TEtraQuinolines: A Missing Link in the Family of Porphyrinoid Macrocycles

Abstract

Porphyrin contains four inwardly oriented nitrogen atoms. It is arguably the most ubiquitous multifunctional naturally occurring macrocycle that has inspired the design of novel nitrogencontaining heterocycles for decades. While cyclic tetramers of pyrrole, indole, and pyridine have been exploited as macrocycles in this category, quinoline has been largely neglected as a synthon. Herein, we report the synthesis of TEtraQuinoline (TEQ) as a 'missing link' in this N4 macrocycle family. In TEQs, four quinoline units are concatenated to produce an S 4 -symmetric architecture. TEQs are characterized by a highly rigid saddle shape, wherein the lone-pair orbitals of the four nitrogen atoms are not aligned in a planar fashion. Nevertheless, TEQs can coordinate a series of transition-metal cations (Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , and Pd 2+ ). TEQs are inherently fluorescence-silent but become strongly emissive upon protonation or complexation of Zn(II) cations (ϕ = 0.71). TEQ/Fe(II) complexes can catalyze dehydrogenation and oxygenation reactions with catalyst loadings as low as 0.1 mol %.

Chemoselective Catalytic Dehydrogenation of Benzylic Amines Driven by the TEtraQuinoline/FeCl2 Complex

“…[25][26][27] Recently, we unveiled TEtraQuinoline (TEQ) as a novel nonflat porphyrinoid tetradentate nitrogen ligand whose iron complex promotes dehydrogenation of tetrahydroquinolines and benzylic oxidation of dihydroanthracene. [28] Given the recent developments in oxidative iron catalysis with peroxides, [29][30][31][32][33][34][35][36] we reasoned that our highly rigid TEQ/Fe(II) complex offers more opportunities in oxidative catalysis. Herein we report the TEQ/FeCl 2 complex as a proficient catalyst to promote chemoselective dehydrogenation of benzylic amines under mild conditions (0 °C to room temperature).…”

“…Iron catalysis has attracted sustained attention due to the abundance of iron in nature, and aerobic oxidative conditions promoted by iron complexes were disclosed [25–27] . Recently, we unveiled TEtraQuinoline (TEQ) as a novel non‐flat porphyrinoid tetradentate nitrogen ligand whose iron complex promotes dehydrogenation of tetrahydroquinolines and benzylic oxidation of dihydroanthracene [28] . Given the recent developments in oxidative iron catalysis with peroxides, [29–36] we reasoned that our highly rigid TEQ/Fe(II) complex offers more opportunities in oxidative catalysis.…”

Oxa-TriQuinoline: A New Entry to Aza-Oxa-Crown Architectures

“…33 Given the vast array of successful applications of porphyrins, including organic solar cells (OSCs), [34][35][36][37][38] various molecular devices, [39][40][41] and photosensitizers (PSs) used for tumor-targeting reagents, [42][43][44][45] the last two decades have witnessed the extensive development of skeletally modified porphyrinoids in pursuit of functional materials. [46][47][48][49][50][51][52][53][54][55][56][57][58][59][60] Recently, our group has reported the synthesis of TriQuinoline (TQ), a quasi-planar quinoline trimer concatenated at the 2-and 8positions in a head-to-tail fashion; this 12-membered macrocycle contains three nitrogen atoms located on the interior of the cycle (Figure 1). 61 Due to the structural rigidity and limited central void space, TQ strongly accommodates a single proton, which causes it to behave as a positively charged aromatic plane, exhibiting supramolecular complexation via both π-π and CH-π interactions as well as DNA intercalation.…”

Oxa‐TriQuinoline: A New Entry to Aza‐Oxa‐Crown Architectures**

“…In addition to oxygen‐based macrocycles, nitrogen counterparts such as porphyrins have emerged in nature that accommodate a variety of transition metals to significantly expand the scope of vital biocatalytic transformations [33] . Given the vast array of successful applications of porphyrins, including organic solar cells (OSCs), [34–38] various molecular devices, [39–41] and photosensitizers (PSs) used for tumor‐targeting reagents, [42–45] the last two decades have witnessed the extensive development of skeletally modified porphyrinoids in pursuit of functional materials [46–60] …”

“…[33] Given the vast array of successful applications of porphyrins, including organic solar cells (OSCs), [34][35][36][37][38] various molecular devices, [39][40][41] and photosensitizers (PSs) used for tumortargeting reagents, [42][43][44][45] the last two decades have witnessed the extensive development of skeletally modified porphyrinoids in pursuit of functional materials. [46][47][48][49][50][51][52][53][54][55][56][57][58][59][60] Recently, our group has reported the synthesis of TriQuinoline (TQ), a quasi-planar quinoline trimer concatenated at the 2-and 8-positions in a head-to-tail fashion; this 12-membered macrocycle contains three nitrogen atoms located on the interior of the cycle (Figure 1). [61] Due to the structural rigidity and limited central void space, TQ strongly accommodates a single proton, which causes it to behave as a positively charged aromatic plane, exhibiting supramolecular complexation via both π-π and CH-π interactions as well as DNA intercalation.…”