N‐Ferrocenyl‐Substituted Planar‐Chiral N‐Heterocyclic Carbenes and Their Pd^II Complexes

Abstract

The N-ferrocenyl-linked N-heterocyclic carbenes 1a and 1b were obtained by treatment of their imidazolium salts 12a and 12b with potassium tert-butoxide. The latter were shown to be accessible from (R)-1-amino-2-methylferrocene (9) and aminoferrocene, respectively, which were converted into the corresponding formamidines and then subjected to a novel cyclization procedure. Treating the ligand precursors 12a and 12b with [Pd(OAc) 2 ] 3 under different reaction conditions afforded the trans-pyridine-substituted Pd II complexes 14a and 14b as well as their trans-triphenylphosphane-substituted

Palladium Complexes of N‐Heterocyclic Carbenes as Catalysts for Cross‐Coupling Reactions—A Synthetic Chemist's Perspective

“…[211] Independently of our research group, Togni and co-workers prepared (NHC)PdI 2 -pyridine complexes with chiral, N-ferrocenyl-substituted NHCs. [212] Even though good yields of 309 were obtained (70 %), the ee value was only 9 %. Similarly, Douthwaite and co-workers observed 90 % yield, but only 11 % ee of 309 when they used complex 76 (Scheme 22).…”

Aus der Sicht des Synthetikers: Palladiumkomplexe N‐heterocyclischer Carbene als Katalysatoren für Kreuzkupplungen

“…[211] Unabhängig von uns stellten Togni und Mitarbeiter (NHC)PdI 2 -Pyridinkomplexe mit chiralen NFerrocenyl-NHCs her. [212] Das Produkt 309 wurde mit diesen Katalysatoren in guter Ausbeute erhalten (70 %), der ee-Wert betrug jedoch nur 9 %. In einem ähnlichen Beispiel beobachteten Douthwaite et al mit Komplex 76 für 309 90 % Ausbeute bei nur 11 % ee (Schema 22).…”

A stereoselective access to a ferrocene-based planar chiral triazolium salt

“…Following the pioneering work of Bildstein, [7] the Togni group presented the first synthesis of imidazolium salts directly linked to planar chiral ferrocenes (2). [8] Their applications in catalysis afforded modest enantioselectivities, because of the free rotation around the C-N bond between the ferrocene substituent and the heterocycle. A logical and practical solution to solve this issue would be to prevent the free rotation by blocking the structure conformation.…”