Conformationally flexible, chiral quaternary ammonium bromides for asymmetric phase-transfer catalysis
Conformationally flexible, chiral quaternary ammonium bromides for asymmetric phase-transfer catalysis
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DOI：
10.1002/1521-3773(20020503)41:9
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发表时间：
2002-01-01
影响因子：
16.6
通讯作者：
Maruoka, K
中科院分区：
文献类型：
--
作者：
Ooi, T;Uematsu, Y;Maruoka, K
4a在08 ℃下在氩气氛下反应36小时，导致以62%产率形成3（R CH 2 Ph）。[5]幸运的是，获得了可观的对映选择性（64%ee），并且绝对构型被指定为R，如表1（条目1）所示。基于该结果，我们接下来检查了在柔性联苯部分的3，3 ′-位上具有β-萘基的催化剂4 b，并且如所希望的那样发现，化学产率和对映选择性都提高到85%，87%ee（R）（表1，条目2）。该反应也被手性溴化铵5顺利催化，其在手性联萘骨架上具有β-萘基（β-Np）取代基，得到87%，83%ee的具有相同R构型的3（R CH 2 Ph）（表1，条目3）。在这里，我们假设所观察到的手性效率的来源可以归因于快速平衡的非对映异构体的同手性和异手性催化剂之间的催化活性的相当大的差异;即，纯手性-4或-5主要负责有效的不对称相转移催化以产生具有高对映体过量的3（R CH 2 Ph），而杂手性-4或-5显示低反应性和立体选择性（方案2）。[6，7]为了获得支持性证据，我们制备了杂手性季铵
4a at 08C for 36 h under an argon atmosphere resulted in formation of 3 (R CH2Ph) in 62% yield.[5] Fortunately, appreciable enantioselectivity (64% ee) was attained and the absolute configuration was assigned to be R as shown in Table 1 (entry 1). Based on this result, we next examined the catalyst 4b, which has a β-naphthyl group at the 3, 3'-position of the flexible biphenyl moiety, and found, as hoped, that both chemical yield and the enantioselectivity were improved to 85%, 87% ee (R)(Table 1, entry 2). The reaction was also smoothly catalyzed by the chiral ammonium bromide 5, which has β-naphthyl (β-Np) substituents on the chiral binaphthyl framework, giving 3 (R CH2Ph) in 87%, 83% ee with the same R configuration (Table 1, entry 3). Here, we assumed that the origin of the observed chiral efficiency could be ascribable to the considerable difference of catalytic activity between the rapidly equilibrated, diastereomeric homo-and heterochiral catalysts; namely, homochiral-4 or-5 is primarily responsible for the efficient asymmetric phase-transfer catalysis to produce 3 (R CH2Ph) with high enantiomeric excess, whereas heterochiral-4 or-5 displays low reactivity and stereoselectivity (Scheme 2).[6, 7] To gain supportive evidence of this, we prepared the heterochiral quaternary ammonium