Aromaticity-Driven Rupture of CN Triple and CC Double Bonds: Mechanism of the Reaction between Cp2Ti(C4H4) and RCN

Abstract

The mechanism of the unique reaction of Cp2Ti(C4H4) with MeCN yielding benzene and pyridine is unraveled using the DFT method. This reaction is characterized by the cleavage of two CC double bonds and one CN triple bond. Two separate mechanisms have been determined for these bond cleavages, wherein the first one involves two and the second one involves three MeCN molecules. In both these mechanisms, aromaticity-driven steps are identified on the basis of delocalized electronic structures, energetic stabilization, and significant NICS values of the intermediates. The aromaticity is a dominant factor for the high exothermicity of 65.8 kcal/mol found in the second mechanism. The third MeCN was very important for the CN triple bond breaking stage of the second mechanism, as it stabilized the titanium nitride intermediate by forming an aromatic TiNCN titanacycle. The π orbitals of the intermediate 4π-electron TiCCC and TiNCN titanacycles are found to be stabilized by participation of a vacant Ti “d” orbital in the 4π-electron conjugation, which is an important factor of aromaticity in metalloaromatic systems. Moreover, the agostic bonding interactions of the type recently identified in ruthenacyclobutanes have also been found in the intermediate 4π-electron titanacycles. Similar bonding interactions also occur in the 4π-electron tungstenacyclobutadiene of the Schrock complex To our knowledge, this reaction is the first theoretical study of an aromaticity-driven organo-transition metal reaction.