Iron-catalyzed carbon-carbon cross-coupling reactions using alkynyl nucleophiles represent attractive approaches to incorporating sp-hybridized functionalities into organic molecules. This presentation will summarize the use of a multitechnique approach combining inorganic spectroscopic methods, inorganic synthesis, and reaction studies to interrogate iron-SciOPP catalyzed Csp-Csp3 cross-couplings reactions, providing the first detailed insight into effects of variation from sp2- to sp-hybridized nucleophiles on accessible iron speciation and reactivity. Neutral mono- and bis-alkynylated iron(II)-SciOPP species as well as a SciOPP-supported tris-alkynylated iron(II) ferrate are accessible in solution upon treatment of FeBr2(SciOPP) with 1 equiv (triisopropylsilyl)ethynylmagnesium bromide (TIPS-CC-MgBr) in 1:1 THF/2-MeTHF, indicating inherent instability of alkynylated iron(II)-SciOPP centers toward alkynyl ligand redistribution pathways. Such instabilities contrast with catalytically relevant arylated iron(II)-SciOPP species studied previously. In contrast, non-polar solvents such as toluene imparted increased stability on these transmetalated species, enabling detailed characterization and assessment of their reactivity with electrophile. While the tris-alkynylated iron(II) species was found to be unreactive with cycloheptyl bromide over the average turnover time of catalysis, in situ generated mono- and bis-alkynylated iron(II)-SciOPP species are consumed upon exposure to electrophile with generation of cross-coupled product at catalytically relevant rates. Reaction solvent and Grignard reagent addition rate were found to have broader implications in overall reaction selectivity, reaction rate, and accessibility of off-cycle iron(I)-SciOPP species. Insight gained from this study provides an essential foundation for the continued development of this important class of reactions.

Intermediates and reactivity in iron-catalyzed cross-couplings of alkynyl Grignard reagents with alkyl halides

255th National Meeting (2018)

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