Low-Valent, High-Spin Chromium-Catalyzed Cleavage of Aromatic Carbon Nitrogen Bonds at Room Temperature: A Combined Experimental and Theoretical Study

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 Caryl)-N bonds in electronically neutral molecules. We report here the first low-valent, high-spin chromium-catalyzed cleavage of Caryl)-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 CrCl2 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(6) 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.