Mechanistic Studies of Catalytic Carbon–Carbon Cross-Coupling by Well-Defined Iron NHC Complexes

The
mechanism of iron-catalyzed carbon–carbon cross-coupling
reactions between Grignard reagents and alkyl halides has been investigated
using well-defined N-heterocyclic carbene (NHC) compounds. The iron­(II)
precatalyst, [Fe2Cl2(μ-Cl)2(IPr)2], was employed in several C–C cross coupling
reactions exhibiting the ability to efficiently couple primary and
secondary alkyl halides with several aryl and alkyl Grignard reagents.
For selected substrates, a 2 mol % catalyst loading (4 mol % Fe) afforded
conversions of >99% and were achieved with <8% homocoupling
of
the electrophile. The mechanism of the coupling reaction was studied
by means of radical clock, radical trap, and single-turnover experiments,
which support a radical-based cycle involving an Fe­(II/III) redox
couple. The implications of this mechanism on the efficacy of iron-NHC-catalyzed
cross-coupling reactions are discussed.