作者:Xiao-Chen Wang、Wei Gong、Li-Zhen Fang、Ru-Yi Zhu、Suhua Li、Keary M. Engle、Jin-Quan Yu
DOI:10.1038/nature14214
日期:2015.3
A combination of norbornene and pyridine-type ligand enables commonly used ortho-directing groups to direct meta-CâH activation with palladium catalysts. This manuscript presents a new alternative approach to the selective activation of remote meta-CâH bonds with the aim of CâH functionalization. This method has the advantage over a recently developed protocol involving a U-shaped template in that it does not require the covalent attachment of a complex template. The authors use norbornene as a transient mediator to help achieve bond activation with a simple and common ortho-directing group. The use of a newly developed pyridine-based ligand is crucial for relaying the palladium catalyst to the meta position by norbornene following initial ortho-CâH activation. This work provides a novel approach to arene synthesis that may have applications in the pharmaceuticals industry. Achieving site selectivity in CâH functionalization reactions is a significant challenge, especially when the target CâH bond is distant from existing functional groups1,2,3,4,5. Coordination of a functional group to a metal is often a key driving force and control element in many important reactions including asymmetric hydrogenation6, epoxidation7,8 and lithiation9. Exploitation of this effect has led to the development of a broad range of directed CâH activation reactions10,11,12,13,14. However, these CâH activation methods are limited to proximal CâH bonds, which are spatially and geometrically accessible from the directing functional group. The development of meta-selective CâH functionalizations remains a significant challenge1,2,3,4,5,15,16,17. We recently developed a U-shaped template that can be used to overcome this constraint and have shown that it can be used to selectively activate remote meta-CâH bonds1,2. Although this approach has proved to be applicable to various substrates and catalytic transformations3,4,5, the need for a covalently attached, complex template is a substantial drawback for synthetic applications. Here we report an alternative approach employing norbornene as a transient mediator to achieve meta-selective CâH activation with a simple and common ortho-directing group. The use of a newly developed pyridine-based ligand is crucial for relaying the palladium catalyst to the meta position by norbornene after initial ortho-CâH activation. This catalytic reaction demonstrates the feasibility of switching ortho-selectivity to meta-selectivity in CâH activation of the same substrate by catalyst control.
降冰片烯和吡啶型配体的结合使常用的正交定向基团能够在钯催化剂的作用下定向活化元-CâH。本手稿介绍了一种以 CâH 功能化为目的,选择性活化远端元-CâH 键的新替代方法。与最近开发的涉及 U 型模板的方案相比,这种方法的优势在于不需要共价连接复杂的模板。作者使用降冰片烯作为瞬时媒介,通过一个简单而常见的正向定向基团帮助实现键活化。在最初的正交-CâH活化之后,使用一种新开发的基于吡啶的配体对于降冰片烯将钯催化剂转移到元位置至关重要。这项工作为炔类化合物的合成提供了一种新方法,可能会应用于制药行业。在 CâH 功能化反应中实现位点选择性是一项重大挑战,尤其是当目标 CâH 键与现有官能团距离较远时1,2,3,4,5。在不对称氢化6 、环氧化7,8 和石化作用9 等许多重要反应中,官能团与金属的配位通常是关键的驱动力和控制元素。利用这一效应开发出了一系列定向 CâH 活化反应10,11,12,13,14。然而,这些 CâH 活化方法仅限于近端 CâH 键,因为这些键在空间和几何上都可以从定向官能团接触到。开发元选择性 CâH 功能化仍然是一项重大挑战1,2,3,4,5,15,16,17。我们最近开发出一种 U 型模板,可用于克服这一限制,并证明它可用于选择性地激活偏远的元 CâH 键1,2。虽然这种方法已被证明适用于各种底物和催化转化3,4,5,但需要共价连接的复杂模板是合成应用的一个重大缺陷。在此,我们报告了一种采用降冰片烯作为瞬时介质的替代方法,通过一个简单而常见的正向定向基团实现元选择性 CâH 活化。使用一种新开发的基于吡啶的配体对于钯催化剂在最初的正交-CâH活化后通过降冰片烯转接到元位置至关重要。这一催化反应证明了通过催化剂控制将同一底物的 CâH 活化的正选择性转换成元选择性的可行性。