Lewis‐Base‐Katalyse in der organischen Synthese

Abstract

Das Erbe von Gilbert Newton Lewis (1875–1946) ist im Wörterbuch der chemischen Bindung und Reaktivität allgegenwärtig. Die Bedeutung seines Konzepts der Donor‐Akzeptor‐Bindung kommt in den eponymen Grundbegriffen für Elektronenpaarakzeptoren (Lewis‐Säuren) und ‐donoren (Lewis‐Basen) zum Ausdruck. Lewis erkannte, dass Säuren nicht automatisch Wasserstoff enthalten müssen (Brønsted‐Säuren) und trug so zum Sturz des “modernen Protonenkults” bei. Seine Entdeckung läutete zunächst die Entwicklung von Lewis‐Säuren als Reagentien und Katalysatoren für organische Reaktionen ein, in den letzten Jahren wurde aber offensichtlich, dass für diese Zwecke auch Lewis‐Basen eingesetzt werden können. Bezüglich der Beschleunigung chemischer Reaktionen ergänzen Lewis‐Basen die entsprechenden Lewis‐Säuren nicht nur elektronisch: Tatsächlich können Lewis‐Basen die Elektrophilie wie auch die Nucleophilie der Verbindungen verstärken, an die sie gebunden werden. Diese unterschiedliche Reaktivität resultiert in einer bemerkenswerten Vielfalt Lewis‐Base‐katalysierter Reaktionen.

Chiral Bicyclic Imidazole Nucleophilic Catalysts: Design, Synthesis, and Application to the Kinetic Resolution of Arylalkylcarbinols

“…The use of toluene and 0.55 equiv. of (i-PrCO) 2 O dramatically increased the s factor from 5.2 (Table 2, entry 12) to 12 ( Table 3, entry 1). Other low polarity solvents such as hexane, diethyl ether, and tert-amyl alcohol were not suitable because of the reduced solubility of the catalyst or higher freezing point.…”

“…Other low polarity solvents such as hexane, diethyl ether, and tert-amyl alcohol were not suitable because of the reduced solubility of the catalyst or higher freezing point. It was found that the s factor increased when the temperature decreased (entries [1][2][3]. At À60 8C, the s factor for 1-phenylethanol reached 22 but the reaction rate was relatively slow and needed 64 h to give the desired product in a yield of 39% (entry 3).…”

“…Therefore a combination of higher DH with a lower dA C H T U N G T R E N N U N G (O/C/N/C) leads to a more effective catalyst. From the calculated data in Table 1, it is implied that an sp 3 -carbon alkyl-substituted DPI would be the best type of catalyst (Table 1, entries [11][12][13][14][15][16][17], compared with catalysts containing sp 2 -carbon alkyl (entry 10), amino (entries 6-9) and alkoxy substituents (entries [1][2][3][4][5].…”

“…[2] Catalysts derived from chiral tertiary amines have received the most attention, [3,4] and a number of them have served as excellent catalysts for the kinetic resolution of arylalkylcarbinols. [5][6][7][8] However, the use of chiral imidazoles as nucleophilic catalysts remains relatively undeveloped [5] compared to the use of chiral pyridines, [6] amidines and isothioureas, [7] mainly due to their lower catalytic activity.…”

Stereoselective Synthesis of Functionalized Chiral 2‐Nitrocyclohexanecarboxylic Esters via Catalytic Dienamine Addition to β‐Substituted β‐Nitroacrylates

“…[8] In the attempt to develop a catalytic methodology for the synthesis of functionalized cyclohexanecarboxylic acids, we have thought to take advantage of the recent progress in the field of organocatalysis, [9] where impressive results have been obtained in the activation of unsaturated ketones via amino catalysis. [10] Recently Melchiorre and List have reported that Cinchona-based primary amines can be exploited to activate a,b-unsaturated ketones and enals, [11] and reacted them via dienamine catalysis with different electrophilic partners, including nitroalkenes.…”