Mechanistic studies on acetylene cyclotrimerization catalyzed by Grubbs first and second generation catalysts

Abstract

Cyclotrimerization of acetylene by Grubbs first and second generation CC bond metathesis catalysts has been attempted by experimentalists with limited success and lack of reliable information about the mechanism of the reaction hinders further progress in the development of efficient catalysts. Herein, we describe a systematic density functional theory study on the probable mechanisms of this reaction and show that it proceeds through either a fully metathesis pathway (pathway 1) or through an alternate non-metathesis pathway (pathway 2). Pathway 1 consists of four metathesis steps characterized by the formation of metallacyclobutene-like intermediate in each step while pathway 2 passes through a metallacyclopentadiene intermediate. In metathesis pathway, the final ring closing metathesis step for benzene formation competes with another cross metathesis propagation step towards polyene chain expansion. Pathways 1 and 2 show a highly exothermic ∼120kcal/mol reaction profile due to the tremendous energy gain in converting acetylene triple bonds to aromatic bonds. The reaction is highly influenced by the choice of ligands in the catalyst design, particularly, by reducing the steric influence of ligands, cyclotrimerization of acetylene to benzene appears as a feasible process by Grubbs-type catalysts.