Chromium- and Cobalt-Catalyzed, Regiocontrolled Hydrogenation of Polycyclic Aromatic Hydrocarbons: A Combined Experimental and Theoretical Study.

Polycyclic aromatic hydrocarbons are difficult substrates for hydrogenation because of the thermodynamic stability caused by aromaticity. We report here the first chromium- and cobalt-catalyzed, regiocontrolled hydrogenation of polycyclic aromatic hydrocarbons at ambient temperature. These reactions were promoted by low-cost chromium or cobalt salts combined with diimino/carbene ligand and methylmagnesium bromide and are characterized by high regioselectivity and expanded substrate scope that includes tetracene, tetraphene, pentacene, and perylene, which have rarely been reduced. The approach provides a cost-effective catalytic protocol for hydrogenation, is scalable, and can be utilized in the synthesis of tetrabromo- and carboxyl-substituted motifs through functionalization of the hydrogenation product. The systematic theoretical mechanistic modelings suggest that low-valent Cr and Co monohydride species, most likely from zerovalent transition metals, are capable of mediating these hydrogenations of fused PAHs.