Synthesis of aromatic dicarbaporphyrinoids from resorcinol and 2-methylresorcinol

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.

What’s in a name? The MacDonald condensation

In 1960, MacDonald and coworkers introduced a new method for porphyrin synthesis that involved the acid-catalyzed condensation of dipyrrylmethane dialdehydes with [Formula: see text],[Formula: see text]-diunsubstituted dipyrrylmethanes or the related dicarboxylic acids, followed by an air oxidation. The key bond forming steps entail electrophilic substitution at two pyrrole units with the aldehyde moieties to generate, following elimination of water, a 5,15-dihydroporphyrin or porphodimethene intermediate. Following addition of sodium acetate, or in later procedures zinc acetate, the dihydroporphyrins readily air oxidize to the fully aromatic porphyrin system. This strategy, which parallels chemistry contemporaneously developed by R. B. Woodward at Harvard for the total synthesis of chlorophyll [Formula: see text], demonstrated that dipyrrylmethanes were sufficiently stable to be utilized as intermediates in porphyrin syntheses and that porphodimethene formation circumvented acidolytic scrambling reactions that might lead to isomeric porphyrin products. This type of chemistry was later adapted by Johnson, Woodward and others to prepare porphyrin-type systems by a “3 + 1” strategy, or expanded porphyrins by “3 + 2” or other combinations of oligopyrrolic precursors. In recent years, the term “MacDonald condensation” has been increasingly used to describe other types of chemistry involving oligopyrrolic intermediates. Following on from a historical review of this area, guidelines for the identification of MacDonald-type reactions are proposed.

adj-Dicarbachlorin, the first free base carbaporphyrinoid system with an internal methylene unit

Base-catalyzed condensation of dicyclopentadienylmethane with a dipyrrylmethane dialdehyde gave a dicarbachlorin with an internal CH₂ group.

Benziporphyrins, a unique platform for exploring the aromatic characteristics of porphyrinoid systems

Benziporphyrins and related systems exhibit a wide range of properties and may possess nonaromatic, strongly aromatic or even antiaromatic characteristics.

Carbaporphyrins, porphyrin isomers and the legacy of Emanuel Vogel

Vogel discovered a large number of bridged annulene structures, including constitutional porphyrin isomers, heteroporphyrinoid species such as the tetraoxaporphyrin dication, corrole isomers and novel expanded porphyrinoid systems. In this review, the impact of Vogel's work on the author's research program is discussed. Carbaporphyrinoid systems with one or two carbocyclic rings replacing the usual pyrrole subunits have been synthesized and their diatropic characteristics can be related to the presence of 18π electron delocalization pathways within these macrocycles. The preparation of a dideazaporphyrin also provides strong support for the concept, espoused by Vogel, that porphyrins are examples of bridged annulenes. Ring contractions of azuliporphyrins to benzocarbaporphyrins with tert-butyl hydroperoxide in the presence of base have been proposed to involve a Cope rearrangement where a cycloheptatriene unit rearranges to give a norcaradiene-fused carbaporphyrinoid, followed elimination of tert-butyl alcohol, and Vogel's investigations into these types of valence tautomerization processes provides insights into these unusual transformations. In addition, a new class of porphyrin isomers has been synthesized and these porphyrin analogs extend the observations previously made for N-confused porphyrins and Vogel's constitutional isomers.

Modifying the porphyrin core — a chemist's jigsaw

Core modified porphyrinoids are porphyrin analogs which differ from these by containing at least one non-pyrrolic aromatic ring. The result of core modification is to produce systems with novel and unique physical and electronic properties. In this review, the various possible core modified porphyrins and their syntheses are summarized starting by porphyrinoids with one pyrrolic nitrogen replaced followed by porphyrin-related macrocycles with two or more pyrrole rings exchanged. In addition the spectroscopic properties, especially the UV-vis absorption profiles and proton NMR shifts emanating from their core modifications are discussed and summarized in a table.

Origin of aromatic character in porphyrinoid systems

The aromatic nature of porphyrins is commonly attributed to the presence of an [18]annulene substructure. However, this viewpoint has been disputed. Drawing on a range of examples of carbaporphyrinoid systems from the author's own studies, the [18]annulene model is shown to be a self-consistent and insightful approach for considering the aromatic properties of these porphyrin analogs. Benziporphyrins provide a continuum of porphyrinoid structures that range from nonaromatic to highly aromatic species and the presence of 18π electron delocalization pathways provides an excellent explanation for the variations in their properties. The same type of analysis is applied to carbaporphyrins, tropiporphyrins, azuliporphyrins, N-confused porphyrins, pyrazoloporphyrins and dicarbaporphyrins. Nevertheless, these properties might also be explained by a 22π electron delocalization model proposed by Schleyer. The [18]annulene model gains further support from the properties of dideazaporphyrins which cannot take part in this type of 22π electron delocalization but nevertheless retain porphyrin-like aromatic properties. These results support the concept that porphyrins are bridged [18]annulenes.