Quaterpyrroles as Building Blocks for the Synthesis of Expanded Porphyrins

The unprecedented strong paratropic ring current of a bis-PdII complex of 5,10,23-trimesityl [28]heptaphyrin(1.1.0.0.1.0.0)

Acid-catalyzed Friedel-Crafts-type cyclization of tetrapyrrolic BF2 complex 1 and α,α'-dibromotripyrrin 2 gave 5,10,23-trimesityl [28]heptaphyrin(1.1.0.0.1.0.0) BF2 complex 3BF2 as a stable and moderate antiaromatic macrocycle. Demetalation of 3BF2 with methanesulfonic acid followed by treatment with HCl gave free-base salt 3HCl that holds a chloride anion at the core. This salt displays a planar structure with an inverted pyrrole and a stronger paratropic ring current. Metalation of neutral free-base 3 with PdCl2 gave bis-PdII complex 3Pd2 as a stable antiaromatic molecule. The 1H NMR spectrum of 3Pd2 displays signals due to pyrrolic β-protons in the range of -1.06 ∼ -1.90 ppm, indicating the unprecedented strong paratropic ring current.

Conformationally rigid, π-extended annulated porphyrinoids derived from the naphthobipyrrole motif

π-Extension in porphyrinoids can be achieved by fusing additional aromatic rings onto the macrocycle's periphery and such porphyrinoids are referred to as annulated porphyrinoids. Annulated porphyrinoids display contrasting properties in comparison with their non-annulated congeners. While an annulation strategy can create π-extended systems, the simultaneous incorporation of conformational rigidity in such porphyrinoids can ensure that they adopt a planar structure, and the advantages associated with the extended π-network can be leveraged. Hence, while synthesizing such porphyrinoids, judicial selection of the precursor becomes important. The ease of synthesis and the presence of a β-β'-linked o-phenylene bridge qualify 3,8-1,10-dihydrobenzo[e]pyrrolo[3,2-g]indole, commonly known as naphthobipyrrole, to be one such precursor suitable for the synthesis of conformationally rigid annulated porphyrinoids. This field of study has started to bloom only in the last decade and the examples reported so far are confined to the naphtho-versions of porphycenes (isomeric porphyrin), a few members of the aromatic/antiaromatic expanded porphyrinoids, and calix[n]bipyrroles. In view of this, the current review article aims to summarize the up-to-date developments in this area and discusses the synthesis, structure, and properties of the reported naphthobipyrrole-derived annulated porphyrinoids.

Iterative Suzuki-Miyaura Cross-coupling/Bromo-desilylation Reaction Sequences for the Assembly of Chemically Well-defined, Acyclic Oligopyrrole/Benzenoid Hybrids Embodying Mixed Modes of Connectivity

Syntheses of a range of chemically well-defined oligopyrrole/benzenoid hybrids are described using tandem Suzuki-Miyaura cross-coupling/bromo-desilyation reaction sequences for linking borylated pyrroles, halogenated pyrroles and/or dibromobenzenes to one another. By such means, including iterative variants, a range of all α-linked, all β-linked oligopyrroles as well as certain combinations thereof have been assembled, some of them for the first time. The conductivities of iodine-treated thin films formed from certain such systems have been determined.

Extended 2,2'-Bipyrroles: New Monomers for Conjugated Polymers with Tailored Processability

The synthesis of 2,2′-bipyrroles substituted at positions 5,5′ with pyrrolyl, N-methyl-pyrrolyl and thienyl groups and their application in the preparation of conducting polymers is reported herein. The preparation of these monomers consisted of two synthetic steps from a functionalized 2,2′-bipyrrole: Bromination of the corresponding 2,2′-bipyrrole followed by Suzuki or Stille couplings. These monomers display low oxidation potential compared to pyrrole because of the extended length of their conjugation pathway. The resulting monomers can be polymerized through oxidative/electropolymerization. Electrical conductivity and electrochromic properties of the electrodeposited polymeric films were evaluated using 4-point probe measurements and cyclic voltammetry to evaluate their applicability in electronics.

Gram-Scale Synthesis of a Bench-Stable 5,5″-Unsubstituted Terpyrrole

The controlled preparation of higher order oligopyrrolic species holds broad utility across the chemical and material sciences. Here, we describe the gram-scale synthesis of a bench-stable 5,5″-unsubstituted terpyrrole in excellent yield via a tandem Suzuki cross-coupling with in situ deprotection. The solution and solid-state stability as well as UV-vis, fluorescence, and single crystal X-ray diffraction structure are also detailed.

Functionalized 2,2'-Bipyrroles: Building Blocks for Pyrrolic Macrocycles

Functionalized N-unsubstituted 2,2'-bipyrroles are basic building blocks for the preparation of pyrrolic macrocycles and natural products, such as prodigiosines. The aim of this review is to provide a description of the most important methodologies used to prepare 2,2'-bipyrroles and their central role as building blocks for the synthesis of porphyrinoids and property-defining structural elements therein.

Selective synthesis of 5-aryl-10-(nitromethyl) substituted 15-azatripyrrane, 15-oxatripyrrane and 15-thiatripyrrane: access to nitromethyl functionalized A3B-porphyrins

A chemical strategy toward the selective synthesis of unsymmetrical tripyrranes was developed for the building of meso-aryl and -nitromethyl substituted A₃B porphyrins.

Naphthylbipyrrole-Containing Amethyrin Analogue: A New Ligand for the Uranyl (UO22+) Cation

Using naphthobipyrrole as a functional building block, a new expanded porphyrin, naphthoisoamethyrin, was prepared in 85% yield under acid-catalyzed [4 + 2] MacDonald coupling conditions. Treatment of naphthoisoamethyrin with the nonaqueous uranyl silylamide salt [UO₂[N(SiMe₃)₂]₂·2THF] yielded the corresponding uranyl complex. Upon metalation, naphthoisoamethyrin undergoes a two-electron oxidation to yield a formal 22 π-electron aromatic species, as inferred from ¹H NMR and UV-vis spectroscopy, as well as cyclic voltammetry.

Stable 5,5'-Substituted 2,2'-Bipyrroles: Building Blocks for Macrocyclic and Materials Chemistry

The preparation and characterization of a family of stable 2,2'-bipyrroles substituted at positions 5 and 5' with thienyl, phenyl, TMS-ethynyl, and vinyl groups is reported herein. The synthesis of these new bipyrroles comprises three steps: formation of the corresponding 5,5'-unsubstituted bipyrrole, bromination, and Stille or Suzuki coupling. The best results in the coupling are obtained using the Stille reaction under microwave irradiation. The new compounds have been fully characterized by UV-vis absorption, fluorescence, and IR spectroscopies and cyclic voltammetry. X-ray single-crystal analysis of four of the synthesized bipyrroles indicates a trans coplanar geometry of the pyrrole rings. Furthermore, the substituents at positions 5,5' remain coplanar to the central rings. This particular geometry extends the π-conjugation of the systems, which is in agreement with a red-shifting observed for the λ_max of the substituted molecules compared to the unsubstituted bipyrrole. All of these new compounds display a moderate fluorescence. In contrast with unsubstituted bipyrroles, these bipyrroles are endowed with a high chemical and thermal stability and solubility in organic solvents.

Computational Studies on Optoelectronic and Nonlinear Properties of Octaphyrin Derivatives

The electronic and nonlinear optical (NLO) properties of octaphyrin derivatives were studied by employing the DFT/TDFT at CAM-B3LYP/6-311++G (2d, 2p) level of the theory. Thiophene, phenyl, methyl and cyano moieties were substituted on the molecular framework of octaphyrin core, in order to observe the change in optoelectronic and nonlinear response of these systems. The frontier molecular orbital studies and values of electron affinity reveals that the studied compounds are stable against the oxygen and moisture present in air. The calculated ionization energies, adiabatic electron affinity and reorganization energy values indicate that octaphyrin derivatives can be employed as effective n-type material for Organic Light Emitting Diodes (OLEDs). This character shows an enhancement with the introduction of an electron withdrawing group in the octaphyrin framework. The polarizability and hyperpolarizability values of octaphyrin derivatives demonstrate that they are good candidates for NLO devices. The nonlinear response of these systems shows enhancement on the introduction of electron donating groups on octaphyrin moiety. However, these claims needs further experimental verification.

A new synthesis of isoamethyrins: A 4+2 route

A concise and versatile strategy for the preparation of isoamethyrins is presented. The novel procedure is predicated on the acid catalyzed condensation of a quaterpyrrole and a bipyrrole dialdehyde. As application of this procedure an aryl substituted isoamethyrin has been synthetized and characterized.

Flexible Porphyrinoids

This review underscores the conformational flexibility of porphyrinoids, a unique class of functional molecules, starting from the smallest triphyrins(1.1.1) via [18]porphyrins(1.1.1.1) and concluding with a variety of expanded porphyrinoids and heteroporphyrinoids, including the enormous [96]tetracosaphyrin(1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0). The specific flexibility of porphyrinoids has been documented as instrumental in the designing or redesigning of macrocyclic frames, particularly in the search for adjustable platforms for coordination or organometallic chemistry, anion binding, or mechanistic switches in molecular devices. A structural prearrangement to coordinate one or more metal ions has been outlined. The coverage of the topic focuses on representative examples of geometry or conformational rearrangements for each selected class of the numerous porphyrinoids accordingly categorized by the number of built-in carbo- or heterocycles.

2,2'-Bipyrrole-Based Porphyrinoids

A large number of porphyrinoids containing 2,2'-bipyrrole subunits have appeared since they were originally found as a component of sapphyrin and corrole, and it was found that the bipyrrole subunit endowed macrocycles with specific geometric features and electronic properties. Synthetic methods for bipyrrole-containing precursors for porphyrinoid are summarized in this review; these include coupling reactions of pyrrole rings, pyrrole ring-forming reactions leading directly to bipyrrole units, and synthetic reactions for oligopyrrolic compounds. Some hybrid oligopyrroles having nonpyrrole (hetero)aromatic ring(s) are also included. This review also describes porphyrinoids composed of bipyrrole subunits. Interesting electronic properties derived from strong cyclo-π-conjugation are highlighted in the bipyrrole-based porphyrinoids with or without meso-like carbons. Anion-binding chemistry is one of the main topics for bipyrrole-based macrocycles with less efficient or deficient cyclo-π-conjugation, such as those linked with electronically localized aromatic ring(s), with sp³ carbon(s), and with amido or imine connection(s). The principal concern in this review is porphyrinoids of relatively large ring size, composed of more than five units of pyrroles and (hetero)aromatic substitutes in total, and so bipyrrole-based porphyrinoids up to five pyrrolic units, such as corroles, porphycenes, sapphyrins, and smaragdyrin, will not be covered here except for some special cases.