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

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 alpha-enaminones from primary arylamines and ketones was also developed. Mechanistic investigation of alpha-enaminone formation suggests that an amine radical cation generated through O-2 singlet energy transfer was involved in initiating alpha-enaminone formation. The reactivity and utility of alpha-enaminones were explored with a [3+3] cycloaddition reaction of enones affording dihydropyridines in good yields (58-85%). alpha-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.

Automated summary

The formation of enamine from primary arylamines was studied and confirmed, with the necessity of a radical quencher for detection. A direct synthesis of alpha-enaminones from primary arylamines and ketones was developed, and a mechanistic investigation suggested the involvement of an amine radical cation. The reactivity and utility of alpha-enaminones were explored, showing different properties from enamines and advancing understanding in organic chemistry.