Hydrogen Transfer to Internal Alkynes Using Secondary Amines on Carbon-Supported Noble Metals

High Resolution Image Download MS PowerPoint Slide The catalytic hydrogen transfer of internal alkynes using carbon-supported noble metals (Pd/C, Pt/C) was systematically studied with various secondary amines, such as indoline (Ind), tetrahydroquinoline (Thq), diisopropylamine (( i Pr) 2 NH)), and, for comparison, diisopropylethylamine (( i Pr) 2 NEt)) as hydrogen donors. The reactions proceeded sequentially, forming (Z) - and (E) -olefins as interim products, having (Z) as a major olefin product, which were sequentially hydrogenated to alkanes. The presence of (E) -alkene isomers was also observed as minor products. Among the tested systems, Pt/C-Ind, Pd/C-Ind, Pt/C-Thq, and Pd/C-Thq exhibited the highest activity and selectivity. Initial reaction rates and activation parameters (activation energy, E a; enthalpy of activation, Δ H ‡; and entropy of activation, Δ S ‡ ) were determined for these systems. To further elucidate the reaction mechanism, density functional theory (DFT) calculations were performed. The computational results revealed that the balance between adsorption strength and binding energy of reactants and intermediates governs the observed selectivity trends. Notably, the strongly negative activation entropies suggest a rigid, highly ordered transition state, independent of the metal catalyst.