Hall Dennis G
Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada.
Chem Soc Rev. 2019 Jul 1;48(13):3475-3496. doi: 10.1039/c9cs00191c.
Although boronic acids are recognized primarily for their utility as reagents in transition metal-catalyzed transformations, other applications are emerging, including their use as reaction catalysts. Few methods are available for the catalytic activation of hydroxy functional groups as a way to promote their direct transformation into useful products under mild conditions. To this end, the ability of boronic acids to form reversible covalent bonds with hydroxy groups can be exploited to enable both electrophilic and nucleophilic modes of activation in various organic reactions. Using the concept of boronic acid catalysis (BAC), electrophilic activation of carboxylic acids leads to the formation of amides from amines, as well as cycloadditions and conjugate additions with unsaturated carboxylic acids. Alcohols can also be activated with boronic acid catalysts to form carbocation intermediates that can be trapped in selective Friedel-Crafts-type reactions with arenes and other nucleophiles. On the other hand, diols and saccharides can form tetrahedral adducts with boronic acids, which increases their nucleophilic character towards electrophiles. Altogether, BAC imparts mild and selective reaction conditions that display high atom-economy by circumventing the need for wasteful stoichiometric activation of hydroxy groups into halides or sulfonates.
尽管硼酸主要因其作为过渡金属催化转化反应试剂的用途而被认可，但其他应用也正在兴起，包括用作反应催化剂。目前很少有方法可用于催化活化羟基官能团，以便在温和条件下将其直接转化为有用的产物。为此，可以利用硼酸与羟基形成可逆共价键的能力，在各种有机反应中实现亲电和亲核活化模式。利用硼酸催化（BAC）的概念，羧酸的亲电活化可导致胺形成酰胺，以及与不饱和羧酸发生环加成和共轭加成反应。醇也可以用硼酸催化剂活化，形成碳正离子中间体，这些中间体可以在与芳烃和其他亲核试剂的选择性傅克型反应中被捕获。另一方面，二醇和糖类可以与硼酸形成四面体加合物，这增加了它们对亲电试剂的亲核性。总之，BAC提供了温和且选择性的反应条件，通过避免将羟基进行浪费的化学计量活化转化为卤化物或磺酸盐，从而显示出高原子经济性。
