Emamian19 has reported that the bulky bromine atom on acarbon
of cyclobutenone destabilizes the exo transition state making
the endo stereo-isomers predominate at the transition state.
However, these calculations show that none of the halogenated
endo/exo activation energy difference exceeds the endo/exo energy
difference of the parent which suggests that the dominance of endo
isomers over the exo isomers does not come about as a result of
atomic size at the reaction center of cyclobutenone (5b). It is of
interest to note that unlike in the cyclobutene where the halogenated
adduct energies increase compared to those of the parent
products, here the energies of the halogenated adducts of compound
5b rather decrease appreciably for both endo and exo stereoisomers.
The decrease in products energies explains the readiness
of the halogenated products to undergo trans alkylation reactions
with other reagents such as alkyl lithium or methyl iodide as
observed in experiments.