¹⁸O Kinetic Isotope Effect Evidence for a Concerted [3+1] Cycloaddition in Phosphetane-Catalyzed Reductive...

Heavy-atom kinetic isotope effect (KIE) measurements are reported for the P^III/P^V=O-catalyzed reductive N-arylation of ¹⁸O-labeled nitrobenzenes with phenylboronic acid. Intermolecular competition between singly labeled ¹⁶O,¹⁸O-nitrobenzene and unlabeled ¹⁶O₂-nitrobenzene yields a KIE of k ₁₆/k ₁₈ = 1.033 ± 0.003. Similarly, competition between doubly labeled ¹⁸O₂-nitrobenzene and ¹⁶O₂-nitrobenzene affords a KIE of 1.066 ± 0.003. These results, interpreted in conjunction with density functional theory (DFT) calculations, support a concerted [3+1] cycloaddition between the nitroarene and the P^III catalyst in the substrate-committing step. The observed KIE values are consistent with synchronous bond reorganization at both oxygen atoms of the nitro group, as predicted for a cheletropic transition state. This study provides new mechanistic evidence for the thermal 1,3-dipolar cycloaddition reactivity of nitroarenes, strengthening the mechanistic foundation of organophosphorus-mediated nitroarene deoxygenation chemistry.