Porphyrins with exocyclic rings. Part 3. A reassessment on the utility of cyclopenta[b]pyrroles in the synthesis of porphyrin molecular fossils. Preparation of three type II porphyrins related to deoxophylloerythroetioporphyrin (DPEP)

Tailor-made aromatic porphyrinoids with NIR absorption

The highlight of this article is the recent progress in the state-of-the-art synthetic design and isolation of artificial porphyrinoids by swapping pyrrole component(s) with diverse functionalized pyrrolic(heterocyclic)/carbacycle building block(s) to compare the impact on the electronic absorption spectra and aromaticity of the incorporated isomeric/expanded porphyrinoids. Attention has been directed towards five distinct criteria of utilizing functionalized pyrrolic(heterocyclic)/aromatic hydrocarbons as synthons for NIR absorbing aromatic isomeric (N-confusion)/expanded porphyrinoids (with five/six heterocycles): (i) fused or annelated pyrrole (heterocycle), (ii) functionalized bi-pyrrole/bi-thiophene/bi-furan building blocks, (iii) azulene based carbacycle building block, (iv) vinylogous aromatic carbacycle/heterocycle(s) building block and (v) N-confused pyrrole ring(s), and N-confused fused pyrrole ring(s) leading to π-extension. These hybrid porphyrinoids are ideal candidates for basic research into macrocyclic aromaticity and for many potential applications owing to NIR absorption.

Deciphering structure and aggregation in asphaltenes: hypothesis-driven design and development of synthetic model compounds

Asphaltenes comprise the heaviest and least understood fraction of crude petroleum. The asphaltenes are a diverse and complex mixture of organic and organometallic molecules in which most of the molecular constituents are tightly aggregated into more complicated suprastructures. The bulk properties of asphaltenes arise from a broad range of polycyclic aromatics, heteroatoms, and polar functional groups. Despite much analytical effort, the precise molecular architectures of the material remain unresolved. To understand asphaltene characteristics and reactivity, the field has turned to synthetic model compounds that mirror asphaltene structure, aggregation behavior, and thermal chemistry, including the nucleation of coke. Historically, molecular asphaltene modeling was limited to commercial compounds, offering little illumination and few opportunities for hypothesis-driven research. More recently, however, rational molecular design and modern organic synthesis have started to impact this area. This review provides an overview of commercially available model compounds but is principally focused on the design and synthesis of structurally advanced and appropriately functionalized compounds to mimic the physical and chemical behavior of asphaltenes. Efforts to model asphaltene aggregation are briefly discussed, and a prognosis for the field is offered. A referenced tabulation of the synthetic compounds reported to date is provided.

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.

Synthetic aspects of 2,2'-bisdipyrrins

The synthesis of open-chain, tetrapyrrolic 2,2'-bisdipyrrin ligands was investigated, starting from a variety of different pyrrolic and 2,2'-bipyrrolic precursors. Four important observations were made: (1) The solubility of 2,2'-bisdipyrrins can easily be tuned through the peripheral substituent pattern, allowing the aimed preparation of both well-soluble and hardly soluble tetrapyrroles. (2) meso-Arylsubstituted 2,2'-bisdipyrrins are easily available from respective p- and m-, but not o-functionalized dibenzoyl bipyrroles due to sterical effects. (3) Unsymmetric derivatives can be obtained by the stepwise acylation of 2,2'-bipyrroles and concomitant condensation reactions, using the new 5-benzoyl-3,3',4,4'-tetraethyl-2,2'-bipyrrole as the key intermediate. (4) meta-Nitrophenyl groups in the periphery of 2,2'-bisdipyrrins can be reduced to aminophenyl groups and further derivatized in analogy to a reaction cascade used in porphyrin chemistry, yielding superstructured 2,2'-bisdipyrrins. The synthetic schemes developed open the way for a large variety of tailor-made 2,2'-bisdipyrrin ligands.

Porphyrins with exocyclic rings. 16. Synthesis and spectroscopic characterization of fluoranthoporphyrins, a new class of highly conjugated porphyrin chromophores

Porphyrins with fused aromatic rings are under detailed investigation due to their unique spectroscopic properties. To gain more insights into the effects due to ring annealation on the porphyrin chromophore, a series of fluoranthoporphyrins have been synthesized. Reaction of 3-nitrofluoranthene with isocyanoacetate esters in the presence of a phosphazene base afforded good yields of the fluorantho[2,3-c]pyrrole esters 8. Cleavage of the ester moiety with KOH in ethylene glycol afforded the parent heterocycle 9, and this condensed with 2 equiv of acetoxymethylpyrroles 10 in refluxing acetic acid-2-propanol to afford tripyrranes 11. Following cleavage of the tert-butyl ester protective groups with TFA, "3 + 1" condensation with pyrrole dialdehyde 12 gave the fluoranthoporphyrins 13 in good overall yields. In addition, reaction of tripyrrane 11 with acenaphthopyrrole dialdehyde 16 gave the mixed acenaphthofluoranthoporphyrin 17 in excellent yields. A difluoranthoporphyrin 18 was also prepared via a "2 + 2" MacDonald condensation. Reaction of fluoranthopyrrole 8a with dimethoxymethane in the presence of p-toluenesulfonic acid gave the symmetrical dipyrrylmethane 19, and following ester saponification, this was condensed with a dipyrrylmethane dialdehyde to afford the adj-difluoranthoporphyrin 18. The UV--vis spectra for these fluoranthoporphyrins gave a series of three broadened absorptions in the Soret band region, although the Q-bands were little effected by ring fusion. The nickel(II), copper(II), and zinc chelates were more unusual, showing strong absorptions near 600 nm. Difluoranthoporphyrin 18 showed many of the same spectroscopic features, although the presence of two ring fusions gave rise to an increase in the spectroscopic shifts. The mixed system 17 gave spectra that showed larger red shifts due to the acenaphthylene unit combined with the features due to the fluoranthene rings. This work further demonstrates the utility of aromatic ring fusion in altering the properties of porphyrinoid systems.