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

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posted on 2008-06-11, 00:00 authored by Alfred J. Wooten, Patrick J. Carroll, Patrick J. Walsh
Heterobimetallic Lewis acids M3(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 Li3(THF)4(BINOLate)3Ln(THF) [Ln = La, Pr, and Eu], Li3(py)5(BINOLate)3Ln(py) [Ln = Eu and Yb], and Li3(py)5(BINOLate)3La(py)2 [py = pyridine]. Solution binding studies of cyclohexenone, DMF, and pyridine with Li3(THF)n(BINOLate)3Ln [Ln = Eu, Pr, and Yb] and Li3(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 Li3(THF)n(BINOLate)3Ln induce relatively large lanthanide-induced shifts on substrate analogues that ranged from 0.5 to 4.3 ppm in the 1H NMR spectrum. X-ray structure analysis and NMR studies of Li3(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 Li3(DMEDA)3(BINOLate)3La in three catalytic asymmetric reactions led to enantioselectivities similar to those obtained with Shibasaki’s Li3(THF)n(BINOLate)3La complex. Also reported is a unique dimeric [Li6(en)7(BINOLate)6Eu2][μ-η11-en] structure [en = ethylenediamine]. On the basis of these studies, it is hypothesized that the lanthanide in Shibasaki’s Li3(THF)n(BINOLate)3Ln complexes cannot bind bidentate substrates in a chelating fashion. A hypothesis is also presented to explain why the lanthanide catalyst, Li3(THF)n(BINOLate)3La, is often the most enantioselective of the Li3(THF)n(BINOLate)3Ln derivatives.

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