Exploration of catalysts for organic reactions in water, which is a nontoxic and nonexplosive liquid, has been studied to achieve environmentally friendly chemical processes. For such a purpose, we utilized click chemistry to incorporate sugar units into transition-metal complexes affording water-soluble complexes. In this study, a series of palladium complexes with C–C–N pincer ligands, each of which consisted of two N-heterocyclic carbenes (NHCs) and a triazole moiety with a d-glucopyranosyl unit were synthesized. These complexes possess various N-substituents, namely, methyl, isopropyl, benzyl, or d-glucopyranosyl units on the terminal NHC moieties. The diastereoselectivity in the formation of the complexes and their catalytic ability toward the Suzuki–Miyaura cross-coupling reactions in water exhibited clear N-substituent effects. The tridentate ligand with an acetylated d-glucopyranosyl unit on the terminal NHC moiety led to high diastereoselectivity in the formation of its complex, while each of the other ligands with achiral substituents on the terminal NHC units afforded both of the diastereomers with low selectivity. Catalytic reactions using the complexes were examined for the Suzuki–Miyaura cross-coupling reactions of phenylboronic acid and 4′-bromoacetophenone in water. The complex possessing an isopropyl group on the terminal NHC moiety exhibited excellent catalytic activity with TON (800000), which is comparable to the best value reported for a similar reaction.
水中有机反应催化剂的探索已经被研究,以实现环保的
化学过程,
水是一种无毒且不易爆炸的液体。为此,我们利用点击
化学将糖单元引入过渡
金属配合物中,从而合成
水溶性配合物。在本研究中,合成了一系列
镍铂配合物,这些配合物具有C–C–N夹心
配体,每个
配体由两个N-杂环卡宾(NHC)和一个含d-
葡萄糖吡喃糖单元的三唑部分组成。这些配合物具有多种N取代基,即在末端NHC部分有甲基、异丙基、苄基或d-
葡萄糖吡喃糖单元。配合物形成过程中的立体选择性及其对
水中铃木-宫浦交叉偶联反应的催化能力表现出明显的N取代基效应。具有末端NHC部分上有乙酰化d-
葡萄糖吡喃糖单元的三齿
配体,在形成其配合物时显示出高的立体选择性,而其他末端NHC单位上具有无鞘取代基的
配体则产生两种对映异构体,但选择性较低。使用这些配合物的催化反应研究了
水中苯基
硼酸与4'-
溴乙酰苯酮的铃木-宫浦交叉偶联反应。末端NHC单位上具有异丙基的配合物表现出优异的催化活性,转化次数(TON)高达800000,接近于类似反应中报道的最佳值。