Why Does Rh Prefer [4 + 1] while Pd Prefer [4 + 4 + 1] Mode in Carbocyclization of Vinylallene with CO? Unveiling Nickel’s Potential for Expanding the Scope Beyond Pd

Rhodium(I) promotes a [4 + 1] assembly mode with a vinylallene substrate and CO, leading to the formation of a five-membered ring structure. In striking contrast, palladium(0) catalyzes a more complex [4 + 4 + 1] mode leading to a nine-membered cyclic ketone. The underlying mechanisms that govern this contrast in assembly modes remain unclear, even in today's date, presenting a major challenge to the further development of efficient reactions. Our computational study reveals that Rh adopts a square pyramidal geometry with CO. This is in contrast to Pd, which prefers a trigonal planar geometry. A stronger metal-CO interaction is detected in Rh than in a Pd intermediate, making the [4 + 1] mode more preferable in Rh than in Pd. The transition state for C-C coupling between two vinylallene has a significantly distorted tetrahedral core in the case of Rh, while Pd retains a stable trigonal planar structure, making the [4 + 4 + 1] reaction less favorable in Rh than in Pd. By conceptualizing and transferring the critical features of Pd to Ni systems, we have rationally designed a Ni-catalyzed [4 + 4 + 1] version that combines the desirable reactivity of Pd with the cost-effectiveness and abundance of Ni, aligning with the growing demand for more sustainable and atom-efficient strategies in synthetic organic chemistry.