Table 2. Catalyst Screening for the Synthesis of 3-Indolylquinones from Hydroquinone.
| entry | catalyst | yieldsa, % |
|---|---|---|
| 1 | 00/86c | |
| 2 | PVP | 00/73c |
| 3 | Fe3O4 | 00/81c |
| 4 | Amberlyst-15 | 12/67c |
| 5 | PWA | 75/9c |
| 6 | PVP–PWA | 83 |
| 7 | PVP–phosphomolybdic acid (PMA) | 72/10c |
| 8 | PVP–silicotungstic acid (STA) | 65/20c |
| 9 | Fe3O4/PVP–PWA (131) | 75/6c |
| 10 | Fe3O4/PVP–PWA (141) | 94 |
| 11 | Fe3O4/PVP–PWA (181) | 42/32c |
| 12 | Fe3O4/PVP–PWA (141) | 95b |
| 13 | Fe3O4/PVP–PWA (141) | 97a,d |
a
Reactions were performed with 1:1 2-methylhydroquinone/2-methylindole, 0.1 g Fe3O4/PVP–PWA (141), 20 mol % Ag2O, 1.5 [quinone]/3 [hydroquinone] mmol H2O2, and 10 mL of THF at RT for 5 h; isolated yields of 3a after column chromatography. Bold values represent maximum product yield under optimized reaction conditions.
b
Reflux condition.
c
Byproduct 2-methylbenzoquinone was obtained (5).
d
2-Methylbenzoquinone was used as the reactant.