Rhodium(I) and Iridium(I) Complexes with Bidentate Phosphine−Pyrazolyl Ligands: Highly Efficient Catalysts for the Hydroamination Reaction

Mechanistic insights into the α-branched amine formation with pivalic acid assisted C-H bond activation catalysed by Cp*Rh complexes

Density functional theory computations revealed a pivalic acid assisted C-H bond activation mechanism for rhodium catalyzed formation of α-branched amines with C-C and C-N bond couplings. The reaction energies of the [Cp*RhCl₂]₂ dimer and silver cations indicate that the Cp*RhCl⁺ cation is the active catalyst. The essential role of pivalic acid is a co-catalyst for the activation of the ortho-C(sp²)-H bond in phenyl(pyrrolidin-1-yl)methanone, while the reaction of NaHCO₃ and HCl reduces the overall barrier of the catalytic cycle. In the presence of both pivalic acid and NaHCO₃ in the reaction, the C(sp²)-H bond is activated through a concerted metallation deprotonation process, and the C-C bond coupling is the rate-determining step with a total free energy barrier of 23.9 kcal mol^-1. Without pivalic acid and NaHCO₃, the C(sp²)-H bond can only be activated through a σ-bond metathesis process and the free energy barrier increases to 32.2 kcal mol^-1. We also investigated the mechanisms of a side reaction for β-branched amine formation and the reaction without styrene and found that their free energy barriers are 33.4 and 30.5 kcal mol^-1, respectively.

Iminophosphanes: Synthesis, Rhodium Complexes, and Ruthenium(II)-Catalyzed Hydration of Nitriles

Highly stable iminophosphanes, obtained from alkylating nitriles and reaction of the resulting nitrilium ions with secondary phosphanes, were explored as tunable P-monodentate and 1,3-P,N bidentate ligands in rhodium complexes. X-ray crystal structures are reported for both κ¹ and κ² complexes with the counterion in one of them being an unusual anionic coordination polymer of silver triflate. The iminophosphane-based ruthenium(II)-catalyzed hydration of benzonitrile in 1,2-dimethoxyethane (180 °C, 3 h) and water (100 °C, 24 h) and under solvent free conditions (180 °C, 3 h) results in all cases in the selective formation of benzamide with yields of up to 96%, thereby outperforming by far the reactions in which the common 2-pyridyldiphenylphosphane is used as the 1,3-P,N ligand.

Homogeneous rhodium(i)-catalysis in de novo heterocycle syntheses

Rh(i)-catalysed reactions often employ mild reaction conditions and offer excellent functional group tolerance, making them ideal transformations for the preparation of complex molecules. This review surveys examples of these synthetically useful transformations as applied to the synthesis of various heterocycles.

Weakly coordinating counter-ions for highly efficient catalysis of intramolecular hydroamination

A series of cationic rhodium(I) and iridium(I) complexes of the type [M(L[symbol: see text]L)(C2)]BAr(F)24 (where M = Rh or Ir, L[symbol: see text]L = bis(pyrazol-1-yl)methane (bpm), bis(N-methylimidazol-2-yl)methane (bim) or 1-(2-(diphenylphosphino)ethyl)-3,5-diphenylpyrazole (Ph2PyP), C2 = 1,5-cyclooctadiene (COD) or (CO)2 and BAr(F)24 = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) were synthesised in good yields. The solid-state structure of a number of complexes, including [Ir(Ph2PyP)(COD)]BAr(F)24, [Ir(bpm)(COD)]BAr(F)24 and [Ir(bim)(COD)]BAr(F)24 was determined using X-ray crystallography. The efficiency of the complexes as catalysts for the intramolecular hydroamination of 4-phenyl-3-butyn-1-amine, 4-pentyn-1-amine and 2-(2-phenylethynyl)aniline was established. The incorporation of the BAr(F)24- counter-ion in the Rh(I) and Ir(I) complexes was found to significantly improve the catalytic activity of the complexes, compared to the analogous Rh(I) and Ir(I) complexes containing BPh4- as the counter-ion. Excellent conversions were achieved for the cyclisation of 2-(2-phenylethynyl)aniline to 2-phenylindole using [Rh(bpm)(CO)2]BAr(F)24 as a catalyst. The use of a microwave reactor for enhancing the catalysed reactions was also investigated.