Table 6.
AZ Reaction of Pre-formed Imine 1a' with Added Bases.a
| ||||||||
|---|---|---|---|---|---|---|---|---|
| entry | pKa | baseb | base x mol% | % yield 5a'c,d | % ee 5a'e | cis/transc | % yield 18a' + 19a'c | unreacted 1a' (%)c |
| 1 | – | none | 0 | 83 (85) | 90 | 100:1 | 10 | <1 |
| 2 | 10.75 | Et3N | 10 | 71 (77) | 86 | 25:1 | 9 | 6 |
| 3 | 20 | <1 | – | – | – | 89 | ||
| 4 | 40 | <1 | – | – | – | 92 | ||
| 5 | 10.60 | Ph2CHNH2 | 10 | 77 (80) | 89 | 27:1 | 10 | <1 |
| 6 | 20 | 10 | _ | 12:1 | 2 | 86 | ||
| 7 | 9.20 | DMAP | 10 | <1 | _ | – | – | 96 |
| 8 | 20 | <1 | – | – | – | 80 | ||
| 9 | 4.60 | C6H5NH2 | 10 | 73 (78) | 88 | 33:1 | 8 | 5 |
| 10 | 20 | 17 (20) | 86 | 23:1 | 6 | 68 | ||
a
Unless otherwise specified, the boroxinate catalyst 9 was prepared in CHCl3 from 10 mol% (S)-VAPOL, 30 mol% B(OPh)3 and x mol% of a base to induce boroxinate formation. After 1h at 25 °C, 100 mol% of imine 1a' and 120 mol% EDA 2 were added to the catalyst and the aziridination reaction carried out with 1.0 mmol of imine (0.5 M) for 24 h at 25 °C.
b
All bases were purified by distillation or sublimation.
c
Determined from the 1H NMR spectrum of the crude reaction mixture with Ph3CH as internal standard.
d
Yields in parentheses are after isolation by silica gel chromatography.
e
Determined on purified 5a' by chiral HPLC.