Bimetallic Ni(II) complex with carboxylate bridging for homogeneous hydrogenation of alkenes with NaBH4

Experimental and Computational Investigation of Facial Selectivity Switching in Nickel-Diamine-Acetate-Catalyzed Michael Reactions

Chiral Ni complexes have revolutionized both asymmetric acid-base and redox catalysis. However, the coordination isomerism of Ni complexes and their open-shell property still often hinder the elucidation of the origin of their observed stereoselectivity. Here, we report our experimental and computational investigations to clarify the mechanism of β-nitrostyrene facial selectivity switching in Ni(II)-diamine-(OAc)₂-catalyzed asymmetric Michael reactions. In the reaction with a dimethyl malonate, the Evans transition state (TS), in which the enolate binds in the same plane with the diamine ligand, is identified as the lowest-energy TS to promote C-C bond formation from the Si face in β-nitrostyrene. In contrast, a detailed survey of the multiple potential pathways in the reaction with α-keto esters points to a clear preference for our proposed C-C bond-forming TS, in which the enolate coordinates to the Ni(II) center in apical-equatorial positions relative to the diamine ligand, thereby promoting Re face addition in β-nitrostyrene. The N-H group plays a key orientational role in minimizing steric repulsion.

Non-noble Nickel-Modified Covalent Organic Framework for Partial Hydrogenation of Aromatic Terminal Alkynes

Developing non-noble metal-based catalysts with excellent performance for selective hydrogenation of alkynes under mild reaction conditions is highly desirable but still faces challenges. Herein, a non-noble nickel-modified covalent organic framework (Ni/COF) had been synthesized through a facile post-modified method and followed by reduction at a different temperature under a H₂/Ar atmosphere. The as-prepared catalysts were characterized by X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller, and Fourier transforms infrared, and the optimal H₃₅₀-Ni/COF presents excellent catalytic performance in the semihydrogenation of a series of aromatic terminal alkyne substrates, particularly in the partial hydrogenation of phenylacetylene with nearly full conversion and 85% selectivity toward styrene under mild reaction conditions (10 bar of H₂, 100 °C, and 1 h). Moreover, such a catalyst also exhibited satisfying stability after three consecutive cycles.