Condensation of indoles and aldehydes in subcritical water without the addition of catalysts

Article abstract of DOI:10.1246/bcsj.20150247

A series of 3,3′-diindolylmethanes were prepared in high yields from indoles and aldehydes under subcritical water conditions without the addition of catalysts. 3-Alkenylindoles were also obtained in good yields from aldehydes bearing benzylic hydrogen atoms.

Full text of DOI:10.1246/bcsj.20150247

Condensation of Indoles and Aldehydes in Subcritical Water
without the Addition of Catalysts
Tsunehisa Hirashita,* Masaki Ogawa, Reina Hattori, Sota Okochi, and Shuki Araki
Omohi College, Graduate School of Engineering, Nagoya Institute of Technology,
Gokiso-cho, Showa-ku, Nagoya, Aichi 466-8555
E-mail: hirasita@nitech.ac.jp
Received: July 21, 2015; Accepted: September 12, 2015; Web Released: September 25, 2015
Tsunehisa Hirashita
Tsunehisa Hirashita is an Associate Professor of Nagoya Institute of Technology (NITech). He received his
Ph.D. degree from NITech (1999, Professor M. Kawai). He was appointed as an Assistant Professor of NITech
(
(
Professor S. Araki’s group) in 2002, and was promoted to an Associate Professor in 2011. He spent one year
2005–2006) in the group of Professor R. A. Sheldon at Delft University of Technology (The Netherlands). His
current research interest is mainly in the development of clean chemical methods for organic reactions,
specifically in the area of “greener” solvents such as (near-critical) water and (meso) ionic liquids.
Abstract
Results and Discussion
A series of 3,3¤-diindolylmethanes were prepared in high
yields from indoles and aldehydes under subcritical water
conditions without the addition of catalysts. 3-Alkenylindoles
were also obtained in good yields from aldehydes bearing
benzylic hydrogen atoms.
We started our study with the reaction of indole 1a and
benzaldehyde (2a) as model partners according to the con-
1
7
ditions used in the reactions between alcohols and indoles.
A
3
mixture of 1a (1.2 mmol) and 2a (1.0 mmol) in water (15 cm )
3
was placed in a Teflon container (30 cm ) supported by an SUS
2
2
3
16 outer jacket and heated in an electric dryer for 1 h at the
3
,3¤-Diindolylmethane (DIM) scaffolds, isolated from natu-
set temperature.
1
ral sources, have been receiving increasing attention because
of their medicinal properties, including anticancer activity. A
At 100 °C, DIM 3a was obtained in 39% yield, and both the
starting materials remained largely intact (Entry 1). With an
increase in temperature, the yield increased and reached a
plateau at 150 °C (Entries 3 and 4). However, at 60 °C, the DIM
3a was obtained only in 66% yield, even after the reaction was
continued for 6 h (Entry 5). The maximum yield was observed
when the reaction was carried out at 100 °C for 6 h (Entry 6).
When a stoichiometric amount of 1a (2 mmol) was used and the
reaction was performed at 170 °C for 1 h, 3a was obtained in
80% yield (Entry 7). In this screening, all reactions gave 3a
alone and excess benzaldehyde was partially recovered. Further
2
convenient approach for the synthesis of DIM and its deriv-
atives is based on the reaction of indoles with aldehydes in the
3
4
5
presence of Brønsted or Lewis acids, iodine, montmorillon-
6
7
8
9
ite, zeolite, silica gel, nanoporous aluminosilicate, surfac-
tants, TiO nanoparticles, benzyltriphenylphosphonium tri-
bromide, glycerol, and amino catalysts. Direct synthesis
of DIM from benzyl alcohols and indoles has been recently
reported. Solvent-free reactions of DIMs have also been
reported.
During our study on environmentally benign organic
synthesis, we became interested in reactions in high-temper-
1
0
11
2
1
2
13
14
1
5
4
f,16
2
3
cyclization from 3a with 2a leading to indolo[3,2-b]carbazole
(2:2 product) was not observed during these reactions.
1
7
1
8,19
ature water.
Water that is super-heated above its boiling
The conditions shown in Table 1 presumably did not reflect
the real temperature in the reaction vessel, because temperature
was adjusted at the electric dryer, and there may have been a
non-negligible delay before the final temperature was attained
by the interior of the reactor. The results in Entries 1 and 6
indicated, however, that 6 h would be sufficient for completion
of the reaction. Thus, we used 170 °C and 6 h for subsequent
reactions performed with a ratio of 2:1 1a to carbonyl com-
pound and proceeded to determine the scope and limitations of
carbonyl substrates. The results are shown in Table 2.
point shows unique characteristics that are not observed in the
case of water at ambient temperature, i.e., a marked increase in
2
0
the ionic product and a decrease in the dielectric constant.
Therefore, superheated water is a very good solvent for most
organic compounds; acid/base reactions can proceed without
the addition of catalysts and organic compounds become
miscible. Aside from regeneration of useful resources from
organic wastes in near-critical water,21 there have been few
reports of organic synthesis in water at high temperatures.
Herein, we report the direct coupling of indoles and aldehydes
in subcritical water without any added catalyst.
With benzaldehyde, 3a was obtained in 90% yield. p-
Anisaldehyde, p-hydroxybenzaldehyde, and p-formylbenzoic
1760 | Bull. Chem. Soc. Jpn. 2015, 88, 1760–1764 | doi:10.1246/bcsj.20150247
© 2015 The Chemical Society of Japan

Table 1. Reactions of indole 1a with benzaldehydea)
Ph
1
1
70 °C, 6 h
+
2a
Ph
2
2
H O
2
N
+
PhCHO
Me
1b
1c
MeN
NMe
H O
2
N
H
3
o: 72%
1a
2a
HN
3a
NH
Entry
1a:2a
Conditions
Yield/%b)
Ph
70 °C, 6 h
1
2
3
4
5
6
7
1.2:1
1.2:1
1.2:1
1.2:1
1.2:1
1.2:1
2:1
100 °C, 1 h
120 °C, 1 h
150 °C, 1 h
170 °C, 1 h
60 °C, 6 h
39
67
88
89
66
95
80
+ 2a
Me
H O
2
N
H
Me NH
Me
p: 77%
HN
3
100 °C, 6 h
170 °C, 1 h
Me
Me
170 °C, 6 h
Me
a) Conditions: all reactions were conducted using 1a (1.2 mmol
+
2a
2
H O
NH
3
2
HN
or 2.0 mmol) and 2a (1.0 mmol) in water (15 cm ), in a Teflon
N
H
3
container (30 cm ). b) Based on 1a.
Ph
q: 48%
1d
3
Table 2. Synthesis of DIM in subcritical watera)
Scheme 1. Reactions of 1-, 2-, and 3-methylindoles.
R
O
1
70 °C, 6 h
+
RCHO
1
a
+
H O
H
3m
N
2
H
HN
NH
OH
1a
3
200 mol%
H SO (5 mol%), 120 °C, 6 h, H O, 92%
OMe
2
4
2
NaOH (10 mol%), 120 °C, 6 h, H O, 7%
2
1
70 °C, 6 h, neat, 30%
Scheme 2. Effect of acid, base, and water.
Ph
acid gave the corresponding DIMs 3b, 3c, and 3e in high
yields, although p-dimethylaminobenzaldehyde afforded 3d in
relatively low yield. The presence of electron-withdrawing
groups such as ester, cyano, and nitro groups, at the ortho and
para positions of benzaldehyde did not affect the reaction, and
3
3
a: 90%
3b: 79%
3c: 58%
NMe2
COOH
CO Me
2
3
f­3h were obtained in high yields. The cyano and ester groups
remained intact under the subcritical conditions. However, the
sterically demanding o-substitution resulted in a lower yield of
d: 17%
3e: 83%
3f: 95%
3
i. The heteroaromatic aldehyde 3-thiophenecarboxaldehyde
proved to be a good substrate, while indolyl aldehyde proved to
be a poor substrate and gave triindolylmethane 3k in low yield.
Aliphatic aldehydes were converted into the corresponding
DIMs 3l­3n in moderate yields.
CN
O N
Ph
2
We next turned our attention towards substituted indoles. 1-,
2
2
-, and 3-Methylindoles were subjected to the reaction with
a. 1-Methylindole and 2-methylindole gave the corresponding
3g: 86%
3h: 80%
3i: 34%
DIMs 3o and 3p, respectively, in good yields whereas 3-
methylindole gave 2,2¤-DIM 3q in only 48% yield (Scheme 1).
In order to examine the role of water, the reaction was
carried out in both DMSO and o-xylene, as representative polar
and nonpolar solvents. When the reaction of 1a and 2a was
performed in o-xylene at 100 °C for 6 h, 3a was obtained in 7%
yield and 74% of 1a were recovered. In DMSO only trace
amounts of 3a were obtained at 100 °C and even at 170 °C 3a
was generated in only 35% yield. These results clearly indicate
that the reaction is promoted by water. Next, the effect of acid/
base addition and water as a solvent was examined to obtain
mechanistic insights into this reaction (Scheme 2). Addition
of sulfuric acid (5 mol %) to the reaction with cyclohexane-
carbaldehyde increased the yield of 3m to 92%, while the
S
HN
Me
3
3
j: 80%
3k: 9%
3l: 44%
Me
5
m: 52%
3n: 69%
a) All reactions were performed with 1a (2.0 mmol) and
aldehyde (1.0 mmol) in water (15 cm ).
3
Bull. Chem. Soc. Jpn. 2015, 88, 1760–1764 | doi:10.1246/bcsj.20150247
© 2015 The Chemical Society of Japan | 1761

p-HOC H CHO
Ph
6
4
Ar
(
1.0 mmol)
170 °C, 6 h
H
H
1
a
+
O
and
H O
(
2.0 mmol)
2
R'
HN
NH
N
R
N
R
R'
p-NCC H CHO
6
4
3c: 3g = 15:85
6
1
Ph
Ph
(
1.0 mmol)
Ph
Scheme 3. Competitive reaction.
R'
R'
N
R
N
H
OH
R'
Me
, 6 h
H O
R
2
5
+
Ph
CHO
N
R
+
2
N
H+
1
1
c: R = H
R
e: R = Me
Ph
Ph
Ph
OH2
R'
Ph
+
Me
Me
N
R
R'
N
N
R
N
R
Me
N
R
R
A
5a: R = H
5b: R = Me
6a: R = H
6b: R = Me
H O
2
Scheme 5. A plausible reaction mechanism.
120 °C, 5a/6a: 92%/trace; 5b/6b: 46%/40%
170 °C, 5a/6a: 74%/21%; 5b/6b: 37%/46%
220 °C, 5a/6a: 18%/61%; 5b/6b: 0%/80%*
Ph
*
With 1e (100 mol%)
,
6 h
Ph
+
1c
Me
Me
N
Scheme 4. Reaction of phenylacetaldehyde with indoles.
N
H
HN
H
Me
5
a
6a
2
1
20 °C (H O/neat), 6a: 76%/76%; 1c: 75%/83%
addition of sodium hydroxide (10 mol %) and the reaction
under solvent-free conditions at 170 °C resulted in lower yields.
In order to gain further information about the mechanism, a
pair of aromatic aldehydes possessing electron-donating and
electron-withdrawing groups was reacted competitively with
2
20 °C (H O/neat), recovery of 5a: 97%/100%
2
Scheme 6. Elimination of 2-methylindole from DIM 5a.
1
a (Scheme 3). The strong preference for more electrophilic 3g
can be explained by a Friedel­Crafts mechanism.
-Alkenylindole derivatives are useful intermediates in the
synthesis of natural products, exemplified by a dienophile for
confirm the elimination of an indole molecule from DIM under
the present conditions, we exposed isolated DIM 5a alone to
high temperatures. At 220 °C, 5a was completely converted
into 3-alkenylindole 6a and 1c within 6 h, with a ratio of 1:1
n good yields in the presence or absence of water, indicating
that elimination of 2-methylindole from 5a occurred effec-
tively at this temperature irrespective of the presence of water
(Scheme 6). At 120 °C, 5a remained intact under both the
conditions. A comparison of the results in Schemes 4 and 6
confirms that multiply substituted indoles tend toward forma-
tion of 3-alkenylindole 6 even at a lower temperatures while
the elimination of mono-substituted indoles requires higher
temperatures. In the case of unsubstituted indole, only the
corresponding DIM was obtained in 67% yield at 170 °C, while
at 220 °C a complex reaction mixture was obtained wherein
neither the corresponding DIM nor 3-alkenylindole could be
found. These observations can be accounted for by assuming
that substitution of indole with alkyl groups increases the
stability of the intermediate A by their electron-donating nature
3
24
Diels­Alder cyclization and a precursor to unsymmetrical
2
5
DIMs. We found that an aldehyde bearing benzylic hydro-
gen atoms gave the corresponding 3-alkenylindoles at higher
reaction temperatures as reported.26 When 1c was treated with
phenylacetaldehyde, a mixture of DIM 5a and 3-styrylindole
6
a was obtained (Scheme 4). At 120 °C, 5a was exclusively
formed, while the formation of 3-alkenylindole 6a become
gradually predominant at higher temperatures. When 1,2-di-
methylindole was subjected to the reaction, 3-alkenylindole 6b
was obtained even at 120 °C, and a selective formation of 6b
was achieved at 220 °C.
A plausible reaction mechanism for the alkylation and
alkenylation is depicted in Scheme 5: indoles undergo acid-
catalyzed alkylation with aldehydes at the 3-position to give
iminium intermediates A, which react with a second indole to
give the corresponding DIMs. In the case of phenylacetalde-
hyde, however, abstraction of the benzylic proton can provide
an alternative path to 3-alkenylindoles by elimination of an
indole moiety from DIM 5 and/or elimination of the benzylic
proton directly from the iminium intermediate A. The above
scenario leading to 3-alkenylindoles has already been proposed
2
6c
and facilitates the elimination of indole and proton.
Finally, we assessed the present reaction with special empha-
2
7
sis on the E-factor. The proposed protocol can obviate the
need for extraction by an organic solvent because water alone
is employed as the reagent/solvent, provided that the reaction
goes to completion and the product is insoluble in water. A
quick look at Table 2 shows that 3f, obtained as a solid, was
2
6
for reactions in the presence of Brønsted or Lewis acids. To
1762 | Bull. Chem. Soc. Jpn. 2015, 88, 1760–1764 | doi:10.1246/bcsj.20150247
© 2015 The Chemical Society of Japan

almost quantitatively obtained under the present conditions.
Thus, we tried to isolate 3f directly from the reaction mix-
ture by simple filtration. After the reaction was carried out for
was filtered off and dried under reduced pressure yielding 3f
(363 mg) in 95% yield. H NMR analysis confirmed the purity
of the product.
1
6
h, the product was filtered off from the aqueous phase and
dried under reduced pressure giving 3f in 95% yield. These
results clearly demonstrate that this process is greener than the
conventional procedures; the amount of materials required for
the reaction and purification is minimized, implying that the
E-factor value is close to zero.
We thank Professor Y. Yamasaki (Hosei University) for
assistance with the operation of the hydrothermal reactions and
for fruitful discussions. This research was partially supported
by Tanikawa Fund Promotion of Thermal Technology.
Supporting Information
Conclusion
Experimental procedures and characterization of products,
and copies of H and C NMR spectra. This material is avail-
able electronically on J-STAGE.
1
13
A variety of 3,3¤-DIM derivatives were synthesized by
the reaction between indoles and aldehydes in water under
uncatalyzed subcritical conditions. In cases of aldehydes
having benzylic hydrogen atoms, 3-alkenylindoles were ob-
tained in good yields. The reaction can be easily conducted by
heating in an electric oven, a commonly available laboratory
device. The reaction is safe and clean, since non-flammable
water alone is used as the reagent/solvent. This work may
expand the synthetic utility of high-temperature water in the
field of organic chemistry. Further study of reactions under
these conditions is now in progress.
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Materials and Methods. All reactions were carried out
¹
1
in ion-exchanged water (<0.08 mS cm ), which was obtained
by an ORGANO PURE LITE PRB-002A. Teflon containers
and SUS 316 outer jackets were purchased from Shikokurika
2
2a
Co., Ltd.
Representative Procedure for Preparation of 3a (Entry 7
in Table 1). A mixture of benzaldehyde (102 ¯L, 1.0 mmol),
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1764 | Bull. Chem. Soc. Jpn. 2015, 88, 1760–1764 | doi:10.1246/bcsj.20150247
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