Mechanistic Investigation of Iron-Catalyzed Coupling Reactions

The mechanism of the iron-catalyzed cross-coupling of aryl electrophiles with alkyl Grignard reagents is studied by a combination of GC monitoring, Hammett competition experiments, and DFT calculations. The reaction follows a pathway where an Fe-I complex, formed in situ, reacts in a rate-limiting oxidative addition with the aryl electrophile. A rapid thermoneutral transmetalation from a Grignard reagent occurs either before or after the oxidative addition, with little to differentiate between the two pathways A reductive elimination of the resulting alkyl aryl Fe-II complex closes the catalytic cycle. Iron in lower oxidation states can act as a competent precatalyst by oxidation into the Fe-I-Fe-III cycle, Fe-I complexes can give Fe-I catalysts through reductive elimination of a bimetallic complex. Added ligands, dilution and powerful aryl electrophiles all serve to increase the stability of the active catalyst presumably by counteracting oligomerization of low valent iron.