Domino rhodium/palladium-catalyzed dehydrogenation reactions of alcohols to acids by hydrogen transfer to inactivated alkenes

Article abstract of DOI:10.1002/chem.200903069

The combination of the d⁸ Rh^I diolefin amide [Rh(trop₂N)(PPh₃)] (trop₂N=bis(5-H-dibenzo-[a, d]cyclohepten-5-yl)amide) and a palladium heterogeneous catalyst results in the formation of a superior catalyst system for the dehydrogenative coupling of alcohols. The overall process represents a mild and direct method for the synthesis of aromatic and heteroaromatic carboxylic acids for which inactivated olefins can be used as hydrogen acceptors. Allyl alcohols are also applicable to this coupling reaction and provide the corresponding saturated aliphatic carboxylic acids. This transformation has been found to be very efficient in the presence of silica-supported palladium nanoparticles. The dehydrogenation of benzyl alcohol by the rhodium amide, [Rh]N, follows the well established mechanism of metal-ligand bifunctional catalysis. The resulting amino hydride complex, [RhH]NH, transfers a H₂ molecule to the Pd nanoparticles, which, in turn, deliver hydrogen to the inactivated alkene. Thus a domino catalytic reaction is developed which promotes the reaction R-CH ₂-OH+NaOH+ 2 alkene→R-COONa+2 alkane.

Full text of DOI:10.1002/chem.200903069

FULL PAPER
DOI: 10.1002/chem.200903069
Domino Rhodium/Palladium-Catalyzed Dehydrogenation Reactions of
Alcohols to Acids by Hydrogen Transfer to Inactivated Alkenes
[
a]
[a]
[b]
Mꢀnica Trincado, Hansjçrg Grꢁtzmacher,* Francesco Vizza, and
[
b]
Claudio Bianchini
8
Abstract: The combination of the d
as hydrogen acceptors. Allyl alcohols
are also applicable to this coupling re-
action and provide the corresponding
cles. The dehydrogenation of benzyl al-
cohol by the rhodium amide, [Rh]N,
follows the well established mechanism
of metal–ligand bifunctional catalysis.
The resulting amino hydride complex,
I
Rh diolefin amide [Rh
ACTHUNGTRNENUN(G trop N) ACHTUNGTRENNNU(G PPh )]
2 3
(
trop N=bis(5-H-dibenzo-
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[a,d]cyclohepten-5-yl)amide) and a pal-
satura AHCTNUGTERNNUNtG ed aliphatic carboxylic acids.
ladium heterogeneous catalyst results
in the formation of a superior catalyst
system for the dehydrogenative cou-
pling of alcohols. The overall process
represents a mild and direct method
for the synthesis of aromatic and
This transformation has been found to
be very efficient in the presence of
silica-supported palladium nanoparti-
[RhH]NH, transfers a H molecule to
2
the Pd nanoparticles, which, in turn,
deliver hydrogen to the inactivated
alkene. Thus a domino catalytic reac-
tion is developed which promotes the
Keywords: alkenes · heterogeneous
catalysis · hydrogen transfer · nano-
particles · palladium
hetero
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
aromatic carboxylic acids for
which inactivated olefins can be used
reaction
R-CH -OH+NaOH+
2
2 alkene!R-COONa+2 alkane.
0
Introduction
Pd metal nanoparticles (Pd /SiO ) form a unique stable
2
entity that speed up the overall hydrogenation of benzene
[3]
The catalytic hydrogenation of alkenes, undoubtedly one of
the most relevant reactions in organic chemistry, is usually
carried out with molecular hydrogen. Homogeneous cata-
lysts have demonstrated high activity and have extended the
scope of catalytic hydrogenation for highly selective trans-
formations. However, many catalytic hydrogenations are
still accomplished by using heterogeneous catalysts and Pd
is the preferred industrial catalyst. Despite the mature stage
of hydrogenation technologies, studying alternative less ex-
plored catalytic systems is still of fundamental importance.
We previously reported an example in which the combina-
to cyclohexane.
In comparison with the catalytic reduction using hydrogen
gas, transfer hydrogenations that employ hydrogen donors
present some advantages, principally that the inherent
hazards associated with the use of hydrogen gas, such as
flammability, containment, and high pressures are avoided.
Metal-promoted hydrogen transfer from small organic mole-
cules has been extensively investigated in the reduction of
activated unsaturated compounds. Comparatively little at-
tention has been paid to the transition-metal catalyzed
transfer hydrogenation of alkenes and alkynes. In their early
pioneering work, Braude and Linstead reduced a range of
olefins and alkynes in a catalytic transfer hydrogenation re-
action using a heterogeneous palladium catalyst under rela-
tively mild conditions and cyclohexene as the hydrogen
donor, which was converted to benzene. Few more exam-
ples were reported in which cyclic ethers were utilized as hy-
drogen transfer agents. Nolan and co-workers reported an
iridium complex that reduces alkenes in excellent yield
using 2-propanol as hydrogen donor. Recently Crabtree
described triazole-derived iridium(I) carbene complexes ca-
pable of promoting the transfer hydrogenation of inactiva-
[1]
[
2]
[4]
tion of the complex [Rh
A
H
U
G
R
N
U
G
A
H
U
G
R
N
U
G
ꢀ
diene; sulfos= O S
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(C H )CH C
A
H
U
G
R
N
N
3
6
4
2
2
2 3
[
a] Dr. M. Trincado, Prof. Dr. H. Grꢀtzmacher
[5]
Department of Chemistry and Applied Biosciences
ETH-Hçnggerberg, 8093 Zꢀrich (Switzerland)
Fax : (+41)44-633-1032
[6]
E-mail: gruetzmacher@inorg.chem.ethz.ch
[
b] Dr. F. Vizza, Prof. Dr. C. Bianchini
[7]
Institute of Chemistry of Organometallic Compounds
ICCOM-CNR, Polo Scientifico Area CNR
Via Madonna del Piano 10, 50019 Sesto Fiorentino (Italy)
[8]
AHCTUNGERTNNUNGt ed olefins in 2-propanol.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200903069.
Chem. Eur. J. 2010, 16, 2751 – 2757
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2751

However, the selective oxidation of alcohols by using
simple olefins as hydrogen acceptors (HA) has rarely been
Results and Discussion
repor
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
ted. Jensen et al., used a PCP “pincer” complex for the
During the course of our study of catalytic systems for hy-
[14]
[15,16]
dehydrogenation of alcohols and an electron-rich alkene as
the hydrogen acceptor, albeit under rather forcing condi-
drogenation and hydrogen transfer processes,
scribed the synthesis, from the precursor [Rh
we de-
A
H
U
G
R
N
U
G
2
[
9]
tions.
A
H
U
G
R
N
N
(PPh )] 1, of the rhodium(I)/diolefin amido complex [Rh-
3
Clean and high-yield methods have been established for
ruthenium-assisted dehydrogenative coupling (DHC) reac-
tions of alcohols to esters and aldehydes to acids in the pres-
A
T
N
T
N
U
G
A
T
N
T
E
N
N
(PPh )] (trop N=bis(5-H-dibenzo
A
H
U
G
R
N
U
[a,d]cyclohepten-
2
3
2
I
5-yl)amide) 2. This tetra-coordinated Rh amido complex 2
adopts a saw-horse-type structure that is a trigonal bipyra-
mid with one missing corner in the equatorial plane. This
structure is created by the combination of two p-acceptor
olefinic binding sites in the equatorial positions and an
amido and phosphane s-donor group each placed mutually
trans in the axial positions.
[10a]
ence of benzalacetone
esters by using crotononitrile as the hydrogen acceptor.
Furthermore, acceptorless alcohol dehydrogenative coupling
or primary alcohols to methyl
[10b]
[11]
reactions initially developed by Shvo and Saito and more
recently by Milstein and co-workers are currently of great
interest. Here, Ru pyridine-based “dearomatized” pincer
complexes promote the DHC between primary alcohols and
primary amines or a second alcohol molecule to carbonic
We believe that this type of structure is essential to pro-
mote bond-cleavage reactions via energetically low transi-
tion states. The HOMO corresponds to the anti-bonding
combination of a filled d-orbital at the metal center with the
filled p-type orbital at the adjacent N center (“lone-pair”)
and is polarized to the open face of the complex. Conse-
quently, the HOMO undergoes a symmetry allowed interac-
tion with the s*-orbital of the HꢀH bond (Figure 1), which
is readily cleaved across the RhꢀN bond even at low tem-
[12]
acids derivatives without a hydrogen acceptor.
We reported recently, that the dehydrogenative coupling
of primary alcohols can be efficiently catalyzed at room
I
temperature with a Rh amide complex (Figure 1), using ke-
[15]
peratures.
This reaction is reversible but the liberation of H from
2
the hydrogenated complex, the amino hydride 3, is too en-
dothermic and slow at ambient temperature to allow effi-
cient dehydrogenation reactions. Complex 2 catalyzes the
direct hydrogenation of ketones and imines (Scheme 1) and
also very efficiently the transfer hydrogenation of ketones
and activated olefins using ethanol as hydrogen source
[14]
which is irreversibly converted to ethylacetate.
Figure 1. The structure of [Rh
2 3
ACHTUNGTRENNUNG( trop N) ACHTUNGRTENNUN(G PPh )]. Plots of the DFT calcula-
tions of the HOMO and LUMO.
[13]
tones and a,b-unsaturated esters as hydrogen acceptors.
These results encouraged us to design a new catalytic
system for an economic process in which a simple olefin is
employed as hydrogen acceptor in a DHC reaction under
mild conditions. This procedure may offer significant advan-
tages with respect to costs and in the work-up procedure in
which the hydrogenated by-product, a (volatile) alkane is
easily separated from a polar carbonic acid derivative. Here,
Scheme 1. Catalytic hydrogenation of imines and ketones by [Rh-
(trop N)(PPh )].
A
H
U
G
R
N
U
G
2
A
H
U
G
R
N
U
G
3
I
we propose a domino catalytic system in which a Rh amide
serves as the homogeneous catalyst for the dehydrogenation
of primary alcohols and heterogeneously suspended metal
In a related process, we studied the possibility of using
this system as catalyst for the DHC of primary alcohols with
water, methanol, or amines to furnish carboxylic acids,
methyl esters, or amides, respectively. A high reaction rate
and catalytic turn over is achieved under mild reaction con-
ditions (room temperature) using cyclohexanone or alterna-
tively, methylmethacrylate (MMA) as hydrogen acceptors.
The Scheme 2 shows representatively some results for the
particles serve as H acceptors and as catalysts for the hy-
2
drogenation of inactivated olefins. To the best of our knowl-
edge, such a type of catalytic system, specifically the transfer
of hydrogen from a homogeneous transfer hydrogenation
catalyst to a heterogeneous hydrogenation catalyst has not
been previously investigated.
2752
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Chem. Eur. J. 2010, 16, 2751 – 2757

Rhodium/Palladium-Catalyzed Hydrogen Transfer
FULL PAPER
Table 1. Conversion of BnOH to PhCO
2
Na by transfer hydrogenation of
1
-hexene under organic/aqueous biphasic conditions catalyzed by [Rh-
[
a]
A
H
U
G
R
N
U
G
2
N)
A
H
U
G
E
N
N
(PPh
3
)]. Screening of the co-catalyst.
Cooperative catalyst
[
b]
Entry
Conversion [%]
1
2
3
4
5
6
7
8
9
1
11
1
1
Pd/SiO
Pd/SiO
Pd on zeolite (0.5 wt%)
Pt on zeolite (0.5 wt%)
Pt on alumina (5 wt%)
Pd/C (10 wt%)
2
2
(9.8 wt%)
(1.5 wt%)
quantitative
quantitative
44
41
71
16
19
37
64
68
48
31
43
Scheme 2. Transfer hydrogenation with various unsaturated systems as
hydrogen acceptors (HA).
Pd/C (5 wt%)
[
c]
A
H
U
G
R
N
U
G
2
A
H
U
G
R
N
U
G
3 3
]-CHCl
2
A
H
U
G
R
N
U
G
4
]
0
PdCl
PdCl
2
2
transfer hydrogenation of benzyl alcohol with different hy-
drogen acceptors. Treatment of a primary alcohol with cy-
clohexanone or acetone with 0.1 mol% or less of the pre-
catalyst 1 and NaOH at room temperature for 4 h gives the
corresponding carboxylic acid with excellent yield. The reac-
tion was found to be influenced by the strength of the base
/SnCl
2
Ph
2
3
[Ir
A
H
U
G
R
N
U
G
2 6
) ]PF
A
H
U
G
R
N
U
G
2
A
H
U
G
R
N
U
G
3 3
) ]
[
1
[
a] Reaction conditions: aqueous mixture containing BnOH (1.37 mmol),
-hexene (13.7 mmol), NaOH (1.6 mmol), co-catalyst (0.0137 mmol) and
Rh(trop NH)(PPh )]OTf (0.00137 mmol). [b] Conversion of BnOH to
PhCO Na measured by H NMR spectroscopy. [c] dba =dibenzylidene-
ACHTUNGTRENNaUNG cetone.
A
H
U
G
R
N
U
2
A
H
U
G
R
N
U
G
3
1
2
used. When a weaker inorganic base (like NaHCO ) or an
3
amine was used, poor conversions are obtained. Further-
more, when we treat benzyl alcohol in water using an inacti-
vated alkene as hydrogen acceptor under otherwise identical
conditions, no conversion is observed and the starting alco-
hol is quantitatively recovered.
To achieve the dehydrogenative coupling reaction by
using inactivated cheap alkenes as hydrogen acceptors, we
have investigated several co-catalysts, which, on one hand,
accepts the hydrogen from the primarily formed amino hy-
that the combination of both catalyst is essential for the pro-
cess (domino catalysis).
We subsequently studied the DHC of benzyl alcohol with
water and hydrogenation of 1-hexene by using a combina-
tion of rhodium amino hydride complex 3 to other well es-
tablished homogeneous transition metal hydrogenation cata-
lysts. An analysis of some of the most significant homogene-
ous catalytic palladium systems in hydrogenation, such as
dride complex [Rh ACHTUNGTRENNUNG( eq-H) ACHTUGNERNTUNG( trop NH) AHCNUTGTREUNNN(G PPh )] (3) to regener-
2 3
ate the amide 2. On the other hand, the hydrogen loaded
co-catalyst must be able to hydrogenate inactivated olefins
to the corresponding hydrocarbons under the used reaction
conditions (basic aqueous media) and show no tendency to
be deactivated (via decarbonylation reactions of the alco-
hols, for example). A screening of some established hetero-
geneous and homogeneous hydrogenation catalysts for the
DHC of benzyl alcohol with water to benzoic acid using 1-
hexene as a hydrogen acceptor is shown in Table 1.
[Pd AHCTNUGTRENUN(GN dba) ], PdCl , PdCl /SnCl and Na2 ACHTURENGNTUGN( PdCl ) turn out to
2 3 2 2 2 4
be reasonably efficient (entries 8–10, , Table 1, Scheme 3).
Some time ago we reported iridium(I) complexes containing
Ph
(5H-dibenzo ACHNTUGRNENUG[ a,d]cyclohepten-5-yl)phosphanes (tropp ) as
rigid, concave-shaped, mixed phosphane-olefin ligands.
These complexes show rather high activity in the hydrogena-
ꢀ
1
[17]
tion of imines [turnover frequency (TOF) of >6000 h ].
On the other hand, no or very-low activity was found in
Ph
transfer hydrogenations. The complex [Ir ACHTUNGTRNENUG( tropp ) ]PF was
2 6
Excellent conversion to the acid is observed when Pd/
tested here and showed moderate conversions (entry 12,
Table 1) in the dehydrogenative process. Finally [RuCl2-
SiO is used as co-catalyst (entries 1 and 2, Table 1). Even
2
with small amounts (1.5 wt%) of nanodeposits of Pd on
ACHTUNGTNERNU(GN PPh ) ], a very active hydrogenation catalyst, was also
3 3
found to be active in the DHC reactions, albeit less effi-
ciently (entry 13, Table 1).
0
SiO (for preparation and analysis of Pd /SiO see the Sup-
2
2
porting Information) excellent results are obtained. Other
catalysts that are commercially available or find industrial
use and contain Pd or Pt on other polar supports (zeolite or
alumina) were less efficient (entries 3–5, Table 1). Remarka-
bly, Pd on an apolar support,
To demonstrate the interplay between [Rh
ACHTUNGTNERUNNG( trop NH)-
2
AHCTUNGTREG(NNUN PPh )]OTf and the silica-supported palladium nanoparticles
3
(Pd/SiO ), some experiments using stoichiometric ratios of
2
that is charcoal, is little active
despite the rather high metal
content. In a blind experiment
we verified that none of these
heterogenous catalysts alone
are efficient promoters of the
DHC. In addition we also de-
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
monstrated that there is no re-
action in absence of [Rh-
(trop NH)
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2 3
(PPh )]OTf, showing Scheme 3. The palladium catalyzed hydrogen transfer from [Rh ACTHUNGTRENUNN(G eq-H) ACHTUNTGRENNUG( trop NH) ACHNUTGTRENNUGN( PPh )] to 1-hexene.
2
3
Chem. Eur. J. 2010, 16, 2751 – 2757
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2753

H. Grꢀtzmacher et al.
the reagents were performed (Scheme 3) and followed by
Table 2. Evaluation of various alkenes as hydrogen acceptors in DHC of
BnOH.
[
a]
31
P NMR spectroscopy (for experimental details see the
Supporting Information). When a green solution of [Rh-
(trop N)(PPh )] (3) in [D ]THF was treated with one equiv-
[
b]
Entry
Alkene (n equiv)
Conversion [%]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
1
2
2
3
4
5
6
7
8
9
cyclohexene (10)
1,3-cyclohexadiene (5)
1-heptene (10)
1-hexene (10)
1-hexene (5)
cis-stilbene (5)
styrene (5)
2-methyl-3-buten-2-ol (5)
ethylvinyl ether (5)
isoprene (5)
90
95
2
3
8
alent of BnOH, the solution immediately turned yellow. A
NMR spectrum showed the presence of [Rh(eq-H)-
(trop NH)(PPh )] exclusively. Subsequently, this solution
quantitative
quantitative
AHCTUNGTRENNUNG
[
c]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
ACHTUNGTRENNUNG
2
3
93
27
43
58
31
39
was divided in two samples. In one of them, only alkene was
added and in the second, Pd/SiO and 1-hexene were added.
2
3
1
During a time period of 6 h, the P NMR spectrum of the
first sample showed no conversion and only the signal of the
starting material [Rh ACHTNUGTRENNUNG( eq-H) ACHTUNREGTNNUNG( trop NH) ACHNUTGERTNNUN(G PPh )] was observed.
2 3
[
a] Reaction conditions: aqueous mixture containing BnOH (1.37 mmol),
alkene, NaOH (1.6 mmol), Pd/SiO (0.0137 mmol) and [Rh(trop NH)-
(PPh )]OTf (0.00137 mmol) was stirred for 12 h at room temperature.
[b] Conversion of BnOH in PhCO
of the reaction mixture. [c] It is possible to reduce the catalyst loading to
.01 mol% of rhodium catalyst with only a slight loss in conversion (de-
The second sample to which Pd/SiO was added turned back
2
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
3
1
to a greenish color and the P NMR spectrum showed clear-
ly the formation of [Rh(trop N)(PPh )] (2).
A
H
U
G
R
N
U
G
3
1
A
H
U
G
R
N
U
G
A
T
N
T
E
N
N
2
Na determined by H NMR analysis
2
3
On the basis of these findings as well as the results of the
experiments summarized in Table 1, we conclude that the
enhanced activity in the DHC of primary alcohols to esters
or with aqueous NaOH to carbonic acids with 1-hexene as
simple olefin and hydrogen acceptor (HA) is indeed owed
0
termined by NMR spectroscopy).
I
0
to the synergy of the Rh -Pd /SiO system as is shown in a
comparison with other transfer hydrogenation methods, no
hydro-dehalogenated product was observed throughout the
reaction period with bromobenzyl alcohol as starting mater-
2
simplified manner in Scheme 4.
[18]
AHCTUNGTRENNUNGi al. Significantly, heteroaromatic substrates as the furan-3-
[19]
methanol, which becomes readily available from biomass,
are also tolerated in the DHC under our reaction conditions
and are converted in substantial amounts (>80%, entry 5,
Table 3). Allylic substrates required shorter reaction times
to reach complete conversion, but the reactions give sub-
stantial amounts of side products. This is exemplified by the
transformation of cinnamyl alcohol (entry 6, Table 3), that
afforded cinnamic acid with moderate yield (50%) along
with phenylpropionic acid (28%) and 3-phenylpropanol
(
13%) as further isolated products. Indeed, the C=C double
bond in b-methallyl alcohol served as intra-molecular hydro-
gen acceptor and the product of DHC and simultaneous
C=C double bond hydrogenation, isobutyric acid, is ob-
tained in 50% yield along with 40% of isobutylalcohol
(
entry 7, Table 3).
Remarkably, aliphatic alcohols like ethanol or 1-octanol
not included in Table 1) are not converted in the domino
Scheme 4. A representation of the catalytic cycle in the dehydrogenative
coupling (DHC) of primary hydroxyl compounds with water.
(
Rh-Pd/SiO catalyzed DHC process using an alkene as hy-
2
drogen acceptor. However, the presence of such an alcohol
does not prevent the transformation of a benzylic hydroxyl
function. This is exemplified by an experiment in which a
1:1 mixture of benzyl alcohol and 1,2-propanediol is coupled
In an attempt to generalize this transformation, Pd/SiO2
was selected for further experiments with several alkenes.
Table 2 summarizes the results with mono and diolefins. Not
unexpected, we find a dependency on the efficiency of the
system on the degree of substitution at the olefinic C=C
double bond. Higher degree of substitution leads to lower
activities.
We investigated also the scope of this rhodium-palladium-
domino catalysis and used a variety of differently substituted
aromatic and heteroaromatic alcohols in the conversion to
the corresponding acids. These results are summarized in
Table 3. High effective transformations were obtained with
benzylic alcohols with electron-donating (entries 1–3,
Table 3) or -withdrawing substituents (entry 4, Table 3). In
with NaOH/H O using 1-hexene as HA. In this reaction the
2
aliphatic diol was recovered unchanged (entries 8 and 9,
Table 3), whereas the benzylic alcohol was converted to ben-
zoic acid in 71% yield (entry 9, Table 3). Finally, the conver-
sion of polyhydroxyaromatic compounds was investigated.
The best results so far we could obtain for the formation of
the monoacids from the corresponding benzenedimethanol
compounds are given in entries 10 and 11 (isolated yields
are given). In these reactions, we used THF as co-solvent,
which led to about 50% conversion while 50% of the start-
ing material is recovered. Note, that no protecting groups
2754
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Chem. Eur. J. 2010, 16, 2751 – 2757

Rhodium/Palladium-Catalyzed Hydrogen Transfer
FULL PAPER
Table 3. DHC of various alcohols with 1-hexene as HA, catalyzed by
SiO as co-catalyst. Transfer of hydrogen from the homoge-
2
[
a]
[
Rh
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(trop
2
NH)
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(PPh
3
)]OTf and Pd/SiO
2
.
neous Rh amino hydride complex [Rh
ACHTUNGTNRENUNG( eq-H) CAHTUNGTRENUNNG( trop NH)-
2
[
b]
Entry Substrate
Product
Yield [%]
AHCTUNGTRENNUNG
(PPh )] (3) to the heterogeneously suspended SiO -support-
3
2
ed palladium nanoparticles has been demonstrated. The de-
tailed mechanism by which this transfer occurs, remains ob-
scure and needs further detailed studies concerning the in-
teraction of the molecular Rh hydride complex and Pd
particles. Most remarkably, although the purely homogene-
1
4b
4c
5b 99
2
3
4
5c 52
5d 50
5e 77
ously catalyzed DHC reaction using the amide [Rh
ACHTUNGTRENNUNG( trop N)-
2
AHCTUNGTRENNUNG
(PPh )] (2) and ketones or activated olefins as hydrogen ac-
3
ceptors (HA) does not allow to discriminate between ali-
[c]
4d
4e
I
phatic and benzylic hydroxyl functions, the domino Rh -Pd/
SiO catalyzed system with simple olefins as HA does. This
2
may become important when single primary hydroxyl func-
tions are to be converted to functionalized carboxylic acid
derivatives in more complex reaction mixtures, as they are
obtained from biomass.
5
6
4 f
4g
5 f 84
5g 50
[
[
d]
e]
Experimental Section
7
8
4h
4i
5h 50
General: All syntheses and catalyses were performed under an atmo-
AHCTUNGTRENNUNG
–
–
4
cohols employed for the synthesis of acids were distilled prior to use. The
[
[
f]
catalysts from Table 1 (entries 3–11 and 13) were used as obtained from
9
5a 71
Ph
2 6
the commercial supplier. [Ir CAHTUNGRTENN(NUG tropp ) ]PF was synthesized according to
[
17] 1
the reported method.
H NMR spectra were recorded at 400, 300, or
2
50 MHz and chemical shifts were referenced to the residual solvent
g]
13
1
1
0
1
4j
5j
48
peak. C NMR spectra are proton decoupled and were recorded at
3
1
7
5.5 MHz; chemical shifts were referenced to the solvent. P NMR spec-
tra were recorded at 162.0, 121.5 or 101.3 MHz; chemical shifts were re-
ferenced to an external standard of H PO . Coupling constants J are
A
H
U
G
R
N
U
G
3
4
given in Hertz [Hz] as absolute values, unless specifically stated. The
data is being reported as: s=singlet, d=doublet, t=triplet, q=quadru-
plet, m=multiplet or unresolved, br=broad signal, coupling constant(s)
in Hz, integration, interpretation. Melting points were determined by
using a Bꢀchi melting point apparatus and are reported uncorrected.
Mass spectrometry (LCMS) was carried out by the ETH Zurich Mass
Spectrometry Facility.
g]
4k
5k 41
[
2
a] The reaction of the alcohol (1.37 mmol) was carried out in H O
(
1.6 mL) in the presence of [Rh
A
T
N
T
E
N
N
2 3
NH) ACHTUNTRGENNUNG( PPh )]OTf (0.00137 mmol),
Pd/SiO
2
1
2 h at room temperature. [b] Isolated yield, unless otherwise specified.
Preparation of silica-supported palladium nanoparticles: The Davicat
(Grace) silica employed in this work was a high-surface-area hydrophilic
mesoporous non-ordered material. The support was finely milled, washed
[
c] 3% of ArCHO was detected. [d] Phenylpropionic acid (28%) and 3-
phenylpropanol (13%) isolated as side products. [e] The reaction was
performed without an external hydrogen acceptor. [f] Referred to the
conversion of benzyl alcohol, complete recovery of 1,2-propanediol.
3
with 1m HNO and, subsequently, with distilled water to neutrality. The
material was dried overnight in an oven at 1008C. Porosity and surface
area were determined by nitrogen adsorption. Nitrogen adsorption/de-
sorption isotherms at liquid nitrogen temperature were measured by
using a Micromeritics ASAP 2010 instrument. All silica samples were de-
[
g] The reactions were carried out under standard conditions with a mix-
ture of THF/H O (2:1).
2
gassed at 3008C. Average pore radius (9.70 nm) and specific pore volume
3
(
1.43 cm g) were calculated according to the Barret–Joyner–Halenda
2
are needed and the mono-acids are formed with >98% se-
(BJH) theory. The specific surface area (295 m g) was obtained using the
Brunauer–Emmett–Teller (BET) equation.
lectivity.
Usually, 1 g batches (Pd content ranging from 1.5 to 10 wt%), were pre-
pared from PdCl
2
/SiO
2
by the following treatment: 1) calcination at
ꢀ
1
5
1
008C for 1 h in an
08Cmin , followed by cooling in argon flow (40 mLmin ) to room
O
2
flow (40 mLmin
)
at
a
heating rate of
Conclusion
ꢀ1
ꢀ1
ꢀ
1
2
temperature; 2) reduction at 3008C for 1 h in a H flow (40 mLmin ) at
ꢀ
1
A new catalytic system for the dehydrogenative coupling
a
(
heating rate of 108Cmin
followed by cooling in argon flow
40 mLmin ) to room temperature. PdCl /SiO (1.96–9.24 wt% Pd) was
prepared by impregnation of 5 g of silica with a solution of PdCl (167–
10 mg) in de-ionized water (50–250 mL) and 36% HCl (2–10 mL). The
ꢀ
1
2
2
(
DHC) of primary alcohols with NaOH/H O has been de-
2
2
veloped. Use of inactivated alkenes as hydrogen acceptors is
now possible with this new methodology. High reactivity is
observed upon addition of supported nanoparticles of Pd/
9
suspension was maintained under stirring for 4 h and then the solvent
was removed by evaporation under reduced pressure. After the solid resi-
Chem. Eur. J. 2010, 16, 2751 – 2757
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2755

H. Grꢀtzmacher et al.
1
due was removed by means of a small amount of water, it was quickly
dried and kept under vacuum overnight.
d=144.1–119.9, 72.2 (s, 2C), 60.6 (d,
J
RhC =8.6 Hz, 2C), 57.8 ppm (d,
1
31
1
J
RhC =8.0 Hz, 2C); P{ H} NMR (121.5 MHz, [D
8
]THF): d=62.2 ppm
1
(
d, JRhP =145 Hz).
General procedure for the catalyzed dehydrogenative coupling of alco-
hols to acids: A mixture of alcohol (1 equiv, 1.37 mmol), water (66 equiv,
9
0 mmol, 1.6 mL), sodium hydroxide (1.2 equiv, 1.6 mmol, 64 mg), and
alkene are combined in a Schlenk-flask under argon atmosphere. The bi-
phasic mixture is degassed by purging with argon for 10 min. prior the
Acknowledgements
addition of [Rh
2 3
ACHTUNGTRENNUNG( trop NH) ACHTNUGRTENNNUG( PPh )]OTf (0.001 equiv, 0.00137 mmol, 1 mg)
and Pd/SiO 10 wt% (0.01 equiv, 0.0137 mmol Pd, 14.6 mg) under a
2
This work was supported by the Swiss National Science Foundation
(SNF) and the ETH Zꢀrich.
stream of argon. After 12 h at RT (reaction followed by NMR spectros-
copy) water is added (20 mL), and the mixture is extracted with diethyl
ether (2ꢂ20 mL). The aqueous phase is then acidified with 10% HC1
and extracted with ethyl acetate (4ꢂ25 mL).The combined extracts were
[
1] a) P. N. Rylander, Hydrogenation Methods, Academic Press, New
York, 1990 and preceding volumes; b) M. Freifelder, Practical Cata-
lytic Hydrogenation, Wiley Interscience, New York, 1971; c) S. Nishi-
mura, Handbook of Heterogeneous Catalytic Hydrogenation for Or-
ganic Synthesis Wiley, New York, 2001; d) Review with emphasis on
hydrogenation technology for the fine chemicals: R. M. Machado,
K. R. Heier, R. R. Broekhuis, Curr. Opinion Drug Discov. Devel.
2001, 4, 745; e) N. B. Johnson, I. C. Lennon, P. H. Moran. J. Rams-
den, Acc. Chem. Res. 2007, 40, 1291.
washed with brine (25 mL), dried over Na
duced pressure, affording the pure acid after purification.
Rh(trop NH)(PPh )]OTf (1): [RhCl(trop NH)(PPh )] (for synthesis de-
tails of this complex see the Supporting Information of [14a]) (1 equiv,
.63 mmol, 500 mg) and (1.03 equiv, 0.65 mmol, 166 mg) AgOTf were sus-
2 4
S0 , and evaporated under re-
[
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
3
A
H
U
G
R
N
U
2
A
H
U
G
R
N
N
3
2
0
pended in DCM (15 mL) and stirred for 12 h. The formed AgCl was re-
moved by filtration over a plug of celite. Addition of n-hexane precipitat-
ed the orange product complex. The mother liquor was decanted off and
the product dried under vacuum. A second fraction was obtained from
[2] a) W. S. Knowles, R. Noyori, K. B. Sharpless, Nobel Lectures,
Angew. Chem. 2002, 114, 2096; Angew. Chem. Int. Ed. 2002, 41,
1998; b) C. Bianchini, A. Meli, F. Vizza in The Handbook of Homo-
geneous Hydrogenation, Vol.1 (Eds: J. G. de Vries, C. J. Elsevier)
Wiley-VCH, Weinheim, 2007.
[3] a) P. Barbaro, C. Bianchini, V. Dal Santo, A. Meli, S. Moneti, R.
Psaro, A. Scaffidi, L. Sordelli, F. Vizza, J. Am. Chem. Soc. 2006, 128,
7065; b) P. Barbaro, C. Bianchini, V. Dal Santo, A. Meli, S. Moneti,
C. Pirovano, R. Psaro, L. Sordelli, F. Vizza, Organometallics 2008,
27, 2809.
[4] a) E. A. Braude, R. P. Linstead, J. Chem. Soc. 1954, 3544. For recent
reviews see: b) J. E. Bꢃckvall, J. Organomet. Chem. 2002, 652, 105;
c) S. Gladiali, E. Alberico in Transition Metals for Organic Synthesis,
Vol.2 (Eds: M. Beller, C. Bolm) Wiley-VCH, Weinheim, 2004;
d) S. E. Clapham, A. Hadzovic, R. H. Morris, Coord. Chem. Rev.
2004, 248, 2201; e) T. Ikariya, K. Murata, R. Noyori, Org. Biomol.
Chem. 2006, 4, 393; f) J. S. M. Samec, J.-M. Backvall, P. G. Ander-
sson, P. Brandt, Chem. Soc. Rev. 2006, 35, 237.
the mother liquor upon re-crystallization from DCM/hexane. Yield:
1
5
42 mg, 0.6 mmol, 95%. H NMR (400.1 MHz, CDCl
3
) d=7.84 (m, 6H,
ar
ar
ar
CH ), 7.70–7.55 (m, 9H, CH ), 7.40–7.15 (m, 8H, CH ), 7.00–6.85 (m,
H, CH ), 6.85–6.70 (m, 4H, CH ), 5.66 (dd,
ar
ar
3
2
4
2
2
J
PH =5.8 Hz,
J
RhH
=
3
2
3
.1 Hz, 1H, NH), 5.43 (ddd,
J
HH =9.3 Hz,
HH =9.3 Hz,
J
RhH =3.3 Hz,
J
PH =2.8 Hz,
olefin
3
3
2
H, CH
), 4.94 (ddd,
J
J
PH =2.7 Hz,
J
RhH =1.8 Hz, 2H,
olefin
4
benzyl
13
CH
CDCl
(
1
), 4.91 ppm (d, JPH =8.4 Hz, 2H, CH
) d=136.9 (s, 2 C, C ), 135.5 (d, JRhC =1.8 Hz, 2 C, C ), 134.6
s, 2 C, C ), 134.6 (s, 2 C, C ), 134.5 (d,
PC =2.3 Hz, 3 C, CH ), 129.6 (s, 2 C, CH ), 129.5 (s, 2 C,
PC =46.4 Hz, 3 C, C ), 128.7 (d,
CH ), 128.5 (s, 2 C, CH ), 128.2 (s, 2 C, CH ), 127.5 (s, 2 C, CH ), 127.2
); C NMR (101.6 MHz,
quart
2
quart
3
quart
quart
2
ar
J
PC =9.2 Hz, 6 C, CH ),
4
ar
ar
31.0 (d,
J
ar
1
quart
3
CH ), 129.0 (d,
J
J
PC =10.1 Hz, 6 C,
ar
ar
ar
ar
ar
ar
ar
1
(
s, 2 C, CH ), 126.4 (s, 2 C, CH ), 126.4 (s, 2 C, CH ), 119.8 (q,
J
FC
=
1
olefin
1
3
6
21.1 Hz, C, CF
.9 Hz, 2 C, CH
3
), 74.2 (d,
), 72.7 ppm (d,
J
RhC =13.3 Hz, 2 C, CH
), 74.0 (d,
J
RhC
=
olefin
3
benzyl
31
J
PC =1.4 Hz, 2 C, CH
); P NMR
1
(
162.0 MHz, CDCl
Rh(trop N)(PPh )] (2): To
1 equiv, 0.188 mmol, 150 mg.) in THF (2 mL) was added a solution of
3
) d=40.6 ppm (d, JRhP =137.7 Hz).
[
(
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
3
2 3
a suspension of [RhCl ACHTUNGTRENNUN(G trop NH) ACHTUNGTRENUNN(G PPh )]
[
5] R. P. Linstead, E. A. Braude, P. W. D. Mitchell, K. R. H. Wooldridge,
L. M. Jackman, Nature 1952, 169, 100.
potassium tert-butoxide (1 equiv, 0.188 mmol, 21.1 mg) in THF (1 mL).
After 30 min. of stirring, toluene (1 mL) was added to the resulting deep-
green solution and the solvents were removed under vacuum. The dark-
green residue was extracted with THF (2ꢂ3 mL), filtered over Celite.
The clear filtrate was reduced under vacuum to a volume of approxi-
[
6] For an example of reduction an alkene with dioxane as hydride
donor and the Wilkinson catalyst, see T. Nishiguchi, K. Fukuzumi, J.
Am. Chem. Soc. 1974, 96, 1893.
[
[
[
7] A. C. Hillier, H. M. Lee, E. D. Stevens, S. P. Nolan, Organometallics
2
001, 20, 4246.
mately 1 mL. Dark-green microcrystals of [Rh
2 3
ACHTUNGTRENUNNG( trop N) AHCTUNGRTENNNUG( PPh )] (110 mg,
8] D. Gnanamgari, A. Moores, E. Rajaseelan, R. H. Crabtree, Organo-
metallics 2007, 26, 1226.
9] D. Morales-Morales, R. Redon, Z. H. Wang, D. W. Lee, C. Yung, K.
Magnuson, C. M. Jensen, Can. J. Chem. 2001, 79, 823.
0
.144 mmol, 77%) grew upon layering of this solution with toluene
1
(
8
1 mL) and hexane (10 mL). H NMR (300 MHz, [D ]THF): d=7.63 (m,
3
6
H), 7.56 (m, 9H), 7.22 (d,
J
J
J
HH =6.7 Hz, 2H), 7.03 (m, 4H), 6.95 (d,
3
3
3
3
J
HH =7.0 Hz, 2H), 6.90 (d,
HH =7.3 Hz, 2H), 6.79 (d,
HH =7.2 Hz, 2H), 6.57 (dd,
J
J
J
HH =7.6 Hz,
HH =7.3 Hz,
[
[
10] a) S. I. Murahashi, T. Naota, K. Ito, Y. Maeda, H. Taki, J. Org.
Chem. 1987, 52, 4319; b) N. A. Owston, A. J. Parker, J. M. J. Wil-
liams, Chem. Commun. 2008, 624.
11] a) Y. Blum, Y. Shvo, J. Organomet. Chem. 1985, 282, C7; b) S. Shino-
da, H. Itagaki, Y. Saito, J. Chem. Soc. Chem. Commun. 1985, 860;
c) H. Itagaki, S. Shinoda, Y. Saito, Bull. Chem. Soc. Jpn. 1988, 61,
3
3
2
H), 6.67 (dd,
J
HH =7.2 Hz,
3
3
2
3
J
HH =7.3 Hz, 2H), 5.62 (ddd,
J
HH =9.0 Hz,
J
RhH =3.3 Hz,
PH =2.9 Hz,
4
3
3
2
6
2
6
6
1
8
H), 4.92 (d,
J
PH =13.5 Hz, 2H), 4.69 ppm (ddd,
J
HH =9.0 Hz,
J
PH
=
2
13
.2 Hz,
J
RhH =1.2 Hz, 2H); C NMR (75 MHz, [D
8
]THF): d=146.8 (s,
2
C), 143.7 (s, 2C), 137.7 (s, 2C), 136.6 (s, 2C), 135.5 (d,
JRhC =11.0 Hz,
1
3
C), 132.1 (d,
JRhP =35.5 Hz, 3C), 131.1 (s, 3C), 129.5 (d, JRhC =8.6 Hz,
2
291; d) T. Fujii, Y. Saito, J. Mol. Catal. 1991, 67. For further exam-
C), 128.7 (s, 2C), 127.9 (s, 2C), 127.3 (s, 2C), 126.4 (s, 2C), 126.4 (s, 2C),
ples of acceptorless, ruthenium-catalyzed dehydrogenative cycliza-
tion of diols to lactones, see: e) Y. R. Lin, X. C. Zhu, Y. F. Zhou, J.
Organomet. Chem. 1992, 429, 269; f) J. Zhao, J. F. Hartwig, Organo-
metallics 2005, 24, 2441.
1
26.3 (s, 2C), 126.0 (s, 2C), 125.8 (s, 2C), 84.5 (d,
J
RhC =14.7 Hz, 2C),
RhC =6.7 Hz, 2C); P NMR (121.5 MHz,
]THF): d=38.0 ppm (d, JRhP =125 Hz).
Rh(eq-H)(trop NH)(PPh )] (3): The [Rh
1
31
2.3 (s, 2C), 76.2 ppm (d,
J
1
[
D
8
[
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
3
A
T
N
T
E
N
N
A
H
U
T
N
N
(trop
2
NH)
A
H
U
G
R
N
U
G
3
)] com-
[12] a) J. Zhang, M. Gandelman, L. J. W. Shimon, H. Rozenberg, D. Mil-
stein, Organometallics 2004, 23, 4026; b) J. Zhang, G. Leitus, Y. Ben-
David, D. Milstein, J. Am. Chem. Soc. 2005, 127, 10840; c) J. Zhang,
M. Gandelman, L. J. W. Shimon, D. Milstein, Dalton Trans. 2007,
107; d) C. Gunanathan, Y. Ben-David, D. Milstein, Science 2007,
317, 790; e) C. Gunanathan, D. Milstein, Angew. Chem. 2008, 120,
8789; Angew. Chem. Int. Ed. 2008, 47, 8661; f) C. Gunanathan,
L. J. W. Shimon, D. Milstein, J. Am. Chem. Soc. 2009, 131, 3146.
plex was prepared by adding BnOH (12 mg, 0.11 mmol) to a green solu-
tion of [Rh(trop N)(PPh )] (2) (76 mg, 0.1 mmol) in [D ]THF (1 mL). A
yellow solution was obtained immediately in which the hydride is the
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
3
8
1
only observed product. H NMR (300 MHz, [D
8
]THF): d=7.90–6.10
PH =7.8 Hz, 2H), 3.91 (dd,
HH =9.3 Hz, 2H), ꢀ8.15 ppm
]THF):
3
4
(
J
31H), 5.56 (d,
J
PH =4.9 Hz, 1H), 4.56 (d,
HH =4.7 Hz, 2H), 3.55 (d,
dd, JRhH =23.0 Hz, JPH =23.0 Hz, 1H); C NMR (75.5 MHz, [D
J
3
3
3
HH =9.3 Hz,
J
J
1
2
13
(
8
2756
www.chemeurj.org
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 2751 – 2757

Rhodium/Palladium-Catalyzed Hydrogen Transfer
FULL PAPER
[
13] T. Zweifel, J. Naubron, H. Grꢀtzmacher, Angew. Chem. 2009, 121,
67; Angew. Chem. Int. Ed. 2009, 48, 559.
14] The complex [Rh(trop N)(PPh )] has demonstrated high activity in
the hydrogenation of ketones and imines with H . a) P. Maire, T.
Bꢀttner, F. Breher, P. Le Floch, H. Grꢀtzmacher, Angew. Chem.
005, 117, 6477; Angew. Chem. Int. Ed. 2005, 44, 6318; b) K. Muniz,
Angew. Chem. 2005, 117, 6780; Angew. Chem. Int. Ed. 2005, 44,
622.
15] T. Zweifel, J. Naubron, T. Bꢀttner, T. Ott, H. Grꢀtzmacher, Angew.
Chem. 2008, 120, 3289; Angew. Chem. Int. Ed. 2008, 47, 3245.
16] a) P. Maire, S. Deblon, F. Breher, J. Geier, C. Bçhler, H. Rꢀegger,
H. Grꢀtzmacher, Chem. Eur. J. 2004, 10, 4198–4205; b) E. Piras, F.
Lꢃng, H. Rꢀegger, D. Stein, M. Wçrle, H. Grꢀtzmacher, Chem. Eur.
J. 2006, 12, 5849.
[17] C. Bçhler, N. Avarvari, H. Schçnberg, M. Wçrle, H. Rꢀegger, H.
Grꢀtzmacher, Helv. Chim. Acta 2001, 84, 3127.
5
[
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
A
H
N
T
E
N
N
3
[18] Besides catalyzed hydrodehalogenation with Pd phosphanes com-
plexes under homogenous conditions, recently, palladium supported
on carbon, and, mesoporous silicate, alumina, or organic polymers
among others have been utilized as catalysts for the reduction of
halogenated arenes with hydrogen , alcohols, hydrides and hydrazine
as reducing agents. For a representative example, see: F. J. Urbano,
J. M. Marinas, J. Mol. Catal. A 2001, 173, 329.
2
2
6
[
[19] D. J. Nowakowski, J. M. Jones, R. M. D. Brydson, A. B. Ross, Fuel
2007, 86, 2389.
[
Received: November 8, 2009
Published online: January 15, 2010
Chem. Eur. J. 2010, 16, 2751 – 2757
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2757