Diboron as a reductant for nickel-catalyzed reductive coupling: rational design and mechanistic studies

Diboron (B(2)pin(2)) has been identified as an efficient and environmentally benign reducing reagent for reductive coupling reactions for the first time, which enables the nickel-catalyzed reductive tetramerization of alkynes to be performed with high efficiency. Mechanistic and kinetic studies indicate that the facile reductive elimination to form the B-B bond from the dinuclear Ni-Ni complexes is responsible for the high efficiency. The activation enthalpy (Delta H-double dagger = 56.5 kJ mol(-1)), entropy (Delta S-double dagger = -128 J mol(-1) K-1) and the substituent effect (rho = 1.43) on this reaction were obtained.