Mechanistic Insights into the Initiation Step of the Base Promoted Direct C–H Arylation of Benzene in the Presence of Additive

Radical coupling of aryl halides to arenes facilitated by Ni(COD)(DQ) and other nickel sources

The air-stable complex Ni(COD)(DQ) (COD = 1,5-cyclooctadiene, DQ = duroquinone) promotes the coupling of aryl halides to arenes in the presence of KOtBu. This complex has recently been shown to perform coupling reactions based on organonickel intermediates, but in this case the coupling reactions proceed via aryl radicals as shown by our newly developed assay for aryl radicals. Coupling with this nickel source is more efficient than with Ni(COD)2, Ni(PPh3)4 and Ni(acac)2, all of which we also show to operate through aryl radical pathways.

Photo- and electro-chemical strategies for the activations of strong chemical bonds

The employment of light and/or electricity - alternatively to conventional thermal energy - unlocks new reactivity paradigms as tools for chemical substrate activations. This leads to the development of new synthetic reactions and a vast expansion of chemical spaces. This review summarizes recent developments in photo- and/or electrochemical activation strategies for the functionalization of strong bonds - particularly carbon-heteroatom (C-X) bonds - via: (1) direct photoexcitation by high energy UV light; (2) activation via photoredox catalysis under irradiation with relatively lower energy UVA or blue light; (3) electrochemical reduction; (4) combination of photocatalysis and electrochemistry. Based on the types of the targeted C-X bonds, various transformations ranging from hydrodefunctionalization to cross-coupling are covered with detailed discussions of their reaction mechanisms.

Modular Counter-Fischer-Indole Synthesis through Radical-Enolate Coupling

A single-electron transfer mediated modular indole formation reaction from a 2-iodoaniline derivative and a ketone has been developed. This transition-metal-free reaction shows a broad substrate scope and unconventional regioselectivity trends. Moreover, important functional groups for further transformation are tolerated under the reaction conditions. Density functional theory studies reveal that the reaction proceeds by metal coordination, which converts a disfavored 5-endo-trig cyclization to an accessible 7-endo-trig process.

C-Alkylation of alkali metal carbanions with olefins

C-Alkylations of alkali metal carbanions with olefins, first reported five decades ago, is a class of reaction undergoing a resurgence in organic synthesis in recent years. As opposed to expectations from classical chemistry and transition metal-catalysis, here olefins behave as closed-shell electrophiles. Reactions range from highly reactive alkyllithiums giving rise to anionic polymerization, to moderately reactive alkylpotassium or alkylsodium compounds that give rise to defined, controlled and bimolecular chemistry. This review presents a brief historical overview on C-alkylation of alkali metal carbanions with olefins (typically mediated by KOtBu and KHMDS), highlights contemporary applications and features developing mechanistic understanding, thereby serving as a platform for future studies and the widespread use of this class of reaction in organic synthesis.

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.

The Chemistry of Acylgermanes: Triacylgermenolates Represent Valuable Building Blocks for the Synthesis of a Variety of Germanium-Based Photoinitiators

The formation of a stable triacylgermenolate 2 as a decisive intermediate was achieved by using three pathways. The first two methods involve the reaction of KOtBu or alternatively potassium with tetraacylgermane 1 yielding 2 via one electron transfer. The mechanism involves the formation of radical anions (shown by EPR). This reaction is highly efficient and selective. The third method is a classical salt metathesis reaction toward 2 in nearly quantitative yield. The formation of 2 was confirmed by NMR spectroscopy, UV–vis measurements, and X-ray crystallography. Germenolate 2 serves as a starting point for a wide variety of organo-germanium compounds. We demonstrate the potential of this intermediate by introducing new types of Ge-based photoinitiators 4b–4f. The UV–vis absorption spectra of 4b–4f show considerably increased band intensities due to the presence of eight or more chromophores. Moreover, compounds 4d–4f show absorption tailing up to 525 nm. The performance of these photoinitiators is demonstrated by spectroscopy (time-resolved EPR, laser flash photolysis (LFP), photobleaching (UV–vis)) and photopolymerization experiments (photo-DSC measurements).

Triacylgermenolate 2 was obtained by using KOtBu or alternatively potassium. The mechanism involves the formation of radical anions (shown by EPR). The one-pot synthetic protocol produces 2 in >95% yield, as confirmed by NMR spectroscopy and X-ray crystallography. Germenolate 2 serves as a starting point for a wide variety of organo-germanium compounds. This was demonstrated by introducing new types of Ge-based photoinitiators 4b−4f. Their performance was analyzed by sophisticated spectroscopic methods and photopolymerization experiments.

The base-induced regioselective radical arylation of 3-aminochromone with aryl hydrazine

A simple and efficient direct radical C-2 arylation of 3-aminochromone derivatives with aryl hydrazine under basic conditions is described.

Generation of Aryl Radicals from Aryl Halides: Rongalite-Promoted Transition-Metal-Free Arylation

A new and practical method for the generation of aryl radicals from aryl halides is reported. Rongalite as a novel precursor of super electron donors was used to initiate a series of electron-catalyzed reactions under mild conditions. These transition-metal-free radical chain reactions enable the efficient formation of C-C, C-S, and C-P bonds through homolytic aromatic substitution or S_RN1 reactions. Moreover, the synthesis of antipsychotic drug Quetiapine was performed on gram scale through the described method. This protocol demonstrated its potential as a promising arylation method in organic synthesis.

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.

KOtBu as a Single Electron Donor? Revisiting the Halogenation of Alkanes with CBr₄ and CCl₄

The search for reactions where KO^tBu and other tert-alkoxides might behave as single electron donors led us to explore their reactions with tetrahalomethanes, CX₄, in the presence of adamantane. We recently reported the halogenation of adamantane under these conditions. These reactions appeared to mirror the analogous known reaction of NaOH with CBr₄ under phase-transfer conditions, where initiation features single electron transfer from a hydroxide ion to CBr₄. We now report evidence from experimental and computational studies that KO^tBu and other alkoxide reagents do not go through an analogous electron transfer. Rather, the alkoxides form hypohalites upon reacting with CBr₄ or CCl₄, and homolytic decomposition of appropriate hypohalites initiates the halogenation of adamantane.

Super electron donors derived from diboron

Single-electron transfer is an important process in organic chemistry, in which a single-electron reductant (electron donor) acts as a key component. Compared with metal-based electron donors, organic electron donors have some unique advantages, such as tunable reduction ability and mild reaction conditions. The development of novel organic electron donors with good reduction ability together with ease of preparation is in high demand. Based on the pyridine-catalyzed radical borylation reaction developed in our laboratory, we have discovered that, the reaction system consisting of a diboron(4) compound, methoxide and a pyridine derivative could smoothly produce super electron donors in situ. Two boryl-pyridine based species, the major one being a trans-2H,2'H-[2,2'-bipyridine]-1,1'-diide borate complex and the minor one being a pyridine radical anion-borate complex, were observed and carefully characterized. These complexes were found to be organic super electron donors unprecedented in literature, and their formation mechanisms were studied by DFT calculations. The diboron/methoxide/pyridine system enables the preparation of organic super electron donors from easily accessible starting materials under mild conditions, which has the potential to be a general and practical single-electron reducing agent in organic synthesis.

Radical Arylation of Anilines and Pyrroles via Aryldiazotates

The radical arylation of anilines and pyrroles can be achieved under transition-metal- and catalyst-free conditions by using aryldiazotates in strongly alkaline aqueous solutions. The aryldiazotates act as protected diazonium ions, which do not undergo azo coupling with electron-rich aromatic substrates, but can still serve as an aryl radical source at slightly elevated temperatures. Based on an improved preparation of aryldiazotates in aqueous solution, homolytic aromatic substitutions of anilines and pyrroles were conducted with good overall yields and high regioselectivity. Moreover, DFT calculations provided further mechanistic insights.

Direct arylation of inactivated benzene with aryl acyl peroxides toward biaryls

A direct arylation of inactivated benzene with acyl peroxides toward biaryls under metal-free, base-free and additive-free conditions was developed.

Direct arylation catalysis with chloro[8-(dimesitylboryl)quinoline-κN]copper(I)

We report direct arylation of arylhalides with unactivated sp² C–H bonds in benzene and naphthalene using a copper(I) catalyst featuring an ambiphilic ligand, (quinolin-8-yl)dimesitylborane. Direct arylation could be achieved with 0.2 mol % catalyst and 3 equivalents of base (KO(t-Bu)) at 80 °C to afford TON ≈160–190 over 40 hours.

N-Methylanilines as Simple and Efficient Promoters for Radical-Type Cross-Coupling Reactions of Aryl Iodides

Activation of the carbon-halogen bonds in aryl halides is a key step in transition-metal-free cross-coupling reactions. In this paper, a new and efficient radical initiation system for the activation of iodoarenes to produce aryl radicals was discovered, which employs the combination of N-methylanilines and tBuOK. This radical initiation system is robust and versatile, enabling various types of aryl-radical-related reactions.

Formation of 1,10-Phenanthroline-N,N'-dioxide under Mild Conditions: The Kinetics and Mechanism of the Oxidation of 1,10-Phenanthroline by Peroxomonosulfate Ion (Oxone)

This paper confirms the unexpected formation of 1,10-phenanthroline-N,N'-dioxide (phenO2) when 1,10-phenanthroline (phen) is oxidized by peroxomonosulfate ion (PMS) in a neutral aqueous solution. The kinetics of oxidation of phen by PMS features a complex pH dependence. In 1.00 M H2SO4, 1,10-phenanthroline-mono-N-oxide (phenO) is the sole product of the reaction. The rate of the N-oxidation is highly dependent on pH with a maximum at pH ∼6.7. The formation of phenO occurs via two parallel pathways: the rate constant of the oxidation of phen (k = 3.1 ± 0.1 M(-1) s(-1)) is significantly larger than that of Hphen(+) [k = (4.1 ± 0.3) × 10(-3) M(-1) s(-1)] because the two N atoms are open to oxidative attack in the deprotonated substrate while an internal hydrogen bond hinders the oxidation of the protonated form. With an excess of PMS, four consecutive oxidation steps were found in nearly neutral solutions. In the early stage of the reaction, the stepwise oxidation results in the formation of phenO, which is converted into phenO2 in the second step. The formation of phenO2 was confirmed by (1)H NMR and ESI-MS methods. The results presented here offer the possibility of designing an experimental protocol for preparing phenO2.

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.