Polystyrene-Supported Acyclic Diaminocarbene Palladium Complexes in Sonogashira Cross-Coupling: Stability vs. Catalytic Activity

Two types of immobilized on the amino-functionalized polystyrene-supported acyclic diaminocarbene palladium complexes (ADC-Pd-II) are investigated under Sonogashira cross-coupling conditions. Depending on substituents in the diaminocarbene fragment immobilized ADC-Pd-II, systems are found to have different catalytic activity and stability regarding Pd-leaching. Pd-II-diaminocarbenes possessing protons at both nitrogen atoms smoothly decompose into Pd-0-containing species providing a catalytic cocktail system with high activity and ability to reuse within nine runs. Polymer-supported palladium (II) complex bearing NBn-C-carbene-NH-moiety exhibits greater stability while noticeably lower activity under Sonogashira cross-coupling. Four molecular ADC-Pd-II complexes are also synthesized and investigated with the aim of confirming proposed base-promoted pathway of ADC-Pd-II conversion through carbodiimide into an active Pd-0 forms.