Syntheses, characterization, solution behavior and catalytic activity of trans-[(guanidine)2PdX2] (X = Cl and OC(O)R; R = Me, Ph and tBu) in Heck–Mizoroki coupling reactions involving chloroarenes/methyl acrylate

Abstract

Trans-[{(ArNH)2CNAr}2PdX2] (Ar = 2,5-Me2C6H3; X = Cl (1) and OC(O)R; R = Me (2), Ph (3), and tBu (4)) were isolated in high yields and characterized by elemental analysis, IR, and NMR (1H and 13C) spectroscopy. The molecular structures of the aforementioned complexes were determined by single crystal X-ray diffraction which revealed trans syn anti-anti (1·CHCl3), trans anti anti-syn (2·CHCl3 and 3·C7H8) and trans anti anti-anti (4) stereochemistry in solid state. Complexes 2·CHCl3, 3·C7H8 and 4 contain a pair of R11(8) rings stabilized by an intramolecular N–H⋯O hydrogen bond between the guanidine ligand and the carboxylate moiety. The influence of shape and size of the anion upon the stereochemistry of the complexes in the solid state and in solution are discussed. VT 1H NMR spectroscopic studies carried out on samples of 1 and 3 revealed the presence of a mixture of two rotamers in solution which arise due to the restricted CN(H)Ar single bond rotation of the guanidine ligand in both complexes. Complexes 1–4 were shown to be active catalysts even when used in 0.001 mol% in Heck–Mizoroki coupling reactions involving chlorobenzene and methyl acrylate. The scope of 1 in Heck–Mizoroki coupling reactions involving methyl acrylate and nine distinct chloroarenes were explored at 0.01 mol% which afforded the coupling products in 81−96% yields.