Bifunctional Asymmetric Catalysis:  A Tandem Nucleophile/Lewis Acid Promoted Synthesis of β-Lactams

Abstract

[reaction: see text]. We describe a superior procedure for the catalytic, asymmetric synthesis of beta-lactams using a bifunctional catalyst system consisting of a chiral nucleophile and an achiral Lewis acid.

Bifunctional Organometallic Catalysts Involving Proton Transfer or Hydrogen Bonding

“…The second catalyst type includes Al III , In III , or Ti IV centers held by ligands containing Brønsted-Lowry bases near enough to act on intermediates, but far enough from the metal to prevent coordination. For examples, see the work of Shibasaki, [28][29][30][31][32] Saa, [33,34] Leckta, [35,36] Snapper and Hoveyda, [37] and Kozlowski. [38] In some systems, even a transition metal (for example, Ru II [39,40] ) may act simply as a Lewis acid, not forming metal-carbon bonds.…”

Development of Dinuclear Vanadium Catalysts for Enantioselective Coupling of 2-Naphthols via a Dual Activation Mechanism

“…Since the diastereomeric catalyst (S a ,S,S)-1a was less active (entry 2), (R a ,S,S)-1a constitutes a matched pair and (S a ,S,S)-1a would be a mismatched pair for oxidative coupling of 2-naphthol. The tropos-type 72) complexes (S,S)-7 and 8, with free rotation of either phenyl-phenyl or phenyl-naphthyl units, were also prepared, though their reaction rates were decreased compared to (R a ,S,S)-1a (entries 7,8). In the case of the complexes (S,S)-7 and 8, the equilibrium between matched and mismatched conformations diminished the reaction rate.…”

“…Dual activation catalyses can be classified into three categories ( Fig. 1): Type 1) dual activation using two different kinds of catalysts [3][4][5][6][7][8][9][10][11][12] ; Type 2) conjugated-type dual activation with a functional group such as a phosphate which has both acidic and basic sites in one functionality [13][14][15] ; and Type 3) dual activation by two catalytic sites in a single catalyst. For efficient dual activation of the substrates, a balance between these two functionalities in the catalyst is required.…”

Hoch effiziente asymmetrische Katalyse durch doppelte Aktivierung von Nucleophil und Elektrophil

“…[86] Lectkas Arbeitsgruppe entwickelte ein bifunktionelles katalytisches System aus dem chiralen Nucleophil 52 und einem achiralen, Lewis-sauren Metallsalz für die Synthese von b-Lactamen in hohen Ausbeuten und mit hohen Stereoselektivitäten (Schema 32). [87] Der große Einfluss des metallischen Cokatalysators auf die Reaktion kann durch drei Reaktionsmechanismen erklärt werden, die entweder über einen metallaktivierten IminKomplex (A; Abbildung 6), ein metallstabilisiertes, zwitterionisches Enolat (B) oder einen intermolekular aktivierten Komplex (C) verlaufen.…”