When exploring the electronic effect of the BCB fragment, we were eager to discover whether a relationship between the aptitude for cycloaddition and the compound oxidation potential could be established (Table ). Firstly, it was discovered that BCB substrates which do not bear an electron-withdrawing group possess a considerably lower oxidation potential and yield only trace product under the developed cycloaddition conditions (5aj). However, ester and amide containing BCB compounds bearing aryl substitution that did not greatly perturb the substrate oxidation potential, could effectively deliver the desired BCH products (5ak-am). Considerably decreasing the electron density of the aromatic system, through the addition of a trifluoromethyl group, resulted in a BCB compound with an oxidation potential of +2.00 V which failed to deliver the desired cycloadduct and thus represents the upper limit of BCB oxidation by [Mes2Acr t Bu2]ClO4 under these conditions (5an).