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

Abstract

Shibasaki's heterobimetallic complexes M 3 (THF) n (BINOLate) 3 Ln [M = Li, Na, K, Ln = lanthanide (III)] are among the most successful asymmetric Lewis acid catalysts. Why does M 3 (THF) n (BINOLate) 3 Ln readily bind substrates when M = Li but not when M = Na or K? Structural studies herein indicate Na-and K-C cation-π interactions and alkali metal radius may be more important than even lanthanide radius. Also reported is a novel polymeric [K 3 (THF) 2 (BINOLate) 3 Yb] n structure that provides the first evidence of interactions between M 3 (THF) n (BINOLate) 3 Ln complexes.Shibasaki's M 3 (THF) n (BINOLate) 3 Ln complexes (Figure 1) are among the most effective asymmetric Lewis acid catalysts known, exhibiting high enantioselectivities over a broad range of reactions. 1-3 Understanding how these heterobimetallic catalysts work, however, has proven challenging. 4-9 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 when M = Na. In contrast, the asymmetric Michael reaction gave 92% ee when M = Na and 29% ee for M = Li. 10 The first step in unraveling the factors that are responsible for these striking differences is understanding the impact of the alkali metal on substrate binding to the lanthanide centers.Reported herein are solution and solid state studies of M 3 (sol) n (BINOLate) 3 Ln complexes that illuminate dramatic differences in Ln binding ability when M = Li vs. Na and K. Also disclosed is an unprecedented helical polymer, [K 3 We recently demonstrated that DMF reversibly binds to paramagnetic lanthanides in Li 3 (THF) n (BINOLate) 3 Ln (Ln = Eu, Pr), exhibiting > 2 ppm lanthanide induced shift (LIS) in the formyl C-H resonance in the 1 H NMR spectrum. 8 Salvadori, on the other hand, reported 7 that Na 3 (THF) 6 (BINOLate) 3 Yb does not bind water in solution or the solid state, which was attributed to the small ionic radius of Yb (La = 1.17, Eu = 1.09, Yb = 1.01). This dichotomy prompted us to examine binding of the lithium analog, Li 3 (THF) n (BINOLate) 3 Yb, with DMF. In the presence of Li 3 (THF) n (BINOLate) 3 Yb, the formyl C-H shifted over 4 ppm, consistent with binding to Yb. Furthermore, crystallization of Li 3 (THF) n (BINOLate) 3 Yb from pyridine yielded 7-coordinate Li 3 (py) 5 (BINOLate) 3 Yb•py, the ORTEP of which is shown in Figure 2. We next examined binding of DMF to Na 3 (THF) 6 (BINOLate) 3 Ln [Ln = Yb, Eu] 11-13 and K 3 (THF) 6 (BINOLate) 3 Yb 14,7 under the same conditions. Surprisingly, no LISs (>0.1 ppm) were observed, indicating that binding to the lanthanide in the Na and K analogs is much less favorable than in the Li series.

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 ).…”

“…(right) Two space-filling models viewed down the C 3 -axes with the 3-H protons in blue. Reproduced with permission from ref . Copyright 2007 American Chemical Society.…”

Cation−π Interactions in Organic Synthesis

“…Walsh and co-workers elucidated the X-ray crystal structures of Na 3 (THF) in (BINOLate) 3 La and K 3 (THF) n (BINOLate) 3 Yb in which respective Na + •••naphthyl and K + •••naphthyl interactions were observed (Scheme 147c). 409 The authors speculated that these interactions are responsible for the coordination ability of the catalysts, causing the alkali metal dependence of the product stereochemistry. 433 4.1.2.…”

Additive Effects on Asymmetric Catalysis

“…Furthermore, use of molecular sieves as coadditive provided 82% enantioselectivity. On the basis of the control experiments and the previous studies, they speculated that after addition of NaI, an alkali metal exchange would occur to afford a La–Li 2 –Na–L 3 complex, which would slightly modify the asymmetric environment, thereby leading to better results.…”