Design and development of POCN-pincer palladium catalysts for C–H bond arylation of azoles with aryl iodides

Abstract

Well-defined and efficient POCN-ligated palladium complexes have been developed for the direct C–H bond arylation of azoles with aryl iodides. The phosphinite-amine pincer ligands 1-(R₂PO)-C₆H₄-3-(CH₂NⁱPr₂) [^R2POCN^iPr2-H; R = ⁱPr (1a), R = ^tBu (1b)] and corresponding palladium complexes {2-(R₂PO)-C₆H₃-6-(CH₂NⁱPr₂)}PdCl [(^R2POCN^iPr2)PdCl; R = ⁱPr (2a), R = ^tBu (2b)] were synthesized in good yields. Treatment of palladium complex 2a with KI and AgOAc afforded the complexes (^iPr2POCN^iPr2)PdI (3a) and (^iPr2POCN^iPr2)Pd(OAc) (4a), respectively. Similarly, the reaction of 2a with benzothiazolyl-lithium produces the (^iPr2POCN^iPr2)Pd(benzothiazolyl) (5a) complex in a quantitative yield. The pincer palladium complex 2a efficiently catalyzes the C–H bond arylation of benzothiazole, substituted-benzoxazoles and 5-aryl oxazoles with diverse aryl iodides in the presence of CuI as a co-catalyst under mild reaction conditions. This represents the first example of a pincer palladium complex being applied for the direct C–H bond arylation of any heterocycle with low catalyst loading. A preliminary mechanistic investigation reveals that palladium nanoparticles are presumably not the catalytically active form of 2a and supports the direct involvement of the catalyst 2a, with complex 5a being a probable key intermediate in the catalytic reaction.