Selective C–H activation has emerged as a powerful method for both the conversion of simple hydrocarbons to higher value molecules and the late-stage functionalization of complex drug-like molecules; however, catalyst control over selectivity has only been realized in a few aliphatic C–H bond functionalization reactions. In particular, chlorine radicals have been established as potent hydrogen-atom abstraction agents in free-radical chlorination reactions and, more recently, in photoredox catalysis, but the high reactivity of these intermediates limits their selectivity. We demonstrate that confining photoeliminated chlorine radicals within the steric environment defined by the secondary coordination sphere of a pyridinediimine iron(iii) chloride complex enforces steric control over their reactivity. Specifically, the iron(iii) complex exhibits activity for the C–H chlorination of alkanes with complete selectivity for primary over tertiary C–H bonds, overriding thermodynamic preference for weaker C–H bonds. Characterization of the iron(iii) complex by single-crystal X-ray diffraction and time-resolved transient absorption spectroscopy experiments suggest that this site-selective reactivity arises from a relatively long-lived charge-transfer complex between the photoeliminated chlorine atom and aromatic groups in the secondary coordination sphere of the iron(iii) complex.

Taming the chlorine radical: Controlling the reactivity of photogenerated chlorine-atom complexes for selective C–H activation

2019 Fall National Meeting

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