Characterization of the Rh I /dppe‐Catalyzed Ring Expansion of 2‐Alkyl‐Benzocyclobutenones by DFT Calculations

The [(dppe)RhCl]‐catalysed ring expansion of 2‐methylbenzocyclobutenone has been proposed to occur by C−C oxidative addition to rhodaindanone, β‐hydride elimination, hydrorhodation and C−C reductive elimination. DFT calculations [IEFPCM(1,4‐dioxane, 383.15 K)/PBE0/DGDZVP level of theory] here confirm this mechanism. As proposed, oxidative addition into CHMe−CO bond is preferred over the alternative CHMe‐aryl insertion. The barriers of oxidative addition, β‐hydride elimination hydrorhodation, and reductive elimination are 23.6 (rate‐determining), 8.9, 10.4, and 13.1 kcal mol −1 , respectively. Therefore, the β‐hydride elimination/hydrorhodation steps to/from an octahedral Rh III ‐hydride serve as a fast equilibrating hydrogen shuffle flanking the two slower C−C bond breaking/making steps. This is consistent with the weak kinetic isotope effect observed experimentally when 2‐CH 3 and 2‐CD 3 benzocyclobutenone react competitively in a 1:1 ratio. The reaction barriers calculated with more modern, dispersion interaction‐corrected methods (SMD/M06 and IEFPCM/ωB97xD) follow identical trends.