In the course of our effort to further develop the application of rare-earth metal amidate complexes, two novel benzamide rare- earth metal amides were successfully prepared via simple metathesis reactions of benzamide HL1 and rare-earth metal precursors. For the first time, the catalytic activities of amidate complexes 1 and 2 on the hydrophosphonylation of both aldehydes and unactivated ketones have been studied. In addition, they were compared with three known divalent rare-earth metal amides 3–5 bearing the same or different amidate ligands. The influence of the central metals and their oxidation states on the catalytic behavior are reported here.
4. Conclusion
In summary, two novel bis(amidate) rare-earth metal amides 1 and 2 have been prepared via the simple silylamine elimination reaction with proligand benzamide in good yields. X-ray diffraction analyses indicate that both complexes contain two chelating amidate groups, one amino group N(SiMe3)2, and one coordinated tetrahydrofuran molecule. Five amidate rare-earth metal amides, including complexes 1 and 2, and three known divalent ones 3–5 were tested in the hydrophosphonylation of aldehydes and unactivated ketones. In general, all complexes displayed excellent catalytic activities for aldehydes, and some showed moderate to good activities for unactivated ketones. Taking account of the low catalyst loading (0.1 mol%), the short reaction time (5 min), mild conditions (room temperature, solvent-free), excellent yields of products (up to 99%) for aldehydes and moderate to good yields for ketones, complex 1 is undoubtedly the most effective catalyst among the five rare-earth metal amides studied. The advantage of this catalyst also lies in the wide substrate scope, including aromatic, aliphatic, alicyclic and heteroaromatic aldehydes and ketones. Overall, in this paper, we demonstrate the first application of amidate rare-earth metal amides as efficient catalysts for hydrophosphonylation of aldehydes and unactivated ketones, which is the well-known atom economic way to prepare the key chemicals a-hydroxyphosphonates.