Novel OˆNˆN Pyrazolyl-imine and Imidazolyl-imine Pincer Palladium Complexes as Heck Coupling Catalysts

Abstract

Five pyrazolyl and imidazolyl compounds, 2,4-di-tert-butyl-6-[2-pyrazol-1-yl-ethylimino-methyl]-phenol (L1), 2,4-di-tert-butyl-6-{[2- (3,5-dimethyl-pyrazol-1-yl)-ethylimino]-methyl}-phenol (L2), 2,4-di-tert-butyl-6-{[2-(1H-imidazol-4-yl)-ethylimino]-methyl}-phenol (L3), 4-tert-butyl-2-{[2-(1H-imidazole-4-yl)-ethylimino]-methly}-phenol (L4) and 2-{[2-(1H-imidazole-4-yl)-ethylimino]-methly}-phenol (L5) were synthesized by condensation of the appropriate 2-hydroxybenzaldehyde and the corresponding alkylamine. Reactions of L1–L5 with either [Pd(NCMe)₂Cl₂] or [Pd(COD)MeCl] led to in situ deprotonation of the phenolic-OH proton to afford the pincer palladium complexes [Pd(L1)Cl] (1), [Pd(L1)Me] (2), [Pd(L2)Cl] (3), [Pd(L3)Cl] (4),[Pd(L4)Cl] (5) and [Pd(L5)Cl] (6). The tridentate coordination modes of the ligands were confirmed by the solid state structures of 1, 2, 3 and 4.H₂O. Complexes 1–6 catalyzed the Heck coupling reactions of iodobenzene and butylacrylate. In addition, complex 4 catalyzed the Heck coupling reaction of butyl acrylate and bromobenzene; giving conversions as high as 70 %.

Keywords: Pincer palladium complexes, pyrazolyl-imine, imidazolyl imine, aryl halides, Heck coupling catalysts