Fragmentations observed in the reactions of α-methoxy-γ-alkoxyalkyl iodide substrates with super-electron-donors derived from 4-DMAP and N-methylbenzimidazole

Facile access to benzofuran derivatives through radical reactions with heteroatom-centered super-electron-donors

The development of suitable electron donors is critical to single-electron-transfer (SET) processes. The use of heteroatom-centered anions as super-electron-donors (SEDs) for direct SET reactions has rarely been studied. Here we show that heteroatom anions can be applied as SEDs to initiate radical reactions for facile synthesis of 3-substituted benzofurans. Phosphines, thiols and anilines bearing different substitution patterns work well in this inter-molecular radical coupling reaction and the 3-functionalized benzofuran products bearing heteroatomic functionalities are given in moderate to excellent yields. The reaction mechanism is elucidated via control experiments and computational methods. The afforded products show promising applications in both organic synthesis and pesticide development.

Multitargeting approaches to cognitive impairment: Synthesis of aryl-alkylpiperazines and assessment at cholinesterases, histamine H3 and dopamine D3 receptors

Multitargeting ligands on enzymes and receptors may generate a profile for a potential treatment of cognitive impairment. Considering this, a set of 21 substituted aryl-alkyl-piperazines were designed, prepared and tested for their binding affinities at histamine H₃ and dopamine D₃ receptors (H₃R and D₃R, respectively) as well as acetyl- and butyrylcholinesterases (AChE/BChE) as potentially synergistic profile. Initial screening of the compounds at H₃R and D₃R was done at 1 or 10 µM and 100 µM at AChE and BChE assays. The most promising compounds were then evaluated in full concentration-response curves to estimate the K_i and IC₅₀ values. Results showed that several compounds were ligands at H₃R (n = 10), D₃R (n = 6), AChE (n = 3), and BChE (n = 9). Compounds LINS05006 (K_i H₃R 2.8 µM; D₃R 0.7 µM; IC₅₀ BChE 26.3 µM) and LINS05015 (K_i H₃R 1.1 µM; D₃R 3.1 µM; IC₅₀ AChE 97.8 µM; BChE 43.7 µM) are highlighted since presented affinity in three different. These results suggest that methylpiperazine moiety led to balanced activity at all three classes of targets, and longer linker provided the best affinities. These compounds presented high ligand efficiency values (LE > 0.3) and may have adequate pharmacokinetic profile as suggested by calculated physicochemical properties.

Switching from Single to Simultaneous Free-Radical and Anionic Polymerization with Enamine-Based Organic Electron Donors

Although most monomers can polymerize through different propagation pathways, polymerization-initiating systems that can switch from one mode to another are rare. In this study, we demonstrate that enamine-based organic electron donors (OEDs) constitute the first systems able to initiate either free-radical or anionic polymerization under simple, mild, and safe conditions. While direct electron-transfer reduction of monomers by OEDs results in the initiation of anionic chain-growth polymerization, introduction of a competing oxidant with a higher reduction potential than the monomer switches the former anionic propagation to a clean radical-propagation process. The benefit of this dual-mode activator is highlighted in the synthesis of an interpenetrating polymer network through simultaneous initiation of radical and anionic propagation processes.

Expanding the Chemical Space of Benzimidazole Dicationic Ionic Liquids

Benzimidazole dicationic ionic liquids (BDILs) have not yet been widely explored in spite of their potential. Therefore, two structurally related families of BDILs, paired with either bromide or bistriflimide anions and bearing alkyl spacers ranging from C3 to C6, have been prepared. Their thermal properties have been studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), while their electrical properties have been assessed by cyclic voltammetry (CV). TG analysis confirmed the higher stability of the bistriflimide BDILs over the bromide BDILs, with minor variation within the two families. Conversely, DSC and CV allowed for ascertaining the role played by the spacer length. In particular, the thermal behavior changed dramatically among the members of the bistriflimide family, and all three possible thermal behavior types of ILs were observed. Furthermore, cyclic voltammetry showed different electrochemical window (C3(C1BenzIm)2/2Tf2N < C4(C1BenzIm)2/2Tf2N, C5(C1BenzIm)2/2Tf2N < C6(C1BenzIm)2/2Tf2N) as well as a reduction peak potential, shape, and intensity as a function of the spacer length. The results obtained highlight the benefit of accessing a more structurally diverse pool of compounds offered by dicationic ILs when compared to the parent monocationic ILs. In particular, gains are to be found in the ease of fine-tuning their properties, which translates in facilitating further investigations toward BDILs as designer solvents and catalysts.

Bis(Iminophosphorano)-Substituted Pyridinium Ions and their Corresponding Bispyridinylidene Organic Electron Donors

Optimized synthetic procedures for pyridinium ions featuring iminophosphorano (-N=PR₃ ; R=Ph, Cy) π-donor substituents in the 2- and 4- positions are described. Crystallographic and theoretical studies reveal that the strongly donating substituents severely polarize the π-electrons of the pyridyl ring at the expense of aromaticity. Moreover, the pyridinium ions are readily deprotonated to generate powerful bispyridinylidene (BPY) organic electron donors. Electrochemical studies show exceptionally low redox potentials for the two-electron BPY/BPY²⁺ couples, ranging from -1.71 V vs the saturated calomel electrode for 3PhPh (with four Ph₃ P=N- groups) to -1.85 V for 3CyCy (with four Cy₃ P=N- groups). These new compounds represent the most reducing neutral organic electron donors (OEDs) currently known. Some preliminary reductions involving 3CyCy showed enhanced capability owing to its low redox potential, such as the thermally activated reduction of an aryl chloride, but purification challenges were often encountered.

Generation of powerful organic electron donors by water-assisted decarboxylation of benzimidazolium carboxylates

In situ and easy generation of organic electron donors from water-activation of carboxylate precursors allows OED-promoted intermolecular radical addition reactions.

Studies toward the oxidative and reductive activation of C-S bonds in 2'-S-aryl-2'-thiouridine derivatives

Studies directed toward the oxidative and reductive desulfurization of readily available 2'-S-aryl-2'-thiouridine derivatives were investigated with the prospect to functionalize the C2'-position of nucleosides. The oxidative desulfurization-difluorination strategy was successful on 2-(arylthio)alkanoate surrogates, while extension of the combination of oxidants and fluoride sources was not an efficient fluorination protocol when applied to 2'-S-aryl-2'-thiouridine derivatives, resulting mainly in C5-halogenation of the pyrimidine ring and C2'-monofluorination without desulfurization. Cyclic voltammetry of 2'-arylsulfonyl-2'-deoxyuridines and their 2'-fluorinated analogues showed that cleavage of the arylsulfone moiety could occur, although at relatively high cathodic potentials. While reductive-desulfonylation of 2'-arylsulfonyl-2'-deoxyuridines with organic electron donors (OEDs) gave predominantly base-induced furan type products, chemical (OED) and electrochemical reductive-desulfonylation of the α-fluorosulfone derivatives yielded the 2'-deoxy-2'-fluorouridine and 2',3'-didehydro-2',3'-dideoxy-2'-fluorouridine derivatives. These results provided good evidence of the generation of a C2'-anion through carbon-sulfur bond cleavage, opening new horizons for the reductive-functionalization approaches in nucleosides.

Pushing the limits of neutral organic electron donors: a tetra(iminophosphorano)-substituted bispyridinylidene

A new ground-state organic electron donor has been prepared that features four strongly π-donating iminophosphorano substituents on a bispyridinylidene skeleton. Cyclic voltammetry reveals a record redox potential of −1.70 V vs. saturated calomel electrode (SCE) for the couple involving the neutral organic donor and its dication. This highly reducing organic compound can be isolated (44 %) or more conveniently generated in situ by a deprotonation reaction involving its readily prepared pyridinium ion precursor. This donor is able to reduce a variety of aryl halides, and, owing to its redox potential, was found to be the first organic donor to be effective in the thermally induced reductive S–N bond cleavage of N,N-dialkylsulfonamides, and reductive hydrodecyanation of malonitriles.

Evolution of neutral organic super-electron-donors and their applications

In recent times, metal-free chemistry has received significant attention due to its inherent qualities and its potential savings in the costs of (i) reagents and (ii) environmental treatments of residues. In this context, recently developed neutral organic electron-donors have shown an ability to perform challenging reductions that are traditionally the preserve of reactive metals and metal-based complexes, under mild reaction conditions. Hence, this feature article is aimed at describing the evolution of neutral organic super-electron-donors and their rapidly developing applications in electron-transfer reactions.

Discovery and development of organic super-electron-donors

Based on simple ideas of electron-rich alkenes, exemplified by tetrakis(dimethylamino)ethene, TDAE, and on additional driving force associated with aromatization, families of very powerful neutral organic super-electron-donors (SEDs) have been developed. In the ground state, they carry out metal-free reductions of a range of functional groups. Iodoarenes are reduced either to aryl radicals or, with stronger donors, to aryl anions. Reduction to aryl radicals allows the initiation of very efficient transition-metal-free coupling of haloarenes to arenes. The donors also reduce alkyl halides, arenesulfonamides, triflates, and triflamdes, Weinreb amides, and acyloin derivatives. Under photoactivation at 365 nm, they are even more powerful and reductively cleave aryl chlorides. They reduce unactivated benzenes to the corresponding radical anions and display original selectivities in preferentially reducing benzenes over malonates or cyanoacetates. Additionally, they reductively cleave ArC-X, ArX-C (X = N or O) and ArC-C bonds, provided that the two resulting fragments are somewhat stabilized.

Reductive decyanation of malononitriles and cyanoacetates using photoactivated neutral organic super-electron-donors

A metal-free reductive procedure for decyanation of malononitriles and cyanoacetates, using photoactivated organic electron donors, is described.

Metal-free reductive cleavage of C-N and S-N bonds by photoactivated electron transfer from a neutral organic donor

A photoactivated neutral organic super electron donor cleaves challenging arenesulfonamides derived from dialkylamines at room temperature. It also cleaves a) ArC–NR and b) ArN–C bonds. This study also highlights the assistance given to these cleavage reactions by the groups attached to N in (a) and to C in (b), by lowering LUMO energies and by stabilizing the products of fragmentation.

Overturning established chemoselectivities: selective reduction of arenes over malonates and cyanoacetates by photoactivated organic electron donors

The prevalence of metal-based reducing reagents, including metals, metal complexes, and metal salts, has produced an empirical order of reactivity that governs our approach to chemical synthesis. However, this reactivity may be influenced by stabilization of transition states, intermediates, and products through substrate-metal bonding. This article reports that in the absence of such stabilizing interactions, established chemoselectivities can be overthrown. Thus, photoactivation of the recently developed neutral organic superelectron donor 5 selectively reduces alkyl-substituted benzene rings in the presence of activated esters and nitriles, in direct contrast to metal-based reductions, opening a new perspective on reactivity. The altered outcomes arising from the organic electron donors are attributed to selective interactions between the neutral organic donors and the arene rings of the substrates.