Abstract
Metal-catalyzed alkyne [2+2+2] cycloadditions provide a variety of substantial aromatic compounds of interest in the chemical and pharmaceutical industries. Herein, the mechanistic aspects of the acetylene [2+2+2] cycloaddition mediated by bimetallic half-sandwich catalysts [Cr(CO)3IndRh] (Ind = (C9H7)−, indenyl anion) are investigated. A detailed exploration of the potential energy surfaces (PESs) was carried out to identify the intermediates and transition states, using a relativistic density functional theory (DFT) approach. For comparison, monometallic parent systems, i.e., CpRh (Cp = (C5H5)−, cyclopentadienyl anion) and IndRh, were included in the analysis. The active center is the rhodium nucleus, where the [2+2+2] cycloaddition occurs. The coordination of the Cr(CO)3 group, which may be in syn or anti conformation, affects the energetics of the catalytic cycle as well as the mechanism. The reaction and activation energies and the turnover frequency (TOF) of the catalytic cycles are rationalized, and, in agreement with the experimental findings, our computational analysis reveals that the presence of the second metal favors the catalysis.
Highlights
The [2+2+2] cycloadditions of small unsaturated molecules, such as alkynes and nitriles, afford a variety of aromatic, heterocyclic and polycyclic compounds of paramount importance in the chemical and pharmaceutical industries [1]
The reaction and activation energies and the turnover frequency (TOF) of the catalytic cycles are rationalized, and, in agreement with the experimental findings, our computational analysis reveals that the presence of the second metal favors the catalysis
The computational mechanistic investigation of acetylene [2+2+2] cycloaddition to benzene catalyzed by the monometallic catalysts CpRh and IndRh was first reported in 2006 [41]
Summary
The [2+2+2] cycloadditions of small unsaturated molecules, such as alkynes and nitriles, afford a variety of aromatic, heterocyclic and polycyclic compounds of paramount importance in the chemical and pharmaceutical industries [1]. In 1867, the synthesis of benzene by thermal cyclotrimerization of acetylene was reported for the first time [2]. Further studies assessed that a wide variety of metals such as Ti, Zr, Ru, Co, Rh, Ir, Ni, and Pd may play an important role as catalysts in the synthesis of benzene and its derivatives and, in the past decades, experimental results and theoretical insight have been reported to establish the correct mechanism and tune the efficiency and regioselectivity [3,4,5,6,7].
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