Vinyl proton chemical shifts in 3-amino-2-cycloalkenones parallel those of cyclic enamines in being inversely ordered relative to the following apparent order of increasing nitrogen lone-pair-electron delocalization: morpholino < dimethylamino < pyrrolidino and 1-cyclohexenylamines < 1-cyclopentenylamines. Similar effects are apparent in 2-amino-2-cycloalkenones, but the actual order of vinyl proton chemical shifts is reversed because of greater electron withdrawal from β positions in 2-cyclopentenones than from those in 2-cyclohexenone systems. Greater nitrogen lone-pair-electron delocalization into five- than six-membered rings is demonstrated by variable temperature nmr data which indicates ~2-3 kcal/mol larger δG for rotation about the C--N bond of the enamino ketone system in five- than in six-membered-ring 3-dimethylamino-2-cycloalkenones.
ASJC Scopus subject areas
- Organic Chemistry