Macrocyclic Palladium(II) Complexes in CC Coupling Reactions: Efficient Catalysis by Controlled Temporary Release of Active Species

Abstract

Stabilization of palladium species against agglomeration is essential for reasonable catalytic activity in C À C coupling reactions. In contrast to common methods of palladium(0) complex or particle stabilization, a new concept is introduced here: it is demonstrated that a controlled release of palladium from an inactive precatalyst provides stability, too, and leads to high catalytic activity. This paper presents surprising catalytic results for Heck and Suzuki reactions with aryl chlorides and bromides, using three highly stable macrocyclic palladium complexes as catalyst precursors. Three different behaviour patterns for the macrocyclic complexes can be deduced from the evaluation of catalytic activities, UV-Vis spectroscopy, recycling studies of immobilized complexes, and ligand addition experiments. (i) Palladium tetraphenylporphyrin reversibly releases only extremely low amounts of palladium during the reactions, and low coupling activities are observed.(ii) Release of palladium from its phthalocyanine complex is irreversible; cumulative release of palladium into the reaction mixtures leads to high catalytic activity. (iii) Extraordinary results were obtained with a Robson-type complex of palladium, which reversibly releases effectual amounts of palladium into solution under reaction conditions. This controlled release prevents the formation of inactive palladium agglomerates under harsh conditions and leads to high catalytic performances. Even strongly deactivated electron-rich aryl chlorides (4-chloroanisole) can be completely and selectively converted by the in situ formed anionic palladium halide complexes; the addition of typical stabilizing additives (TBAB) was found to be unnecessary. The bimetallic palladium complex is regenerated at the end of the reaction. These results contribute to the current understanding of the active species in C À C coupling reactions of Heck and Suzuki types.

A macrocyclic Pd(II)–Ni(II) complex in Heck and Suzuki reactions

“…The inhibiting effect of ammonium salt was also observed in the Heck reaction of styrene and chloroanisole catalyzed by a macrocyclic Pd(II) complex [Pd 2 L](ClO 4 ) 2 [33,34]. It was explained by the formation of palladium black in reaction with tributylamine originated from the decomposition of the salt at 160°C.…”

“…10. Similarly, the formation of two products in the Heck reaction of styrene was reported for a macrocyclic palladium precursor [33,34] and phospha-palladacycle [40].…”

“…It was based on the slow and controlled release of palladium from the precursor to the solution. As a result, the concentration of palladium in the solution remained very low, and agglomeration was limited, which allowed a high catalytic activity even for deactivated substrates [33,34].…”

“…Another concept was presented by Köhler and co-workers [33,34]. It was based on the slow and controlled release of palladium from the precursor to the solution.…”

Immobilization of Molecular Catalysts on Silica <i>via</i> New Tripodal Linker Unit

“…1-2 nm). These results suggest that catalyst deactivation is attributable to the deposition and aggregation of the palladium species to form clusters and inactive large particles on the silica surface 16) . Presumably the better recyclability of 5Pd-1.3 is associated with less formation of palladium metal particles on the silica surface during the reaction.…”