Ab initio and DFT study of prototropic and metallotropic 1,5-shifts of isolobal cyclopentadienyl derivatives

The structures, energies and sigmatropic behaviour of eta(1)-cyclopenta-2,4-dienylborane (1) and eta(1)-cyclopentadiene-three carbonyl cobalt (I) (2) were examined using DFT-B3LYP/6-311+G** level of theory. Both BH2 and Co(CO)(3) fragments in these compounds are isolobal. Results indicate that -BH2 and -Co(CO)(3) in sigmatropic shifts had similar behaviour. Furthermore, in compounds 1 and 2, prototropic shifts have similar mechanisms. In compounds 1 and 2, metallotrotropic shift acts faster than prototropic shift. Migration of proton and Co(CO)(3) took place through suprafacial [1,5]-sigmatropic mechanism, but -BH2 shift took place through antrafacial [1,3]-rearrangement. The barrier energies of prototropic shift in compounds 1 and 2 are 15.94 kcal mol(-1) and 33.13 kcal mol(-1) respectively. These energies are lower than those of brotropic shifts in compound 1 (0.16 kcal mol(-1)) or migration of CO(CO)(3) in compound 2 (3.76 kcal mol(-1)).