Chiral Benzamidinate Ligands in Rare‐Earth‐Metal Coordination Chemistry

The treatment of the recently reported potassium salt (S)‐N,N′‐bis‐(1‐phenylethyl)benzamidinate ((S)‐KPEBA) and its racemic isomer (rac‐KPEBA) with anhydrous lanthanide trichlorides (Ln=Sm, Er, Yb, Lu) afforded mostly chiral complexes. The tris(amidinate) complex [{(S)‐PEBA}₃Sm], bis(amidinate) complexes [{Ln(PEBA)₂(μ‐Cl)}₂] (Ln=Sm, Er, Yb, Lu), and mono(amidinate) compounds [Ln(PEBA)(Cl)₂(thf)_n] (Ln=Sm, Yb, Lu) were isolated and structurally characterized. As a result of steric effects, the homoleptic 3:1 complexes of the smaller lanthanide atoms Yb and Lu were not accessible. Furthermore, chiral bis(amidinate)–amido complexes [{(S)‐PEBA}₂Ln{N(SiMe₃)₂}] (Ln=Y, Lu) were synthesized by an amine‐elimination reaction and salt metathesis. All of these chiral bis‐ and tris(amidinate) complexes had additional axial chirality and they all crystallized as diastereomerically pure compounds. By using rac‐PEBA as a ligand, an achiral meso arrangement of the ligands was observed. The catalytic activities and enantioselectivities of [{(S)‐PEBA}₂Ln{N(SiMe₃)₂}] (Ln=Y, Lu) were investigated in hydroamination/cyclization reactions. A clear dependence of the rate of reaction and enantioselectivity on the ionic radius was observed, which showed higher reaction rates but poorer enantioselectivities for the yttrium compound.     

Intramolecular enantioselective hydroamination catalyzed by rare earth binaphthylamides

Asymmetric intramolecular hydroamination reaction is a stately way to prepare chiral nitrogen-containing heterocyclic compounds. We report in this account our personal contribution in this field with the synthesis of chiral amido rare-earth complexes. A new family of structurally defined heterobimetallic rare earth lithium ate complexes based on N-substituted binaphthylamido ligands was discovered that promoted the hydroamination/cyclization of aminoolefins with up to 87% ee.Neutral rare earth amido and amido alkyl complexes could also be prepared and led to very active species. A more simple and reliable synthetic procedure was discovered towards the preparation of heterobimetallic rare earth amido alkyl ate complexes. They proved to be also active and enantioselective catalysts, as a good compromise between efficiency and practicability issues.     

Enantioselective Synthesis of α‐Aminosilanes by Copper‐Catalyzed Hydroamination of Vinylsilanes

The synthesis of α‐aminosilanes by a highly enantio‐ and regioselective copper‐catalyzed hydroamination of vinylsilanes is reported. The system employs Cu‐DTBM‐SEGPHOS as the catalyst, diethoxymethylsilane as the stoichiometric reductant, and O‐benzoylhydroxylamines as the electrophilic nitrogen source. This hydroamination reaction is compatible with differentially substituted vinylsilanes, thus providing access to amino acid mimics and other valuable chiral organosilicon compounds.