Aromatization of enamines promoted by a stoichiometric amount of palladium(II) salts: A novel method for the synthesis of aromatic amines

Enamines (1a-r) prepared from cyclohexanones, cyclohexane-1,3-diones, or tetralones led to arylamines (2a-r) in one pot when treated with a stoichiometric amount of palladium salts [PdCl₂-(MeCN)₂] in acetonitrile in the presence of triethylamine at room temperature or at elevated temperature, in some cases for 5 min to 2 h. The initial electrophilic attack of palladium chloride on the β-carbon of the enamines led to a σ-palladium species (8) which triggered a series of reactions (→ 9 → 19 → 11 → 12) destined for aromatization to give 2a-r in good yields. The intervention of such a σ-palladium species has been attested by a trapping experiment. On the basis of this reaction mechanism, we have developed another new process capable of transforming acyclic compounds having 6-en-2-one frameworks (16, 23, 25) to arylamines (2s-u) when their enamines were treated under the similar conditions as above, featuring again the formation of σ-palladium species such as 8 as the initial key intermediate.