Table 1 Optimization of the catalytic system for formal [3+2] cycloaddition of α-ketoeseter 1a with (E)-nitrone 2a.
From: Naked d-orbital in a centrochiral Ni(II) complex as a catalyst for asymmetric [3+2] cycloaddition
| ||||||
|---|---|---|---|---|---|---|
Entry | Metal source | (R,R )-diamine 4R | Additive (x mol%) | Yield (%)* | d.r.† | e.e. (%) |
1 | Ni(OAc)2·4H2O | 4a: benzyl | — | 91 | 20/1 | 50 |
2 | Cu(OAc)2·2H2O | 4a: benzyl | — | Trace | ND | ND |
3 | Zn(OAc)2·2H2O | 4a: benzyl | — | 19 | 3/1 | 70 |
4 | Pd(OAc)2 | 4a: benzyl | — | Trace | ND | ND |
5 | Ni(octanoate)2·nH2O | 4a: benzyl | — | 87 | 20/1 | 44 |
6 | Ni(benzonoate)2·nH2O | 4a: benzyl | — | 72 | 20/1 | 44 |
7 | NiBr2 | 4a: benzyl | — | 63 | 15/1 | 24 |
8 | Ni(OAc)2·4H2O | 4b: isobutyl | — | 88 | 30/1 | 40 |
9 | Ni(OAc)2·4H2O | 4c: isopropyl | — | 94 | 20/1 | 64 |
10 | Ni(OAc)2·4H2O | 4d: 3-pentyl | — | 83 | >50/1 | 61 |
11 | Ni(OAc)2·4H2O | 4e: cyclohexyl | — | 88 | >50/1 | 83 |
12‡ | Ni(OAc)2·4H2O | 4e: cyclohexyl | Et3N (10) | 88 | >50/1 | 83 |
13‡ | Ni(OAc)2·4H2O | 4e: cyclohexyl | iPr2NH (10) | 89 | >50/1 | 91 |
14§ | Ni(OAc)2·4H2O | 4e: cyclohexyl | iPr2NH (10) | 71 | >50/1 | 91 |
