Neutral versus cationic Group 3 metal alkyl catalysts: performance in intramolecular hydroamination/cyclisation

Abstract

The relative catalytic activity of neutral dialkyl versus cationic monoalkyl Group 3 metal catalysts in the intramolecular hydroamination/cyclisation of the 2,2-dimethyl-4-pentenylamine reference substrate was investigated. This was found to depend strongly on the nature of the monoanionic ancillary ligand. With a bidentate amidinate ligand, the neutral catalysts were quite effective, but their cationic derivatives showed a much lower activity. The reaction kinetics suggest that this reflects an intrinsically higher activation barrier for the insertion of the olefinic moiety into the metal–amide bond for the cationic catalysts. In contrast, the neutral catalysts with tetradentate triamine–amide ligands showed a much lower activity than their cationic derivatives. It appears that this higher activity of the cationic triamine–amide catalysts reflects the beneficial effect of the additional available coordination space relative to the neutral species. The cationic triamine–amide yttrium catalysts are more active than the cationic amidinate catalysts of the same metal, possibly reflecting a stronger Y–amide bond in the latter, which is the more electron-deficient system.