Improving intramolecular hydroaminationRh(i) and Ir(i)catalysts through targeted ligand modification

Abstract

A series of complexes containing the phosphino-pyrazolyl ligand 1-(2-(diphenylphosphino)phenyl)pyrazole (PyPhP) and 1-(2-(diphenylphosphino)ethyl)pyrazole (PyP) were synthesized: [Ir(PyPhP)(COD)]BPh₄, [Ir(PyPhP)(CO)₂]BPh₄, Rh(PyPhP)(CO)Cl and Ir(PyPhP)(CO)Cl. The complexes Rh(PyP)(CO)OSO₂CF₃ and Rh(PyPhP)(CO)OSO₂CF₃ were also synthesized, using the parent complexes Rh(PyP)(CO)Cl and Rh(PyPhP)(CO)Cl. The solid-state structures of the new complexes were determined by X-ray diffraction analysis. The cationic Ir(I) complex [Ir(PyPhP)(COD)]BPh₄ was found to be a highly efficient catalyst for the intramolecular hydroamination of 4-pentyn-1-amine, achieving a turnover rate of 1500 h⁻¹, with >98% conversion in 6 minutes. The efficiency of the catalyzed hydroamination of 4-pentyn-1-amine using the neutral Ir(I) and Rh(I) complexes as catalysts was significantly improved by generating active catalysts in situ through abstraction of a chloride ligand by reaction with AgOSO₂CF₃, TMSOSO₂CF₃ or NaBPh₄. The catalytic efficiency of the catalysts generated from Ir(PyP)(CO)Cl and a sodium salt were found to be inversely proportional to the coordinating strength of the counter-ion of the sodium salt. Rh(PyP)(CO)OSO₂CF₃ is a more efficient catalyst for the cyclization of 4-pentyn-1-amine than the complex generated in situ from AgOSO₂CF₃ and chloride complex Rh(PyP)(CO)Cl indicating that the higher lability of the triflate co-ligand of Rh(PyP)(CO)OSO₂CF₃, compared to the chloride co-ligand of Rh(PyP)(CO)Cl, enhances the catalytic activity of the Rh(I) complexes.