With the development and widespread use of transition metal catalysts, C-H activation has become a hot topic in organic synthesis, especially in the construction of CC bond of organic compounds. As an important and cheap catalyst, manganese complex has shown great potential for catalyzing C-H activation both in academic and industrial applications. In this paper, the mechanism of manganese-catalyzed dehydrogenative [4+2] annulation by C-H/N-H activation was investigated systematically with the aid of density functional theory (DFT) calculations in 1,4-dioxane solvent. In detail, we use M06-L/[SDD:6-311+G(d,p)(SMD)]//M06-L/[LANL2DZ:6-31G(d)] to examine the Gibbs free energy, structure and other properties of possible intermediates and transition states in this catalytic cycle. By comprehensive comparison and discussion, we obtained a favorable pathway consisting of five steps: (1) catalyst initiation occurred with the assistance of bromine anion rather than imide to form active catalyst; (2) alkyne inserted into the active catalyst to generate a seven-membered manganacycle after dissociation of a carbon monoxide; (3) double bond migration happened in this seven-membered manganacycle to form a product precursor; (4) the product precursor would dissociate by β-H elimination and generated product isoquinoline and active Mn-H complex; (5) the active Mn-H complex was subsequently combined with an imine followed by dehydrogenative C-H activation to complete the whole catalytic cycle. In this context, the reason for the highly atom-economical C-H activation by direct dehydrogenation (eliminates the necessity for oxidants or additives) has been clarified by this mechanism. The present study was aimed at further understanding of Mn(I)-catalyzed dehydrogenative C-H activation, and provided more theoretical basis for future more Mn-catalyzed C-H activation.
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