Evidence for the powerful catalytic ability of imidozirconocene complex from its epoxide ring cleavage reactions – A DFT mechanistic view#

Abstract

Imidozirconocene complex is known for its bifunctional reactivity and catalytic ability and this complex mediates ring cleavage of epoxides. Cyclooctene oxide (1) Norbornene oxide (2) and 2,5-dimethyl cyclohexene oxide (3) undergo ring cleavage in the presence of imidozirconocene complex. Epoxide 1 has accessible β-hydrogens (type I) while epoxide 2 and 3 do not have them (type II). Normally type I epoxides undergo elimination while type II epoxides prefer insertion. All the insertion reactions lead to five-membered metallacycle formation and elimination results in thermodynamically stable allyl-alkoxy product. The insertion is a two-step process following either diradical or zwitterionic pathway, while elimination is a one-step concerted reaction. DFT (density functional theory) modelling of these reactions at B3LYP/LANL2DZ level show that epoxide 1 undergoes elimination in agreement with experiment. However, calculations indicate that epoxide (2) proceeds through diradical intermediate in contrast to experimental observations. Surprisingly, epoxide (3) that has both the β positions blocked by methyl groups undergoes elimination rather than insertion. AIM and EDA analyses offer further insights on the reaction mechanism and bifunctional reactivity of imidozirconozene complex. DFT calculations provide clear evidence for the bifunctional reactivity of imidozirconocene through bonding changes in the ring cleavage of epoxides Cyclooctene epoxide undergoes elimination and norbornene oxide follows insertion. Surprisingly, cyclohexene oxide which has both β positions blocked by methyl groups undergoes elimination instead of insertion due to carbocation stability.