Insight into Substrate Binding in Shibasaki’s Li₃(THF)_n(BINOLate)₃Ln Complexes and Implications in Catalysis

Abstract

Heterobimetallic Lewis acids M 3(THF) n (BINOLate) 3Ln [M = Li, Na, K; Ln = lanthanide(III)] are exceptionally useful asymmetric catalysts that exhibit high levels of enantioselectivity across a wide range of reactions. Despite their prominence, important questions remain regarding the nature of the catalyst-substrate interactions and, therefore, the mechanism of catalyst operation. Reported herein are the isolation and structural characterization of 7- and 8-coordinate heterobimetallic complexes Li 3(THF) 4(BINOLate) 3Ln(THF) [Ln = La, Pr, and Eu], Li 3(py) 5(BINOLate) 3Ln(py) [Ln = Eu and Yb], and Li 3(py) 5(BINOLate) 3La(py) 2 [py = pyridine]. Solution binding studies of cyclohexenone, DMF, and pyridine with Li 3(THF) n (BINOLate) 3Ln [Ln = Eu, Pr, and Yb] and Li 3(DMEDA) 3(BINOLate) 3Ln [Ln = La and Eu; DMEDA = N, N'-dimethylethylene diamine] demonstrate binding of these Lewis basic substrate analogues to the lanthanide center. The paramagnetic europium, ytterbium, and praseodymium complexes Li 3(THF) n (BINOLate) 3Ln induce relatively large lanthanide-induced shifts on substrate analogues that ranged from 0.5 to 4.3 ppm in the (1)H NMR spectrum. X-ray structure analysis and NMR studies of Li 3(DMEDA) 3(BINOLate) 3Ln [Ln = Lu, Eu, La, and the transition metal analogue Y] reveal selective binding of DMEDA to the lithium centers. Upon coordination of DMEDA, six new stereogenic nitrogen centers are formed with perfect diastereoselectivity in the solid state, and only a single diastereomer is observed in solution. The lithium-bound DMEDA ligands are not displaced by cyclohexenone, DMF, or THF on the NMR time scale. Use of the DMEDA adduct Li 3(DMEDA) 3(BINOLate) 3La in three catalytic asymmetric reactions led to enantioselectivities similar to those obtained with Shibasaki's Li 3(THF) n (BINOLate) 3La complex. Also reported is a unique dimeric [Li 6(en) 7(BINOLate) 6Eu 2][mu-eta (1),eta (1)-en] structure [en = ethylenediamine]. On the basis of these studies, it is hypothesized that the lanthanide in Shibasaki's Li 3(THF) n (BINOLate) 3Ln complexes cannot bind bidentate substrates in a chelating fashion. A hypothesis is also presented to explain why the lanthanide catalyst, Li 3(THF) n (BINOLate) 3La, is often the most enantioselective of the Li 3(THF) n (BINOLate) 3Ln derivatives.

[Ln(binolam)3]⋅(OTf)3, a New Class of Propeller‐Shaped Lanthanide(III) Salt Complexes as Enantioselective Catalysts: Structure, Dynamics and Mechanistic Insight

“…Keywords: binol derivatives · circular dichroism · enantioselectivity · lanthanides · NMR spectroscopy [ enantioselective catalysts, [4] and prompted a set of structural and dynamic investigations owing to their intriguing properties. [5][6][7][8] One of the keys to the success of heterobimetallic systems has been attributed to the coexistence in the same molecule of Lewis acid and Brønsted base centres. The latter arise because the organic ligand becomes deprotonated by a base before or during synthesis of the complex.…”

“…Single-crystal X-ray diffraction structure of [ScA C H T U N G T R E N N U N G (binolamo) 3 ]·-A C H T U N G T R E N N U N G (OTf)3 (8) in which, for the sake of clarity, triflate counterions, solvents (CH 3 CN and H 2 O) and hydrogen atoms have been removed.…”

Alkali Metal Yttrium neo‐Pentoxide Double Alkoxide Precursors to Alkali Metal Yttrium Oxide Nanomaterials

“…While the contamination of [Ln(OR) 3 ] with alkali metals is often reported as problematic, few reports detailing the structural aspects of molecular ‘[AY(OR) 4 ]’ complexes are available . Of the structurally characterized A−Y‐OR species, several solvated alkali metals/ligands (A/Ligand) have been disseminated, including: K/salen; Na/2,2’‐ethane‐1,1‐diylbis(4,6‐di‐ tert ‐butylphenoxide), Li or Na/binaphtholate;− Li or K/2,6‐dimethylphenoxide; and Na/ tert ‐butoxide . The later species was identified as the trace by‐product [YNa 8 (OCMe 3 ) 10 (OH)] from the reaction between YCl 3 and three equivalents of impure [Na(OBu t )] n .…”

“…‐ The limited number of structural reports for these compounds may be due to the fact that contaminated alkali metal co‐crystallization is often reported as a problem for the synthesis of ‘clean’ [M(OR) n ]. The only previously reported A−Y‐(OR) structure that possesses an alkyl chain resulted from an unintended co‐crystallization; however, rational synthesis routes have been reported for the lighter alkali metal congener derivative based on the binaptholate (BINO) ligand ,…”

Malonamide-Functionalized Gold Nanoparticles for Selective, Colorimetric Sensing of Trivalent Lanthanide Ions

“…The increased use of lanthanides in agricultural fertilizer, 31 catalysts, 32,33 and medical diagnostics 34 has raised concern about their potential for pollution and warrants new methods for their detection. [35][36][37][38][39] In addition, there is interest in monitoring groundwater near storage tanks used to hold high level radioactive waste (HLRW), 40 and recent increased measures of national security are elevating demands for methods to sense and monitor radionuclides.…”