Modular synthesis of triphenylphosphine-derived cage ligands for rhodium-catalyzed hydroformylation applications

Abstract

Four new triphenylphosphine-derived cage ligands were modularly synthesized via dynamic imine chemistry (DIC), and their absolute structures were characterized by single crystal X-ray diffraction (SXRD), nuclear magnetic resonance (NMR) and high resolution mass spectroscopy (HRMS). In contrast to small-molecule analogues, cage ligands demonstrate superior activity and selectivity. The Rh/Cage-L2 catalyst exhibits remarkable performance with an aldehyde selectivity of 89%, accompanied by a TOF value of 2665 h⁻¹ and an l/b ratio of 2.60, thereby showcasing leading activity, chemical selectivity, and regioselectivity in the realm of homogeneous catalysts that rely on triphenylphosphine ligands. The reason for the formation of a higher l/b ratio, with a 3.84 kJ mol⁻¹ difference in the energy of the rate-determining step, has been explained through density functional theory (DFT) calculations. In addition, a Janus-type PPh₃-Au complex has been discovered during the study of the coordination of cage ligands, offering partial corroboration for the single coordination mechanism of these cage ligands. The large steric hindrance effect of cage ligands is believed to play a pivotal role in the hydroformylation reaction. This work highlights the potential application of cage ligands and inspires future efforts in the search of highly selective and efficient organometallic catalysts.