Abstract
Palladium catalysts supported on silk-fibroin (Pd/Fib) and polyethyleneimine (Pd/PEI) were developed. Both catalysts were prepared by adsorption of a rust-colored methanol solution of Pd(OAc)2 in polymers and subsequent reduction of Pd(II) to Pd(0) in situ by methanol with or without the help of hydrogen gas. The Pd/Fib catalyst showed an efficient chemoselectivity in hydrogenation of alkynes, alkenes, and azides in the presence of aromatic ketones, aldehydes, and halides, benzyl esters, and N-Cbz protective groups. These functionalities are readily reduced under the Pd/C- or Pd/C(en)-catalyzed hydrogenation conditions. The Pd/PEI could catalyze the partial hydrogenation of di-substituted alkynes to cis-alkenes in a 1:1 mixed solvent of methanol and 1,4-dioxane. It is noteworthy that even mono-substituted alkynes could be reduced to the corresponding alkenes under the Pd/PEI-catalyzed hydrogenation conditions by simply controlling the solvent ratio of methanol and 1,4-dioxane; such reduction was extremely difficult to achieve with any earlier methods.
