Bifunctional Asymmetric Catalysis Based on Dinuclear Schiff Base Complexes

Abstract

Catalytic asymmetric processes are potentially more economical and environmentally benign than those using stoichiometric amounts of chiral reagents. 1 The development of asymmetric catalysts that exhibit high activity, high stereoselectivity, and broad substrate generality is highly desirable, and is a major theme of modern organic synthesis. In approaches to this issue, the concept of bifunctional asymmetric catalysis, wherein both partners of a bimolecular reaction are simultaneously activated (dual activation, Figure 1), is often utilized for the design of metallic catalysts as well as organocatalysts. 2Since our fi rst report on a lanthanum/lithium/binolate heterobimetallic complex in the early 1990 s, 3 our group has been working intensively on heterobimetallic Lewis acid/Brønsted base bifunctional catalysis by rare earth metal/alkali metal complexes using BINOL, Pybox, and their derivatives as ligands. 4 When designing bimetallic asymmetric catalysts, the construction of a suitable chiral environment for each targeted reaction is important for achieving the effective dual activation of nucleophiles and electrophiles through the cooperation of two metal centers. Optimization of enantioselectivity and reactivity requires a strategy for constructing a fl exible and diverse chiral environment. 5 In order to broaden the scope of bimetallic bifunctional asymmetric catalysis, the development of a heterobimetallic system not using BINOLs or alkali metals is desirable. We believe that new metal combinations will lead to novel catalytic activity and selectivity.Chiral Schiff base ligands, such as salens, are one class of privileged ligands in asymmetric catalysis. Over the last two decades, various transition metal salen and group 13 metal salen complexes have been widely utilized for a broad range of catalytic enantioselective transformations. 6 In the fi eld of bifunctional catalysis, Jacobsen and coworkers have developed an elegant intramolecular cooperative catalysis using two metal salen complexes.6a Attemps to further strengthen the catalytic activity by linking two salen complexes have also been reported. 7 In addition, several groups have nicely expanded the potential of Schiff base ligands by using them for heterobimetallic transition metal/alkali metal catalysts and transition metal/group 13 metal catalysts.8 However, the combination of transition metal and rare earth metal in chiral heterobimetallic Schiff base catalysts had not been investigated at the time we started our project. In this account, our own efforts toward the design and development of bifunctional heterodinuclear and homodinuclear Schiff base catalysts since 2007 are described. For reports from other groups, see a recent review article 7 and leading references. Heterobimetallic Cu/Sm/Schiff base Complex as a Lewis acid/Brønsted base Bifunctional CatalystWhen developing heterobimetallic complexes, the design of a suitable multidentate ligand is important for controlling the position of the two different metals in the complex. These posit...

Enantioselective Synthesis of 2,2-Disubstituted Terminal Epoxides via Catalytic Asymmetric Corey-Chaykovsky Epoxidation of Ketones

“…On the basis of the working hypothesis in Figure 2 , several chiral multimetallic catalysts developed in our group [ 19 , 20 , 21 ] were screened, and the initial screening revealed that heterobimetallic rare earth-alkali metal RE-M 3 -tris(binaphthoxide) complexes (REMB, Figure 1 ) [ 22 , 23 , 24 , 25 ] were the most promising candidates for the addition of a sulfur ylide to ketones. Optimization studies using REMB complexes are summarized in Table 1 .…”

Recent Advances in Direct Catalytic Asymmetric Transformations under Proton‐Transfer Conditions

“…A dinucleating Schiff base 29 and its congeners recently emerged as useful ligands for incorporating two identical or different metal cations into the N 2 O 2 inner cavity and the O 2 O 2 outer cavity, thus generating a wide range of bimetallic asymmetric catalysts. [35,36] A palladium/lanthanum heterodinuclear catalyst 30 prepared from Schiff base 29, Pd(OAc) 2 , La(OiPr) 3 , and p-bromophenol exhibited high anti-diastereoselectivity and enantioselectivity in the nitroaldol reaction (Scheme 7). [37] Considering that monometallic asymmetric catalysts generally promote the nitroaldol reaction in a synselective manner, a bimetallic entity is likely key to attaining a transition state leading to anti-selectivity.…”

Direkte katalytische asymmetrische Reaktionen unter Protonentransferkatalyse

“…Die Metallzentren koordinieren in der inneren N 2 O 2 -und der äuße-ren O 2 O 2 -Höhlung, und ein breites Spektrum von asymmetrischen Dimetallkatalysatoren ist auf diese Weise erhält-lich. [35,36] Ein heteronuklearer Palladium/Lanthan-Katalysator 30, der aus der Schiff-Base 29, Pd(OAc) 2 , La(OiPr) 3 und p-Bromphenol hergestellt wurde, zeigte hohe Enantioselektivitäten und anti-Diastereoselektivitäten in der Nitroaldol-reaktion (Schema 7). [37] Bedenkt man, dass asymmetrische Monometallkatalysatoren in Nitroaldolreaktionen normalerweise syn-selektiv wirken, so scheint die Dimetallstruktur entscheidend dafür zu sein, dass ein zum anti-Produkt füh-render Übergangszustand erreicht wird.…”

“…Die Metallzentren koordinieren in der inneren N 2 O 2 -und der äußeren O 2 O 2 -Höhlung, und ein breites Spektrum von asymmetrischen Dimetallkatalysatoren ist auf diese Weise erhältlich. [35,36] Ein heteronuklearer Palladium/Lanthan-Katalysator 30, der aus der Schiff-Base 29, Pd(OAc) 2 , La(OiPr) 3 und p-Bromphenol hergestellt wurde, zeigte hohe Enantioselektivitäten und anti-Diastereoselektivitäten in der Nitroaldol-…”