Single-pot template transformations of cyanopyridines on a PdII centre: syntheses of ketoimine and 2,4-dipyridyl-1,3,5-triazapentadiene palladium(II) complexes and their catalytic activity for microwave-assisted Suzuki–Miyaura and Heck reactions

Abstract

[2 + 3] cycloaddition of the pyrrolin N-oxide⁻O⁺NCHCH₂CH₂CMe₂2 with 2-cyanopyridine NC₅H₄(2-CN) 1a in the presence of PdCl₂ at room temperature in acetone gives the ketoimine palladium(II) complex cis-[PdCl₂{N(=CCH₂CH₂C(Me)₂NH)C(=O)C₅H₄N}] 3a containing the (E)-N-(5,5-dimethylpyrrolidin-2-ylidene)picolinamide ligand, in good yield (85%). The reaction of 2 with 3-cyanopyridine NC₅H₄(3-CN) 1b or 4-cyanopyridine NC₅H₄(4-CN) 1c under the same experimental conditions gives the simple adducts of cyanopyridinestrans-[PdCl₂{NC₅H₄(3-CN)}₂] 4b or trans-[PdCl₂{NC₅H₄(4-CN)}₂] 4c, respectively. When the cyanopyridines NC₅H₃R¹R²1 (R¹ = 2-CN, R² = H 1a; R¹ = 3-CN, R² = H 1b; R¹ = 4-CN, R² = H 1c; R¹ = 4-CN, R² = 2-Cl 1d; R¹ = 3-CN, R² = 5-Me 1e) are heated at 100 °C for 12 h with 2-butanone oxime (used as a reagent and solvent) in the presence of PdCl₂, the 2,4-dipyridyl-1,3,5-triazapentadienate (or bis-imidoylamidinate) palladium(II) complexes [Pd{HNC(NC₅H₃R²)NC(NC₅H₃R²)NH}₂] 5 (2-NC₅H₃, R² = H 5a; 3-NC₅H₃, R² = H 5b; 4-NC₅H₃, R² = H 5c; 4-NC₅H₃, R² = 2-Cl 5d; 3-NC₅H₃, R² = 5-Me 5e) are synthesized by one-pot template transformations followed by deprotonation by a base (n-PrNH₂). The compounds were characterized by IR, ¹H and ¹³C NMR spectroscopies, FAB⁺-MS, elemental analyses and, in the cases of 3a, 5a, 5b and 5d, also by X-ray diffraction analyses. The catalytic activity of complexes 3 and 5 was studied under microwave irradiation for C–C coupling reactions, in the solid phase on a silica gel support, and it was found that complex 3a is the most active one with the maximum TON of 2.0·10⁵ and TOF of 1.3·10⁴ min⁻¹ for Suzuki–Miyaura, and TON of 800 and TOF of 27 min⁻¹ for Heck reactions.