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 3

Comparing the efficiency of P(4-VPH)HSO₄ with some reported acid catalysts for the synthesis of bis(3-indolyl)phenylmethane (3a).

Entry^a	Catalyst (g)	Reaction conditions	Time (min)	Yield (%) [reference]
1	Boric acid (0.0124 g)	Neat, 80°C	60	94 [8]
2	Oxalic acid (0.09 g)	H2O, 80°C	40	96 [26]
3	Sulfamic acid (0.0485 g)	MeOH, RT	180	90 [10]
4	PEG-SO3H (0.045 g)	H2O, RT	20	92 [27]
5	Dodecyl sulfonic acid (0.025 g)	H2O, RT	25	95 [17]
6	Polyvinylsulfonic acid (0.0216 g)	EtOH, RT	120	93 [16]
7	Silica sulfuric acid (0.1 g)	Neat, RT	40	92 [12]
8	Cellulose sulfuric acid (0.1 g)	Grinding, RT	8	93 [13]
9	P(4-VPH)HSO4 (0.015 g)	Grinding, RT	12	94 [present work]

Note: For comparison, mole percentages were converted into grams.