Silicon-Tethered Strategies for C-H Functionalization Reactions.
Silicon-Tethered Strategies for C-H Functionalization Reactions.
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DOI：
10.1021/acs.accounts.7b00306
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发表时间：
2017-08-15
影响因子：
18.3
通讯作者：
Gevorgyan V
中科院分区：
文献类型：
--
作者：
Parasram M;Gevorgyan V
对普遍存在的C-H键进行选择性和高效的官能化是有机合成的圣杯。这一领域的大多数进展依赖于强配位或弱配位的导向基团的使用，这些基团已被证明对过渡金属催化的C(Sp2)-H和C(Sp3)-H键的官能化是有效的。虽然大多数指导小组本身都是重要的职能部门，但在某些情况下，副秘书长变成了静态实体，几乎没有综合影响力。此外，所使用的一些DG难以去除或不切实际，这排除了在合成中使用这种方法的可能性。人们认为，开发一套易于安装和拆卸/修饰的C-H官能化DG将为不断增长的定向官能化领域增加巨大的价值，从而促进其在复杂分子的合成和后期官能化中的应用。特别是，长期以来，硅系链在有机合成中作为易于安装和可拆卸/可修改的助剂，可用于各种过程，包括自由基转化、环加成反应和许多TM催化的方法，包括闭环歧化(RCM)和交叉偶联反应。使用硅系绳非常有吸引力，原因有几个：(1)它们易于处理/合成，并且相对稳定；(2)它们利用廉价和丰富的硅前体；以及(3)硅系绳易于安装和拆卸/修改。因此，开发用于C-H官能化反应的硅链不仅从实用的角度而且从合成的角度都是有吸引力的，因为硅链可以为C-H官能化后的有机分子的多样化提供额外的处理。在过去的几年里，我们开发了一套用于C-H官能化反应的硅系留方法。所开发的硅系链可分为四种类型：(类型-1)具有反应基团的硅-链条，其中反应基团被传递到官能化位置；(类型-2)具有DG的硅-链条，其设计用于芳烃的选择性C(Sp2)-H官能化；(类型-3)用于有机分子的C-H硅烷化的反应性硅-链条；以及(类型-4)含有DG的反应性硅-链条，用于挑战C(SP3)-H键的选择性C-H硅化/羟基化。在本文中，我们概述了硅助剂在定向C-H官能化反应中的应用进展。还将讨论不同硅系链的使用策略，硅系链的官能化/改性，以及通过硅系链进行C-C、C-X、C-O和C-Si键形成反应的方法学进展。虽然这里描述的工作为C-H功能化领域提供了实质性的进展，但挑战仍然存在。这里提出的C-H官能化方法需要使用贵金属。此外，需要按化学计量使用高分子量硅助剂是所提出的概念的缺点。
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(sp2)–H and C(sp3)–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(sp2)–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(sp3)–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.
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