Palladium-Catalyzed C-N Coupling-Hydroamination Route to 4-Quinolones

Significance

Reported is an efficient palladium-catalyzed C-N cross-coupling-hydroamination route to 4-quinolones from ortho-halo aryl alkynyl ketones. After catalyst optimization, the reaction was extended to a range of commercially available amines bearing ortho-, para- and meta-EDGs and -EWGs to give product 2 in moderate to good yields. Aside from anilines, the reaction is successful for naphthalene- and pyrimidine-containing substrates. A variety of ynones 1 (with EDGs and EWGs) were investigated. Only for substrates bearing R^² = 4-FC₆H₄, 4-ClC₆H₄ or pyridine, low yields of product 2 were obtained. For less reactive chloro-substituted substrates a higher temperature (140 ˚C) was required to achieve modest yields. Two mechanistic pathways, either oxidative addition of palladium(0) to the C-Br bond or conjugate addition of the amine, followed by activation of the triple bond by coordination to palladium (R. Bernini, S. Cacchi, G. Fabrizi, A. Sferrazza Synthesis 2009, 1209) were proposed without provision of evidence.