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.