Neue Entwicklungen bei asymmetrischen Phasentransferreaktionen

Abstract

Die Phasentransferkatalyse bildet eine effiziente Grundlage zur Einführung praktischer Methoden in die organische Synthese, denn sie bietet mehrere Vorteile wie einfache Handhabung, milde Reaktionsbedingungen, die Eignung für großtechnische Synthesen und ein umweltfreundliches Reaktionssystem. Seit den wegweisenden Arbeiten zu hoch enantioselektiven Alkylierungen mit chiralen Phasentransferkatalysatoren ist dieses Forschungsgebiet im Hinblick auf eine umweltverträgliche, nachhaltige Chemie von Interesse, und in den vergangenen Jahrzehnten, vor allem in den letzten Jahren, wurden zahlreiche asymmetrische, durch chirale Oniumsalze und Kronenether katalysierte Umwandlungen für die Synthese wertvoller organischer Verbindungen entwickelt.

Asymmetric Carboxycyanation of Aldehydes by Cooperative AlF/Onium Salt Catalysts: from Cyanoformate to KCN as Cyanide Source

“…KCN likely improves the reactivity by generation of {Al-F}CN. [25] The aldehyde substrate 1 could be activated by coordination to the Al center,f acilitating aq uasi-intramolecular attack of cyanide.T he absence of an on-linear effect (see the Supporting Information) is in agreementw ith am echanismi nw hicho nly one catalyst molecule is involved. Product release and catalyst regeneration would be achieved by carboxylation with 2a.…”

Catalytic Enantioselective Synthesis of Atropisomeric Biaryls: A Cation‐Directed Nucleophilic Aromatic Substitution Reaction

“…), and the major enantiomer was R configured, as obtained previously (Scheme 2 a). The products of the nucleophilic aromatic substitution reaction (13)(14)(15)(16)(17)(18)(19)(20)(21)(22) contain an activated aryl chloride, which serves as a useful handle for further functionalization. [12] To further expand the synthetic utility of the reaction, we wished to demonstrate that the products of the reaction (as exemplified by 2) can be selectively manipulated (Scheme 3).…”

“…[6] The product, an enantioenriched cyclophane, is rendered chiral by virtue of restricted rotation of a linker about a plane of chirality ( Figure 1). [13] For such an approach to prove successful, we rationalized that the electrophile should possess 1) a plane of symmetry containing the biaryl axis, and 2) an arene which is sufficiently electron-poor to undergo a nucleophilic aromatic substitution under mild reaction conditions. [7,8] Given the central role occupied by atropisomeric materials within the fields of asymmetric catalysis and synthesis, [9] enantioselective methods for the construction of such compounds are of great value.…”

Phase‐Transfer and Other Types of Catalysis with Cyclopropenium Ions

“…PTC methods offer a number of important advantages, namely: (1) decreased dependence on organic solvents; (2) excellent scalability and inherent compatibility with moisture; (3) enhancement of reactivity, which permits shortened reaction times and increased yields; (4) ability to substitute costly and inconvenient reagents (such as LDA) for simple aqueous bases (such as KOH); and (5) amenability to enantioselective variants. 2, 3 For these reasons, phase transfer catalysis has emerged as a widely used technology throughout the domains of pharmaceutical, agrochemical, and materials chemistry.…”