Gold(III) N-heterocyclic carbene complexes mediated synthesis of β-enaminones from 1,3-dicarbonyl compounds and aliphatic amines

A series of gold(III) N-heterocyclic carbene complexes [1-(R ₁)-3-(R₂)imidazol-2-ylidene]AuBr₃ [R ₁ = i-Pr, R₂ = CH₂Ph (1c); R₁ = mesityl, R₂ = CH₂Ph (2c); R₁ = i-Pr, R ₂ = CH₂COt-Bu (3c), and R₁ = t-Bu, R ₂ = CH₂COt-Bu (4c)] act as effective precatalysts in the synthesis of β-enaminones from 1,3-dicarbonyl compounds and primary amines under ambient conditions. Specifically the 1c-4c complexes efficiently catalyzed the condensation of a variety of cyclic as well as acyclic 1,3-dicarbonyl compounds, namely, acetyl acetone, benzoylacetone, 2-acetylcyclopentanone, and ethyl-2-oxocyclopentanecarboxylate with primary aliphatic amines, viz., methylamine, ethylamine, n-propylamine, i-propylamine, and n-butylamine, yielding β-enamines at room temperature. Interestingly enough, the more electrophilic gold(III) 1c-4c complexes exhibited superior activity in comparison to the gold(I) counterparts 1b-4b. A comparison along a representative 4a-c series further underscored the importance of gold in the reaction as both the gold(I) 4b and gold(III) 4c complexes were more effective than the silver analogue 4a. The density functional theory (DFT) study revealed that the strong σ-donating nature of the N-heterocyclic carbene ligand results in a strong C_carbene-Au(III) interaction in the 1c-4c complexes.