Cooperative catalysis by iridium complexes with a bipyridonate ligand: Versatile dehydrogenative oxidation of alcohols and reversible dehydrogenation-hydrogenation between 2-propanol and acetone

Article abstract of DOI:10.1002/anie.201206987

Going into reverse: An efficient and versatile catalytic system for the dehydrogenative oxidation of alcohols under extremely mild conditions has been developed using a Cp*Ir complex with bipyridonate ligand as catalyst (see scheme, Cp=pentamethylcyclopentadienyl). Reversible and repetitive transformation between 2-propanol and acetone by catalytic dehydrogenation- hydrogenation is also achieved. Copyright

Full text of DOI:10.1002/anie.201206987

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Angewandte
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DOI: 10.1002/anie.201206987
Dehydrogenation
Cooperative Catalysis by Iridium Complexes with a Bipyridonate
Ligand: Versatile Dehydrogenative Oxidation of Alcohols and
Reversible Dehydrogenation–Hydrogenation between 2-Propanol and
Acetone**
Ryoko Kawahara, Ken-ichi Fujita,* and Ryohei Yamaguchi*
[
12]
The oxidation of alcohols to carbonyl compounds is one of the
most important and fundamental reactions in organic chemis-
try. Conventionally, this reaction is carried out by using
stoichiometric amounts of harmful oxidants, for example,
intermediate a-pyridonate species. Therefore, an a-pyrido-
nate-based ligand could act as a proton acceptor in the
activation step and as a proton donor in the dehydrogenation
step, and thus play a dual role in cooperative catalysis
(Scheme 1). Moreover, bidentate functional ligands that form
[
1]
a chromium reagent. Recently, a number of transition-
metal-catalyzed oxidation reactions of alcohols have been
developed using environmentally acceptable oxidants, such as
[2]
[3]
[4]
molecular oxygen,
hydrogen peroxide,
or acetone.
However, oxidant-free or acceptor-free oxidation based on
the catalytic dehydrogenation of alcohols accompanied by the
evolution of hydrogen gas must be desirable from the
standpoint of atom economy and environmental concerns.
Dehydrogenative oxidation of alcohols is important for the
production of synthetically useful aldehydes and ketones
[5]
from readily available alcohols with high atom efficiency,
and also for the production of hydrogen gas, which is one of
the most promising energy carriers in energy plans for the
[
6,7]
future.
Up to now, various homogeneous catalytic systems
Scheme 1. Dual role of a-pyridonate ligand in cooperative catalysis.
have been reported for the dehydrogenative oxidation of
[8,9]
alcohols to carbonyl compounds.
Most of these catalytic
reactions have to be carried out under reflux in organic
stable five-membered chelate structures in 2 and 3 are
[
10]
solvent at high temperature (> 1008C). Additionally, most
of these systems can be utilized only for the oxidation of
secondary alcohols, and the systems that effectively oxidize
important for the dehydrogenation of both primary and
[
12b,c]
secondary alcohols.
On the basis of these assumptions, we designed new
catalysts 4 and 5 bearing a,a’-bipyridonate ligands, anticipat-
ing that the activation of alcohols would be much accelerated
and lead to high catalytic performance in the dehydrogenative
oxidation of various kinds of alcohols.
[8j,11]
primary alcohols to aldehydes are rare.
Currently, there is
no catalytic system that is able to dehydrogenatively oxidize
a wide variety of both primary and secondary alcohols at
relatively low temperatures (< 908C).
We have previously reported the dehydrogenative oxida-
tion of alcohols catalyzed by Cp*Ir complexes 1–3, which bear
[12]
functional ligands with an a-hydroxypyridine skeleton.
Regarding the mechanisms of these catalytic systems, it has
been proposed that the crucial steps are the intramolecular
reaction of hydride on the metal with protic hydrogen on the
a-hydroxypyridine-based ligand (ligand-promoted dehydro-
genation), and the activation of the alcohol with the
Here, we report the synthesis of a series of new Cp*Ir
complexes that bear bipyridonate ligands and a study of their
catalytic activity in the dehydrogenative oxidation of alcohols.
Oxidation of alcohols under extremely mild conditions was
accomplished by using a new Cp*Ir catalyst that bears
a bipyridonate and an aquo ligand, and oxidations under
solvent-free conditions were also achieved. Furthermore, the
reversible transformation between 2-propanol and acetone by
catalytic dehydrogenation and hydrogenation as a prototype
for hydrogen storage is also described.
[
*] R. Kawahara, Dr. K. Fujita, Prof. Dr. R. Yamaguchi
Graduate School of Human and Environmental Studies
Kyoto University
Sakyo-ku Kyoto 606-8501 (Japan)
E-mail: fujita.kenichi.6a@kyoto-u.ac.jp
yamaguchi.ryohei.75s@st.kyoto-u.ac.jp
[
**] This work was partially supported by KAKENHI (no. 19750075 and
22550098). We thank Yui Tanaka for valuable technical assistance.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206987.
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First, we planned to prepare a series of new Cp*Ir
catalysts by the reaction of the dicationic complex 3 with base.
Treatment of 3 with two equivalents of NaOtBu in water gave
neutral complex 4a bearing a N,N-chelated a,a’-bipyridonate
ligand and an aquo ligand in 83% yield [Eq. (1)]. Similarly,
reflux in pentane (368C) for 5 h, acetophenone (8a) was
obtained quantitatively (Table 1, entry 1), showing an
extremely high catalytic activity of 4a for the dehydrogen-
ative oxidation of an alcohol at very low temperature. Other
catalysts 4b and 4c, which bear pyridine or dimethylsulfoxide
as a ligand, showed lower activity (Table 1, entries 2 and 3),
thus indicating the superiority of the labile aquo ligand in 4a.
Employment of catalysts 5a and 5b with a phenanthroline-
based functional ligand gave inferior results (Table 1,
entries 4 and 5). The loading of highly active catalyst 4a
could be reduced to 0.01 mol% (Table 1, entry 7), while
maintaining an excellent yield of 8a. The turnover number
(TON) reached up to 9500, even at 368C. Performance of the
complexes 4b and 4c were prepared by reaction of 3 with
NaOtBu in dichloromethane in the presence of pyridine and
dimethylsulfoxide, respectively. Complexes 4 were soluble in
various organic solvents (alkanes, aromatic solvents, ethers,
haloalkanes, esters, amides, or alcohols) and sparingly soluble
in water. Related complexes 5a and 5b, which bear a phenan-
throline-based functional ligand, were also prepared from
dicationic complex 6 [Eq. (2)].
reaction under air did not influence the outcome of the
reaction at all (Table 1, entry 8), thus showing the operational
advantage of this system. The evolution of hydrogen gas was
confirmed by analysis of the gas phase using a hydrogen
sensor. Additionally, the volume of the evolved hydrogen gas
was measured using a gas burette, giving a 96% yield of
hydrogen gas (Table 1, entry 9). When the reaction of 7a
was carried out under reflux in p-xylene (b.p. 1388C) in the
presence of 0.0002 mol% of 4a for 48 h, the TON reached up
to 275000 (Table 1, entry 10). To the best of our knowledge,
this is the highest TON that was reported thus far for
a catalytic system for the dehydrogenative oxidation of
alcohols.
[14]
Results of the dehydrogenative oxidation of various
secondary alcohols to the corresponding ketones catalyzed
by 4a are shown in Table 2. The reactions of 1-arylethanols
(7a–f) bearing electron-donating and electron-withdrawing
With the new Cp*Ir complexes 4 and 5 in hand, we
substituents at the aromatic ring proceeded smoothly under
reflux in pentane to give the corresponding acetophenone
derivatives in good to excellent yields (Table 2, entries 1–6).
The reaction of a sterically hindered substrate 7c also
proceeded well (Table 2, entry 3). Aliphatic secondary alco-
hols could be oxidized successfully, although a slightly higher
reaction temperature (reflux in hexane) led to better results
investigated their catalytic performance in the dehydrogen-
[
13]
ation of 1-phenylethanol (7a; Table 1). When the reaction
of 7a was carried out in the presence of 4a (0.5 mol%) under
Table 1: Dehydrogenative oxidation of 1-phenylethanol (7a) to aceto-
[
a]
phenone (8a) under various conditions.
(Table 2, entries 7–12).
Having established the unprecedented high catalytic
performance of 4a for the dehydrogenative oxidation of
secondary alcohols, we next investigated the reactions of
primary alcohols. In order to find optimum conditions for
primary alcohols, the dehydrogenative oxidation of benzyl
alcohol (9a) was conducted under various conditions
[
b]
Entry
Catalyst
mol%)
Solvent
(b.p.)
t
[h]
Yield [%]
(TON)
(
1
2
3
4
5
6
4a (0.5)
4b (0.5)
4c (0.5)
5a (0.5)
5b (0.5)
4a (0.1)
4a (0.01)
4a (0.5)
4a (0.5)
4a (0.0002)
pentane
pentane
pentane
pentane
pentane
pentane
pentane
pentane
pentane
p-xylene
(368C)
(368C)
(368C)
(368C)
(368C)
(368C)
(368C)
(368C)
(368C)
(1388C)
5
5
5
5
5
100
7
16
36
8
(
Table 3). When the reaction of 9a was carried out in the
presence of 4a (1.5 mol%) under reflux in benzene (b.p.
08C) for 20 h, benzaldehyde (10a) was obtained in excellent
8
yield (96%, Table 3, entry 1). Employment of tBuOH (b.p.
20
48
5
96
[
[
[
c]
d]
e]
[
7
8
9
1
95 (9500)
100
100
55 (275000)
828C) as a ’greener’ solvent also led to a high yield of 10a
[15]
(92%, Table 3, entry 2).
The TON reached up to 3100,
[f]
5
when the reaction of 9a was carried out in the presence of
d,g]
0
48
[16]
0
.01 mol% of 4a for 48 h (Table 3, entry 3). The presence
[
0
a] Reaction was carried out with 7a (1.0 mmol) and Cp*Ir catalyst (0.1–
.5 mol%) in pentane (3 mL). [b] Determined by GC analysis. Selectivity
of air did not affect the reaction at all (Table 3, entry 4).
Quantitative analysis showed that the yield of evolved
toward the formation of 8a was higher than 95%. [c] Reaction was
carried out with 7a (10.0 mmol) and 4a (0.01 mol%) in pentane
(
out with 7a (5.0 mmol) in pentane (15 mL). [f] Yield of the evolved
hydrogen gas was 96%. [g] Reaction was carried out with 7a (500 mmol)
and 4a (0.0002 mol%) in p-xylene (500 mL).
[14]
hydrogen gas was 89% (Table 3, entry 5).
The reaction
proceeded at much lower temperature under reflux in THF
b.p. 668C), although the yield of 10a was moderate (54%,
30 mL). [d] Reaction was carried out under air. [e] Reaction was carried
(
Table 3, entry 6). The highest TON (47500) was obtained
when the reaction of 9a was carried out under reflux in
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Table 2: Dehydrogenative oxidation of various secondary alcohols to ketones
catalyzed by 4a under mild reaction conditions.
were tolerated (Table 4, entries 2 and 6–10). The
reaction of 9j, which has a phenolic hydroxy group,
gave the corresponding aldehyde 10j in 79% yield
at higher temperature (reflux in toluene, Table 4,
entry 10). While dehydrogenative oxidation of ali-
[
a]
Ent. Alcohol
Cat.
mol%]
Solvent
(b.p.) Prod.
t
Yield
phatic primary alcohols to aldehydes has been
[
b]
[17]
[
[h] [%]
known to be very difficult,
this reaction pro-
ceeded smoothly using 4a as the catalyst under
reflux in toluene (Table 4, entries 12 and 13).
The present catalytic system was also applicable
to the dehydrogenative transformation of biologi-
cally important and complex molecules. When the
reaction of b-estradiol (7j) was conducted in the
presence of 4a (0.5 mol%) under reflux in tBuOH
for 20 h, the selective dehydrogenation proceeded
to give estrone (8j) quantitatively [Eq. (3)].
The reaction of pregnenolone (7k) proceeded
with concomitant isomerization of C=C bond and
quantitatively afforded progesterone (8k), which
has an a,b-unsaturated ketone moiety (see Eq. (S1)
in the Supporting Information).
1
2
3
4
5
6
R’=H
(7a)
(7b)
(7c)
(7d)
(7e)
(7 f)
0.5
0.5
1.0
1.0
1.0
2.0
pentane (368C)
pentane (368C)
pentane (368C)
pentane (368C)
pentane (368C)
pentane (368C)
8a
8b
8c
8d
8e
8 f
5
5
5
5
5
100 (98)
100 (99)
92
92 (89)
90 (88)
85
R’=4-OMe
R’=2-OMe
R’=4-Cl
R’=4-Br
R’=4-NO2
20
7
8
1.0
2.0
hexane
(698C)
20 100
20
(7g)
(7h)
(7i)
8g
8h
8i
pentane (368C)
93
9
1
1.0
2.0
hexane
(698C)
20
20
94
82
0
pentane (368C)
1
1
1
2
2.0
3.0
hexane
(698C)
20
20
88 (84)
80
We next studied the solvent-free dehydrogen-
ative oxidation of alcohols (Table S2 in the Support-
ing Information). When the reaction of 7a was
carried out in the presence of 4a (3.0 mol%) at
pentane (368C)
[a] Reaction was carried out with a secondary alcohol (1.0 mmol) and 4a (0.5–
3
7
.0 mol%) in solvent (3 mL) under reflux. [b] Determined by GC analysis (entries 1,
–12) or H NMR spectroscopy (entries 2–6). Selectivity toward the formation of 8
1
6
08C for 20 h, 8a was obtained in 93% yield
was higher than 99%. Yield of isolated product is in parenthesis.
(Table S2, entry 1). Reactions of other secondary
Table 3: Dehydrogenative oxidation of benzyl alcohol (9a) to benzalde-
[
a]
hyde (10a) catalyzed by 4a under various conditions.
[
b]
Entry
Cat.
mol%]
Solvent
(b.p.)
Yield [%]
(TON)
[
1
2
1.5
1.5
0.01
1.5
1.5
benzene
tBuOH
tBuOH
tBuOH
tBuOH
THF
(808C)
(828C)
(828C)
(828C)
(828C)
(668C)
(1108C)
96
92
alcohols (7g and 7i) proceeded at 908C to give the corre-
sponding ketones in good yields (Table S2, entries 2 and 3),
while the reaction of a primary alcohol 9a resulted in a low
yield of aldehyde 10a (Table S2, entry 4).
Anticipating the possibility of an efficient hydrogen gas
production from alcohols, we next investigated the dehydro-
genation of 2-propanol [7l; Eq. (4)]. The reaction of 7l
[
[
[
c]
d]
e]
3
4
5
6
7
31 (3100)
92
92
[f]
1.5
0.002
54
[
d,g]
toluene
95 (47500)
[a] Reaction was carried out with 9a (0.25 mmol) and 4a (1.5 mol%) in
(1.4 mL; 30 mmol) in the presence of 4a (1.5 mol%) under
solvent (5 mL). [b] Determined by GC analysis. Selectivity toward the
formation of 10a was higher than 97%. [c] Reaction was carried out with
9
a (10 mmol) and 4a (0.01 mol%) in tBuOH (200 mL) under reflux for
4
8 h. [d] Reaction was carried out under air. [e] Reaction was carried out
with 9a (2.0 mmol) in tBuOH (40 mL). [f] Yield of the evolved hydrogen
gas was 89%. [g] Reaction was carried out with 9a (80 mmol) and 4a
(
0.002 mol%) in toluene (270 mL) under reflux for 48 h.
reflux for 4 h gave acetone (8l) quantitatively with the
[
14]
toluene (b.p. 1108C) for 48 h with a very small catalyst loading
evolution of 695 mL (95% yield) of hydrogen gas.
It is
[16]
(
0.002 mol%; Table 3, entry 7).
Results of the dehydrogenative oxidation of various
worth noting that an efficient production of hydrogen gas was
achieved under neutral and mild reaction conditions from
easily handled and readily available 2-propanol as a starting
primary alcohols to the corresponding aldehydes under mild
conditions are summarized in Table 4. Reactions of benzylic
alcohols (9a–k) bearing electron-donating and electron-with-
drawing substituents at the aromatic ring proceeded smoothly
to give the corresponding aldehydes (10a–k) in good to high
yields (Table 4, entries 2–11). A variety of functional groups
[
7f–h]
material.
Interestingly, the reverse reaction, that is, the hydro-
genation of 8l to 7l could be accomplished using the same
catalyst 4a [Eq. (5)]. When the hydrogenation of 8l was
carried out in the presence of 4a (0.5 mol%) at 408C for 4 h
1
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Table 4: Dehydrogenative oxidation of various primary alcohols to aldehydes
catalyzed by 4a under mild reaction conditions.
On the basis of the above-mentioned results, we
designed a new prototype for a hydrogen storage
system, based on the interconversion between 7l
and 8l (Table 5). First, the dehydrogenation of 7l was
conducted under the same conditions as those in
Eq. (4) to afford 8l (100%) and hydrogen gas
[
a]
[18]
Ent. Alcohol
Cat.
Solvent
(b.p.)
Prod. Yield
[
b]
[mol%]
[%]
(
(
95%). Next, a balloon filled with hydrogen gas
1 atm) was attached to the flask and the solution
was stirred at 408C for 4 h. This simple procedure
transformed 8l back to 7l in 100% yield. Moreover,
the reversible transformation between 7l and 8l
could be repeated eight times without loss of the high
1
2
3
4
5
6
7
8
9
1
1
R’=H
(9a)
(9b)
(9c)
(9d)
(9e)
(9 f)
(9g)
(9h)
(9i)
1.5
1.5
2.5
1.5
1.5
1.5
1.5
3.0
5.0
1.0
1.5
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
heptane
tBuOH
toluene
tBuOH
(828C) 10a
(828C) 10b
(828C) 10c
(828C) 10d
(828C) 10e
(828C) 10 f
(828C) 10g
(988C) 10h
(828C) 10i
(1108C) 10j
(828C) 10k
92
98 (96)
85
R’=4-OMe
R’=2-Me
R’=3-Me
R’=4-Me
R’=4-Cl
R’=4-Br
R’=4-CF3
R’=4-CO Me
R’=4-OH
R’=4-Ph
91
96 (94) catalytic activity of 4a, thus demonstrating a promis-
90 (87) ing prototype for a hydrogen storage system using 2-
88 (86)
88
80
79
propanol, a simple and harmless molecule, as storage
[19]
material.
2
In summary, we have developed a new, efficient,
and versatile catalytic system for the dehydrogen-
ative oxidation of primary and secondary alcohols
under extremely mild conditions (reflux in pentane
for secondary alcohols and reflux in tBuOH for
primary alcohols) by using the new Cp*Ir catalyst 4a,
which bears a functional bipyridonate ligand. The
TONs reached up to 9500 (275000 at 1388C) for
alcohol 7a, and 3100 (47500 at 1108C) for alcohol
0
1
(9j)
(9k)
93 (91)
1
1
2
3
(9l)
2.5
5.0
toluene
toluene
(1108C) 10l
(1108C) 10m
81
87
(9m)
[a] Reaction was carried out with a primary alcohol (0.5 mmol) and 4a (1.0–
5
.0 mol%) in solvent (10 mL) under reflux for 20 h. [b] Determined by GC analysis
1
(
entries 1–8, 11–13) or H NMR spectroscopy (entries 9 and 10). Selectivity toward
the formation of 10 was higher than 97%. Yield of isolated product is in parenthesis.
9
a, which are the highest TONs reported for known
homogeneous catalysts, thus demonstrating a high
catalytic performance of 4a. Furthermore, reversible and
repetitive transformations between 2-propanol (7l) and
acetone (8l) by dehydrogenation–hydrogenation catalyzed
by 4a were achieved, providing a promising prototype for
a hydrogen storage system.
under a hydrogen atmosphere (1 atm), 7l was obtained
almost quantitatively (98%).
Received: August 29, 2012
Published online: November 7, 2012
Keywords: alcohols · dehydrogenation ·
homogeneous catalysis · iridium · oxidation
Table 5: Reversible transformation between 2-propanol (7l) and acetone
.
(
8l) in one flask by dehydrogenation and hydrogenation reactions
[
a]
catalyzed by 4a.
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Dehydrogenation
Hydrogenation
yield of 7l [%]
[
b]
[c]
[b]
yield of 8l [%]
yield of H [%]
2
1
2
3
4
5
6
7
8
100
99
98
96
97
96
95
95
95
94
94
92
93
92
91
91
100
98
98
97
95
94
96
96
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353, 3023.
[
(
4
a] The dehydrogenation was carried out with 7l (30.0 mmol), 4a
1.5 mol%), and internal standard (p-xylene, 1.0 mmol) under reflux for
h. Next, the atmosphere of the flask was replaced with hydrogen, and
[4] a) M. L. S. Almeida, M. Beller, G.-Z. Wang, J.-E. Bꢂckvall,
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a balloon filled with hydrogen gas was connected to the flask. The
mixture was stirred for 4 h at 408C. [b] Determined by GC analysis.
[
c] Yield of hydrogen gas that was collected in a gas burette.
Angew. Chem. Int. Ed. 2012, 51, 12790 –12794
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www.angewandte.org 12793

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Angewandte
Communications
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[
10] Only a few catalytic systems for dehydrogenation at lower
temperature (70 to 1008C) have been reported (see referen-
ces [8b,d,f,i, and j]). Hydrogen production from ethanol or 2-
propanol at reflux (78 or 828C) has also been reported (see
references [7f–h]).
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