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 reagent, previously proposed to involve cyclometalated iron(III) intermediates and an iron(III)/(I) reaction cycle. Detailed spectroscopic studies (Fe Mössbauer, EPR) during catalysis and in stoichiometric reactions identify iron(II) complexes, including cyclometalated iron(II) intermediates, as the major iron species formed 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.