Oxidative homocoupling of terminal alkynes under palladium-, ligand- and base-free conditions using Cu(II)-clay as a heterogeneous catalyst

Abstract

Homocoupling of terminal alkynes to 1,3-diynes has been investigated with Cu(II)modified clay under mild and operationally simple conditions without the use of any ligand or a stabilizing agent. The catalyst is robust, ecofriendly, efficient, furnishes good to excellent yields of the desired products and can be reused several times with negligible loss in catalytic activity.

Cu Catalyst Supported on Nitrogen and Phosphorus Co-Doped Carbon Nanosheets for Homocoupling of Terminal Alkynes Using CO2 as a Soft Oxidant

“…32,33 Moreover, various Cu catalysts were investigated for the oxidative homocoupling of propargylic alcohols in air or under O 2 . 53,58 Notably, in addition to the oxidative coupling reaction, carboxylative cycloaddition of CO 2 into propargylic alcohols 6 was extensively examined with various Cu catalysts to produce cyclic carbonates 8 (Table 4). 47,59−64 atmosphere with carbonates 8 as the predominant products, 47 while quantitative diyne−diols 7 can be obtained with the Cu catalyst only under O 2 rather than a CO 2 atmosphere.…”

“…For example, Jiang and co-workers reported CuCl 2 -promoted oxidative homocoupling of various propargylic alcohols 6 in sc -CO 2 (14 MPa) to give diyne–diol 7 (Table ). , Moreover, various Cu catalysts were investigated for the oxidative homocoupling of propargylic alcohols in air or under O 2 . , Notably, in addition to the oxidative coupling reaction, carboxylative cycloaddition of CO 2 into propargylic alcohols 6 was extensively examined with various Cu catalysts to produce cyclic carbonates 8 (Table ). ,− For instance, we reported that the NHC–Cu complex immobilized on carbon materials can parallelly promote both carboxylative cycloaddition and oxidative homocoupling of 6 under a CO 2 atmosphere with carbonates 8 as the predominant products, while quantitative diyne–diols 7 can be obtained with the Cu catalyst only under O 2 rather than a CO 2 atmosphere.…”

“…In contrast to homogeneous catalysts, Cu-based heterogeneous catalysts are more attractive from the viewpoint of sustainable chemistry owing to their recyclability. The reported heterogeneous Cu catalysts with O 2 as the oxidant for the coupling generally involve nano-Cu 2 O, Cu nanotubes, Cu–Cu 2 O–C nanocomposites, Cu/TiO 2 , Cu/SiO 2 , Cu-doped TiO 2 , Cu­(OH) x /TiO 2 , Cu­(OH) x /OMSs (OMS, manganese oxide-based octahedral molecular sieve), Cu–Al layered double hydroxides (CuAl–LDHs), Cu-based metal–organic frameworks, , NHC–Cu complexes immobilized on carbon materials, and various heterogenized Cu salts. − …”

Mesoporous Copper/Manganese Oxide Catalyzed Coupling of Alkynes: Evidence for Synergistic Cooperative Catalysis

“…Copper-based heterogeneous catalysts, in particular supported copper catalysts, are especially attractive, as they provide similar or higher performance in comparison to palladium. − Nevertheless, these catalysts often lack reusability, require molecular oxygen or other chemical oxidants, use high copper loadings, and need both base and ligand that are indispensable for achieving high efficiency. − In addition, cross-coupling of two different terminal alkynes remains a challenging issue due to the competing homocoupling reactions. , Alkyne prefunctionalization, the presence of precious metals, and assistance of nitrogen-based ligands were identified as crucial factors to promote the transformation. , Moreover, the current understanding of the exact mechanism of the copper-mediated alkyne coupling reaction still remains unsatisfactory . In spite of some efforts to describe the copper-mediated coupling reaction mechanism by computational techniques, proper design of catalytic active sites has not been done. − Therefore, considering the aforementioned disadvantages, the design of copper-based heterogeneous catalysts with a proper explanation of the reaction pathways for alkyne coupling reactions is highly desirable.…”