DFT conformational studies of chiral bis-binaphthyl porphyrins and their metal complexes employed as cyclopropanation catalysts

Three C2-symmetrical chiral porphyrins, derived from α,α,β,β-tetrakis(2-aminophenyl)porphyrin and holding two binaphthyl handles through dimethylene, methylene, or direct connections between the binaphthyl moieties and the amidophenyl pickets, have been submitted to a theoretical study to investigate their conformational properties. The porphyrin with no methylene spacer was shown to be a very rigid molecule, whereas its higher homologues showed a certain degree of conformational freedom resulting in two molecular arrangements that give significant contributions to the equilibrium population of each compound. The effect of complexation with zinc(II) was studied through the modeling of the corresponding complexes as well as the role of two N-methylimidazoles (NMI) coordinating the apical positions of zinc in these complexes. The computational data showed that only one of the two molecular architectures accessible for the free ligands can easily accommodate NMI, indicating that the presence of an additional group on the apical coordination positions selects the geometry most suitable to host this group. Finally, the supposed intermediate radical active species in the cyclopropanation catalyzed by a cobalt(II) porphyrin complex, in which the central metal ion coordinates one NMI and the CHCOOEt carbene, were modeled together with the transition states leading to them. It was shown that the cavity originated by the binaphthyl moiety surmounting the porphyrin is not large enough to host the carbene ethyl group, suggesting the opportunity to increase the size of the cavity in order to confer it hosting capability toward the alkyl group. With the ethyl group outside the cavity, the relatively high mobility of CHCOOEt allows it to assume conformations exposing both the carbene Re and Si faces to the approaching alkene, thus leading to a poorly selective process.