Direct Hydrogenation of a Broad Range of Amides under Base-free Conditions using an Efficient and Selective Ruthenium(II) Pincer Catalyst

Article abstract of DOI:10.1002/cctc.201700952

The ruthenium(II) complex, [fac-PN_HN]RuH(η¹-BH₄)(CO) (B; PN_HN=8-(2-diphenylphosphinoethyl)aminotrihydroquinoline), is a highly versatile and effective catalyst (loadings of 0.1–1 mol %) for the hydrogenation of a multitude of amides, which include primary, secondary, and tertiary amides, to give their corresponding alcohols and amines in high yields under base-free conditions. All products were confirmed by using GC and GC–MS.

Full text of DOI:10.1002/cctc.201700952

Accepted Article
Title: Direct Hydrogenation of a Broad Range of Amides under Base-
free Conditions using an Efficient and Selective PNN-Ru(II)
catalyst
Authors: Wen-Hua Sun, Zheng Wang, Yong Li, Qingbin Liu, Gregory
Solan, and Yanping Ma
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201700952
Link to VoR: http://dx.doi.org/10.1002/cctc.201700952
A Journal of
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10.1002/cctc.201700952
ChemCatChem
COMMUNICATION
Direct Hydrogenation of a Broad Range of Amides under Base-
free Conditions using an Efficient and Selective PNN-Ru(II)
catalyst
[a],[b],[c]
Zheng Wang,
Sun*,[b],[c]
Yong Li,[a] Qing-bin Liu,*,[a] Gregory A. Solan,*,[b],[d] Yanping Ma,[b] Wen-Hua
Abstract: The ruthenium(II) complex, [fac-PNHN]RuH(η1-BH4)(CO)
(B) (PNHN = 8-(2-diphenylphosphinoethyl)-aminotrihydroquinoline),
serves as a highly versatile and effective catalyst (loadings of 0.1 - 1
mol%) capable of selectively hydrogenating a multitude of amides,
including primary, secondary and tertiary, to give their corresponding
alcohols and amines in high yields under base-free conditions. All
products were confirmed by GC and GC-MS.
Scheme 1. Possible reaction pathways for the amide hydrogenation.
Amines can be deemed key intermediates in the chemical
industry that can be employed in a diverse range of applications
including as dyes, drugs, agrochemicals, plastics, surfactants,
textiles through to uses in the paper industry.1 Traditionally,
amines are prepared by the hydrogenation of amides using
stoichiometric or excess amounts of metal-hydride reagents (e.g.,
LiAlH4, NaBH4),2 boranes3 or silanes;2,4 this synthetic approach
being largely driven by the ready availability of the precursor
amides obtainable through chemical and biomass processes.5
Nevertheless, one limitation of this type of hydrogenation is the
formation of by-products that can be toxic in nature which in turn
poses problems with disposal.2,6 As an alternative, the direct
hydrogenation of amides into amines using molecular hydrogen
in the presence of a suitable transition metal catalyst presents a
more environmentally friendly method.2 However, the low
electrophilicity of the carbonyl carbon, and consequent high
thermodynamic and kinetic stability, renders this class of
carboxylic derivative2,7a difficult to hydrogenate and as a result
quite forcing conditions (e.g., higher temperatures and pressures)
tend to be required. Unsurprisingly heterogeneous catalysts
have led the way with numerous systems developed,2,5a,8
nonetheless these more robust systems can suffer from
incompatibility problems associated with functional group
intolerance. Consequently, there has been interest in developing
homogeneous variants that can operate under milder conditions
whilst offering more control on the hydrogenation process.6,7
Indeed a wide variety of molecular catalysts have been reported
for amide hydrogenation with two types of reaction pathway
prevalent namely de-oxygenative hydrogenation (C-O cleavage,
pathway A) and de-aminative hydrogenation (C−N cleavage,
pathway B) (Scheme 1).2,9
A decade ago, Cole-Hamilton and co-workers reported the
first
Ru(acac)3/triphos (triphos = 1,1,1-tris(diphenylphosphinomethyl)-
ethane),10
through predominantly de-oxygenative
homogeneous
hydrogenation
of
amides
using
a
hydrogenation pathway that displayed low selectivity. To
enhance this selectivity, the presence of an acid as co-catalyst
has been disclosed.11 Subsequently, selective C–N bond
hydrogenation of amides has been demonstrated using catalysts
developed by the groups of Milstein,12 Ikariya,13 Saito,14
Bergens,15 Mashima,16 Beller17 and others,18 in which the
majority of these examples require an excess amount of base to
ensure good activities. In a manner akin to Noyori’s work on
metal-mediated carbonyl group hydrogenation under base-free
conditions,19 two examples have also emerged that can operate
under similar such conditions.15c,17a However, these base-free
catalysts, disclosed by Bergen and Beller, are limited in their
substrate scope with some amides (e.g., primary) not conducive
to reduction. Hence, there is a considerable drive to develop a
general catalyst capable of efficiently hydrogenating a multitude
of common amide-types into amines without the use of base and
under mild conditions.
[a]
Z. Wang, Y. Li, Prof. Q.-B. liu.
College of Chemistry and Material Science, Hebei Normal University,
Shijiazhuang 050024, China
E-mail: liuqingb@sina.com
[b]
Z. Wang, Prof. G. A. Solan, Y. Ma, Prof. W.-H. Sun.
Key Laboratory of Engineering Plastics and Beijing National Laboratory
for Molecular Science, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, China
Chart 1. Ruthenium(II) complexes, A – D, to be investigated as catalysts for
the hydrogenation of amides in this study
E-mail: whsun@iccas.ac.cn;gas8@le.ac.uk
[c]
[d]
CAS Research/Education Center for Excellence in Molecular Sciences,
University of Chinese Academy of Sciences, Beijing 100049, China
Department of Chemistry, University of Leicester, University Road,
Leicester LE1 7RH, UK
In recent work we have found the PNN-ruthenium(II) complex,
[fac-PNN]RuH(PPh3)(CO) (PNN = 8-(2-diphenylphosphinoethyl)-
amidotrihydroquinoline) (A, Chart 1) to be highly efficient in the
coupling-cyclization of γ-amino alcohols with secondary alcohols
with catalyst loadings of as low as 0.025 mol%.20 Furthermore, A
Supporting information for this article is given via a link at the end of the
document.
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10.1002/cctc.201700952
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and its derivative [fac-PNHN]RuH(η1-BH4)(CO) (B) (PNHN = 8-(2-
diphenylphosphinoethyl)aminotrihydroquinoline) (Chart 1) could
promote the catalytic hydrogenation of esters in the presence of
a 5 mol% amount of NaBH4.21 Moreover, both A and B along
with the related ruthenium(II) complexes, [fac-PNHN]RuCl2(PPh3)
(C) and [fac-PNHN]RuH(η1-BH4)(PPh3) (D) (Chart 1), were
capable of the acceptorless dehydrogenation of alcohols with
high conversions.22 Given the adaptability of A – D to both
dehydrogenation and hydrogenation processes, we were
interested in exploring their potential as catalysts for the
hydrogenation of the more challenging amides.
of the substrate being reduced to benzyl alcohol (42%) and
aniline (43%) (entry 2). Doubling the amount of the base, NaBH4,
to 10 mol% had the effect of increasing the conversion to 65%
(entry 3). More notably, running the reaction in i-PrOH instead of
THF under the same conditions, afforded benzyl alcohol and
aniline in quantitative yield (entry 4), while performing the
reaction in the complete absence of NaBH4 in i-PrOH led to
conversions of 80% and 85% in 24 and 36 hours, respectively
(entry 5).
Using B (1 mol%) in place of A with the quantity of NaBH4 set
at 10 mol%, the reaction afforded quantitative yields of benzyl
alcohol and aniline in 16 hours (entry 6). Substituting NaBH4 with
10 mol% of t-BuOK or KHMDS (potassium bis(trimethylsilyl)-
amide), resulted in 95% and 98% of benzanilide being reduced
over 20 hours, respectively (entries 7 and 8). Significantly, when
B was used in the absence of base, quantitative yields of benzyl
alcohol and aniline were obtained at 50 bar of H2 and 120 °C in
To assess the catalytic activity, the PNN-bound ruthenium
complex A was in the first instance evaluated in amide
hydrogenation using benzanilide as the test substrate (Table 1).
Using the same conditions as previously employed in ester
hydrogenation21 (catalyst loading = 0.05 mol%, p(H2) = 50 bar, 5
mol% of NaBH4, T = 120 °C, solvent = THF, t = 24 hours), only
12% of benzanilide was hydrogenated (entry 1). However, with a
catalyst loading of 1 mol%, the conversion increased with 45%
Table 1. Optimization of the reaction conditions for the hydrogenation of benzanilide with ruthenium complexes, A - Da
Entry
Catalyst
Loading (mol%)
Solvent
THF
T (oC)
120
Base (mol%)
NaBH4 (5)
t (h)
24
24
24
18
24
36
4
Conv. (%)c
12
alcohol (%)c
12
amine (%)c
10
1d
2
A
A
A
A
0.05
1
1
1
THF
120
NaBH4 (5)
45
42
43
3
THF
120
NaBH4 (10)
NaBH4 (10)
65
64
63
4
i-PrOH
120
100
80
>99
78
>99
75
5
6
A
B
1
1
i-PrOH
i-PrOH
120
120
none
85
84
83
30
27
28
NaBH4 (10)
16
20
20
4
100
95
>99
94
>99
93
7
8
B
B
1
1
i-PrOH
i-PrOH
120
120
t-BuOK (10)
KHMDSe (10)
98
97
96
36
34
35
9
B
1
i-PrOH
120
none
16
24
14
24
24
24
16
24
24
24
100
75
>99
74
>99
73
10
11
B
B
0.1
1
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
120
140
80
none
none
100
90
>99
90
>99
87
12
B
1
none
13f
14g
15h
16
B
1
120
120
120
120
120
120
none
85
85
84
B
1
none
45
43
42
B
1
NaBH4 (100)
none
90
86
87
C
1
45
43
44
17
D
1
none
95
95
95
-
-
-
18
none
0
NaBH4 (10)
a
b
c
Conditions: benzanilide (1 mmol), A - D (0.01 mmol), solvent (30 mL) under H2 (50 bar). Pressure of hydrogen at room temperature in all cases.
Conversion of benzanilide and yields of benzyl alcohol and aniline were determined by GC (using hexadecane as an external standard) and GC-MS. d The
reaction was conducted with B (0.0005 mmol) and 5 mol% of NaBH4. e KHMDS is potassium bis(trimethylsilyl)amide. f The reaction was conducted under H2
(30 bar). g The reaction was conducted under H2 (10 bar). h In the absence of H2.
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10.1002/cctc.201700952
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16 hours (entry 9). Even with the catalyst loading reduced to 0.1
mol%, B still performed to a high level allowing a 75%
conversion of benzanilide in 24 hours (entry 10). Raising the
temperature to 140 oC also gave quantitative conversion over 14
hours while lowering it to 80 oC, the conversion dropped to 90%
(entries 11 and 12). With the temperature restored to 120 oC, the
conversion was also affected by the pressure of hydrogen
employed, which decreased to 85% and 45% at 30 bar and 10
bar, respectively in 24 hours (entries 13 and 14).
with aprotic solvents such as THF, 1,4-dioxane and toluene only
32%, 30% and 25% conversion of benzanilide could be
achieved after 24 hours using B, respectively. Hence, i-PrOH
proved the most suitable solvent for the transformation and
following some further optimization studies (see SI, Tables S2
and S3), the best overall conditions for effective hydrogenation
were established as: B (0.1 – 1 mol%), i-PrOH as solvent,
temperature of 120 °C and 50 bar of hydrogen pressure.
Next, with a view to exploring the substrate scope of B as a
base-free catalyst and hence its generality, we screened it for
the hydrogenation of a broad range of alkyl- and aryl-substituted
amides using the optimized conditions identified (Tables 2 and 3).
Firstly, a family of benzanilides functionalized at either the N-aryl
or the benzoyl group was evaluated; the complete list of
substrates along with hydrogenation results are collected in
Table 2. Pleasingly, most of the substrates were reduced
affording excellent yields of the desired amine and/or alcohol. As
a general observation, electron-withdrawing groups at the para-
position of the N-aryl group enhanced the reactivity of the
substrate. For example, the benzanilides bearing electron-
withdrawing p-F, p-Cl and p-Br substituents afforded quantitative
yields of benzyl alcohol and the corresponding substituted
aniline in 10 - 16 hours (entries 2, 4, 7). Notably the aryl
bromides were tolerant to the reaction conditions with no
dehalogenation products detected. Even at catalyst loadings as
low as 0.1 mol%, N-(4-chlorophenyl)benzamide and N-(4-
bromophenyl)benzamide underwent hydrogenation with high
conversions over 24 hours with >99% selectivity for C−N bond
cleavage (entries 4, 7). In general, the benzanilides containing
para-substituted halides are more easily reduced than their
meta-substituted counterparts (entries 2 vs. 1 and 4 vs. 3), while
increasing the number of halide substituents on the N-aryl ring
did not accelerate the reaction rate (entries 4 vs. 5 and 6). In
With C and D as catalyst, under base-free conditions, 45% and
95% of benzanilide were hydrogenated over 24 hours,
respectively (entries 16 and 17), which contrasts with
quantitative conversion observed for B over 16 hours (entry 9).
Moreover, with a stoichiometric amount of NaBH4 (1 mmol) but
no hydrogen (0 bar H2, T = 120 oC), B facilitated 90% conversion
of benzanilide in 16 hours. Hence NaBH4 can also serve as the
source of hydrogen for the hydrogenation of amides (entry 15);
this observation is entirely consistent with that found for the
catalytic hydrogenation of esters by A.20 For purposes of
comparison, no benzanilide was hydrogenated when the
reaction was performed without any Ru-complex (entry 18).
Overall these preliminary experiments (Table 1, see also SI
Table S2) suggest that
B is a credible catalyst for the
hydrogenation of amides under base-free conditions.
As mentioned above the choice of solvent employed can have
a significant effect on the conversion of benzanilide to benzyl
alcohol and aniline with i-PrOH superior to THF when using A.
To explore more thoroughly these effects, we also probed the
catalytic performance of
B (0.1 mol%) under base-free
conditions using a range of different solvents (Fig. 1, see Table
S3 in the SI). When MeOH, EtOH or i-BuOH were employed, the
conversion observed over 24 hours was respectively 15%, 25%
and 28%, notably less than that found with i-PrOH (75% over the
same time period). In comparison using A (0.1 mol%), this time
in the presence of 10 mol% NaBH4, but with i-PrOH as solvent,
addition, at
a
catalyst loading of 0.1 mol%, N-(3,4-
dichlorophenyl)benzamide needs a longer reaction time (40
hours) to reach a comparable conversion to that seen with N-(4-
chlorophenyl)benzamide (entry 5 vs. 4). Unexpectedly, the aryl-
substituted substrates containing electron-donating groups also
achieved high conversions despite the decreased electrophilicity
of the carbonyl carbon, however these conversions were over
longer reaction times (entries 10, 12-15). Moreover, the
presence of substituents with some modest steric hindrance at
the meta- or ortho-positions on the aromatic ring substrates were
well tolerated (entries 12 and 16). However, amides with either
highly hindered N-aryl groups (e.g., mesityl) or containing N-
pyridyl groups were not amenable to reduction (entries 11 and
17).
The hydrogenation capability of this catalytic system is not
limited to aryl-substituted secondary amides (entries 18 – 22).
Indeed, we have shown that primary and tertiary amides can
also be reduced. For example, benzamide, 4-chlorobenzamide,
4-bromobenzamide and 4-methoxybenzamide gave reasonable
conversions to their corresponding alcohols over 24 hours. It is
Fig. 1. Influence of solvent on the hydrogenation of benzanilide using Ba
a Standard reaction conditions: benzanilide (2.5 mmol), B (2.5 μmol), solvent
(30 mL) under H2 (50 bar) at 120 °C in 24 hours. b Conversion of benzanilide
and yields of benzyl alcohol and aniline were determined by GC (using
c
hexadecane as an external standard) and GC-MS.
The reaction was
conducted with A (2.5 μmol), 10 mol% of NaBH4 at 120 °Cin 36 hours.
the conversion lowered to 54% in 36 hours. On the other hand,
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Table 2. Hydrogenation of aryl-substituted amides catalyzed by Ba
alcohol/amine
(%/%)c
alcohol/amine
Conv. (%)c
75
Entry
Substrate
t (h)
16
Conv. (%)c
Entry
Substrate
t (h)
16
(%/%)c
1
2
70/73
12
86
99
85/84
16
36d
99
75
99/99
75/72
13
14
15
24
24
75/99
65/75
3
4
16
99
99/99
75
10
24d
99
80
95/98
80/76
24
48e
55
99
50/48
95/96
16
40d
99
75
96/99
75/74
5
6
7
16
17
18
24
24
24
95
trace
65
95/90
nd
16
99
99/95
10
24d
99
85
99/99
85/83
65/nd
8
9
10
99
99/99
19f
20f
24
24
55
65
50/nd
62/nd
20
48d
99
65
95/92
65/60
10
11
24
24
99
<1
95/93
n.d.
21f
22f
24
24
64
45
60/nd
42/nd
a Standard reaction conditions: amide (1 mmol), B (0.01 mmol), i-PrOH (30 mL) under H2 (50 bar) in 10 - 48 hours. b Pressure of H2 at room temperature in all
c
cases. Conversion of amide and yields of alcohol and amine were determined by GC (using hexadecane as an external standard) and GC-MS (nd = not
detected). d Standard reaction conditions: amide (10 mmol), B (0.01 mmol), i-PrOH (30 mL) under H2 (50 bar) in 24 - 48 hours. e Standard reaction conditions:
amide (10 mmol), B (0.5 mmol), i-PrOH (30 mL) under H2 (50 bar) in 48 hours. f The reaction temperature is 140 oC
smoothly, while the longer chain amide, N-(4-bromophenyl)-
decanamide, gave high yields of decanol and 4-bromoaniline
(entry 8). For the primary fatty amide, octanamide,
hydrogenation to give octan-1-ol was more sluggish at 35% yield
noteworthy that such unactivated primary amides have been
scarcely investigated in this type of hydrogenation but, where
reported, tend to require an excess of base to provide
satisfactory activity.14,16,17a
Secondly, the capacity of B to reduce alkyl-substituted amides
was explored; the specific substrate types and the results of the
hydrogenation are documented in Table 3. Acetanilide and its
derivatives underwent hydrogenation in quantitative yield using
B at a 1 mol% loading (entries 1 - 5). Even at 0.1 mol%, N-
methylacetamide and N,N-dimethylacetamide could be readily
hydrogenated affording high yields of ethanol and the
corresponding amines after 20 hours (entries 1 and 2). Likewise,
the reduction of 3-phenyl-N-(p-tolyl)propanamide and N-(4-
bromophenyl)-3-phenylpropanamide (entries 6 and 7) proceeded
o
even with the reaction temperature raised to 140 C (entry 9).
Meanwhile, hydrogenation of the cyclic amide, ε-caprolactam,
proceeded cleanly to afford 6-amino-1-hexanol in 98% yield
(entry 10). Overall, these results reveal that alkyl-substituted
amides like their aryl-amide counterparts can be readily reduced
with B under a set of general reaction conditions.
The mechanistic details of how
B operates in amide
hydrogenation raises some intriguing questions, particularly
because we have previously highlighted the cooperative role of
NaBH4 (5 mol%) in the hydrogenation of esters with the same
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Table 3. Hydrogenation of alkyl-substituted amides catalyzed by Ba
Conv.
(%)c
alcohol
(%)c
amine
(%)c
Entry
1d
Substrate
t (h)
20
99
99
99
99
nd
nd
2d
3
20
16
90
99
85
16
99
80
99
74
99
80
4
36d
16
99
83
99
80
99
83
5
6
7
36d
24
16
99
99
98
99
95
99
Scheme 2. Proposed catalytic cycles for amide reduction promoted by B
8
16
24
99
35
98
35
97
nd
9f
release of ½B2H6. A fac-mer isomerization of the PNHN ligand
then ensues generating trans-dihydride 1, which in previous
work we have considered as the actual catalyst in ester
hydrogenation.21 Hence, 1 can then enter two linked but distinct
catalytic cycles.21 At the start of Cycle 1, an amide molecule
approaches 1 resulting in a transfer of the hydride directly from
the ruthenium center to its unsaturated carbonyl carbon to form
intermediate 3 via an outer-sphere bifunctional mechanism.2, 12a,
10
36
99
trace
98
a Standard reaction conditions: amide (1 mmol), B (0.01 mmol), i-PrOH (30
mL) under H2 (50 bar) in 10 - 36 hours.
temperature in all cases. Conversion of amide and yields of alcohol and
amine were determined by GC (using n-hexadecane as an external
b
Pressure of H2 at room
c
d
standard) and GC-MS (nd = not detected) Standard reaction conditions:
amide (10 mmol), B (0.01 mmol), i-PrOH (30 mL) under H2 (50 bar) in 20
hours. f The reaction temperature is 140 oC.
Reorganization of the CH(NHR2)R1O anion forms O-
12e, 21, 23, 24
catalyst.21 Importantly, we have shown in this work that the
addition of 10 mol% of NaBH4 to an amide hydrogenation
promoted by B has little or no effect on the conversion observed
when compared with the reaction performed in the absence of
NaBH4 (entries 6 vs. 9, Table 1). By contrast, the addition of 10
mol% of NaBH4 to an amide hydrogenation mediated by A did
show some increase in the conversion (entries 4 vs. 5, Table 1),
a finding that can be attributed to the in-situ generation of B from
A.20 The implication is that the base-free amide hydrogenation
reported herein follows a modified mechanism to that seen in
ester hydrogenation.
To understand the effect of amine formation on the catalytic
performance of B, we added one molar equivalent of aniline to
our original ester hydrogenation using a catalyst composed of A
and NaBH4 (50 bar H2, T = 120 oC) (Table S4 in SI). With methyl
benzoate as the substrate, A/NaBH4 promoted almost
quantitative conversion to methyl benzoate over 24 hours (see
SI, Table S4), which is significantly slower than that observed
without aniline (only 4 hours).21 This would suggest that catalyst
deactivation is occurring and likely by the coordination of the
aniline to the ruthenium catalyst. By implication, it would
therefore seem probable that those amides that proved less
conducive to hydrogenation by B (e.g., lower conversions or
slower) are inhibited by the formation of Ru-amine type species.
bound 4 which can then undergo elimination of a molecule of
amine and aldehyde to afford 2. The active catalyst 1 is
regenerated through alcohol-assisted H2 activation.21, 24 Cycle 2
represents the hydrogenation of the aldehyde to alcohol which
starts with a hydride transfer from ruthenium to the unsaturated
carbonyl carbon via an outer-sphere bifunctional mechanism
forming intermediate 5 which can then transform into hemiacetal
6. 6 then decomposes by the release of alcohol to form 2 which
can then re-form 1 via H2 activation.
In summary, we have demonstrated that ruthenium(II)
complex B bearing a PNHN ligand, is an effective catalyst for the
hydrogenation of both aromatic and aliphatic amides including
unactivated examples under base-free conditions affording
selectively the corresponding alcohols and amines in high yields.
The catalytic activity for the reduction of substituted-benzanilides
was greatly affected by electronic, conjugative and steric effects.
Electron withdrawing groups on the aromatic ring increase the
reactivity while electron-donating groups decrease it. For N,N-
dimethylacetamide, a very high conversion (catalyst loading =
0.1 mol%, 99% yield) is achieved in 20 hours at 120 °C. Unlike
ester hydrogenation with A, there is no evidence for cooperation
between NaBH4 and A in the hydrogenation of amides; the
function of adding 10 mol% of NaBH4 is purely to form B.
A
proposed mechanism for the amide hydrogenation
developed in this work is shown in Scheme 2. Initially, B can be
-
converted to cis-dihydride 1' involving BH4 dissociation and the
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[3] (a) J. Seyden-Penne, Reductions by the Alumino- and Boro-hydrides in
Organic Synthesis; John Wiley & Sons, Inc: New York, 1997; (b) N. L.
Lampland, M. Hovey, D. Mukherjee, A. D. Sadow, ACS Catal. 2015, 5,
4219-4226.
Experimental section
General information. Unless otherwise stated, all manipulations were
performed under an atmosphere of argon or using standard Schlenk
techniques. Hydrogen gas (99.99%) was purchased from Shijiazhuang
Xisanjiao. Solvents were dried using standard procedures and degassed
with nitrogen. For example, tetrahydrofuran (THF) was distilled over
sodium/benzophenone, 1,4-dioxane was distilled over sodium and
diglyme (diethylene glycol dimethyl ether) was distilled over sodium
under reduced pressure. Analytical grade isopropanol, ethanol, methanol
and toluene were degassed by bubbling argon through them before use.
GC–MS was carried out on DSQII instrument (column: HP-5MS):
injector temp. 300 ºC; detector temp. 300 ºC; column temp. 40 ºC;
withdraw time 2 min, then 20 ºC/min to 230 ºC over 5 min. and then 20
ºC/min to 300 ºC; withdraw time 5 min. GC analysis was carried out on an
Agilent 6820 instrument using a polar capillary column (part number
19091N-113 HP-INNOWAX): injector temp. 300 °C; detector temp.
300 °C; column temp. 40 ºC; withdraw time 2 min, then 20 ºC/min to 230
ºC over 5 min. and then 20 ºC/min to 300 ºC; withdraw time for 5 min.
The purity of all the amide substrates was greater than 97% and these
were purchased from Beijing Innochem, Science & Technology. CAS
numbers for the amide substrates and products are included in Table S1
in the SI. All the liquid substrates and solid substrates were used directly.
Complexes A - D were prepared using previously reported routes.20-22
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Acknowledgements
We acknowledge support from the National Natural Science
Foundation of China (21476060 and U1362204) and the Nature
Science Foundation of Hebei Province (B2014205049). GAS
thanks the Chinese Academy of Sciences for
fellowship.
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Z．Wang, Y. Li, Q.-B. Liu, G. A. Solan,
Y. Ma, W.-H. Sun.
Page No. – Page No.
Direct Hydrogenation of a Broad
Range of Amides under Base-free
Conditions using an Efficient and
Selective PNN-Ru(II) catalyst
Amide hydrogenation: A catalyst loading of between 0.1 - 1 mol% of the PNN-
Ru(II) complex B efficiently and selectivity catalyzes the hydrogenation of a broad
range of amides under base-free conditions to afford the corresponding alcohols
and amines in high yields.
This article is protected by copyright. All rights reserved.