Expanding the Rare-Earth Metal BINOLate Catalytic Multitool beyond Enantioselective Organic Synthesis

Abstract

Shibasaki's rare earth alkali metal BINOLate (REMB) framework has provided chemists with a general catalyst platform to access a range of enantioenriched small molecules from the single, commercially available pro-ligand (R)- or (S)-BINOL. A defining feature of these heterobimetallic frameworks is the high level of catalyst tunability, achieved through the simple modulation of the central rare-earth cation and peripheral alkali metal cations. While this family of multifunctional catalysts displays impressive generality and catalytic capability, detailed mechanistic understanding of these complex, multimetallic systems was lacking prior to our investigations. This backdrop served as initial inspiration for our investigations of this privileged class of complexes over the past decade, which have led to new and exciting advances in catalysis and beyond.In this Account, we describe our investigations using Shibasaki's framework focusing on the central metal-ion, the BINOLate ligands, and the secondary sphere cations. Our studies began with an investigation into the Lewis acidity of the complexes, where we demonstrated that Lewis bases readily coordinate to REMB frameworks when lithium occupies the secondary coordination sphere. This observation was contrasted by the complexes containing sodium or potassium in the secondary coordination sphere, as the rare earth cation is evidently less accessible for substrate binding. Our efforts in understanding the ligand exchange of the complexes enabled the discovery that associative processes dominate the mechanism of ligand exchange and LA/LA (Lewis acid/Lewis acid) and LA/BB (Lewis acid/Brønsted base) catalysis by the REMB frameworks. Replacing metal cations in the secondary coordination sphere with the N,N,N',N'-tetramethylguanidinium cation delivered an effective precatalyst that is air and water stable over the course of 6 months.To expand the reactivity of the REMB, we investigated the ability of UIV cations to occupy the primary coordination sphere and ZnEt+ and Cu(DBU)+ cations to occupy the secondary coordination sphere. Synthesizing the REMB complexes using the thiol congener monothioBINOL provided an unusual anionic REMB framework, driven by the oxophilicity of the lithium cations. Using the REMB as a platform for investigating the CeIII/CeIV redox couple, we demonstrated that, while oxidative cerium functionalization is observed in the case of lithium containing REMBs, salt elimination is observed in the sodium, potassium, and cesium containing REMBs. Furthermore, we found that while the rate of heterogeneous electron transfer for CeIII was ks(CsI) > ks(KI) > ks(NaI) > ks(LiI), the rates of reaction with the oxidant trityl chloride trended in the opposite order with kobs(LiI) ≫ kobs(NaI) > kobs(KI) > kobs(CsI). We attribute this to the ability to form inner-sphere complexes with the oxidant, rather than differences in redox potential or reorganization energies.Applying our knowledge in ligand exchange and redox behavior of Ce containing REMB complexes, we detailed the mechanism for oxidation of the heterochiral cerium REMB frameworks, reiterating the importance of the formation of inner-sphere complexes in the oxidation chemistry of cerium. There are many different avenues for both organic and inorganic investigation of Shibasaki's REMB framework, and our works have demonstrated the richness of the structural chemistry and properties of this framework that inform mechanism and properties of these privileged catalysts.