane)iridium
complex, which is expected to be formed from the catalyst
precursor (Table 3, entry 6). This complex indeed showed
similar performance to that of the catalyst formed in situ.
Interestingly,
2-(diphenylphospanyl)-1-phenyl-1H-pyr-
role (L2) gave n-nonanal in the highest yield and octane in
only 12% yield (Table 3, entry 10). Most of the ligands used
(Scheme 2; Table 3) gave the linear/branched regioisomeric
products in similar ratios of between 67:33 and 77:23. The
Angew. Chem. Int. Ed. 2011, 50, 280 –284
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www.angewandte.org
281

Communications
Table 4: Comparison of catalyst precursors containing different metals
for the hydroformylation of 1-octene.[a]
Entry Catalyst
Yield[b] n/iso
[%]
Octene
isomers[c] [%] [%] [hÀ1
H[d] TOF[e]
]
1[f]
2[f]
3[g]
[Ir(cod)acac]
PPh3 (8 equiv)
[Ru3CO12]
PPh3 (8 equiv)
[Rh(cod)acac] 75
PPh3 (8equiv)
65
–
76:24
–
2
–
19
–
163
–
60:40 21
3
1255
Figure 1. Molecular structure of the binuclear species [Ir2(CO)6(PPh3)2].
Thermal ellipsoids correspond to 30% probability. Hydrogen atoms
are omitted for clarity.
[a] Reaction conditions: 1-octene (10.2 mmol), metal precursor
(0.02 mol%), PPh3 (8 equiv), CO/H2 (2:1, 20 bar), THF (6 mL), 1208C:
CO (7 bar) was introduced at room temperature, then syngas (13 bar)
was introduced at 1008C. [b] The yield was determined by GC. [c] Yield of
isomers of 1-octene. [d] Yield of octane. [e] Turnover frequency based on
the aldehyde yield. [f] Reaction time: 20 h. [g] Reaction time: 3 h.
iridium catalyst systems. However, iridium-catalyzed hydro-
formylation reactions will only be of broader interest if the
activity of the catalyst is competitive with that of rhodium-
based catalysts and if the catalysts tolerate various substrates.
Hence, we carried out rhodium- and ruthenium-catalyzed
hydroformylation reactions in the presence of triphenylphos-
phane under identical reaction conditions (Table 4).
Whereas the ruthenium catalyst did not show any activity
at all, the iridium and rhodium complexes performed
similarly: [Ir(cod)acac]/PPh3 gave the aldehyde products in
65% yield (n/iso 76:24), and [Rh(cod)acac]/PPh3 gave the
aldehydes in 75% yield with a regioselectivity of 60:40. The
iridium catalyst showed an overall turnover frequency of
163 hÀ1, which is only eight times lower than that of the
rhodium/triphenylphosphane catalyst. However, prior to this
study, rhodium catalysts were considered to be 10000 times
faster than iridium catalysts.[15] If the price advantage of
iridium, a factor of 5–10, is taken into account, iridium is
clearly a promising metal for novel hydroformylation cata-
lysts.[16]
Finally, after suitable process conditions had been estab-
lished, we studied the scope and limitations of the present
reaction protocol and found that the catalyst system is broadly
applicable. Various aliphatic and aromatic olefins were
hydroformylated in high yield (Table 5). To the best of our
Table 5: Scope and limitations of the iridium-catalyzed hydroformylation.[a]
Entry
1
Substrate
Product
t [h]
Yield[b] [%]
n/iso
H[c] [%]
Substrate
isomers [%]
24
62
26:74
6
–
2
3
16
16
85
68:22
8
–
90[d]
68:22[d]
9[b]
<1
4[e]
5
48
16
90
90
–
5
7
–
2
84:16
6
7
8
16
20
20[f]
20
82
71
79:21
75:25
9
5
4
2
69
62
87
89
96:4
97:3
75:25
76:24
9
7
7
6
–
–
1
1
9[g]
10
20
16
11[f]
20
14
4:96
2
2
[a] Reaction conditions: substrate (10.2 mmol), [Ir(cod)acac] (0.2 mol%), PPh3 (2.2 equiv), NMP (6 mL), CO (7 bar), room temperature, then syngas
(13 bar) introduced at 1008C. [b] The yield was determined by GC analysis. [c] Yield of the hydrogenated product ar 1308C. [d] The yield and n/iso
product ratio were determined by NMR spectroscopy. [e] The reaction was carried out with [Ir(cod)acac] (0.4 mol%) in 1308C. [f] The reaction was
carried out in THF. [g] The substrate contained dodecane (2%), ethyloctene (3%), and methyldodecene (2%).
282
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Angew. Chem. Int. Ed. 2011, 50, 280 –284

knowledge, these are the highest aldehyde yields observed
with iridium catalysts for all substrates shown. For example,
the hydroformylation of 1-octene gave aldehydes in 89%
yield (Table 5, entry 10), whereas the best yield observed
previously was only 40% ([(cod)Ir(pnnp)Ir(cod)]BF4
(2 mol%), 808C, CO/H2 (1:1, 30 bar), 6 h; pnnp = 3,5-bis(di-
phenylphosphanylmethyl)pyrazole).[17] Interestingly, no or
very little isomerization occurred; thus, the catalyst system
is highly selective for terminal olefins. In agreement with this
observation, the hydroformylation of 2-octene gave the
corresponding branched aldehyde in low yield (Table 5,
entry 11). Cyclic olefins, such as cyclooctene, reacted well:
cyclooctanecarbaldehyde was obtained in very good yield
(90%; Table 5, entry 4). The hydroformylation of aromatic
olefins[18] proceeded to give the branched aldehyde preferen-
tially (Table 5, entry 1) owing to the increased thermody-
namic stability of the intermediate benzyliridium complex.
In conclusion, we have demonstrated that iridium is a
suitable metal for hydroformylation catalysts. Despite pre-
vious reservations, iridium/phosphine complexes form active
hydroformylation catalysts under mild conditions. These
catalysts promote the efficient hydroformylation of a variety
of olefins. On the basis of our findings, we believe that iridium
complexes will become more popular for this important
transformation in organic synthesis and industrial chemistry.
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Experimental Section
General procedure: The olefin (10.2 mmol) was added under argon to
a solution of the metal precursor (0.2 mol% Ir) and the correspond-
ing ligand (2.2 equiv) in THF or NMP (6 mL). The reaction mixture
was transferred into an autoclave, which was then charged with CO
and heated at the indicated reaction temperature. Syngas was then
introduced. After the indicated reaction time, the autoclave was
cooled to 08C, and the pressure was released. The reaction mixture
was analyzed immediately by gas chromatography; isooctane was
used as an internal standard. The yield was determined by NMR
spectroscopy with benzaldehyde as an internal standard.
X-ray
C42H30Ir2O6P2, Mr = 1077.00, trigonal, space group R3, a =
15.2149(3), c = 28.7461(6) ꢀ, V= 5763.0(2) ꢀ3, Z = 6, 1calcd
crystal-structure
analysis
of
[Ir2(CO)6(PPh3)2]:
¯
=
1.862 gcmÀ3, T= 200 K; 30527 reflections measured, 2819 independ-
ent reflections (Rint = 0.0289), 2448 reflections observed (I > 2s(I));
numerical absorption correction (m = 7.05 mmÀ1, max./min. trans-
mission: 0.4920/0.2372); final R indices (I > 2s(I)): R1 = 0.0158, wR2 =
0.0299; R indices (all data): R1 = 0.0228, wR2 = 0.0308; 157 parame-
ters. Data were collected on a STOE IPDS II diffractometer with
graphite-monochromated MoKa radiation. The structure was solved
by direct methods (SHELXS-97) and refined by full-matrix least-
squares techniques on F2 (SHELXL-97). XP (Bruker AXS) was used
for graphical representation. CCDC 775742 contains the supplemen-
tary crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data request/cif.
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Received: April 2, 2010
Published online: December 3, 2010
Delayed at authorꢁs request
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Keywords: aldehydes · alkenes · homogeneous catalysis ·
hydroformylation · iridium
.
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