Phosphine Ligand-Free Ruthenium Complexes as Efficient Catalysts for the Synthesis of Quinolines and Pyridines by Acceptorless Dehydrogenative Coupling Reactions

Article abstract of DOI:10.1002/cctc.201900435

A series of phosphine-free Ru(III)/Ru(II) complexes of NH functionalized N^?N^?N pincer ligands exhibit excellent activity for acceptorless dehydrogenative coupling (ADC) of secondary alcohols with 2-aminobenzyl or γ-amino alcohols to quinolines and pyridines. Ru(III) complexes [LRuCl₃] (L=6-(3-R¹,5-R²-1H-pyrazol-1-yl)-N-(pyridin-2-yl)pyridin-2-amine; 1 a: R¹=R²=H (L1); 1 b: R¹=R²=Me (L2); 1 c: R¹=H, R²=CF₃ (L3); 1 d: R¹=H, R²=Ph (L4); 1b^Me: L=6-(3,5-dimethyl-1H-pyrazol-1-yl)-N-methyl-N-(pyridin-2-yl)pyridin-2-amine (L2^Me)) were obtained by refluxing RuCl₃ ? xH₂O with the corresponding ligand in EtOH. Five Ru(II) complexes [LRu(DMSO-κS)Cl₂] (2 a: L=L1; 2 b: L=L2; 2 c: L=L3; 2 d: L=L4; 2b^Me: L=L2^Me) were formed by reducing the corresponding Ru(III) complex in refluxing EtOH. The latter complexes could also be prepared directly by refluxing Ru(DMSO)₄Cl₂ with the corresponding ligand in EtOH. These Ru(III) and Ru(II) complexes, especially 1 b/2 b, exhibited high catalytic efficiency and broad functional group tolerance in ADC reactions of secondary alcohols with 2-aminobenzyl or γ-amino alcohols to quinolines and pyridines. A detail mechanistic study indicated the Ru(III) complex was reduced into the Ru(II) species, which is the active catalytic center for ADC via a Ru?H/N?H bifunctional outer-sphere mechanism. This protocol provides a reliable, atom-economical and environmentally benign procedure for C?N and C?C bond formation.

Full text of DOI:10.1002/cctc.201900435

Accepted Article
Title: Phosphine Ligand-free Ruthenium Complexes as Efficient
Catalysts for the Synthesis of Quinolines and Pyridines by
Acceptorless Dehydrogenative Coupling Reactions
Authors: Bin Guo, Tian-Qi Yu, Hong-Xi Li, Shi-Qi Zhang, Pierre
Braunstein, David J. Young, Hai-Yan Li, and Jian-Ping Lang
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10.1002/cctc.201900435
ChemCatChem
FULL PAPER
Phosphine Ligand-free Ruthenium Complexes as Efficient
Catalysts for the Synthesis of Quinolines and Pyridines by
Acceptorless Dehydrogenative Coupling Reactions
Bin Guo,[a] Tian-Qi Yu,[a] Hong-Xi Li,*[a] Shi-Qi Zhang,[a] Pierre Braunstein,[b] David J. Young,[c] Hai-Yan
Li,[a] and Jian-Ping Lang*[a]
Abstract: A series of phosphine-free Ru(III)/Ru(II) complexes of NH functionalized N˄N˄N pincer ligands exhibit excellent activity for
acceptorless dehydrogenative coupling (ADC) of secondary alcohols with 2-aminobenzyl or γ-amino alcohols to quinolines and pyridines.
Ru(III) complexes [LRuCl3] (L = 6-(3-R1,5-R2-1H-pyrazol-1-yl)-N-(pyridin-2-yl)pyridin-2-amine; 1a: R1 = R2 = H (L1); 1b: R1 = R2 = Me (L2); 1c:
R1 = H, R2 = CF3 (L3); 1d: R1 = H, R2 = Ph (L4); 1bMe: L = 6-(3,5-dimethyl-1H-pyrazol-1-yl)-N-methyl-N-(pyridin-2-yl)pyridin-2-amine (L2Me))
were obtained by refluxing RuCl3·xH2O with the corresponding ligand in EtOH. Five Ru(II) complexes [LRu(DMSO-κS)Cl2] (2a: L = L1; 2b: L =
L2; 2c: L = L3; 2d: L = L4; 2bMe: L = L2Me) were formed by reducing the corresponding Ru(III) complex in refluxing EtOH. The latter complexes
could also be prepared directly by refluxing Ru(DMSO)4Cl2 with the corresponding ligand in EtOH. These Ru(III) and Ru(II) complexes,
especially 1b/2b, exhibited high catalytic efficiency and broad functional group tolerance in ADC reactions of secondary alcohols with 2-
aminobenzyl or γ-amino alcohols to quinolines and pyridines. A detail mechanistic study indicated the Ru(III) complex was reduced into the
Ru(II) species, which is the active catalytic center for ADC via a Ru-H/N-H bifunctional outer-sphere mechanism. This protocol provides a
reliable, atom-economical and environmentally benign procedure for C-N and C-C bond formation.
Introduction
catalysis[64-67] can dramatically improve catalytic performance of
ADC reactions using pincer ligands bearing NH, OH or CH2
functionality.[40-44,68-81] Milstein,[40] Sun,[41] Yu,[42] Kundu[43] and
Chen[44] have independently developed bifunctional Ru(II)
catalysts for the synthesis of pyridine and quinoline derivatives
(Scheme 1), but these catalysts were often air/water sensitive,
involved a somewhat complicated synthesis, or required toxic
phosphine ligands. The air- and moisture-stable phosphine-free
Ru(III) complexes are easily prepared in a large scale via
reaction of RuCl3 with pincer ligands, but have been paid much
less attention for ADC or auto-transfer-hydrogen (ATH)
reactions.[82] Recently, we have been interested in ADC and
ATH reactions catalyzed by Cu(I)/Ni(II) N-heterocycle thiolate
clusters via ligand−metal cooperative catalysis.[59,72,83] In this
Quinolines and pyridines are an important class of heterocycles
found in pharmaceuticals, agrochemicals and functional
materials.[1-5] Numerous synthetic protocols for the quinoline
nucleus have been reported, including the Skraup, Comps,
Conrad-Limpach, Gould-Jacobs, Doebner-von Miller and
Povarov reactions.[6-12] Pyridine derivatives can be synthesized
by transition metal catalyzed cycloaddition reactions.[13-22]
However, harsh reaction conditions, low stereoselectivity,
multiple steps and low yields limit the general applicability of
these methods. Green and atom-economic acceptorless
dehydrogenative coupling (ADC) reactions are a competitive
methodology for the synthesis of quinolines (a variation on the
work, we have chosen and prepared
a family of NH
Friedlander protocol) and pyridines from 2-aminobenzyl or γ-
̈
functionalized N˄N˄N pincer ligands (L1 = 6-(1H-pyrazol-1-yl)-N-
(pyridin-2-yl)pyridin-2-amine; L2 = 6-(3,5-dimethyl-1H-pyrazol-1-
yl)-N-(pyridin-2-yl)pyridin-2-amine;[84] L3 = N-(pyridin-2-yl)-6-(3-
amino alcohols with secondary alcohols or ketones.[23-28] These
metal-catalyzed catalytic cycle involves dehydrogenation of
alcohols, followed by cyclo-condensation with amines, with only
H2 and H2O as by-products. Among various homogeneous
transition metal catalysts,[29-59] Ru-complexes are the most
efficient reported to date.[29-44,60-63] Metal-ligand cooperative
(trifluoromethyl)-1H-pyrazol-1-yl)pyridin-2-amine; L4
Phenyl-1H-pyrazol-1-yl)-N-(pyridin-2-yl)pyridin-2-amine) and the
methyl-substituted derivative 6-(3,5-dimethyl-1H-pyrazol-1-yl)-N-
methyl-N-(pyridin-2-yl)pyridin-2-amine (L2Me
=
6-(3-
[84]
)
for the catalytic
performance comparison. Reactions of these ligands with
equimolar RuCl3·xH2O in refluxing EtOH yielded five Ru(III)
complexes [LRuCl3] (1a: L = L1; 1b: L = L2; 1c: L = L3; 1d: L =
L4; 1bMe: L = L2Me). Further treatment of 1a-1d or 1bMe in the
presence of LiCl, Et3N and DMSO in refluxing EtOH or reactions
of Ru(DMSO)4Cl2 with equimolar corresponding N˄N˄N pincer
ligand in refluxing EtOH gave rise to five corresponding Ru(II)
complexes [LRu(DMSO-κS)Cl2] (2a: L = L1; 2b: L = L2; 2c: L =
L3; 2d: L = L4; 2bMe: L = L2Me). These Ru(III) and Ru(II)
complexes could catalyze ADC reactions of secondary alcohols
with 2-aminobenzyl or γ-amino alcohols to give quinolines and
pyridines. Particularly, complexes 1b and 2b are highly efficient
at low molar ratios under mild conditions and tolerant of various
functional groups. A mechanistic study was performed to reveal
that the Ru(III) complex was reduced into the Ru(II) active
[a]
B. Guo, T.-Q. Yu, Prof. Dr. H.-X. Li, S.-Q. Zhang, Prof. H.-Y. Li, Prof.
Dr. J.-P. Lang
College of Chemistry, Chemical Engineering and Materials Science
Soochow University
Suzhou 215123 (P.R. China)
E-mail: lihx@suda.edu.cn
jplang@suda.edu.cn
Prof. Dr. P. Braunstein
Institut de Chimie (UMR 7177 CNRS)
Université de Strasbourg
Strasbourg 67081 (France)
[b]
[c]
Prof. Dr. D. J. Young
College of Engineering, Information Technology and Environment
Charles Darwin University
Northern Territory 0909 (Australia)
Supporting information for this article is given via a link at the end of
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10.1002/cctc.201900435
ChemCatChem
FULL PAPER
species in the presence of base and alcohol under the reaction
conditions to realize the ADC reaction and the formation of
quinolines and pyridines.
P(Ph)3
N
N
HN
N
N
Cl
Cl
N
N
N
P(tBu)2
N
X-
(But)2P
N
N
OC
Ru
N
Ru
Ru
H
Ru
(Ph)3P
(Ph)3P
OH
CO
H
Cl
CO
H
Ph
Ph
X = Cl, PF6
Kundu
2018
Sun
2016
Yu
2017
Milstein
2013
R1
R1
O
NH
Cl
NH
N
N
N
N
N
N
Cl
Cl
N
N
Ru
Ru
Ru
N
N
N
R2
R2
S
Cl
OH
Cl
Cl
CO
Cl
O
Ru(III)
Ru(II)
Chen
2018
R1 = H, R2 = H, CF3, Ph; R1 = R2 = Me
This work
Scheme 1. The reported bifunctional pincer Ru(II) catalysts for the synthesis
of quinolines and pyridines via ADC reaction.
Results and Discussion
Synthesis and characterization of ligands and complexes
The functional N˄N˄N pincer ligands reported in this work were
obtained by modified literature procedures[84,85] through a simple
two-step synthesis starting from reactions of 2,6-dibromopyridine
with 1H-pyrazole, 3,5-dimethyl-1H-pyrazole, 3-(trifluoromethyl)-
1H-pyrazole or 3-phenyl-1H-pyrazole (Scheme 2). The Ullmann
condensation reaction of the substituted pyrazole and 2,6-
dibromopyridine with K2CO3
catalyzed by CuI/1,10-
phenanthroline (CuI/phen) at 110 ºC in toluene for 24 h,
generated 2-bromo-6-(pyrazolyl)pyridine derivatives in 83-85%
yields. Ligands L1-L4 were prepared by Buchwald-Hartwig
cross-coupling of 2-bromo-6-(pyrazolyl)pyridine derivatives with
Figure 1. The ESI-MS spectra of 1b (a) and 2b (b). The calculated isotope
patterns (up) and observed patterns (bottom) of [M + Na]+ for 1b and [M - Cl]+
for 2b.
2-aminopyridine catalyzed by Pd2(dba)3/dppf (dba
dibenzylideneacetone; dppf
=
1,1'-
=
bis(diphenylphosphino)ferrocene) in the presence of KOtBu in
refluxing toluene for 24 h. L1, L2, L3 or L4 was then reacted with
equimolar RuCl3·xH2O in refluxing EtOH under a N2 atmosphere,
giving the corresponding Ru(III) complex [LRuCl3] (L = L1 (1a), L
= L2 (1b), L = L3 (1c), L = L4 (1d)) in 71-78% yields. Complex
[LRu(DMSO-κS)Cl2] (L = L1 (2a), L = L2 (2b), L = L3 (2c), L = L4
(2d)) was obtained as yellow solids in 86-92% yields by refluxing
complex 1a, 1b, 1c or 1d in anhydrous ethanol and DMSO in the
presence of LiCl and Et3N. Complexes 2a-2d could also be
directly synthesized via reactions of Ru(DMSO)4Cl2 with L1-L4 in
72-77% yields. The methyl-substituted derivative (L2Me) was
prepared to examine the mechanism of reaction. Reaction of
L2Me with RuCl3·xH2O or Ru(DMSO)4Cl2 resulted in the
Complexes
1 and 2 were characterized by electrospray
ionization mass spectrometry (ESI-MS), elemental analysis, IR
spectroscopy or/and NMR spectroscopy. The positive-ion ESI-
MS analyses revealed signals at m/z = 466.9005 (1a), 494.9308
(1b), 476.9297 (1c), 484.9757 (1d), 450.9907 (1bMe), 451.9892
(2a), 480.0181 (2b), 519.9769 (2c), 528.0205 (2d) and 494.0354
(2bMe), which are ascribed to the cationic species [(L1)RuCl3 +
Na]+ (1a), [(L2)RuCl3 + Na]+ (1b), [(L3)RuCl2]+ (1c), [(L4)RuCl2]+
(1d),
[(L2)Ru(DMSO)Cl]+
[(L2Me)RuCl2]+
(1bMe),
(2b),
[(L1)Ru(DMSO)Cl]+
[(L3)Ru(DMSO)Cl]+
(2a),
(2c),
[(L4)Ru(DMSO)Cl]+ (2d), and [(L2Me)Ru(DMSO)Cl]+ (2bMe), and
matched well with their simulated isotopic patterns (Figure 1 and
Figure S1 ). The IR spectra of 1a-1d and 2a-2d revealed weak
absorption bands at ca. 3300 cm-1 corresponding to the
stretching vibrations of the N-H bonds. Moderately intense
absorption bands at 1080 cm-1 correspond to the stretching
vibrations of the S=O bonds of DMSO in the IR spectra of 2a-2d
formation of [(L2Me)RuCl3] (1bMe
,
in 75% yield) and
[(L2Me)Ru(DMSO-κS)Cl2] (2bMe, in 74% yield), respectively.
Similarly, complex 1bMe could also be reduced into 2bMe in 89%
yield.
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10.1002/cctc.201900435
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1
and 2bMe. The H NMR spectra of 2a-2d and 2bMe showed two
2.882/2.892 ppm (2bMe), which are assignable to the six protons
singlets at δ = 2.796/2.807 ppm (2a), δ = 2.866/2.873 ppm (2b),
δ = 2.741/2.935 ppm (2c), δ = 2.642/2.802 ppm (2d), δ =
of the two methyl groups from coordinated DMSO molecules.
Scheme 2. Synthesis of ligands and Ru-N^N^N pincer complexes.
Crystal structures of 1a-1d, 1bMe, 2a-2d, and 2bMe
[RuCl2(bpea)(DMSO-κS)] (2.2412(5) Å).[87] The N(5)-Ru(1)-N(1),
The solid-state structures of 1a·H 2O, 1b·DMF, 1c·DMF·MeOH,
1d·2DMF, 1bMe, 2a, 2b·CH2Cl2, 2c·CH2Cl2, 2d·CH2Cl2 and
2bMe·0.5H2O were determined by single crystal X-ray
Cl(2)-Ru(1)-Cl(3)/S(1), N(3)-Ru(1)-Cl(1) angles in 1a·H 2O-1bMe
,
2a-2bMe·0.5H2O, N(10)-Ru(2)-N(6), Cl(5)-Ru(2)-Cl(6), N(8)-
Ru(2)-Cl(4) angles in 1bMe and N(10)-Ru(2)-N(6), N(8)-Ru(2)-
Cl(3), S(2)-Ru(2)-Cl(4) angles in 2bMe·0.5H2O are in the range of
168.17(14)-178.66(5)°, indicating almost linear alignment (Table
S1 and S2 in the Supporting Information).
crystallography.
Each
Ru(III)
in
1a·H 2O,
1b·DMF,
distorted
1c·DMF·MeOH, 1d·2DMF and 1bMe exhibits
a
octahedral coordination geometry surrounded by three N atoms
and three Cl atoms. Each Ru(II) atom in 2a, 2b·CH2Cl2,
2c·CH2Cl2, 2d·CH2Cl2 and 2bMe·0.5H2O is octahedrally
coordinated by three N atoms, one S atom from DMSO and two
cis-Cl atoms. As shown in Table S1 and S2 (in the Supporting
Information), the Ru-N (center pyridyl ring) bond length is shorter
than other Ru-N bond distances in all cases. The Ru-Cl lengths
in Ru(II) complexes range from 2.4217(13) Å to 2.4685(12) Å,
which are longer than those in Ru(III) complexes (2.3283(4)-
2.3959(11) Å). This elongation is likely due to the steric
hindrance caused by bulky DMSO ligand. In Ru(II) complexes,
Catalysis
In order to examine the catalytic activity of the as-prepared
Ru(III)
and
Ru(II)
complexes,
we
chose
(2-
aminophenyl)methanol (3a) and 1-phenylethan-1-ol (4a) as
model substrates for ADC reaction (Table 1). The initial
experiment was performed by stirring a mixture of 3a (1 mmol),
4a (1.2 mmol), 1a (0.01 mmol) and KOH (0.5 mmol) in toluene at
110 °C under a closed N2 atmosphere. After 24 h, the desired
product 2-phenylquinoline (5aa) could be isolated in 77% yield
(entry 1). For a comparative purpose, the precatalysts 1b-1d
and various (nonfunctional) Ru complexes such as RuCl3,
RuCl2(DMSO)4, Ru2Cl4(p-Cy)2 (p-Cy = p-cymene), RuCl2(PPh3)3
the Ru-S lengths are 2.1972(14)
Å (2a), 2.2003(16) Å
(2b·CH2Cl2), 2.2006(14)/2.2050(15) Å (2c·CH2Cl2), 2.2054(19) Å
(2d·CH2Cl2) and 2.2006(11) Å (2bMe·0.5H2O) (Table S1 and S2),
which are shorter than those found in trans-[RuCl2(DMSO-
κS)(terpy)] (terpy = 2,2’:6’,2’’-terpyridine) (2.2734(10) Å),[86]
(PPh3
=
triphenylphosphine),
Ru3(CO)12, RuCl3(terpy),
RuCl3(pzpypz) (pzpypz
=
2,6-di(1H-pyrazol-1-yl)pyridine),
RuCl3(pzpypzMe) (pzpypzMe = 2,6-bis(3,5-dimethyl-1H-pyrazol-1-
yl)pyridine) were examined under similar conditions (Table 1,
entries 2-12). Complexes 1a-1d showed higher activity for this
trans,mer-[RuCl2(bpea)(DMSO-κS)]
(bpea
=
N,N-bis-(2-
pyridylmethyl)ethylamine) (2.2236(13) Å)[87] and cis,fac-
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10.1002/cctc.201900435
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ADC reaction, and 1b was proved the most efficient. The
screening of different bases revealed that strong bases were the
more conducive, and KOtBu gave the highest yields of product,
while weak bases were inactive (Table 1, entries 13-17). Yields
were slightly lower in 1,4-dioxane, mesitylene and t-amyl alcohol
(entries 18-20). The reaction performed in DMSO gave 5aa in
9% yield (entry 21). The ADC reaction did not proceed in DMF
(entry 22). Decreasing the catalyst loading from 1 mol% to 0.5
mol% or 0.25 mol%, led to lower yields of 5aa but higher TONs
(entries 23 and 24). When the reaction was performed under a
N2 flow, 5aa was obtained in higher yields with higher TONs,
even at 1b loading of only 0.5 mol% or 0.25 mol%. Lowering the
reaction temperature from 110 °C to 100 °C had almost no effect
(entries 25 vs 27), however, the yield of 5aa got decreased from
91% to 73% (entries 26 vs 28) with 0.25 mol% 1b loading at this
lower temperature. Finally, blank experiments indicated that both
the catalyst and the base were indispensable for this ADC
reaction. The optimized reaction conditions were therefore fixed
to be 1b (0.5 mol%), KOtBu (0.5 eq.), 100 °C for 24 h under an
N2 atmosphere in toluene (1 mL). Under these standard reaction
conditions, RuCl3/L2 gave 5aa in 69% yield with TON of 138
(entry 31). 5aa could also be obtained in high yields (92-94%)
using 2a-2d as catalysts (entries 32-35).
The 1-phenylethan-1-ols with a -Me or -OMe group substituted
at different positions of the phenyl ring showed different reaction
rates in the order ortho- < meta- < para-substituted (5aj < 5al <
5ab, 5ak < 5am < 5ac). The reaction of 3a with 1-([1,1'-
biphenyl]-4-yl)ethan-1-ol (4i), 1-(naphthalen-1-yl)ethan-1-ol (4o)
and 1-(naphthalen-2-yl)ethan-1-ol (4p) proceeded efficiently,
forming 5ai and 5ao-5ap in 82-92% yields. Interestingly, the
heteroatom-containing
secondary
alcohols
1-(pyridin-3-
yl)ethanol and 1-(thiophen-2-yl)ethanol, also underwent these
ADC reactions with 3a to produce the desired quinolines (5aq,
5ar) in 65-69% yields. Secondary alcohols with different alkyl
chains led to the desired products 5as-5au in 76-78% yields with
a catalyst loading of 1.0 mol%. 1-Phenylpropan-1-ols with -CH3
or -Cl substituents could also be converted to the corresponding
3-methyl-2-(p-tolyl)quinolone (5av) and 2-(4-chlorophenyl)-3-
methylquinoline (5aw) with satisfactory yields under the
standard
conditions.
Cyclohexanol
(4x)
and
bicyclo[2.2.1]heptan-2-ol (4y) reacted to give 5ax-5ay in
moderate yields (51-53%). The cross-coupling of butan-2-ol (4z)
with (2-aminophenyl)methanol (3a) gave 2-ethylquinoline (5az)
in 43% yield and 2,3-dimethylquinoline (5az’) in 24% yield. We
next turned our attention to explore the scope of 2-aminobenzyl
alcohols. (2-Amino-5-methylphenyl)methanol (3b) and (2-amino-
4-chlorophenyl)methanol (3c) were smoothly converted to 6-
methyl-2-phenylquinoline (5ba) and 7-chloro-2-phenylquinoline
(5ca), respectively, in 62-82% yields, under our standard
conditions. Interestingly, 1-(2-aminophenyl)ethan-1-ol (3d) was
also a suitable substrate for the ADC reaction giving 3-methyl-2-
phenylquinoline (5da) in 58% yield. The scale-up reaction of 3a
(1.23 g, 10 mmol) with 4a (1.46 g, 12 mmol) produced 5aa (1.52
g) in 74% isolated yield.
This efficient formation of quinoline derivatives encouraged us
to further investigate the synthesis of pyridines via ADC
reactions of secondary alcohols with γ-amino alcohols (Table 3).
Both electron-donating (-Me, -OMe, -NMe2) and electron-
withdrawing (-F, -Cl, -Br) substituents on 1-phenylethan-1-ols
reacted with 3-amino-3-phenylpropan-1-ol (6a) to yield the
desired products in good to moderate yields. 1-Phenylethan-1-
ols with electron-rich or electron-neutral groups reacted
smoothly with 6a in 55-68% isolated yields (7aa-7ad and 7aj-
7am). Using the electron-withdrawing substituted 1-phenylethan-
1-ols (4f-4h) as coupling partners, the corresponding products
(7af-7ah) were obtained in 51-58% yields. Compared with para-
or meta-substituted substrates, the ortho-substituted 1-(2-
methylphenyl)ethan-1-ol (4j) and 1-(2-methoxyphenyl)ethan-1-ol
(4k) could afford 2-(2-methylphenyl)-6-phenylpyridine (7aj) and
2-(2-methoxyphenyl)-6-phenylpyridine (7ak), respectively, in
slightly lower isolated yields. Notably, [1,1'-biphenyl]-4-yl and 2-
naphthyl substituted ethan-1-ol gave the corresponding
pyridines in 67-81% yields. Heteroatom-containing alcohols also
went smoothly to give the corresponding pyridines in 51% yield
(7aq) and 53% yield (7ar). 2,3,6-Trisubstituted pyridines were
obtained by using 1-(4-methylphenyl)propan-1-ol (4v) and 1-(4-
chlorophenyl)propan-1-ol (4w) as substrates. Notably, 3-
aminobutan-1-ol (6b) with 4a provided the desired product 2-
methyl-6-phenylpyridine (7ba) in 52% yield.
Figure 2. Molecular structures of 1b (a), 2b (b), 1bMe (c) and 2bMe (d).
A variety of secondary alcohols and 2-aminobenzyl alcohols
were employed to explore the substrate scope of this reaction
(Table 2). Moderate to excellent yields of quinolines were
obtained with a series of secondary alcohols tolerating various
functional groups. 1-Phenylethan-1-ol (4) bearing an electron-
donating -CH3, -OCH3, or -N(CH3)2 substituent group reacted
with 3a to form the desired products 5ab-5ad and 5aj-5am in
73-89% yields. 1-Phenylethan-1-ols with electron-withdrawing
substituents -CF3, -F, -Cl, and -Br proceeded with a good
conversion to the corresponding quinolines (5ae-5ah and 5an).
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10.1002/cctc.201900435
ChemCatChem
Table 1. Optimization of reaction conditions for the synthesis of 2-
phenylquinoline (5aa).
FULL PAPER
Table 2. Synthesis of quinolines.[a]
Entry[a]
Cat.
mol%
Ru
Base
Solvent
Yield
(%)
TON
1
1a
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
KOH
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
1,4-dioxane
mesitylene
77
77
90
81
87
53
51
57
66
31
52
72
68
80
95
-
2
1b
KOH
90
3
1c
KOH
81
4
1d
KOH
87
5
RuCl3
KOH
53
6
RuCl2(DMSO)4
Ru2Cl4(p-Cy)2
RuCl2(PPh3)3
Ru3(CO)12
RuCl3(terpy)
RuCl3(pzpypz)
KOH
51
7
KOH
57
8
KOH
66
9
KOH
31
10
11
12
13
14
15
16
17
18
19
20
KOH
52
KOH
72
RuCl3(pzpypzMe
)
KOH
68
1b
1b
1b
1b
1b
1b
1b
1b
NaOH
KOtBu
Na2CO3
K2CO3
Cs2CO3
KOtBu
KOtBu
KOtBu
80
95
trace
trace
trace
90
[a] Reaction conditions: 3 (1 mmol), 4 (1.2 mmol), 1b (0.5 mol%), KOtBu
(0.5 eq.), toluene (1 mL), 100 °C, under a slow N2 flow, 24 h, isolated yield.
[b] Gram scale reaction (10 mmol 3a and 12 mmol 4a). [c] 1b (1 mol%).
-
-
Table 3. Synthesis of pyridines.[a]
90
86
90
86
t-amyl
90
alcohol
21
1b
1
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
-
DMSO
DMF
9
9
22
1b
1
trace
88
-
23
1b
0.5
0.25
0.5
0.25
0.5
0.25
0.5
-
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
176
300
192
364
190
292
-
24
1b
75
25[b]
26[b]
27[b,c]
28[b,c]
29[b,c]
30[b,c]
31[b,c]
32[b,c]
33b,c]
34[b,c]
35[b,c]
1b
96
1b
91
1b
95
1b
73
1b
trace
trace
69
-
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
-
[a] Reaction conditions: 6 (1 mmol), 4 (1.2 mmol), 1b (1 mol%), KOtBu (0.5
eq.), toluene (1 mL), 110 °C, under a slow N2 flow, 24 h, isolated yield.
RuCl3/L2
2a
0.5
0.5
0.5
0.5
0.5
138
186
188
184
186
93
The catalytic performance of 1b was compared to those of
other known catalysts using the synthesis of 2-
phenylquinoline from (2-aminophenyl)methanol and 1-
phenylethan-1-ol as a model reaction (Table S3 in the
2b
94
2c
92
2d
93
[a] Reaction conditions: 3a (1 mmol), 4a (1.2 mmol), cat., base (0.5 equiv.),
solvent (1 mL), 110 °C, closed N2, 24 h, HPLC yield. [b] Under a slow N2 flow. [c]
At 100 °C.
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10.1002/cctc.201900435
ChemCatChem
FULL PAPER
Supporting Information). Catalyst 1b showed better activity with
higher TOFs than nonfunctionalized catalysts including
RuCl(CO)(PPh3)2(saz) (H2saz = salicylaldazine)[38] and Ru(η6-p-
cymene)Cl(pad) (pad = pyrrole-2-aldehydehydrazones).[39] It was
also superior to the functionalized phosphine-containing
same ADC reactions, indicating that a Ru(II) species is the true
catalytically active species, consistent with previous reports.[82,88]
The precatalyst Ru(III) complex 1 is reduced in situ to an active
Ru(II) species in the presence of KOtBu and alcohol under the
reaction conditions, that was further confirmed by electrospray
ionization mass spectrometry (ESI-MS), see following. Only 4a
as a substrate under standard reaction conditions generated the
dehydrogenative product acetophenone (4a’) and homo-
coupling product 1,3-diphenylbutan-1-one (4a’’) (Scheme S1 in
the Supporting Information). A Ru-H signal was detected at δ = -
complexes like RuClH(CO)(dpbp) (dpbp
butylphosphanyl)methyl)-2,2'-bipyridine),[40]
=
6-((di-tert-
Mn(CO)3(btpea)Br
(btpea
=
2-(tert-butylthio)-N-(pyridin-2-ylmethyl)ethan-1-
(E)-2-(2-(1-(pyridin-2-
amine),[54] MnBr(CO)2(pehp) (pehp
=
yl)ethylidene)hydrazinyl)pyridine),[53] but inferior to the more
sophisticated complexes such as RuClH(CO)(dpmppa) (dpmppa
1
10.60 ppm in the H NMR spectrum of the mixture formed from
=
N-(di-tert-butylphosphanyl)-6-(1-methyl-1H-pyrazol-3-
the dehydrogenation of 4a using 2b (5 mol%), KOH (0.5 mmol)
in toluene-d8 (Equation 3, Scheme 3, and Scheme S2).
Meanwhile, using 1b as catalyst, a cationic species at m/z =
486.0877 was observed in the ESI-MS, corresponding to the
formation of Ru-H species [HRuCl(L2)(CH3OH)2(H2O) + H]+
(calcd for m/z = 486.0840) (Figure 3). These results confirmed
the dehydrogenative step in the ADC reaction sequence and the
involvement of a cooperative Ru-H/N-H bifunctional mechanism.
yl)pyridin-2-amine),[42] RuCl(PPh3)2(phpy) (phpy
=
6-(1,10-
phenanthrolin-2-yl)pyridin-2-ol)[43] and RuCl2(CO)(bphm) (bphm
= [2,2'-bipyridin]-6-yl(6-hydroxypyridin-2-yl)methanone).[44] The
catalytic activity of 1b for the synthesis of 2,6-diphenylpyridine
was slightly lower than the phosphine-supported complexes
such
as
RuH(CO)(PPh3)(dppeqa)
(dppeqa
=
N-(2-
(diphenylphosphanyl)ethyl)-5,6,7,8-tetrahydroquinolin-8-
amine),[41] RuClH(CO)(dpbp),[40] RuClH(CO)(dpmppa),[42] and
RuCl2(CO)(bphm)[44] (Table S4 in the Supporting Information).
Figure 3. The ESI-MS spectrum of Ru-H species. The calculated isotope
patterns (up) and observed patterns (bottom) of [RuClH(L2)(CH3OH)2(H2O) +
H]+.
Reaction mechanism studies
The Pd/C catalyzed hydrogenation of 2-vinylnaphthalene was
used as a probe for the liberation of molecular hydrogen in the
ADC reactions (Equation 1, Scheme 3). The Ru catalyzed
synthesis of 5aa was carried out in one reactor under standard
conditions with a second, connected reactor containing 2-
vinylnaphthalene and Pd/C. Approximately one equivalent of 2-
vinylnaphthalene was converted to 2-ethylnaphthalene as
determined by GC-MS (see Supporting Information), indicating
the generation of molecular hydrogen in the ADC reaction. The
influence of the NH functionality was investigated with
complexes 1bMe and 2bMe. Only moderate yields of 5aa were
isolated using either 1bMe or 2bMe as a catalyst (Equation 2,
Scheme 3). This lower activity suggests a positive influence of
the N-H group through an outer-sphere mechanism of reaction.
Complexes 1b and 2b showed similar catalytic activity for the
Scheme 3. Control experiments.
In the presence of KOtBu, the condensation of
aminobenzaldehyde (3a’) with 4a’ resulted in the formation of
5aa with or without 1b in 61-63% yields, indicating that 4a’ and
3a’ were the intermediates of the ADC reaction (Equation 4 in
Scheme 3). Under the standard condition, reaction of 3a with 4a’
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10.1002/cctc.201900435
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FULL PAPER
or 4a with 3a’ yielded 5aa in 93% and 67% yields, respectively
(Equations 5 and 6 in Scheme 3). Additionally, we found the
loading of KOtBu had a significant influence on the activity of 1b
(Equation 7 in Scheme 3). When the proportion of KOtBu
decreased from 50 mol% to 20 mol%, the yield of 5aa
decreased from 95% to 31%. The addition of 4Å molecular sieve
(0.7 g) increased the yield to 86% in the presence of 20 mol%
KOtBu.
complexes by reactions of RuCl3 or RuCl2(DMSO)4 with NH-
containing N˄N˄N-pincer ligands. The Ru(III)-type complexes 1a-
1d and 1bMe could be in situ converted to Ru(II)-type ones 2a-2d
and 2bMe under reducing conditions in the presence of DMSO.
These bifunctional N˄N˄N-Ru(III)/Ru(II) complexes, particularly
1b and 2b, showed high catalytic efficiency and broad functional
group tolerance in the ADC reactions of secondary alcohols with
2-aminobenzyl and γ-amino alcohols to quinolines and pyridines.
These catalysts are easy to synthesize on a large scale and
stable to air and moisture. Mechanism studies indicate a Ru-
H/N-H bifunctional process. For Ru(III) complexes, the in situ
generated Ru(II) species is the active catalytic center for
acceptorless dehydrogenation coupling and auto-transfer-
hydrogen reactions. It is anticipated that the phosphine-free
adducts of simple metal salts with NH-containing pincer ligands
might have high catalytic activity towards other organic
transformations.
Taken previous reports[40-44,82,88] into account, we propose a
possible mechanism for Ru(II) catalyzed ADC reaction as
depicted in Scheme 4. Initially, the precatalyst Ru(III) complex
like 1b is reduced in situ to a Ru(II) species I under the reaction
conditions. Such a Ru(II) species I is converted into Ru(II) alkoxy
intermediate II in the presence of KOtBu and alcohol. The
intermediate II transfers the α-H from alkoxy group to Ru(II)
atom, thus producing one hydrido species III and ketone 4’. The
intermediate III subsequently evolves H2 to yield I. Reaction of 4’
with 3 or 6 yields the intermediate imine alcohol IV, which is then
dehydrogenated to imine ketone intermediate V followed by
cyclization into the intermediate tertiary alcohol VI and
dehydration, or dehydration and hydrogenation to the final
products 5 or 7.
Experimental Section
General information: 2-Bromo-6-(1H-pyrazol-1-yl)pyridine, 2-bromo-6-
(3,5-dimethyl-1H-pyrazol-1-yl)pyridine, 6-(3,5-dimethyl-1H-pyrazol-1-yl)-
N-(pyridin-2-yl)pyridin-2-amine (L2) and 6-(3,5-dimethyl-1H-pyrazol-1-yl)-
N-methyl-N-(pyridin-2-yl)pyridin-2-amine (L2Me) were prepared according
to published procedures.[84,85] All other reagents obtained from
commercial sources were directly used without further purification. All
solvents were obtained from commercial sources and were purified
according to standard procedures. Column chromatography was
1
performed on silica gel. H NMR spectra were recorded at 400 MHz and
13C NMR spectra were measured at 150 MHz or 100 MHz using Varian
UNITY plus-400 spectrometer or Bruker Avance III HD spectrometer with
CDCl3, [D6]DMSO or CD3OD or [D8]toluene as the solvents. Elemental
analyses for C, H, and N were performed on a Carlo-Erbo CHNO-S
microanalyzer. The IR spectra (KBr disk) were recorded on a Nicolet
MagNa-IR550 FT-IR spectrometer (4000-400 cm-1). Electrospray ion
mass spectra (ESI-MS) were recorded on an Agilent 1200/6200 mass
spectrometer using MeCN as the mobile phase. High resolution mass
spectra (HRMS) were obtained with a GCT Premier (Micromass UK
Limited) chemical ionization time-of-flight mass spectrometer (CI-TOF).
The uncorrected melting points were determined on a Mel-Temo II
apparatus.
Synthesis of 2-bromo-6-(3-(trifluoromethyl)-1H-pyrazol-1-yl)pyridine:
This compound was prepared according to the published procedures.[85]
To a toluene solution (30 mL) of 2,6-dibromopyridine (1.18 g, 6.0 mmol)
was added 3-(trifluoromethyl)-1H-pyrazole (0.68 g, 5.0 mmol), 1,10-
phenanthroline (0.18 g, 1.0 mmol), CuI (0.10 g, 0.5 mmol) and K2CO3
(0.69 g, 5.0 mmol) under a N2 atmosphere. The mixture was stirred at
110 °C for 24 h. After cooling to ambient temperature, water and ethyl
acetate were added into the mixture. The organic layer was separated
and the aqueous layer was extracted with ethyl acetate three times. The
combined organic phase was washed with brine, dried over anhydrous
Na2SO4 and filtered. The solvent was removed under reduced pressure
and the residue was purified by silica gel column chromatography using
petroleum ether and ethyl acetate as eluents to afford 2-bromo-6-(3-
Scheme 4. Proposed mechanism.
(trifluoromethyl)-1H-pyrazol-1-yl)pyridine as a white solid (1.24 g, 85%).
+
Mp: 53.1-54.0 °C. HRMS (CI-TOF) m/z: [M + H]+, calcd for C9H6BrF3N3
:
291.9692; found: 291.9705. Anal. calcd for C9H6BrF3N3: C, 37.01; H,
1.73; N, 14.39%. Found: C, 36.89; H, 1.67; N, 14.13%. 1H NMR (400
MHz, CDCl3, ppm): δ 8.59 (s, 1H), 7.99 (d, J = 8.1 Hz, 1H), 7.71 (t, J =
7.9 Hz, 1H), 7.46 (t, J = 5.9 Hz, 1H), 6.72 (s, 1H). 13C NMR (151 MHz,
Conclusions
In summary, we have synthesized a series of phosphine-free
tridentate ligands coordinated N˄N˄N-Ru(III) and Ru(II)
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10.1002/cctc.201900435
ChemCatChem
FULL PAPER
CDCl3, ppm): δ 141.2, 140.3, 140.2, 129.1, 128.6, 127.3, 126.9, 111.6,
111.5, 106.5, 106.4. IR (KBr disk, cm-1): 3142 (w), 3100 (w), 1587 (s),
1567 (s), 1474 (m), 1446 (s), 1389 (s), 1278 (s), 1177 (m), 1126 (s), 970
(m), 794 (m), 777 (m).
Anal. calcd for C19H15N5: C, 72.83; H, 4.83; N, 22.35%. Found: C, 73.01;
1
H, 4.76; N, 22.45%. H NMR (400 MHz, DMSO-d6, ppm): δ 9.89 (s, 1H),
8.60 (s, 1H), 8.27 (d, J = 4.1 Hz, 1H), 7.97 (d, J = 7.5 Hz, 2H), 7.84 (dd, J
= 15.0, 7.8 Hz, 2H), 7.77 (t, J = 7.5 Hz, 1H), 7.60 (d, J = 8.2 Hz, 1H), 7.48
(t, J = 7.5 Hz, 3H), 7.39 (t, J = 7.1 Hz, 1H), 7.09 (s, 1H), 6.94 (t, J = 5.7
Hz, 1H). 13C NMR (151 MHz, DMSO-d6, ppm): δ 153.9, 153.2, 152.6,
149.0, 147.5, 140.4, 137.8, 132.4, 128.8, 128.3, 125.6, 116.4, 111.9,
109.2, 105.5, 102.7. IR (KBr disk, cm-1): 3257 (w), 3079 (w), 3004 (w),
1618 (m), 1597 (w), 1570 (w), 1529 (m), 1473 (m), 1455 (w), 1437 (s),
1365 (w), 1334 (w), 1295 (w), 1231 (w), 1156 (m), 1044 (w), 984 (w), 792
(w), 777 (m), 746 (m).
Synthesis of 2-bromo-6-(3-phenyl-1H-pyrazol-1-yl)pyridine: White
solid 2-bromo-6-(3-phenyl-1H-pyrazol-1-yl)pyridine was obtained in a
similar method described above by using 3-phenyl-1H-pyrazole (0.72 g,
5.0 mmol) and 2,6-dibromopyridine (1.18 g, 6.0 mmol). Yield: 1.25 g
(83%). Mp: 88.5-89.2 °C. HRMS (CI-TOF) m/z: [M + H]+, calcd for
C14H11BrN3+: 300.0131; found: 300.0139. Anal. calcd for C14H10BrN3: C,
56.02; H, 3.36; N, 14.00%. Found: C, 56.08; H, 3.46; N, 14.21%. 1H NMR
(400 MHz, CDCl3, ppm): δ 8.56 (d, J = 2.3 Hz, 1H), 8.04 (d, J = 8.1 Hz,
1H), 7.92 (d, J = 7.5 Hz, 2H), 7.65 (t, J = 7.9 Hz, 1H), 7.45 (t, J = 7.4 Hz,
2H), 7.38 (d, J = 7.2 Hz, 1H), 7.34 (d, J = 7.8 Hz, 1H), 6.79 (d, J = 2.3 Hz,
1H). 13C NMR (151 MHz, CDCl3, ppm): δ 154.5, 151.6, 140.8, 140.0,
132.8, 129.0, 128.9, 128.7, 126.2, 125.2, 111.1, 106.0. IR (KBr disk, cm-
1): 3029 (w), 1585 (m), 1564 (s), 1545 (s), 1503 (w), 1465 (w), 1454 (w),
1417 (s), 1159 (w), 1136 (m), 980 (m), 793 (s), 775 (m), 753 (s).
Synthesis of L1: This compound was prepared in a similar manner to 6-
Synthesis of 1a: A mixture of ligand (L1) (0.237 g, 1.0 mmol) and
RuCl3•xH2O (0.262 g, 1.0 mmol) was placed in a two-necked flask
equipped with a stirring bar and 15 mL anhydrous ethanol was added
under a N2 atmosphere. The mixture was refluxed for 6 h and then
cooled to ambient temperature. The mixture was filtered and the brown
solid filtrate was washed with diethyl ether (3 × 10 ml) and dried in vacuo.
Yield: 0.347
g (78%). HRMS (ESI) m/z: [M +
Na]+, calcd for
C13H11Cl3N5NaRu+: 466.9016; found: 466.9005. Anal. calcd for
C13H11Cl3N5Ru: C, 35.11; H, 2.49; N, 15.75%. Found: C, 35.22; H, 2.73;
N, 15.58%. IR (KBr disk, cm-1): 3287 (w), 3196 (w), 3109 (m), 1637 (m),
1580 (m), 1528 (w), 1469 (s), 1424 (w), 1399 (m), 1251 (w), 1179 (w),
1070 (w), 1021 (w), 801 (w), 766 (m).
(3,5-dimethyl-1H-pyrazol-1-yl)-N-(pyridin-2-yl)pyridin-2-amine (L2).[84]
A
mixture of 2-bromo-6-(1H-pyrazol-1-yl)pyridine (1.12 g, 5.0 mmol),
pyridin-2-amine (0.71 g, 7.5 mmol), Pd2(dba)3 (0.09 g, 0.1 mmol), dppf
(0.11 g, 0.2 mmol) and KOtBu (1.12 g, 10.0 mmol) was added into a
Schlenk tube equipped with a stirring bar and 25 mL toluene was added
under a N2 atmosphere. The reaction mixture was stirred at 110 °C for 12
h before cooling to ambient temperature. The mixture was then filtered
through a short pad of Celite and rinsed with 50 mL of CH2Cl2. The
combined filtrate was concentrated under reduced pressure and the
residue was purified by column chromatography on silica gel using
petroleum ether and ethyl acetate as eluents to afford L1 as a white solid
(0.87 g, 73%). Mp: 113.2-113.7 °C. HRMS (CI-TOF) m/z: [M + H]+, calcd
for C13H12N5+: 238.1087; found: 238.1093. Anal. calcd for C13H11N5: C,
65.81; H, 4.67; N, 29.52%. Found: C, 65.70; H, 4.81; N, 29.33%. 1H NMR
(400 MHz, DMSO-d6, ppm): δ 9.87 (s, 1H), 8.54 (s, 1H), 8.26 (d, J = 4.1
Hz, 1H), 7.80 (t, J = 5.7 Hz, 3H), 7.75 (t, J = 7.5 Hz, 1H), 7.59 (d, J = 8.2
Hz, 1H), 7.37 (d, J = 7.7 Hz, 1H), 6.93 (t, J = 5.7 Hz, 1H), 6.58 (s, 1H).
13C NMR (151 MHz, DMSO-d6, ppm): δ 153.9, 153.2, 149.1, 147.5, 141.8,
140.4, 137.8, 126.7, 116.4, 111.9, 109.2, 107.9, 102.7. IR (KBr disk, cm-
1): 3252 (m), 3180 (w), 2991 (w), 1618 (w), 1600 (m), 1570 (w), 1532 (m),
1467 (m), 1435 (s), 1349 (w), 1315 (w), 1205 (w), 1145 (w), 1036 (w),
775 (m), 751 (m).
Synthesis of 1b: This complex was prepared in a similar method
described above by using L2 (0.265 g, 1.0 mmol) and RuCl3•xH2O (0.262
g, 1.0 mmol) in EtOH. Yield: 0.359 g (76%). HRMS (ESI) m/z: [M + Na]+,
calcd for C15H15Cl3N5NaRu+: 494.9329; found: 494.9308. Anal. calcd for
C15H15Cl3N5Ru: C, 38.11; H, 3.20; N, 14.81%. Found: C, 37.84; H, 3.54;
N, 14.80%. IR (KBr disk, cm-1): 3294 (w), 3191 (w), 3100 (m), 1642 (m),
1583 (m), 1530 (m), 1463 (s), 1426 (m), 1364 (w), 1278 (w), 1246 (m),
1179 (m), 1168 (m), 793 (m), 775 (m).
Synthesis of 1c: This complex was prepared in a similar method
described above for 1a, using L3 (0.305 g, 1.0 mmol) and RuCl3•xH2O
(0.262 g, 1.0 mmol) in EtOH. Yield: 0.364 g (71%). HRMS (ESI) m/z: [M -
Cl]+, calcd for C14H10Cl2F3N5Ru+: 476.9303; found: 476.9297. Anal. calcd
for C14H14Cl3F3N5Ru: C, 32.80; H, 1.97; N, 13.66%. Found: C, 31.52; H,
2.05; N, 13.38%. IR (KBr disk, cm-1): 3295 (w), 3205 (w), 3108 (w), 1636
(m), 1582 (m), 1529 (m), 1470 (s), 1423 (w), 1381 (w), 1253 (m), 1183
(m), 1156 (m), 806 (w), 768 (m).
Synthesis of 1d: This complex was prepared in a similar method
described above for 1a, using L4 (0.313 g, 1.0 mmol) and RuCl3•xH2O
(0.262 g, 1.0 mmol) in EtOH. Yield: 0.375 g (72%). HRMS (ESI) m/z: [M -
Cl]+, calcd for C19H15Cl2N5Ru+: 484.9743; found: 484.9757. Anal. calcd
for C19H15Cl3N5Ru: C, 43.82; H, 2.90; N, 13.45%. Found: C, 43.69; H,
3.18; N, 13.48%. IR (KBr disk, cm-1): 3300 (w), 3205 (w), 3095 (w), 1640
(m), 1582 (m), 1529 (m), 1471 (s), 1450 (m), 1396 (w), 1359 (m), 1251
(m), 1168 (m), 1063(w), 1021 (w), 798 (w), 778 (w), 761 (m).
Synthesis of 1bMe: This complex was prepared in a similar method
described above for 1a, using L2Me (0.279 g, 1.0 mmol) and RuCl3•xH2O
(0.262 g, 1.0 mmol) in EtOH. Yield: 0.365 g (75%). HRMS (ESI) m/z: [M -
Cl]+, calcd for C16H17Cl2N5Ru+: 450.9899; found: 450.9907. Anal. calcd
for C16H17Cl3N5Ru: C, 39.48; H, 3.52; N, 14.39%. Found: C, 39.00; H,
3.68; N, 14.11%. IR (KBr disk, cm-1): 3135 (w), 3087 (w), 1601 (w), 1576
(m), 1456 (s), 1427 (m), 1353 (w), 1312 (w), 1235 (w), 1194 (w), 1169
(w), 1061 (m), 786 (w), 773 (m).
Synthesis of L3: White solid L3 was obtained using a procedure similar
to that employed for L1, but using 2-bromo-6-(3-(trifluoromethyl)-1H-
pyrazol-1-yl)pyridine (1.46 g, 5.0 mmol) and pyridin-2-amine (0.71 g, 7.5
mmol) as starting materials. Yield: 1.02 g (67%). Mp: 136.9-137.8 °C.
HRMS (CI-TOF) m/z: [M + H]+, calcd for C14H11F3N5+: 306.0961; found:
306.0969. Anal. calcd for C14H10F3N5: C, 55.08; H, 3.30; N, 22.94%.
Found: C, 55.11; H, 3.60; N, 22.83%. 1H NMR (400 MHz, DMSO-d6,
ppm): δ 10.00 (s, 1H), 8.71 (s, 1H), 8.28 (d, J = 4.4 Hz, 1H), 7.88 (t, J =
8.0 Hz, 1H), 7.77 (s, 2H), 7.71 (d, J = 8.3 Hz, 1H), 7.39 (d, J = 7.7 Hz,
1H), 7.07 (s, 1H), 6.96 (s, 1H). 13C NMR (151 MHz, DMSO-d6, ppm): δ
153.7, 153.3, 148.1, 147.5, 143.0, 142.8, 140.8, 140.5, 137.9, 129.3,
116.7, 112.0, 110.8, 106.2, 103.2. IR (KBr disk, cm-1): 3279 (m), 3053 (w),
2998 (w), 1617 (m), 1596 (w), 1570 (w), 1474 (m), 1462 (w), 1437 (s),
1389 (m), 1338 (w), 1299 (s), 1275 (m), 1147 (m), 1122 (s), 798 (w), 787
(w), 774 (s).
Synthesis of complexes 2. Method A: Under a nitrogen atmosphere,
Et3N (0.3 mL), DMSO (0.5 mL) and anhydrous ethanol (10 mL) were
added into a mixture of complex 1 (0.2 mmol) and LiCl (0.0168 g, 0.4
mmol). The mixture was then refluxed with stirring overnight. After it was
cooled to ambient temperature, it was filtered and a light yellow powder 2
was collected, which was washed with Et2O (5 × 10 mL) and dried in
Synthesis L4: White solid L4 was obtained using a procedure similar to
that employed for L1, but using 2-bromo-6-(3-phenyl-1H-pyrazol-1-
yl)pyridine (1.50 g, 5.0 mmol) and pyridin-2-amine (0.71 g, 7.5 mmol) as
starting materials. Yield: 1.12 g (72%). Mp: 132.9-133.6 °C. HRMS (CI-
TOF) m/z: [M + H]+, calcd for C19H16N5+: 314.1400; found: 314.1413.
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10.1002/cctc.201900435
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vacuo. Method B: Under
a
nitrogen atmosphere,
a
mixture of
13.23%. Found: C, 41.02; H, 4.16; N, 13.33%. 1H NMR (400 MHz,
CD3OD, ppm): δ 9.61 (s, 1H), 8.22–8.09 (m, 1H), 8.06 (d, J = 4.7 Hz, 1H),
7.71 (d, J = 7.0 Hz, 1H), 7.62–7.51 (m, 1H), 7.37 (d, J = 6.9 Hz, 1H), 7.29
(d, J = 5.1 Hz, 1H), 6.58 (s, 1H), 3.75 (s, 3H), 3.14 (s, 3H), 2.88 (s, 6H),
2.83 (s, 3H). 13C NMR (151 MHz, CD3OD, ppm): δ 161.0, 158.9, 158.2,
155.2, 153.2, 146.7, 141.3, 140.9, 121.0, 117.5, 115.1, 113.0, 107.6,
47.8, 46.1, 43.2, 15.4, 15.2. IR (KBr disk, cm-1): 3130 (w), 3095 (w), 2995
(w), 1598 (m), 1577 (m), 1479 (w), 1458 (s), 1429 (s), 1392 (w), 1371 (w),
1347 (m), 1229 (w), 1080 (m), 1003 (m), 988 (w), 801 (w), 795 (m), 774
(m).
[RuCl2(DMSO)4] (0.097 g, 0.2 mmol) and L (0.2 mmol) was refluxed in
ethanol (10 mL) with stirring for 6 h. After cooling to ambient temperature,
it was filtered and a light yellow powder 2 was collected, which was
washed with Et2O (5 × 10 mL) and dried in vacuo.
Compound 2a: Yield: 0.088 g (90%, method A), 0.070 g (72%, method
2). HRMS (ESI) m/z: [M - Cl]+, calcd for C15H17ClN5ORuS+: 451.9880;
found: 451.9892. Anal. calcd for C15H17Cl2N5ORuS: C, 36.97; H, 3.52; N,
14.37%. Found: C, 37.05; H, 3.47; N, 14.28%. 1H NMR (400 MHz,
CD3OD, ppm): δ 9.70 (d, J = 6.1 Hz, 1H), 9.08 (d, J = 2.8 Hz, 1H), 8.46 (s,
1H), 8.04 (t, J = 8.1 Hz, 1H), 7.93 (t, J = 7.8 Hz, 1H), 7.75 (d, J = 7.9 Hz,
1H), 7.30 (d, J = 8.3 Hz, 1H), 7.17 (d, J = 8.3 Hz, 1H), 7.09 (t, J = 6.4 Hz,
1H), 7.02 (s, 1H), 2.80 (d, J = 4.3 Hz, 6H). 13C NMR (101 MHz, CD3OD,
ppm): δ 154.1, 151.5, 146.8, 140.9, 140.2, 132.6, 118.7, 116.1, 112.3,
112.2, 104.5, 45.7, 45.6. IR (KBr disk, cm-1): 3312 (w), 3293 (w), 3191
(w), 3079 (m), 1640 (m), 1581 (m), 1533 (m), 1483 (s), 1464 (s), 1418
(m), 1409 (m), 1346 (m), 1251 (m), 1180 (m), 1100 (w), 1086 (m), 1014
(w), 804 (w), 782 (m), 771 (m).
Typical procedure for the synthesis of quinolones: A mixture of 2 (1
mmol), 3 (1.2 mmol), 1b (0.5 mol%) and KOtBu (0.5 mmol) was added
into a 25 mL Schlenk tube equipped with a stirring bar. Toluene (1 mL)
was added under a N2 atmosphere and the reaction mixture stirred at
100 °C for 24 h. After cooling to ambient temperature, 5 mL water was
added and the mixture was extracted three times with ethyl acetate (3 × 5
mL). The combined organic layer was washed with brine, dried over
anhydrous Na2SO4 and concentrated under reduced pressure. The crude
product was purified by flash column chromatography using petroleum
ether and ethyl acetate as eluents.
Compound 2b: Yield: 0.095 g (92%, method A), 0.075 g (73%, method
2). HRMS (ESI) m/z: [M - Cl]+, calcd for C17H21ClN5ORuS+: 480.0193;
found: 480.0181. Anal. calcd for C17H21Cl2N5ORuS: C, 39.62; H, 4.11; N,
13.59%. Found: C, 39.68; H, 3.98; N, 13.65%. 1H NMR (400 MHz,
CD3OD, ppm): δ 9.88 (dd, J = 6.2, 1.2 Hz, 1H), 8.00 (t, J = 8.3 Hz, 1H),
7.93–7.86 (m, 1H), 7.56 (d, J = 8.1 Hz, 1H), 7.28 (d, J = 8.4 Hz, 1H), 7.14
(d, J = 8.3 Hz, 1H), 7.10–7.03 (m, 1H), 6.57 (s, 1H), 2.93 (s, 3H), 2.87 (d,
J = 2.8 Hz, 6H), 2.76 (s, 3H). 13C NMR (101 MHz, CD3OD, ppm): δ 163.4,
160.6, 153.7, 146.2, 140.6, 140.1, 118.5, 115.7, 115.6, 111.4, 105.6,
46.8, 45.7, 15.8, 15.6. IR (KBr disk, cm-1): 3308 (w), 3193 (w), 3075 (m),
3019 (m), 1647 (m), 1597 (w), 1580 (m), 1533 (m), 1488 (s), 1463 (m),
1437 (m), 1394 (w), 1363 (w), 1247 (m), 1176 (w), 1087 (m), 805 (w),
771 (m).
Typical procedure for the synthesis of pyridines: A mixture of 5 (1
mmol), 3 (1.2 mmol), 1b (1.0 mol%) and KOtBu (0.5 mmol) was added
into a 25 mL Schlenk tube equipped with a stirring bar. Toluene (1 mL)
was added under a N2 atmosphere. The reaction mixture was stirred at
110 °C for 24 h, cooled to ambient temperature and 5 mL of water was
added into the mixture which was extracted three times with ethyl acetate
(3 × 5 mL). The combined organic layer was washed with brine, dried
over anhydrous Na2SO4 and concentrated under reduced pressure. The
crude product was purified by flash column chromatography using
petroleum ether and ethyl acetate as eluents.
Single crystal X-ray crystallography: Dark green crystals of 1a•H2O,
1b•DMF, 1c•DMF•MeOH, 1d•2DMF, and 1bMe were obtained by slow
diffusion of Et2O onto the solution of each complex in CHCl3 or
DMF/MeOH. Orange crystals of 2a, 2b•CH2Cl2, 2c•CH2Cl2, 2d•CH2Cl2
and 2bMe•0.5H2O were obtained by diffusion of diethyl ether into their
solution in CH2Cl2/MeOH (1:1). Single crystals were mounted on glass
fibers with grease and cooled in a liquid nitrogen stream to 210 K
(1a•H2O, 1b•DMF, 1c•DMF•2H2O, 1d•2DMF, 1bMe, 2a, 2b•CH2Cl2, and
2d•CH2Cl2) or 193 K (2c•CH2Cl2 and 2bMe•0.5H2O). Crystallographic
measurements were made on a Bruker APEX-II CCD diffractometer
Compound 2c: Yield: 0.095 g (86%, method A), 0.083 g (75%, method
2). HRMS (ESI) m/z: [M - Cl]+, calcd for C16H16ClF3N5ORuS+: 519.9754;
found: 519.9769. Anal. calcd for C16H16Cl2F3N5ORuS: C, 34.60; H, 2.90;
N, 12.61%. Found: C, 34.73; H, 3.06; N, 12.48%. 1H NMR (400 MHz,
CD3OD, ppm): δ 10.06 (d, J = 6.2 Hz, 1H), 9.34 (d, J = 2.6 Hz, 1H), 8.12
(t, J = 8.2 Hz, 1H), 7.93 (t, J = 7.9 Hz, 2H), 7.40 (d, J = 2.7 Hz, 1H), 7.29
(dt, J = 12.0, 5.8 Hz, 2H), 7.10 (t, J = 6.5 Hz, 1H), 2.93 (s, 3H), 2.74 (s,
3H). 13C NMR (101 MHz, CD3OD, ppm): δ 154.1, 141.0, 140.5, 134.6,
131.6, 118.8, 116.2, 116.2, 113.6, 106.2, 45.5, 40.6. 19F NMR (377 MHz,
CD3OD) δ -59.06. IR (KBr disk, cm-1): 3315 (w), 3196 (w), 3092 (m),
3073 (w), 1645 (m), 1584 (m), 1537 (m), 1499 (m), 1483 (s), 1461 (m),
1419 (w), 1383 (m), 1295 (m), 1271 (w), 1253 (w), 1159 (s), 1135 (w),
1087 (m), 1017 (w), 974 (w), 917 (w), 792 (w), 783 (m), 771 (w).
using graphite-monochromated Mo-Kα (λ
= 0.71070 Å) radiation.
Diffraction data were collected at a ω mode with a detector distance of 35
mm to the crystal. The collected data were reduced using the program
Bruker APEX2 and an absorption correction (multi-scan) was applied.
The reflection data were also corrected for Lorentz and polarization
effects.
Compound 2d: Yield: 0.101 g (90%, method A), 0.087 g (77%, method
2). HRMS (ESI) m/z: [M - Cl]+, calcd for C21H21ClN5ORuS+: 528.0193;
found: 528.0205. Anal. calcd for C21H21Cl2N5ORuS: C, 44.76; H, 3.76; N,
12.43%. Found: C, 44.95; H, 3.61; N, 12.63%. 1H NMR (400 MHz,
CD3OD, ppm): δ 9.81 (d, J = 6.0 Hz, 1H), 9.12 (d, J = 2.5 Hz, 1H), 8.05 (t,
J = 8.2 Hz, 1H), 7.87 (t, J = 7.8 Hz, 1H), 7.75 (d, J = 7.9 Hz, 1H), 7.61 (d,
J = 7.1 Hz, 2H), 7.52–7.46 (m, 1H), 7.43 (t, J = 7.1 Hz, 2H), 7.26 (d, J =
8.3 Hz, 1H), 7.16 (d, J = 8.4 Hz, 1H), 6.99 (t, J = 6.5 Hz, 1H), 6.90 (d, J =
2.5 Hz, 1H), 2.80 (s, 3H), 2.64 (s, 3H). 13C NMR (101 MHz, CD3OD,
ppm): δ 154.1, 153.9, 133.2, 131.5, 130.7, 128.9, 116.0, 111.7, 46.8,
45.3. IR (KBr disk, cm-1): 3308 (w), 3192 (w), 3099 (m), 3078 (w), 1639
(m), 1595 (w), 1581 (m), 1533 (m), 1485 (s), 1470 (s), 1451 (m), 1439
(w), 1415 (w), 133 (w), 1367 (w), 1250 (m), 1171 (m), 1088 (m), 1015 (w),
790 (m), 771 (m), 752 (m).
All crystal structures were solved by direct methods and refined on F2
by full-matrix least-squares techniques with SHELXTL-2014/7
program.[89] In 1a•H2O, one H2O molecule occupied two sites with an
occupancy ratio of 0.5/0.5 for O1/O1A. In 1c•DMF•MeOH, the
uncoordinated MeOH molecule occupied two sites with an occupancy
ratio of 0.5/0.5 for O2/O2A and C18/C18A. In 1bMe, the three methyl
groups of L2Me occupied two sites with an occupancy ratio of 0.5/0.5 for
C14/C14A, C15/C15A, and C16/C16A. In 2b•CH2Cl2, one Cl atom of
CH2Cl2 occupied two sites with an occupancy ratio of 0.5/0.5 for Cl3/Cl3A.
In 2c•CH2Cl2, the two Cl atoms from CH2Cl2 occupied two sites with an
occupancy ratio of 0.5/0.5 for Cl7/Cl7A and Cl8/Cl8A. Except for O1 in
1a•H2O, C14, C15 and C16 atoms in 1bMe, Cl3 in 2b•CH2Cl2, Cl7 and Cl8
in 2c•CH2Cl2, all non-H atoms were refined on F2 anisotropically by the
full-matrix least square method. Hydrogen atoms of the disordered H2O
molecule (O1 in 1a•H2O), disordered MeOH molecule (O2 and C18 in
Compound 2bMe: Yield: 0.094 g (89%, method A), 0.078 g (74%, method
2). HRMS (ESI) m/z: [M - Cl]+, calcd for C18H23ClN5ORuS+: 494.0350;
found: 494.0354. Anal. calcd for C18H23Cl2N5ORuS: C, 40.83; H, 4.38; N,
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10.1002/cctc.201900435
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1c•DMF•MeOH), one methyl group of L2Me (C14 in 1bMe) were not
located. The H atom of each N2 atom from L1-L4 ligands in all complexes,
N7 from L3 in 2c•CH2Cl2 and the H atoms of O2 from uncoordinated H2O
molecule 2bMe•0.5H2O were located from Fourier maps. All other H
atoms were introduced at the calculated positions and included in the
structure-factor calculations. Crystal data along with data collection and
refinement parameters for 1a•H2O, 1b•DMF, 1c•DMF•MeOH, 1d•2DMF,
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10.1002/cctc.201900435
ChemCatChem
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Bifunctional pincer Ru(III) and Ru(II)
complexes catalysed synthesis of
quinolones and pyridines through
acceptorless dehydrogenative
coupling (ADC) reactions of γ-amino
alcohols and secondary alcohols via a
Ru-H/N-H bifunctional outer-sphere
mechanism.
Bin Guo, Tian-Qi Yu, Hong-Xi Li,* Shi-Qi
Zhang, Pierre Braunstein, David J.
Young, Hai-Yan Li, and Jian-Ping Lang*
Page No. – Page No.
Phosphine Ligand-free Ruthenium
Complexes as Efficient Catalysts for
the Synthesis of Quinolines and
Pyridines by Acceptorless
Dehydrogenative Coupling Reactions
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