Co-catalysis over Bi-functional Ligand Based Ir-catalyst for Tandem Hydroformylation-acetalization Reaction

Abstract

The ionic bi-functional ligands of L2 and L3 in combination of a phosphine with a Lewis acidic phosphonium were synthesized through selective quaternized by MeI at the one P-position of the bi-dentate ligands. Due to the separation by the rigid ring, the incorporated -PPh2 and the Lewis acidic phosphonium were well retained without quenching problem. In the bi-functional ligands, the phosphine group could coordinate with transition metals, the phosphonium group could present Lewis acidic. The Ir(I)-complex modified by the "phosphino-phosphonium" bi-functional ligand (L2) was used to catalyze the tandem hydroformylation-acetalization of olefins. Under the optimized conditions ([Ir(COD)Cl]2 0.025 mmol, L2 0.025 mmol, 1-octene 5.0 mmol, methanol 5.0 mL, N-methyl pyrrolidone 2.0 mL, VCO/VH2＝4∶1 4.0 MPa, temperature 120 ℃, reaction time 8 h), the conversion of 1-octene was 97% with the selectivity of acetals up to 86%, which was analyzed by GC and GC-MS. In this system, the activity of Ir catalyst was better than that of Rh catalyst at same conditions. It was found that L2-modified [Ir(COD)Cl]2 could also efficiently accelerate this tandem reaction, which proved more effective than the ligands lacking phosphonium or the mechanical mixtures of the individual functional groups independently. It was believed that, in L2, the phosphonium not only acted as a Lewis acid organocatalyst to drive the sequential acetalization of aldehydes, but also contributed to the synergetic catalysis for the preceding hydroformylation through stabilizing the Ir-acyl intermediate with the phosphine cooperatively. The L2-[Ir(COD)Cl]2 system is also generally applied to the tandem hydroformylation-acetalization reaction of a wide range of olefins in different alcohols, give the yields of products in the range of 45%～87%. Advantageously, as an ionic ligand with high polarity, L2-based Ir(I)-catalyst could be precipitated by n-hexane upon completion to fulfil the recovery and recycling uses of transition metal catalysts.