Exploring the Reactivity and Activation Barrier Origin of Group 14 = Phosphorus Nitrile Analogues in [2 + 2] Cycloaddition with Diphenylketene

The chemical reactivity of the [2 + 2] cycloaddition reaction between diphenylketene and base-supported, double-bonded nitrile-like G14 = P-Rea species was theoretically investigated using density functional theory calculations. Our findings indicate that only the heavier analogues, namely, the G14 = P-Rea species with G14 = Si, Ge, Sn, and Pb, are energetically capable of undergoing [2 + 2] cycloaddition with diphenylketene to form four-membered heterocyclic adducts. In contrast, the lightest analogue (C = P-Rea) does not favor this transformation. Moreover, these four heavier nitrile-like molecules tend to favor a reversible [2 + 2] cycloaddition pathway. Activation strain model analysis reveals that the deformation energy of diphenylketene, associated with its transformation from a linear to a bent geometry, is the dominant factor governing the reaction barrier. Consequently, heavier G14 elements, which possess larger atomic radii, facilitate better orbital overlap with the bent diphenylketene, leading to lower activation barriers and higher reaction rates. Our theoretical findings reveal that the dominant orbital interaction is identified as p-π(G14=P) → p-π*(O═C). In contrast, the back-donation interaction, although weaker, involves p-π*(G14=P) ← p-π(O═C).