Electrochemical Proton Reduction over Nickel Foam for Z‐Stereoselective Semihydrogenation/deuteration of Functionalized Alkynes

Abstract

Selective reduction strategies based on abundant-metal catalysts are very important in the production of chemicals. In this paper, a method for the electrochemical semihydrogenation and semideuteration of alkynes to form Z-alkenes was developed, using a simple nickel foam as catalyst and H 3 O + or D 3 O + as sources of hydrogen or deuterium. Good yields and excellent stereoselectivities (Z/E up to 20 : 1) were obtained under very mild reaction conditions. The reaction proceeded with terminal and nonterminal alkynes, and also with alkynes containing easily reducible functional groups, such as carbonyl groups, as well as aryl chlorides, bromides, and even iodides. The nickelfoam electrocatalyst could be recycled up to 14 times without any change in its catalytic properties.

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

“…The semihydrogenation of alkynes to form alkenes (Figure a) is a prominent example of catalytic hydrogenation, , used industrially in acetylene removal from ethylene streams , but also in fine chemical synthesis. ,, The reaction is conventionally performed over heterogeneous catalysts based on rare metals, such as Lindlar Pd, although efficient catalytic systems made of nanoparticles or molecular complexes of earth-abundant metals (e.g., Co, , Cu, , and Ni − ) were recently disclosed. The electrochemical counterpart, electrocatalytic alkyne semihydrogenation, has to date been explored only with heterogeneous catalytic systems, − often based on noble metals. − …”

Stereoselective Semi‐Hydrogenations of Alkynes by First‐Row (3d) Transition Metal Catalysts

“…[100] Different Ni-to-Fe ratios have been prepared and investigated: While the pure iron catalyst Scheme 37. a) Substrate scope of the electrochemical Z-selective semihydrogenation of internal alkynes using a Ni foam cathode. [95] CC = carbon cloth. Reaction times, 1 H-NMR yields vs. 2,3,5,6-tetrachloronitrobenzene and Z/E ratios are given below each substrate.…”

“…2,3,5,6‐tetrachloronitrobenzene and Z / E ratios are given below each substrate. b) Proposed mechanism for the electrochemical Z ‐selective semi‐hydrogenation of internal alkynes using a Ni foam cathode [95] …”

“…A novel approach to Z ‐selective nickel‐catalyzed semi‐hydrogenation was presented in 2021 by Valiente and co‐workers. [95] The authors employed dilute sulfuric acid as the hydrogen source, while the reaction itself is carried out electrochemically using a nickel foam cathode at room temperature within 3–8 h. Generally good yields and selectivities were obtained for a variety of functionalized substrates (Scheme 37 a). The reaction most likely occurs at the surface of the nickel cathode, which is initially protonated due to the acidic medium.…”

“… a) Substrate scope of the electrochemical Z‐selective semi‐hydrogenation of internal alkynes using a Ni foam cathode. [95] CC=carbon cloth. Reaction times, 1 H‐NMR yields vs .…”