Optimization of reaction conditionsa.
| |||||
|---|---|---|---|---|---|
| Entry | VB1 (x mol%) | Base (y equiv.) | Solvent | T (°C) | Yieldb (%) |
| 1 | 20 | DABCO (2) | EtOH | 40 | 58 |
| 2 | 30 | DABCO (2) | EtOH | 40 | 49 |
| 3 | 20 | DABCO (3) | EtOH | 40 | 47 |
| 4 | — | DABCO (2) | EtOH | 40 | 0 |
| 5 | 20 | — | EtOH | 40 | 0 |
| 6 | 20 | Na2CO3 (2) | EtOH | 40 | <10 |
| 7 | 20 | Et3N (2) | EtOH | 40 | 10 |
| 8 | 20 | NaOH (2) | EtOH | 40 | <10 |
| 9 | 20 | DABCO (2) | EtOH | 80 | 65 c |
| 10 | 20 | DABCO (2) | CHCl3 | 40 | 49d |
| 11 | 20 | DABCO (2) | DMSO | 40 | 22d |
| 12 | 20 | DABCO (2) | Toluene | 40 | Traced |
| 13 | 20 | DABCO (2) | THF | 40 | Traced |
| 14 | 20 | DABCO (2) | Acetone | 40 | 57d |
| 15 | 20 | DABCO (2) | Acetone | 60 | 63 d |
| 16 | 20 | DABCO (2) | Acetone | 60 | 61d,e |
a
Reaction conditions: 4-nitrobenzaldehyde (1a, 1 mmol), VB1 (0–0.6 mmol), and base (0–3 mmol) in EtOH (2a, 2 mL) at 40–80 °C (oil bath) under air for 6 h.
b
Isolated yield.
c
12 h.
d
Reaction conditions: 4-nitrobenzaldehyde (1a, 3 mmol), EtOH (2a, 1 mmol), VB1 (0.2 mmol), and DABCO (2 mmol) in solvent (2 mL) at 40–60 °C (oil bath) under air for 12 h.
e
O2 is used as an oxidant.