Thiamine hydrochloride as a recyclable organocatalyst for the synthesis of bis(indolyl)methanes, tris(indolyl)methanes, 3,3-di(indol-3-yl)indolin-2-ones and biscoumarins

Article abstract of DOI:10.1039/c9ob02090j

Thiamine hydrochloride was identified as an eco-friendly organocatalyst for the synthesis of a broad range of bis(indolyl)methanes in good to excellent yields. Green chemistry matrix calculations showed high atom economy and a small E-factor for the reaction. Moreover, the simple and easy operational protocol using a small amount of thiamine hydrochloride (1 mol%) makes this procedure an alternative approach for this transformation industrially. This protocol was also extended to the synthesis of naturally occurring alkaloids such as tris(indolyl)methane, arundine, vibrindole A, 3,3-di(indol-3-yl)indolin-2-ones and biscoumarin derivatives.

Full text of DOI:10.1039/c9ob02090j

Organic &
Biomolecular Chemistry
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PAPER
Thiamine hydrochloride as a recyclable
organocatalyst for the synthesis of bis(indolyl)
methanes, tris(indolyl)methanes, 3,3-di(indol-3-yl)
indolin-2-ones and biscoumarins†
Cite this: Org. Biomol. Chem., 2019,
7, 9620
1
Sivagami Mathavan, Keerthana Kannan and Rajesh B. R. D. Yamajala
*
Thiamine hydrochloride was identified as an eco-friendly organocatalyst for the synthesis of a broad
range of bis(indolyl)methanes in good to excellent yields. Green chemistry matrix calculations showed
high atom economy and a small E-factor for the reaction. Moreover, the simple and easy operational pro-
tocol using a small amount of thiamine hydrochloride (1 mol%) makes this procedure an alternative
approach for this transformation industrially. This protocol was also extended to the synthesis of naturally
occurring alkaloids such as tris(indolyl)methane, arundine, vibrindole A, 3,3-di(indol-3-yl)indolin-2-ones
and biscoumarin derivatives.
Received 26th September 2019,
Accepted 23rd October 2019
DOI: 10.1039/c9ob02090j
rsc.li/obc
expensive reagents, photosensitive catalysts, high catalyst load-
ings, high temperatures, chromatographic purifications, etc.
Introduction
6
The development of alternative strategies for controlling the
Another fused heterocycle is coumarin, which has received
1
pollution and damage to the environment is inevitable. In much attention due to its wide range of biological activities.
this sense, green technology aims to replace traditional prac- Besides, biscoumarins, the bridge substituted dimers of cou-
tices of using toxic metal catalysts and organic solvents by marins, have antiseptic, antipyretic, antifungal, anti-inflamma-
7
employing sustainable and cost-effective alternatives. Hence, tory and urease inhibitor properties. These are usually syn-
the synthesis of drugs and drug intermediates using environ- thesized via the condensation of 4-hydroxycoumarin and alde-
ment friendly solvents and catalysts has become one of the hydes through the Knoevenagel–Michael reaction in the pres-
2
6e,l,8
most fascinating areas in academia and industry.
In the plethora of indole derivatives, C3 substituted indole
ence of a variety of catalysts.
Although the literature-reported methodologies provide
alkaloids such as bis(indolyl)methanes (BIMs) have found a efficient access to bis(indolyl)methanes (BIMs) and biscoumar-
great deal of interest and have been identified as a key struc- ins, many of them have several disadvantages, such as harsh
3
tural moiety because of their numerous biological activities.
reaction conditions, usage of toxic catalysts and solvents, high
For example, these have been used as drugs having antibacter- temperatures, high catalyst loadings and limited substrate
ial, antileishmanial, antihyperlipidemic, anti-inflammatory scope. Considering the above limitations and the synthetic
4
and anticancer properties. Besides, a few molecules such as demand on developing a simple, efficient and eco-friendly pro-
trisindoline, arundine, vibrindole and 1,1,3-tri(3-indolyl) cedure, our aim is to employ a reusable organocatalyst without
3
d
butane are naturally occurring bioactive BIMs. Owing to the any apparent loss in its activity in order to solve some of the
wide range of biological activities of BIMs, the synthesis of key problems related to the synthesis of BIMs and biscoumar-
BIMs received much attention which led to numerous syn- ins (Fig. 1).
thetic protocols. A variety of catalysts have been used for the
condensation of indoles with aromatic (or aliphatic) alde-
5
hydes. However, the reported protocols for the synthesis of
BIMs involved either toxic metal salts or catalytic systems with
Department of Chemistry, School of Chemical & Biotechnology, SASTRA Deemed
University, Thanjavur-613 401, Tamil Nadu, India. E-mail: ybrdrajesh@gmail.com
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9ob02090j
1
Fig. 1 Thiamine hydrochloride (VB ).
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change in the yield (85%) (Table 1, entry 2). The model reac-
tion produced a yield of 92% of the desired product with
5
mol% of the catalyst with water as the solvent (Table 1,
entry 3). In the reaction with water as the solvent, the product
was precipitated out after completion. A reduction in the cata-
lyst loading from 5 to 3 and 1 mol% caused the reaction to
produce 94% of the product which was the highest yield
among the previous conditions (Table 1, entries 4 and 5). All
the abovementioned reactions were conducted at room temp-
erature in open air. To check the effect of the temperature on
the reaction, the model reaction was conducted at 50 °C, and
this resulted in the reduction of the yield from 94 to 85%
1
Scheme 1 VB catalysed synthesis of bis(indolyl)methanes.
(
Table 1, entry 6). From the viewpoint of green chemistry, grati-
As an eco-friendly catalyst, thiamine hydrochloride
vitamin-B ) (VB ) is a non-toxic, biodegradable and in-
expensive catalyst. Many organic reactions were carried out by
using VB as a catalyst, including the synthesis of formamide
fyingly, when the reaction was conducted under solvent-free
conditions with 1 mol% of the catalyst, it gave an excellent
yield (96%) (Table 1, entry 7). But the yield was slightly
reduced when 0.5 mol% of the catalyst was used (Table 1,
entry 8). An increase in the temperature of the reaction
reduced the yield with 1 mol% of the catalyst (Table 1,
(
1
1
1
derivatives, 1,4-dihydropyridines, 2,4,6-triarylpyridines, amido-
alkyl naphthols, benzo[4,5]imidazo[1,2-a]pyrimidine and
9
[1,2,4]triazolo[1,5-a]pyrimidine derivatives.
Herein, we
entry 9). Furthermore, to ascertain the effect of VB , the model
1
describe the eco-friendly synthesis of BIMs and biscoumarin
reaction was conducted without the catalyst either in water or
under solvent-free conditions at room temperature; however
the reaction did not proceed at all (Table 1, entries 10 and 11).
Thus, the optimized reaction conditions to prepare BIMs
under solvent-free conditions were the use of indole 1a
derivatives via the VB activation of aldehydes (Scheme 1).
1
Results and discussion
(
1.0 mmol), aldehyde 2a (0.5 mmol), and VB (1 mol%) at RT
1
The catalytic activity of VB
1
was examined by reacting indole
in an open atmosphere for 1 h (Table 1, entry 7).
(0.5 mmol) 1a and benzaldehyde (0.25 mmol) 2a using
The model reaction between 1a and 2a was then attempted
to synthesize the BIMs by grinding the components using a
mortar and pestle without any solvent. To our delight, the reac-
tion afforded 94% of the desired product within 20 min of
grinding. Subsequently, all the BIMs (3b–r) were prepared
under grinding conditions as well for the specified time men-
tioned in Table 3. The yields of the BIMs under grinding con-
ditions are comparable to the solvent-free method as shown in
Table 3. The crude products produced by solvent-free and
grinding methods were purified by recrystallization from
EtOH.
5
mol% of the catalyst as a model reaction to obtain the
desired product phenyl bis(indolyl)methane 3a in 87% yield at
room temperature in EtOH as the solvent (Table 1, entry 1). A
reduction in the catalyst loading did not produce much
a
Table 1 Optimization of the reaction conditions for the synthesis of 3a
1
In order to investigate the scope and limitations of VB cat-
alysis, the methodology was extended to various substituted
indoles and aromatic aldehydes as substrates. Various differ-
ently substituted aromatic aldehydes were reacted with indole
under solvent-free conditions at RT which afforded BIMs in
good to excellent yields after 1–3 h (Table 2, 3a–3k). The elec-
tron-donating and electron-withdrawing substituents on the
aromatic ring made no remarkable difference in the reactivity.
Furthermore, under the optimized conditions, the heteroaro-
matic aldehydes such as pyridine-2-carboxaldehyde and thio-
phenecarboxaldehyde were also converted into their corres-
ponding BIMs and indole-3-carboxaldehdye was converted into
tris(indolyl)methane (TIM) in good yields (Table 2, 3l–3n). The
presence of electron-donating and electron-withdrawing sub-
stituents on the indole ring had no significant effect on the
reactivity for the synthesis of BIMs and afforded the desired
product in excellent yields (Table 2, 3o and 3p). Interestingly,
the reaction proceeded with an excellent yield with alkyl sub-
Catalyst
load
(mol%)
b
Solvent
Entry Catalyst (1 mL)
Temp.
(°C)
Time
(h)
Yield
(%)
1
2
3
4
5
6
7
8
9
1
1
VB
VB
VB
VB
VB
VB
VB
VB
VB
—
1
1
1
1
1
1
1
1
1
EtOH
EtOH
5
3
5
3
1
1
1
0.5
1
RT
RT
RT
RT
RT
50
RT
RT
50
2
2
2
2
1
2
1
1
87
85
92
94
94
85
96
90
92
0
2
H O
2
H O
2
H O
2
H O
—
—
—
1
24
24
0
1
2
H O
—
—
RT
RT
—
—
0
a
Optimization studies were performed with indole 1a (1.0 mmol),
benzaldehyde 2a (0.5 mmol) and the catalyst in open air. Isolated
yields without column chromatography.
b
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a
Table 2 Substrate scope with various aldehydes and indoles
stitution on indole nitrogen (Table 2, 3r). In addition to this,
the reaction proceeded excellently with aliphatic aldehydes; in
particular, formaldehyde and acetaldehyde produced the
corresponding BIMs in good to excellent yields (3s and 3t).
Tris(indolyl)methane, 3n, arundine, 3s and vibrindole A, 3t are
1
0
naturally occurring indole alkaloids. To test the robustness
of the catalytic system, we synthesized dibis(indolyl)methane
(
3u) from terephthalaldehyde in an excellent yield (Scheme 2).
To explore the scope of the catalyst system, we tried to syn-
thesize the ketone derived BIMs. To our delight, the reaction
of indole with cyclohexanone under solvent-free conditions at
room temperature afforded 75% of the desired product
(
Table 4, 5a) after 4 h. Furthermore, the catalytic potential of
VB1 has been explored for the synthesis of isatin derived
BIMs and 3,3-di(indol-3-yl)indolin-2-ones, as these compounds
possess remarkable biological activities and are important
structural motifs found in various natural products and
1
1
drugs. A few catalytic systems were reported for the synthesis
1
2
of 3,3-di(indol-3-yl)indolin-2-ones. The optimized reaction
conditions which were used for the synthesis of BIMs were
used even to synthesize 3,3-di(indol-3-yl)indolin-2-ones. The
reaction of isatins and indoles containing electron donating,
electron-withdrawing and alkyl substituents on nitrogen pro-
ceeded smoothly with good to excellent yields in 10 h (Table 4,
1
0
5b–5f). 5b and 5e are naturally occurring indole alkaloids.
Slow reactivity of ketones and isatins was observed compared
to aldehydes for the synthesis of BIMs (Table 4). The reaction
of cyclohexanone and isatins with indoles was also tested in
water and produced comparable yields within the specified
time.
a
Table 3 Synthesis of BIMs under different conditions
Solvent-free method
Grinding method
b
Entry
Product
Time (h)
Yield (%)
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
1
3
2
1
1
1
1
2
2
3
2
1
1
2
2
3
2
3
2
2
96
86
90
84
86
89
89
87
85
86
87
84
84
80
89
86
82
90
87
78
20
30
30
30
20
20
20
30
30
30
30
20
20
30
20
30
20
30
30
30
94
82
88
85
88
84
86
90
86
83
85
87
80
84
82
89
85
92
83
78
0
1
2
3
4
5
6
7
a
Reaction conditions: Indole (1.0 mmol), aldehyde (0.5 mmol) and 18
1
VB
Isolated yields without chromatography.
(1 mol%) under neat conditions at RT in open air for 1–3 h. 19
3s
3t
20
a
Reaction conditions: Indole (1.0 mmol), aldehyde (0.5 mmol) and
1
(1 mol%) at RT in open air for 1–3 h. Isolated yields without
b
VB
chromatography.
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1
After the successful synthesis of BIMs using VB as a eco-
friendly catalyst, we further explored its catalytic activity for
the reaction of aldehydes with 4-hydroxycoumarin to form bis-
coumarins (3,3′-(arylmethylene)bis(4-hydroxycoumarins)). To
find the optimum conditions, the reaction between 4-hydroxy-
Table 6 Substrate
scope
with
various
aldehydes
and
Scheme 2 Synthesis of dibis(indolyl)methanes. Reaction conditions:
a
4-hydroxycoumarins
Indole (1.0 mmol), aldehyde (0.25 mmol) and VB
conditions at RT in open air for 5 h.
1
(1 mol%) under neat
a
Table 4 Substrate scope with ketones
a
Reaction conditions: Indole (1.0 mmol), ketone/isatin (0.5 mmol)
(1 mol%) under solvent-free conditions at RT in open air for
–10 h. Isolated yields without chromatography.
and VB
4
1
Table 5 Optimization of the reaction conditions for the synthesis of
a
biscoumarins
Catalyst
load
(mol%)
b
Solvent
(1 mL)
Temp.
(°C)
Time
(h)
Yield
(%)
Entry
Catalyst
1
2
3
4
5
6
7
VB
VB
VB
VB
VB
—
1
1
1
1
1
EtOH
3
3
1
1
1
—
—
RT
RT
RT
RT
50
RT
RT
2
1
1
1
1
24
24
82
89
87
92
83
0
H
H
2
O
O
2
—
—
2
H O
—
—
0
a
a
Optimization studies were performed with 4-hydroxycoumarin 6a
Reaction conditions: 4-Hydroxycoumarin (1.0 mmol), aldehyde
(1 mol%) under solvent-free conditions at RT in
(1.0 mmol), benzaldehyde 2a (0.5 mmol) and the catalyst in open air. (0.5 mmol) and VB
1
b
Isolated yields without column chromatography.
open air for 3–10 h. Isolated yields without column chromatography.
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coumarin and benzaldehyde was studied as a model reaction large-scale industrial production of highly significant mole-
under different conditions. The reaction was conducted with cules at room temperature under solvent-free conditions.
3
mol% of the catalyst in EtOH and in water as the reaction
According to the green chemistry matrices, for a standard
medium at room temperature, and among these two, water green chemistry reaction, the atom economy for mass
was found to afford a good yield (89%) (Table 5, entries 1 efficiency should be high, and the environmental factor and
6
f,13
and 2).
process mass intensity should be low.
Green chemistry
No significant reduction in the yield was observed when the matrices were calculated for the optimized reaction, and it was
reaction was conducted with 1 mol% of the catalyst in water found that the reaction has a small E-factor (E-factor = 0.098),
(Table 5, entry 3). As seen from the synthesis of BIMs, the a high reaction mass efficiency (R.M.E. = 91.0%), a high atom
solvent-free reaction with 1 mol% of the catalyst at room temp- economy (A.E. = 94.7%), and a high process mass intensity (P.
erature gave the best yield (95%) of the desired product M.I. = 1.098). These values evidently indicate the eco-friendly
(
Table 5, entry 5 and Table 6, 7a). An increase in the tempera- nature of the current method (refer the ESI† for green metrics
ture of the reaction led to a slight reduction in the yield calculations). In order to highlight the efficiency of the green
Table 5, entry 5). The formation of biscoumarin was not catalyst VB , we compared the activity of the catalyst with
(
1
observed without the catalyst either in water or under solvent- various reported catalysts for the synthesis of BIMs, 3,3-di
free conditions (Table 5, entries 6 and 7). Having found the (indol-3-yl)indolin-2-ones and biscoumarins (Table 7). The
optimal reaction conditions for the synthesis of biscoumarins, yields of the products such as BIMs, 3,3-di(indol-3-yl)indolin-
1
the scope of the reaction was tested by employing various alde- 2-ones and biscoumarins with the VB catalyst were compar-
hydes and 4-hydroxycoumarin. First, a differently substituted able to the reported catalysts. Table 7 presents the literature
aromatic aldehyde was reacted with 4-hydroxycoumarin 6a reports of the target compounds which shows that the syn-
under the optimal conditions. Aromatic aldehydes bearing thesis of the above compounds resulted in good yields with
substituents like nitro, chloro, bromo, fluoro, methoxy, shorter reaction times, but those methods involved harsh reac-
methyl, trimethoxy and cyano substituents on phenyl rings tion conditions, use of metal catalysts, toxic organic solvents
were well tolerated to afford the desired products in excellent and high catalyst loadings. Hence, from the above comparison,
yields (Table 6, 7b–7i). Moreover, heteroaromatic aldehydes it is evident that the VB
such as pyridine-2-carboxaldehyde reacted smoothly with friendly, and recyclable, and has many advantages over the
-hydroxycoumarin to give the corresponding product in a reported methods.
good yield (Table 6, 7j). Finally, recycling and reusability of the catalyst were also
1
based protocol is mild, efficient, eco-
4
Furthermore, in order to find the utility of the developed explored. For this purpose, the solvent-free based model reac-
protocol for industrial applications, a gram-scale reaction was tion was studied under the optimized reaction conditions.
conducted. For this, 1 g of benzaldehyde 2a was reacted with After completion of the reaction, the reaction mixture was tri-
indole 1a (2.207 g) under solvent-free conditions with constant turated with EtOAc (2 × 3 mL) and filtered. The catalyst was
stirring at room temperature in open air for 1 h which collected from the residue and washed with EtOAc (5 mL),
afforded the desired bis(indolyl)methane 3a in 92% yield. dried and used for the next cycle. This process was repeated
These results suggest the efficacy of the organocatalyst VB for and the recycled catalyst was reused for ten runs without any
1
1
Table 7 Comparison of catalytic efficacy of VB with other reported catalysts for the synthesis of BIMs, 3,3-di(indol-3-yl)indolin-2-ones and
biscoumarins
Entry
Catalyst
Solvent
Temp. (°C)
Time (min)
Yield (%)
Ref.
BIMs
1
2
3
4
5
6
GO (150 mg)
Sulfated polyborate (10 wt%)
H
Neat
H
H
Neat
2
O
40
RT
50
RT
RT
RT
180
5
60
120
20
60
92
98
97
97
99
96
6n
6m
6g
6f
Ammonium niobium oxalate (5 mol%)
MoS –RGO (5 wt%)
AlCl ·H O–SDS–SiO
VB (1 mol%)
2
O/glycerol
O
2
2
3
2
2
6p
1
Neat/H
2
O
This work
3
7
8
9
,3-Di(indol-3-yl)indolin-2-ones
I
I
2
(10 mol%) TBHP
(10 mol%)
Cyclohexanone
Propylene carbonate
RT
RT
RT
540
10
60
87
97
90
12c
12b
This work
2
VB
1
(1 mol%)
2
H O
Biscoumarins
10
11
12
13
14
SDS (20 mol%)
VPy–CuO–NPs (20 mg)
[P(4-VPH)ClO ] (30 mg)
(10 mol%)
VB (1 mol%)
2
H O
2
H O
2
H O
2
H O
2
H O
60
Reflux
80
100
RT
180
15
15
25
98
93
90
97
95
8c
6l
6e
8a
P
4
4
I
2
1
60
This work
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mechanism for the synthesis of 3,3-di(indol-3-yl)indolin-2-
ones and biscoumarins also follows a similar pathway as
shown in Scheme 3.
Conclusions
Thiamine hydrochloride (VB ) has been proved to be a green,
1
efficient, selective and reusable catalyst for the synthesis of bis
(
indolyl)methanes with stirring under water, solvent-free and
grinding conditions with low catalyst loadings. The desired
products were isolated by a chromatography-free protocol and
recycling of the catalyst was demonstrated for the reaction
with water, solvent-free and grinding conditions. Green matrix
Fig. 2 Recyclability of the catalyst.
pre-treatment, with good yields (Fig. 2). The yield of the calculations showed high atom economy and small E-factor
product 3a was between 96 and 78% after ten successive cycles values for all the reactions. Industrial applications of the cata-
(
Fig. 2).
On the basis of the results obtained and the literature gram-scale. In addition, the catalyst was also found to be
reports, we have proposed a plausible mechanism for the syn- efficient to prepare several alkaloids, isatin derived BIMs and
lyst were also demonstrated by conducting the reaction on a
9
a,e,f
thesis of BIMs in the presence of VB (Scheme 2).
Initially, biscoumarins. The application of the green catalyst with atom
1
the catalyst VB activates the aldehyde through the NH proton economy and the use of eco-friendly conditions (water,
1
so that the electrophilic nature of the carbonyl group solvent-free, and grinding) can be recognized as a potential
increases, and thereby it facilitates the nucleophilic attack by green alternative to synthesize bis(indolyl)methanes, 3,3-di
indoles through the Friedel–Craft pathway and forms the (indol-3-yl)indolin-2-ones and biscoumarins selectively.
3
-phenylindolyl methanol intermediate I, which was converted
into intermediate II by the loss of a water molecule. The inter-
mediate II, an enamine intermediate, which is further acti-
vated by the catalyst undergoes another nucleophilic attack by
Conflicts of interest
the second indole moiety to obtain bis(indolyl)methane as There are no conflicts to declare.
shown in Scheme 3. To support the proposed mechanism, we
have synthesized the intermediate I (3-phenylindolylmethanol)
1
4
by the reaction of indole with benzaldehyde. The intermedi-
ate I, when reacted with 1 equiv. of indole in the presence of
Acknowledgements
the VB
1
catalyst under solvent-free conditions at RT afforded R. B. R. D. Y. is grateful to DST-SERB, New Delhi, for the Early
the desired BIM, which indicates that the reaction proceeds via Career Research Award (Grant No. ECR/2016/001041) and S. M.
the formation of intermediate I. Similarly, the plausible is thankful to DST-SERB for the fellowship. We also thank the
SASTRA Deemed University for providing lab space and NMR
facility.
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Scheme 3 Plausible mechanism for the synthesis of BIMs.
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