Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia StrategyP-09

Friedel–Crafts Reaction of Indoles with Aldehydes Catalyzed by a ScandiumBased Coordination Polymer
Pennapa Tungjratthitikan,1 and Hiroshi Furuno2
1

Interdisciplinary Graduate School of Engineering Sciences, Kyushu University
2
Green Asia Education Center, Kyushu University
Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan,Abstract
Air-stable scandium-based coordination polymer, trivalent scandium biphenyl-4,4'-disulfonate[Sc2(BPDS)3], effectively catalyzed the Friedel–Crafts reaction of indoles with aromatic
aldehydes under heterogeneous conditions, and aryl(diindolyl)methanes were obtained in highchemical yields. After the reaction, the catalyst could be recovered and reused for the next
reaction without serious loss of the activity.
aldehydes under heterogeneous conditions, in order toextend usefulness of the coordination polymer as areusable catalyst in organic syntheses.

1. Introduction
Utilization of heterogeneous catalysts in organic
transformations has attracted much attention fromenvironmentally-friendly practical and economical
points of view because they generally have anadvantage in recover and reuse over homogeneous
ones. In particular, coordination polymer-type
catalysts have recently attracted much attention due toeasy preparation by simply mixing a metal ion, amulti-way ligand, and other components, relatively
large surface areas with regularly dispersed activecenters, and so on.
We have focused on the unique characteristics ofthe rare earth metal salts of strong acids, which could
easily handle in air and effectively work as a Lewis
acid catalyst even in the presence of Brønsted basessuch as water and amines, and have synthesized rareearth arylsulfonates complexes, RE2(BPDS)3, as arare earth metal-based coordination polymer (Scheme1) [1–3]. The scandium complex could effectively
catalyze the ring-opening of epoxides with amines,
and the hetero-Diels–Alder reaction of aldehydes or
ketones with the Danishefsky’s diene underheterogeneous conditions (Scheme 2). These
reactions smoothly proceeded at room temperature,
particularly under solvent-free conditions, and theproducts were obtained in high yields. After thereaction, the catalysts could be easily recovered bysimple filtration or centrifugation, and reused for thenext reaction for several times without any loss of thecatalytic activity.
We therefore applied the scandium catalyst to theFriedel–Crafts reaction of indoles with aromatic

Scheme 1. Preparation of rare earth coordination
polymer complexes RE2(BPDS)3

Scheme 2. Sc2(BPDS)3-catalyzed reactions: (a) the ringopening of epoxides with amines; (b) the hetero-Diels–Alder reaction of carbonyl compounds with theDanishefsky’s diene

2. Results and Discussion
The Friedel–Crafts reaction was carried out bymixing pre-dried Sc2(BPDS)3 (2.5 mol%: 5 mol%for Sc), an indole (2 equiv) and an aldehyde in asolvent under argon. The scandium catalyst
effectively promoted the reaction between variousindoles and aromatic aldehydes under24

Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

P-09
heterogeneous conditions, and the corresponding
aryl[di(3-indolyl)]methanes were obtained in highyields (Tables 1 and 2). The reaction of 3methylindole with benzaldehyde afforded di(2indolyl)phenylmethane in high yield (Scheme 3).
The Sc2(BPDS)3 also catalyzed the reaction of 7azaindole (Scheme 3).
Table 1. Sc2(BPDS)3-catalyzed Friedel–Crafts
reaction of indoles with benzaldehyde

EntryR1

R2

R3Conditions

Scheme 3. Sc2(BPDS)3-catalyzed Friedel–Crafts
reaction of 3-methylindole or 7-azaindole withbenzaldehyde

The reusability of Sc2(BPDS)3 was also checked inthe Friedel–Crafts reaction of 5-methylindole withbenzaldehyde (30 oC, 6 h). After the reaction, thecatalyst was separated from the product and substrates
by centrifugation, and then reused for the next reaction
after drying by heating. It was confirmed thatSc2(BPDS)3 could be used at least three times without
serious loss of the catalytic activity (1st use: 99%, 3rduse: 82%).

Yield(%)[a]

1H

H

H

30 oC, 0.5 h>992

Me

H

H

60 C, 2 h393

H

Me

H

100 oC, 25 h

424

H

H

Me

60 C, 0.5 h>995

H

HOMe

60 C, 0.5 h>996

H

H

Br

30 C, 12 h>997

H

H

NO2100 C, 48 h50ooooo3. Conclusion
We succeed in applying the scandium coordination
polymer Sc2(BPDS)3 to the Friedel–Crafts reaction ofvarious indoles with aromatic aldehydes. We also
confirmed that the catalyst could be recovered andreused without serious loss of the catalytic activity
even in the 3rd use.

[a] Determined by NMR.
Table 2. Sc2(BPDS)3-catalyzed Friedel–Crafts
reaction of indole with aromatic aldehydes

Acknowledgment
Entry

ArConditions

We thank to Advanced Graduate Program in
Global Strategy for Green Asia for research financial
support. We also much appreciate Prof. Hideo
Nagashima for his material support, and Nagashimalaboratory members for their kindly help.

Yield(%)[a]

12-HO-C6H430 oC, 24 h

72

24-HO-C6H430 C, 24 h>99

34-MeO-C6H430 C, 24 h>99

44-Br-C6H460 C, 48 h>99

54-F-C6H460 C, 48 h>9964-NC-C6H460 oC, 24 h

95

74-O2N-C6H4o

100 C, 24 h

54

82-HO-4-Br-C6H330 C, 24 h

99ooooo

Reference[1] S. Ishida, T. Hayano, H. Furuno, J. Inanaga,Heterocycles, 66, 645–649 (2005).
[2] S. Ishida, S. Suzuki, T. Hayano, H. Furuno, J.
Inanaga, J. Alloys Compd, 408–412, 441–443(2006).
[3] H. Furuno, S. Ishida, S. Suzuki, T. Hayano, S.
Onitsuka, J. Inanaga, Heterocycles, 77, 1007–1018(2009).

[a] Determined by NMR.

Email: 3ES15030Y@ s.kyushu-u.ac.jp

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