| Entry | Cobalt salt | Lewis acid | Yield/% | Selectivity | |
| 2 | 1 | 2/1 | |||
| 1 | Co(BF4)2∙6H2O | - | 10.3 | 32.0 | 24.3/75.7 |
| 2 | - | Sm(OTf)3 | 1.2 | 76.6 | 1.5/98.5 |
| 3 | Co(BF4)2∙6H2O | Sm(OTf)3 | 69.6 | 21.1 | 76.7/23.3 |
| 4 | - | Yb(OTf)3 | 4.8 | 79.1 | 5.7/94.3 |
| 5 | Co(BF4)2∙6H2O | Yb(OTf)3 | 68.3 | 24.1 | 73.9/26.1 |