Flyover Compounds and Bridging Bent Benzene Derivatives as Intermediates in the Cobalt Carbonyl Cyclotrimerization of Alkynes

The so-called flyover complexes (η^3,1,η^3,1-μ-R₃R′₃C₆)­Co₂(CO)₄ are important because of their role as intermediates in the cobalt carbonyl-catalyzed cyclotrimerization of alkynes. A density functional study of such flyover complexes has led to the discovery of isomers with bent benzene rings bridging a pair of cobalt atoms. Such complexes are likely to be involved in the process of forming a carbon–carbon bond in closing the six-carbon chain in the flyover complexes to give the corresponding arene derivatives. The relative energies of the flyover complexes and the bridging bent benzene derivatives depend on the substituents on the six-carbon chain. Thus, for [C₆(CF₃)₆]­Co₂(CO)₄ and [1,3,6-<i>t</i>Bu₃C₆H₃]­Co₂(CO)₄ with bulky CF₃ and <i>tert</i>-butyl substituents at the ends of the six-carbon chain, the experimentally known flyover complexes are preferred energetically over the isomeric bridging bent benzene ring structure by 23.3 and 1.1 kcal/mol, respectively. However, for the less sterically hindered (C₆R₆)­Co₂(CO)₄ (R = H, CH₃) and 1,3,6-(CF₃)₃C₆H₃]­Co₂(CO)₄ derivatives the bridging bent benzene ring structures are preferred energetically over the flyover structures by 3.6 to 14.3 kcal/mol, respectively