Transition-metal aqua complexes of 4,6-dibenzofurandiyl-2,2′-bis(4-phenyloxazoline).: Effective catalysis in Diels-Alder reactions showing excellent enantioselectivity, extreme chiral amplification, and high tolerance to water, alcohols, amines, and acids
Transition-metal aqua complexes of 4,6-dibenzofurandiyl-2,2′-bis(4-phenyloxazoline).: Effective catalysis in Diels-Alder reactions showing excellent enantioselectivity, extreme chiral amplification, and high tolerance to water, alcohols, amines, and acids
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DOI：
10.1021/ja973519c
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发表时间：
1998-04-08
影响因子：
15
通讯作者：
Curran, DP
中科院分区：
文献类型：
--
作者：
Kanemasa, S;Oderaotoshi, Y;Curran, DP
由反式螯合三齿配体（R，R）-4，6-二苯并呋喃二基-2，2 '-双（4-苯基恶唑啉）（DBFOX/Ph）和各种过渡金属（II）高氯酸盐制备阳离子水络合物。这些配合物在环戊二烯与3-烯酰基-2-恶唑烷酮亲二烯体的Diels-Alder反应中是有效的催化剂，并显示出优异的对映选择性。由高氯酸镍（II）六水合物制备的活性催化剂络合物具有三个水配体的八面体结构，并且它可以被分离和储存数月而不损失催化活性。铁（II）、钴（II）、铜（II）和锌（II）络合物具有类似的活性。在反应中诱导的绝对构型可以很容易地预测的基础上的C-2-对称结构的配合物，以及简单的结构的基板配合物。由高氯酸镍（II）或高氯酸锌（II）制备的水络合物在Diels-Alder反应中产生高效的手性放大。使用20%ee的低对映体纯度的DBFOX/Ph配体导致内环加合物的96%ee。这种显著的手性放大涉及两种机制：（1）DBFOX/Ph和Ni（II）离子之间的S-4-对称内消旋2：1络合物的沉淀和（2）通过基于水配体的氢键的辅助缔合形成1：1异手性络合物，以产生稳定的内消旋低聚物。
Cationic aqua complexes are prepared from a trans-chelating tridentate ligand, (R,R)-4,6-dibenzofurandiyl-2,2'-bis(4-phenyloxazoline) (DBFOX/Ph), and various transition-metal(II) perchlorates. These complexes are effective catalysts in the Diels-Alder reactions of cyclopentadiene with 3-alkenoyl-2-oxazolidinone dienophiles and show excellent enantioselectivities. The active catalyst complex prepared from nickel(II) perchlorate hexahydrate has an octahedral structure with three aqua ligands, and it can be isolated and stored for months without loss of catalytic activity. Iron(II), cobalt(II), copper(II), and zinc(II) complexes are similarly active. The absolute configuration induced in the reaction can be readily predicted on the basis of the C-2-symmetric structure of the complexes as well as the simple structure of the substrate complex. The aqua complex prepared from Ni(II) or Zn(II) perchlorate results in highly effective chiral amplification in the Diels-Alder reaction. Use of the DBFOX/Ph ligand of a low enantio purity of 20% ee leads to a 96% ee for the endo cycloadduct. Two mechanisms for amplification are involved in this remarkable chiral amplification: (1) precipitation of an S-4-symmetric meso 2:1 complex between DBFOX/Ph and Ni(II) ion and (2) associative formation of 1:1 heterochiral complexes by the aid of hydrogen bonds based on aqua ligands to produce stable meso oligomers.