Electrocatalytic Semihydrogenation of Alkynes with [Ni(bpy)₃]²⁺

Abstract

Electrifying the production of base and fine chemicals calls for the development of electrocatalytic methodologies for these transformations. We show here that the semihydrogenation of alkynes, an important transformation in organic synthesis, is electrocatalyzed at room temperature by a simple complex of earth-abundant nickel, [Ni­(bpy)3]2+. The approach operates under mild conditions and is selective toward the semihydrogenated olefins with good to very good Z isomer stereoselectivity. (Spectro)­electrochemistry supports that the electrocatalytic cycle is initiated in an atypical manner with a nickelacyclopropene complex, which upon further protonation is converted into a putative cationic Ni­(II)–vinyl intermediate that produces the olefin after electron–proton uptake. This work establishes a proof of concept for homogeneous electrocatalysis applied to alkyne semihydrogenation, with opportunities to improve the yields and stereoselectivity.

Dual Photoredox and Nickel Catalysed Reductive Coupling of Alkynes and Aldehydes

“…Based on the spectroscopical studies, and thanks to the insightful and careful analysis published on the reductive coupling of alkynes mediated by nickel in the presence of stoichiometric amount of reductant, we can propose a tentative catalytic cycle for the reaction (Figure 4). In previous reports, different mechanisms have been proposed for metal catalysed reductive coupling reactions [39] . Jamison and Houk, [30] and Montgomery and Houk [40] have undertaken a mechanistic investigations of the process promoted by reductant such as BEt 3 or HSiR 3 .…”

“…In previous reports, different mechanisms have been proposed for metal catalysed reductive coupling reactions. [39] Jamison and Houk, [30] and Montgomery and Houk [40] have undertaken a mechanistic investigations of the process promoted by reductant such as BEt 3 or HSiR 3 . The resting state of the catalyst was calculated being the 16e À alkyne(bisphosphane) nickel(0) complex.…”

Electrocatalysis with Molecular Transition-Metal Complexes for Reductive Organic Synthesis

“…While (pre)catalysts displaying a readily open coordination sphere are challenging to handle, this difficulty is bypassed with the electroreductive dissociation of spectator ligands that leaves the catalyst in an activated, coordinatively unsaturated state ( Figure 14 b). This case is easily achieved from oxidized complexes bearing halides, as with [Ni(dppe)Cl 2 ] in electrocatalytic Ullmann coupling (see section III.A.2 ), or strong σ-donor L ligands, such as bpy units in [Ni(bpy) 3 ] 2+ catalyzing EHC (see section III.C ) or alkyne hydrogenation, 98 that are expulsed when electron density at the metal increases.…”

“…First insights in that direction have recently been disclosed for the electrocatalytic selective semihydrogenation of alkynes with [Ni(bpy) 3 ] 2+ . 98 There, the hydrogenation cycle does not require the involvement of a hydride complex but can activate the alkyne at the reduced Ni(0) complex into a nickelacyclopropene intermediate that is further protonated. This metallacycle route opens up original activation ways toward hydrogenation and hydroelementation exempt of the need for hydrides ( Figure 15 a).…”

“…In the presence of an alkyne (e.g., 4-octyne), , electrochemical observations suggest that a fast ligand exchange follows the reduction to [Ni 0 (bpy) 2 ] and affords a formal [Ni 0 (bpy)­(alkyne)] complex (Figure ). The structure of this intermediate is supported by isolated [Ni 0 (bpy)­(alkyne)] samples, − which display a square planar geometry and are best described as Ni­(II) cyclopropene compounds.…”