Transfer hydrogenation of ketones catalyzed by 2,6‐bis(triazinyl)pyridine ruthenium complexes: The influence of alkyl arms

Article abstract of DOI:10.1016/j.tetlet.2019.150993

The transfer hydrogenation of ketones catalyzed by transition metal complexes has attracted much attention. A series of ruthenium(II) complexes bearing 2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)pyridine ligands (R-BTPs) were synthesized and characterized by NMR analysis and X-ray diffraction. These ruthenium(II) complexes were applied in the transfer hydrogenation of ketones. Their different catalytic activity were attributed to the alkyl arms on the 2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)pyridine. As the length of the alkyl arms rising, the catalytic activities of the complex catalysts decreased. By means of 0.4 mol % catalyst RuCl₂(PPh₃)(3-methylbutyl-BTP) in refluxing 2-propanol, a variety of ketones were reduced to their corresponding alcohols with >95% conversion over a period of 3 h.

Full text of DOI:10.1016/j.tetlet.2019.150993

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Transfer hydrogenation of ketones catalyzed by 2,6-bis(triazinyl)pyridine ruthe-
nium complexes: The influence of alkyl arms
Liandi Wang, Tingting Liu
PII:
S0040-4039(19)30748-8
DOI:
https://doi.org/10.1016/j.tetlet.2019.150993
TETL 150993
Reference:
Tetrahedron Letters
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Accepted Date:
22 May 2019
24 July 2019
27 July 2019
Please cite this article as: Wang, L., Liu, T., Transfer hydrogenation of ketones catalyzed by 2,6-
bis(triazinyl)pyridine ruthenium complexes: The influence of alkyl arms, Tetrahedron Letters (2019), doi: https://
doi.org/10.1016/j.tetlet.2019.150993
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Transfer hydrogenation of ketones catalyzed by
2,6‐ bis(triazinyl)pyridine ruthenium complexes: The
influence of alkyl arms
Liandi Wanga,*, Tingting Liua,b
a Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Dalian, Liaoning 116023,
China
b Institute of Chemistry Henan Academy of
Sciences, Zhengzhou 450002, China

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Tetrahedron Letters
1
TETRAHEDRON
LETTERS
Pergamon
Transfer hydrogenation of ketones catalyzed by
2,6‐ bis(triazinyl)pyridine ruthenium complexes: The influence of alkyl
arms
Liandi Wanga,*, Tingting Liua,b
a Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning 116023, China
b Institute of Chemistry Henan Academy of Sciences, Zhengzhou 450002, China
Abstract—The transfer hydrogenation of ketones catalyzed by transition metal complexes has attracted much attention. A series of
ruthenium(II) complexes bearing 2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)pyridine ligands (R-BTPs) were synthesized and characterized by
NMR analysis and X-ray diffraction. These ruthenium(II) complexes were applied in the transfer hydrogenation of ketones. Their different
catalytic activity were attributed to the alkyl arms on the 2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)pyridine. As the length of the alkyl arms
rising, the catalytic activities of the complex catalysts decreased. By means of 0.4 mol % catalyst RuCl2(PPh3)(3-methylbutyl-BTP) in
refluxing 2-propanol, a variety of ketones were reduced to their corresponding alcohols with >95% conversion over a period of 3 h. © 2019
Elsevier Science. All rights reserved.
(americium and curium) from trivalent lanthanides, which
represents a challenging goal for the definition of new
Introduction
methods in the disposal of nuclear wastes [6]. In addition,
other metals such as Pd(II), Co(II), Ni(II) and Ca(II) etc.
could also be extracted from the highly active liquid wastes
with the BTP ligands [7]. We have been interested in
developing N-heterocyclic ruthenium(II) complex catalysts
for a long time. Various pyridyl-based NNN ruthenium(II)
complexes have been synthesized and applied in the TH of
ketones [8]. Our group have also reported ruthenium(II)
complex based on iBu-BTP ligand, exhibiting moderate to
excellent catalytic efficiency in TH of ketones [9]. As
different alkyl-substituted BTPs displayed diverse stability
towards acidic hydrolysis and radiolytic degradation in the
extraction of An(III) from acidic solutions [10], in this
paper, we synthesized a series of ruthenium(II) complexes
bearing 2,6-bis(triazinyl)pyridine ligands with diverse
substituted alkyl groups (R-BTPs), and their catalytic
behaviors in the transfer hydrogenation reactions of ketones
were investigated.
Alcohols, especially secondary alcohols, are a class of
important organic compounds and have been extensively
utilized in organic synthesis and fine chemicals [1]. Among
the many methods for the synthesis of alcohols, the transfer
hydrogenation (TH) of carbonyl compounds is one of the
most effective strategies and has been considered to be a
useful alternative method to the widely used catalytic
hydrogenation by molecular hydrogen [2]. Transition-
metal-catalyzed TH reactions of ketones have made great
success over the past few decades and ruthenium(II)
complexes are usually used as the most useful potential
catalysts. Much research work has been devoted to the
exploration of new ligands for construction of ruthenium(II)
catalysts. For example, the versatile ruthenium(II) 2-
aminomethylpyridine (ampy) complexes reported by
Baratta et al. have been used in TH and ATH of ketones as
efficient catalysts [3]; Noyori’s ruthenium(II) complexes
containing N-sulfonylated 1,2-diamines chiral ligands have
demonstrated very high catalytic activity in the ATH of
ketones [4]; Moreover, the NH functions have possessed
beneficial effects on catalytic transformation, ligand
assembly, and/or catalyst formation in TH of ketones [5].
Although various ruthenium(II) complex catalysts have
been synthesized for TH, chemists are still committed to
the development of new and efficient catalytic systems.
2,6-Bis(triazinyl)pyridines (BTPs) are a kind of pyridyl-
based tridentate NNN ligands and have been reported as
effective extractants to separate trivalent minor actinides
________________
Conditions: (i) 4Å MS, toluene, N2 (0.1 MPa), 110 °C, 23 h; (ii)
RuCl2(PPh3)3, toluene, N2 (0.1 MPa), 110 °C, 3 h.
* Corresponding author.
E-mail address: wangliandi@dicp.ac.cn (L. D. Wang).
Scheme 1. Synthesis of ligands 3a-d and complexes 4a-d.

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Tetrahedron Letters
We chose complex 4d with cyclopentylethyl as the side
Results and discussions
arms to further investigate the structure of the ruthenium(II)
complex by X-ray crystallographic study. In the solid state,
complex 4d exhibits a neutral molecular structure and acts
as a planar pseudo-N3 ligand, and the metal center is
surrounded by the tridentate pseudo-N3 ligand, two
chlorides, and one PPh3 ligand. The two chlorides in 4d are
closely linear to each other (Cl(1)–Ru–Cl(2), 172.83(7)°)
and positioned on the two sides of the pseudo-NNN ligand
plane (Figure 1). The three Ru–N, two Ru–Cl and Ru–P
bond distances are 1.980(7), 2.029(6), 2.047(6),
2.3798(19), 2.4103(19) and 2.347(2) Å respectively, with
Ligands 3a-d were prepared by means of a modified
literature procedure (Scheme 1). Dehydrated by heating,
pyridine-2,6-dicarbohydrazide imide 1 reacted with 1,2-
diketones 2a-d to afford 2,6-bis(5,6-dialkyl-1,2,4-triazin-3-
yl)pyridines 3a-d. Then, the reaction of 3a-d with 1.0 equiv
of RuCl2(PPh3)3 [11] in refluxing toluene afforded
complexes RuCl2(PPh3)(R-BTP) 4a-d. Complexes 4a-d
were stable when exposed to air at ambient temperature.
The NMR spectra of 4a-d reveal 3a-d to be the
coordinating ligands. The chemical shifts of the pyridyl CH
hydrogen atoms in complexes 4a-d were shifted upfield by
0.2-0.3 ppm in the proton NMR spectrum compared with
the ligand precursors 3a-d. The 31P{1H} NMR signals of
ruthenium(II) complexes 4a-d reveal a singlet at 41.9, 41.8,
41.7, 41.9 ppm, respectively, suggesting one PPh3 ligand is
presented in the complex.
0.0007–0.0161
RuCl2(PPh3)(iBu-BTP)
Å
shortened by comparing with
[40]
(Table 1), revealing that the
metal center in 4d was in a much tighter environment than
the metal center in RuCl2(PPh3)(iBu-BTP). These data
suggest that the complex 4d may act as a less catalytically
active catalyst than RuCl2(PPh3)(iBu-BTP).
Table 2
Optimizing of reaction conditions for transfer hydrogenation .
Ketone/base/cat.
Entry Ru(II) cat. Base
Time (h) Yield (%)a
(molar ratio)
500/30/1
250/30/1
250/12.5/1
250/25/1
250/40/1
250/30/1
250/30/1
250/30/1
250/30/1
250/30/1
250/30/1
250/30/1
1
2
NaOH
NaOH
NaOH
NaOH
NaOH
iPrOK
tBuOK
KOH
2
2
65
97
73
94
97
97
98
95
97
96
95
97
4a
4a
4a
4a
4a
4a
4a
4a
4b
4c
4d
A
3
2
4
2
Figure 1. Molecular structure of complex 4d.
5
2
6
2
Table 1
Selected bond distances (Å) and angles (°) for complexes 4d and
RuCl2(PPh3)(iBu-BTP).
7
2
8
2
Selected bond
Ru–N(1)
RuCl2(PPh3)(iBu-BTP)
1.982(3)
4d
9
NaOH
NaOH
NaOH
NaOH
3
1.980(7)
2.029(6)
2.047(6)
2.347(2)
2.3798(19)
2.4103(19)
1.341(9)
172.83(7)
78.8(3)
10
11
12
8
Ru–N(2)
2.032(3)
9
Ru–N(5)
2.058(3)
1.5
Ru–P
2.3631(11)
2.3919(13)
2.4110(13)
1.341(4)
Conditions: ketone, 2.0 mmol (0.1 M in 20 mL iPrOH); 0.1 MPa, 82 oC; a
GC yield of the corresponding alcohol.
Ru–Cl(1)
Ru–Cl(2)
Ruthenium(II) complexes 4a-d have been used as the
potential catalysts for TH of ketones. Using 4a as the
catalyst, TH of acetophenone was carried out in 2-propanol
N(5)–N(6)
Cl(1)–Ru–Cl(2)
N(1)–Ru–N(2)
N(1)–Ru–N(5)
N(2)–Ru–N(5)
N(1)–Ru–P
N(1)–Ru–Cl(1)
N(1)–Ru–Cl(2)
N(2)–Ru–Cl(2)
P–Ru–Cl(1)
N(2)–Ru–P
174.91(3)
78.85(12)
78.67(12)
156.03(12)
175.80(8)
90.05(8)
o
at 82 C (Table 2). When NaOH was used as the base, the
best molar ratio for ketone/base/catalyst is 250/30/1 (Table
2, entries 1–5). iPrOK, tBuOK and KOH were also tested
as the bases. Over a period of 2 h, the corresponding
alcohol product from acetophenone reached 97%, 97%,
98%, and 95% yields by GC analysis in the reactions using
NaOH, iPrOK, tBuOK and KOH as the base, respectively
(Table 2, entries 2 and 6–8). Thus, NaOH was selected as
the reaction promoter. Under optimized conditions,
complexes RuCl2(PPh3)(R-BTP) with diverse alkyl groups
were used as the catalysts in the transfer hydrogenation of
acetophenone. To reach >95% yield by GC analysis, the
corresponding complexes 4a, 4b, 4c, 4d and
78.7(3)
157.2(3)
176.45(19)
89.43(19)
85.32(19)
95.31(19)
93.18(7)
98.81(19)
84.96(8)
93.88(8)
93.83(4)
99.77(9)
S2