Iron(II) Active Species in Iron鈥揃isphosphine Catalyzed Kumada and Suzuki鈥揗iyaura Cross-Couplings of Phenyl Nucleophiles and Secondary Alkyl Halides
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- 作者：Stephanie L. Daifuku ; Jared L. Kneebone ; Benjamin E. R. Snyder ; Michael L. Neidig
- 刊名：Journal of the American Chemical Society
- 出版年：2015
- 出版时间：September 9, 2015
- 年：2015
- 卷：137
- 期：35
- 页码：11432-11444
- 全文大小：716K
- ISSN：1520-5126
文摘
While previous studies have identified FeMes2(SciOPP) as the active catalyst species in iron鈥揝ciOPP catalyzed Kumada cross-coupling of mesitylmagnesium bromide and primary alkyl halides, the active catalyst species in cross-couplings with phenyl nucleophiles, where low valent iron species might be prevalent due to accessible reductive elimination pathways, remains undefined. In the present study, in situ M枚ssbauer and magnetic circular dichroism spectroscopic studies combined with inorganic syntheses and reaction studies are employed to evaluate the in situ formed iron species and identify the active catalytic species in iron鈥揝ciOPP catalyzed Suzuki鈥揗iyaura and Kumada cross-couplings of phenyl nucleophiles and secondary alkyl halides. While reductive elimination to form Fe(畏6-biphenyl)(SciOPP) occurs upon reaction of FeCl2(SciOPP) with phenyl nucleophiles, this iron(0) species is not found to be kinetically competent for catalysis. Importantly, mono- and bis-phenylated iron(II)鈥揝ciOPP species that form prior to reductive elimination are identified, where both species are found to be reactive toward electrophile at catalytically relevant rates. The higher selectivity toward the formation of cross-coupled product observed for the monophenylated species combined with the undertransmetalated nature of the in situ iron species in both Kumada and Suzuki鈥揗iyaura reactions indicates that Fe(Ph)X(SciOPP) (X = Br, Cl) is the predominant reactive species in cross-coupling. Overall, these studies demonstrate that low-valent iron is not required for the generation of highly reactive species for effective aryl-alkyl cross-couplings.