| Abstract
| - We propose and examine a comprehensive mechanism of the [(η5-C5H5)Rh]-catalyzed [2+2+2] cycloadditions of acetylene to benzene and of acetylene and acetonitrile to 2-methylpyridine, based on an extensive and detailed exploration of the potential energy surfaces and solvent effects thereon, using density functional theory.
- We propose and examine a comprehensive mechanism of the [(η5-C5H5)Rh]-catalyzed [2+2+2]cycloadditions of acetylene to benzene and of acetylene and acetonitrile to 2-methylpyridine, based onan extensive and detailed exploration of the potential energy surfaces using density functional theory.Both processes involve the formation of a coordinatively unsaturated 16-electron metallacycle, occurringafter the replacement of the ancillary ligands L of the catalyst precursor of general formula [(η5-C5H5)RhL2] (typically L = C2H4, CO, PH3 or L2 = 1,5-cyclooctadiene) by two acetylene molecules. Thefacile coordination of a third acetylene molecule, and its subsequent addition to the π electron system ofthe rhodacycle, leads to the formation of an intermediate, which is characterized by a six-memberedarene ring coordinated to the metal in η4 fashion. The release of benzene occurs by stepwise addition oftwo acetylene molecules, which regenerates the catalyst. In the presence of acetonitrile, a nitrile moleculecoordinates to the rhodacycle, and different stages are outlined for the process, leading to the eventualrelease of 2-methylpyridine. The steric and electronic effects of the π ligand coordinated to the metal arealso included in our exploration by addressing the whole mechanism of the [(η5-C9H7)Rh]-catalyzedalkyne self-trimerization to benzene. The kinetic parameters, i.e., the energies in vacuum and in differentsolvents, and the geometries of the intermediates and of the transition states are analyzed in detail.
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