Wang Long, Carrow Brad P
Department of Chemistry, Princeton University, Princeton, NJ 08544, United States.
ACS Catal. 2019 Aug 2;9(8):6821-6836. doi: 10.1021/acscatal.9b01195. Epub 2019 Jun 24.
Oxidative C-H/C-H coupling is a promising synthetic route for the streamlined construction of conjugated organic materials for optoelectronic applications. Broader adoption of these methods is nevertheless hindered by the need for catalysts that excel in forging core semiconductor motifs, such as ubiquitous oligothiophenes, with high efficiency in the absence of metal reagents. We report a (thioether)Pd-catalyzed oxidative coupling method for the rapid assembly of both privileged oligothiophenes and challenging hindered cases, even at low catalyst loading under Ag- and Cu-free conditions. A combined experimental and computational mechanistic study was undertaken to understand how a simple thioether ligand, MeS(CH)SONa, leads to such potent reactivity toward electron-rich substrates. The consensus from these data is that a concerted, base-assisted C-H cleavage transition state is operative, but thioether coordination to Pd is associated with decreased synchronicity (bond formation exceeding bond breaking) versus the "" concerted metalation-deprotonation (CMD) model that was formalized by Fagnou in direct arylation reaction. Enhanced positive charge build-up on the substrate results from this perturbation, which rationalizes experimental trends strongly favoring π-basic sites. The term CMD (CMD) is introduced to distinguish this mechanism from the standard model, even though both mechanisms locate in a broad concerted continuum. More O'Ferrall-Jencks analysis further suggests CMD should be a general mechanism manifested by many metal complexes. A preliminary classification of complexes into those favoring CMD or standard CMD is proposed, which should be informative for studies toward tunable catalyst-controlled reactivity.
氧化C-H/C-H偶联是一种很有前景的合成路线，可用于高效构建用于光电子应用的共轭有机材料。然而，这些方法的更广泛应用受到阻碍，因为需要在无金属试剂的情况下高效锻造核心半导体基序（如普遍存在的低聚噻吩）的催化剂。我们报道了一种（硫醚）钯催化的氧化偶联方法，可快速组装具有优势的低聚噻吩以及具有挑战性的受阻情况，即使在无银和无铜条件下低催化剂负载量时也能实现。我们进行了实验和计算相结合的机理研究，以了解简单的硫醚配体MeS(CH)SONa如何对富电子底物产生如此强的反应活性。这些数据的共识是，协同的、碱辅助的C-H裂解过渡态起作用，但与Fagnou在直接芳基化反应中形式化的“协同金属化-去质子化（CMD）”模型相比，硫醚与钯的配位与同步性降低（键形成超过键断裂）有关。这种扰动导致底物上增强的正电荷积累，这解释了强烈有利于π-碱性位点的实验趋势。引入术语“CMD（CMD）”以将此机制与标准模型区分开来，尽管这两种机制都位于广泛的协同连续体中。更多的O'Ferrall-Jencks分析进一步表明CMD应该是许多金属配合物表现出的一般机制。我们提出了将配合物初步分类为有利于CMD或标准CMD的那些，这对于可调催化剂控制的反应性研究应该是有帮助的。
