Homogeneous catalytic transfer semihydrogenation of alkynes – an overview of hydrogen sources, catalysts and reaction mechanisms

Abstract

Chemoselective semihydrogenation of alkynes to alkenes with E- or Z-stereoselectivity is among the most important transformations in the synthesis of highly functional organic building blocks. Use of hydrogen transfer reagents...

An Amine-Assisted Ionic Monohydride Mechanism Enables Selective Alkynecis-Semihydrogenation with Ethanol: From Elementary Steps to Catalysis

“…Partly due to the fact that 1,2-disubstituted alkenes have the propensity to undergo both stereo-and regioisomerization, most reported catalytic TH systems are unsuitable for dialkyl alkynes or provide low selectivity, to name a few. 14,24,34 The TH of dialkyl alkynes with 1-tBuNH 2 -EtOH furnished high yields and selectivities, although certain substrates needed a relatively high temperature (85 °C). In addition to those with simple alkyl substituents (3b), alkynes bearing Cl (3ae) and OH (3af) functionalities in the alkyl chains were reduced smoothly.…”

“…Catalytic cis -semihydrogenation of internal alkynes is one of the most important approaches for synthesis of Z -alkenes. − A number of heterogeneous and homogeneous catalysts have been developed for direct hydrogenation of alkynes with H 2 − or transfer hydrogenation (TH) with HCOOH, − NH 3 BH 3 , − hydrosilanes, , or alcohols − as hydrogen donors. Although effective for reduction of low-functionality alkynes, the chemo-, regio-, and stereoselectivity of the state-of-the-art catalysts, in many cases, are insufficient to produce pure Z -alkenes in polyfunctionalized bioactive molecules. , Moreover, in order to avoid Z – E alkene stereoisomerization and overreduction (two common side reactions), most catalytic systems require careful handling of the reaction conditions, which may pose a challenge for industrial application . Thus, despite tremendous progress in this field, there is still room to develop more selective, general, and practical catalysts for alkyne semihydrogenation. , …”

“…Although effective for reduction of low-functionality alkynes, the chemo-, regio-, and stereoselectivity of the state-of-the-art catalysts, in many cases, are insufficient to produce pure Z -alkenes in polyfunctionalized bioactive molecules. , Moreover, in order to avoid Z – E alkene stereoisomerization and overreduction (two common side reactions), most catalytic systems require careful handling of the reaction conditions, which may pose a challenge for industrial application . Thus, despite tremendous progress in this field, there is still room to develop more selective, general, and practical catalysts for alkyne semihydrogenation. , …”

“…35 Thus, despite tremendous progress in this field, there is still room to develop more selective, general, and practical catalysts for alkyne semihydrogenation. 3,34 Low-carbon alcohols, such as methanol, ethanol, and isopropanol, are ideal hydrogen sources, as they are much easier and safer to handle than hydrogen gas, 36−43 and their use as green solvents is prevalent industrially. 44 Bioderived ethanol is particularly attractive due to its sustainability, accessibility, and environmentally friendly nature with the formation of low-toxic byproducts (acetaldehyde or ethyl acetate) in the TH reactions.…”

Copper(0) nanoparticle catalyzed Z‐Selective Transfer Semihydrogenation of Internal Alkynes

“…A wide range of homogeneous and heterogeneous catalysts has been successfully employed to pursue this aim (Pd 0 , Ru(II), Ni 0 , Au(I), Cu(I), and Fe(II)). [2] However, this transformation often involves Z/E isomerization, a shift in double-bond position, overhydrogenation to alkanes and poor tolerance of other reducible groups. The Lindlar catalyst is a well-established and effective system for alkyne semihydrogenation, but the presence of lead in its composition makes its replacement desirable in terms of sustainability.…”