Tandem rhodium-catalyzed hydroformylation-hydrogenation of alkenes by employing a cooperative ligand system

Article abstract of DOI:10.1002/anie.201108946

Dual action: A multifunctional rhodium catalyst system enables the simultaneous catalysis of two distinct transformations, hydroformylation of an alkene and reduction of an aldehyde, in a highly selective manner. This one-pot/two-step process is controlled by the cooperative action of two different supramolecular ligand systems and transforms terminal alkenes into C₁-chain-elongated linear alcohols. Copyright

Full text of DOI:10.1002/anie.201108946

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DOI: 10.1002/anie.201108946
Cooperative Catalysis
Tandem Rhodium-Catalyzed Hydroformylation–Hydrogenation of
Alkenes by Employing a Cooperative Ligand System**
Daniela Fuchs, Gꢀraldine Rousseau, Lisa Diab, Urs Gellrich, and Bernhard Breit*
The hydroformylation of olefins is one of the largest industrial
applications of homogeneous catalysis and results in the
production of millions of tons of aldehydes per year.[1] These
aldehydes constitute useful intermediates, but are rarely the
final objective of the industrial chemist, because they are
usually reduced to the corresponding alcohols. In particular
linear alcohols have tremendous industrial applications as
solvents but also as raw materials for plasticizers and
detergents.[2] In most of the cases, these valuable materials
are produced in two separate steps from the terminal alkenes,
namely by a hydroformylation that employs syngas as an
inexpensive one-carbon source, followed by a reduction step
that uses molecular hydrogen and a second catalyst.[3] The
cost of the alcohol is further increased by the requirement to
purify the aldehyde.
Many approaches have been investigated in an attempt to
shorten this sequence, ideally by designing a one-pot tandem
hydroformylation/hydrogenation protocol, in which the alco-
hol would be directly isolated from the reaction mixture. In
1966, chemists from the Shell Oil Company pioneered such
a process and reported the use of cobalt catalysts that are
capable of converting alkenes into alcohols under CO/H2
atmosphere.[4] The main limitations are the moderate yields
and the somewhat harsh reaction conditions that are required.
Many other catalytic systems that are composed of
a ligand associated with a metal, such as Co,[5] Pd,[6] Rh,[7] or
Ru,[8] have been reported as potential solution to this highly
relevant industrial issue.[9] However, none of these systems
operates with satisfying chemoselectivity (alcohol vs. alkane;
the latter resulting from competing direct reduction of the
alkene) and regioselectivity (linear/branched (l/b) regioisom-
ers resulting from unselective hydroformylation). Recently,
Nozaki and co-workers described an elegant approach that
relies on the cooperative use of rhodium- and ruthenium-
based catalysts and results in the formation of the desired
linear alcohols with excellent linear/branched selectivities in
good yields.[10] Our group also reported an example that
fulfills these requirements by using a supramolecular rhodium
catalyst based on the bifunctional ligand L. While the system
showed high activity, its regioselectivity toward the linear
alcohol product was not optimal.[11]
In recent years, the use of cooper-
ative catalysis has enabled the devel-
opment of many tandem processes or
cascade reactions that combine the use
of two or more separate catalytic
systems, which work either in a coop-
erative or successive manner. Such
tandem protocols can be the result of combining Lewis acid
catalysis with Brønsted acid catalysis or Lewis base catalysis
with Brønsted base catalysis, but also any other possible
combination involving organometallic catalysis and organo-
catalysis.[12] For example, Cole-Hamilton and co-workers have
reported an elegant approach for the synthesis of alcohols
from alkenes by combining use of rhodium, Xantphos, and
triethylphosphine.[13]
We herein report a unique multifunctional rhodium
catalyst system that enables the simultaneous catalysis of
two distinct transformations in a highly selective manner,
controlled by the cooperative action of two different ligands
1a and 2a (Scheme 1). These ligands stem from two
Scheme 1. Ligand cooperation for tandem hydroformylation/hydroge-
nation of alkenes.
conceptually different supramolecular catalyst systems that
were developed in our group.[14] Ligand 1a (6-DPPon = 6-
diphenylphosphanylpyridone) has been designed to self-
assemble in the presence of a rhodium(I) center to form
a chelating catalyst system that acts as a highly active and
regioselective hydroformylation catalyst.[15] The acylguani-
dine ligand 2a, which was also developed in our group, is an
enzyme-inspired bifunctional ligand and enables highly
chemoselective hydrogenation of aldehydes by relying on
supramolecular aldehyde activation through hydrogen-bond-
ing.[11] We report the combination of these two approaches in
order to fulfill the following objectives:
[*] D. Fuchs, Dr. G. Rousseau, Dr. L. Diab, U. Gellrich, Prof. Dr. B. Breit
Institut fꢀr Organische Chemie und Biochemie
Freiburg Institute for Advanced Studies (FRIAS)
Albert-Ludwigs-Universitꢁt Freiburg
Albertstrasse 21, 79104 Freiburg i. Brsg. (Germany)
E-mail: bernhard.breit@chemie.uni-freiburg.de
[**] D.F., G.R., and L.D. contributed equally to this work. Support from
the Freiburg Institute for Advanced Studies (FRIAS), the Deutsche
Forschungsgemeinschaft (IRTG 1038), and the Fonds der chem-
ischen Industrie (fellowship to U.G.) is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108946.
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1) one-pot conversion of alkenes to linear alcohols through
hydroformylation/hydrogenation by using a single metal-
lic catalyst;
2) high linear/branched regioselectivity;
3) simultaneous chemoselective reduction of the intermedi-
ate aldehyde with molecular hydrogen gas (no alkene
hydrogenation).
ligand 1a than with 2a (Figure 1). The situation is reversed for
the reduction step, because 1a is unable to initiate the
reduction. These results suggested that a mix of both ligand
types in the presence of a rhodium salt may result in
a favourable cooperation between both catalyst systems,
which may solve the problematic chemo- and regioselectivity
issues of the desired transformation.
To our delight, the product was obtained in excellent yield
(95%, Table 1, entry 4) and with high regioselectivity (97:3)
For such a scenario to be successful, it is important that
both complexes of type I and II exist in solution, and that the
rate of hydroformylation with complex I is significantly faster
than with any other rhodium complex present (Scheme 1).
However, the rhodium/1a catalytic system is unable to reduce
an aldehyde under hydroformylation conditions, and the
hydrogenation step requires the presence of a rhodium
catalyst of type II, which is modified with the acyl guanidine
ligand 2a. In other words, these complexes will have to be
present in an equilibrium and the relative rates of hydro-
formylation and hydrogenation will have to be efficiently
different and simultaneously balanced to allow an efficient
and highly regioselective tandem process.
Initial experiments were carried out with 1-octene as
a model substrate in toluene at 808C with 20 bar syngas
pressure and a catalyst loading of 0.5 mol%. Employing both
PPh3 as well as ligand 1a (6-DPPon) efficient hydroformyla-
tion with excellent regioselectivity for the 6-DPPon system in
favor of the linear aldehyde was noted (Table 1). No traces of
hydrogenation to the corresponding alcohols were detected
(Table 1, entries 1 and 2).[15f] Conversely, nearly complete
conversion to the alcohol was noted when a rhodium catalyst
modified with the acylguanidine ligand 2a was employed
(Table 1, entry 3). However, the regioselectivity was low
(81:19; Table 1, entry 3). Thus, neither the rhodium catalyst
derived from 1a nor that derived from 2a could solve the
chemo- and regioselectivity issues of the reaction on its own.
Interestingly, kinetic data from side-by-side experiments
showed that hydroformylation was significantly faster with
Figure 1. Kinetics of hydroformylation with ligand 1a and 2a independ-
ently.
after 72 hours by using 0.5 mol% of rhodium catalyst and
5 mol% of both ligands 1a and 2a. Unfortunately, the overall
reaction time to achieve quantitative conversion to the
alcohol was too long.
Kinetic studies[16] of this tandem process by employing the
ligand mixture 1a/2a showed that hydrogenation is the rate
limiting step, while hydroformylation is fast (90% conversion
after 3 h with 1a (5 mol%), 2a (5 mol%), [Rh(CO)2acac]
(0.5 mol%), toluene (1m), 808C).
Table 1: Tandem hydroformylation/hydrogenation of 1-octene.
A detailed analysis of the data showed that the hydro-
genation rate for the 1a/2a catalyst system is significantly
slower compared to the activity of the rhodium/2a catalyst on
its own. A reasonable explanation for this behavior is that the
presence of the self-assembling ligand 1a shifts the equilib-
rium between the different rhodium complexes toward
complex I, thus reducing the concentration of catalyst II
and resulting in a lower rate of hydrogenation (Scheme 1).
To solve this problem, we decided to tune the electronic
properties of each ligand according to the following hypoth-
esis. A reduction of the s-donor strength and an increase of
the p-acceptor abilities at the P donor of self-assembling
ligand 1 should furnish a more active hydroformylation
catalyst I.[17] Conversely, an increase of the s-donor capabil-
ities of the P donor of acylguanidine ligand 2 should increase
both the binding constant toward rhodium(I) and the hydro-
genation rate.[18] To test this hypothesis, both electron-rich
and electron-poor 6-DPPon derivatives 1b and 1c, respec-
Entry
L1
L2
RCHO [%][a]
l:b[a]
ROH [%][a]
l:b[a]
1
2
3
PPh3
1a
2a
1a
1a
–
–
–
2a
2b
2b
2b
99
99
2
5
1
82:18
95:5
–
–
–
0
0
–
–
98
95
99
95
95
81:19
97:3
93:7
91:9
96:4
4[b]
5
6
7
1b
1c
5
5
33:67
37:63
[a] Determined by GC analysis. [b] Reaction time was increased to
72 hours. acac=acetylacetonate.
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tively, as well as the more electron-rich acylguanidine ligand
2b were prepared (Table 1).[16]
Indeed, replacing 2a in our 1a/2a catalyst system with the
more electron-rich pyrrole-derived phosphine 2b led to
complete conversion to the alcohol in less than 24 hours
under standard conditions (Table 1). However, the regiose-
lectivity dropped slightly to 93:7 (Table 1, entry 5). Use of
a mixture of more electron-rich 1b and 2b, led to an even
lower regioselectivity of 91:9 (Table 1, entry 6). However, by
employing electron-poor ligand 1c with electron-rich acyl-
guanidine ligand 2b in a 1:1 ratio, we observed nearly full
conversion from the alkene to the desired alcohol in 24 hours
(instead of 72 hours for the parent system 1a/2a) with
excellent regioselectivity in favor of the linear alcohol (96:4;
Table 1, entry 7).
In order to gain deeper insight into the role of each ligand
and the variety of complexes formed during the catalytic
process, we performed kinetics studies by in situ ReactIR
experiments as well as NMR experiments after pressurizing
with syngas.
The reaction kinetics for rhodium catalysts derived from
ligands 1c and 2b independently were studied (Figure 2). The
rhodium catalyst derived from self-assembling ligand 1c is
a significantly faster hydroformylation catalyst than the
rhodium catalyst derived from 2b.[16]
Figure 3. Kinetics of the tandem hydroformylation/hydrogenation of
1-octene with the optimal mixed 1c/2b rhodium catalyst.
and hydrogenation operate simultaneously (after 5 h, 80% of
octene was consumed, 40% aldehyde and 40% alcohol were
formed), which proves the cooperate action of both ligand
systems under these reaction conditions.[19]
That the self-assembled 1c/rhodium catalyst is the kineti-
cally competent hydroformylation catalyst became more
obvious when the regioselectivities were studied during the
course of the reaction. The mixed catalyst system operates
with the same regioselectivity throughout the reaction, as
observed for the single 1c/rhodium catalyst (96/4 = l/b),
compared to the low regioselectivity observed for the 2b/
rhodium catalyst (75/25 = l/b).[16,19]
Figure 2. Kinetics of the tandem hydroformylation/hydrogenation of
1-octene with ligand 1c and 2b independently.
Figure 4. Complexes in the reaction of [HRh(1c)3(CO)] with ligand 2b
under CO/H2 pressure, observed by in situ IR spectroscopy.
Conversely, by studying the hydrogenation step starting
from n-nonanal, it becomes evident that the catalyst derived
from 1c is unable to reduce the aldehyde function (as with
1a). However, the electron-rich acylguanidine 2b/rhodium
catalyst performed with significantly higher activity (turnover
frequency after 50 min = 84 hÀ1) than the parent system with
2a (turnover frequency after 50 min = 40 hÀ1), and thus, 2b is
so far the best ligand for this aldehyde hydrogenation step
under hydroformylation conditions.[16]
In situ IR spectroscopic experiments (Figure 4) corrobo-
rated by DFT calculations support the presence of an
equilibrium between complexes I, II, and III (Scheme 2).[16]
Therefore, we synthesized and isolated a [HRh(1c)3(CO)]
1
complex (characterized by H and 31P NMR spectroscopy).
When this complex was dissolved in toluene, characteristic
bands at 2025 cmÀ1 and 1936 cmÀ1 emerged. Interestingly,
after 2b (3 equiv) was added to the solution, the intensity of
these bands decreased and a new band was detectable at
Additionally, the reaction kinetics of the tandem process
employing the 1c/2b catalyst system was studied and the
results are shown in Figure 3. Obviously, hydroformylation
1996 cmÀ1. This indicates the formation of a [HRh(1c)3Àx
-
(2b)x(CO)] complex. After the solution was pressurized with
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Chemie
[HRh(1c)3Àx(2b)x(CO)] with syngas can be assigned to the
eq–eq and ax–eq conformer of the [HRh(1c)2(CO)2] com-
plex. The band at 1952 cmÀ1 may arise from a [HRh(1c)x-
(2b)y(CO)4-x-y] complex. This supports our suggestion, which
is based on the kinetic studies, that an independent [HRh-
(1c)2(CO)2] complex is responsible for the outstanding
hydroformylation activity of the ligand system presented
herein, because such a complex was detectable by in situ IR
spectroscopy in the presence of 2b.
With the optimized reaction conditions in hand, we
studied the scope of our method. For each substrate both
catalyst combinations 1a/2a and 1c/2b were screened and
compared. A wide range of substrates were evaluated, and
many common functional groups were compatible with the
reaction conditions (Table 2). Other aliphatic terminal
alkenes, such as 1-decene, gave results identical to our
model substrate, and undecen-1-ol was isolated in 99%
yield and 93:7 regioselectivity after 24 hours with method B
(Table 2, entry 2). An increased substitution at the allylic
position provided the corresponding product with even higher
selectivities (Table 2, entry 3). Acetals, esters, benzyl and silyl
ethers, carbamates, and free hydroxyl groups are well
tolerated (Table 2, entries 4–11). Furthermore, 1,2-disubsti-
tuted alkenes are completely unreactive under these reaction
conditions because of high chemoselectivity for the terminal
alkene in the hydroformylation step (Table 2, entry 12).
In conclusion, we have developed a unique multifunc-
tional rhodium catalyst system that enables the simultaneous
catalysis of two distinct transformations (hydroformylation of
alkenes and aldehyde hydrogenation) in a highly selective
manner controlled by the cooperative action of two different
ligands 1 and 2 (Scheme 1). Thus, terminal alkenes are
transformed into highly valuable C1-chain-elongated linear
alcohols in high yields and excellent selectivities. Even though
this catalyst system relies on subtle hydrogen-bonding
interactions to enable its performance, the system is compat-
ible with a wide range of functional groups present in the
alkenic substrate, thus rendering this method attractive for
synthetic as well as industrial applications.
Scheme 2. Equilibrium of potential complexes formed in situ.
CO/H2 (5 bar), five distinct absorptions were observed in the
carbonyl area. To allow assignment of these five bands, we
performed
a control experiment in which the [HRh-
(1c)3(CO)] complex alone was pressurized with CO/H2
(5 bar). Additionally, we computed the bands of the equato-
rial–equatorial (eq–eq) and axial–equatorial (ax–eq) con-
formers of the [HRh(1c)2(CO)2] complex by DFT calcula-
tions.[16] Four of the five bands observed after pressurizing the
Table 2: Scope and limitations.
Entry
Cond. ROH:RCHO[a] l:b (ROH)[a] Yield [%][b]
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
95:5
95:5
95:5
99:1
100:0
97:3
90:10
100:0
96:4
98:2
100:0
100:0
99:1
99:1
97:3
97:3
96:4
93:7
93:7
99:1
99:1
91:9
93:7
94:6
96:4
92:8
93:7
91:9
96:4
90:10
96:4
91:9
85:15
94[c]
94[c]
94
1
2
3
4
5
6
7
8
99
99[c]
89[c]
90[c]
99[c]
94[c]
90
Received: December 19, 2011
Published online: January 30, 2012
98[c]
97[c]
98
Keywords: alcohols · cooperative catalysis · hydroformylation ·
99
.
97[c]
85
hydrogenation · tandem reactions
93:7
96:4
99:1
96
99[c]
9
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96:4
89:11
96:4
93
87
70
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Angew. Chem. Int. Ed. 2012, 51, 2178 –2182