Synthesis Of N-,P-containing Ligand Functionalized Ionic Liquids And The Application In Palladium-Catalyzed Heck Reaction

In this work three types of ligand-functionalized ionic liquids (ILs), including the P-ligand functionalized IL ([BDPPMM]PF₆), the hybrid N-,P-ligand functionalized IL ([EPDDMIM]PF₆), and porphyrin-functionalized ILs ([H₂TMPyP]][I]₄, [H₂TBCMMimP][I]₄), were design and synthesized, which were applied respectively in the palladium-catalyzed Heck cross-coupling reaction in conjunction with the similar structured common ILs as the solvents. Due to the available coordination and the good solubility of the Pd catalyst in the compatible IL compositions composed of the IL solvent and the ligand-functionalized IL, not only did the activity and stability of the Pd catalyst for the Heck reaction were guaranteed with a wide array of substrates, but also the expensive and toxic Pd catalyst-ligand were tightly locked in the IL phase without leaching, which avoided contaminating the organic products and realized the recyclability of the catalyst-ligand system with advantageous merits of green chemistry. Comparing to the conventional P-contaning ligands like PPh₃, the developed P-ligand functionalized ILs of [BDPPMM]PF₆ and [EPDDMIM]PF₆ exhibite improved oxidation tolerance due to the strong electron-withdrawing effect coming from the imidazoliumn cation, giving rise to the stability of the corresponding Pd catalyst dramatically. The use of N-containing tetradentate porphyrin ligands ([H₂TMPyP]][I]₄ and [H₂TBCMMimP][I]₄) with characters of non-toxicity and insensitivity to moisture/ oxygen also ensure the activity and stability of Pd catalyst, which can be qualified as the much greener processes. 1. Synthesis of the P-ligand functionalized IL ([BDPPMIMlPF₆) and its application in the Pd-catalyzed Heck reactionThe coupling reaction of iodobenzene with ethyl acrylate was chosen as a model reaction to evaluate the activity, stability and recyclability of the derived Pd catalyst in the functional IL compositions (MFIL) composed of the ligand-functionalized IL of [BDPPMIM]PF₆, the basic IL of [PEMIM]PF₆ (as the acid scavenger) and the common IL of [BMIM]PF₆ (as the solvent). In which each component not only exhibits the individual functions as the ligand, base and solvent respectively, but also embodies the synergistic effect on the catalytic performance as one unit. Due to more available coordinating sites in such MFIL system, the palladium catalyst immobilized in the which combines the advantageous activity and stability of the N, P-containing ligand coordinated Pd complexes and Pd-carbene complexes. Due to the strong hydrophobicity of the MFIL system, the recovery of [PEMIM]PF₆ and the recycling uses of the palladium catalyst can be guaranteed, and can be reused at least for 7 cycles without activity loss. The replacement/omission of any individual in the MFIL system can result in the deteriorated activity or stability/recyclability. 2. Synthesis of the hybrid N, P-ligand functionalized IL ([EPDDMIM]PF₆) and its application in the Pd-catalyzed Heck reactionWith the presence of the hybrid N,P-ligand functionalized IL, [EPDDMIM]PF₆ (3) as a ligand, PdCl₂ exhibites excellent activity and stability in the Heck cross-coupling reaction of iodobenzene with ethyl acrylate and can be reused at least for 7 cycles without activity loss, along with the common IL of [BMIM]PF₆ as the solvent. Comparing to the monodentate P-ligand ([BDPPMIM]PF₆, 2) and N-ligand ([PEMIM]PF₆, 1), [EPDDMIM]PF₆ as a bidentate ligand and with a hemilabile ligation to metal center could offer a necessary unsantuation site for the substrate at N-coordinating site and simultaneously hold the Pd atom by the P-ligand arm. On the other hand, as for [EPDDMIM]PF₆, the binding of the phosphine moiety at 2C position of imidazolium cation with strong electron-withdrawing nature, results in the decreased electron density at the phosphorous centre, leading to the improved oxidation tolerance of [EPDDMIM]PF₆ and the consequently favorable stability of the palladium catalyst.3. Synthesis of the porphyrin-functionalized ILs ([H2TBCMMimP][I]4, [H₂TMPyP]][I]₄) and the application in the Pd-catalyzed Heck reaction In contrast to the common N-ligand, the porphyrin base as a tetradentate pincer ligand can provide more stable Pd complexes suitable for Heck reactions and thereby avoid the use of exogenous ligands which could degrade the activity of the palladium catalyst. In order to match the compatibility of the porphyrin base with the IL and avoid the leaching of the porphyrin-ligand and the Pd catalyst out of the IL phase, the efforts have been made to enhance the ionopholicity of the porphyrin base through incorporating imidazolium or pyridinium tags into them. It was found that the derived ionic palladium porphyrin in conjunction with the similar structured ionic liquid (as the solvent) was a highly efficient and recyclable catalytic system for the Heck cross-coupling of iodobenzene (derivatives) and ethyl acrylate, with high turnover frequency (TOF) of 56000 h^-1 under mild (100℃, 1 h) and aerobic conditions. And the catalyst can be reused at least for 7 cycles without activity loss.