Characterization of the RhI /dppe-Catalyzed Ring Expansion of 2-Alkyl-Benzocyclobutenones by DFT Calculations.

Abstract

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 RhIII -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-CH3 and 2-CD3 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.