Catalytic and stoichiometric reactions of Arylpalladium(II) complexes bearing a trans-chelating dinitrogen ligand with arylboronic acids

Abstract

Pd(II) complexes with 1,2-bis(2′-pyridylethylnyl)benzene (bpeb) ligand, PdCl2(bpep) and Pd(OAc)2(bpep), catalyze cross-coupling of arylboronic acids and aryl iodide, in spite of the trans-chelation of the ligand. Arylpalladium iodo and trifluoroacetate complexes with bpep ligand, Pd(C6H4X-4)(I)(bpep) (3a-3h, X = OMe, Me, H, COMe, CHO, COOEt, NO2, F), Pd(C6H3F2-2,4)(I)(bpep) (3i), Pd(C6H4X-4)(OCOCF3)(bpep) (4a-4e, X = OMe, Me, H, COMe, CHO) and Pd(C6F5)(OCOCF3)(bpep) (4j), were obtained by ligand exchange of the Pd complexes having tmeda ligand (tmeda = N,N,N′,N′-tetramethylethylenedi-amine). X-ray crystallographic results of arylpalladium(II) trifluoroacetate complex, Pd(C6H4OMe-4)(OCOCF3)(bpeb) (4a), and bis(trifluoroacetate)palladium(II) complex, Pd(OCOCF3)2(bpeb) (5) revealed the structures with two pyridyl groups of bpeb coordinated to Pd(II) in a trans-chelating bidentate mode. 2-Pyridyl hydrogens and an acetate oxygen of 5 are at close positions to each other (2.52 and 2.66 Å). The reaction of (4-MeC6H4)B(OH)2 (7b) with Pd(C6H4OMe-4)(OCOCF3)(bpeb) (4a) in the presence of Ag2O yields Pd(C6H4Me-4)(OCOCF3)(bpeb) (4b) via exchange of the aryl group between the palladium complex and arylboronic acid. The reaction of (4-CHOC6H4)B(OH)2 (7e) yields not only Pd(C6H4CHO-4)(OCOCF3)(bpeb) (4e) but also biaryls, 4-MeOC6H4-C6H4Y-4 (Y = OMe (8a), CHO (8e)). Hammett plots of the reaction exchange suggest higher reactivity of the Pd complex with more electron-donating aryl ligand.