Carboxylic acids are not readily applied as carbon-based nucleophiles due to their intrinsic acidic group. Here, we demonstrate that free (E,E)-2,4-dienoic acids form electron-neutral and highest occupied molecular orbital-raised η2-complexes with Pd(0) and undergo Friedel-Crafts-type additions to imines with exclusive α-regioselectivity, giving formal dienylated products after decarboxylation. Unusual and switchable (E,E)- and (Z,E)-selectivity, along with excellent enantioselectivity, is achieved via ligand-controlled outer-sphere or inner-sphere reaction modes, respectively, which are well supported by comprehensive density functional theory calculation studies. An unprecedented formal reductive Mannich reaction between (E,E)-dienoic acids and imines is also developed to furnish enantioenriched β-amino acid derivatives.