Access to Well-Defined Heterogeneous Catalytic Systems via Ring-Opening Metathesis Polymerization (ROMP): Applications in Palladium(II)-Mediated Coupling Reactions

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Abstract

The preparation of a new heterogeneous palladium(II)-based catalyst and its homogeneous analogue and their use for Heck-type, alkyne and amine couplings are described. The heterogeneous catalytic system is based on a polymer-bound dichloropalladium di(pyrid-2-yl)amide and was prepared via ring-opening metathesis copolymerization of norborn-2-ene-5-(N,N-di(pyrid-2-yl))carbamide with 1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo-endo-dimethanonaphthalene and subsequent loading of the resulting resin with palladium(II) chloride. The heterogeneous catalyst is air, moisture, and temperature stable up to 150 °C and highly active (94−99% yields) in the vinylation of aryl iodides and aryl bromides (Heck-type couplings) with turn-over numbers (TONs) of up to 210000. Even higher TON's (up to 350000) may be achieved in the arylation of alkynes. High yields (≤95%) and TONs (≤24000) may additionally be achieved in the tetrabutylammonium bromide (TBAB) assisted vinylation of aryl chlorides. Moderate yields (<65%) and TONs (<4000) are observed in the amination of aryl bromides. A soluble analogue of the heterogeneous system, N,N-di(pyrid-2-yl)acetamide palladium dichloride (2), was prepared from palladium(II) chloride and N,N-di(pyrid-2-yl)acetamide (1) and characterized by X-ray analysis. 2 crystallizes in the monoclinic space group P21/n, a = 835.0(1) pm, b = 1494.3(4) pm, c = 1199.3(2) pm, α = 90°, β = 109.05(2)°, γ = 90°, Z = 4. It served as a model compound for the elucidation of the actual geometry of the catalytic species. The unstrained geometry of 2 with almost ideal angles and bond lengths explaines the high stability of the catalytic center. The significantly higher catalytic activity of the heterogeneous system compared to the homogeneous one suggests well-defined ligand-bound catalytic sites rather than polymer-supported palladium colloids.

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