Described is the in situ formation of triorganocerium reagents and their application in catalyst‐free Zweifel olefinations. These unique cerium species were generated through novel exchange reactions of organohalides with n‐Bu3Ce reagents. The adequate electronegativity of cerium allowed for compensating the disadvantages of both usually functional‐group‐sensitive organolithium species and less reactive organomagnesium reagents. Exchange reactions were performed on aryl and alkenyl bromides, enabling enantiospecific transformations of chiral boron pinacol esters. Finally, these new organocerium species were engaged in selective 1,2‐additions onto enolisable and sterically hindered ketones.
Wirb eschreiben die In-situ-Synthese von Triorganocer-Reagentien und deren Anwendung in katalysatorfreien Zweifel-Olefinierungen. Diese einzigartigen Cer-Spezies wurden durch neuartige Austauschreaktionen von Organohalogeniden mit n-Bu 3 Ce-Reagentien erzeugt. Durch die geeignete Elektronegativitätv on Cer konnten sowohl die Empfindlichkeit von Organolithium-Verbindungen fürfunktionelle Gruppen als auch die geringere Reaktivitätv on Organomagnesium-Verbindungen ausgeglichen werden. Austauschreaktionen wurden an Aryl-und Alkenylbromiden durchgeführt, die mit chiralen Borpinakolestern enantiospezifische Transformationen eingingen. Abschließend wurden diese neuen Organocer-Spezies eingesetzt, um selektive 1,2-Additionen an enolisierbaren und sterischg ehinderten Ketonen durchzuführen.
Zweifel olefination is a catalyst-free reaction that serves alkene functionalization. While most methods employ commercially available boron pinacol esters, we have assembled a sequence in which the two partners of the formal coupling reaction are installed successively, starting from inexpensive boron alkoxides. The in situ formation of bisorganoborinates was accomplished by consecutive reaction of two different organometallic species. This single-pot procedure represents a great advancement in the generation of organoborinates and their involvement in C−C bond formation.
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