Department of Chemistry, University of Illinois at Chicago , 845 West Taylor Street, Chicago, Illinois 60607, United States.
Acc Chem Res. 2017 Aug 15;50(8):2038-2053. doi: 10.1021/acs.accounts.7b00306. Epub 2017 Aug 3.
Selective and efficient functionalization of ubiquitous C-H bonds is the Holy Grail of organic synthesis. Most advances in this area rely on employment of strongly or weakly coordinating directing groups (DGs) which have proven effective for transition-metal-catalyzed functionalization of C(sp)-H and C(sp)-H bonds. Although most directing groups are important functionalities in their own right, in certain cases, the DGs become static entities that possess very little synthetic leverage. Moreover, some of the DGs employed are cumbersome or unpractical to remove, which precludes the use of this approach in synthesis. It is believed, that development of a set of easily installable and removable/modifiable DGs for C-H functionalization would add tremendous value to the growing area of directed functionalization, and hence would promote its use in synthesis and late-stage functionalization of complex molecules. In particular, silicon tethers have long provided leverage in organic synthesis as easily installable and removable/modifiable auxiliaries for a variety of processes, including radical transformations, cycloaddition reactions, and a number of TM-catalyzed methods, including ring-closing metathesis (RCM) and cross-coupling reactions. Employment of Si-tethers is highly attractive for several reasons: (1) they are easy to handle/synthesize and are relatively stable; (2) they utilize cheap and abundant silicon precursors; and (3) Si-tethers are easily installable and removable/modifiable. Hence, development of Si-tethers for C-H functionalization reactions is appealing not only from a practical but also from a synthetic standpoint, since the Si-tether can provide an additional handle for diversification of organic molecules post-C-H functionalization. Over the past few years, we developed a set of Si-tether approaches for C-H functionalization reactions. The developed Si-tethers can be categorized into four types: (Type-1) Si-tethers possessing a reacting group, where the reacting group is delivered to the site of functionalization; (Type-2) Si-tethers possessing a DG, designed for selective C(sp)-H functionalization of arenes; (Type-3) reactive Si-tethers for C-H silylation of organic molecules; and finally, (Type-4) reactive Si-tethers containing a DG, developed for selective C-H silylation/hydroxylation of challenging C(sp)-H bonds. In this Account, we outline our advances on the employment of silicon auxiliaries for directed C-H functionalization reactions. The discussion of the strategies for employment of different Si-tethers, functionalization/modification of silicon tethers, and the methodological developments on C-C, C-X, C-O, and C-Si bond forming reactions via silicon tethers will also be presented. While the work described herein presents a substantial advance for the area of C-H functionalization, challenges still remain. The use of noble metals are required for the C-H functionalization methods presented herein. Also, the need for stoichiometric use of high molecular weight silicon auxiliaries is a shortcoming of the presented concept.
选择性和高效的 C-H 键功能化是有机合成的圣杯。该领域的大多数进展都依赖于使用强或弱配位导向基团(DG),这些基团已被证明可有效用于过渡金属催化的 C(sp)-H 和 C(sp)-H 键功能化。尽管大多数导向基团本身就是重要的官能团,但在某些情况下,DG 成为具有很少合成杠杆作用的静态实体。此外,所使用的一些 DG 体积庞大或不实用,难以去除,这排除了该方法在合成中的应用。人们相信,开发一套易于安装和可去除/可修改的用于 C-H 功能化的 DG 将为不断发展的导向功能化领域增添巨大价值,并因此促进其在复杂分子的合成和后期功能化中的应用。特别是,硅键合基团作为各种过程(包括自由基转化、环加成反应和许多 TM 催化方法,包括环封闭复分解(RCM)和交叉偶联反应)中易于安装和可去除/可修改的辅助剂,作为有机合成中的杠杆已有很长时间了。硅键合基团的使用具有以下几个吸引人的原因:(1)它们易于处理/合成且相对稳定;(2)它们利用廉价且丰富的硅前体;(3)硅键合基团易于安装和可去除/可修改。因此,从实用和合成的角度来看,开发用于 C-H 功能化反应的硅键合基团不仅具有吸引力,因为硅键合基团可以为 C-H 功能化后有机分子的多样化提供额外的处理方式。在过去的几年中,我们开发了一套用于 C-H 功能化反应的硅键合基团方法。所开发的硅键合基团可以分为四类:(类型 1)具有反应基团的硅键合基团,其中反应基团被输送到功能化位点;(类型 2)具有 DG 的硅键合基团,设计用于芳环的选择性 C(sp)-H 功能化;(类型 3)用于有机分子 C-H 硅烷化的反应性硅键合基团;最后,(类型 4)含有 DG 的反应性硅键合基团,用于选择性 C-H 硅烷化/羟基化挑战性 C(sp)-H 键。在本报告中,我们概述了我们在使用硅助剂进行导向 C-H 功能化反应方面的进展。还将讨论使用不同硅键合基团的策略、硅键合基团的功能化/修饰以及通过硅键合基团形成 C-C、C-X、C-O 和 C-Si 键的方法学发展。虽然本文所述的工作代表了 C-H 功能化领域的重大进展,但挑战仍然存在。本文所述的 C-H 功能化方法需要使用贵金属。此外,所提出的概念的缺点是需要使用化学计量的高分子量硅助剂。
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