Catalyzed Tandem C–N/C–CBond 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)-catalyzedHuisgen cycloaddition reactions between 1-propargylpyrazoles and 4-substitutedphenyl azides. The electron-withdrawing ability of the substituentsfollows the trend PhCH2< p-CH3Ph < Ph < p-CF3Ph < p-NO2Ph, as illustrated in the gradual downfieldshift of the 1,2,3-triazolyl-C4′ 13C NMR resonances. A series of Rh and Ir complexes containing thesepyrazolyl-1,2,3-triazolyl or bis(pyrazol-1-yl)methane donor ligandsof general formulae [Ir(N–N′)Cp*Cl]X (X = BArF4, BPh4; 5–8), [Rh(N–N′)Cp*Cl]X (X = BArF4, BPh4; 9–11), and [Rh(N–N′)(CO)2]BArF4(13–16) (BArF4= tetrakis[3,5-bis(trifluoromethyl)phenyl]borate)were synthesized and fully characterized. The solid-state structuresof 5, 6a′, 6b, 7b, 8, 9, 10a, 10a′, 11, and 15cwere determinedby X-ray diffraction studies. As the electron-withdrawing strengthof the phenylene substituent on the triazolyl ring is increased, theM–N3′(triazole) bond length becomes longer. The efficiencyof these Rh and Ir complexes as catalysts for the synthesis of tricyclicindoles via tandem C–N and C–C bond formation reactionsfrom 2-(hydroxyalk-1-ynyl)anilines (17S–20S) was assessed. The Ir(III) catalysts were the most efficientfor the C–C bond formation step, and the Rh(I) complexes 13–16were the most efficient catalystsfor C–N bond formation, where TOFs >1000 h–1were reached. However, the Ir(III) complexes 5–8were found to be the only active catalysts for the tandemC–N and C–C bond formation, as the Rh(I) complexes werenot active catalysts for the C–C bond formation step. The C–Nbond formation leading to the formation of indoles was found to proceedvia two reaction pathways with 2-(hydroxyalk-1-ynyl)aniline substrates:(a) hydroamination and (b) hydroalkoxylation–Lewis acid mediatedisomerization. Pathway (b) is likely to be the main pathway in theformation of indoles starting with 2-(hydroxyalk-1-ynyl)aniline substrates 17S, 18S, and 20S. [ABSTRACT FROM AUTHOR]