| Entry | Brønsted acid (mol%) | Cobalt salt | Time/h | Yield/% | |
| 2 | 1 | ||||
| 1 | H2SO4(20) | Co(BF4)2∙6H2O | 1 | 0.0 | 66.4 |
| 2 | H2SO4(20) | Co(BF4)2∙6H2O | 1.5 | 7.6 | 63.0 |
| 3 | H2SO4(20) | Co(BF4)2∙6H2O | 2 | 56.7 | 22.5 |
| 4 | H2SO4(20) | Co(BF4)2∙6H2O | 6 | 61.2 | 33.8 |
| 5 | H2SO4(20) | Co(BF4)2∙6H2O | 12 | 62.9 | 27.4 |
| 6 | H2SO4(20) | Co(BF4)2∙6H2O | 18 | 75.7 | 7.3 |
| 7 | H2SO4(20) | Co(BF4)2∙6H2O | 24 | 86.3 | 6.6 |