Can Primary Arylamines Form Enamine? Evidence, α-Enaminone, and [3+3] Cycloaddition Reaction

Abstract

The formation of enamine from primary arylamines was detected and confirmed by nuclear magnetic resonance spectroscopy. The presence of a radical quencher, e.g., (2,2,6,6-tetramethylpiperidin-1-yl)­oxidanyl, was found to be essential for the detection of enamine formation. A direct synthesis of α-enaminones from primary arylamines and ketones was also developed. Mechanistic investigation of α-enaminone formation suggests that an amine radical cation generated through O2 singlet energy transfer was involved in initiating α-enaminone formation. The reactivity and utility of α-enaminones were explored with a [3+3] cycloaddition reaction of enones affording dihydropyridines in good yields (58–85%). α-Enaminones displayed a set of reactivities that is different from that of enamines. The knowledge gained in this work advances our basic understanding of organic chemistry, providing insights and new opportunities in enamine catalysis.

Substrate‐Dependent Divergent Oxyamination of beta‐Tetralones Enabling Modular Synthesis of 2‐Amino‐para‐Iminonaphthoquinones and 4‐Amino‐1,2‐Naphthoquinone Derivatives

“…Followed by the cleavage of O−O bond and dehydration, 2‐amino‐enone E was afforded. Subsequently, intermediate E probably undergoes a single electron transfer (SET) process with O 2 to form a nitrogen‐centered cation radical F [11j] . The hydrogen atom attached at the benzylic position of species F is abstracted by the superoxide anion radical, giving the intermediate G .…”