A mechanistic study of the manganese porphyrin-catalyzed C–H isocyanation reaction

We explored the mechanism and the origin of chemoselectivity of the manganese porphyrin-catalyzed aliphatic C–H isocyanation reaction with density functional theory (DFT) calculations. In this reaction, a diradical intermediate complex of a hydroxomanganese porphyrin and a substrate-derived radical is formed by the H-abstraction process. Then the axial ligand exchange of OH with NCO leads to a di-isocyanate manganese porphyrin intermediate, which provides the possibility for the NCO-rebound process to form the alkyl isocyanate product. The competition of different axial ligands rebounding to substrate-derived radicals is the origin of different products. The computational results suggest that the NCO-rebound pathway is more favorable than the OH-rebound pathway by 2.7 kcal mol−1, which is consistent with the experimental results that the major product is an alkyl isocyanate instead of the oxygenation product. The NCO-rebound pathway through the di-isocyanate manganese porphyrin intermediate is more favorable than that through the hydroxomanganese porphyrin by 5.9 kcal mol−1 due to the stronger trans effect of the axial ligand NCO compared to OH. We also found that the electron-donating aryl ligand can promote the C–H bond isocyanation. The mechanism and the controlling factors are also applicable to the manganese porphyrin-catalyzed C–H bond azidation reaction.