Intermediates and mechanism in iron-catalyzed C-H methylation with trimethylaluminum

A mechanistic study is performed on the reaction method for iron-catalyzed C-H methylation with AlMe 3 reagent, previously proposed to involve cyclometalated iron( iii ) intermediates and an iron( iii )/( i ) reaction cycle. Detailed spectroscopic studies ( 57 Fe Mössbauer, EPR) during catalysis and in stoichiometric reactions identify iron( ii ) complexes, including cyclometalated iron( ii ) intermediates, as the major iron species formed in situ under catalytic reaction conditions. Reaction studies identify a cyclometalated iron( ii )-methyl species as the key intermediate leading to C-H methylated product upon reaction with oxidant, consistent with a previously proposed iron( ii )/iron( iii )/iron( i ) reaction manifold for C-H arylation. Iron-catalyzed C-H methylation with organoaluminum reagents accesses reactive cyclometalated iron( ii ) intermediates.