N-Heterocyclic carbene-based well-defined ruthenium hydride complexes for direct amide synthesis from alcohols and amines under base-free conditions

Article abstract of DOI:10.1016/j.tet.2015.02.016

Readily synthesized, well-defined N-heterocyclic carbene-based ruthenium(II) hydride complexes were developed for amide synthesis from alcohols and amines under base-free conditions. Diverse amides were synthesized in fair-to-excellent yields. In the case of secondary amines, where direct dehydrogenative amidation is not feasible, a catalytic amount of a base was required to promote the transamidation of esters, which are byproducts of alcohol dimerization.

Full text of DOI:10.1016/j.tet.2015.02.016

Tetrahedron xxx (2015) 1e5
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Tetrahedron
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N-Heterocyclic carbene-based well-defined ruthenium hydride
complexes for direct amide synthesis from alcohols and amines
under base-free conditions
,
,
,b,
Kunsoon Kim a b, Byungjoon Kang a b, Soon Hyeok Hong a
*
a Department of Chemistry, College of Natural Sciences, Seoul National University, Seoul 151-747, Republic of Korea
b Korea Carbon Capture Sequestration R&D Center (KCRC), Daejeon 305-303, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Readily synthesized, well-defined N-heterocyclic carbene-based ruthenium(II) hydride complexes were
developed for amide synthesis from alcohols and amines under base-free conditions. Diverse amides
were synthesized in fair-to-excellent yields. In the case of secondary amines, where direct de-
hydrogenative amidation is not feasible, a catalytic amount of a base was required to promote the
transamidation of esters, which are byproducts of alcohol dimerization.
Received 14 January 2015
Accepted 4 February 2015
Available online xxx
Keywords:
Amide
Ó 2015 Elsevier Ltd. All rights reserved.
Ruthenium
Homogeneous catalysis
Alcohol
Amine
1. Introduction
a Ruehydride complex, which is believed to be the key catalytic
species, from readily available Ruehalide complexes and also gen-
Amide bonds are key components in various important chem-
icals such as peptides, pesticides, polymers, and many other bi-
ologically important materials.1 Traditional methods for amide
synthesis such as reactions of carboxylic acid derivatives with
amines1d,2 require more than stoichiometric amounts of activating
reagents and generate wastes. To solve these problems, more en-
vironmentally benign synthetic methods for amides are being de-
veloped.3 In particular, catalytic acceptorless dehydrogenative
amide synthesis from amines and alcohols has received much at-
tention, because readily available alcohols are used, and hydrogen,
a clean source of energy, is generated as the sole byproduct.4 In-
spired by the pioneering studies by Murahashi5 and Milstein,6
many research groups have developed the dehydrogenative cou-
pling of amines and alcohols using Ru,7e9 Rh,10 Re,11 and Ag12 cat-
alysts. Our group has focused on N-heterocyclic carbene (NHC)-
erate free NHC ligands from the imidazolium precursors (Scheme
1). However, although some catalytic systems use a Ru-hydride
source such as RuH2(PPh3)4 as the precatalyst, a higher amount of
a strong base to generate free NHC from imidazolium precursors
was required.7b,8,13a Confirming the involvement of a Ru-hydride
species as the actual active catalytic intermediate, we have made
significant efforts to find an NHC-based Ruehydride complex that
can catalyze amide synthesis under base-free conditions.
2. Results and discussion
Recently, our group reported redox-neutral amide synthesis
from alcohols and nitriles using
a Ru-dihydride precatalyst
(Ru(PPh3)3(CO)H2), an NHC precursor, 1,3-diisopropylimidazolium
bromide, and sodium hydride.13a The studies on catalytic in-
termediates showed that RueIiPr (RueIiPr¼Ru(IiPr)(PPh3)2(CO)H2,
IiPr¼1,3-diisopropylimidazol-2-ylidene), which was first synthe-
sized and characterized by Whittlesey and Williams et al.,14 is
generated in situ. RueIiPr was further investigated for the for-
mylation of nitriles and amines with methanol under neutral
conditions.13b Inspired by the results, we synthesized analogous
RueNHC complexes with various substituents on NHC to use in the
direct acylation of amines with alcohols (Fig. 1) following the re-
ported procedures.14,15
based Ru(II) complexes for this transformation;7 Madsen,8 Glori-
9c
us,9a Bera,9b and Keul and Moller, have also studied NHC-based
€
Ru(II) complexes for the purpose.
Most of the NHC-based Ru catalytic systems operate under
basic conditions for effective amide synthesis, whereas the dear-
omatized PNN pincer Ru based Milstein catalyst operates under
neutral reaction conditions. It was proposed that bases generate
* Corresponding author. Tel.: þ82 2 880 6655; fax: þ82 2 889 1568; e-mail ad-
dress: soonhong@snu.ac.kr (S.H. Hong).
http://dx.doi.org/10.1016/j.tet.2015.02.016
0040-4020/Ó 2015 Elsevier Ltd. All rights reserved.
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K. Kim et al. / Tetrahedron xxx (2015) 1e5
Table 1
Catalyst screening for amide synthesisa
Entry
RueNHC
Ratio 2a/1a
Loading (mol %)
Yield (%)
1
2
3
4
5
6
7
8
9b
RueIMe
RueIiPr
RueICy
RueIPr
RueICy2
RueIiPr
RueIiPr
RueIiPr
RueIiPr
1
1
1
1
5
5
5
5
5
5
5
2.5
5
14
86
42
2
74
94
>97
63
42
1
1.1
1.2
1.2
1.2
a
Reaction conditions: 0.5 mmol amine, toluene 0.6 mL, 110 ꢀC, 24 h. The yields
were measured by GC using dodecane as the internal standard.
b
Benzene 0.6 mL, 90 ꢀC, 24 h.
Next, the reactions of 1a with different alcohols were in-
vestigated (Table 3). The reactions of aliphatic alcohols smoothly
afforded the corresponding amides in moderate-to-excellent yields
(entries 1e3). Electron-rich, p-methoxybenzyl alcohol (2e) gave
3ae in 74% yield (entry 4). Electron-deficient, p-fluorobenzyl alco-
hol produced 3af in a moderate yield of 69% (entry 5). Furfuryl
alcohol (2g) furnished 3ag in 65% yield (entry 6).
Scheme 1. Proposed mechanism for acceptorless dehydrogenative amide synthesis
starting from Ruechloride complexes.
In the case of secondary amines (Table 2, entries 9 and 10), the
presence of a strong base significantly increased the yields. Our
group previously reported that another pathway involving esters as
the intermediate may exist, particularly when the direct reactions
of alcohols and amines are hindered such as in the case of the re-
actions with secondary amines.7c From the observed enhanced
reactivity in the cases of 1i and 1j when a base was added, we as-
sumed that the base-catalyzed the transamidation of esters, as
reported previously.7c,16 Indeed, the transamidation reactions of 1a
with benzylbenzoate (4) could be distinguished depending on the
presence of KOtBu (Table 4). Therefore, we believe that the role of
base in our case is to help the transamidation of esters especially
when the direct dehydrogenation is not facile, as we reported
(Scheme 2).7c
Fig. 1. Well-defined RueNHC complexes.
The reaction of 3-phenyl-1-propylamine (1a) and 2-
phenylethanol (2a) was chosen to investigate the catalyst activity
and reaction conditions (Table 1). The catalytic activity of the
RueNHC complexes highly depends on the structure of the NHC.
RueIMe and RueIPr exhibited poor yields under neutral conditions
(entries 1 and 4). As previously reported in the in situ generated
NHCeRu catalytic systems,7,8 IiPreNHC-based RueIiPr showed the
best yield (entry 2). Notably, the RueICy2 complex bearing two NHC
ligands showed higher activity than the corresponding mono-
substituted RueICy (entries 3 and 5). However, attempts to syn-
thesize other bis-NHC Ru-hydride complexes such as RueIiPr2 were
unsuccessful. Using RueIiPr as the precatalyst and a slightly higher
equivalent of alcohol 2a, product 3aa was obtained in excellent
yields (entries 6 and 7). A lowered catalyst loading of 2.5 mol %
decreased the product yield (entry 8). Toluene was identified as the
best solvent as in other reported cases.
3. Conclusion
Based on the mechanistic insight that a Ru hydride is the actual
catalytic intermediate, a well-defined NHCeRuehydride complex
was developed for amide synthesis from alcohols and amines under
neutral conditions. The reactions of various alcohols and amines
smoothly afforded the corresponding amides. However, in the case
of secondary amines, a catalytic amount of a base was required to
promote the transamidation of esters, which are byproducts of al-
cohol dimerization.
With the optimized conditions, various amines were reacted
with 2-phenylethanol as the coupling partner (Table 2). The re-
action of aliphatic amines and simple benzylamines afforded the
corresponding amides in good-to-excellent yields under neutral
conditions (entries 1e5). Electron-rich benzylamines were good
substrates (entries 4 and 5), whereas electron-deficient benzyl-
amines did not perform well (entries 6 and 7). The reaction of 3-
picorylamine (1h) smoothly afforded the corresponding amide
3ha in 70% yield (entry 8). The reaction of secondary amines 1i and
1j resulted in poor yields (entries 9 and 10). Moreover, the addition
of 15 mol % base increased the product yields from 41% to 71% in the
case of 1i and from 31% to 75% in the case of 1j (entries 9 and 10).
Unfortunately, aromatic amines such as aniline were not effective
as reported previously.7a,8
4. Experimental section
4.1. General considerations
Unless otherwise noted, all the reactions were carried out using
standard Schlenk techniques or in an argon-filled glove box. Tolu-
ene, benzene, and tetrahydrofuran were purified by a solvent col-
umn system. Ethanol and dimethyl sulfoxide were purchased from
commercial suppliers and degassed with dry Ar before use. Deu-
terated solvents were purchased from Cambridge Isotope Labora-
tories, Inc. and SigmaeAldrich Corporation. NMR spectra were
recorded in CDCl3 or benzene-d6 using a Bruker DPX300, AMX400,
Agilent 400-MR, JEOL ECA400, or JEOL ECA400SL spectrometer, and
TMS (tetramethylsilane) was used as the reference. Chemical shifts
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K. Kim et al. / Tetrahedron xxx (2015) 1e5
3
Table 2
Table 3
Amide synthesis from amines and 2-phenylethanola
Amide synthesis from 3-phenyl-1-propylamine (1a) and alcoholsa
Entry
1
Amine
Product
3aa
Yieldb (%)
90
Entry
1
Alcohol
2b
Product
Yieldb (%)
82
1a
3ab
2
3
81
85
2
3
4
92
67
74
1b
1c
2c
3ba
3ac
3ad
3ca
2d
2e
4
>97
1d
3da
3ea
3fa
3ae
5
6
82
42
5
6
69
65
1e
1f
2f
3af
2g
3ag
a
Reaction conditions: 0.5 mmol 3-phenyl-1-propylamine, 0.6 mmol alcohol,
0.025 mmol RueIiPr, toluene 0.6 mL, 110 ꢀC, 24 h.
b
7
8
43
70
Isolated yield.
1g
1h
3ga
3ha
Table 4
Transamidiation of 4 with 1aa
Entry
KOtBu (mol %)
RueIiPr (mol %)
Yieldb (%)
9
41
69c
71d
1
2
3
4
0
15
0
0
0
5
5
Trace
86c
1i
1j
3d
3ia
3ja
15
>97d
a
Reaction conditions: 0.5 mmol 3-phenyl-1-propylamine, 0.3 mmol benzylben-
zoate, toluene 0.6 mL, 110 ꢀC, 24 h.
10
31
75c
b
Isolated yield.
c
Calculated based on benzylbenzoate.
Calculated based on 3-phenyl-1-propylamine.
d
a
Reaction conditions: 0.5 mmol amine, 0.6 mmol 2-phenylethanol, 0.025 mmol
RueIiPr, toluene 0.6 mL, 110 ꢀC, 24 h.
b
Isolated yield.
Additional 15 mol % of potassium tert-butoxide.
Additional 15 mol % of sodium hydride.
4.2. General procedure for amide synthesis
c
d
Inside an Ar-filled glove box, RueIiPr (20.2 mg, 0.025 mmol), and
toluene (0.6 mL) were added to an oven-dried 25 mL Schlenk tube.
If solid amine (0.50 mmol) or alcohol (0.60 mmol) substrates were
used, they were also added to the reaction mixture inside the glove
box. Liquid amine or alcohol substrates were added into the vial
using a microsyringe under Ar flow after the Schlenk tube equipped
with septum was removed from the glove box. Then, the reaction
mixture was stirred at 115 ꢀC for 24 h in closed condition before
cooling to room temperature. All the volatiles were removed under
vacuum. The crude product was purified by silica gel column
chromatography using a mixture of hexane and ethyl acetate as the
eluant to afford the pure amide.
are reported in parts per million (ppm) and coupling constants are
reported in hertz (Hz). High-resolution mass spectrometry (HRMS)
was performed by the Organic Chemistry Research Center from
Sugang University. GC analysis was carried out using a 7890A GC
system (Agilent Technologies) equipped with an HP-5 column and
FID detector. Ru(PPh3)3(CO)H2, Ru(PPh3)3(CO)HF,18 imidazolium
17
salt,19 and the corresponding carbene9a,14,20 were synthesized by
literature procedures or purchased from SigmaeAldrich. Other
chemicals were purchased from commercial suppliers and used as
received without further purification.
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4
K. Kim et al. / Tetrahedron xxx (2015) 1e5
2H), 3.77 (s, 2H), 2.89 (s, 3H) ppm; minor rotamer:
d 7.35e7.20 (m,
9H), 7.08 (s, 1H), 4.51 (s, 2H), 3.74 (s, 2H), 2.94 (s, 3H) ppm.
4.2.10. 1-(Phenylacetyl)piperidine (3ja).7d Yellow oil (76.0 mg, 75%).
1H NMR (400 MHz, CDCl3)
d 7.25e7.21 (m, 2H), 7.18e7.14 (m, 3H),
3.65 (s, 2H), 3.49 (t, J¼5.4 Hz, 2H), 3.28 (t, J¼5.4 Hz, 2H), 1.52e1.40
(m, 4H), 1.29e1.23 (m, 2H) ppm.
4.2.11. N-(3-Phenylpropyl)benzamide (3ab).24 White solid (98.0 mg,
82%). 1H NMR (500 MHz, CDCl3)
d 7.68e7.66 (m, 2H), 7.49e7.48 (m,
Scheme 2. Proposed mechanism for dehydrogenative amidation via a direct pathway
or via a base-catalyzed transamidation pathway through esters.
1H), 7.42e7.38 (m, 2H), 7.31e7.30 (m, 2H), 7.24e7.20 (m, 3H), 6.05
(br s, 1H), 3.53 (q, J¼6.60 Hz, 2H), 2.75 (t, J¼7.59 Hz, 2H), 2.02e1.96
(m, 2H) ppm.
4.2.1. 2-Phenyl-N-(3-phenylpropyl)acetamide (3aa).21 White solid
(113.4 mg, 90%). 1H NMR (500 MHz, CDCl3)
d 7.39e7.36 (m, 2H),
4.2.12. N-(3-Phenylpropyl)pentanamide
(100.4 mg, 92%). 1H NMR (500 MHz, CDCl3)
(3ac).13a Brown
oil
7.33e7.30 (m, 1H), 7.28e7.24 (m, 4H), 7.20e7.17 (m, 1H), 7.10 (d,
J¼7.25 Hz, 2H), 5.32 (br s, 1H), 3.56 (s, 2H), 3.25 (q, J¼6.71 Hz, 2H),
2.56 (t, J¼7.68 Hz, 2H), 1.80e1.74 (m, 2H) ppm.
d
7.31e7.28 (m, 2H),
7.22e7.18 (m, 3H), 5.44 (br s, 1H), 3.31 (q, J¼6.65 Hz, 2H), 2.66 (t,
J¼7.65 Hz, 2H), 2.14 (t, J¼7.64 Hz, 2H), 1.88e1.82 (m, 2H), 1.62e1.56
(m, 2H), 1.38e1.31 (m, 2H), 0.96e0.88 (m, 3H) ppm.
4.2.2. N-Hexyl-2-phenylacetamide (3ba).7a White solid (89.3 mg,
81%). 1H NMR (400 MHz, CDCl3)
d 7.33e7.21 (m, 5H), 5.61 (br s, 1H),
4. 2.13 . N-( 3-P henyl pro pyl )cycl opropa nec ar boxa mid e
(3ad).25 White solid (68.4 mg, 67%). 1H NMR (400 MHz, CDCl3)
3.52 (s, 2H), 3.15 (q, J¼6.70 Hz, 2H), 1.39e1.35 (m, 2H), 1.23e1.19 (m,
6H), 0.82 (t, J¼6.90 Hz, 3H) ppm.
d
7.28e7.24 (m, 2H), 7.18e7.15 (m, 3H), 5.85 (br s, 1H), 3.28 (q,
J¼6.63 Hz, 2H), 2.63 (t, J¼7.68 Hz, 2H), 1.86e1.78 (m, 2H), 1.31e1.25
4.2.3. N-Benzyl-2-phenylacetamide (3ca).21 White solid (95.9 mg,
(m, 1H), 0.94e0.91 (m, 2H), 0.70e0.66 (m, 2H) ppm.
85%). 1H NMR (500 MHz, CDCl3)
d 7.38e7.35 (m, 2H), 7.32e7.24 (m,
(3ae).26 White
7.64 (d,
6H), 7.19 (d, J¼7.22 Hz, 2H), 5.68 (br s, 1H), 4.43 (d, J¼5.80 Hz, 2H),
4.2.14. 4-Methoxy-N-(3-phenylpropyl)benzamide
solid (100.2 mg, 74%). 1H NMR (500 MHz, CDCl3)
3.65 (s, 2H) ppm.
d
J¼8.82 Hz, 2H), 7.32e7.29 (m, 2H), 7.24e7.20 (m, 3H), 6.91 (d,
J¼8.82 Hz, 2H), 5.97 (br s, 1H), 3.85 (s, 3H), 3.51 (q, J¼6.60 Hz, 2H),
2.74 (t, J¼7.59 Hz, 2H), 2.01e1.95 (m, 2H) ppm.
4.2.4. N-(4-Methoxybenzyl)-2-phenylacetamide
(3da).22 White
d 7.38e7.35 (m,
solid (128.8 mg, >97%). 1H NMR (500 MHz, CDCl3)
2H), 7.31e7.26 (m, 3H), 7.12 (d, J¼8.52 Hz, 2H), 6.85e6.82 (m, 2H),
5.61 (br s, 1H), 4.36 (d, J¼5.80 Hz, 2H), 3.79 (s, 3H), 3.63 (s,
2H) ppm.
4.2.15. 4-Fluoro-N-(3-phenylpropyl)benzamide (3af).13a Colorless
oil (88.9 mg, 69%). 1H NMR (400 MHz, CDCl3)
d 7.65e7.62 (m, 2H),
7.31e7.25 (m, 2H), 7.20e7.16 (m, 3H), 7.04e6.98 (m, 2H), 6.32 (br s,
1H), 3.44 (q, J¼6.67, 2H), 2.68 (t, J¼7.44 Hz, 2H), 1.96e1.89 (m,
2H) ppm.
4.2.5. N-(2-Methylbenzyl)-2-phenylacetamide (3ea). Yellow solid
(98.1 mg, 82%). 1H NMR (400 MHz, CDCl3)
d 7.35e7.31 (m, 2H),
7.29e7.25 (m, 3H), 7.19e7.08 (m, 4H), 5.82 (br s, 1H), 4.38 (d,
J¼5.52 Hz, 2H), 3.58 (s, 2H), 2.22 (s, 3H) ppm. 13C NMR (100 MHz,
4.2.16. N-(3-Phenylpropyl)-2-furancarboxamide (3ag).13a Yellow oil
(74.4 mg, 65%). 1H NMR (500 MHz, CDCl3)
d
7.39 (t, J¼0.86 Hz, 1H),
CDCl3)
d 170.76, 136.25, 135.67, 130.44, 129.36, 128.98, 128.00,
127.61, 127.33, 126.10, 43.69, 41.79 ppm; HRMS (ESI) calcd for
7.28e7.25 (m, 2H), 7.18 (d, J¼7.20 Hz, 3H), 7.07 (d, J¼3.58 Hz, 1H),
6.47e6.45 (m, 1H), 6.38 (br s, 1H), 3.44 (q, J¼6.68 Hz, 2H), 2.69 (t,
J¼7.64 Hz, 2H), 1.96e1.88 (m, 2H).
C
16H17NONa: 262.1202, found: 262.1202 [MNaþ].
4.2.6. N-(4-Chlorobenzyl)-2-phenylacetamide (3fa).13a White solid
(54.0 mg, 42%). 1H NMR (500 MHz, CDCl3)
7.38e7.35 (m, 2H),
d
Acknowledgements
7.32e7.26 (m, 5H), 7.12 (d, J¼8.27 Hz, 2H), 5.67 (br s, 1H), 4.39 (d,
J¼5.95 Hz, 2H), 3.64 (s, 2H) ppm.
This research was supported by the National Research Founda-
tion
of
Korea
(NRF-2014R1A2A1A11050028;
Korea CCS R&D Center;
NRF-
NRF-
4.2.7. 2-Phenyl-N-(4-(trifluoromethyl)benzyl)acetamide
(3ga). -
7.56 (d,
White solid (62.6 mg, 43%). 1H NMR (500 MHz, CDCl3)
d
2014M1A8A1049347,
2014R1A5A1011165, Center for New Directions in Organic Synthe-
sis), funded by the Korean Government.
J¼8.18 Hz, 2H), 7.40e7.37 (m, 2H), 7.33e7.32 (m, 1H), 7.30e7.27 (m,
4H), 5.76 (br s, 1H), 4.48 (d, J¼5.99 Hz, 2H), 3.67 (s, 2H) ppm. 13
C
NMR (75 MHz, CDCl3)
d 171.22, 142.41, 134.71, 129.38, 129.08,
127.56, 127.48, 129.89, 125.53 (q, J¼11.36 Hz), 122.28, 43.66, 42.96.
19F NMR (376 MHz, CDCl3)
ꢁ62.59 ppm; HRMS(ESI) calcd for
16H14F3NONa: 316.0920, found: 316.0920 [MNaþ].
Supplementary data
d
C
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.tet.2015.02.016.
4.2.8. 2-Phenyl-N-(pyridin-3-ylmethyl)acetamide (3ha).23 Yellow
solid (79.1 mg, 70%). 1H NMR (500 MHz, CDCl3)
8.52e8.50 (m, 2H),
d
References and notes
7.65 (d, J¼7.53 Hz, 1H), 7.39e7.36 (m, 2H), 7.32e7.27 (m, 4H), 5.88
(br s, 1H), 4.46 (d, J¼6.13 Hz, 2H), 3.66 (s, 2H) ppm.
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Please cite this article in press as: Kim, K.; et al., Tetrahedron (2015), http://dx.doi.org/10.1016/j.tet.2015.02.016