Extended porphyrinoid chromophores: heteroporphyrins fused to phenanthrene and acenaphthylene

Serendipitous Discovery of Conjugated Dipyrromethene-Linked Nitronaphthoporphyrins

Acid-catalyzed condensation of a nitronaphthalene-fused dipyrrylmethane with dipyrrylmethane dialdehydes afforded unique dipyrromethene-naphthoporphyrin conjugates together with expected nitronaphthoporphyrins. The unusual conjugated system retained aromatic porphyrin-type characteristics but afforded highly modified UV-vis spectra with multiple absorptions throughout the visible region.

Organometallic Chemistry within the Structured Environment Provided by the Macrocyclic Cores of Carbaporphyrins and Related Systems

The unique environment within the core of carbaporphyrinoid systems provides a platform to explore unusual organometallic chemistry. The ability of these structures to form stable organometallic derivatives was first demonstrated for N-confused porphyrins but many other carbaporphyrin-type systems were subsequently shown to exhibit similar or complementary properties. Metalation commonly occurs with catalytically active transition metal cations and the resulting derivatives exhibit widely different physical, chemical and spectroscopic properties and range from strongly aromatic to nonaromatic and antiaromatic species. Metalation may trigger unusual, highly selective, oxidation reactions. Alkyl group migration has been observed within the cavity of metalated carbaporphyrins, and in some cases ring contraction of the carbocyclic subunit takes place. Over the past thirty years, studies in this area have led to multiple synthetic routes to carbaporphyrinoid ligands and remarkable organometallic chemistry has been reported. An overview of this important area is presented.

Synthesis of Chrysoporphyrins and a Related Benzopyrene-Fused System

Reaction of 6-nitrochrysene with ethyl isocyanoacetate in the presence of a non-nucleophilic base gave a c-annulated pyrrole ethyl ester that was used to prepare chrysene-fused tripyrranes and a chrysopyrrole dialdehyde. Chrysene-fused tripyrranes were reacted with a pyrrole dialdehyde, but poor yields of chrysoporphyrins were obtained. However, condensation of the chrysopyrrole dialdehyde with a series of tripyrranes afforded excellent yields of chrysoporphyrins and an acenaphtho-chrysoporphyrin. Iron(III) chloride mediated oxidative cyclization of a dihexylchrysoporphyrin afforded a benzopyrene-fused porphyrin that exhibited a strongly red-shifted electronic absorption spectrum. DFT calculations show that both chrysoporphyrins and the benzopyrene-fused porphyrin have tautomers that possess 34π electron delocalization pathways that pass through the porphyrin nucleus and the fused polycyclic aromatic hydrocarbon (PAH) units. Protonation gave dications that favor 36-atom 34π electron circuits. c-Annulated pyrrole dialdehydes were also condensed with a carbatripyrrin to generate PAH-fused carbaporphyrins that retained fully aromatic properties.

Synthesis of a Series of Tropone-Fused Porphyrinoids

A series of tropone-fused porphyrinoids with unique spectroscopic features has been prepared. A dimethyl tropone-fused pyrrole was reacted with lead tetraacetate to give a bis(acetoxymethyl) derivative that condensed with an α-unsubstituted pyrrole tert-butyl ester to form a tripyrrane intermediate. Cleavage of the tert-butyl ester protective groups, followed by condensation with a series of aromatic dialdehydes and oxidation with DDQ, afforded the tropone-fused porphyrinoid systems. Reactions with pyrrole, furan, thiophene, and selenophene dialdehydes gave tropone-fused porphyrins and related heteroporphyrins. In addition, indene, 4-hydroxybenzene, and 3-hydroxypyridine dicarbaldehydes generated examples of carba-, oxybenzi-, and oxypyriporphyrins. The electronic absorption spectra of the tropone-fused porphyrinoids were greatly altered, showing shifts to longer wavelengths and the appearance of strong Q bands between 600 and 800 nm. The proton nuclear magnetic resonance spectra were also very unusual, as the internal protons were strongly shifted upfield, in some cases giving rise to resonances that approached -10 ppm. However, the external protons showed reduced downfield shifts compared to porphyrinoids that do not have tropone ring fusion. The profound changes observed for these macrocycles demonstrate that the introduction of fused tropone units, together with other structural changes such as core modification, can provide the means by which porphyrinoids with unique spectroscopic properties can be accessed.