Celebrating 20 Years of SYNLETT - Special Account On Palladium(II)-Catalyzed Additions of Arylboronic Acids to Electron-Deficient Alkenes, Aldehydes, Imines, and Nitriles

The enantioselective synthesis of cyclic and acyclic beta-aryl ketones, esters, and amides by palladium(II)-Catalyzed 1,4-addition of an arylboronic acid or a potassium aryltrifluoroborate to unsaturated carbonyl compounds is described. The protocol provides simple access to biologically and pharmaceutically active compounds such as optically active chromenes, (R)-tolterodine, (R)-CDP 840, and the endothelin receptor antagonist. The palladium(II) catalysts are also efficient for Grignard-type addition of arylboronic acids to carbon-heteroatom double bonds and triple bonds of aldehydes, ketones, aldimines, and nitriles. The catalytic cycle involves transmetalation between arylboronic acid and palladium(II) complexes as the key process, the mechanism of which is discussed on the basis of characterization of the transmetalation intermediates and electronic effects of the substituents. The enantio-selection mechanism and the efficiency of a chiraphos ligand for structurally planar unsaturated ketones are discussed on the basis of the results of DFT computational studies on the modes of. coordination of the substrates to the phenylpalladium(II)/(S,S)-chiraphos intermediate. This review focuses mainly on reactions of arylboronic acids; there are very few such reports for other organoboronic acids or esters, but pinacol allylboronate undergoes 1,4-addition to unsaturated N-acylpyrroles.