Gold‐Catalyzed Functionalization of Semicarbazides with Terminal Alkynes to Achieve Substituted Semicarbazones.

Gold‐catalyzed functionalization of semicarbazides (ArNHCONHNH2) with various terminal alkynes R2C≡CH (R2 = Alk or Ar) in the presence of Ph3PAuNTf2 (3 mol‐%) grants a range of substituted semicarbazones (16 examples; 64–93 %). This novel metal‐catalyzed coupling proceeds under mild conditions (PhCl, 60 °C) and demonstrates a high functional group tolerance; the reaction is sensitive to the electronic effects of the substituents in terminal alkynes. Inspection of XRD solid‐state structures of three obtained semicarbazones proved their dimerization via weak to moderate strength hydrogen bonding. The structures of the semicarbazones in solutions were studied by concentration dependent 1H NMR and 2D NMR and these experiments proved the presence of H‐bonds between two molecules. The nature and energy of H‐bonding (3.8–5.1 kcal/mol), degree of electron density delocalization, and rotational barriers via various bonds in the equilibrium model structures were studied by DFT calculations at the M06‐2X/6‐311++G** level of theory. [ABSTRACT FROM AUTHOR]