Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University , Chengdu 610064, China.
Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Photochemistry, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences , Beijing 100190, China.
J Am Chem Soc. 2017 Oct 25;139(42):15182-15190. doi: 10.1021/jacs.7b08579. Epub 2017 Oct 12.
The cleavage of aromatic carbon-nitrogen bonds catalyzed by transition metals is of high synthetic interest because such bonds are common in organic chemistry. However, few metal catalysts can be used to selectively break C(aryl)-N bonds in electronically neutral molecules. We report here the first low-valent, high-spin chromium-catalyzed cleavage of C(aryl)-N bonds in electronically neutral aniline derivatives at room temperature. By using simple and inexpensive chromium(II) chloride as precatalyst, accompanied by an imino auxiliary, the selective arylative and alkylative C-C coupling of C(aryl)-N bonds can be achieved. Crossover experiments indicate that a low-valent chromium species, formed in situ by reduction of CrCl with Grignard reagent, is responsible for the catalytic cleavage of C(aryl)-N bonds. DFT calculations show that facile insertion of the C(aryl)-N bond by chromium(0) can take place in a high-spin quintet (S = 2) ground state, whereas the lower-spin singlet (S = 0) and triplet (S = 1) states are inaccessible in energy. It was found that both donation of the sole paired d electrons in the d shell of high-spin chromium(0) to the antibonding orbital of the C(aryl)-N bond and the nitrogen ligating interaction to the metal center with its lone pair play important roles in the cleavage of the C(aryl)-N bond by the zerovalent chromium species.
过渡金属催化的芳香碳-氮键的断裂具有很高的合成价值，因为这种键在有机化学中很常见。然而，很少有金属催化剂可以用于选择性地断裂电子中性分子中的 C(芳基)-N 键。我们在这里报道了首例低价、高自旋铬催化电子中性苯胺衍生物中 C(芳基)-N 键在室温下的断裂。通过使用简单且廉价的二价氯化铬作为前催化剂，并配合亚氨基助剂，可以实现 C(芳基)-N 键的选择性芳基化和烷基化 C-C 偶联。交叉实验表明，由 Grignard 试剂还原 CrCl 原位形成的低价铬物种负责催化 C(芳基)-N 键的断裂。DFT 计算表明，铬(0)通过易于插入 C(芳基)-N 键，可以在高自旋 quintet（S = 2）基态中发生，而较低的自旋单重态（S = 0）和三重态（S = 1）状态在能量上是不可及的。研究发现，高自旋铬(0)的 d 壳中单对 d 电子向 C(芳基)-N 键的反键轨道的捐赠以及氮配体与金属中心的孤对电子的相互作用对于零价铬物种断裂 C(芳基)-N 键都起着重要作用。
