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

The so-called flyover complexes (η3,1,η3,1-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-tBu3C6H3]­Co2(CO)4 with bulky CF3 and tert-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.