First evidence of the oxidative addition of Fe~0(N,N)_2 to aryl halides: This precondition is not a guarantee of efficient iron-catalysed C-N cross-coupling reactions

摘要

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 FeCl_3 in the presence of N,N′- dimethylethylenediamine (dmeda; 2 equiv.) in DMF leads to [Fe ~0(phen)_2] and [Fe~0(dmeda)_2], respectively. They undergo unprecedented oxidative addition to aryl halides to generate [ArFe~(II)X(phen)_2] (X = I, Br) and [ArFe ~(II)X(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: [Fe~0(phen)_2] < [Fe ~0(dmeda)_2]. [ArFe~(II)X(phen)_2] and [ArFe~(II)X(dmeda)_2] are nucleophilic reagents (reaction with H+ and CO_2) 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 {FeCl_3 + 2 dmeda} cannot be reduced in situ to Fe~0(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 {FeCl_3 + 2 dmeda} are inefficient precatalysts for C-N cross-couplings reactions, as recently reported in the literature.