The Chemistry Behind and Beyond the Molecular Engineering of 1,7-Naphthiporphyrin

Abstract

The discovery of 1,7-naphthiporphyrin was based on a specific naphthalene connectivity that disrupts naphthalene’s local aromaticity, and its global aromaticity was experimentally confirmed without dispute. Its unique conjugated pathways inspired our molecular engineering approaches─an unprecedented strategy for modifying the core structure of porphyrinoids. The first synthesis of o-styreniporphyrin was achieved in a truncated form, and further truncation yielded allyliporphyrin. Both compounds exhibited dynamic behavior in solution, including observable vinylenic bond rotation. Similarly, 1,7-naphthiporphyrin was converted into naphthioxacorrole by removing one meso-like carbon; its oxidized form featured an enedione unit that underwent selective sulfur or nitrogen nucleophilic addition, accompanied by color change. To obtain a larger macrocycle, we synthesized dimeric forms of 1,7-naphthiporphyrin, which exhibited a conjugation pathway approximating a 22π system (although a 34π system is also possible). As another possible approach, the naphthalene moiety of 1,7-naphthiporphyrin was expanded to anthracene, whose central ring acted as a diene and reacted with DMAD (a dienophile) to yield a new phlorin structure. Similarly, o-styreniporphyrin was expanded with naphthalene and anthracene; interestingly, its aromaticity decreased with increasing polycyclic aromatic hydrocarbon (PAH) size. Its aromaticity was evaluated by analyzing conformers and tautomers at equilibrium and considering their calculated energy levels. Functionalized allyliporphyrins were used to address an unresolved question in physical science: do electron-donating (ED) or electron-withdrawing (EW) groups reduce aromaticity? Allyliporphyrins─which offer more space at one meso-like position due to the partial removal of the naphthalene unit and the presence of both inner and outer CH groups─facilitated the evaluation of global aromaticity upon ED or EW substitution. Our experiments support the conclusion that both ED and EW groups reduce aromaticity. We then examined whether Clar’s sextets always contradict resonance theory. Some porphyrinoids derived from 1,7-naphthiporphyrin contain PAHs that allow the identification of two distinct Clar’s sextets─shared and independent─relative to the global conjugated pathway. To enable direct comparison, regioisomers of o-vinylnaphthiporphyrins were synthesized using our synthetic strategy. Sextet analysis of three regioisomers predicted that 3,4-vinylnaphthiporphyrin is the most aromatic (with an independent sextet), 1,2-vinylnaphthiporphyrin is next (with an independent sextet but some steric hindrance between thiophene and naphthalene), and 2,3-vinylnaphthiporphyrin is the least aromatic or nearly nonaromatic (with shared sextets). This ordering, along with other examples, verifies that an independent sextet stabilizes the global conjugation pathway, whereas shared sextets destabilize it. Overall, the modifications described above, along with their emerging properties and applications, demonstrate the usefulness of the molecular engineering approach to 1,7-naphthiporphyrin. Despite extensive studies on 1,7-naphthiporphyrin, additional modifications remain possible. Moreover, the molecular engineering strategy is broadly applicable to other porphyrinoids, provided that the associated synthetic challenges are manageable.