An air and moisture tolerant iminotrihydroquinoline-ruthenium(ii) catalyst for the transfer hydrogenation of ketones

Article abstract of DOI:10.1039/c8dt01919c

Reaction of 8-amino-5,6,7,8-tetrahydroquinoline with RuCl₂(PPh₃)₃ at room temperature affords the ruthenium(ii) chelate (8-NH₂-C₉H₁₀N)RuCl₂(PPh₃)₂ (E), in which the two triphenylphosphine ligands are disposed mutually cis. By contrast, when the reaction is performed at reflux ligand oxidation/dehydrogenation occurs along with cis-trans reorganization of the triphenylphosphines to form the 8-imino-5,6,7-trihydroquinoline-ruthenium(ii) complex, (8-NH-C₉H₉N)RuCl₂(PPh₃)₂ (F). Complex F can also be obtained in higher yield by heating a solution of E alone to reflux. Comparison of their molecular structures highlights the superior binding properties of the bidentate imine ligand in F over its amine-containing counterpart in E. Both complexes are highly effective in the transfer hydrogenation of a wide range of alkyl-, aryl- and cycloalkyl-containing ketones affording their corresponding secondary alcohols with loadings of as low as 0.1 mol%. Significantly, F can deliver excellent conversions even in bench quality 2-propanol in reaction vessels open to the air, whereas the catalytic efficiency of E is diminished by the presence of air but only operates efficiently under inert conditions.

Full text of DOI:10.1039/c8dt01919c

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Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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DOI: 10.1039/C8DT01919C
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ARTICLE
An air and moisture tolerant iminotrihydroquinoline‐ruthenium(II)
catalyst for the transfer hydrogenation of ketones
Received 00th January 20xx,
Accepted 00th January 20xx
Jiaoyan Li,a,# Yingmiao Ma,a,# Zheng Wang,a,b,c,# Qingbin Liu,a,* Gregory A. Solan,d,* Yanping Mab
and Wen‐Hua Sunb,c,
*
DOI: 10.1039/x0xx00000x
Abstract: Reaction of 8‐amino‐5,6,7,8‐tetrahydroquinoline with RuCl2(PPh3)3 at room temperature affords the ruthenium(II)
chelate (8‐NH2‐C9H10N)RuCl2(PPh3)2 (E), in which the two triphenylphosphine ligands are disposed mutually cis. By contrast,
when the reaction is performed at reflux ligand oxidation/dehydrogenation occurs along with cis‐trans reorganization of
the triphenylphosphines to form the 8‐imino‐5,6,7‐trihydroquinoline‐ruthenium(II) complex, (8‐NH‐C9H9N)RuCl2(PPh3)2 (F).
Complex F can also be obtained in higher yield by heating a solution of E alone to reflux. Comparison of their molecular
structures highlights the superior binding properties of the bidentate imine ligand in F over its amine‐containing
counterpart in E. Both complexes are highly effective in the transfer hydrogenation of a wide range of alkyl‐, aryl‐ and
cycloalkyl‐containing ketones affording their corresponding secondary alcohols with loadings of as low as 0.1 mol%.
Significantly, F can deliver excellent conversions even in bench quality 2‐propanol in reaction vessels open to the air,
whereas the catalytic efficiency of E is diminished by the presence of air but only operates efficiently under inert
conditions.
www.rsc.org/
Introduction
However, to the best of our knowledge, there are only limited
The hydrogenation of unsaturated carbonyl and imine substrates
has attracted considerable attention with regard to developing more
sustainable, efficient and environmentally friendly processes.1,2
Besides the significant breakthrough made by Noyori,3 some recent
developments in metal‐mediated transfer hydrogenation4‐7 have
been achieved through ligand modification of Ru‐1b,4i‐n,6 and Os‐
catalysts,1b,4b,c,5e and in particular those based on phosphine and
amino‐containing alkylpyridines by the groups of Baratta,4 Yu6 and
Mezzetti7 (see A – D in Chart 1). Driven by demand for such
processes that can operate effectively in the presence of moisture
and oxygen,8 good stability, air/water tolerance and straightforward
preparation represent desirable features of a potential catalyst.
reports of efficient transfer hydrogenation catalysts that are
capable of tolerating such conditions.1a‐b
R2
Cl
N
N
N
Ph3P
Ph2P
R1
NH2
Cl
Cl
P
Ru
Cl
P
M
N
M
Cl
P
H
P
NH2
A4o
C4b,4d,4k
B4l
M = Ru, Os,
N
Ph3P
M = Ru, Os,
N
N
N
N
Cl
Cl
N
N
NH2
NH
Ru
H
Ru
Cl
Ph3P
Ru
Cl
a College of Chemistry and Material Science, Hebei Normal University, Shijiazhuang
050024, China
PPh3
PPh3
Cl
PPh3
Cl
D6d
F
E
b
Key Laboratory of Engineering Plastics and Beijing National Laboratory for
this work
Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing
Chart 1. Ruthenium and osmium complexes that have been used
effectively in transfer hydrogenation (A – D) along with the systems to
be developed in this work (E and F)
100190, China
c CAS Research/Education Center for Excellence in Molecular Sciences, University of
Chinese Academy of Sciences, Beijing 100049, China
d
Department of Chemistry, University of Leicester, University Road, Leicester LE1
7RH, UK
# Jiaoyan Li, Yingmiao Ma and Zheng Wang made equal contributions in this work.
†Corresponding Authors: whsun@iccas.ac.cn; liuqingb@sina.com; gas8@leicester.
ac.uk (G.A.S.)
Electronic Supplementary Information (ESI) available: Figures, tables, and giving
NMR spectra of the new complexes and a selected substrate; CCDC 1586459 for E,
In this article, we are concerned with the preparation of two
structurally related ruthenium(II) complexes namely E and F (Chart
1), with the aim to explore their independent use in the transfer
hydrogenation of ketones to secondary alcohols. In particular, we
are interested in investigating how the distinct imine and primary
amine donors impact on catalytic efficiency as well as on their
tolerance to air and moisture conditions. Full details of the
synthetic procedures for complexes and ligand synthesis are
CCDC 1586458 for
format see DOI:
F. For ESI and crystallographic data in CIF or other electronic
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presented as is an in‐depth investigation of their catalytic
performance in transfer hydrogenation of a diverse range of alkyl‐,
aryl‐ and cycloalkyl‐containing ketones.
View Article Online
DOI: 10.1039/C8DT01919C
Results and discussion
Synthesis and characterization
Figure 1. ORTEP representation of E. The thermal ellipsoids are shown
at the 30% probability level; hydrogen atoms have been omitted for
clarity. Bond lengths [Å] and angles [deg]: Ru1‐Cl1 2.4239(17), Ru1‐Cl2
2.4294(17), Ru1‐P1 2.3258(16), Ru1‐P2 2.3535(16), Ru1‐N1 2.152(5),
Ru1‐N2 2.161(5), N2‐C6 1.473(8); P2‐Ru1‐Cl2 96.80(6), P2‐Ru1‐Cl1
87.97(6), Cl1‐Ru1‐Cl2 167.74(6), P1‐Ru1‐P2 98.09(6), N1‐Ru1‐Cl1
82.45(15), N1‐Ru1‐Cl2 90.47(15), N2‐Ru1‐Cl2 84.47(16), N2‐Ru1‐Cl1
84.07(16), N2‐C6‐C5 108.2(5).
Scheme 1. Synthetic routes to E and F
In the first instance we focused on synthesizing (8‐NH2‐
C9H10N)RuCl2(PPh3)2
(E)
by
reacting
8‐amino‐5,6,7,8‐
tetrahydroquinoline9 with RuCl2(PPh3)3.10 Pleasingly, with
dichloromethane as the solvent and under ambient conditions, E
was isolated in 85% yield after one hour (Scheme 1). With a view to
potentially forming the trans geometrical isomer of E, we also
performed the reaction in toluene at reflux over 16 hours.
Unexpectedly, on work‐up the oxidized/dehydrogenated imine‐
containing product, (8‐NH‐C9H9N)RuCl2(PPh3)2 (F), was isolated in
65% yield (Scheme 1). Moreover, heating E in toluene at reflux for 4
hours also resulted in this ligand oxidation/dehydrogenation to give
Figure 2. ORTEP representation of F. The thermal ellipsoids are shown
at the 30% probability level; hydrogen atoms have been omitted for
clarity. Bond lengths [Å] and angles [deg]: Ru1‐Cl1 2.4451(9), Ru1‐Cl2
2.4440(8), Ru1‐P1 2.3606(8), Ru1‐P2 2.3678(8), Ru1‐N1 2.057(3), Ru1‐
N2 2.059(3), C6‐N2 1.319(5); P1‐Ru1‐Cl1 89.37(3), P1‐Ru1‐Cl2 88.16(2),
Cl1‐Ru1‐Cl2 96.77(3), P1‐Ru1‐P2 176.56(3), N1‐Ru1‐Cl1‐168.53(8), N1‐
Ru1‐Cl2 94.70(8), N2‐Ru1‐Cl1 89.46(9), N2‐Ru1‐Cl2,173.75(10), N2‐C6‐
C7 114.8(3).
F in 90% yield (Scheme 1). Indeed, monitoring of this reaction by 31
P
NMR spectroscopy in CDCl3 showed that full conversion to F could
be achieved after just one hour at 100 oC (in a closed reactor) (see
1
SI). Both ruthenium complexes have been characterized by H, 13C,
and 31P NMR spectroscopy, elemental analysis and have been the
Both structures consist of a ruthenium center surrounded by
two nitrogen atoms belonging to a neutral N,N‐chelating ligand, two
chlorides and two triphenylphosphines to complete a distorted
octahedral arrangement. The key difference between the structures
relates to the nature of the N,N ligand (viz. 8‐amino‐5,6,7,8‐
tetrahydroquinoline in E and 8‐imino‐5,6,7‐trihydroquinoline in F)
and the disposition of each pair of phosphines or chlorides.
Specifically in E the phosphines are cis [P1‐Ru1‐P2 98.09(6)o] and
the chlorides trans [Cl1‐Ru1‐Cl2 167.74(6) o], while in F the
phosphines are trans [P1‐Ru1‐P2 176.56(3)o] and the chlorides cis
[Cl2‐Ru1‐Cll 96.77(3) o]. With regard to the N,N ligand, the N1‐C5‐
C6‐N2 torsion angles (‐30.14o E, 3.85o F) highlight the deviation
from co‐planarity in E as a result of the sp3‐hybridized CH‐NH2
carbon (N2‐C6‐C5 108.2(5)o); in F this distortion is minimized with
the incorporation of an imine C=NH unit into the chelate ring.
Comparison of the C6‐N2 distances in E (1.473(8) Å) and F (1.319(5)
Å) further supports the presence of an imine unit in F. This variation
in donor atoms of the N,N‐ligand also affects the Ru‐N distances
subject of single crystal X‐ray diffraction studies.
The 8‐amino‐5,6,7,8‐tetrahydroquinoline ligand itself, is not
commercially available and can be prepared in two steps from 6,7‐
dihydro‐5H‐quinolin‐8‐one using a literature procedure.9,11‐13 As an
additional purification step, the resulting oil was converted to its
salt, 8‐amino‐5,6,7,8‐tetrahydroquinoline hydrochloride, before
being neutralized to afford 8‐amino‐5,6,7,8‐tetrahydroquinoline in
high purity in 42% yield.
Crystals of E and F suitable for the X‐ray determinations were
grown by the slow diffusion of n‐pentane into their corresponding
dichloromethane solutions. Views of each structure are shown in
Figures 1 and 2; selected bond distances and angles are given in the
figure captions.
2 | J. Name., 2012, 00, 1‐3
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Table 1. The effect of base and catalyst on tVhieew AtrrtaicnlesOfenrline
with those in E (2.152(5), 2.161(5) Å) longer than in F (2.057(3),
2.059(3) Å), underlining the more effective binding of the 8‐imino‐
5,6,7‐trihydroquinoline in F. There are also some differences in the
Ru‐P distances with those in E [2.3258(16), 2.3535(16) Å] shorter
than those in F (2.3606(8), 2.3678(8) Å). Likewise, the Ru‐Cl
distances in E (2.4294(17), 2.4239(17) Å) are shorter than those in F
(2.4451(9), 2.4440(8) Å). There are no intermolecular contacts of
note.
hydrogenation of acetophenonea
DOI: 10.1039/C8DT01919C
Entry
Catalyst
Base
Conversion (%)b
1
2
3
E
E
E
None
t‐BuOK
t‐BuONa
‐‐
94
87
The 31P{1H} NMR spectrum (recorded in CDCl3) of E shows two
mutually coupled doublets at  42.98 and 39.09 with a two‐bond
coupling constant of ca. 31 Hz, consistent with a cis arrangement of
4
5
6
7
8
9
10
E
E
E
E
NaOMe
i‐PrONa
KOH
NaOH
t‐BuOK
t‐BuOK
84
63
65
63
‐‐
92
44
1
the two phosphine ligands.4o,10 In its H NMR spectrum, signals for
the aliphatic CH2 and CH protons belonging to 8‐amino‐5,6,7,8‐
tetrahydroquinoline ligand are seen as multiplets in the range 1.5
‐ 3.5. In the 13C{1H} NMR spectrum, the CHN carbon is seen at δ
58.97 for E which is only shifted slightly downfield with respect to
that seen in the free ligand (δ 47.09). For F the 31P{1H} NMR
spectrum shows two relatively close doublets at δ 69.31 and 66.49
with a 2J(PP) mutual coupling of ca. 113 Hz indicating that the two P
atoms are in slightly different environments; this inequivalency is
likely due to non‐planarity of the saturated ring in the 8‐imino‐
None
F
RuCl2(PPh3)3/ t‐BuOK
8‐NH2‐C9H10N
a Conditions: 8 mmol acetophenone, 8 μmol catalyst (0.1 mol%), 0.8
mmol base (10 mol%), 20 mL i‐PrOH, 82 oC, 30 minutes.
b
Determined by GC: based on acetophenone consumption with
dodecane as the internal standard.
To ascertain the optimal quantity of t‐BuOK required, the
conversion of acetophenone was monitored with the amount of E
fixed at 0.1 mol% and the loading of t‐BuOK varied between 2 and
20 mol% (Table 2). A peak of 94% conversion was observed after 30
minutes using 10 mol% of t‐BuOK.
1
5,6,7‐trihydroquinoline chelating ligand. As with E, the H NMR
spectrum of F shows signals for the CH2 protons of the N,N ligand as
multiplets between δ 1.57 and 2.61, while the C=NH proton is
assigned as a 1H‐singlet at  8.47. In the 13C{1H} NMR spectrum, the
C=NH carbon is seen clearly at δ 160.60. Related ligand
dehydrogenation involving the transformation of a R2CH‐NH2 unit to
a R2C=NH group has been previously reported for complexes
containing pyridylalkylamines and is likely that conversion of E to F
follows in a similar manner.14
Table 2. Transfer hydrogenation of acetophenone using E at
different loadings of t‐BuOKa
OH
O
O
OH
0.1 mol% E
oC
t-B
uO
K, i-P
Ph
Me
Ph
rOH
, 82
Me
Catalytic evaluation in transfer hydrogenation
Entry
mol% of t‐BuOK
Conversion (%)b
To explore the potential of the 8‐amine‐containing E and imine F to
serve as catalysts for the transfer hydrogenation of ketones, E was
used in the first instance to allow an optimization of the conditions
(Table 1). The transfer hydrogenation of acetophenone to 1‐
phenylethanol was chosen as the transformation to be screened
and a preliminary study initiated to determine the optimal catalyst
loading, type of base as well as the most suitable loading of base.
The reactions were typically performed with freshly distilled
and degassed 2‐propanol at 82 oC over 30 minutes under an
atmosphere of nitrogen. Initially, a selection of different bases, t‐
BuOK, t‐BuONa, NaOMe, KOH and NaOH, was investigated with the
loading of base set at 10 mol% and the loading of E fixed at 0.1
mol% (Table 1). Of the five bases, t‐BuOK was found to achieve the
best conversion of 94% (entry 2, Table 1); a similarly high
conversion was notably achieved when F was used in place of E
with t‐BuOK again as the base (entry 9, Table 1). Meanwhile the
blank tests performed in the absence of base (entry 1, Table 1) or
without ruthenium catalyst showed no conversion to 1‐
phenylethanol (entry 8, Table 1). In addition, it was found that the
1
2
3
4
2
5
10
65
85
94
92
20
a Experimental conditions: 8 mmol acetophenone, 8 μmol E, 20 mL i‐PrOH,
monitored at 82 oC after 30 minutes.
b Determined by GC analysis: based on acetophenone consumption with
dodecane as the internal standard.
With the amount of t‐BuOK now fixed at 10 mol%, the amount
of E was changed between 0.1 and 0.025 mol% resulting in
conversions of 94% (0.1 mol%), 93% (0.05 mol%), 58% (0.03 mol%)
and 16% (0.025 mol%) (Table 3). This lowering in conversions may
be ascribed to the quicker decomposition of the lower
concentration active species under strongly basic conditions.8
Overall the optimal amounts of catalyst and base for the transfer
hydrogenation were established as 0.1 mol% E and 10 mol% t‐BuOK.
In order to investigate the tolerance of the ruthenium catalyst to
air and moisture, the dry and degassed 2‐propanol used initially was
replaced with bench 2‐propanol (analytical reagent) and the
transfer hydrogenation of acetophenone carried out in the air. It
was observed that the efficiency of E was greatly affected and the
conversion dramatically decreased to 51% after 30 minutes with
little improvement after 60 minutes (entry 1, Table 4). By contrast,
combination
of
RuCl2(PPh3)2
and
8‐amino‐5,6,7,8‐
tetrahydroquinoline as catalyst exhibited only a modest conversion
(44%, entry 10, Table 1).
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BuOK at 82 oC over 30 minutes with bench 2V‐pierwoApraticnleoOl.nliTnoe
complement this study, E was employed inDaOIp: a10ra.1l0le3l9i/nCv8eDsTt0ig1a9t1i9oCn
using the same ratio of substrate: catalyst: base, but under inert
conditions and using dry and degassed solvent. Notably, the
hydrogenation of all twenty ketones was achieved using F with
conversions between 60 and 100% (entries 1‐20, Table 5). Ketones
incorporating aryl groups containing both electron withdrawing
(entries 1‐4, Table 5) and donating groups (entry 7, Table 5) were
equally well hydrogenated. Similarly, ketones containing n‐alkyl
groups with and without halide substituents could be readily
transformed (entries 8, 11, 12, 13, Table 5). In the same way, the
cyclic ketones, cyclohexanone, cyclopentanone, adamantan‐2‐one,
cyclododecanone along with cyclic systems appended with ester
and acetal groups could be transformed to their corresponding
alcohols with good conversions (entries, 14‐17, 19, 20, Table 5).
Indeed, the lowest conversion of 60% was obtained with the
bis(arene)‐fused cyclopentanone, 9‐fluorenone. Inspection of the
results obtained using E show good conversions albeit obtained
under more rigorous conditions. Furthermore, high isolated yields
of 90 and 88% of tert‐butyl‐4‐hydroxypiperidine‐1‐carboxylate were
obtained using both E and F, respectively (entry 17, Table 5). Clearly,
both electronic and steric effects associated with the particular
ketone influence the reactivity of E and F. For example, the
electron‐rich aromatic ketones (entries 8 ‐ 10, Table 5) catalyzed by
E gave somewhat higher yields of the corresponding alcohols when
compared to that seen with F. Overall, this study highlights not only
the versatility of imine‐containing F as a transfer hydrogenation
catalyst but also its ability to operate effectively in air and moisture‐
containing environments, conditions that lend themselves to
industrial applications.
Table 3. Transfer hydrogenation of acetophenone at different
catalyst loadings of E with t‐BuOK as basea
Entry
mol% of E
0.1
Conversion (%)b
1
2
3
4
94
93
58
16
0.05
0.03
0.025
a Reaction conditions: 8 mmol acetophenone, 0.8 mmol t‐BuOK, 20 mL
i‐PrOH, monitored at 82 oC after 30 minutes.
b
The conversion to the product was measured by GC: based on
acetophenone consumption with dodecane as the internal standard.
Table 4. Effects of air and water on the transfer hydrogenation of
acetophenone using E or F
Conversion (%)
Entry
Cat.
atm.
H2O
10 min
44
75
92
91
30 min
51
92
94
93
60 min
54
92
94
93
1a
2a
3a
4b
5b
E
F
E
E
E
air
air
N2
N2
N2
none
none
none
5 L
83
88
91
10 L
a Conditions: 8 mmol acetophenone, 8 mol catalyst (0.1 mol%), 0.8 mmol
t‐BuOK, 20 mL i‐PrOH, 82 oC and open to the air or N2; conversion was
monitored by GC after 10, 30 and 60 minutes.
b Conditions as in ‘a’ but with distilled and degassed i‐PrOH.
replacing E with F with the conditions of the transfer hydrogenation
otherwise the same (bench 2‐propanol and an air atmosphere),
acetophenone was converted to 1‐phenylethanol in 92% yield
(entry 2, Table 4). It is apparent that F is less sensitive to oxygen and
apparently more tolerant to the conditions than E. Indeed, when
oxygen gas was separately passed through a solution of E and t‐
Conclusions
In summary, synthetic routes to 8‐amino‐5,6,7,8‐tetrahydro‐
quinoline‐containing E and imino‐5,6,7‐trihydroquinoline‐containing
o
F
have been developed with the latter accessible by a
BuOK in 2‐propanol for 1 hour at 82 C, triphenylphosphine oxide
dehydrogenative pathway involving E. Each complex has been
independently assessed as a catalyst in the transfer hydrogenation
of ketones to give secondary alcohols. Imine‐containing F has
proved an oxygen‐stable catalyst for transfer hydrogenation
allowing the transformations to be effectively carried out in the
open air with bench solvent. By contrast, amine‐containing E
undergoes catalyst deactivation when exposed to air but is
nevertheless an efficient catalyst in the absence of air and under
dry conditions. Furthermore, the scope of E and F to mediate the
transfer hydrogenation of more than twenty examples of ketones
including aryl and alkyl ones as well as cycloalkyl ketones have been
studied resulting in the formation of their corresponding alcohol
products in good to high yields.
(ca. 95%) was isolated (see SI) and what was presumed to be
insoluble ruthenium oxide; similar deactivation of iridium catalysts
has been reported.8 In addition, monitoring of the 31P{1H} NMR
spectrum of E during this catalyst deactivation at intervals of 1 min,
5 min, 10 min and 30 min (see SI), also showed the gradual
formation of triphenylphosphine oxide. Alternatively, if the transfer
hydrogenation using E was conducted under nitrogen with dry
degassed 2‐propanol, a 94% conversion of acetophenone was noted
(entry 3, Table 4). To examine the influence of water, the reaction
mediated by E was performed under nitrogen with controlled
amounts of water introduced (5 μL: entry 4, Table 4) and (10 μL:
entry 5, Table 4). Only slightly lower conversions (entries 4 and 5,
Table 4) were observed suggesting that E displays some tolerance
to moisture.
To examine the general applicability of F as an air and moisture
tolerant catalyst, a broad range of ketone substrates were screened
including aryl, alkyl and cycloalkyl examples differing in their
electronic and steric properties (entries 1‐20, Table 5). Typically, the
catalytic runs were performed in the air using the optimal
conditions established of 8 mmol ketone, 0.1 mol% F, 10 mol% t‐
4 | J. Name., 2012, 00, 1‐3
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DOI: 10.1039/C8DT01919C
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Table 5. Exploring the substrate scope of F as an air and moisture tolerant catalyst; the corresponding data for E under inert conditions
are also tabulateda
Entry
R1
Me
Me
Me
Me
Et
R2
Conversion (%) using Fb
TOF (h‐1)
1860
1920
1820
1900
1880
2000
Conversion (%) using Ec
TOF (h‐1)
1940
1920
1840
1980
1800
1440
1
2
3
4
5
6
3‐BrC6H4
4‐BrC6H4
4‐FC6H4
2,4‐Cl2C6H3
Ph
93
96
91
95
94
97
96
92
99
90
86
Et
3‐ClC6H4
100
7
8
Et
Me
4‐MeC6H4
PhCH2CH2
90
91
1800
1820
89
96
1780
1920
9
Ph
Me
Me
Me
Et
Ph
91
88
96
1820
1760
1920
1860
2000
2000
1840
1520
1920
93
94
93
93
94
100
94
70
99
1860
1880
1860
1860
1880
2000
1880
1400
1980
10
11
12
13
14
15
16
17
1‐naphthyl
CH3(CH2)5
Cl(CH2)4
Et
93
100
100
92
76
96
‐CH2(CH2)3CH2‐
‐CH2(CH2)2CH2‐
‐CH2(CH2)9CH2‐
18
19
20
60
95
99
1200
1900
1980
63
96
99
1260
1920
1980
a Reaction conditions: 8 mmol ketone, 8 mol E or F, 0.8 mmol t‐BuOK, 20 mL i‐PrOH, monitored at 82 oC over 30 minutes.
b Using bench i‐PrOH in the air; the conversion was determined by GC: based on ketone consumption with dodecane as the internal standard.
c Using distilled and degassed i‐PrOH under nitrogen; the conversions was determined by GC: based on ketone consumption with dodecane as the
internal standard.
number 19091N‐113 HP‐INNOWAX): injector temp. 300 oC; detector
o
o
Experimental section
temp. 300 C; column temp. 40 C; withdraw time 2 min, then 20
oC/min to 270 C over 20 min. The percentage conversions were
o
determined based on the consumption of the ketone, with
dodecane as an internal standard.15‐17 Complex RuCl2(PPh3)3 was
synthesized according to the literature procedure.10
General information.
All manipulations involving ruthenium complexes were carried out
under a nitrogen atmosphere using standard Schlenk techniques. 2‐
Propanol (analytical reagent) was either used directly from the
bottle or was dried over sodium wire, distilled and stored under
Synthesis of 8‐amino‐5,6,7,8‐tetrahydroquinoline
nitrogen before being degassed prior to use. 1H NMR (500 MHz), 13
C
The two‐step procedure was based on that described in the
literature.9,11‐13 In the first step, 1‐phenylethanamine (10.4 mL, 82.2
mmol) was added to a stirred solution containing 6,7‐dihydro‐5H‐
quinolin‐8‐one (12.0 g, 82.2 mmol) and NaBH(OAc)3 (25.6 g, 120.8
NMR (125 MHz) and 31P (202 MHz) spectra were recorded on a
Bruker AVШ–500 NMR spectrometer. GC analysis was carried out
on an Agilent 6820 instrument using a polar capillary column (part
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1
solid (0.220 g, 60%). H NMR (500 MHz, CDCl3) δ 8.47 (s, 1H, NH),
mmol) in 1,2‐dichloroethane (100 mL) under a nitrogen atmosphere.
The reaction mixture was stirred at room temperature for 16 h
(monitored by TLC) before being quenched with a saturated
aqueous solution of NaHCO3 until basic. The mixture was diluted
with water and extracted with ethyl acetate (3 x 50 mL). The
organic layer was dried with anhydrous Na2SO4 and concentrated
under reduced pressure to afford amine intermediate 1‐phenylethyl
(5,6,7,8‐tetrahydroquinolin‐8‐yl)amine as a yellow liquid (20.0 g,
95%). In the second step, the amine was taken up in acetic acid (4
mL) and dry MeOH (200 mL) and the solution flushed with nitrogen
and transferred to a stainless steel 250 mL autoclave, equipped
with a magnetic stirring bar. 10% Palladium on carbon (5.5 g) was
then added to the mixture. The autoclave was purged by three
cycles of pressurization/venting with hydrogen gas (10 bar), then
pressurized with hydrogen (35 bar), sealed and disconnected from
View Article Online
DOI: 10.1039/C8DT01919C
7.96 – 7.86 (m, 1H, HPy), 7.69 – 7.48 (m, 8H, HPh), 7.24 – 7.14 (m,
12H, HPy), 7.09 – 6.86 (m, 1H, HPy), 6.63 – 6.39 (m, 1H, HPy), 2.61 (d, J
= 35.0 Hz, 2H, CH2), 1.93 (q, J = 24.5 Hz, 2H, CH2), 1.57 (d, J = 15.0 Hz,
2H, CH2). 13C NMR (125 MHz, CDCl3) δ 160.60 (CC=NH), 156.23 (Cpy),
136.64, 136.38, 135.39, 135.32, 134.84, 134.77, 134.37, 133.99,
133.83, 133.49, 133.42, 132.61, 132.24, 132.11, 128.96, 128.69,
128.63, 127.63, 127.47, 127.40, 127.27, 127.20, 122.18, 56.62
(CH2CH), 33.22, 27.80, 21.28. 31P NMR (202 MHz, CDCl3) δ 69.31 (d,
J = 111.2 Hz), 66.49 (d, J = 114.9 Hz). Anal. Calcd for C45H41Cl2N2P2Ru:
C, 64.06; H, 4.90; N, 3.32%. Found: C, 64.18; H, 4.98; N, 3.37%.
Catalyst Deactivation
Oxygen gas was bubbled through a mixture of E (200 mg, 0.23
mmol), t‐BuOK (46 mg, 0.40 mmol) and 2‐propanol (15 mL) using
ultrasound stirring at 82 oC for 1 h. The reaction mixture was
filtered and the resulting black polymeric solid washed with
dichloromethane. The filtrate was then immediately cooled to 0 oC
forming white crystals, which was shown by 1H/13C/31P NMR
spectroscopy as triphenylphosphine oxide (125 mg, 95%).18 1H NMR
(500 MHz, CDCl3) δ 7.63 (dd, J = 11.9, 7.7 Hz, 6H), 7.51 (t, J = 7.4 Hz,
3H), 7.43 (td, J = 7.6, 2.4 Hz, 6H). 13C NMR (125 MHz, CDCl3): δ
132.16, 132.08, 131.94, 131.92, 128.56, 128.46. 31P NMR (202 MHz,
CDCl3) δ 29.14 (s).
o
the hydrogen source. The vessel was stirred and heated to 50 C
(bath temperature) for 18 h. After cooling to room temperature and
venting the hydrogen pressure, the reaction mixture was filtered
and concentrated under reduced pressure to afford a green oil.
Concentrated HCl (35% wt%) (8 ‐ 10 mL) was then added dropwise
followed by cold MeOH to give 8‐amino‐5,6,7,8‐tetrahydroquinoline
hydrochloride as a white solid. The free base could be formed by
treating the HCl salt with an ammonium hydroxide solution and
dichloromethane to give 8‐amino‐5,6,7,8‐tetrahydroquinoline as a
yellow oil (5.5 g, 45%). 1H NMR (400 MHz, CDCl3) δ 8.40 (d, J = 4.7
Hz, 1H, HPy), 7.36 (dd, J = 7.7, 1.5 Hz, 1H, HPy), 7.06 (dd, J = 7.7, 4.7
Hz, 1H, HPy), 4.00 (dd, J = 7.8, 5.4 Hz, 1H, CH), 2.86 – 2.70 (m, 2H,
CH2), 2.23‐2.16 (m, 1H, CH2), 2.03 (s, 2H, NH2), 2.00 – 1.91 (m, 1H,
CH2 ), 1.83 – 1.65 (m, 2H, CH2 ). 13C NMR (100 MHz, CDCl3) δ 159.61,
147.08, 136.80, 131.63,121.73, 51.43 (CH2CH), 32.06, 29.07, 19.99.
General procedure for the transfer hydrogenation of ketones
under nitrogen or air
(a) Under nitrogen. The selected ketonic substrate (8.0 mmol) was
dissolved in dry and degassed 2‐propanol (15 mL) under a nitrogen
atmosphere and the solution stirred and heated to 82 oC. On
reaching this temperature, a solution of base (0.16 – 1.6 mmol) in 2‐
propanol (4 mL) was introduced followed by a solution of either E, F
Synthesis of E4o
(2.0
– 8.0 μmol) or RuCl2(PPh3)3/8‐amino‐5,6,7,8‐tetrahydro‐
10
RuCl2(PPh3)3
(400 mg, 0.42 mmol) and 8‐amino‐5,6,7,8‐
quinoline in 2‐propanol (1 mL), taking the total volume of solvent to
20 mL. At the specified reaction time (10 – 60 min), 0.1 mL of the
reaction mixture was sampled and immediately diluted with 0.5 mL
of 2‐propanol precooled to 0 oC, dodecane introduced, before being
analyzed by GC. The composition of the reaction mixture was
confirmed by running GC of a mixture of pure ketone, alcohol and
dodecane.
tetrahydroquinoline (64.5 mg, 0.44 mmol) were treated with
dichloromethane (5 mL) and the suspension stirred at room
temperature for 1 h. The mixture was then concentrated and n‐
pentane (4 mL) added to afford a yellow precipitate. The precipitate
was filtered, washed with n‐heptane (3 × 1 mL) and dried to give E
as a yellow solid (0.302 g, 85%). 1H NMR (500 MHz, CDCl3) δ 7.67 (d,
J = 7.9 Hz, 1H, HPy), 7.55 (t, J = 7.9 Hz, 5H, HPh), 7.43 (t, J = 8.2 Hz, 5H,
HPh), 7.37 – 7.31 (m, 2H, HPh ), 7.20 (dd, J = 15.7, 7.7 Hz, 7H, HPh),
7.08 – 7.01 (m, 7.6 Hz, 11H, HPh), 6.97 (t, J = 7.7 Hz, 1H, HPy ), 6.58 –
6.50 (m, 1H, HPy), 3.35 (dd, J = 5.0, 2.7 Hz, 1H, CH), 3.06 (dd, J = 7.5,
6.1 Hz, 1H, CH2), 2.78‐2.65 (m, 2H, CH2), 2.05 – 1.74 (m, 3H, CH2).
13C NMR (125 MHz, CDCl3) δ 136.49, 135.23, 135.15, 134.68, 134.61,
133.82, 133.33, 133.26, 128.80, 127.46, 127.39, 127.30, 127.23,
127.10, 127.03, 58.97 (CH2CH), 32.57, 27.08, 21.65. 31P NMR (202
MHz, CDCl3) δ 42.98 (d, J = 31.5 Hz), 39.09 (d, J = 30.9 Hz). Anal.
Calcd for C45H41Cl2N2P2Ru: C, 64.06; H, 4.90; N, 3.32%. Found: C,
64.26; H, 4.95; N, 3.37%.
(b) Under air. The selected ketonic substrate (8.0 mmol) was
dissolved in bench 2‐propanol (15 mL) in a vessel open to the air
and the solution stirred and heated to 82 oC. On reaching this
temperature, a solution of t‐BuOK (0.8 mmol) in 2‐propanol (4 mL)
was introduced followed by a solution of either E or F (6.75 mg, 8.0
μmol) in 2‐propanol (1 mL), taking the total volume of solvent to 20
mL. At the specified reaction time (10 – 60 min), 0.1 mL of the
reaction mixture was sampled and immediately diluted with 0.5 mL
of 2‐propanol precooled to 0 oC, dodecane introduced, before being
analyzed by GC. The composition of the reaction mixture was
confirmed by running GC of a mixture of pure ketone, alcohol and
dodecane.
Synthesis of F
RuCl2(PPh3)3 (400 mg, 0.42 mmol) and 8‐amino‐5,6,7,8‐
tetrahydroquinoline (64.5 mg, 0.44 mmol) were treated with
toluene (10 mL) and stirred for 16 h at 110 oC. On cooling to room
temperature the suspension was filtered and dried to give F as a red
Synthesis of tert‐butyl 4‐hydroxypiperidine‐1‐carboxylate (entry
17, Table 5)
6 | J. Name., 2012, 00, 1‐3
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(a) Using F as catalyst. tert‐Butyl‐4‐oxopiperidine‐1‐carboxylate
(1.59 g, 8.0 mmol) was dissolved in bench 2‐propanol (15 mL) in a
vessel open to the air and the solution stirred and heated to 82 oC.
t‐BuOK (89.6 mg, 0.8 mmol) in 2‐propanol (4 mL) was introduced
followed by a solution of F (6.75 mg, 8.0 μmol) in 2‐propanol (1 mL).
After 30 min of stirring at 82 C, the solution was cooled to room
temperature and concentrated to afford the crude product. The
residue was purified by silica gel chromatography with petroleum
Simon, C. J Li, Chem. Soc. Rev. 2012, 41, 1415; (d) F. Alonso, P.
Riente, M. Yus, Acc. Chem. Res. 2011,D4O4I,: 3107.910;3(9e/)CM8D.TB0a19rt1ó9kC,
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(3) (a) C. A. Sandoval, Y. Li, K. Ding, R. Noyori, Chem. Asian J. 2008,
3, 1801; (b) R. Noyori, Angew. Chem. Int. Ed. 2002, 41, 2006; (c)
K. J. Haack, S. Hashiguchi, A. Fujii, T. Ikariya, R. Noyori, Angew.
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2521; (e) N. Uematsu, A. Fujii, S. Hashiguchi, T. Ikariya, R.
Noyori, J. Am. Chem. Soc. 1996, 118, 4916.
o
ether/EtOAc (v:v
= 3:1) as eluent to afford tert‐butyl‐4‐
hydroxypiperidine‐1‐carboxylate19 as a light yellow oil (1.45 g, 90%).
1H NMR (500 MHz, CDCl3): δ 3.87 – 3.81 (m, 3H), 3.02 (ddd, J = 13.2,
9.7, 3.3 Hz, 2H), 2.17 (s, 1H), 1.92 – 1.76 (m, 2H), 1.48 (d, J = 12.6 Hz,
2H), 1.45 (s, 9H). 13C NMR (125 MHz, CDCl3) δ 154.83, 79.55, 67.79,
41.24, 34.20, 28.44.
(4) (a) A. Volpe, S. Baldino, C. Tubaro, W. Baratta, M. Basato, C.
Graiff, Eur. J. Inorg. Chem. 2016, 247; (b) W. Baratta, L. Fanfoni,
S. Magnolia, K. Siega, P. Rigo, Eur. J. Inorg. Chem. 2010, 1419;
(c) G. Chelucci, S. Baldino, W. Baratta, Acc. Chem. Res. 2015, 48,
363; (d) S. Zhang, W. Baratta, Organometallics 2013, 32,
3339−3342; (e) W. Baratta, M. Ballico, A. Del Zotto, K. Siega, S.
Magnolia, P. Rigo, Chem. Eur. J. 2008, 14, 2557−2563; (f) W.
Baratta, G. Chelucci, S. Magnolia, K. Siega, P. Rigo, Chem. Eur. J.
2009, 15, 726; (g) W. Baratta, M. Ballico, G. Chelucci, K. Siega,
P. Rigo, Angew. Chem., Int. Ed. 2008, 47, 4362; (h) W. Baratta,
M. Ballico, S. Baldino, G. Chelucci, E. Herdtweck, K. Siega, S.
Magnolia, P. Rigo, Chem. Eur. J. 2008, 14, 9148; (i) W. Baratta,
G. Bossi, E. Putignano, P. Rigo, Chem. Eur. J. 2011, 17, 3474; (j)
W. Baratta, F. Benedetti, A. Del Zotto, L. Fanfoni, F. Felluga, S.
Magnolia, E. Putignano, P. Rigo, Organometallics 2010, 29,
3563; (k) S. Facchetti, V. Jurcik, S. Baldino, S. Giboulot, H. G.
Nedden, A. Zanotti‐Gerosa, A. Blackaby, R. Bryan, A. Boogaard,
D. B. McLaren, E. Moya, S. Reynolds, K. S. Sandham, P.
Martinuzzi, W. Baratta, Organometallics, 2016, 35, 277; (l) W.
Baratta, M. Ballico, A. Del Zotto, E. Herdtweck, P. Rigo,
Organometallics 2007, 26, 5636; (m) W. Baratta, G. Chelucci, E.
Herdtweck, S. Magnolia, K. Siega, P. Rigo, Angew. Chem., Int.
Ed. 2007, 46, 7651; (n) E. Putignano, G. Bossi, P. Rigo, W.
Baratta, Organometallics 2012, 31 1133; (o) W. Baratta, E.
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24, 1660.
(b) Using E as catalyst. Using the same procedure and molar ratios
as described in (a) above, but under an atmosphere of nitrogen and
with dry and degassed 2‐propanol as solvent and E as catalyst, tert‐
butyl‐4‐hydroxypiperidine‐1‐carboxylate19 was isolated as a light
1
yellow oil (1.41 g, 88%). The H and 13C NMR data obtained of the
product were as given above.
X‐ray crystallographic studies
The single crystal X‐ray diffraction studies for E and F were carried
out on a Rigaku Saturn 724+ CCD with graphite‐monochromatic Mo‐
K radiation (λ = 0.71073 Å) at 173(2) K. Cell parameters were
obtained by global refinement of the positions of all collected
reflections (See SI, Table S1). Intensities were corrected for Lorentz
and polarization effects and empirical absorption. The structures
were solved by direct methods and refined by full‐matrix least
squares on F2. All hydrogen atoms were placed in calculated
positions. The structural solution and refinement were performed
using the SHELXL‐97 package.20
Associated Content
The authors declare no competing financial interest.
Acknowledgments
(5) X. Liu, C. Chen, A. Chen, L. Gao, R. Zhang, J. Chen, Z. Huo, Catal
Commun. 2015, 67, 90.
This work is supported by the National Natural Science Foundation
of China (Nos. 21476060, and U1362204) and the Nature Science
Foundation of Hebei Province (No. B2014205049), China. G.A.S.
thanks the Chinese Academy of Sciences for a President’s
International Fellowship for Visiting Scientists.
(6) (a) H. Chai, T. Liu, Q. Wang, Z.‐K. Yu, Organometallics 2015, 24,
5278; (b) W. Du, Q. Wang, L. Wang, Z.‐K. Yu, Organometallics
2014, 33, 974; (c) W. Du, P. Wu, Q. Wang, Z.‐K. Yu,
Organometallics 2013, 32, 3083; (d) W. Jin, L. Wang, Z.‐ K. Yu,
Organometallics 2012, 31, 5664; (e) W. Ye, M. Zhao, Z.‐ K. Yu,
Chem. Eur. J. 2012, 18, 10843; (f) W. Du, L. Wang, P. Wu, Z.‐ K.
Yu, Chem. Eur. J. 2012, 18, 11550.
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DOI: 10.1039/C8DT01919C
Graphical for Table of Contents
An air and moisture tolerant iminotrihydroquinoline‐ruthenium(II) catalyst for the
transfer hydrogenation of ketones
Jiaoyan Li,a,# Yingmiao Ma,a,# Zheng Wang,a,b,c,# Qingbin Liu,a,* Gregory A. Solan,d,* Yanping Mab and Wen‐Hua Sunb,c,
*
(#Jiaoyan Li, Yingmiao Ma and Zheng Wang made an equal contribution in this work.)
a College of Chemistry and Material Science, Hebei Normal University, Shijiazhuang 050024, China
b Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, China
c CAS Research/Education Center for Excellence in Molecular Sciences, University of Chinese Academy of Sciences, Beijing
100049, China
d Department of Chemistry, University of Leicester, University Road, Leicester LE1 7RH, UK.
O
OH
N
E or F (0.1 mol%)
R2 i-PrOH, t-BuOK, 82 oC
N
Cl
Ph3P
Cl
R1
NH2
N
Ru
H
Ru
R1
R2
Ph3P
Cl
PPh3
R1, R2 = alkyl, aryl, cycloalkyl
PPh3
Cl
E
F
Both amine‐ and imine‐containing E and F have been prepared by reactions of 8‐amino‐5,6,7,8‐tetrahydroquinoline with RuCl2(PPh3)3, the
latter via a thermally induced route involving ligand oxidation/dehydrogenation. Both E and F are highly effective in the transfer
hydrogenation of a wide range of ketones with F notably operating in bench quality 2‐propanol and in vessels open to the air.
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