Nichtlineare Effekte in der asymmetrischen Katalyse

Abstract

Zahlreiche Anwendungen in der Chemie erfordern die Synthese enantiomerenreiner Verbindungen. Einen effizienten Ansatz hierfür bietet die asymmetrische Katalyse. Hierbei wird der chirale Katalysator gewöhnlich aus einem chiralen Auxiliar erzeugt, das z. B. von einem Naturstoff abgeleitet wurde oder aus der Racematspaltung racemischer Vorstufen stammt. Die Verwendung nicht‐enantiomerenreiner chiraler Auxiliare in der asymmetrischen Katalyse scheint aus präparativer Sicht ungünstig, da man erwarten sollte, dass der Enantiomerenüberschuss (ee) des Reaktionsprodukts zum ee‐Wert des chiralen Auxiliars proportional ist (Linearität). Tatsächlich kann es jedoch zu Abweichungen von der Linearität kommen. Derartige nichtlineare Effekte können sehr aufschlussreich für den Mechanismus einer Reaktion sein und außerdem in der Synthesechemie genutzt werden (asymmetrische Verstärkung). Dieser Aufsatz fasst die Fortschritte der letzten zehn Jahre bei der Anwendung des nichtlinearen Effekts in der metallorganischen und organischen Katalyse zusammen.

Construction of Chiral Tri‐ and Tetra‐Arylmethanes Bearing Quaternary Carbon Centers: Copper‐Catalyzed Enantioselective Propargylation of Indoles with Propargylic Esters

“…The molecular structure of 4 was unambiguously characterized by X-ray crystallography, and an ORTEP drawing of the cationic part of 4 is shown in A proposal of the dicopper complex, such as 4, as a reactive species in the present propargylation is also supported by a nonlinear relationship [15] between the ee value of optically active ligand L1 and the ee value of 3 a, which is produced by treatment of 1 a with 2 a in methanol at 0 8C for 48 hours in the presence of 5 mol % of CuOTf·1/2 C 6 H 6 and 10 mol % of L1 as shown in Scheme 1 c. This result provides direct evidence that a dinuclear copper complex works as a key reactive species in the propargylation.…”

Enantioselective and Regioselective Friedel–Crafts Alkylation of Pyrroles with Nitroalkenes Catalyzed by a Tridentate Schiff Base–Copper Complex

“…Interestingly, a negative nonlinear effect was observed (Figure 3 a). [10] This phenomenon is quite different from most literature examples of dimeric metal-complex catalysis. [11] Kinetic studies revealed that the enantiopure catalyst (homochiral dimer) afforded a slower reaction than the racemate (heterochiral dimer), although both exhibited very good solubility in toluene (Figure 3 b).…”

Substrate‐Dependent Nonlinear Effects in Proline–Thiourea‐Catalyzed Aldol Reactions: Unraveling the Role of the Thiourea Co‐Catalyst

“…[16] These findings cast serious doubts upon the extent and relevance of proline solubilization by the thiourea co-catalyst in the aldol reactions explored by Reis et al [4] and by us, [5] and we felt that this important issue deserved more detailed consideration. We envisaged that the comparative study of nonlinear effects [18] in the heterogeneous prolinecatalyzed aldol reaction, both in the absence and in the presence of a thiourea co-catalyst, could shed some light on this problem. As demonstrated in the pioneering contributions of Hayashi [19] and Blackmond, [20] the higher solubility of enantiopure proline with respect to the racemic gives rise to characteristic nonlinear effects (positive for low ees and negative for high ees of the solid proline catalyst, joined by a plateau reflecting the formation of an eutectic for intermediate ees of proline) that give a direct and clear-cut indication of the presence of solid proline as a catalyst source in the reaction medium.…”