Site-Selectivity of C(sp2)–H Oxidative Addition of Fluorinated Arenes with Pyridine(dicarbene) Cobalt(I) Complexes and Aryl Isomerization

The rate, site-selectivity, and product isomerization of the C(sp2)–H oxidative addition of arenes with pyridine(dicarbene) cobalt methyl and phenyl complexes have been investigated with four representative arenes of varying electronic and steric properties. The rates of C(sp2)–H activation to yield cobalt-aryl products and subsequent aryl isomerization were influenced by the electronic properties of the arene; the relatively electron-poor arene 3-fluorobenzotrifluoride underwent C(sp2)–H activation and isomerization of the cobalt-aryl more than 70 times faster than the more electron-rich substrate, 3-fluoro-N,N,α-trimethylbenzeneacetamide. In all cases, meta-to-fluorine C(sp2)–H oxidative addition was the major product at low conversion, which subsequently isomerized to the ortho isomer over time. Deuterium-labeling experiments and measurement of methane isotopologues establish that the major cobalt-aryl product at early conversion arises from kinetically preferred, meta-selective oxidative addition. Density functional theory calculations support pathways involving cobalt(I)–(III) redox cycles with oxidative addition to cobalt(I) occurring with a relatively high barrier followed by faster reductive elimination. Despite the strong σ-donating properties of the pyridine(dicarbene) pincer ligand, the π-accepting character of the carbene donors lowers the barrier for reductive elimination, and hence, cobalt(III) intermediates have not been observed.