Efficient Synthesis of 5,6-Dihydroxyindole Dimers, Key Eumelanin Building Blocks, by a Unified o-Ethynylaniline-Based Strategy for the Construction of 2-Linked Biindolyl Scaffolds

An iterative synthesis of poly-substituted indole oligomers reveals a short effective conjugation length in eumelanin model compounds

Eumelanin is a multifunctional biomaterial that colors the skin, hair and eyes of mammals. Despite years of effort, its molecular structure remains unknown, limiting our understanding of its biological function and the design of synthetic mimics. In an effort to address this challenge, we report an Iterative Chain Growth (ICG) of well-defined 5,6-dihydroxyindole (DHI) model compounds that provides direct, experimental evidence of a short effective conjugation length in the resulting oligomers. Our ICG highlights the C2-selective borylation of N-H indoles in complex settings, and the utility of Suzuki-Miyaura Coupling (SMC) to grow the chain. The resulting C2-C7' linkage is installed selectively with good yields, affording products with up to 5-indole units. Access to these oligomers allows us to probe how DHI chain extension contributes to the emergence of sun screening in eumelanin. Our oligomers guarantee the absence of oxidized by-products that may otherwise complicate analysis, without substantially altering the photophysics of the indolic-backbone. Steady-state absorption and emission spectroscopy coupled with excited-state calculations reveal pronounced vibronic structure and excited state planarization, but only a moderate red shift with increasing chain length because of poor orbital coupling between adjoined π-systems. We conclude that eumelanin's characteristic ability to absorb visible light does not derive from long chains of fully reduced DHI sub-units. Our work takes an important step towards a more systematic exploration of eumelanin's structure through iterative synthesis, with the long-term goal of explaining the molecular origins of its properties.

Highly Functionalized SWCNTs with a Dopamine Derivative as a Support for Pd Nanoparticles: A Recyclable Catalyst for the Reduction of Nitro Compounds and the Heck Reaction

Single-walled carbon nanotubes (SWCNTs) were functionalized with a dopamine derivative in which the amine group was converted to azide (dopamine azide). The direct reaction of SWCNTs and dopamine azide in o-dichlorobenzene at high temperature (160 °C) led to very highly functionalized CNTs (≈60 wt.%). Surprisingly, despite this high degree of functionalization, Raman spectroscopy detected a low disruption of the π-network of the carbonaceous support. This finding was justified by the rehybridization from sp3 to sp2 of the sidewall carbon atoms of CNTs involved in the functionalization process. Further characterization by means of different techniques such as X-ray photoelectron spectroscopy (XPS) analysis and transmission electron microscopy (TEM) allowed to shed some light on the chemical composition and morphology of the obtained material. Moreover, the estimation of the total content of phenolic units and their reducing potential after CNTs functionalization was also assessed using Folin and Ciocalteu and 2,2-diphenyl-1-picryl hydrazide (DPPH) assays. The functionalization of CNTs was exploited to immobilize palladium(II) species that were subsequently reduced with NaBH4 leading to the formation of Pd nanoparticles (NPs). The so obtained hybrid material was used as a recyclable heterogeneous catalyst for the reduction of nitro compounds and the Heck reaction.

Catalyst-Controlled C-H Transformation of Pyrazolidinones with 1,3-Diynes for Highly Selective Synthesis of Functionalized Bisindoles and Indoles

An efficacious synthetic solution to offer functionalized bisindoles and indoles has been developed based on catalyst-controlled C-H functionalization of pyrazolidinones and 1,3-diynes with highly selective control of both chemoselectivity and regioselectivity. This straightforward pathway conquers chemo- and regioselectivity challenges concerning the use of 1,3-diynes.

Thiazolocatechol: Electron-Withdrawing Catechol Anchoring Group for Dye-Sensitized Solar Cells

Anchoring groups adopting a five-membered bidentate chelating are attractive to realize high power conversion efficiency (η) and long-term durability in dye-sensitized solar cells (DSSCs). In this regard, we chose catechol as an anchoring group that can adopt the chelating. However, the DSSCs with catechol-based sensitizers have never exceeded an η-value of 2 %. These poor photovoltaic performances may be associated with the electron-donating ability of the hydroxy groups in catechol. Considering these, we envisioned that fusing an electron-withdrawing thiazole moiety with a catechol anchoring group would improve its photovoltaic performance. Herein, we report a push-pull porphyrin sensitizer ZnPTC with a thiazolocatechol anchoring group. The DSSC with ZnPTC exhibited η=4.87 %. This value is the highest ever reported for catechol-anchor based DSSCs. Meanwhile, the long-term cell durability was not improved, although the robust anchoring properties were attained under harsh conditions.

Expedient synthesis of eumelanin-inspired 5,6-dihydroxyindole-2-carboxylate ethyl ester derivatives

Dihydroxyindoles such as 5,6-dihydroxyindole-2-carboxylic acid (DHICA) are the main monomer units of eumelanin, the black to brown pigment in humans, and have emerging biological roles beyond melanin. Elaboration of commercially available 5,6-dimethoxy-2-carboxylate ethyl ester provides ready access to DHICA-inspired small molecules, including 3-(hetero)aryl-indoles and 4,7-di-(hetero)aryl-indoles.

Two concise syntheses of novel aryl- and heteroaryl-substituted 5,6-dimethoxyindole-2-carboxylate ethyl esters utilizing regioselective halogenation/dehalogenation and Suzuki coupling are presented.

Palladium(II)-Catalyzed Transformation of 3-Alkylbenzofurans to [2,3'-Bibenzofuran]-2'(3'H)-ones: Oxidative Dimerization of 3-Alkylbenzofurans

An unprecedented oxidative dimerization by palladium catalysis has been developed using PhI(OPiv)₂ as a by-standing oxidant. This provides a facile method for the synthesis of quaternary 2,3'-bibenzofuran-2'(3')-ones from readily accessible substrates. A plausible mechanism involving a Pd(II)-Pd(IV) catalytic cycle is proposed; a trace amount of water is required for subsequent oxidation.

Melanin-Inspired Organic Electronics: Electroluminescence in Asymmetric Triazatruxenes

The oxidative polymerization of 5,6-dihydroxyindoles and related hydroxyindoles at pH<3 is diverted from the usual eumelanin-forming pathway to produce mixtures of symmetric and asymmetric triazatruxenes (TATs), which could be separated and characterized for their opto-electronic properties with the aid of TD-DFT calculations. Data showed that the asymmetric isomers exhibit higher fluorescence quantum efficiencies, lower HOMO-LUMO gaps, better film homogeneity, and a more definite aggregation behavior than the symmetric counterparts, suggesting promising applications in organic electronics. The enhanced luminance exhibited by the OLED devices fabricated with blends of the synthesized TATs in poly-9-vinylcarbazole confirmed the potential of the asymmetric skeleton as new versatile platform for light-emitting materials.

Eumelanin-inspired core derived from vanillin: a new building block for organic semiconductors

An eumelanin-inspired core derived from the natural product, vanillin (vanilla bean extract) was utilized for the synthesis of eumelanin-inspired small molecules and polymer via Sonogashira cross coupling.

Oxidative dimerization of N-protected and free indole derivatives toward 3,3-biindoles via Pd-catalyzed direct C-H transformations

An oxidative homo dimerization of N-protected and free indole derivatives toward bioactive 3,3-linked biindolyl scaffolds via Pd-catalyzed direct C-H transformations was first successfully demonstrated.

Palladium-catalyzed regioselective oxidative coupling of indoles and one-pot synthesis of acetoxylated biindolyls

Mild conditions have been developed to achieve Pd-catalyzed homocoupling of indoles with excellent regioselectivity in the presence of Cu(OAc)(2) x H(2)O in DMSO at room temperature in high efficiency. This method provides a simple route to 2,3'-biindolyls from the electron-rich to moderately electron-poor indoles. The reaction tolerates the bromide substituent on indoles. In addition, a one-pot approach to C3-position acetoxylated biindolyls is realized via palladium catalysis by the use of AgOAc under oxygen atmosphere as oxidants.