Directionally aligned crown ethers as superactive organocatalysts for transition metal ion-free arylation of unactivated arenes

When one-dimensionally aligned to the same side, multiple non-covalently associated crown ether groups could act as a whole to yield a higher catalytic activity than an individual poorly active crown ether group, delivering the lowest catalyst loading of 1 - 2 mol% among all hitherto known organocatalysts for catalyzing direct arylation of unactivated arenes with haloarenes.

Synthesis of Highly Substituted Biaryls by the Construction of a Benzene Ring via In Situ Formed Acetals

Herein, we present an interesting method for the construction of a benzene ring using propargylic alcohols and 1,3-dicarbonyls, which involves three new C-C bond formations via cascade alkylation, formylation, annulation, and aromatization to make substituted biaryls. This one-pot Brønsted acid-promoted protocol utilizes the unique reactivity of the acetal formed under the reaction conditions. Alkynyl methyl ketones could be employed instead of 1,3-dicarbonyls as they are converted to 1,3-dicarbonyls by hydration under the reaction conditions.

A Bottleable Imidazole-Based Radical as a Single Electron Transfer Reagent

Reduction of 1,3-bis(2,6-diisopropylphenyl)-2,4-diphenyl-1H-imidazol-3-ium chloride (1) resulted in the formation of the first structurally characterized imidazole-based radical 2. 2 was established as a single electron transfer reagent by treating it with an acceptor molecule tetracyanoethylene. Moreover, radical 2 was utilized as an organic electron donor in a number of organic transformations such as in activation of an aryl-halide bond, alkene hydrosilylation, and in catalytic reduction of CO₂ to methoxyborane, all under ambient temperature and pressure.

Mechanism of the t-BuOM (M = K, Na, Li)/DMEDA-Mediated Direct C–H Arylation of Benzene: A Computational Study

Over the past ten years, a combination of organic additive and t-BuOK/t-BuONa has been successfully used for the direct C–H arylation of arenes. Conceptually different from transition-metal-catalyzed cross-coupling reactions, these t-BuOK-mediated reactions have raised significant curiosity among organic chemists. Herein, a systematic computational study of each elementary step of the t-BuOM (M = K, Na, Li)/N 1,N 2-dimethylethane-1,2-diamine (DMEDA) mediated direct C–H arylation of benzene is detailed. The presented mechanistic proposal relies on the complexation and reaction of t-BuOM with DMEDA (additive), which leads to the formation of different complexes such as SED(M+)…PhI. These complexes mainly involve coordination of the metal ion (from t-BuOM) to the additive and iodobenzene via stabilizing cation–lone pair and cation–π interactions. Such complexation of a metal ion to an additive and iodobenzene not only ensures facile electron transfer to iodobenzene but also provides a lowest energy pathway for the subsequent radical addition and deprotonation step.

NaH-mediated direct C-H arylation in the presence of 1,10-phenanthroline

Transition-metal-free coupling of haloarenes with unactivated arenes has been developed in the presence of NaH and 1,10-phenanthroline. Various haloarenes bearing methyl, methoxy, halogen (fluoride, chloride, and bromide), cyano, trifluoromethyl, ester, and amide groups can be cross-coupled with unactivated arenes, or heteroarenes in this reaction.

Tetramethylammonium Fluoride Tetrahydrate-Mediated Transition Metal-Free Coupling of Aryl Iodides with Unactivated Arenes in Air

Biaryls are important compounds with widespread applications in many fields. Tetramethylammonium fluoride tetrahydrate was found to promote the biaryl coupling of aryl iodides bearing electron-withdrawing substituents with unactivated arenes. The reaction takes place at temperatures between 100 and 150°C and can be applied to a wide range of aromatic and heteroaromatic rings, affording the products in moderate to high yields. The reaction does not require strong bases or expensive additives that are employed in the existing methods and can be conducted in air and moisture without any precautions.

Photoinduced Transition-Metal-Free Cross-Coupling of Aryl Halides with H-Phosphonates

Photoinduced transition-metal- and photosensitizer-free cross-coupling of aryl halides (including Ar-Cl, Ar-Br, and Ar-I) with H-phosphonates (including dialkyl phosphonates and diarylphosphine oxides) is reported. Various functional groups were tolerated, including ester, methoxy, dimethoxy, alkyl, phenyl, trifluoromethyl, and heterocyclic compounds. This simple and green strategy provides a practical pathway to synthesize arylphosphine oxides.

Electron Transfer Reactions: KO tBu (but not NaO tBu) Photoreduces Benzophenone under Activation by Visible Light

Long-standing controversial reports of electron transfer from KO tBu to benzophenone have been investigated and resolved. The mismatch in the oxidation potential of KO tBu (+0.10 V vs SCE in DMF) and the first reduction potential of benzophenone (of many values cited in the literature, the least negative value is -1.31 V vs SCE in DMF), preclude direct electron transfer. Experimental and computational results now establish that a complex is formed between the two reagents, with the potassium ion providing the linkage, which markedly shifts the absorption spectrum to provide a tail in the visible light region. Photoactivation at room temperature by irradiation at defined wavelength (365 or 400 nm), or even by winter daylight, leads to the development of the blue color of the potassium salt of benzophenone ketyl, whereas no reaction is observed when the reaction mixture is maintained in darkness. So, no electron transfer occurs in the ground state. However, when photoexcited, electron transfer occurs within a complex formed from benzophenone and KO tBu. TDDFT studies match experimental findings and also define the electronic transition within the complex as n → π*, originating on the butoxide oxygen. Computation and experiment also align in showing that this reaction is selective for KO tBu; no such effect occurs with NaO tBu, providing the first case where such alkali metal ion selectivity is rationalized in detail. Chemical evidence is provided for the photoactivated electron transfer from KO tBu to benzophenone: tert-butoxyl radicals are formed and undergo fragmentation to form (acetone and) methyl radicals, some of which are trapped by benzophenone. Likewise, when KOC(Et)₃ is used in place of KO tBu, then ethylation of benzophenone is seen. Further evidence of electron transfer was seen when the reaction was conducted in benzene, in the presence of p-iodotoluene; this triggered BHAS coupling to form 4-methylbiphenyl in 74% yield.

Simple and Efficient Generation of Aryl Radicals from Aryl Triflates: Synthesis of Aryl Boronates and Aryl Iodides at Room Temperature

Despite the wide use of aryl radicals in organic synthesis, current methods to prepare them from aryl halides, carboxylic acids, boronic acids, and diazonium salts suffer from limitations. Aryl triflates, easily obtained from phenols, are promising aryl radical progenitors but remain elusive in this regard. Inspired by the single electron transfer process for aryl halides to access aryl radicals, we developed a simple and efficient protocol to convert aryl triflates to aryl radicals. Our success lies in exploiting sodium iodide as the soft electron donor assisted by light. This strategy enables the scalable synthesis of two types of important organic molecules, i.e., aryl boronates and aryl iodides, in good to high yields, with broad functional group compatibility in a transition-metal-free manner at room temperature. This protocol is anticipated to find potential applications in other aryl-radical-involved reactions by using aryl triflates as aryl radical precursors.

Transition-Metal-Free C3 Arylation of Indoles with Aryl Halides

We report an unprecedented transition metal-free coupling of indoles with aryl halides. The reaction is promoted by KOtBu and is regioselective for C3 over N. The use of degassed solvents devoid of oxygen is necessary for the success of the transformation. Preliminary studies implicate a hybrid mechanism that involves both aryne intermediates and non-propagative radical processes. Electron transfer is also a distinct possibility. These conclusions were substantiated by EPR data, isotopic labeling studies, and the use of radical scavengers and electron transfer inhibitors.

Synthesis and Evaluation of New Bis(imino) Pyridine Based Catalysts for Ethylene Polymerization

Three-component catalytic systems based on 2,6-bis(imino) pyridine iron(II) chloride were synthesized from different ligands, which provided new alternative catalysts for polymerization of ethylene. The synthesized catalysts were characterized by Fourier transform infrared spectroscopy (FTIR), and the lack of absorption bands was observed in the region related to the carbonyl, as well as the presence of bands in the region of imino groups corresponding to C=N bonds. Coordination with Fe was also carried out. The structure of the ligands and the new catalysts were confirmed by the elemental analysis (CHN), and 1H- and 13C-nuclear magnetic resonance spectroscopy. In ethylene polymerization with methylaluminoxane as a cocatalyst, the activity of catalyst C1 was high. Although this catalyst structure contains sterically bulky ligands, the metal center was not sufficiently protected allowing transfer reactions, producing polyethylene with a low molar mass and melting temperature.

KOtBu: A Privileged Reagent for Electron Transfer Reactions?

Many recent studies have used KOtBu in organic reactions that involve single electron transfer; in the literature, the electron transfer is proposed to occur either directly from the metal alkoxide or indirectly, following reaction of the alkoxide with a solvent or additive. These reaction classes include coupling reactions of halobenzenes and arenes, reductive cleavages of dithianes, and SRN1 reactions. Direct electron transfer would imply that alkali metal alkoxides are willing partners in these electron transfer reactions, but the literature reports provide little or no experimental evidence for this. This paper examines each of these classes of reaction in turn, and contests the roles proposed for KOtBu; instead, it provides new mechanistic information that in each case supports the in situ formation of organic electron donors. We go on to show that direct electron transfer from KOtBu can however occur in appropriate cases, where the electron acceptor has a reduction potential near the oxidation potential of KOtBu, and the example that we use is CBr4. In this case, computational results support electrochemical data in backing a direct electron transfer reaction.

Homolytic cleavage of the O–Cu(ii) bond: XAFS and EPR spectroscopy evidence for one electron reduction of Cu(ii) to Cu(i)

In this work, we demonstrate that the ^tBuO⁻ anion serves not only as the base but also as a mediator to promote the reduction of Cu(ii) to Cu(i) in copper catalysis. XAFS and EPR spectroscopy evidence the [Cu(O^tBu)₃]⁻ ate complex as the key intermediate which undergoes homolytic cleavage of the O–Cu(ii) bond generating [Cu(O^tBu)₂]⁻ ate complex.

Microwave assisted synthesis of phenanthridinones and dihydrophenanthridines by vasicine/KOtBu promoted intramolecular C–H arylation

Vasicine and KOtBu promoted intramolecular C–H arylation has been demonstrated for synthesis of phenanthridinones and dihydrophenanthridines with various aryl halides under microwave conditions.

Reductive cleavage of inert aryl C–O bonds to produce arenes

Reductive cleavage of aryl C–O bonds in phenols and aryl ethers was achieved using LiAlH₄–KOtBu, producing arenes in high yields.

Evidence for the interaction between tBuOK and 1,10-phenanthroline to form the 1,10-phenanthroline radical anion: a key step for the activation of aryl bromides by electron transfer

Electron paramagnetic resonance and electrochemistry are utilized to the interaction between 1,10-phenanthroline (Phen) and KO^tBu to form the radical anion Phen˙⁻via an inner-sphere electron transfer. An aryl radical is then formed from aryl bromide via outer-sphere electron transfer from Phen˙⁻.

Multi-bond forming and iodo-selective base-promoted homolytic aromatic substitution

Base-promoted homolytic aromatic substitution (BHAS) has been applied as a means to effect multi-bond forming reactions.

Room-temperature and transition-metal-free Mizoroki-Heck-type reaction. Synthesis of E-stilbenes by photoinduced C-H functionalization

We report a conceptually different approach toward E-stilbene syntheses by photoinduced direct C-H arylation of alkenes at rt without the addition of transition metals, with a broad range of aryl halides, including ArI, ArBr, and even ArCl. This is the first time that this reaction has been produced without extra solvent but with 18-crown-6 ether and t-BuOK in only 15 min of reaction.