Catalyzed tandem C-N/C-C bond formation for the synthesis of tricyclic indoles using Ir(III) pyrazolyl-1,2,3-triazolyl complexes

A series of new pyrazolyl-1,2,3-triazolyl N-N′ bidentate donor ligands (2a-c, 3a-d) were prepared via Cu(I)-catalyzed Huisgen cycloaddition reactions between 1-propargylpyrazoles and 4-substituted phenyl azides. The electron-withdrawing ability of the substituents follows the trend PhCH ₂ < p-CH ₃Ph < Ph < p-CF ₃Ph < p-NO ₂Ph, as illustrated in the gradual downfield shift of the 1,2,3-triazolyl-C4′ ¹³C NMR resonances. A series of Rh and Ir complexes containing these pyrazolyl-1,2,3-triazolyl or bis(pyrazol-1-yl)methane donor ligands of general formulae [Ir(N-N′)Cp*Cl]X (X = BAr ^F ₄, BPh ₄; 5-8), [Rh(N-N′)Cp*Cl]X (X = BAr ^F ₄, BPh ₄; 9-11), and [Rh(N-N′)(CO) ₂]BAr ^F ₄ (13-16) (BAr ^F ₄ = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) were synthesized and fully characterized. The solid-state structures of 5, 6a′, 6b, 7b, 8, 9, 10a, 10a′, 11, and 15c were determined by X-ray diffraction studies. As the electron-withdrawing strength of the phenylene substituent on the triazolyl ring is increased, the M-N3′(triazole) bond length becomes longer. The efficiency of these Rh and Ir complexes as catalysts for the synthesis of tricyclic indoles via tandem C-N and C-C bond formation reactions from 2-(hydroxyalk-1-ynyl)anilines (17S-20S) was assessed. The Ir(III) catalysts were the most efficient for the C-C bond formation step, and the Rh(I) complexes 13-16 were the most efficient catalysts for C-N bond formation, where TOFs >1000 h ^-1 were reached. However, the Ir(III) complexes 5-8 were found to be the only active catalysts for the tandem C-N and C-C bond formation, as the Rh(I) complexes were not active catalysts for the C-C bond formation step. The C-N bond formation leading to the formation of indoles was found to proceed via two reaction pathways with 2-(hydroxyalk-1-ynyl)aniline substrates: (a) hydroamination and (b) hydroalkoxylation-Lewis acid mediated isomerization. Pathway (b) is likely to be the main pathway in the formation of indoles starting with 2-(hydroxyalk-1-ynyl)aniline substrates 17S, 18S, and 20S.