Low Temperature Studies of Iron-Catalyzed Cross-Coupling of Alkyl Grignard Reagents with Aryl Electrophiles

The title reaction has been studied under low temperature conditions. Coupling with active substrates can be done even at dry ice temperature. Initial rate studies at -25 degrees C indicate that high concentrations of any reagent can lead to either complete or partial catalyst deactivation. Under strongly reducing conditions, iron seems to form less active complexes that only slowly re-enter the catalytic cycle, possibly through bimolecular coupling of iron(II) complexes. Computational studies support the experimental observations, and indicate that oxidation states below +I cannot be reached by reductive elimination.