Ring-Walking of Zerovalent Nickel on Aryl Halides.

While ring-walking is a critical step in transition metal catalyzed cross-coupling reactions, the associated metastable intermediates are often difficult to isolate and characterize. In this work, theoretical structures and energetics for ring-walking and oxidative addition of zerovalent nickel with 1-bromo-2-methylbenzene, 2-bromopyridine, 2-bromo-3-methyl-thiophene, and 2-bromopyrrole were computed at the B3LYP-D3/TZ2P-LANL2TZ(f)-LANL08d level. The mechanisms vary qualitatively with substrate ring size and type-the catalyst weaves along the edges of the benzene and pyridine rings, cuts through the interior of the thiophene ring, and arcs along the bond opposite the nitrogen atom in the pyrrole ring. Analogous computations on the ring-walking and oxidative addition of zerovalent palladium with 1-bromo-2-methylbenzene reveal an energetic profile similar to that of Ni but with much weaker overall binding to the arene. In all cases, dispersion corrections are found to be very important for computing accurate metal-substrate binding energies.