Recent Progress in Methyl-Radical-Mediated Methylation or Demethylation Reactions

CH3 Radical Generation in Microplasmas for Up-Conversion of Methane

The conversion of methane, CH4, into higher value chemicals using low temperature plasmas is challenged by both improving efficiency and selectivity. One path toward selectivity is capturing plasma-produced methyl radicals, CH3, in a solvent for aqueous processing. Due to the rapid reactions of methyl radicals in the gas phase, the transport distance from the production of the CH3 to its solvation should be short, which then motivates the use of microplasmas. The generation of CH3 in Ar/CH4/H2O plasmas produced in nanosecond pulsed dielectric barrier discharge microplasmas is discussed using results from a computational investigation. The microplasma is sustained in the channel of a microfluidic chip in which the solvent flows along one wall or in droplets. CH3 is primarily produced by electron-impact of and dissociative excitation transfer to CH4, as well as CH2 reacting with CH4. CH3 is rapidly consumed to form C2H6 which, in spite of being subject to these same dissociative processes, accumulates over time, as do other stable products including C3H8 and CH3OH. The gas mixture and electrical properties were varied to assess their effects on CH3 production. CH3 production is largest with 5% CH4 in the Ar/CH4/H2O mixture due to an optimal balance of electron-impact dissociation, which increases with CH4 percentage, and dissociative excitation transfer and CH2 reacting with CH4, which decreases with CH4 percentage. Design parameters of the microchannels were also investigated. Increasing the permittivity of the dielectrics in contact with the plasma increased the ionization wave intensity, which increased CH3 production. Increased energy deposition per pulse generally increases CH3 production as does lengthening pulse length up to a certain point. The arrangement of the solvent flow in the microchannel can also affect the CH3 density and fluence to the solvent. The fluence of CH3 to the liquid solvent is increased if the liquid is immersed in the plasma as a droplet or is a layer on the wall where the ionization wave terminates. The solvation dynamics of CH3 with varying numbers of droplets was also examined. The maximum density of solvated methyl radicals CH3aq occurs with a large number of droplets in the plasma. However, the solvated CH3aq density can rapidly decrease due to desolvation, emphasizing the need to quickly react with the solvated species in the solvent.

[Cu]/NFSI-Mediated Cascade Diels-Alder Radical Annulations Using Norbornene as H-Acceptor under Redox-Neutral Conditions

An oxidative cascade [4 + 2] radical cycloaddition/dehydroaromatization reaction of aryl alkenes to access α-aryl substituted naphthalenes under redox-neutral conditions was achieved. This reaction was found to require the addition of [Cu] catalyst along with stoichiometric concentrations of NFSI as a trigger of radical series of steps. Norbornene (NBE), rather than the conventional oxidant, manifested optimal performances as a H-acceptor in this procedure. The results herein might shed encouraging insight into the transition-metal-catalyzed dehydrogenative C-H activation protocols.

C-O Coupling/[4+2] Cycloaddition Tandem Reactions via Oxidative Dearomatization of BINOLs: Access to Bridged Polycyclic Compounds

Intramolecular C-H activation/C-O coupling, dearomatization, and [4+2] cycloaddition of BINOL units have been well developed in a one-pot approach with maleimide derivatives as the dienophiles. This tandem catalytic system generates a variety of functionalized bridged polycyclic products in a step-economical manner, which greatly enriches the modification methods and strategies for the BINOL skeletons.

Metal-Free Carbon-Based Covalent Organic Frameworks with Heteroatom-Free Units Boost Efficient Oxygen Reduction

Accurate identification of carbon-based metal-free electrocatalyst (CMFE) activity and enhancing their catalytic efficiency for O₂ conversion is an urgent and challenging task. This study reports a promising strategy to simultaneously develop a series of covalent organic frameworks (COFs) with well-defined heterocyclic-free biphenyl or fluorenyl units. Unlike heteroatom doping, the developed method not only supplies methyl-induced molecular configuration to promote activity, but also provides a direct opportunity to identify heteroatom-free carbon active centers. The introduction of methyl groups (MGs) with reversible valence bonds into a pristine biphenyl-based COF results in an excellent performance with a half-wave potential of 0.74 V versus the reversible hydrogen electrode (RHE), which is among the highest values for CMFE-COFs as oxygen reduction reaction (ORR) electrocatalysts. Combined with in situ Raman spectra and theoretical calculations, the MG-bound skeleton (DAF-COF) is found to produce ortho activation, confirming the ortho carbon (site-5) adjacent to MGs as active centers. This may be attributed to the opening and binding of MGs, which effectively regulate the molecular configuration and charge redistribution, as well as improve charge transfer and reduce the energy barrier. This study provides insight into the design of highly efficient metal-free organic electrocatalysts via the regulation of valence bonds.

Theoretical Study for Evaluating and Discovering Organic Hydride Compounds as Potential Novel Methylation Reagents

Methylation reaction is a fundamental chemical reaction that plays an important role in the modification of drug molecules, DNA, as well as proteins. This work focuses on seeking potential novel methylation reagents through a systematic investigation of the thermodynamics and reactivity of methyl-substituted organic hydride radical cations (XH^•+s). In this work, 45 classical and important XH^•+s were designed to investigate the relationship between their structure and reactivity, to find excellent or potential methylation reagents. The Gibbs free energy and activation free energy of XH^•+ to release the methyl radical in MeCN at 298.15 and 355 K are calculated with the density functional theory (DFT) method to quantitatively measure the reactivity of XH^•+ as a methylation reagent in this work. The relationships between structures and reactivities on XH^•+s as methylation reagents are well examined. Since we have calculated the Gibbs free energy and activation free energy of trifluoromethyl-substituted organic hydride compound radical cations (X'H^•+) releasing trifluoromethyl radicals in MeCN with the DFT method in our previous work, accordingly, the relationship of thermodynamics and reactivity between X'H^•+ releasing trifluoromethyl radical and XH^•+ releasing methyl radical is discussed in detail. Excitingly, 4 XH^•+s (1H^•+, 3H^•+∼4H^•+, and 44H^•+) are found to be excellent methyl radical reagents, while 9 XH^•+s (5H^•+, 6H^•+, 9H^•+, 10H^•+, 12H^•+, 13H^•+, 15H^•+, 43H^•+, and 45H^•+) are found to be potential methyl radical reagents in chemical synthesis. The molecular library and reactivity database of novel methylation reagents could be established for synthetic chemists to query and use. Our work may offer a theoretical basis and reference experience for screening different substituted organic hydride compounds (YRHs) as alkylation reagents.

Silyl Radical Cascade Cyclization of 2-Isocyanothioanisole toward 2-Silylbenzothiazoles through Radical Initiator-Inhibitor Symbiosis

A demethylative silyl radical cascade cyclization of 2-isocyanothioanisoles toward 2-silylated benzothiazole building blocks has been developed. The development of a "radical initiator-inhibitor symbiosis" system solves the challenge of otherwise dominant methyl radical-triggered side reactions brought about by kinetically unfavored generation of reactive silyl radical species. The products accessed in this protocol are amendable to various downstream functionalization reactions, including the quick construction of a topoisomerase II inhibitor via a Hiyama cross-coupling reaction and of an antiviral agent via a fluoride-/hydroxide-free nucleophilic substitution to acyl chloride.

Rhodium-Catalyzed C-H Methylation and Alkylation Reactions by Carbene-Transfer Reactions

In this combined computational and experimental study, the C-H functionalization of 2-phenyl pyridine with diazoalkanes was investigated. Initial evaluation by computational methods allowed the evaluation of different metal catalysts and diazoalkanes and their compatibility in this C-H functionalization reaction. With these findings, suitable reaction conditions for the C-H methylation reactions were quickly identified by using highly reactive TMS diazomethane and C-H alkylation reactions with donor/acceptor diazoalkanes, which is applied to a broad scope on alkylation reactions of 2-aryl pyridines with TMS diazomethane and donor/acceptor diazoalkane (51 examples, up to 98 % yield).

Palladium-catalyzed nucleomethylation of alkynes for synthesis of methylated heteroaromatic compounds

Herein, we disclosed a novel and efficient palladium-catalyzed nucleomethylation of alkynes for the simultaneous construction of the heteroaromatic ring and methyl group. The 3-methylindoles, 3-methylbenzofurans and 4-methylisoquinolines were obtained in moderate to excellent yields. Notably, this methodology was employed as a key step for synthesis of a pregnane X receptor antagonist, zindoxifene, bazedoxifene and AFN-1252. The kinetic studies revealed that reductive elimination might be the rate-determining step.

A novel palladium-catalyzed nucleomethylation of alkynes is developed, affording 3-methylindoles, 3-methylbenzofurans and 4-methylisoquinolines in moderate to excellent yields.

Recent advance in the reactions of silacyclobutanes and their applications

Silacyclobutanes (SCBs), as a key member of organosilicon family, have received considerable attention in synthetic chemistry since the silicon-carbon bond can be activated. Followed by ring-opening and ring expansion process,...

A photocatalytic radical relay reaction of 2-methylthiolated phenylalkynones and potassium metabisulfite

The generation of methylsufonyl-containing thioflavones through a radical relay reaction of methylthiolated phenylalkynones and potassium metabisulfite in the presence of sodium methylsulfinate under visible light irradiation is developed.