Neue Ansätze für die Synthese von Metallcarbenen

Abstract

Metallcarbene sind vielseitige Verbindungen, deren Reaktivität sowohl durch das Metall als auch den Liganden gesteuert werden kann. Zur Synthese von Metallcarbenen nutzt man im Allgemeinen die Stickstoff‐Eliminierung von Diazoverbindungen, die allerdings häufig instabil, explosiv und toxisch sind, was ihren Einsatz in großskaligen Synthesen einschränkt. Demzufolge gibt es eine intensive Suche nach nachhaltigen und ungefährlichen Ersatzsubstraten für die zuverlässige Herstellung von Metallcarbenen. In diesem Aufsatz fassen wir die wichtigsten Fortschritte bei der Erzeugung, katalytischen Reaktionen und der Selektivitätskontrolle von Metallcarbenen zusammen, die nicht aus Diazoverbindungen gebildet wurden.

The Carbocation-Catalyzed Intermolecular Formal [2 + 2 + 1] Cycloaddition of Ynamides with Quinoxaline N-Oxides

“…Gold-catalyzed oxidative cyclizations have attracted enormous attention as a powerful and flexible tool for the construction of heterocyclic frameworks. , Therein, the facile alkyne oxidation by N -oxides to access α-oxo gold carbenes, avoiding hazardous diazo carbonyl compounds as carbene precursors, significantly expands the substrate scope and the synthetic potential of gold catalysis. , However, the published cyclization processes mostly involve intramolecular trapping of α-oxo gold carbenes by tethered nucleophiles . Currently, intermolecular cyclization patterns for α-oxo gold carbenes are still limited to the reaction with external N -, O -nucleophiles (i.e., nitriles and amides), leading to oxazole derivatives (Scheme A).…”

Palladium‐Catalyzed Oxygenative Cross‐Coupling of Ynamides and Benzyl Bromides by Carbene Migratory Insertion

“…In addition to diazo compounds,t he unsaturated compounds that can be utilized as metal carbene precursors in transition-metalcatalyzed reactions have attracted significant attention in recent years. [3] Thus,i np rinciple these unsaturated compounds can also be used as reaction partners in carbene-based cross-coupling reactions.I nt his context, we have previously demonstrated the palladium-catalyzed cross-coupling of enynones and arylhalides.The enynones readily undergo cyclization with aryl palladium species to generate palladium carbenes,w hich further undergo migratory insertion (Scheme 1b). [4,5] In another example,wedemonstrated that allenyl ketones react similarly with organoboronic acids in the presence of palladium catalysts.…”

“…Our initial investigations were focused on the reaction of the ynamide 1a and 4-methylbenzyl bromide (2b)w ith the treatment of pyridine N-oxide (3a)a nd K 2 CO 3 in toluene (Table 1). Pd(OAc) 2 and P(2-furyl) 3 were utilized as the catalyst and ligand, respectively. [16] Ther eaction was carried out at 90 8 8Cf or 4hours,a nd the target product 4a was obtained in 38 %yield (entry 1).…”