Efficient hydrogenation of organic carbonates, carbamates and formates indicates alternative routes to methanol based on CO2 and CO

Article abstract of DOI:10.1038/nchem.1089

Catalytic hydrogenation of organic carbonates, carbamates and formates is of significant interest both conceptually and practically, because these compounds can be produced from CO₂ and CO, and their mild hydrogenation can provide alternative, mild approaches to the indirect hydrogenation of CO₂ and CO to methanol, an important fuel and synthetic building block. Here, we report for the first time catalytic hydrogenation of organic carbonates to alcohols, and carbamates to alcohols and amines. Unprecedented homogeneously catalysed hydrogenation of organic formates to methanol has also been accomplished. The reactions are efficiently catalysed by dearomatized PNN Ru(II) pincer complexes derived from pyridine- and bipyridine-based tridentate ligands. These atom-economical reactions proceed under neutral, homogeneous conditions, at mild temperatures and under mild hydrogen pressures, and can operate in the absence of solvent with no generation of waste, representing the ultimate 'green' reactions. A possible mechanism involves metal-ligand cooperation by aromatization-dearomatization of the heteroaromatic pincer core.

Full text of DOI:10.1038/nchem.1089

ARTICLES
PUBLISHED ONLINE: 22 JULY 2011 | DOI: 10.1038/NCHEM.1089
Efficient hydrogenation of organic carbonates,
carbamates and formates indicates alternative
routes to methanol based on CO and CO
Ekambaram Balaraman , Chidambaram Gunanathan , Jing Zhang , Linda J. W. Shimon
2
1
1
1
2
and David Milstein1
*
Catalytic hydrogenation of organic carbonates, carbamates and formates is of significant interest both conceptually and
practically, because these compounds can be produced from CO and CO, and their mild hydrogenation can provide
2
alternative, mild approaches to the indirect hydrogenation of CO and CO to methanol, an important fuel and synthetic
2
building block. Here, we report for the first time catalytic hydrogenation of organic carbonates to alcohols, and carbamates
to alcohols and amines. Unprecedented homogeneously catalysed hydrogenation of organic formates to methanol has also
been accomplished. The reactions are efficiently catalysed by dearomatized PNN Ru(II) pincer complexes derived from
pyridine- and bipyridine-based tridentate ligands. These atom-economical reactions proceed under neutral, homogeneous
conditions, at mild temperatures and under mild hydrogen pressures, and can operate in the absence of solvent with no
generation of waste, representing the ultimate ‘green’ reactions. A possible mechanism involves metal–ligand cooperation
by aromatization–dearomatization of the heteroaromatic pincer core.
1
6,17
he hydrogenation of polar bonds, in particular organic carbonyl of methanol to methyl formate
or oxidative carbonylation to
occurs under relatively mild conditions,
decades1 , mainly due to its synthetic significance as an envir- and their mild hydrogenation could provide a desirable route to
onmentally benign approach to fundamental synthetic building methanol (Fig. 1).
1
8,19
groups, has captured much attention during the past four dimethyl carbonate
–3
T
blocks such as alcohols and amines. Much progress has been
Production of methanol from CO2 by direct hydrogenation
made in the hydrogenation of ketones and aldehydes and, more under mild conditions is an attractive goal, but a practical catalytic
2
0–22
recently, rare examples of the significantly more difficult hydrogen- process has not yet been developed
. On the other hand, efficient
ation of esters4 and amides have also been reported. However, the transformation of CO2 to formic acid and its derivatives (for
,5
6,7
2
3–29
hydrogenation of organic carbonates and carbamates remains a example, methyl formate) is known and well investigated
.
major challenge. Indeed, as far as we know, catalytic hydrogenation Heterogeneous hydrogenation of methyl formate to methanol in
3
0–33
of these important families of compounds has never been reported, the gas phase
be it under heterogeneous or homogeneous catalysis. In fact, organic
carbonates have been used as solvents in catalytic hydrogenation
as well as the liquid phase using a semi batch
8
a CO
+
MeOH
2H2
reactions . Moreover, the popular amine protecting groups, benzyl
O
2
MeOH
carbamates, undergo deprotection by heterogeneous hydrogenation,
which involves cleavage of the benzyl–O bond, but with the
carbonyl group not being reduced . In addition, hydrogenation of
MeOH
CO2
+
H2
H
OMe
OMe
9
cyclic N-acylcarbamates leads to cleavage of the C–N bond
b CO
CO2
2 MeOH
2 MeOH
1/2 O2
10
without affecting the carbamate group .
+
+
O
–
H O
3H2
2
Hydrogenation of carbonates and carbamates is of considerable
interest conceptually and practically, because these compounds can
be readily formed from CO2 or from CO, and their mild
hydrogenation would effectively mean indirect hydrogenation of the
latter compounds to methanol. In addition, mild hydrogenation of
3 MeOH
2 MeOH
MeO
c
O
methyl formate, which can be produced from CO or CO, is of great
MeOH
3H2
2
CO2
interest as an alternative route to the conversion of these gases to
methanol. Heterogeneously catalysed hydrogenation of methyl formate
R
N
OMe
R
NH2
H
–
R
NH2
1
1,12
under high pressure and temperature has been reported , but we
are unaware of reports of homogeneously catalysed hydrogenations. Figure 1 | Alternative routes to methanol based on methyl formate,
Attempts at homogeneous hydrogenations of formate esters resulted dimethyl carbonate and organo-carbamates. a, Synthesis of methyl formate
1
3
in decomposition to CO and alcohol, with no methanol formation .
either from CO or CO followed by hydrogenation. b, Synthesis of dimethyl
2
Methanol is industrially produced from syngas at high tempera- carbonate either from CO or CO followed by unprecedented hydrogenation.
2
tures (250–300 8C) and high pressures (50–100 atm) using a c, Synthesis of organo-carbamates from CO followed by
2
copper-zinc-based oxide catalyst1 . In contrast, the carbonylation unprecedented hydrogenation.
4,15
1
2
Department of Organic Chemistry, The Weizmann Institute of Science, Rehovot, 76100, Israel, Department of Chemical Research Support, The Weizmann
Institute of Science, Rehovot, 76100, Israel. *e-mail: david.milstein@weizmann.ac.il
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1089
ARTICLES
reactor34 has been reported at elevated temperatures, with low
selectivity. However, this method suffers from side reactions such
as the formation of CO by-product from the decomposition of
methyl formate, making it less attractive.
Table 1 | Hydrogenation of dimethyl carbonate to methanol.
O
1
or 2
+
3H2
3 MeOH
MeO
OMe
Δ
The synthesis of dimethyl carbonate from CO is well documen-
2
ted3 , but to the best of our knowledge the hydrogenation of organic
carbonates (such as dimethyl carbonate) to methanol remains
unknown2 . Methods for the synthesis of methyl carbamates
5,36
Entry Cat. Solvent
pH2 Time
Conv.
(%)*
Yield
(%)*
TON
(h)
5,37
†
1
1
1
2
2
2
2
1,4-dioxane
1,4-dioxane
THF
40
60
3.5
1
.99
.99
96
89
89
.99
.99
96
88
89
2,500
2,500
960
4,400
890
from CO , methanol and amines have also been reported 6,38,39.
3
†
2
2
‡
§
Thus, catalytic hydrogenation of the organic carbonates, carbamates
and formates to methanol under mild conditions could provide an
3
10 48
4
THF
Neat
Neat
50
10
10
14
2
8
}
5
6
indirect method to obtain methanol from CO and from CO (Fig. 1).
2
}
.99
.99
.990
We have developed several reactions catalysed by pincer complexes
based on pyridine4
,40–46
and acridine backbones , including the
47,48
*Yields of methanol and conversion of dimethyl carbonate were determined by gas chromatography
†
4
(GC) using m-xylene as an internal standard. Complex
1
(0.01 mmol), dimethyl carbonate
hydrogenation of esters catalysed by 1 and, very recently, selective
‡
(25 mmol) and 1,4-dioxane (20 ml) were heated in a Parr apparatus at 145 8C. Complex 2
7
hydrogenation of amides to the corresponding alcohols and amines
(
0.01 mol) and dimethyl carbonate (10 mmol) were heated in a Fischer-Porter tube at 110 8C.
§
by cleavage of the C–N bond under homogeneous conditions, cata-
lysed by bipyridine-based pincer complex 2. These reactions are
thought to involve a new mode of metal–ligand cooperation based
on ligand aromatization–dearomatization, which has also led to con-
secutive water splitting promoted by complex 1 (ref. 49).
Complex 2 (0.005 mmol), dimethyl carbonate (25 mmol) and dry THF (5 ml) were heated in an
}
autoclave at 110 8C. Complex 2 (0.01 mmol) and dimethyl carbonate (10 mmol) were heated at
100 8C.
of dimethyl carbonate with H2 (10 atm) at 110 8C in
tetrahydrofuran (THF) for 48 h yielded 96% of methanol (Table 1,
entry 3). It is quite remarkable that this novel hydrogenation
reaction can proceed under such mild pressure (10 atm) and with
high TON (960). Even higher TONs (4,400) were obtained using
complex 2 as a catalyst with 50 atm of dihydrogen (Table 1, entry 4).
Remarkably, the hydrogenation of dimethyl carbonate proceeds
smoothly under solvent-free conditions with catalyst 2. Thus,
performing the reaction at 100 8C resulted in 89% conversion of
dimethyl carbonate to methanol after 2 h, and quantitative
conversion was observed after 8 h (Table 1, entries 5 and 6). This
represents an ultimate green transformation—no solvent, no waste,
complete selectivity, quantitative yield and under very mild conditions.
Complex 2 also efficiently catalyses the hydrogenation of other
organic carbonates to the corresponding alcohols. For example,
diethyl carbonate was selectively hydrogenated to ethanol (91%)
and methanol (89%) after 8 h in almost complete conversion
tBu
tBu
H
H
N
P
N
P
tBu
tBu
Ru
Ru
N
N
CO
CO
1
2
tBu
tBu
tBu
H
Ru
H
H
Ru
Cl
N
P
N
P
tBu
N
CO
N
CO
(
93%) and with a good TON (910, based on ethanol). The
3
4
Table 2 | Hydrogenation of methyl carbamates to methanol
and amines.
Here we report the first examples of catalytic hydrogenation of
organic carbonates to alcohols and organic carbamates to alcohols
and amines, and homogeneously catalysed hydrogenation of alkyl
formates to methanol and the corresponding alcohols. The reactions
are selective, proceed efficiently under mild, neutral conditions, have
high turnover numbers, and generate no waste. In addition, they
also proceed very efficiently under solvent-free conditions. These
new reactions provide interesting alternatives for the mild hydro-
O
2
+
3H2
2 MeOH
R
N
H
OMe
OMe
Δ
–
R
NH2
Entry
Carbamate
Amine
Alcohol Yield*
O
1
MeOH
97
genation of CO and CO to methanol, which is a timely topic in
2
N
H
NH2
50
the context of the ‘methanol economy’ . Some key mechanistic
insight is also provided.
O
Results and discussion
2
MeOH
98
Catalytic hydrogenation of organic carbonates. Complexes 1 and 2
were investigated as catalysts for the catalytic hydrogenation of
organic carbonates to alcohols. Dimethyl carbonate was selected as
a benchmark substrate for the hydrogenation reactions. Thus,
treatment of dimethyl carbonate (25 mmol) with dihydrogen
N
H
OMe
NH2
MeO
MeO
O
3
MeOH
94
(
(
40 atm) at 145 8C for 3.5 h with a catalytic amount of 1
0.01 mmol), using 1,4-dioxane as solvent, resulted in complete
Pentyl
NH2
N
OMe
H
conversion with selective formation of methanol, with a turnover
number (TON) of 2,500 (Table 1, entry 1). With 60 atm of H2,
quantitative formation of methanol was observed even after
Complex 2 (0.01 mmol), carbamate (1 mmol), H2 (10 atm) and dry THF (2 ml) were heated in a
Fischer-Porter tube at 110 8C (bath temperature). *Yields of product (based on MeOH) were
analysed by GC using m-xylene as an internal standard.
2
1
1
h (TOF ¼ 2,500 h ), with corresponding consumption of
dihydrogen (Table 1, entry 2). Using catalyst 2 (0.1 mol%), reaction
610
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1089
ARTICLES
O
complexes 1–4 as catalysts. Reaction of methyl formate and
dihydrogen (7 atm) catalysed by 1 (0.1 mol%) at 145 8C in 1,4-
dioxane yielded 71% of methanol after 30 h (Table 3, entry 1).
Cat. 2
Ph CH OH
+
+ MeOH
2
3H2
N
H
a
b
O
Performing the reaction under 9 atm of H resulted in complete
2
conversion of methyl formate to methanol after 36 h with complete
selectivity (Table 3, entry 2). Using a lower temperature, complex
Ph
O
N
1
(0.01 mmol) was heated with methyl formate (15 mmol) and
O
O
dihydrogen (10 atm) at 110 8C in THF for 48 h to yield 77% of
methanol (Table 3, entry 3). With catalyst 2, under the same
Pd/C
H2
Ph CH3
+
+ CO2
N
H
conditions (methyl formate/2 ¼ 1,500/1, 10 atm of H , 110 8C in
2
THF for 48 h), 96% conversion of methyl formate into methanol was
observed (Table 3, entry 4). Importantly, GC analysis of the reaction
mixtures indicated the absence of CO, which is a problematic
by-product in heterogeneously catalysed hydrogenation of methyl
Figure 2 | Hydrogenation of a benzyl carbamate. a, Unprecedented
hydrogenation to methanol, amine and benzyl alcohol catalysed by 2.
b, A common amine deprotection by heterogeneous hydrogenation of benzyl
carbamate with cleavage of the benzyl–O bond and liberation of free CO2.
35–39
formate
. Using 50 atm of H , even higher TONs (Table 3, entries
2
5
and 6) were achieved (up to 4,700) using complex 2. Notably, the
7
reaction was performed under solvent-free conditions under mild air-stable hydrido chloride complex 4 in the presence of one
pressure (pH ¼ 10 atm) and low temperature (100 8C). In none equivalent of base (relative to Ru) also efficiently catalysed the
2
of these hydrogenations did we observe any formation of hydrogenation of methyl formate; in this reaction the actual catalyst
alkyl formates.
2 is generated in situ by deprotonation of 4. Thus, upon heating a
THF solution of complex 4 (0.01 mmol) with KO Bu (0.01 mmol)
t
Catalytic hydrogenation of carbamates. In still another and methyl formate (15 mmol) at 110 8C under H (10 atm) for
2
unprecedented reaction, complex 2 catalyses the hydrogenation of 48 h, methanol was formed in 91% yield (Table 3, entry 8). No
methyl carbamates to methanol and the corresponding amines. reaction took place in the absence of base (Table 3, entry 9).
Thus, heating
a solution of methyl N-benzyl carbamate Importantly, hydrogenation of methyl formate is efficiently catalysed
(1.0 mmol) and dihydrogen (10 atm) with a catalytic amount of by 2 under solvent-free conditions, as in the case of dimethyl
complex 2 (0.01 mmol) at 110 8C in THF for 48 h selectively carbonate. Thus, heating a solution of methyl formate (10 mmol)
yielded methanol and benzylamine in quantitative yields. Other and a catalytic amount of 2 (0.01 mmol) at 80 8C under H (10 atm)
2
examples are listed in Table 2.
resulted in quantitative conversion of methyl formate selectively
Remarkably, benzyl carbamates were also selectively hydrogen- to methanol after 8 h, with the corresponding consumption of
ated to yield methanol, the corresponding amines and benzyl dihydrogen (Table 3, entry 10). Like in the case of dimethyl
alcohol, without cleavage of the benzyl–O bond. Thus, upon treat- carbonate, this is an ultimate green transformation—no solvent, no
ment of benzyl morpholine-4-carboxylate with H2 (10 atm) at waste, complete selectivity, quantitative yield, and under very mild
1
(
10 8C in dry THF for 52 h with a catalytic amount of 2 conditions. Under the same solvent-free conditions, catalyst 1 is less
1 mol%), 88% of benzyl alcohol, 87% of morpholine and 81% of effective and affords only ꢀ16% conversion of methyl formate.
methanol were obtained (Fig. 2a). It is important to note that in
Hydrogenation of other formate esters also proceeds efficiently.
the hydrogenolysis of benzyl carbamates, used for deprotection of Thus, reaction of ethyl formate with H (10 atm) at 110 8C in
2
carbamate-N functional groups catalysed by Pd/C, the deprotected THF for 48 h catalysed by 2 (formate/2 ¼ 1,500/1) resulted in
9
amine and free CO (via a carbamic acid intermediate) are formed . 92% of ethanol and 91% of methanol without formation of CO or
2
The reaction occurs by cleavage of the benzyl–O bond, but the car- ethyl acetate. In the same way, hydrogenation of n-butyl formate
bonyl group is not reduced, unlike the benzyl carbamate hydrogen- catalysed by 2 resulted in 86% conversion and selectively yielded
ation reported here (Fig. 2b).
methanol (82%) and n-butanol (84%).
Homogeneous hydrogenation of alkyl formates. Finally, we Mechanistic studies of the hydrogenation of organic carbonates
examined the possibility of homogeneous hydrogenation of alkyl and formates. The mechanism of these unusual hydrogenation
formates to methanol and the corresponding alcohols using reactions is of interest. As we reported, the trans-dihydride 3 is
Table 3 | Homogeneously catalysed hydrogenation of methyl formate to methanol.
O
Cat.
+
2H2
2 MeOH
Time (h)
H
OMe
Δ
Entry
Cat.
Solvent
Temp. (8C)
pH2
Conv. (%)*
Yield (%)*
TON
710
1
1
1
1
2
2
2
3
4
4
2
1,4-dioxane
1,4-dioxane
THF
THF
THF
THF
THF
THF
THF
145
145
110
110
110
110
110
110
110
80
7
9
30
36
48
48
8
14
48
48
48
8
72
.99
78
96
87
94
84
93
–
71
ꢀ99
77
96
85
94
81
91
–
98
2
3
4
ꢀ990
1,155
1,440
4,250
4,700
1,215
1,365
–
10
10
50
50
10
10
10
10
†
5
†
6
7
8
‡
9
1
0§
Neat
ꢀ99
980
Complexes 1, 2, 3 or 4 (0.01 mmol), methyl formate (10 mmol for entries 1–2; 15 mmol forentries 3–4, 7–9), H
2
and dry solvent (2 ml) were heated in a Fischer-Porter tube at the specified (oil bath) temperature.
†
*
Yields of methanol and conversion of methyl formate were analysed by GC using m-xylene as an internal standard. Complex 2 (0.005 mmol), methyl formate (25 mmol), and THF (5 ml) were heated under H
2
‡
t
§
pressure in an autoclave. One equiv (relative to Ru) of KO Bu was used. Complex 2 (0.01 mmol) and methyl formate (10 mmol) were used neat.
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1089
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dimethyl carbonate to deliver methyl formate, the in situ generated
complex 1 (in the absence of dihydrogen) reacts further with excess
of dimethyl carbonate, leading to 5 (see Supplementary Information
for reaction of 1 with dimethyl carbonate) by the decomposition of
dimethyl carbonate to methanol, CO and H (note that no CO was
2
observed in the catalytic hydrogenation reactions, because 1 reacts
with H to form 3).
2
P3
Analogousstepwise hydrogenationwasalsoobservedforthecaseof
methyl formate. Upon reaction of methyl formate (ꢀ2.5 equiv.) with 3
at room temperature, formationof methanol and acomplex tentatively
assigned as a formaldehyde Ru(II) intermediate 6 was observed after
1HB
O1
C1
51
N1
RU1
1
10 min (Fig. 4b) . The aldehyde proton of 6 appears as a singlet at
9
.21 ppm (ꢀ0.5 ppm downfield relative to free formaldehyde, which
HA
appears at 8.73 ppm in toluene-d ). After prolonged standing at
8
room temperature (ꢀ6 h), the signal of 6 slowly diminished, together
with the formation of the fully characterized, methoxy Ru(II) species 7,
which was independently prepared by the addition of methanol to
complex 1 (see Supplementary Information).
N2
On the basis of the above results and the metal–ligand
cooperation by aromatization–dearomatization prevalent in the
chemistry of pincer complexes 1 (ref. 44) and 2 (ref. 7), we
propose a possible mechanism for the hydrogenation of dimethyl
carbonate and methyl formate catalysed by 1 (Fig. 5), although
more studies are needed for mechanistic interpretation. Initially,
dihydrogen addition by metal–ligand cooperation to complex 1
results in aromatization, to form the coordinatively saturated,
Figure 3 | X-ray structure of complex 3 (50% probability level). Hydrogen
atoms (except hydrides) are omitted for clarity. Selected bond distances (Å)
and angles (deg): Ru1–N1 2.101 (3), Ru1–N2 2.251 (3), Ru1–P3 2.252 (1),
Ru1–C1 1.821 (4), H1A–Ru1 1.71 (4) Å, (H1B–Ru1 was not refined). N1–Ru1–C1
4
trans-dihydride complex 3, as previously observed . Subsequent
hydride transfer to the carbonyl group of the carbonate ligand
can lead to intermediate A. This process may involve direct
hydride attack on the ester or alternatively dissociation of the
pyridyl arm to provide a site for dimethyl carbonate coordination.
1
75.0 (2), N1–Ru1–P3 82.6 (1), N1–Ru1–N2 78.0 (1), N2–Ru1–P3 160.6 (1),
N2–Ru1–C1 104.8 (2), C1–Ru1–P3 94.6 (1).
4
formed by treatment of the dearomatized 1 with dihydrogen . A Deprotonation of the benzylic arm by the adjacent methoxy
single-crystal X-ray diffraction study of 3 (Fig. 3) reveals a group can result in liberation of methanol and the formation of a
distorted octahedral geometry around the ruthenium centre, with dearomatized intermediate B, bearing a coordinated methyl
the CO ligand coordinated trans to the pyridyl nitrogen atom. formate. Dihydrogen addition to B (which may also involve
Complex 3 also catalyses the hydrogenation of methyl formate to amine arm opening), followed by hydride transfer to methyl
methanol (Table 3, entry 7).
formate, can generate a intermediate C. Deprotonation of the
Reaction of the trans-dihydride 3 with dimethyl carbonate benzylic arm by the methoxy group can generate the product
(
ꢀ2.5 equiv.) in toluene-d in a sealed NMR tube at 50 8C resulted methanol and a formaldehyde intermediate 6, which undergoes
8
in the formation of free methyl formate and methanol in 5 min, as hydrogenation to the characterized methoxy intermediate 7.
observed by H NMR, together with the fully characterized, new Methanol liberation from complex 7 regenerates catalyst 1.
1
dearomatized dicarbonyl complex 5 (Fig. 4a and Supplementary
Similarly, in the case of hydrogenation of methyl formate, reac-
Information). Presumably, after reaction with one equivalent of tion with the dihydride 3 can give intermediate C. Deprotonation
a
O
tBu
tBu
tBu
tBu
O
H
Ru
H
2
MeO
H
N
P
N
P
OMe
H
OMe
+
2MeOH
+
Ru
CO
Δ, –H , –CO
2
N
CO
N
CO
3
5
H
CH3
b
H
O
tBu
tBu
tBu
tBu
H
Ru
H
N
O
O
N
P
P
N
P
H
OMe
MeOH
tBu
tBu
1
Ru
H
+
MeOH
Ru
H
N
CO
N
CO
N
CO
3
6
7
Figure 4 | Reactivities of the saturated Ru(II)–trans-dihydride complex 3, suggesting its intermediacy in the catalytic hydrogenation mechanism. a,
Reaction with dimethyl carbonate leads to methyl formate, methanol and complex 5. b, Reaction of complex 3 with methyl formate resulted in methanol,
formaldehyde intermediate 6 and methoxy intermediate 7.
612
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1089
ARTICLES
Me
O
H2 MeOH
H
PtBu2
O
t
MeO
OMe
N
N
P Bu
2
H2
Ru
H
Ru
H
N
CO
N
CO
Et
Et
2
2
3
7
H
OMe
H
H
H
OMe
H
O
Ru
H
PtBu2
N
PtBu2
N
O
t
N
P Bu
2
Ru
Ru
H
N
N
CO
CO
N
CO
Et
Et
2
Et
2
2
A
1
6
H
H
H
OMe
OMe
O
Ru
H
PtBu2
O
Ru
H
N
N
PtBu2
MeOH
MeOH
N
CO
N
CO
Et
H2
2
Et
2
B
C
Figure 5 | Postulated mechanism for novel, homogeneous hydrogenation of dimethyl carbonate and methyl formate to methanol catalysed by complex 1.
The mechanism involves metal–ligand cooperation by aromatization–dearomatization of the heteroaromatic pincer ligand and hydride transfer to the
carbonyl group.
of the benzylic arm by the methoxy group (as with intermediate B in 5. Saudan, L. A., Saudan, C. M., Debieux, C. & Wyss, P. Dihydrogen reduction of
carboxylic esters to alcohols under the catalysis of homogeneous ruthenium
complexes: high efficiency and unprecedented chemoselectivity. Angew. Chem.
Int. Ed. 46, 7473–7476 (2007) and references therein.
N u´ n˜ ez Magro, A. A., Eastham, G. R. & Cole-Hamilton, D. J. The synthesis of
amines by the homogeneous hydrogenation of secondary and primary amides.
Chem. Commun. 43, 3154–3156 (2007).
Balaraman, E., Gnanaprakasam, B., Shimon, L. J. W. & Milstein, D. Direct
hydrogenation of amides to alcohols and amines under mild conditions.
J. Am. Chem. Soc. 132, 16756–16758 (2010).
the case of dimethyl carbonate) leads to methanol and intermediates
6
and 7, as indicated experimentally (Fig. 4b). Further mechanistic
studies are in progress.
6.
7.
8.
In conclusion, for the first time, selective hydrogenations of
the CO (and CO)-derived organic carbonates, formates and carba-
2
mates to methanol were demonstrated using soluble, well-defined
metal complexes as catalysts. The reactions, catalysed by PNN
complexes 1–3, proceed efficiently and selectively under mild,
neutral conditions using mild hydrogen pressure, without the
Schaffner, B., Schaffner, F., Verevkin, S. P. & Borner, A. Organic carbonates as
solvents in synthesis and catalysis. Chem. Rev. 110, 4554–4581 (2010).
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This research was supported by the European Research Council under the FP7 framework
(ERC no. 246837), by the Israel Science Foundation, and by the MINERVA Foundation.
D.M. is the Israel Matz Professorial Chair of Organic Chemistry.
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Author contributions
E.B. carried out catalytic experiments, mechanistic studies and contributed to writing the
manuscript. C.G. carried out catalytic experiments and contributed to writing the
manuscript. J.Z. prepared and crystallized complex 3. L.J.W.S. conducted the X-ray
structural study of complex 3. D.M. carried out the design and direction of the project
and contributed to writing the manuscript.
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carbon dioxide to urethanes by homogeneous catalysis. Chem. Commun. 37,
238–2239 (2001).
3
The authors declare no competing financial interests. Supplementary information
2
accompanies this paper on www.nature.com/naturechemistry, including synthesis and
X-ray structural determination of 3, mechanistic stoichiometric reactions, general
procedures for the catalytic hydrogenation reactions, and cif file for 3 (CCDC #826775)
and chemical compound information accompanies this paper at www.nature.com/
naturechemistry. Reprints and permission information is available online at http://www.
nature.com/reprints. Correspondence and requests for materials should be addressed to D.M.
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of CO to urethanes. Green Chem. 6, 524–525 (2004).
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into esters and dihydrogen catalyzed by new ruthenium complexes. J. Am. Chem.
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