An Eco-Friendly Improved Protocol for the Synthesis of Bis(3-indolyl)methanes Using Poly(4-vinylpyridinium)hydrogen Sulfate as Efficient, Heterogeneous, and Recyclable Solid Acid Catalyst

Table 2

Synthesis of bis(3-indolyl)methanes catalyzed by P(4-VPH)HSO₄ under solvent-free conditions.

Entry^a	Aldehyde	Product	Time (min)	Yield^b (%)	Melting points (°C)
					Found	Lit. [Reference]
1	Benzaldehyde	3a	12	94	150–152	150–152 [7]
2	2-Chlorobenzaldehyde	3b	14	92	71–73	72–74 [7]
3	4-Chlorobenzaldehyde	3c	12	96	76–78	76-77 [7]
4	4-Fluorobenzaldehyde	3d	10	90	78–80	80–82 [9]
5	2-Nitrobenzaldehyde	3e	14	88	140–142	140-141 [18]
6	3-Nitrobenzaldehyde	3f	14	89	262–264	264-265 [20]
7	4-Methylbenzaldehyde	3g	10	92	95–97	94–96 [7]
8	4-Methoxybenzaldehyde	3h	10	92	186–188	187–189 [7]
9	3,4-Dimethoxybenzaldehyde	3i	10	94	195–197	197-198 [19]
10	2-Hydroxybenzaldehyde	3j	12	89	342–344	340–342 [18]
11	4-Hydroxybenzaldehyde	3k	10	90	124–126	124–126 [20]
12	4-Hydroxy-3-methoxybenzaldehyde	3l	10	91	126–128	126-127 [22]
13	4-(Dimethylamino)benzaldehyde	3m	14	90	168–171	170–172 [20]
14	Furan-2-carbaldehyde	3n	16	89	321–323	322–325 [7]

aReaction conditions: indole (2 mmol), aldehyde (1 mmol), and P(4-VPH)HSO₄ (15 mg), grinding at RT.
bIsolated yields.