Table 1. Optimization of the Reaction Conditionsa.
| entry | catalyst (mol %) | additive | solvent | T (°C) | time (h) | 5af yield | 5a:4a ratio |
|---|---|---|---|---|---|---|---|
| 1 | ZnI2 (60) | neat | 120 | 16 | (24) | ||
| 2b | ZnI2 (60) | Ti(OEt)4 | neat | 120 | 16 | 58 | 77:23 |
| 3b | ZnBr2 (60) | Ti(OEt)4 | neat | 120 | 16 | 45 | 94:6 |
| 4b | ZnCl2 (60) | Ti(OEt)4 | neat | 120 | 16 | 36 | 89:11 |
| 5b | Ti(OEt)4 | neat | 120 | 16 | 0 | ||
| 6b | ZnI2 (80) | Ti(OEt)4 | neat | 120 | 16 | 64 | 90:10 |
| 7b | ZnI2 (60) | Ti(OEt)4 | p-cymene | 120 | 16 | 16 | 19:81 |
| 8 | ZnI2(60) | Ti(OEt)4 | neat | 120 | 16 | 77 (64) | 91:9 |
| 9c | ZnI2 (60) | Ti(OEt)4 | neat | 120 | 16 | 43 | 54:46 |
| 10 | ZnI2 (40) | Ti(OEt)4 | neat | 120 | 16 | 67 | 85:15 |
| 11 | ZnI2 (60) | Ti(OEt)4 NaI | neat | 120 | 16 | 55 | 73:27 |
| 12 | ZnI2 (60) | Ti(OEt)4 Bu4NI | neat | 120 | 16 | 17 | 29:71 |
| 13 | ZnI2 (60) | Ti(OEt)4 | neat | 120 | 8 | 16 | 41:59 |
| 13 | ZnI2 (60) | Ti(OEt)4 | neat | 110 | 16 | 44 | 83:17 |
| 14 | ZnI2 (60) | Ti(OEt)4 | neat | 130 | 16 | 65 | 97:3 |
| 15d | ZnI2(60) | Ti(OEt)4 | neat | 120 | 1 | 86 (71) | 95:5 |
| 16e | ZnI2 (60) | Ti(OEt)4 | neat | 120 | 1 | 74 | 96:4 |
| 17d | ZnI2 (60) | Ti(OEt)4 | neat | 110 | 1 | 46 | 67:33 |
| 18d | ZnI2 (60) | Ti(OEt)4 | neat | 130 | 1 | 79 | 95:5 |
| 19d | ZnI2 (60) | Ti(O-i-Pr)4 | neat | 120 | 1 | (18) | |
| 20 | ZnI2 (60) | Ti(O-i-Pr)4 | neat | 120 | 18 | 7 |
a
Unless otherwise mentioned, all reagents and additives were employed in 1 equiv.
b
2 equiv of Ti(OEt)4 were used.
c
0.5 equiv of Ti(OEt)4 were used (conventional heating).
d
The reaction was performed under microwave irradiation (MW) at 300 W.
e
The reaction was performed under microwave irradiation (MW) at 200 W.
f
Yield of allene 5a in the crude mixture (isolated yields in brackets).