Ab initio and DFT Study of Prototropic and Metallotropic 1,5-Shifts of Isolobal Cyclopentadienyl Derivatives

The structures, energies and sigmatropic behaviour of η¹-cyclopenta-2,4-dienylborane (1) and η¹-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).