Donald P. and Katherine B. Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, 837 Bloom Walk, Los Angeles, California 90089-1661, USA.
Acc Chem Res. 2012 Apr 17;45(4):565-77. doi: 10.1021/ar2002039. Epub 2011 Dec 9.
Green chemical processes play a crucial role in sustainable development, and efficient recyclable catalysts that can be conveniently applied in various chemical reactions are the key elements for the development of sustainable synthetic processes. Many organic transformations rely on Lewis and Brønsted acid catalysts, and such molecules have been widely studied in organic synthesis. Over the years, researchers have looked for Lewis acid catalysts that provide high selectivity and high turnover frequency but are also stable in aqueous media and recoverable. Since the first preparation of trifluoromethanesulfonic acid by Hazeldine (triflic acid, HOTf), researchers have synthesized and used numerous metal triflates in a variety of organic reactions. Even though the rare earth metal triflates have played a major role in these studies, the majority of rare earth triflates lack one or more of the primary properties of sustainable catalysts: low cost and easy availability of the metals, easy preparation of triflates, aqueous/thermal stability, recyclability, and catalytic efficiency. In this Account, we describe the synthetic applications of Ga(OTf)(3) and its advantages over similar catalysts. Ga(OTf)(3) can be conveniently prepared from gallium metal or gallium chloride in excess of trifluoromethanesulfonic acid (triflic acid) under reflux. Among many Lewis acid catalysts recently studied, Ga(OTf)(3) is water tolerant and soluble and requires very low catalyst loading to drive various acid-catalyzed reactions including Friedel-Crafts alkylation, hydroxyalkylation, and acylation selectively and efficiently. In many reactions Ga(OTf)(3) demonstrated high chemo- and regioselectivity, high yields, excellent stability, and recyclability. We successfully synthesized many biologically active heterocycles and their fluoroanalogs under mild conditions. Many challenging reactions such as the ketonic Strecker reactions proceed efficiently via Ga(OTf)(3) catalysis. Because it is stable in water, this catalyst provides the opportunity to study substrates and develop new synthetic protocols in aqueous media, significantly reducing the production of hazardous waste from organic solvents and toxic catalyst systems.
绿色化学过程在可持续发展中起着至关重要的作用，而能够方便地应用于各种化学反应的高效可回收催化剂则是开发可持续合成工艺的关键要素。许多有机转化依赖于路易斯酸和布朗斯台德酸催化剂，这些分子在有机合成中得到了广泛的研究。多年来，研究人员一直在寻找提供高选择性和高转化频率的路易斯酸催化剂，同时在水介质中稳定且可回收的路易斯酸催化剂。自 Hazeldine 首次制备三氟甲磺酸（三氟磺酸，HOTf）以来，研究人员已经在各种有机反应中合成并使用了许多金属三氟甲磺酸酯。尽管稀土金属三氟甲磺酸酯在这些研究中发挥了重要作用，但大多数稀土三氟甲磺酸酯缺乏可持续催化剂的一个或多个主要特性：金属的低成本和易得性、三氟甲磺酸酯的简便制备、水/热稳定性、可回收性和催化效率。在本综述中，我们描述了 Ga(OTf)(3)的合成应用及其相对于类似催化剂的优势。Ga(OTf)(3)可以方便地从镓金属或过量的三氟甲磺酸（三氟磺酸）与三氟甲烷磺酸镓（三氟甲磺酸镓）在回流下制备。在最近研究的许多路易斯酸催化剂中，Ga(OTf)(3)耐水且可溶于水，并且需要非常低的催化剂负载量即可选择性和有效地驱动各种酸催化反应，包括 Friedel-Crafts 烷基化、羟烷基化和酰化反应。在许多反应中，Ga(OTf)(3)表现出高的化学和区域选择性、高收率、优异的稳定性和可回收性。我们成功地在温和条件下合成了许多具有生物活性的杂环及其氟代类似物。许多具有挑战性的反应，如酮斯特雷克反应，通过 Ga(OTf)(3)催化高效进行。由于它在水中稳定，该催化剂提供了在水介质中研究底物和开发新合成方案的机会，显著减少了有机溶剂和有毒催化剂体系产生的危险废物。
