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

• Published in: ACS catalysis Vol. 5; no. 10; pp. 5938 - 5946
• Main Authors: Przyojski, Jacob A, Veggeberg, Kevin P, Arman, Hadi D, Tonzetich, Zachary J
• Format: Journal Article
• Language: English
• Published: American Chemical Society 02.10.2015
• Subjects: cross-coupling; iron catalysis; three-coordinate complexes; radical mechanisms; N-heterocyclic carbenes
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/acscatal.5b01445

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.