Manganese‐Catalyzed Selective <i>ortho</i>‐Deuteration of Aromatic Amidines with D₂O

Abstract We report a manganese‐catalyzed ortho ‐selective deuteration of aromatic amidines using D 2 O as the deuterium source. The reaction, catalyzed by Mn(CO) 5 Br in the presence of NaOAc as an additive, proceeds with high regioselectivity, achieving efficient deuterium incorporation at the ortho ‐position with excellent yields. The reaction scope was extended to a wide range of substituted aromatic amidines, demonstrating consistently high levels of deuterium incorporation and broad substrate compatibility. Furthermore, the method was successfully scaled up to gram‐scale, and the resulting deuterated products were subsequently transformed into heterocycles, including triazoles and imidazoles.