Highly active bidentate N-heterocyclic carbene/ruthenium complexes performing dehydrogenative coupling of alcohols and hydroxides in open air

Article abstract of DOI:10.1039/c9cc03519b

Eight bidentate NHC/Ru complexes, namely [Ru]-1-[Ru]-8, were designed and prepared. In particular, [Ru]-2 displayed extraordinary performance even in open air for the dehydrogenative coupling of alcohols and hydroxides. Notably, an unprecedentedly low catalyst loading of 250 ppm and the highest TON of 32 800 and TOF of 3200 until now were obtained.

Full text of DOI:10.1039/c9cc03519b

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DOI: 10.1039/C9CC03519B
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Highly active bidentate N-heterocyclic carbene/ruthenium
complexes performing dehydrogenative coupling of alcohols
and hydroxides in open air
Received 00th January 20xx,
Accepted 00th January 20xx
Zhi-Qin Wang , Xiao-Sheng Tang , Zhao-Qi Yang , Bao-Yi Yu, Hua-Jing Wang , Wei Sang a,b, Ye
a,b
c
d
e
a
DOI: 10.1039/x0xx00000x
a
a
a, f, g
Yuan , Cheng Chen* and Francis Verpoort*
Eight bidentate NHC/Ru complexes, namely [Ru]-1~[Ru]-8, were number of primary alcohols were directly transformed into
designed and prepared. Particularly,
extraordinary performance even in open air for the handful of Ru,
[Ru]-2 displayed carboxylic acids in aqueous NaOH solution. Afterwards, a
22-31
32
33 34, 35
36
37
37, 38
39
Rh, Pd, Ir
, Ag, Fe, Mn,
and Zn
dehydrogenative coupling of alcohols and hydroxides. Notably, an complexes were also reported for this catalysis. Especially, Ru
unprecedentedly low catalyst loading of 250 ppm and the highest complexes have been most accessed and investigated.
TON of 32800 and TOF of 3200 until now were obtained.
With catalyst loadings ranging from 0.1 mol% to 5.0 mol%, a
number of Ru complexes were reported to be active for this
approach. Among these catalysts, N-heterocyclic carbene
Transforming primary alcohols into the corresponding
1
,
carboxylic acids is a fundamental process in organic chemistry.
(NHC)-based Ru complexes have attracted our growing interest
2
Traditionally, direct oxidation of primary alcohols was utilized
owing to the attractiveness of NHCs as ligands in organometallic
3
for the production of acids. However, stoichiometric amounts
40-44
catalysis.
Hence, two monodentate NHC/Ru complexes
of strong and toxic oxidants were usually required and
enormous by-products were generated as waste.
though in some cases oxygen were used as the sole oxidant,
problems may occur as oxygen could form explosive mixtures
with various organic reagents and solvents. Therefore, it is of
vital importance to develop green and sustainable alternatives
for producing carboxylic acids. Recently, the dehydrogenative
coupling of alcohols and water or hydroxides has been
demonstrated as an environmental-benign and atom-economic
transformation with the concomitant generation of dihydrogen
have also been developed and proven to be active for this
4
-6
Even
23, 26
transformation (Figure 1a).
Despite the above two NHC/Ru
7
-17
complexes, the discovery of versitile and highly active NHC/Ru
complexes for this process is still in high demand. Inspired by
the high activities of our previously presented bidentate
45
NHC/Ru complexes in dehydrogenative amidation of alcohols
and by the recent developments for the dehydrogenative
coupling of alcohols to form carboxylic acids, we thereby
NHC
designed and synthesized several Ru complexes bearing C
C
bidentate NHC ligands (Figure 1b). Gratifyingly, selective
formation of carboxylic acids was acheived through extensive
screening of various conditions. Notably, one of these
complexes ([Ru]-2) demonstrated outstanding efficiency, with
the applied catalyst loading at a low level of 250 ppm. Moreover,
the turnover number (TON) could be up to 32800 and the
maximum turnover frequency (TOF) could be 3200. To the best
of our knowledge, this is the lowest catalyst loading and highest
TON and TOF ever reported for this reaction (as listed in Table
S1 of the supporting information). In addition, this catalytic
dehydrogenation was performed in open air and did not require
2
(H ) as the sole byproduct, showing a prosperous potential for
synthetic applications. At the outset, Grützmacher et al.
accomplished the catalytic dehydrogenative synthesis of
carboxylic acids from primary alcohols promoted by hydrogen
18-20
acceptors.
The acceptorless version of this transformation
was pioneered by Milstein and co-workers.21 In this work, 0.2
mol% of a PNN-type Ru pincer complex could realize this
catalytic process without a hydrogen acceptor, thereby a
a.State Key Laboratory of Advanced Technology for Materials Synthesis and
Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, P. special handling or purification, demonstrating great potential
R. China[phone: (+86)-27-88016035; fax: (+86)-27-87879468; E-mail addresses:
chengchen@whut.edu.cn, francis.verpoort@ghent.ac.kr].
for its practical applications.
b.
c.
With the above considerations in mind, five bidentate
NHC/Ru complexes ([Ru]-1~[Ru]-5, as shown in the scheme of
School of Materials Science and Engineering, Wuhan University of Technology,
1
22 Luoshi Road, Wuhan 430070, P. R. China;
Key Laboratory of Optoelectronic Technology and Systems (Ministry of Education)
College of Optoelectronic Engineering, Chongqing University, Chongqing 400044,
P. R. China;
School of Pharmaceutical Sciences, Jiangnan University, Wuxi 214122, China;
Key Laboratory of Urban Agriculture (North China), Ministry of Agriculture, Beijing
University of Agriculture, Beinong Road 7, Beijing 102206, P. R. China;
National Research Tomsk Polytechnic University, Lenin Avenue 30, Tomsk 634050,
Russian Federation;
Table 1) were easily prepared and purified from a mixture of
NHC
[RuCl
2 2
(p-cymene)] , a C C bidentate NHC precursor and
d.
e.
Cs CO in refluxing THF. Then the acceptorless dehydrogenative
2
3
coupling of benzyl alcohol (1a) and a hydroxide was selected as
a typical reaction for optimization of reaction conditions. Varied
hydroxides and solvents were screened, and KOH combined
f.
g.
Ghent University Global Campus, 119 Songdomunhwa-Ro, Yeonsu-Gu, Incheon
2
1985, Korea.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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Journal Name
Table 1 Optimization of reaction conditions.a
View Article Online
DOI: 10.1039/C9CC03519B
Yields of
Entry
[Ru]
x
n
b
TON
TOF
2
a (%)
94
93
92
91
59
70
52
76
94
89
91
92
5
1
2
3
4
5
6
[Ru]-1
[Ru]-1
[Ru]-1
[Ru]-2
[Ru]-3
[Ru]-4
[Ru]-5
[Ru]-1
[Ru]-2
[Ru]-6
[Ru]-7
[Ru]-8
-
0.10
0.05
6
6
940
1860
3680
3640
2360
2800
2080
3040
3760
3560
3640
3680
-
157
310
640
637
393
467
347
507
647
593
607
613
-
Figure 1. The design strategy of this work.
with m-xylene (at a refluxing temperature) were selected to be
optimum (as detailed in Table S2 of the supporting information).
In refluxing m-xylene, 94% of 2a was obtained after 6 h (entry
0.025
0.025
0.025
0.025
0.025
0.025
0.025
0.025
0.025
0.025
-
6
6
1
0
). Interestingly, variation of the Ru loading from 0.1 mol% to
.05 mol% and 0.025 mol% did not trigger substantial loss of the
6
product yields (entries 2-3 vs. entry 1). With a Ru loading of
.025 mol% (as listed in entry 3), the catalytic performance of
6
0
7
6
the other four NHC/Ru complexes was also evaluated (as shown
in entries 4-7). Compared with [Ru]-1, the imidazole-based
analogue [Ru]-2 exhibited a similar activity (entry 4 vs. entry 3).
For [Ru]-3 which was testified to be active for the
8c
6
9c
6
dehydrogenative alcohol amidation with amines,45
considerable decrease of the product yield was obtained (entry
vs. entry 3). With regard to the thiophene-containing [Ru]-4
a
10c
6
1
1
1
1c
6
5
2c
6
and [Ru]-5, 52-70% yields of 2a were observed (entries 6-7).
Therefore, [Ru]-1 and [Ru]-2 displayed outstanding activities at
the current conditions (entries 3-4). In order to further examine
the practical values of these two complexes and make a better
comparison of their activities, reactions applying both
complexes were conducted in open air (entries 8-9). The results
indicated that [Ru]-2 afforded the product in 94% yield (entry
3c
6
1
4c,d
[Ru]-2
[Ru]-2
0.025
0.01
24
16
36
90
86
3600
8600
32800
600
538
911
c
15
6c,e
82
1
[Ru]-2 0.0025
(
1)
(8)
(3200) (3200)
9
). However, it seemed that air was detrimental to [Ru]-1 since
the yield deteriorated to 76% for this complex in open air (entry
). Furthermore, the superiority of the imidazole-based NHC
ligand over the benzimidazole counterpart inspired us to
1
7 c,f
[Ru]-2 0.025
6
87
3480
580
a
Conditions: 1a (1.00 equiv.), KOH (1.20 equiv.) and [Ru] (x mol%) at a refluxing m-
8
b
xylene under an open argon atmosphere;
NMR yields using 1,3,5-
c
trimethoxybenzene as an internal standard (average of two consistent runs); in
d
e
f
conduct further optimization of NHC ligands. Different open air instead of argon; under a neat condition; KOH (2.00 equiv) was used;
the reaction was run at a scale of 100 mmol (10.4 g of 1a).
substituents were introduced on the imidazole framework of
NHC ligands to afford [Ru]-6
comparable activities with [Ru]-2 in open air (entries 10-12 vs.
). In addition, the structures of [Ru]-4 [Ru]-8 were further
confirmed by X-ray crystallography (as shown in Figure S1 of the
supporting information). Accordingly, [Ru]-2 was selected as
the optimal catalyst due to its high activity and robustness. In
the absence of a Ru complex, only 5% of 2a was detected with
~[Ru]-8, which exhibited
the majority of 1a remaining (entry 13), which excluded the
pathway of base-promoted aerobic oxidation of primary
9
~
10, 13, 17
alcohols.
In addition, [Ru]-2 could also efficiently catalyze
the reaction under a neat condition, but a longer period of time
was required (entry 14). Further reducing the catalyst loading
to 0.01 mol% and 0.0025 mol% led to a maximum TON of 32800
2
| J. Name., 2012, 00, 1-3
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Journal Name
COMMUNICATION
and a maximum TOF of 3200 (entries 15-16). Finally, a 10 gram-
scale reaction was also performed, leading to 87% of the desired
product (entry 17). Therefore, the optimized parameters,
recognized as 0.025 mol% (equal to 250 ppm) of [Ru]-2, 1.00
equiv. of 1a and 1.20 equiv. of KOH in refluxing m-xylene for 6
h (as listed in entry 9 of Table 1), were used for further
investigations.
View Article Online
DOI: 10.1039/C9CC03519B
With the optimized reaction conditions in hand, the substrate
scope and limitations were thoroughly investigated (as depicted
in Figure 2). First of all, a number of aromatic alcohols were
subjected to this dehydrogenation, affording the corresponding
carboxylic acids in moderate to good yields. Electron-rich benzyl
alcohols 1b-1d gave acids 2b-2d in exellent yields, while the
electron-deficient counterparts (1e-1h) generally afforded the
corresponding acids in lower yields (45-73%) with longer
reaction time (16 h) or a higher catalyst loading (500 ppm).
Particularly, 1e (or 1f) not only produced expected product 2e
(or 2f), but also yielded the dehalogenated acid 2a in 10% (or
1
2%) yield. Moreover, the substituent position on the phenyl
ring was found to affect the yields of the products. Under the
standard conditions, 3-methylbenzyl alcohols (1i) and 2-
methylbenzyl alcohol (1j) generated the acid products in 83%
and 73% yields, respectively. Impressively, phenyl- and aniline-
containing alcohols 1k and 1l could also be tolerated to give
acids 2k and 2l in 51% and 10% yields, respectively. Additionally,
the reactions of substrates (1m and 1n) bearing a naphthyl
group or a sulfur atom also proceeded smoothly. Except
aromatic alcohols, we also extended this Ru-catalyzed protocol
to aliphatic alcohols under analogous conditions. Sterically
nonhindered alcohols including linear alcohols
cyclopentylmethanol (1q), cyclohexylmethanol (1r) and 3-
phenylpropan-1-ol (1s) gave rise to aliphatic acid products (2o
(1o-1p),
-
2
1
s
) in 81-93% yields after 16 h. In addition, the bulky substrate
-adamantanemethanol (1t) was efficiently converted to acid 2t
2v) were
in 65% yield, and 73-85% of the desired products (2u
-
given for heterocycle-containing substrates. For the reaction of
KOH and hex-5-en-1-ol (1w) comprising a terminal C=C bond, a
mixture of 57% of product 2w, 18% of 2w’ generated by olefin
isomerization of 2w, and 7% of the reduced product 2o were
obtained. Substrate 1x, which contains both primary and
Figure 2. Synthesis of carboxylic acids from various alcohols and KOH
catalyzed by [Ru]-2.
secondary OH groups, was also attempted. It was indicated that intermediate
acid 2x, with the secondary OH intact, was obtained in 48% yield. form the hydroxide ion (OH ). Afterwards,
Finally, treatment of amino alcohols 1y 1z with KOH furnished undergo a similar pathway as proposed by our previous report
the corresponding amino acids 2y 2z in good yields.
for [Ru]-3-catalyzed dehydrogenative coupling of alcohols and
Generally, it has been postulated that the generation of Ru amines, by replacing the amine nucleophile with H O (or OH ).
I and water molecule, which could solvate KOH to
-
26
I
could probably
-
-
-
45
2
hydride species is crucial for this Ru-catalyzed dehydrogenative Through β-hydride elimination, intermediate
I can be converted
2
1, 26, 30
transformation.
Therefore, NMR studies deciphering the to a hydrido complex II bound with the corresponding aldehyde,
-
mechanistic insights were carried out. Expectedly, several which reacts with H O (or OH ) to produce intermediate III and
2
hydride resonances were observed, which was probably liberate one molecule of H . Furthermore, III is accompanied
2
indicative of the essential role of Ru-hydride species in the with another β-hydride elimination to provide Ru monohydride
catalytic cycle (as detailed in Fig. S2 of the supporting species IV, followed by ligand substitution with
(which is converted to carboxylate [2-H]- under
information). Based on these experiments and the reported Ru carboxylic acid
we suggested a basic condition) and hydrido complex
. Eventually,
1 to deliver
2
2
1, 26, 30
catalysts for this transformation,
V
V
plausible pathway for this [Ru]-2-catalyzed protocol (as shown eliminates another H molecule to regenerate intermediate
I,
2
in Figure S3 of the supporting information). Initially, treatment allowing for the fulfilment of the catalytic cycle.
of [Ru]-2 with alcohol
1
and KOH gives key Ru-alkoxide
In summary, we have successfully prepared eight Ru
complexes bearing C C bidentate NHC ligands, namely [Ru]-
NHC
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J. Name., 2013, 00, 1-3 | 3
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~[Ru]-8. Through a systematic investigation of these 20.
Journal Name
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T. L. Gianetti, S. P. Annen, G. Santiso-Quinones,VMiew. RAreticihleeOr,nlMine.
Driess and H. Grützmacher, Angew. DChOeI:m10. .I1n0t3.9E/dC.9,C2C010365,1595B,
complexes and other parameters, optimized reaction
conditions applying [Ru]-2 demonstrated excellent activities for
the acceptorless dehydrogenative coupling of primary alcohols
and KOH. Accordingly, numerous aromatic and aliphatic
alcohols were efficiently transformed into the corresponding
carboxylic acids. Interestingly, this catalytic process could be
performed in open air, and no special handling or further
purification was necessary to achieve satisfactory results.
Notably, [Ru]-2 is effective for this transformation even at a low
1
854-1858.
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catalyst loading of 250 ppm. In addition, a TON of up to 32800 25.
and a TOF of up to 3200 were obtained. It is worth mentioning
that these values are superior over all the literature reports
L. Zhang, D. H. Nguyen, G. Raffa, X. Trivelli, F. Capet, S. Desset,
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This research was supported by the National Natural Science
Foundation of China (No. 21502062), F.V. acknowledges the
1
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support
from
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Polytechnic
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2
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Conflicts of interest
There are no conflicts to declare.
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View Article Online
DOI: 10.1039/C9CC03519B
O
R1 OH
KOH
=
R3
R1
R2
N
OK
Ru
I
N
S
2H2
S
The first bidentate NHC/Ru complex for this process
As low as 250 ppm [Ru] and TONs (TOFs) up to 32800 (3200)
In open air and easy operation
A highly active and robust bidentate NHC/Ru complex for the acceptorless dehydrogenative
coupling of alcohols and hydroxides in open air.