Blue Light Irradiated Metal-, Oxidant-, and Base-Free Cross-Dehydrogenative Coupling of C(sp²)–H and N–H Bonds: Amination of Naphthoquinones with Amines

Herein, we report a blue-light-driven amination of C(sp2)-H bond of naphthoquinones and quinones with the N-H bond of primary and secondary amines for the synthesis of 2-amino-naphthoquinones and 2-amino-quinones. The coupling of naphthoquinones with a wide array of aliphatic, aromatic, chiral, primary, and secondary amines having electron donating (-CH3, -OCH3, -SCH3), withdrawing (-F, -Cl, -Br, -I), and CO2H, -OH, -NH2 groups with acidic protons selectively occurred to afford C-N coupled 2-amino-naphthoquinones in 60-99% yields and hydrogen gas as a byproduct in methanol solvent without using any additional reagents, additives, and oxidant under the blue light irradiation. Mechanistic insight by DFT computation, controlled experiments, kinetic isotopic effect, and substitution effect of the substrates suggest that the reaction proceeds by radical pathway in which naphthoquinone forms a highly oxidizing naphthoquinonyl biradical upon irradiation of blue light (457 nm). Consequently, electron transfer from electron-rich amine to an oxidizing naphthoquinonyl biradical leads to a naphthoquinonyl radical anion and aminyl radical cation, followed by proton transfer and delocalization leading to a carbon-centered naphthoquinonyl radical. The cross-coupling of naphthoquinonyl carbon-centered and aminyl nitrogen radicals forms a C-N bond, with subsequent elimination of hydrogen gas (which was also confirmed by GC-TCD), affording 2-amino-1,4-naphthoquinone under metal-, reagent-, base-, and oxidant-free conditions.