Experimental and computational studies of the mechanism of iron-catalysed C-H activation/functionalisation with allyl electrophiles.
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Synthetic methods that utilise iron to facilitate C-H bond activation to yield new C-C and C-heteroatom bonds continue to attract significant interest. However, the development of these systems is still hampered by a limited molecular-level understanding of the key iron intermediates and reaction pathways that enable selective product formation. While recent studies have established the mechanism for iron-catalysed C-H arylation from aryl-nucleophiles, the underlying mechanistic pathway of iron-catalysed C-H activation/functionalisation systems which utilise electrophiles to establish C-C and C-heteroatom bonds has not been determined. The present study focuses on an iron-catalysed C-H allylation system, which utilises allyl chlorides as electrophiles to establish a C-allyl bond. Freeze-trapped inorganic spectroscopic methods (57Fe Mssbauer, EPR, and MCD) are combined with correlated reaction studies and kinetic analyses to reveal a unique and rapid reaction pathway by which the allyl electrophile reacts with a C-H activated iron intermediate. Supporting computational analysis defines this novel reaction coordinate as an inner-sphere radical process which features a partial iron-bisphosphine dissociation. Highlighting the role of the bisphosphine in this reaction pathway, a complementary study performed on the reaction of allyl electrophile with an analogous C-H activated intermediate bearing a more rigid bisphosphine ligand exhibits stifled yield and selectivity towards allylated product. An additional spectroscopic analysis of an iron-catalysed C-H amination system, which incorporates N-chloromorpholine as the C-N bond-forming electrophile, reveals a rapid reaction of electrophile with an analogous C-H activated iron intermediate consistent with the inner-sphere radical process defined for the C-H allylation system, demonstrating the prevalence of this novel reaction coordinate in this sub-class of iron-catalysed C-H functionalisation systems. Overall, these results provide a critical mechanistic foundation for the rational design and development of improved systems that are efficient, selective, and useful across a broad range of C-H functionalisations.
