Expansion and contraction: Shaping the porphyrin boundary via diradical reactivity

Porphyrins have remarkable utility in optical and materials applications due to their extensive pi-network and the corresponding electronic properties associated with this delocalization. Recent strategies have focused on methods to modulate these properties by chemically extending the conjugation. Our approach to periphery modification has centered on the application of diradicals generated through unique diyne-ene and diazo-keto functionalization at the beta,beta'-position of the macrocycle. In the former case, Bergman cyclization of the enediyne motif generates a new aromatic ring via a 1,4-phenyl diradical intermediate. In the absence of bimolecular quenchers in very high concentrations, the diradical adds across the adjacent phenyl rings of the meso-positions to extend the aromaticity across three rings. In the second case, dione or diazo-keto functionalization and subsequent reaction with nucleophiles in the presence of Ag+ generates acetals that lead to modified chemical properties and potentially, additional reactivity at the periphery. Diazoketochlorin photolysis on the other hand, leads to rapid N-2 extrusion and initial carbene formation. The carbene is short lived and cannot be trapped even at extremely low temperatures (2 K). The subsequent Wolff ring-contracted ketene, however, is detectable, demonstrating that the out-of-plane electronic configuration is initially generated and reacts to give the nucleophile-quenched species, as well as relaxing to form exocyclic ring addition and other radical-based products. In addition to generating unique infrared and electronic spectral properties, together these strategies offer unusual synthetic opportunities, to markedly modulate macrocycle electronic structure in a highly asymmetric manner to form truly distinct porphyrinoid constructs. (C) 2012 Elsevier B.V. All rights reserved.