Photoredox Catalysis for the Coupling Reaction of Nitrones with Aromatic Tertiary Amines

Abstract

Carbon-carbon bond formation at α-amino carbon based on α-aminoalkyl radicals is an essential transformation in the synthesis of nitrogen-containing compounds. Recently, some novel photoredox catalytic protocols for this goal have been developed, in which α-aminoalkyl radicals were generated from a sequential oxidation/α-deprotonation of aromatic tertiary amines. Inspired by these studies and based on our previous works, we have developed the cross-coupling reaction of nitrones with aromatic tertiary amines via visible light photoredox catalysis. This method features a radical addition of α-aminoalkyl radicals to nitrones with advantages of simple operation, mild conditions, atom economy, a broad scope of nitrone substrates; and allows for an easy access to β-amino hydroxylamines, which could be readily converted into vicinal diamines. Compared with the UV-excited organophotosensitizer-promoted coupling reaction of nitrones with tertiary amines, visible light is a more safe and convenient light source, the photo-excited electron transfer (PET) by 1 mol% of Ir-photocatalyst is more efficient. In addition, nitrones exclusively server as excellent radical acceptors thus with a broader range of structures. A general procedure of this coupling reaction is as follows:To a 25 mL Schlenk tube equipped with a magnetic stir bar were added a nitrone (0.30 mmol), a tertiary amine (0.90 mmol), Ir(ppy)2(dtbbpy)PF6 (0.003 mmol, 1.0 mol%) and K2HPO4 (0.06 mmol, 20 mol%). After being evacuated and backfilled with argon for three times, DMSO (3 mL) was added to the tube. Then the tube was placed approximately 7 cm away from a 12 W blue LEDs, and the reaction mixture was stirred at r.t. under an argon atmosphere for 24 h. The reaction was quenched with saturated aqueous NaHCO3 (25 mL), and the mixture was extracted with dichloromethane (DCM, 20 mL×3). The combined organic layers were washed with brine, dried over anhydrous Na2SO4, filtered and concentrated under reduced pressure. The residue was purified by flash column chromatography on silica gel to afford the desired cross-coupling product β-amino hydroxylamine.