Breslow‐Intermediate aromatischer N‐heterocyclischer Carbene (Benzimidazolin‐2‐ylidene, Thiazolin‐2‐ylidene)

Abstract

Wirb erichten über die erstmalige Herstellung und Charakterisierung von Breslow-Intermediaten aromatischer N-heterocyclischer Carbene (NHCs), nämlichB enzimidazolin-2-ylidenen (NMR, Rçntgenkristallstruktur) und Thiazolin-2-ylidenen (NMR). Im erstgenannten Fall wurden die Diaminoenole durch die Reaktion der freien N,N-Bis-Dipp-und N,N-Bis-Mes-Benzimidazolin-2-ylidene mit Aldehyden generiert, während im zweiten Fall das Dimer des 3,4,5-Trimethylthiazolin-2-ylidenes als Ausgangspunkt diente.Z ur eindeutigen NMR-Identifizierung des Thiazolin-2-yliden-basierten Breslow-Intermediats diente eine 13 C-Doppelmarkierung der NHC-und der Aldehyd-Komponente.Z udem wurded ie Acylanion-Reaktivität über die Bildung von Benzoin mit einem Überschuss an Aldehyd nachgewiesen. Schema 1. Katalysezyklus der Benzoinkondensation nach Breslow. [1,2] [*] Dr.

Stable Organic Radicals Derived from N‐Heterocyclic Carbenes

“…Traditionally, organocatalytic application of NHCs involves the generation of Breslow intermediates, which then undergo various useful reactions with two‐electron acceptors such as aldehydes, Michael acceptors, and external oxidants . However, several organocatalytic systems that operate through a new single‐electron‐transfer pathway have recently been developed.…”

Formation of Breslow Intermediates from N‐Heterocyclic Carbenes and Aldehydes Involves Autocatalysis by the Breslow Intermediate, and a Hemiacetal

“…Key to success in these early experiments was the use of saturated imidazolidin‐2‐ylidenes as NHC component, such as SIPr ( 1 a ), which smoothly affords the Breslow intermediate BI 1a,2a when combined with benzaldehyde ( 2 a ), as shown in Scheme 1, top [7, 8] . Later studies from our laboratory extended the range of BIs produced in this way, and characterized by NMR and X‐ray crystallography, to unsaturated, aromatic NHCs, including thiazolin‐2‐ylidenes, the catalytic principle of Nature's Umpolung catalyst thiamin (vitamin B1) [9–11] …”

“…[7,8] Later studies from our laboratory extended the range of BIs produced in this way, and characterized by NMR and X-ray crystallography, to unsaturated, aromatic NHCs, including thiazolin-2-ylidenes, the catalytic principle of Nature's Umpolung catalyst thiamin (vitamin B1). [9][10][11] As the initial step of BI formation, it is generally assumed that nucleophilic attack of the NHC on the aldehyde results, in equilibrium, in the formation of a zwitterionic primary adduct (PA; Scheme 1, bottom). [4,12]] For the completion of the BI formation, a formal 1,2-C-to-O H-shift must follow.…”