In situ catalysts for the intramolecular hydroamination of aminoalkynes - What ligand properties determine catalyst activity?

Insight into the iridium(I)/rhodium(I)-catalyzed hydroamination of 4-pentyn-1-amine was established through comparison of catalysts formed in situ from metal precursors and selected ligands with analogous catalysts which had been synthesized and isolated as pure complexes. The isolated metal complexes used were [Rh(bim)(CO) ₂]BPh ₄ (1) and [M(bpm)(CO) ₂]BPh ₄ (2 M = Rh, 3 M = Ir), which contain the bidentate nitrogen donor ligands bis(1-methylimidazol-2-yl)methane (bim, 4) and bis(1-pyrazolyl)methane (bpm, 5). The metal precursors used for formation of in situ catalysts included [Rh(μ-Cl)(CO) ₂] ₂, [Ir(μ-Cl)(CO) ₂] _n, [Ir(μ-Cl)(C ₂H ₄) ₂] ₂, [Ir(μ-Cl)(COE) ₂] ₂, and [Ir(μ-Cl)(COD)] ₂. The sp ²-hybridised N-donor ligands investigated included heterocyclic bim (4) and bpm (5); and the diimines glyoxal bis(4-methylphenylimine) (p-tol-DAD, 8), bis(2,4,6- trimethylphenylimino)acenapthene (Mes-BIAN, 9) and bis(4-methylphenylimino) acenapthene (p-tol-BIAN, 10). The weakly coordinating p-tol-DAD (8) ligand was found to yield the most active Rh ^I catalyst which opens up a new, and synthetically very facile, ligand class for the hydroamination reaction. The most active catalysts were formed from a combination of N-donor ligand, co-ligand and counterion that resulted in the most electron-deficient metal centre. This indicated that the most likely mechanism of action of the catalysts was via alkyne binding and activation as a first step. The rhodium(I)/ iridium(I)-catalyzed hydroamination reaction was investigated by generation of the catalysts in situ using various combinations of metal precursor, N-donor ligand and counterion. It was found that ligand combinations which afforded the most electrophilic metal centres furnished the highest reaction rates.