An Improved PIII/PV=O-Catalyzed Reductive C-N Coupling of Nitroaromatics and Boronic Acids by Mechanistic Differentiation of Rate- and Product-Determining Steps
An Improved PIII/PV=O-Catalyzed Reductive C-N Coupling of Nitroaromatics and Boronic Acids by Mechanistic Differentiation of Rate- and Product-Determining Steps
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DOI：
10.1021/jacs.0c01666
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发表时间：
2020-04-08
影响因子：
15
通讯作者：
Radosevich, Alexander T.
中科院分区：
文献类型：
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作者：
Li, Gen;Nykaza, Trevor, V;Radosevich, Alexander T.
报道了实验、光谱和计算研究，为氧化还原活性主基催化剂（1，2，2，3，4，4-六甲基磷杂环丁烷P-氧化物，即，1个中心点[O]）。中心点[O]催化还原为P-III磷杂环丁烷1在催化条件下是快速的，磷杂环丁烷1代表催化静止状态，N-15 NMR光谱没有观察到长寿命的硝基芳烃部分还原中间体，N-15 NMR光谱没有观察到长寿命的硝基芳烃部分还原中间体，N-15 NMR光谱没有观察到长寿命的硝基芳烃部分还原中间体。（4）该反应对溶剂介电敏感，在中等极性溶剂（即环戊基甲基醚）中表现最好;和（5）该反应对于普通的氢化硅烷还原剂很大程度上不敏感。在上述研究的基础上，描述了新的改进的催化条件，其扩大了反应范围并提供了温和的温度（T >= 60 ℃）、低催化剂负载（>= 2mol%）和无害的末端还原剂（聚甲基氢硅氧烷）。DFT计算定义了还原C-N偶联的两阶段脱氧序列。最初的脱氧涉及一个速率决定步骤，由（3+1）螯合添加之间的硝基芳烃基板和磷杂环丁烷1;能量分解技术突出的biphilic字符的磷杂环丁烷在这一步。虽然动力学上看不见，但第二脱氧阶段被认为是关键的C-N产物形成事件，其中假定的氧氮磷杂环丙烷中间体从芳基氮烯解离转向与芳基硼酸的异裂开环;所得偶极中间体通过有机硼残基向氮的反周面1，2-迁移而演变，导致1个中心点[O]的位移，并在原位水解时形成目标C-N偶联产物。因此，所描述的方法构成了一个机械定义明确和操作稳健的主族补充目前的主力过渡金属为基础的方法，用于催化分子间C-N偶联。
Experimental, spectroscopic, and computational studies are reported that provide an evidence-based mechanistic description of an intermolecular reductive C-N coupling of nitroarenes and arylboronic acids catalyzed by a redox-active main-group catalyst (1,2,2,3,4,4-hexamethylphosphetane P-oxide, i.e., 1 center dot [O]). The central observations include the following: (1) catalytic reduction of 1 center dot[O] to P-III phosphetane 1 is kinetically fast under conditions of catalysis; (2) phosphetane 1 represents the catalytic resting state as observed by P-31 NMR spectroscopy; (3) there are no long-lived nitroarene partial-reduction intermediates observable by N-15 NMR spectroscopy; (4) the reaction is sensitive to solvent dielectric, performing best in moderately polar solvents (viz. cyclopentylmethyl ether); and (5) the reaction is largely insensitive with respect to common hydrosilane reductants. On the basis of the foregoing studies, new modified catalytic conditions are described that expand the reaction scope and provide for mild temperatures (T >= 60 degrees C), low catalyst loadings (>= 2 mol%), and innocuous terminal reductants (polymethylhydrosiloxane). DFT calculations define a two-stage deoxygenation sequence for the reductive C-N coupling. The initial deoxygenation involves a rate-determining step that consists of a (3+1) cheletropic addition between the nitroarene substrate and phosphetane 1; energy decomposition techniques highlight the biphilic character of the phosphetane in this step. Although kinetically invisible, the second deoxygenation stage is implicated as the critical C-N product-forming event, in which a postulated oxazaphosphirane intermediate is diverted from arylnitrene dissociation toward heterolytic ring opening with the arylboronic acid; the resulting dipolar intermediate evolves by antiperiplanar 1,2-migration of the organoboron residue to nitrogen, resulting in displacement of 1 center dot[O] and formation of the target C-N coupling product upon in situ hydrolysis. The method thus described constitutes a mechanistically well-defined and operationally robust main-group complement to the current workhorse transition-metal-based methods for catalytic intermolecular C-N coupling.