Dehydrogenative amide synthesis from alcohols and amines utilizing N-heterocyclic carbene-based ruthenium complexes as efficient catalysts: The influence of catalyst loadings, ancillary and added ligands

Article abstract of DOI:10.1016/j.poly.2020.114979

The metal-catalyzed dehydrogenative coupling of alcohols and amines to access amides has been recognized as an atom-economic and environmental-friendly process. Apart from the formation of the amide products, three other kinds of compounds (esters, imines and amines) may also be produced. Therefore, it is of vital importance to investigate product distribution in this transformation. Herein, N-heterocyclic carbene-based Ru (NHC/Ru) complexes [Ru-1]-[Ru-5] with different ancillary ligands were prepared and characterized. Based on these complexes, we selected condition A (without an added NHC precursor) and condition B (with an added NHC precursor) to comprehensively explore the selectivity and yield of the desired amides. After careful evaluation of various parameters, the Ru loadings, added NHC precursors and the electronic/steric properties of ancillary NHC ligands were found to have considerable influence on this catalytic process.

Full text of DOI:10.1016/j.poly.2020.114979

Polyhedron 195 (2021) 114979
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Dehydrogenative amide synthesis from alcohols and amines utilizing
N-heterocyclic carbene-based ruthenium complexes as efficient
catalysts: The influence of catalyst loadings, ancillary and added ligands
b,
c,
Wan-Qiang Wang a, Zhi-Qin Wang b, Wei Sang b, Rui Zhang a, Hua Cheng a, , Cheng Chen , Da-Yong Peng
⇑
⇑
⇑
a Department of Chemical Engineering and Food Science, Hubei University of Arts and Science, 296 Longzhong Road, Xiangyang 441053, PR China
b State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, PR China
c College of Science, Jiangxi Agricultural University, Nanchang 330045, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The metal-catalyzed dehydrogenative coupling of alcohols and amines to access amides has been recog-
nized as an atom-economic and environmental-friendly process. Apart from the formation of the amide
products, three other kinds of compounds (esters, imines and amines) may also be produced. Therefore, it
is of vital importance to investigate product distribution in this transformation. Herein, N-heterocyclic
carbene-based Ru (NHC/Ru) complexes [Ru-1]-[Ru-5] with different ancillary ligands were prepared
and characterized. Based on these complexes, we selected condition A (without an added NHC precursor)
and condition B (with an added NHC precursor) to comprehensively explore the selectivity and yield of
the desired amides. After careful evaluation of various parameters, the Ru loadings, added NHC precur-
sors and the electronic/steric properties of ancillary NHC ligands were found to have considerable influ-
ence on this catalytic process.
Received 14 October 2020
Accepted 11 December 2020
Available online 19 December 2020
Keywords:
Amide
Ruthenium
N-Heterocyclic carbene
Ancillary ligand
Added ligand
Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction
second pathway, III eliminates one molecule of H2 to directly yield
amide 3 [2]. Therefore, the design and preparation of active cata-
The metal-catalyzed dehydrogenative coupling of alcohols and
amines without a hydrogen acceptor has been recognized as a
green and alternative approach for the fabrication of the amide
linkage. This attractive approach has the characteristics of environ-
mental friendliness and high atom economy since dihydrogen gas
is evolved as the exclusive by-product [1–8]. Generally, treatment
of alcohols with amines via a metal catalyst or catalytic system
could result in several potential compounds (as shown in Fig. 1).
Presumably, with the assistance of a catalyst, two alcohol mole-
cules undergo dehydrogenative coupling to deliver an ester inter-
mediate (I), which then reacts with amine 2 to afford amide 3
[2]. Alternatively, alcohol 1 is dehydrogenated to an aldehyde or
metal-bound aldehyde II, followed by the reaction of II and amine
2 to give hemiaminal intermediate III [2]. Subsequently, two path-
ways are postulated from III. The first one is the transformation of
III into imine IV by the release of one water molecule, and IV then
reacts with the generated H2 to produce amine V. This pathway is
the so-called hydrogen borrowing (HB) strategy [9–14]. For the
lysts are crucial for the selective formation of amide 3. Among
the versatile catalysts, Ru-based ones have been extensively
accessed and investigated [8]. At the beginning, the Murahashi
group reported
a
RuH2(PPh3)4-catalyzed protocol for the
intramolecular conversion of 1,3- or 1,4-amino alcohols into the
corresponding lactams [15]. The intermolecular alcohol amidation
with amines was pioneered by Milstein et al., who developed a
highly active PNN-type Ru complex, namely the Milstein catalyst
[16]. Inspired by these examples, many other groups have since
made their own contributions in this catalytic process [17–54].
It has been well documented that N-heterocyclic carbenes
(NHCs) are known as promising ancillary ligands for plenty of
metal complexes [55–62], thus we have also been working on
the preparation and exploration of efficient NHC/Ru catalysts or
catalytic systems for this transformation [27,28,49–53]. In our pre-
vious work, a well-defined NHC/Ru complex ([Ru-1]) was prepared
and its catalytic activity was examined [53]. It was found that [Ru-
1] exhibited high selectivity and excellent yield of the amide prod-
ucts with a Ru loading of 1 mol% (Fig. 2a). Further invesigations
revealed that addition of 3-ethyl-1-methyl-1H-benzo[d]imidazol-
3-ium iodide (L1) led to a catalytic system with considerably
improved activity, with only 0.25 mol% of Ru to achieve
⇑
Corresponding authors.
E-mail addresses: cch510@126.com (H. Cheng), chengchen@whut.edu.cn
(C. Chen), dayongpeng@163.com (D.-Y. Peng).
https://doi.org/10.1016/j.poly.2020.114979
0277-5387/Ó 2020 Elsevier Ltd. All rights reserved.

Wan-Qiang Wang, Zhi-Qin Wang, W. Sang et al.
Polyhedron 195 (2021) 114979
Fig. 1. The possible compounds from the metal-catalyzed reactions of alcohols and amines.
Fig. 2. The design strategy of this work.
satisfactory results (Fig. 2b). With a goal to have a comprehensive
understanding of the above-mentioned catalytic systems and
potential selectivity concerns for the dehydrogenative alcohol ami-
dation with amines, we intended to prepare a few more NHC/Ru
complexes bearing different NHC ligands to evaluate the steric
and electronic effects on the selectivity and yield of the desired
amide products (as shown in Fig. 2c). On the other hand, the influ-
ence of other factors such as the catalyst loadings and added
ligands was also evaluated (Fig. 2c). Hopefully, this study could
provide useful information for the researchers who are working
on related dehydrogenative transformations.
pled), respectively. For the NMR analyses, CDCl3 was used as the
deuterated solvent. The following abbreviations were used to des-
ignate multiplicities: s = singlet, brs = broad singlet, d = doublet,
t = triplet, q = quartet, p = pentet, hept = heptet, m = multiplet,
dd = doublet of doublets, dt = doublet of triplets, dq = doublet of
quartets, td = triplet of doublets. All the solvents and common
reagents, including the alcohol and amine substrates, were pur-
chased from commercial suppliers and used as received. The syn-
thesis of NHC precursors L1 [53], L2 [63], L3 [63], L4 [63] and L5
[63] as well as NHC/Ru complexes [Ru-1] [53], [Ru-2] [63], [Ru-
3] [63], [Ru-4] [63] and [Ru-5] [63] followed the previously
reported procedures. All the amide products (3a–3t) are known
compounds, and their NMR data and spectra are included in the
Supporting information.
2. Materials and methods
2.1. General considerations
2.2. General procedure for the NHC/Ru-catalyzed amide synthesis
under condition A
All the catalytic reactions and the synthesis of [Ru-1]-[Ru-5]
were carried out using standard Schlenk techniques. The weighing
or measurement of air- or moisture-sensitive chemicals, and anhy-
drous solvents was performed inside an argon-filled glovebox. The
NMR spectra were recorded on a Bruker Avance 500 spectrometer,
with 500 MHz for 1H NMR and 126 MHz for 13C NMR (1H-decou-
For the procedure applying a Ru loading of 1 mol%, an NHC-Ru
complex (one of [Ru-1]-[Ru-5], 0.005 mmol), Cs2CO3 (3.3 mg,
0.01 mmol) and dry toluene (0.75 mL) were added to a 25 mL Sch-
lenk flask inside an argon-filled glovebox. The Schlenk flask was
2

Wan-Qiang Wang, Zhi-Qin Wang, W. Sang et al.
Polyhedron 195 (2021) 114979
equipped with a reflux condenser, which was sealed with a rubber
septum. The flask was taken out and subjected to three evacuating-
refilling (with argon) cycles in a Schlenk line before being heated to
reflux under argon for 0.5 h to facilitate the formation of the corre-
sponding NHC-Ru complex bearing a carbonate moiety [53]. Sub-
sequently, alcohol 1 (0.50 mmol), amine 2 (0.55 mmol) were
added into the flask, and the whole system was purged with Ar
for 5 min. Finally, the flask was heated to reflux for another 24 h.
For the other Ru loadings, similar procedures were adopted. For
a Ru loading of 0.5 mol%, an NHC-Ru complex (0.005 mmol), Cs2-
synthesized via structural modification from [Ru-1] (as shown in
the chemical structures of Table 1). To modulate the steric property
of the ancillary NHC ligand, replacing the Et group in one N-termi-
nus of [Ru-1] with Me or iPr led to [Ru-2] or [Ru-3]. For the tuning
of the electronic nature of the NHC ligand, incorporating two Me or
Cl groups on the backbone of the benzimidazole framework
resulted in [Ru-4] or [Ru-5]. Huynh’s electronic parameter (HEP),
the 13C NMR chemical shift of a carbene signal, has been widely
used to indicate the electronic property of an NHC ligand
[55,59,60,62]. In general, an NHC ligand possessing strong elec-
tron-donating nature exhibits a upfield signal for the carbene car-
CO3 (3.3 mg, 0.01 mmol), dry toluene (0.75 mL), 1a (103
1.00 mmol) and 2a (120 L, 1.10 mmol) were used. For a Ru load-
ing of 2.0 mol%, an NHC-Ru complex (0.01 mmol), Cs2CO3 (6.6 mg,
0.02 mmol), dry toluene (0.4 mL), 1a (52 L, 0.50 mmol) and 2a
(60 L, 0.55 mmol) were used. For a Ru loading of 5.0 mol%, an
NHC-Ru complex (0.01 mmol), Cs2CO3 (6.6 mg, 0.02 mmol), dry
lL,
l
bon atom. Accordingly, the 13Ccarbene-NMR signals of the five NHC/
13
Ru complexes were listed in Table 1. The
C
-NMR chemical
carbene
l
shifts of [Ru-1]-[Ru-3] were measured as 188.6, 189.1 and
188.3 ppm, respectively (entries 1–3). These results revealed that
the three complexes displayed marginally distinct electronic prop-
l
toluene (0.4 mL), 1a (21
were used.
lL, 0.20 mmol) and 2a (25
l
L, 0.22 mmol)
erties, while their major differences depended on the steric hin-
13
drance. With regard to the
C
-NMR signals of [Ru-1], [Ru-
carbene
4] and [Ru-5], their respective chemical shifts were determined
as 188.6, 186.1 and 193.5 ppm (entries 1, 4, 5). Therefore, the elec-
2.3. General procedure for the NHC/Ru-catalyzed amide synthesis
under condition B
tron-donating capability follows the order of [Ru-4]
1] > [Ru-5], which is consistent with our expectation.
> [Ru-
With the above five complexes in hand, we started to investi-
gate their catalytic performance under various conditions. The
reaction of benzyl alcohol (1a) and benzyl amine (2a) was selected
as a model reaction. It appeared that only amide 3a and imine 4a
were observed under all circumstances, without the detection of
the possible ester or amine by-product. Therefore, product
For the procedure with a Ru loading of 0.25 mol%, an NHC-Ru
complex (one of [Ru-1]-[Ru-5], 0.005 mmol), an NHC precursor
(one of L1-L5, 0.015 mmol), Cs2CO3 (11.5 mg, 0.035 mmol) and
dry toluene (1.00 mL) inside an argon-filled glovebox. The Schlenk
flask was equipped with a reflux condenser, which was sealed with
a rubber septum. The flask was taken out and subjected to three
evacuating-refilling (with argon) cycles in a Schlenk line before
being heated to reflux under argon for 0.5 h to facilitate the forma-
tion of the corresponding NHC-Ru complex bearing a carbonate
moiety [53]. Subsequently, alcohol 1 (2.00 mmol) and amine 2
(2.20 mmol) were added to the flask, and the whole system was
purged with Ar for 5 min. Finally, the flask was heated to reflux
for another 24 h.
Table 1
13
The
C
-NMR chemical shifts of the five NHC-Ru complexes bearing electron-
carbene
ically and/or sterically different properties.
For the other Ru loadings, similar procedures were adopted. For a
Ru loading of 0.125 mol%, an NHC-Ru complex (0.0025 mmol), an
NHC precursor (0.0075 mmol), Cs2CO3 (5.8 mg, 0.0175 mmol), dry
toluene(1.00mL),1a(204lL,2.00mmol)and2a(240lL,2.20mmol)
were used. For a Ru loading of 0.5 mol%, an NHC-Ru complex
(0.005 mmol), an NHC precursor (0.015 mmol), Cs2CO3 (11.5 mg,
0.035 mmol), dry toluene (0.75 mL), 1a (103
2a (120 L, 1.10 mmol) were used. For a Ru loading of 1.0 mol%, an
NHC-Ru complex (0.005 mmol), an NHC precursor (0.015 mmol),
Cs2CO3 (11.5 mg, 0.035 mmol), dry toluene (0.50 mL), 1a (52 L,
0.50 mmol) and 2a (60 L, 0.55 mmol) were used.
lL, 1.00 mmol) and
l
l
l
2.4. The general procedure for the calculation of the NMR and isolated
yield
With regard to the NMR yield, 1,3,5-trimethoxybenzene
(0.5 mmol, 84.0 mg) and CDCl3 (1.0 mL) were added after the reac-
tion was complete, 0.4–0.5 mL of the above solution were injected
into an NMR tube, which was directly subjected to 1H NMR analy-
sis. The NMR yield of each compound was determined based on the
exact amount of 1,3,5-trimethoxybenzene. For the isolated yield of
the amide products, the reaction mixture was directly purified by
silica gel flash column chromatography to afford the pure products.
The isolated yield of the amides was calculated from the ratio of
the measured weight and the theoretical weight.
13
Entry
NHC/Ru complex
C
-NMR chemical shift
carbene
1
2
3
4
5
[Ru-1]
[Ru-2]
[Ru-3]
[Ru-4]
[Ru-5]
188.6 ppm
189.1 ppm
188.3 ppm
186.1 ppm
193.5 ppm
3. Results and discussion
With the above considerations in mind, [Ru-1] was firstly pre-
pared and four other NHC/Ru complexes ([Ru-2]-[Ru-5]) were also
3

Wan-Qiang Wang, Zhi-Qin Wang, W. Sang et al.
Polyhedron 195 (2021) 114979
distribution among 3a, 4a and unreacted 1a was thoroughly
explored. Initially, the influence of the Ru loadings (ranging from
0 to 5 mol%) on the catalytic performance was evaluated under
condition A (an NHC/Ru complex, Cs2CO3 and toluene was refluxed
for 0.5 h without an added NHC precursor, as shown in Fig. 3). In
the absence of a catalyst, 6% of imine 4a and 92% of unreacted 1a
were observed, without any formation of amide 3a. If [Ru-1] was
used as a catalyst, a Ru loading of 0.5 mol% gave rise to 57% of
3a, 10% of 4a and 32% of unreacted 1a. On the contrary, increasing
the catalyst dosage to 1.0 mol% or even higher values provided
excellent selectivity and yield of 3a (Fig. 3a). Under condition B
(an NHC/Ru complex, an added NHC precursor, Cs2CO3 and toluene
was refluxed for 0.5 h), higher Ru loadings also triggered higher
amide/imine selectivity and improved yield of 3a. It was also worth
noting that the added NHC precursor L1 played a central role in all
the reactions, providing similar results with much lower Ru load-
ings (1–5 mol% for Fig. 3a vs. 0.25–1.00 mol% for Fig. 3b). Similar
tendency was observed for the other four NHC/Ru complexes
under condition A (Fig. S1) and condition B (Fig. S2), respectively.
In order to explore the electronic and steric effects of the ancil-
lary NHC ligands, clear comparison regarding the catalytic perfor-
mance of [Ru-1]-[Ru-5] was made (as shown in Figs. 4 and 5). In
the absence of an NHC precursor, the five complexes demonstrated
varied catalytic performance (as shown in Fig. 4). With 0.5 mol% of
Ru, the activity trend follows the order of [Ru-1] > [Ru-2] ꢀ [Ru-
4] > [Ru-5] > [Ru-3] (Fig. 4a). An enhanced dosage of 1.0 mol% pro-
vided [Ru-1] ꢀ [Ru-4] > [Ru-2] > [Ru-5] > [Ru-3] (Fig. 4b), while
even more Ru loadings (2.0–5.0 mol%) gave [Ru-1] ꢀ [Ru-
2] ꢀ [Ru-4] ꢀ [Ru-5] > [Ru-3] (Fig. 4cd). As a consequence, [Ru-
1] is superior over the other four NHC/Ru complexes in the
perspectives of the amide/imine selectivity and amide yield. Com-
pared with [Ru-1], [Ru-2] (having a similar donor ligand and
slightly smaller steric hindrance) and [Ru-4] (possessing a stronger
donor ligand and similar steric hindrance) exhibited comparable or
slightly lower activity. It appeared that the weaker donor ligand of
[Ru-5] in comparison with [Ru-1] resulted in lower stability
of this complex, which displayed moderate performance at low
loadings (0.5–1.0 mol%) and satisfactory results at high loadings
(2.0–5.0 mol%). Obviously, the more hindered iPr in the wingtip
group of [Ru-3] has a detrimental effect on the selective amide
formation, providing unsatisfactory results for all the tested load-
ings. Next, the potency trend of [Ru-1]-[Ru-5] combined with
the corresponding NHC precursors L1-L5 was also investigated
(Fig. 5). The results indicated that the activity order of [Ru-1]-
[Ru-5] under condition B was almost identical with those under
condition A (Figs. 5 vs. 4).
To provide a more comprehensive understanding about the cat-
alytic performance of [Ru-1]-[Ru-5] under both conditions, a vari-
ety of alcohol and amine substrates were attempted (as listed in
Table 2). In most cases, long-chain aliphatic alcohol 1b exhibited
lower reactivity than benzyl alcohol 1a, with the formation of
amide 3b in moderate yield (entry 2 vs. entry 1). For the activity
of the five complexes under both conditions, [Ru-1], [Ru-2], [Ru-
4] and [Ru-5] exhibited comparable activity, while [Ru-3] mani-
fested slightly lower activity than the above four complexes (entry
2). In terms of the product yield and catalytic activity trend, the
coupling of 1a with long-chain aliphatic amine 2c is analogous to
that of 1a with 2a (entry 3 vs. entry 1). In addition, the moderately
congested amides 3d–3e and furan-containing amides 3f–3g were
also obtained in low to moderate yield (entries 4–7). For the
Fig. 3. The product distribution applying different loadings of [Ru-1] under (a) condition A (without an added ligand L1) and (b) condition B (with an added ligand L1).
4

Wan-Qiang Wang, Zhi-Qin Wang, W. Sang et al.
Polyhedron 195 (2021) 114979
Fig. 4. Comparison of the catalytic performance of [Ru-1]-[Ru-5] applying the Ru loading of (a) 0.5 mol%, (b) 1.0 mol%, (c) 2.0 mol%, (d) 5.0 mol% without an added ligand.
dehydrogenative coupling of 1d or 1f with benzyl amine (1a), the
activity trend follows the order of [Ru-1] ꢀ [Ru-2] ꢀ [Ru-
4] > [Ru-5] > [Ru-2] under both conditions (entries 4 and 6). With
regard to the treatment of benzyl alcohol (1a) with 2e or 2g, the
tendency of [Ru-1] ꢀ [Ru-2] ꢀ [Ru-4] ꢀ [Ru-5] > [Ru-2] was iden-
tified (entries 5 and 7). Actually, the crude NMR spectra for the
reactions of 1d and 2a (as detailed in Table S1 of the Supporting
information), 1a and 2e (as detailed in Table S2) were investigated.
The results indicated that some amounts of imines were observed
for these two reactions.
In order to further verify the steric effects of the substrates on
the reactivity and selectivity, a few more congested alcohols or
amines were tested under both conditions. Firstly, two more ster-
ically hindered alcohols (neopentyl alcohol and 2,2-diphenylethan-
1-ol) were attempted under both conditions utilizing [Ru-1] (as
shown in Scheme S1). Unfortunately, trace amounts of the corre-
sponding amide products and the majority of both starting materi-
als were detected from TLC and crude NMR spectra, which
indicated that our catalytic systems were quite sensitive to steric
hindrance on the alcohol substrates. Due to the limited reactivity
of the sterically bulky alcohols, it was not meaningful to further
investigate the influence of different Ru complexes on the reac-
tions. Furthermore, two more sterically bulky amines (2h and 2i)
were explored under both conditions using all the five NHC-Ru
complexes (as listed in entries 8–9 of Table 2 and detailed in Tables
S3 and S4). Apart from the formation of the amides and imines,
ester 5a (generated via the dehydrogenative homo-coupling of
alcohols) was also observed for these reactions. Presumably, the
sterically hindered amines, which possess relatively low reactivity,
have to compete with the alcohols so that a mixture of amides, imi-
nes and esters is delivered. Under either condition A or condition B,
no considerable difference was detected for the catalytic activity of
the five complexes in the intramolecular lactam synthesis from
amino alcohol 1j (entry 10).
Furthermore, substituted benzyl alcohols and benzyl amines
were selected as the reactants (as listed in Table 3). Firstly, the
reactions of substituted benzyl alcohols and 2a were examined
(entries 2–6). For all the five complexes (with or without an added
ligand), 4-methylbenzyl alcohol (1k) displayed comparable reac-
tivity with 1a (entry 2 vs. entry 1), whereas 4-fluorobenzyl alcohol
(1l) exhibited similar or lower reactivity than 1a (entry 3 vs. entry
1). An even more electron-deficient alcohol 1m furnished the cor-
responding amide (3m) in merely 10–55% yield (entry 4). It
appeared that the substituent positions significantly affected the
generation of the amide products (entries 3, 5 and 6). In all the sit-
uations, 3-fluorobenzyl alcohol (1n) exhibited similar reactivity
with 4-fluorobenzyl alcohol (1l), whereas 2-flurobenzyl alcohol
(1o) is less reactive than 1l and 1n. Subsequently, the coupling of
1a with a substituted benzyl amine was examined (entries 7–11).
Except the highly electron-deficient substrate (2r) which provided
only 12–61% yield (entry 9), the other benzyl amines displayed
comparable or marginally lower reactivity in comparison with 2a
(entries 7–8, 10–11). From the above reactions, it was concluded
that the activity trend followed the order of [Ru-1]
>
5

Wan-Qiang Wang, Zhi-Qin Wang, W. Sang et al.
Polyhedron 195 (2021) 114979
Fig. 5. Comparison of the catalytic performance of [Ru-1]-[Ru-5] applying the Ru loadings of (a) 0.125 mol%, (b) 0.25 mol%, (c) 0.5 mol%, (d) 1.0 mol% with an added ligand.
[Ru-2] ꢀ [Ru-4] > [Ru-5] > [Ru-3]. In most cases, the electron-defi-
cient NHC ligand in [Ru-5] and the sterically hindered NHC ligand
in [Ru-3] are not beneficial for the selective amide formation.
evaluation of various alcohol and amine reactants, the overall
activity trend was discovered as [Ru-1] > [Ru-2] ꢀ [Ru-4] > [Ru-
5] > [Ru-3] under both conditions. In general, [Ru-1] exhibited
the best catalytic performance. Moreover, no obvious difference
among the activity of [Ru-2] and [Ru-4] was observed, while the
weaker donor ligand in [Ru-5] and the more congested NHC ligand
in [Ru-3] had negative effects on the selective amide formation.
4. Conclusion
In summary, five NHC/Ru complexes bearing distinct electronic
and/or steric nature, denoted as [Ru-1]-[Ru-5], were reported. Two
sets of conditions, condition
A (one of the above NHC/Ru
CRediT authorship contribution statement
complexes alone) and condition B (a combination of one NHC/Ru
complex and the identical NHC precursor as an added ligand), were
used to investigate product distribution in the metal-catalyzed
dehydrogenative coupling of alcohols and amines. Among the four
possible compounds generated from the two substrates, only
amides and imines were detected in most experiments. Accord-
ingly, a systematic investigation on the selectivity and yield of
the amides was conducted under both conditions. It was found that
the Ru loadings, the electronic and steric properties of an ancillary
NHC ligand as well as an added NHC precursor had significant
impacts on this transformation. For each NHC/Ru complex, use of
a higher Ru loading generally led to better selectivity and yield of
the amides, while incorporation of an added NHC precursor consid-
erably influenced the performance of the resulting catalytic sys-
tem. In addition, the influence of the ancillary NHC ligands in
[Ru-1]-[Ru-5] on the catalytic potency was studied. After careful
Wan-Qiang Wang: Investigation, Formal analysis, Methodol-
ogy, Validation. Zhi-Qin Wang: Investigation, Formal analysis.
Wei Sang: Investigation, Validation. Rui Zhang: Formal analysis,
Validation, Project administration. Hua Cheng: Resources, Investi-
gation, Supervision, Writing - review & editing. Cheng Chen:
Resources, Supervision, Conceptualization, Visualization, Writing
- original draft, Writing - review & editing. Da-Yong Peng:
Resources, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
6

Wan-Qiang Wang, Zhi-Qin Wang, W. Sang et al.
Polyhedron 195 (2021) 114979
Table 2
Table 3
Amide synthesis from various alcohols and amines.
Amide synthesis from substituted benzyl alcohols and benzyl amines.
Acknowledgements
This research was supported by the National Natural Science
Foundation of China (Nos. 21502062, 21562022), Scientific
Research Program Guiding Project of Hubei Provincial Department
of Education (B2019135), the Teachers’ Scientific Research Ability
Cultivation Fund, Hubei University of Arts and Science (Natural
Science, Grant No. 2019kypy003) and the Open Foundation of Dis-
cipline of Hubei University of Arts and Science (Grant Nos.
XK2019038), the China National Key R & D Program during the
13th Five-year Plan Period (Grant No. 2017YFD0301604).
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