Jianguo Liu,Nan Wang,Jianan Liu,Ming Li,Ying Xu,Chenguang Wang,Yanzhi Wang,Haoquan Zheng,Longlong Ma
Highly selective catalytic hydrogenation of alkynes to alkenes is a highly important reaction owing to its industrial and commercial application. Specifically, semihydrogenation of terminal alkynes has been more challenging than internal alkenes even using Lindlar catalysts. Also, the high reduction degree state metal-supported catalysts like Pd0/C, Pt0/C, and Ru0/C have been well-known to be used widely in hydrogenation due to their super activity. However, charcoal can absorb a large amount of water; Pd/C with 50% water is convenient on a large-scale synthesis. Charcoal generally bears oxygen groups on its surface, which are responsible for low selectivity and undesired products. Even typically, only 10–60% of the Pd metal atoms are exposed, they still suffer from poor stability in acids owing to leaching. Herein, we intend to design active and stable metal catalysts with features as the following to avoid leaching: having strong interaction with the support and coordinatively unsaturated metal sites or low valence state metals physically isolated from the acid environment. Herein, a highly efficient semihydrogenation of terminal alkynes to produce alkenes has been realized using a heterogeneous Pd(II)/POP-GIEC catalyst, imine-linked, crystalline, and porous organic polymer supporter modified by coordination of Pd(OAc)2 to its walls under mild conditions. Surprisingly, for the first time, modified POP-supported low reduction degree PdII catalysts were synthesized efficiently, and they were successfully used in semihydrogenation of terminal alkynes. The substrate scope was studied and included both unfunctionalized as well as functionalized substituents on the para, ortho, and meta position of aromatic alkynes. The substrate having a substituent with the functionality of fluoro protected at the meta position was semihydrogenated with a high alkyne conversion of 100% and olefin selectivity (up to 99%).
A micron-scale palladium(II) material has been successfully prepared using Schiff base-modified hexagonal boron nitride as a support and used for the first time as an efficient and recyclable catalyst in organic synthesis. The morphology, composition, metal loading and thermal stability of the catalyst were studied using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), infrared spectroscopy (FT-IR), inductively coupled plasma (ICP) and thermogravimetric (TG) analyses. Then, the micron material was tested in various C–C cross-coupling reactions and exhibited excellent catalytic activities in the Suzuki and Heck reactions. Moreover, the catalyst could be easily recovered by simple filtration and reused at least ten times without significant loss of its catalytic activity. In general, this work demonstrates the possibility of using Schiff-base@hexagonal boron nitride as an efficient support for heterogeneous catalysts.
The development of single chiral source-derived ligands to fine-switch enantioselectivity has continuously attracted the attention in asymmetric catalysis. Herein, we use the same chiral source L-prolinamide as the starting material, synthesizing chiral bidentate pyridine-type N-oxide ligands (denoted as Py-NO) and chiral monodentate phenyl N-oxide ligands (denoted as Ph-NO); their effectiveness was demonstrated in the switch of enantioselectivity in palladium(II)-catalysed Friedel-Crafts (FC) alkylation of indoles and 2,3-dioxopyrrolidines, and both enantiomers of structurally important indole derivatives can be prepared in good yields (up to 91% yield) and high enantioselectivities (up to 99% ee). Key to the success of switch of enantioselectivity is the existence of two different activation patterns in this palladium(II) catalytic system. Experiments demonstrated that a counteranion would plausibly coordinate to the palladium(II) center in the stereoselectivity-determining step. Preliminary mechanistic studies provide a rational explanation for the above observations. This study was the first example of monodentate ligand in asymmetric catalytic Friedel-Crafts alkylation.
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