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

Abstract

Metrics & MoreArticle Recommendations CONSPECTUS: 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 U IV 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 Ce III /Ce IV 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 Ce III was k s (Cs I ) > k s (K I ) > k s (Na I ) > k s (Li I ), the rates of reaction with the oxidant trityl chloride trended in the opposite order with k obs (Li I ) ≫ k obs (Na I ) > k obs (K I ) > k obs (Cs I ). 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 knowle...

Strong Circularly Polarized Luminescence at 1550 nm from Enantiopure Molecular Erbium Complexes

“…In addition, we hypothesized that the loss of any C sp3 –H vibrations and overall rigidity of the system could prevent some quenching. Recently, other air-stable binolate-lanthanide complexes with nonalkali metal cations (notably tetramethylguanidinium) have been reported; however, to minimize additional quenching, we decided to prioritize the use of an alkali-metal and targeted [(Binol) 3 ErNa 3 ].…”

Synthesis and Reactivity of Redox-Active Cerium(IV) Aryloxide Complexes

“…Examples of structurally characterized Ce­(IV) complexes with simple aryloxide ligands include [Ce IV (Odpp) 4 ] (dpp = 2,6-diphenylphenyl), [Cp′ x Ce IV (ODipp) 4– x ] (Cp′ = substituted cyclopentadienyl; x = 1–3; Dipp = 2,6-diisopropylphenyl), and [Ce­(L OEt )­(OC 6 H 2 Cl 3 -2,4,6) 3 ] (L OEt – = [Co­(η 5 -C 5 H 5 )­{P­(O)­(OEt) 2 } 3 ] − ) . On the other hand, Ce­(IV) complexes with aryloxy-containing chelating ligands, such as bis-phenolate, , BINOLate, quinolate, phosphinoaryloxide, polydentate Schiff base, and calixarene, as well as Ce­(IV) catecholate and 1,4-dihydroquinolate complexes are well documented. [Ce IV (Odpp) 4 ] exhibited a rather low Ce­(IV/III) potential (−0.54 V vs Fc +/0 , where Fc = ferrocene, CH 2 Cl 2 ), which is, however, more anodic than those of [Ce IV {N­(SiHMe 2 ) 2 } 4 ] (−1.14 V, CH 2 Cl 2 ) and [Ce IV (O t Bu) 4 (py) 2 ] (py = pyridine) (−1.99 V, CH 2 Cl 2 ), indicating the aryloxide is a strong electron-donating group that can stabilize Ce­(IV), but it is less electron-rich than the alkoxide and silylamide.…”

Catalytic atroposelective synthesis of axially chiral benzonitriles via chirality control during bond dissociation and CN group formation

“…Our axially chiral biaryl products contain both a carbonitrile and a hydroxyl group that are both useful in many settings. For example, both CN 25 – 27 and OH 28 – 32 groups in our products can directly serve as ligands for transition metals in asymmetric catalysis. These groups can also be easily transformed into other moieties such as amines and carboxylic acids with a broad presence in functional molecules such as medicines.…”