SEARCH

SEARCH BY CITATION

Keywords:

  • cross-coupling;
  • density functional calculations;
  • homogeneous catalysis;
  • iron;
  • reaction mechanisms

Abstract

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 FeI 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 FeIII complex closes the catalytic cycle. Iron in lower oxidation states can act as a competent precatalyst by oxidation into the FeI–FeIII cycle. FeII complexes can give FeI 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.