Semireduction of Alkynes Using Formic Acid with Reusable Pd-Catalysts

Renaissance in Alkyne Semihydrogenation: Mechanism, Selectivity, Functional Group Tolerance, and Applications in Organic Synthesis

Alkenes constitute a significant class of chemical compounds with applications in the bulk, pharmaceutical, or perfume industry. Among the known methods of olefin production, semihydrogenation of the C-C triple bond seems to be the most straightforward one. Nonetheless, the success of this reaction requires full control over diastereoselectivity, eradication of a parasitic process of over-reduction or migration of the C-C double bond formed, and achieving satisfactory functional-group compatibility. The review demonstrates developments in the field of alkyne semihydrogenation over the period 2010-2022, with selected papers published in 2023 and 2024, emphasizing solutions to the above-mentioned limitations. We discuss mechanistic aspects of this transformation, including those related to unconventional systems. The review includes examples of applications of alkyne semihydrogenation in organic synthesis, confirming the considerable utility of this process. Finally, strategies to enhance catalyst selectivity are summarized. For the reader's convenience, we provided a graphical guidebook to catalytic systems, illustrating the efficiency of the particular method.

A simple and efficient in situ generated copper nanocatalyst for stereoselective semihydrogenation of alkynes

Development of a simple, effective, and practical method for (Z)-selective semihydrogenation of alkynes has been considered necessary for easy-to-access applications at organic laboratory scales. Herein, (Z)-selective semihydrogenation of alkynes was achieved using a copper nanocatalyst which was generated in situ simply by adding ammonia borane to an ethanol solution of copper sulfate. Different types of alkynes including aryl-aryl, aryl-alkyl, and aliphatic alkynes were selectively reduced to (Z)-alkenes affording up to 99% isolated yield. The semihydrogenation of terminal alkynes to alkenes and gram-scale applications were also reported. In addition to eliminating catalyst preparation, the proposed approach is simple and practical and serves as a suitable alternative method to the conventional Lindlar catalyst.

Bis-NHC–Ag/Pd(OAc)2 Catalytic System Catalyzed Transfer Hydrogenation Reaction

The bis-NHC–Ag/Pd(OAc)2 catalytic system (NHC = N-heterocyclic carbene), a combination of bis-NHC–Ag complex and Pd(OAc)2, was found to be a smart catalyst in the Pd-catalyzed transfer hydrogenation of various functionalized arenes and internal/terminal alkynes. The catalytic system demonstrated high efficiency for the reduction of a wide range of various functional groups such as carbonyls, alkynes, olefins, and nitro groups in good to excellent yields and high chemoselectivity for the reduction of functional groups. In addition, the protocol was successfully exploited to stereoselectivity for the transformation of alkynes to alkenes in aqueous media under air. This methodology successfully provided an alternative useful protocol for reducing various functional groups and a simple operational protocol for transfer hydrogenation.

Catalytic, Transition-Metal-Free Semireduction of Propiolamide Derivatives: Scope and Mechanistic Investigation

We report a transition-metal-free trans-selective semireduction of alkynes with pinacolborane and catalytic potassium tert-butoxide. A variety of 3-substituted primary and secondary propiolamides, including an analog of FK866, a potent nicotinamide mononucleotide adenyltransferase (NMNAT) inhibitor, are reduced to the corresponding (E)-3-substituted acrylamide derivatives in up to 99% yield with >99:1 E/Z selectivity. Mechanistic studies suggest that an activated Lewis acid-base complex transfers a hydride to the α-carbon followed by rapid protonation in a trans fashion.

Cobalt catalyzed stereodivergent semi-hydrogenation of alkynes using H2O as the hydrogen source

Cobalt-catalyzed stereodivergent semi-hydrogenation of internal alkynes to alkenes is developed.