Thieme Chemistry Journal Awardees - Where Are They Now? Bifunctional Silver Acetate Catalyzed Asymmetric Mannich-Type Reactions

Abstract

Efficient bifunctional silver acetate catalyzed three asymmetric Mannich-type reactions with high reactivity, up to >95:5 diastereoselectivities and moderate to excellent enantioselectivities (up to 97% ee). External base is not required in this bifunctional catalytic procedure, in which acetate promotes the deprotonation.Key words: bifunctional, silver acetate, Mannich reaction, asymmetric catalysis, iminesThe asymmetric Mannich reaction is one of the most powerful and widely utilized chemical transformation for the preparation of optically active b-amino carbonyl compounds and related compounds, 1 which are key chiral building blocks found in various natural products and pharmaceutically important compounds. 2 Therefore, various enantioselective and diastereoselective Mannich-type reactions catalyzed by either organometallic complexes 3-7 or organocatalysts [8][9][10][11] have been developed over the past decade. However, for the transition-metal-catalyzed asymmetric Mannich reactions, a catalytic or stoichiometric amount of base is usually crucial for achieving full conversion in most cases of these reactions. Thus, developing an efficient asymmetric Mannich reaction without extra base under mild conditions is attractive.Recently, we demonstrated that bifunctional silver acetate catalyzed asymmetric cycloaddition of azomethine ylides with electron-deficient alkenes using ferrocenyloxazoline-derived N,P-ligands. 12 The reactive metal-bound azomethine ylide dipole is formed through deprotonation by the acetate, which plays the role of base, meaning that extra base is not necessary. We envisioned that the introduction of an ion with proper basicity to the Lewis acid might be an appealing choice for the enolization of carbonyl compounds, and that the same strategy could be applied to the Lewis acid catalyzed Mannich reaction. Herein, we describe three examples of asymmetric Mannich-type reactions catalyzed by chiral silver acetate complexes, which acts as a bifunctional catalyst, with low to high diastereoselectivities and moderate to excellent enantioselectivities.Based on such considerations and on other groups' work, 13 we set out to establish whether the catalytic system depicted in Table 1 could be used to effect catalytic asymmetric Mannich reactions between acetyl acetone and N-Boc-protected arylimines 1.In our initial investigation, we found that the silver acetate-L1 system can efficiently catalyze the Mannich reaction of aldimines 1a with acetyl acetone in diethyl ether with high activity and moderate enantioselectivity (67% ee) in the absence of base (entry 1, Table 1). Encouraged by this result, an evaluation of a range of solvents on the activity and enantioselectivity was carried out (entries 1-4). The results showed that diethyl ether was superior to other solvents.When ligand (S,S P )-L2, with the same central chirality and opposite planar chirality to (S,R P )-L1, was used in this reaction to investigate the effect of planar chirality on the enantioselectivity and absolute configuratio...

Catalytic asymmetric synthesis of 1,2-diamines

“…Silver acetate in combination with chiral ligands such as 178 122 and 179 123 have been used as catalysts for the aza-Mannich reaction of glycine methyl imino esters 160 and N -tosyl aldimines 161 (Fig. 6).…”

Self-assembled coordination thioether silver(I) macrocyclic complexes for homogeneous catalysis

“…This successful chemistry was usually conducted in the presence of commercially available inexpensive salts (AgOTf, AgNO 3 ) and eventually a (chiral) ligand. Such silver(I) complexes were prepared by using (bi)pyridine [19,23–26], phosphine [22,27–30], ditopic N/P [22] ligands and a few S/P and S/N ones [31–34]. None of the silver(I) catalysts based on sulfur ligands were reported so far, although alkyl thioethers are soft σ-donor ligands such as crown thioethers that were largely developed as macrocyclic ligands for silver(I) [35–43].…”