Rhodium-catalyzed synthesis of quinolines and imines under mild conditions

Article abstract of DOI:10.1039/c5ra08887a

An environmentally benign protocol for the synthesis of quinolines in aqueous medium by using a Rh(ii)acetate/TPPTS recyclable catalytic system has been developed. Anilines reacted smoothly with allyl alcohols furnishing the corresponding quinolines in moderate to good yields. This catalytic system was recycled up to five runs without much loss in its catalytic activity. Furthermore, imines were also synthesized from benzylamines in moderate to good yields.

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ARTICLE TYPE
Rhodium-Catalyzed Synthesis of Quinolines and Imines under Mild
Condition
Dilip Kumar T. Yadav and Bhalchandra M. Bhanage*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Ru,22 Pd,23 Ni,24 Co,25 and Fe26 were also developed for the
An environmentally benign protocol for the synthesis of
50 quinolines synthesis. Despite the advances in methodology for
the quinoline synthesis, most of the reported protocols have one
or more shortcomings such as use of hazardous organic solvents,
harsh reaction conditions, limited functionalꢀgroup tolerance and
the homogeneous nonꢀrecyclable catalytic system which has the
55 problem of catalystꢀproduct separation thereby limits the
attractiveness of these transformations for their general
applications. Thus, to develop an economical and sustainable
methodology for the quinolines synthesis which should operate
under environmentally favourable condition remains an active
60 area of research.
Now a days, the use of water as a reaction media for organic
reactions have been made attention, as water is an inexpensive,
readily available, safe and nonꢀtoxic solvent which provides
remarkable advantages over common organic solvents from
65 environmental and economic points of view.27 Hence, the
progress towards the development of new waterꢀsoluble catalytic
protocol for the quinoline synthesis which allows an easy
separation of the catalyst from product, and recyclability of
aqueous solution containing the catalyst by employing water as a
70 reaction media could be the answer for the future of green
chemistry. In continuation of our ongoing research on the
development of a new facile protocol in water as a green reaction
media.28 Herein, we report a simple and efficient recyclable
rhodium catalyzed protocol for quinolines synthesis in water
75 using the triphenylphosphine trisulfonate sodium salt (TPPTS)
(Scheme 1).
quinolines in aqueous medium by using Rh(II)acetate/TPPTS
recyclable catalytic system has been developed. The anilines
were reacted smoothly with allyl alcohols furnishing the
10 corresponding quinolines in moderate to good yields. This
catalytic system was recycled up to five runs without much
loss in its catalytic activity. Furthermore, imines were also
synthesized from benzylamines in moderate to good yields.
Introduction
15 Quinolines are important class of compounds which display
attractive applications in pharmaceuticals, agrochemicals,
fungicides, herbicides, complexing agents and dyes.1
Furthermore, substituted quinoline structural frameworks are
often found in many biological active molecules and they exhibits
20 antibacterial,2 antimalarial,3 antiinflammatory,4 antiprotozoan,5
antiasthmatic,6 antituberculosis,7 antihypertensive,8 anticancer,9
antiꢀHIV,10 and antihelmintic activities.11 Moreover, quinolines
are also vital synthons for the synthesis of many important
materials such as nanoꢀmeso structures and polymers which are
25 investigated for applications in electronics, optoelectronics and
nonlinear optics.12 Hence, the development of simple and
efficient protocol for the synthesis of quinolines has been the
subject of continued interest from the several decades.
Traditionally, quinolines were synthesized by the Skraup
30 reaction,13 the ConradꢀLimpach reaction,14 the Pfitzinger
reaction,15 the DoebnerꢀVon Miller reaction,16 and the
Friedlaender reaction.17 However, these reactions suffer from one
or more drawbacks such as use of environmentally unfriendly
strong acids or bases, high temperatures, low yields and harsh
35 reaction conditions. The other alternative methods for the
synthesis of quinolines by transition metalꢀcatalyzed process
were also reported in the literature. Some research groups have
reported the rhodium complex catalyzed reaction of aniline with
aliphatic aldehyde,18 aliphatic olefin,19 styrene,20 and the reaction
40 of nitrobenzene with aliphatic alcohol21 for the synthesis of
quinolines. Subsequently, other transition metal precursors of
80
Scheme 1: Rh(II)acetate /TPPTS catalyzed synthesis of quinolines.
Results and discussion
Initially to optimize the reaction conditions aniline (1a) and allyl
alcohol (2a) was chosen as a model substrate for the rhodiumꢀ
85 catalyzed synthesis of quinoline derivative.
A series of
*Department of Chemistry, Institute of Chemical Technology, Matunga,
Mumbai-400019. India.
Tel.: +91-22-33612601; fax: +91-22-33611020;
45 E-mail: bm.bhanage@gmail.com, bm.bhanage@ictmumbai.edu.in
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
experiments were performed in order to study the effect of
various reaction parameters such as catalyst screening, catalyst
loading, time and temperature for the synthesis of quinolines
(Table 1). Firstly, we have screened various rhodium precursors
90 such as RhCl3, Rh(PPh3)3Cl, Rh(COD)Cl and Rh(II)acetate for
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45 Table 2: Substrate study for the synthesis of quinolinesa
the synthesis of 2ꢀethylꢀ3ꢀmethylquinoline (3a) (Table 1, entries
1–4) and it was found that Rh(II)acetate furnished the best yield
of the desired product 3a and hence was used for further studies
(Table 1, entry 4). However, other rhodium precursors like RhCl3,
Rh(PPh3)3Cl and Rh(COD)Cl provided 3a in moderate yields
(Table 1, entries 1–3). However, the reaction of 1a with 2a did
not give desired product 3a in the absence of the catalyst which
shows that catalyst is necessary for the progress of reaction
(Table 1, entry 5). Furthermore, when the reaction was performed
5
Entry
Aniline
Product
Yield (%)b 3
NH2
1
86
10 in the absence of TPPTS ligands, formation of 3a was observed
in low yield (Table 1, entry 6). Next, we have studied the effect
of catalyst loading and it was observed that 0.03 mmol of Rh(II)
acetate gave 3a in good yield (Table 1, entry 7). Furthermore, we
have also studied the effect of TPPTS loading and it was
15 observed that 0.06 mmol of it gave 3a in 82% yield (Table 1,
entry 8). Subsequently, we have studied the effect of reaction
time and it was found that 6 h was the optimum time required for
completion of the reaction and further increase in time has found
no effect on reaction outcome (Table 1, entry 9). Furthermore,
20 we have also examined the temperature effect on the formation of
3a and it was found that increase in temperature has no effect on
reaction outcome whereas with decrease in reaction temperature
from 100 °C to 90 °C decrease in yield of 3a observed (Table 1,
entries 10 and 11).
1a
2
62
3
4
88
77
5
64
6
7
89
79
25 Table 1: Optimization of the reaction conditionsa
Entry
Catalyst (mmol)
Time (h)
Yield (%)b 3a
F
NH2
8
68
1
2
3
RhCl3
4
4
4
64
63
67
Rh(PPh3)3Cl
Rh(COD)Cl
F
1h
9
69
62
4
5
6c
Rh(II)acetate
ꢀ
4
4
4
4
4
6
6
6
72
ꢀ
Rh(II)acetate
57
79
82
88
88
78
10
7
8d
9d
10d,e
11d,f
Rh(II)acetate (0.03)
Rh(II)acetate (0.03)
Rh(II)acetate (0.03)
Rh(II)acetate (0.03)
Rh(II)acetate (0.03)
11
12
72
81
a
Reaction conditions: 1a (1.0 mmol), 2a (2.5 mmol), Catalyst (0.02
30 mmol), TPPTS (0.04 mmol), water (4 mL), 100 °C. GC yield. No
TPPTS. d TPPTS (0.06 mmol). e 110 °C. f 90 °C.
b
c
NH2
With these optimized reaction conditions, we have studied the
scope and limitations of present methodology for the preparation
of different kinds of structurally diverse quinolines. Firstly, the
35 reaction of 1a with 2a provided corresponding product 3a in 86%
isolated yield (Table 2, entry 1). Next, it was found that orthoꢀ
substituted electron donating aniline derivative (1b and 1e) shows
the steric effect and furnished the corresponding product 3b and
3e in moderate yields (Table 2, entries 2 and 5). The reaction of
40 pꢀsubstituted and disubstituted electron donating anilines (–CH3,
–OMe) with 2a provided the respective products 3c-3g in good
yields (Table 2, entries 3–4 and 6–7). Subsequently, aniline
derivatives bearing haloꢀsubstituents (–F, –Cl, –Br) were also
well tolerated under the present reaction conditions and afforded
87
13
1m
14
15
ꢀ
1a
74
a
Reaction conditions: 1 (1 mmol), 2 (2.5 mmol), Rh(II)acetate (0.03
50 mmol), TPPTS (0.06 mmol), water (4 mL), 100 °C, 6 h, N2 atmosphere. b
isolated yield.
2
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the corresponding products 3h–3k in moderate yields (Table 2,
entries 8–11). Furthermore, it was observed that when the simple
aromatic ring of aniline was replaced by more hindered biphenyl
and naphthyl groups, the respective quinolines were also obtained
It was found that when benzyl amine (4a) was heated with
Rh(II)acetate at 100 °C for 10 h under neat condition, it
undergoes self condensation providing corresponding imine 5a in
good yield (Table 3, entry 1). Subsequently, the scope of imines
5
in good yields (Table 2, entries 12 and 13). However, the aniline 35 synthesis was advance towards other benzyl amine derivatives
bearing electron withdrawing group (ꢀNO2) did not furnished the
corresponding product 3n under the present reaction condition
might be due to low nucleophilicity of such aniline (Table 2,
entry 14). Furthermore, the reaction of 1a with crotyl alcohol (2b)
10 afforded respective product 3o in 74% yield (Table 2, entry 15).
Imines have been used as valuable synthons for many
advantageous chemicals such as nitrogen heterocycles,
pharmaceuticals, fine chemicals, agricultural chemicals and in
natural product synthetic applications.29 In addition, imines also
(Table 3). It was observed that electronꢀrich benzylamine
derivatives were well tolerated and affording respective products
in good yields (Table 3, entries 2–5). Next, it is noteworthy that
benzylamine bearing haloꢀsubstituents also performed well by
40 providing corresponding imines in good yields (Table 3, entries
6–8), which could be used for further transformations along with
the imine functionality. Subsequently, the heterocyclic benzyl
amine gave respective imine 5i in low yield (Table 3, entry 9).
However, 1ꢀphenylethanamine (4j) did not provide corresponding
15 shows antiꢀbacterial antiꢀviral, antiꢀinflammatory and antiꢀfungal 45 imine under the present reaction condition (Table 3, entry 10).
activities.29e Furthermore, imines also serve as a electrophiles in
different type of reactions such as condensations, addition,
cyclization, reductions and multiꢀcomponent reactions.30 Hence,
developing newer methods for the synthesis of imines is an main
Based on our experimental observation and literature
report,18,34,35 we propose a plausible reaction mechanism for the
synthesis of quinoline (Scheme 2). Initially, allyl alcohol (2)
isomerized to corresponding propanal (A) in the presence of
20 task for the organic synthetic community. Consequently, much 50 rhodium catalyst34 which undergoes self aldol condensation to
progress has been made for the imines synthesis such as selfꢀ
condensation of primary amines with oxidants,31 oxidation of
secondary amines32 and condensation of amines with alcohols.33
form corresponding unsaturated aldehyde (B).35 The formation of
unsaturated aldehyde (B) was confirmed by GCꢀMS. In the next
step, aniline (1a) adds to compound B giving intermediate C
which undergoes ring closure with elimination of water molecule
55 forming intermediate E and in the last step dehydrogenation of
intermediate E provides corresponding quinoline derivative
(Scheme 2).
Table 3: Substrate study for the synthesis of iminesa
25
Entry
Benzylamine
Product
Yield (%)b 5
1
84
2
3
4
77
72
74
5
6
62
75
N
Scheme 2: Plausible mechanism for the synthesis of quinoline.
F
F
5f
7
8
79
76
9
43
ꢀ
10
a Reaction conditions: 4 (2 mmol), Rh(II)acetate (0.03 mmol), 100 °C, 10
60
h. b isolated yield.
Fig. 1 Catalyst recyclability study. Reaction conditions: 1a (1
mmol), 2a (2.5 mmol), Rh(II)acetate (0.03 mmol), TPPTS (0.06
30
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mmol), water (4 mL), 100 °C, 6 h, N2 atmosphere. GC yield.
It is important to reveal that, catalyst separation and recyclability
with homogeneous reactions have serious concern in industrial
applications. So, in order to make present protocol more
economical, here we have studied the reusability of developed
catalytic system for a model reaction of 1a with 2a (Fig. 1). Here,
we have extracted the product in organic phase from the catalyst
containing aqueous phase and the catalyst containing aqueous
phase was recycled for five times without much loss in its activity
product was purified by column chromatography (basic alumina
saturated with Et3N, 100ꢀ200 mesh, PE) to provide the desired
pure product. The identity of product was confirmed by
comparison with those of authentic compounds from literature.
5
60 Acknowledgements
The author D. K. T. Yadav is greatly thankful to the UGC
(University Grant Commission), India for providing fellowship.
10 and the decrease in yield is might be due to handling loss during
work up of the reaction.
Notes and references
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Conclusion
In conclusion, we have developed a simple, highly efficient
rhodium catalyzed aqueous phase recyclable methodology for the
15 synthesis of quinolines from easily available amines and allyl
alcohols. The various quinolines were efficiently synthesized by
present method in moderate to good yields. The homogeneous
water soluble Rh(II)acetate/TPPTS catalytic system could be
reused for five runs after simple extraction of the product with
20 ethyl acetate. Imines were also synthesized in good yields by
rhodium catalyzed selfꢀcondensation of primary benzyl amines
under mild reaction conditions. Due to the simplicity of
developed protocol, it will cover a wide application in industry as
well as academics. Thus, the developed protocol sounds to be
25 highly efficient for the synthesis of quinolines and imines.
2
3
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Experimental Section
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combined ethyl acetate layers were dried over anhydrous Na2SO4
and solvent was evaporated under reduce pressure. The obtained
crude product was directly purified by column chromatography
40 (silica gel, 100ꢀ200 mesh, PE–EtOAc) to afford the pure product.
The identity of product was confirmed by 1H and 13C NMR
spectroscopic analysis.
8
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The reaction was performed as stated above in a typical
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phase containing catalyst was subjected further for catalyst
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DOI: 10.1039/C5RA08887A
Rhodium-Catalyzed Synthesis of Quinolines and Imines under Mild Condition
Dilip Kumar T. Yadav and Bhalchandra M. Bhanage*
Department of Chemistry, Institute of Chemical Technology,
Matunga, Mumbai- 400019, India
A simple and efficient protocol for the synthesis of quinolines in aqueous medium by using
Rh(II)acetate/TPPTS recyclable catalytic system has been developed. This catalytic system
was recycled up to five runs without much loss in its catalytic activity. Furthermore, imines
were also synthesized from benzylamines in moderate to good yields under solvent free
condition.