Table 1 Optimization of aerobic oxidation reaction conditions of diethyl 2,6-dimethyl-4-phenyl-1,4-dihydropyridine-3,5-dicarboxylate in the presence of Fe–N–C/DDQ catalyst system and compare it with Fe–N–C catalyst precursors.

Entry	Catalyst	Amount of catalyst (mg)	DDQ (mol %)	Solvent	Time (h)	GC yield (%)
1	Fe–N–C	100	20	MeCN	1.96	99
2	Fe–N–C	80	20	MeCN	2.42	99
3	Fe–N–C	60	20	MeCN	6.24	99
4	Fe–N–C	50	20	MeCN	18	93
5	Fe–N–C	70	20	MeCN	4.25	99
6	Fe–N–C	70	15	MeCN	5.32	97
7	Fe–N–C	70	5	MeCN	21	88
8	Fe–N–C	70	10	MeCN	6.42	99
9	Fe–N–C	70	10	H₂O/MeCN	6.51	99
10	Fe–N–C	70	10	H₂O	24	71
11	Fe–N–C	70	10	EtOH	18	95
12	Fe–N–C	0	10	MeCN	24	23
13	Fe–N–C	70	0	MeCN	24	20
14	Fe-ZIF	70	10	MeCN	24	31
15	FeC1₂.4H₂O	49.7 (0.25 mmol)	10	MeCN	24	24
16	Fe₃O₄	20	10	MeCN	24	23
17	CoFe₂O₄	20	10	MeCN	24	21
18	Fe–N–C	70	10	H₂O/MeCN	12	99^a

Reaction condition: 1,4-dihydropyridines (1 mmol), DDQ, Fe–N–C, and solvent (2 mL) under O₂ (balloon) at room temperature.
^aThe reaction was performed in an open flask.

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