Site-Selectivityof C(sp²)–H OxidativeAddition 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-<i>N</i>,<i>N</i>,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.