Imidozirconocene‐Mediated Ring Cleavage of Epoxides – Evidence for Bifunctional Reactivity from DFT

Abstract

AbstractThe ring cleavage of epoxides mediated by bis(cyclopentadienyl)(tert‐butylimido)zirconium (Cp2Zr=N–tBu)(THF) has been investigated using DFT calculations with a view to understand the mechanism of epoxide ring cleavage and the role of the imidozirconocene complex. Two types of epoxides have been chosen; some with and some without accessible β‐hydrogen atoms. Epoxides without accessible β‐hydrogen atoms undergo only insertion, whereas those with undergo both insertion and elimination. Although insertion is found to follow a zwitterionic mechanism, the elimination passes through a concerted intramolecular hydrogen transfer step. The role of the imidozirconocene complex has been elucidated by EDA and NBO analysis, which show that the vacant 1a1 molecular orbital of the Lewis acidic metal center of Cp2Zr=N–tBu fragment initiates the reaction as a Lewis acidic center and the Lewis basic imido nitrogen atom of the Cp2Zr=N–tBu fragment completes the reaction. The coordinate to covalent bond conversion and the involvement of the 1a1 and 2a1 molecular orbitals of the Cp2Zr=N–tBu fragment in the reaction have been examined. The formation of a five‐membered metallacycle in the insertion and the alkoxy allyl product in the elimination predicted by calculations concur with experimental observations. Topological analysis has been carried out to further elucidate the mechanism and to reveal the bifunctional reactivity of imidozirconocene in epoxide ring opening reactions.