Optimization of the reaction conditions of benzimidazole^a.

Entry	Variations from the standard conditions	Yieldb (%)
1	None	93
2	4-Amino-TEMPO instead of TEMPO	57
3	4-Hydroxy-TEMPO instead of TEMPO	64
4	4-Oxo-TEMPO instead of TEMPO	58
5	No TEMPO	Trace
6	TBAB instead of TBAPF6	36
7	TBAI instead of TBAPF6	23
8	DMSO/H2O (v/v = 1 : 2) as solvent	76
9	DMF/H2O (v/v = 1 : 2) as solvent	33
10	THF/H2O (v/v = 1 : 2) as solvent	46
11	1,4-Dioxane/H2O (v/v = 1 : 2) as solvent	Trace
12	H2O as solvent	73
13	No current	Trace
14	40 mA	49
15	60 mA	64
16	C (+)/Pt (−)	31
17	5 h	92

^aStandard conditions: undivided cell, Pt anode (1.0 cm × 1.0 cm), Pt cathode (1.0 cm × 1.0 cm), 1a (0.5 mmol), 2a (0.8 mmol), TEMPO (0.1 mmol), TBAPF₆ (0.15 M), CH₃CN/H₂O (v/v = 1 : 2, 3 mL), I = 80 mA at room temperature for 3 h.

^bIsolated yield.