First Evidence of the Oxidative Addition of Fe0(N,N)2 to Aryl Halides: This Precondition Is Not a Guarantee of Efficient Iron-Catalysed C-N Cross-Coupling Reactions
Résumé
It has been shown by cyclic voltammetry (CV) that the electrochemical reduction of [Fe(acac)3] in the presence of 1,10-phenanthroline (phen; 2 equiv.) or FeCl3 in the presence of N,N -dimethylethylenediamine (dmeda; 2 equiv.) in DMF leads to [Fe0(phen)2] and [Fe0(dmeda)2], respectively. They undergo unprecedented oxidative addition to aryl halides to generate [ArFeIIX(phen)2] (X = I, Br) and [ArFeIIX(dmeda)2] (X = Br), characterized by CV, ESI-MS and DFT. The order of reactivity in the oxidative addition was deduced by CV and supported by DFT: [Fe0(phen)2] [Fe0-(dmeda)2]. [ArFeIIX(phen)2] and [ArFeIIX(dmeda)2] are nucleophilic reagents (reaction with H+ and CO2) and do not react with the investigated N-nucleophiles (imidazole or pyrazole) often tested as reagents in catalytic C-N cross-coupling reactions, even in the presence of a base. Moreover, it has been established that {[Fe(acac)3] + 2 phen} and {FeCl3 + 2 dmeda} cannot be reduced in situ to Fe0(N,N)2 precondition required to activate ArX) by N-nucleophiles (pyrazole, imidazole) even in the presence of a base. All this explains why {[Fe(acac)3] + 2 phen} and {FeCl3 + 2 dmeda} are inefficient precatalysts for C-N cross-couplings reactions, as recently reported in the literature.