NHC-Rare Earth Organometallic Adducts And Base-Catalyzed Silane Transformations

In this dissertation, the dehydrogenative coupling of amines and silanes catalyzed by NHC-rare earth amides adducts (NHC=N-heterocyclic carbenes), hydrosilylation of imines catalyzed by NHC and reduction of carbonyl compounds catalyzed by cesium carbonate were investigated. The details are summarized as below:1. NHC-ytterbium amides were prepared by reactions of NHCs with homoleptic rare earth metal silylamides Yb[N(SiMe3)2]2(THF)2or Yb[N(SiMe3)2]2. The adducts {(I’Pr)Yb[N(SiMe3)2]2}(1),{(IMes)Yb[N(SiMe3)2]2}(2) and {(IPr)Yb[N(SiMe3)2]2}(5) were prepared and fully characterized.2is an efficient catalyst for the dehydrogenative coupling of amines and silanes. The product distributions can be effectively controlled by changing the molar ratios of the two substrates. Subsequent studies indicated that NHC-ytterbium amides can also be employed to catalyze hydrosilation of alkenes and imines. The preliminary mechanism studies indicated that NHCs may stabilize the ytterbium hydride intermediate generated in the system and prevent it from oligomerization, and thus the catalytic performances were significantly improved. The steric factors of NHCs have significant effects on the catalytic reactions, demonstrating the important roles of NHCs in the catalytic reactions. This is the first time that NHCs were employed to tune the catalytic performances of rare earth amides.2. The NHC-catalyzed (NHC=1,3-Diisopropyl-4,5-dimethyl-imidazol-2-ylidene, I’Pr) hydrosilylation of imines were studied. The catalytically active intermediate8(aza-Breslow intermediate) was isolated and characterized by X-ray single crystal analysis. The NMR scale reactions indicated that NHCs initially reacted with imines to give aza-Breslow intermediates, which subsequently reacted with hydrosilanes to give hypervalent hydrosilicates followed by the transfer of one of the hydride donor to the carbonaton. The intermediate8only survived for3h in solution at298K. 3. In the course of our investigation on hydrosilylation reactions, we found that Cs2CO3can catalyze the oxidation of hydrosilanes in the presence of DMF to yield siloxanes at room temperature with0,1-0.5%molar loading of the catalyst. Based on this finding, we investigated the Cs2CO3-catalyzed reduction of various amides with hydrosilanes. Cs2CO3was an effective catalyst for the reduction of tertiary carboxamides with the commercially available PhSiH3under solvent-free conditions. The catalytic system can effectively promote the reduction of a range of amides under relatively mild conditions (from room temperature to80℃) to yield the corresponding amines in good to excellent yields (71%-100%), and thus has the potential for practical applications. It is quite possible that the "naked" carbonate acts as a strong base to initiate the catalytic process. The formation of a hypervalent hydrosilicate might be operative during the reduction process.