Palladium-Catalysed Double Carbonylation of Aryl Halides To Give α-Keto Amides. Mechanistic Studies

Article abstract of DOI:10.1021/ja00297a033

Various aryl halides are catalytically converted into α-keto amides and amides on treatment with secondary amines and carbon monoxide.Palladium complexes containing tertiary phosphine ligands, particularly diphenylmethylphosphine and 1,4-bis(diphenylphosphino)butane, are most effective among other transition-metal complexes.Detailed examination of factors controlling the reaction rates and selectivity for α-keto amide formation revealed the following characteristics of the reactions. (a) Reactivity of phenyl halide decreases in the order PhI > PhBr >> PhCl. (b) Oxidative addition of phenyl bromide constitutes the rate-determining step in double carbonylation of phenyl bromide, whereas in the reaction of phenyl iodide the rate determinig step is associated with the reaction of a catalitically active palladium species with carbon monoxide. (c) Introduction of an electron-withdrawing substituent into the para position of phenyl halide enhances the reactivity but decreases the selectivity for α-keto amide. (d) Employment of amines of high basicity (pK_b Et2NH > piperidine > hexamethyleneimine > Me2NH > pyrrolidine probably reflects the decrease in steric bulkiness of amines. (f) Although primary amines are in general not suitable for the double carbonylation, tert-butylamine can be used because of its inertness to the product α-keto amide.Reactivity of trans-PdPh(I)(PMePh2)2 and trans-Pd(COPh)I(PMePh2)2, supposed intermediates in the catalytic reaction of PhI, toward amines and CO was examined.The relative reactivity of six secondary amines with the benzoylpalladium complex increasing in the order Pr2NH < Et2NH < piperidine < Me2NH < hexamethyleneimine < pyrrolidine was found to be inversely correlated with decreasing selectivity order for α-keto amide formation in the catalytic systems.On the basis of the experimental results, a mechanism consisting of two catalytic cycles to produce α-keto amides and amides has been proposed.