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

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.