Evidence for Substrate Binding by the Lanthanide Centers in [Li₃(thf)_n(binolate)₃Ln]: Solution and Solid‐State Characterization of Seven‐ and Eight‐Coordinate [Li₃(sol)_n(binolate)₃Ln(S)_m] Adducts

Abstract

Ln] (binol = 1,1'-bi-2-naphthol; M = Li, Na, K; thf = tetrahydrofuran) catalysts (Figure 1) developed by Shibasaki and co-workers have proven to be among the most versatile and successful asymmetric Lewis acid catalysts introduced to date. [1][2][3] Central to mechanistic proposals involving [M 3 A C H T U N G T R E N N U N G (thf) n -A C H T U N G T R E N N U N G (binolate) 3 Ln] heterobimetallic compounds is Lewis acid activation of the substrate by the lanthanide. [2][3][4] Prior efforts to detect lanthanide-substrate interactions, however, have not been successful. Herein, we disclose solid-state and solution evidence that [Li 3 A C H T U N G T R E N N U N G (thf) n A C H T U N G T R E N N U N G (binolate) 3 Ln] catalysts can bind organic substrates and substrate analogues to form seven-and eight-coordinate Ln centers. , 6] and , 5, 7, 8] complexes have been characterized crystallographically.[9] The lanthanide centers in these structures are either six-coordinate (Figure 1) or sevencoordinate when bound to a water molecule. Each maingroup-metal center is bound by one or two organic solvent molecules. The lanthanide centers in sterically hinderedLn] complexes have a much higher affinity for water than for organic ligands, evidence for which was found by crystallization of the hydrates above from THF and Et 2 O. In this regard, the absence of structurally characterizedLn(S)] (S = substrate or solvent) complexes with Lewis basic organic ligands bound to the lanthanide atom raises questions concerning the ability of the lanthanide center to bind and activate substrates in asymmetric catalysis. One approach used by prior investigators to address these concerns involved probing Ln-substrate binding interactions in solution by using paramagnetic Pr, Eu, and Yb analogues. Paramagnetic complexes provide greater chemical-shift dispersions than their diamagnetic counterparts, thereby allowing the study of normally unobservable metal-substrate interactions.[10] Previously reported 1 H NMR spectroscopic binding studies of [Eu] with cyclohexenone showed small chemicalshift differences for the a-vinyl proton of 0.1 and 0 ppm, respectively. [8] Similarly, addition of pivalaldehyde to 3 Pr] resulted in a shift of the signal for the formyl hydrogen atom of only 0.1 ppm. A C H T U N G T R E N N U N G [Li 3 A C H T U N G T R E N N U N G (thf) 6 A C H T U N G T R E N N U N G (binolate)[11] These small lanthanide-induced shifts (LISs) could be attributed to coordination of the carbonyl groups to the main-group atoms. Alternatively, the small LIS may indicate weak, reversible substrate binding to the lanthanide center. A related NMR spectroscopic study with paramagnetic 3 Yb] showed that the small Yb center does not even bind water. [6] To reconcile some of the differences in proposed mechanisms and experimental observations for this important class of catalysts, we set out 1) to assess whether the lanthanide in [Li 3 A C H T U N G T R E N N U N G (sol) n A C H T U N G T R E N N U N G (binolate) 3 Ln] complexes could bind org...

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

“…Salvadori reported that YbNaB does not bind water, and related observations were made by Shibasaki and Aspinall . We found that the DMF formyl C–H shifted over 4 ppm in the presence of YbLiB in THF, but no LIS was observed in the presence of RENaB (RE = Yb and Eu) or YbKB under identical conditions . Further insight came from the SC-XRD studies of anhydrous RENaB (RE = La III and Eu III ).…”

“…For example, substrate binding at a paramagnetic RE III center (e.g., Pr III , Eu III ) would induce significant changes in substrate chemical shifts (lanthanide-induced shifts, LISs); however, minimal shifts were observed for the formyl C–H of pivaldehyde in the presence of PrNaB and EuNaB . Therefore, we undertook a series of binding studies to understand the activation mode of REMB. , …”

Impact of Na− and K−C π-Interactions on the Structure and Binding of M3(sol)n(BINOLate)3Ln Catalysts

“…Shibasaki's M 3 (THF) n (BINOLate) 3 Ln complexes (Figure ) are among the most effective asymmetric Lewis acid catalysts known, exhibiting high enantioselectivities over a broad range of reactions. − Understanding how these heterobimetallic catalysts work, however, has proven challenging. − In particular, seemingly subtle alterations in the catalyst composition result in dramatic changes in selectivity. For example, in the nitro aldol reaction with M 3 (THF) n (BINOLate) 3 Ln, 94% ee was obtained when M = Li and 2% ee was obtained when M = Na.…”

“…We recently demonstrated that DMF reversibly binds to paramagnetic lanthanides in Li 3 (THF) n (BINOLate) 3 Ln (Ln = Eu, Pr), exhibiting a >2 ppm lanthanide induced shift (LIS) in the formyl C−H resonance in the 1 H NMR spectrum . Salvadori, on the other hand, reported 7 that Na 3 (THF) 6 (BINOLate) 3 Yb does not bind water in solution or in the solid state, which was attributed to the small ionic radius of Yb (La = 1.17, Eu = 1.09, Yb = 1.01).…”

A Career in Catalysis: Masakatsu Shibasaki

“…The highly ordered structure of RE-M [1] 3 -tris­(BINOLate) has attracted sustained interest in the chemical community, and in-depth studies were conducted to dissect the coordination dynamics of this class of complexes . Walsh et al revealed hepta- and octa-coordinated RE in the crystal structures of these complexes with ligated guest molecules, while NMR experiments suggested a dynamic exchange of Li + and BINOL units in solution. − The exchange kinetics are faster than the reactions, and the involvement of a putative bis­(BINOLate) species (or oligomeric multimetallic species) was proposed. Although the mode of action of RE-M [1] 3 -tris­(BINOLate) is mostly Lewis acid/Brønsted base cooperative catalysis, Lewis acid/Lewis acid cooperative catalysis would be operative in aza-Michael reactions, Corey–Chaykovsky cyclopropanation, and epoxidation, − where both RE and M [1] function as Lewis acids to activate nucleophiles and electrophiles.…”