Stereoelectronic effects on type I 1,2-dyotropic rearrangements in vicinal dibromides

The effects of different substituents on type I-dyotropic rearrangements of open-chain and cyclic 1,2-dibromo hydrocarbons have been studied by means of DFT calculations. The activation energy (Ea) of this transformation decreases with the pi-donor ability of the substituent attached to the reacting ethylenic system. This is due to donation of electronic density by conjugation or hyperconjugation. This donation leads to longer C-C and C-Br bond lengths in the corresponding four-membered transition states (TSs). Linear relationships between the E. and either the up Hammett substituent constants and the C-C bond length of the TSs were also found. In all cases, the processes have a high value of synchronicity, which is mainly independent on the substituents. A model based on the second-order perturbational analysis for one ethylene unit with two apical bromine radicals accounts for all the computed results.