Heterogeneous gold-catalyzed selective reductive transformation of quinolines with formic acid

Article abstract of DOI:10.1002/adsc.201400721

Single phase rutile titania supported gold nanoparticles (Au/TiO₂-R) are found to be efficient and versatile catalysts for chemo- and regioselective transfer hydrogenation of quinoline derivatives to 1,2,3,4-tetrahydroquinolines (THQs) using formic acid (FA) as a safe and convenient hydrogen source under mild conditions. The activity and chemoselectivity of the Au/TiO₂-R catalyst towards THQs is excellent, with a substrate to catalyst ratio (S/C) of 1000 being feasible. Furthermore, a straightforward and selective route to N-formyltetrahydroquinolines (FTHQ) directly from quinoline compounds and FA by one-pot, gold-catalyzed reductive N-formylation protocol is also established.

Full text of DOI:10.1002/adsc.201400721

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DOI: 10.1002/adsc.201400721
Very Important Publication
Heterogeneous Gold-Catalyzed Selective Reductive Transforma-
tion of Quinolines with Formic Acid
a
a
a
a
a
a,
Lei Tao, Qi Zhang, Shu-Shuang Li, Xiang Liu, Yong-Mei Liu, and Yong Cao *
a
Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University,
Shanghai 200433, Peopleꢀs Republic of China
Fax : (+86)-21-65643774; e-mail: yongcao@fudan.edu.cn
Received: July 24, 2014; Revised: November 14, 2014; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400721.
Abstract: Single phase rutile titania supported gold
nanoparticles (Au/TiO -R) are found to be efficient
2
and versatile catalysts for chemo- and regioselective
transfer hydrogenation of quinoline derivatives to
1,2,3,4-tetrahydroquinolines (THQs) using formic
acid (FA) as a safe and convenient hydrogen source
under mild conditions. The activity and chemoselec-
tivity of the Au/TiO -R catalyst towards THQs is
2
excellent, with a substrate to catalyst ratio (S/C) of
1
000 being feasible. Furthermore, a straightforward
and selective route to N-formyltetrahydroquinolines
FTHQ) directly from quinoline compounds and
(
Figure 1. THQ derivatives as prescription or potential drugs.
FA by one-pot, gold-catalyzed reductive N-formyla-
tion protocol is also established.
try of H is difficult to control, oftentimes leading to
2
Keywords: gold; heterogeneous catalyst; nanoparti-
cles; N-formylation; quinolines; transfer hydrogena-
tion
substrate over-reduction. A more attractive alterna-
tive is 11]to use catalytic transfer hydrogenation
[
(CTH), which can be efficiently performed without
the use of sophisticated instruments (autoclave) or H2
gas. In this regard, a number of homogeneous as well
as heterogeneous catalytic systems in combination
with different hydrogen donors have been devel-
[
7–11]
1,2,3,4-Tetrahydroquinolines (THQs), which structur- oped.
However, despite their potential utility,
ally exist in numerous natural products and synthetic these methods suffer from one or more drawbacks
bioactive compounds, have attracted much attention such as the use of expensive ligands, hazardous or
[
1]
in the chemical and pharmaceutical industries. For toxic reducing agents, limited substrate scope, and
instance, among well-known prescription or potential poor chemoselectivity. Therefore, it remains a great
drugs are the THQ derivatives Oxamniquine used for challenge to develop a ligand-free, heterogeneous
[
2]
treatment of Schistosoma mansoni infection, Nicai- CTH system that can reduce quinolines in a general,
[
3]
noprol for cardiac arrhythmis, Viratmycin as a novel efficient, selective, safe and convenient manner.
[
4]
antibiotic, l-689,560 as one of the most potent
N-Formyltetrahydroquinolines (FTHQs), an impor-
[
5]
NMDA antagonists and a natural product, isolated tant class of derivatives to THQs, also show a variety
from Martinella iquitosensis, as a hradykinin antago- of pharmacological properties and minor changes of
nist (Figure 1). Given their immense pharmaceutical their structure offer a high degree of diversity that
utility, the synthesis of THQs has been extensively has proven useful for the search of new therapeutic
[6]
[7]
[12]
studied. The most common route to THQs is the hy- agents.
Although FTHQ compounds occur less
drogenation of readily accessible quinolines. Such often in nature and offers less potential structural
type of reactions are usually carried out using molecu- multiplicity, the broad spectrum of pharmacological
[8]
[9,10]
lar hydrogen, with two major drawbacks:
high activity of these heterocycles led nevertheless to the
H pressure is typically required and the stoichiome- elaboration of concise and flexible synthetic methods
2
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Lei Tao et al.
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[
2–6]
useful in the production of specific structures.
compounds to FTHQs by reductive N-formylation in
FTHQs can be directly accessed via N-formylation the presence of a gold catalyst.
from THQs, with formic acid (FA) being the most
First, we investigated the selective reduction of
commonly used formylating reagents. FA is one of quinoline (1a) in the presence of FA and a range of
the major by-products formed during biomass proc- Au NPs (average diameter of approximately 2-3 nm)
essing and is also easily accessible by methyl formate deposited on various inorganic oxide materials. We
[
13]
hydrolysis or CO hydrogenation. As a result of an found that such reactions proceeded at a S/C ratio of
2
ongoing demand for new strategies that can allow 100:1 when 10 equivalents of FA in a mixed solvent,
[19]
safe and renewable energy storage, the use of FA as Et N/DMF in a 1:8 v/v ratio, was used at 1008C.
3
a hydrogen storage material has received much atten- We initially investigated the previously reported zir-
[
14]
tion over the past decade. As a hydrogen source, it conia supported gold catalyst system comprising small
has many advantages with regards to storage and han- Au NPs (approx. 1.8 nm) deposited on acid-tolerant
dling, and therefore has been frequently used in CTH zirconia (Au/ZrO ), which has been identified as
2
[15]
reactions.
Given the versatile nature of FA, the a very effective catalyst for low temperature dehydro-
direct one-pot reductive N-formylation of quinolines genation of FA to produce CO-free hydrogen. Un-
using FA is envisioned as an appealing approach to fortunately, the reduction hardly occurred and only
access FTHQs. With regard to green synthesis, this a very limited amount of THQ (2a) was formed
approach is particularly attractive given the use of FA (Table 1, entry 1). Attempts to extend the reaction
as an inexpensive and sustainable reagent as well as time or varying the reaction temperature failed to
the step economy as compared to traditional multi- promote the reduction of 1a (Table 1, entries 2-4). In-
step procedures. A very recent report describes the terestingly, when applying a commercial biphasic tita-
use of pyrophoric palladium on activated charcoal nia (Evonik P25) supported gold catalyst (Au/TiO2-
(
Pd/C) as the catalyst for this particular transforma- P25, average gold particle size ~2.6 nm), which has
[16]
tion. However, the inherent limitations of high cat- been widely studied and proven to be highly effective
alyst loading, high excess of FA and necessity of spe- for a variety of organic transformations including che-
cial handling have restricted the utility of this proce- moselective nitro reduction, selective alcohol oxida-
dure.
tion, and nitrile hydration reactions for amide forma-
[20]
In the course of our continuing efforts in develop- tion,
appreciable levels of reduction activity was
ing green catalysis for sustainable synthesis, we have observed, thus furnishing the desired 2a in a yield of
discovered an efficient approach for mild and clean approximately 37% (Table 1, entry 5). Further evalu-
hydrogenation of a wide range of quinoline com- ation of a series of titania polymorphs supported gold
pounds to the corresponding THQs using titania sup- catalysts with identical average Au particle size re-
[
8d]
ported gold nanoparticles (NPs).
An interesting vealed that an impressive yield of 2a of about 78%
feature of this heterogeneous Au-catalyzed quinoline can be attained with a catalyst comprised of gold de-
hydrogenation reaction is that an unusual reactant- posited on single-phase rutile titania (Au/TiO -R)
2
promoted H activation has been identified, which is (Table 1, entry 6).
2
in stark contrast to what prevails in the traditional
Encouraged by these promising results, a more de-
[
17]
platinum-group-metal-based catalytic systems,
in tailed study on the effects of reaction conditions has
which quinolines and their derivatives typically act as been carried out with the Au/TiO -R catalyst. We fo-
2
poisons. We have also made great efforts in the ex- cused on the effect of the molar ratio of FA to 1a
ploitation of FA activation, wherein we found that zir- under otherwise identical reaction conditions. It turns
conia supported gold NPs can promote efficient H2 out that the reaction of 1a with 15 equiv of FA gave
generation via selective FA dehydrogenation under the best results (Table 1, entry 15). Use of 5 equiv of
[
18]
ambient conditions. With the aim to develop inno- FA led to incomplete conversion of 1a, and only a lim-
vative catalytic processes that enable chemical trans- ited amount of the desired 2a could be obtained
formations to be performed under mild and sustaina- (entry 14). Upon increasing the FA/1a ratio from 5 to
ble conditions with high efficiency, we decided to 15, the reaction proceeds to completion with a signifi-
evaluate the catalytic activity of supported gold for cant increase in the yield of the product (2a) (en-
the partial reduction of quinolines with FA. Herein, tries 6, 14 and 15). This result suggests that the com-
we report that by using a single-phase rutile titania position of the reaction medium was critical for the
supported gold catalyst, it is possible to realize a ver- desired reaction pathway. However, lower yields as
satile transfer reduction of a range of functionalized a result of 3a formation via further N-formylation of
quinolines to their corresponding THQs utilizing FA 2a were observed when 20 equiv of FA were em-
as a safe and economically more favored hydrogen ployed (Table 1, entry 16). Notably, further improve-
source. To the best of our knowledge, this study re- ments were achieved by performing the reaction at an
ports the first direct one-pot conversion of quinoline elevated temperature of 1308C using 15 equiv of FA,
2
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Gold-Catalyzed Selective Reductive Transformation
[a]
Table 1. Reduction of quinoline to THQ under CTH conditions.
[b]
Yield [%]
o
Entry
Catalyst
FA [equiv]
Time [min]
Temp. [ C]
Conversion [%]
2
a
3a
1
2
3
4
5
6
7
8
9
Au/ZrO2
Au/ZrO2
Au/ZrO2
Au/ZrO2
10
10
10
10
10
10
10
10
10
10
10
10
10
5
15
20
15
15
15
13
60
120
60
60
60
30
60
30
180
180
180
180
180
30
30
30
10
10
100
100
80
16
16
14
24
37
79
16
0
16
16
6
24
37
78
16
0
0
10
0
2
1
37
92
91
96
94
89
80
0
0
0
0
0
1
0
0
1
0
3
1
9
0
2
7
3
4
6
8
130
100
100
100
100
100
100
100
100
100
100
100
100
130
130
130
130
Au/TiO -P25
2
Au/TiO -R
2
Au/TiO -A
2
TiO -R
2
Au/ZnO
Au/Al O
6
10
5
10
11
12
13
14
15
16
17
18
19
20
2
3
Pd/C
Ru/C
Ir/CaCO3
7
13
37
94
98
99
98
95
88
Au/TiO -R
2
Au/TiO -R
2
Au/TiO -R
2
Au/TiO -R
2
[
[
[
c]
d]
e]
Au/TiO -R
2
Au/TiO -R
30
60
2
Au/TiO -R
2
[
[
[
[
[
a]
Reaction Conditions: quinoline (0.5 mmol), catalyst (metal: 1 mol%), Et N/DMF (1:8 v/v, 3 mL).
Conversion and yield were determined by GC and GC-MS.
Result for the third run.
Au: 0.25 mol%
Au: 0.1 mol%
3
b]
c]
d]
e]
which permit access to 2a in 96% yield in a very oline compounds of the Au/TiO -R catalyst. The high
2
short time of 10 min (Table 1, entry 17).
efficiency of Au/TiO -R for the chemoselecive reduc-
2
Upon decreasing the catalyst loading to 0.25 mol%, tion of 1a using FA relative to other supported gold
2
a was formed in nearly quantitative yield albeit catalysts was shown clearly (Table 1). Au/Al O re-
2
3
a longer reaction time of ca. 30 min was required sulted in poor yields, whereas Au/ZnO did not pro-
Table 1, entry 19). The transfer reduction was still mote the reduction at all (Table 1, entries 9 and 10).
(
operative with only 0.1 mol% of the Au/TiO -R, and These results show that the combination of Au NPs
2
under these conditions remarkable values of the turn- with suitable polymorphs of titania is essential for
over number (TON=800) and average turnover fre- achieving a high catalytic activity for the selective re-
À1
quency (TOF=800 h ) were calculated (Table 1, duction of 1a into 2a using FA under mild conditions.
entry 20). These values are two orders of magnitude Moreover, consistent with the case identified for cata-
greater than those in the previously reported catalyst lytic dehydrogenation of liquid FA under mild condi-
[
18]
systems, such as the nanoporous gold-organosilane tions,
we found that gold is uniquely active for
À1
combinations (TOF: 1.8 h , TON: 43, reaction at quinoline reduction with FA compared with other
[11b,21]
808C).
At this juncture, it is important to note noble metals (Table 1, entries 11-13). Among the vari-
that gold deposited on single-phase anatase titania ous catalysts examined here small Au NPs deposited
(
2
Au/TiO -A) can only afford a very moderate yield of on single-phase rutile titania (Au/TiO -R) affords, by
a (Table 1, entry 7), in line with the broad literature far, the best catalytic performance.
2
2
documenting the effect of polymorphic structure on
the activity of titania-based catalysts. The superior formed with Au/TiO -R, the time-course plot for the
To gain more detailed insight into the reaction per-
[
22]
2
activity found for Au/TiO -R with respect to Au/TiO - above mentioned FA-mediated quinoline reduction
2
2
P25 and Au/TiO -A in this reaction could be due to was examined at a S/C ratio of 400:1 (Supporting In-
2
a higher adsorption capacity of both FA and the quin- formation Figure S1). It was found that the reaction
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[a]
Table 2. CTH of quinolines to THQs with FA in the presence of Au/TiO -R.
2
[b]
Entry
1
Substrate
FA [equiv]
15
Time [min]
Conversion [%]
99
Yield [%]
96 (90)
10
20
2
20
99
94 (89)
3
4
5
6
20
35
20
20
25
40
15
20
99
93
95
90
98 (92)
90 (82)
90 (84)
88 (80)
7
8
20
30
20
30
98
95
96 (88)
94 (85)
[
c]
9
1
20
45
25
50
99
97
98 (93)
90 (80)
0
[
[
[
a]
Reaction Conditions: quinoline (0.5 mmol), Au (1 mol%), Et N/DMF (1:8 v/v, 3 mL), 1308C.
Conversion and yield were determined by GC and GC-MS; values in parenthesis are the yields of the isolated products.
The product is 2-phenethyl-1,2,3,4-tetrahydroquinoline.
3
b]
c]
proceeds smoothly and goes to completion within TiO -R. Furthermore, the Au/TiO -R catalyst was re-
2
2
3
0 min. During the whole reaction process, 2a was the coverable by simple filtration after the transfer reduc-
predominant product with only very trace amount of tion without any loss in their activity; images of Au/
a detected as a by-product. To determine whether TiO -R after the reuse reveal that the average diame-
3
2
Au/TiO -R worked as a heterogeneous catalyst, Au/ ter and size distribution of the gold NPs were similar
2
TiO -R was removed from the reaction mixture by to those of the fresh Au/TiO -R (Supporting Informa-
2
2
simple filtration at 50% conversion of 1a. Continued tion Figure S2). Also no aggregation of the used gold
stirring of the filtrate under similar conditions did not NPs was apparent. In addition, XPS analysis of Au/
give any products. The absence of Au ions in the fil- TiO -R showed virtually no change in the metallic
2
trate was verified using ICP analysis (detection limit: state or dispersion of gold (Supporting Information
0.10 ppm). These results clearly demonstrate that re- Figure S3), corroborating the observation that the
duction took place only on the Au NPs deposited on gold NPs after reuse were the same size after recy-
4
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Gold-Catalyzed Selective Reductive Transformation
[a]
Table 3. Direct reductive N-formylation of quinolines with FA using Au/TiO2-R.
[b]
Entry
1
Substrate
Time [h]
5
Conversion [%]
Yield [%]
97 (92)
>99
[
c]
2
10
>99
95 (89)
3
4
5
8
6
5
98
92 (84)
94 (88)
96 (90)
>99
>99
6
9
95
90 (80)
[
d]
7
8
4
>99
86 (78)
95 (90)
11
96
9
10
95
93 (85)
[
[
[
[
a]
Reaction Conditions: quinoline (0.5 mmol), Au (1 mol%), FA (12.5 mmol), Et N/H O (3:10 v/v, 3 mL), 1008C.
Conversion and yield were determined by GC and GC-MS; values in parenthesis are the yields of the isolated products.
3
2
b]
c]
d]
1
1
308C, 6.5 mmol FA was added after 6 h.
308C
cling compared to that in the original sample. These quinolines were cleanly reduced to the corresponding
results are consistent with the retention of the catalyt- chloro or fluoro THQs without any dehalogenation
ic activities of Au/TiO -R during recycling experi- over Au/TiO -R (Table 2, entries 5 and 6), which was
2
2
[
23]
ments.
often encountered on other metal surfaces.
Note
Building upon these results, we started to extend that the reaction chemistry described herein exempli-
this novel Au-FA-based transfer reduction protocol to fies once more the unique nature of gold in catalysis.
a structurally diverse range of quinoline derivatives. The selective reduction of 8-hydroxyquinolines af-
As depicted in Table 2, the reaction was remarkably forded the corresponding biologically active THQ
selective for the synthesis of various THQs regardless (Table 2, entries 7 and 8), which could inhibit leuko-
[
24]
of the presence of electron donor or acceptor sub- triene formation in macrophages. Quinoline deriva-
stituents (Table 2, entries 2-11). Quinolines bearing tives with an alkenyl moiety did not show good che-
a methyl group at the 2- and 4-position could react moselectivity; 2-styryquinoline afforded a major prod-
smoothly, providing the corresponding THQs in high uct of 2-phenethyltetrahydroquinoline in 98% yield
yields (Table 2, entries 2 and 3). Halogen-substituted (Table 2, entry 9). 2,4-dimethylquinoline showed a rel-
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Lei Tao et al.
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atively lower reactivity, giving the corresponding titania supported gold nanoparticles as catalyst and
THQ in 90% yield (Table 2, entry 10). renewable FA as hydrogen source, various quinoline
Having established an efficient protocol for the Au- derivatives were reduced to give the THQs in high
FA-mediated quinoline reduction to prepare THQs, yields with excellent regioselectivity. More interest-
we were then interested in whether this protocol was ingly, this gold-FA-based CTH system could also ac-
applicable to the tandem direct transformation of qui- complish the direct conversion of quinoline com-
nolines to FTHQs. With this goal in mind, we have pounds bearing different functional groups to the cor-
re-explored the effect of solvent on the Au-FA-medi- responding FTHQs chemoselectively, providing a val-
ated selective reduction of 1a at 1008C. Interestingly, uable alternative to currently used methods for het-
upon switching the mixed organic solvents to a Et N/ erocycle transformation.
3
H O mixture (1:8 v/v ratio), appreciable formation of
2
FTHQ with a moderate yield well above 60% can be
attained even in the presence of just 15 equiv of Experimental Section
[19]
FA. This result was encouraging and led us to be-
lieve that the desired reductive N-formylation path- General Procedure for THQ Synthesis via Gold-
way may be favored in aqueous-based media. Given Catalyzed Selective Reduction of Quinolines
the potentially large scale of this conversion, it is im-
Supported gold catalyst (0.005 mmol Au) was placed in
portant to develop a selective process for the direct
transformation of quinolines into FTHQs that can
minimize the required amount of FA. We thus decid-
a 10 mL round-bottom flask, followed by the addition of
quinoline (0.5 mmol), formic acid (the given amount), and
Et N/DMF (1:8 v/v, 3 mL). The reaction mixture was then
3
ed to investigate the reaction in the cosolvent of vigorously stirred (800 rpm with a magnetic stir bar) at the
Et N/H O with various volume ratios. To our delight, designated temperature for the given reaction time. After
3
2
the use of a 3:10 Et N/H O mixture together with completion of the reaction, the reaction mixture was filtered
3
2
and the catalyst was washed thoroughly with ethanol. Then
the filtrate was washed with water and extracted by ethyl
acetate. The mixture was concentrated and dried under re-
duced pressure using a rotatory evaporator. The crude prod-
uct was purified by column chromatography [silica gel (200–
2
5 equiv of FA gave excellent results, with 99% con-
version and a high yield well above 97% towards
FTHQ. By using this newly established procedure,
a variety of quinolines bearing electron-withdrawing
and electron-donating subsituents can be converted to
their corresponding FTHQs under the aqueous-based
conditions as depicted in Table 3.
3
00 mesh); petroleum ether (60~908C)/ethyl acetate mix-
ture] to afford the product. All the products were identified
by GC-MS and the spectra obtained were compared with
The utility of this direct reductive N-formylation the standard spectra. The conversion and yields were deter-
methodology is further exemplified in the straight-for- mined by Agilent 7820 A gas chromatograph equipped with
ward and efficient one-pot synthesis of a diversity of a HP-WAX column (30 m ꢂ 0.32 mm) and a flame ioniza-
tion detector (FID).
formyl-substituted aromatic N-heterocycles by using
isoquinoline and 7,8-benzoquinoline and again only
General Procedure for FTHQ Synthesis via Gold-
Catalyzed Reductive N-Formylation of Quinolines
2
5 equiv of FA as starting materials (Table 3, entries 8
[25]
and 9). To further showcase the effectiveness of this
direct reductive N-formylation system, we carried out
Supported gold catalyst (0.005 mmol Au) was placed in
the direct reductive N-formylation of isoquinoline on a 10 mL round-bottom flask, followed by the addition of
a one-gram scale (Scheme 1). Direct one-pot N-for- quinoline (0.5 mmol), formic acid (12.5 mmol), and Et N/
3
mylation of isoquinoline (1.29 g) in the 3:10 Et N/
H O (3:10 v/v, 3 mL). The reaction mixture was then vigo-
2
3
rously stirred (800 rpm with a magnetic stir bar) at the des-
ignated temperature for the given reaction time. After com-
pletion of the reaction, the reaction mixture was filtered and
the catalyst was washed thoroughly with ethanol. The fil-
trate was washed with water and extracted by ethyl acetate.
Then the mixture was concentrated and dried under reduced
pressure using a rotatory evaporator. The crude product was
purified by column chromatography [silica gel (200–300
H O mixture with an even lower catalyst loading of
2
0
.025 mol% (100 mg Au/TiO -R) in air provided the
2
valuable N-formyltetrahydroisoquinoline in 97% iso-
lated yield. This appears to us to be the most efficient
way for accessing this type of compound, regarding
cost, effectiveness, practicability and scalability.
In conclusion, we have developed a simple, efficient
and robust protocol for the chemoselective transfer mesh); petroleum ether (60~908C)/ethyl acetate mixture] to
reduction of quinolines. By using single phase rutile afford the product. All the products were identified by GC-
MS and the spectra obtained were compared with the stan-
dard spectra. The conversion and yields were determined by
Agilent 7820 A GC equipped with a HP-WAX column
(
30 m ꢂ 0.32 mm) and a FID.
Scheme 1. Direct reductive N-formylation of isoquinoline on
a one-gram scale.
6
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