A Bidentate Ru(II)-NC Complex as a Catalyst for Semihydrogenation of Alkynes to (E)-Alkenes with Ethanol

Abstract

Four Ru­(II)-NC complexes were tested as catalysts for semihydrogenation of internal alkynes to (E)-alkenes with ethanol, and the complex {(C5H4N)­(C6H4)}­RuCl­(CO)­(PPh3)2 (1a) showed the highest activity. The reactions proceeded well with 1 mol % catalyst loading and 0.1 equiv of t-BuONa at 110 °C for 1 h, and 32 alkenes were synthesized with excellent E:Z selectivity. This is the first ruthenium-catalyzed semihydrogenation of internal alkynes to (E)-alkenes using ethanol as the hydrogen donor.

Cyclometalated (NNC)Ru(ii) complex catalyzed β-methylation of alcohols using methanol

“…13,17,25,26 Later on, cyclometalated (NC)Ru( ii ) complexes were developed by the Chen group and applied towards the acceptorless dehydrogenation of primary alcohols to carboxylic acids, the α-alkylation of unactivated amides and esters and the semihydrogenation of internal alkynes to E -alkenes. 16,27,28 In addition, metal complexes cyclometalated using an indolyl fragment also disclosed great potential towards catalysing hydrogen transfer reactions. 11 Recently, the Yu group reported the synthesis of indolyl fragment containing cyclometalated (NNC)Ru( ii ) complexes which showed excellent reactivity towards the acceptorless dehydrogenation of N-heterocycles and alcohols.…”

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

“…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 .…”

“…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 . While catalytic TH of C–O or C–N double bonds with alcohols has been thoroughly studied, − the variant for reduction of unactivated C–C multiple bonds had remained elusive until very recently. − ,− There are two known pathways for the TH of C–C triple bonds with alcohols: an inner-sphere dihydride mechanism and outer-sphere metal–ligand bifunctional catalysis . In 2020, an outer-sphere Mn-ligand bifunctional system was reported for cis -semihydrogenations of aryl alkynes, wherein the cooperation of metal and ligand is essential for MeOH activation. , Another proposed pathway involves the intermediacy of a metal dihydride complex (Scheme a), through which alkyne hydrogenation occurs via insertion of the triple bond into a M–H bond (M = metal); the subsequent reductive elimination at the resulting vinyl hydride complex forms Z -alkenes .…”

Catalytic and mechanistic studies of a highly active and E-selective Co(ii) PNNH pincer catalyst system for transfer-semihydrogenation of internal alkynes

“…Alcohols are commonly used as reagents and/or solvents in alkyne semihydrogenation. 15 In a unique study of alkyne coupling/TSH, Liao, Wang have presented a half sandwich Co catalyst that selectively produces dienes from terminal alkynes with amine borane acting as the only source of H 2 (Fig. 1, III).…”