The
antipodal introduction of two bromine atoms on the 2,12 β-pyrrolic
position of 5,10,15,20-tetra(3,5-di-<i>tert</i>-butylphenyl)porphyrin
was successfully achieved by a light-induced reaction of the substrate
with excess NBS. Complexation with Ni<sup>II</sup> of the major regioisomer
led to good quality crystals, suitable for X-ray structure determination
with unprecedented probability levels. The regiospecific character
of the synthetic procedure and the exactness of the bromine atom position
assignment were thus confirmed, suggesting an unexpected electrophilic
aromatic substitution pathway rather than a free-radical halogenation
process. A QTAIM topological analysis on the DFT-optimized wave function
of the monosubstituted free-base porphyrin intermediate carrying a
bromine atom in C2 β-pyrrolic position confirmed the largest
negative charge for the C12 carbon atom in antipodal position, in
agreement with the proposed electrophilic aromatic substitution mechanism