The Immobilization of Pd(II) on Porous Organic Polymers for Semihydrogenation of Terminal Alkynes.

Abstract

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%).