A density functional study of [2+3] versus [2+2] addition of ethylene to chromium-oxygen bonds in chromyl chloride

Density functional theory (DFT) calculations have been carried out on the [2+2] and [2+3] addition of ethylene to Cr=O bonds in CrO2Cl2. The [2+3] addition was found to be more favorable with an electronic reaction enthalpy of -9.6 kcal/mol and an activation barrier of 15.8 kcal/mol. The corresponding values for the [2+2] addition were 15.4 and 27.9 kcal/mol, respectively. It was concluded that epoxides formed in the reaction between CrO2Cl2 and olefins are unlikely to originate from a [2+2] addition path to a CrO bond as previously suggested. The two addition reactions were analyzed by the intrinsic reaction coordinate method, and comparisons were made to a previous study of [2+2] and [2+3] addition by ethylene to OsO4. Qualitative considerations were used to rationalize why the addition of ethylene to metal-oxygen bonds in tetrahedral d(0) oxo complexes seems more prone to proceed by a [2+3] mechanism.