Table 2. Investigation of Background Rate at Low Temperature.
| conv,b (%) (ketone recovered) |
|||||
|---|---|---|---|---|---|
| entry | N-Het-SPh | temp, °C | Brønsted acida | 2 h | 24 h |
| 1 | 3 | –10 | CF3CO2H | 89.9 (9.2) | 88.4 (11.6) |
| 2 | 3 | –78 | CF3CO2H | 61.5 (6.4) | 73.7 (8.4) |
| 3 | 4 | 25 | – | 47.6 (12.3) | 78.6 (21.3) |
| 4 | 4 | –10 | – | 31.9 (6.1) | 67.3 (8.7) |
| 5 | 4 | –10 | Cl3CCO2H | 50.1 (27.2) | 49.8 (28.6) |
| 6 | 4 | –20/–25 | – | 3.1 (3.6) | 19.6 (4.5) |
| 7 | 4 | –50 | – | 3.9 (2.8) | 12.7 (2.9) |
| 8 | 4 | –78 | – | 5.4 (7.5) | 8.8 (7.2) |
| 9 | 4 | –78 | Cl3CCO2H | 41.3 (52.9) | 44.8 (51.1) |
a
1.0 equiv was used and added 10 min after the addition of the silyl enol ether.
b
Monitored by GC with internal standard.