The 50 most-cited articles in Organic Letters, 2018

)-H/N-H cross-dehydrogenative coupling (CDC)-amination with both primary and secondary aliphatic amines at room temperature in air is developed. This photocatalytic reaction allows the direct formation of 3-aminoquinoxalin-2(1 H)-ones via CDC-amination in the absence of any external oxidant added from outside. Preliminary mechanistic studies reveal that the present reaction proceeds through a radical process.

An efficient synthesis of cyanoalkylated heteroarenes via iron-catalyzed direct C-H cyanoalkylation of heteroarenes has been developed. Structurally diverse cyanoalkyl motifs generated through C-C bond cleavage of cyclobutanone oxime esters have been introduced into quinoxalin-2(1H)-ones, flavone, benzothiazoles, and caffeine in good to excellent yields. Remarkably, less-strained cyclopentanone and unstrained cyclohexanone oxime esters were also amenable substrates in this cyanoalkylation reaction.

in combination with 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos) under irradiation of blue LEDs efficiently catalyzes a decarboxylative Heck reaction of vinyl arenes and vinyl heteroarenes with aliphatic N-(acyloxy)phthalimides at room temperature. A broad scope of secondary, tertiary, and quaternary carboxylates, including α-amino acid derived esters, can be applied as amenable substrates with high stereoselectivity. The experimental observation was explained by excitation-state reactivity of the palladium complex under irradiation to induce single-electron transfer to activate N-(acyloxy)phthalimides, and to suppress undesired β-hydride elimination of alkyl palladium intermediates.

A visible-light-initiated α-perfluoroalkyl-β-heteroarylation of various alkenes with perfluoroalkyl iodides and quinoxalin-2(1 H)-ones is presented. This three-component radical cascade reaction allows an efficient synthesis of a range of perfluoroalkyl containing quinoxalin-2(1 H)-one derivatives in moderate to excellent yields under mild conditions. Reactions proceed via acidic aminyl radicals that are readily deprotonated to give the corresponding radical anions able to sustain the radical chain as single electron transfer reducing reagents. Hence, the overall cascade classifies as an electron-catalyzed process.

Abstract An electrochemically driven, nickel-catalyzed reductive coupling of N-hydroxyphthalimide esters with aryl halides is reported. The reaction proceeds under mild conditions in a divided electrochemical cell and employs a tertiary amine as the reductant. This decarboxylative C(sp3)–C(sp2) bond-forming transformation exhibits excellent substrate generality and functional group compatibility. An operationally simple continuous-flow version of this transformation using a commercial electrochemical flow reactor represents a robust and scalable synthesis of value added coupling process.

The efficient Pd-catalyzed Heck reaction of diverse tertiary alkyl halides with alkenes has been developed. Unactivated tertiary alkyl halides efficiently react at room temperature under visible light irradiation with no exogenous photosensitizers required. For activated tertiary alkyl halides, the same catalytic system works well without light. These methods offer a general access to electronically diverse alkenes possessing quaternary and functionalized tertiary allylic carbon centers. The substituents at these centers include alkyl-, carbalkoxy-, tosyl-, phosphonyl-, and boronate groups. It was also shown that the end-game mechanism of this transformation may vary depending on the type of the substrates used.

Electrochemical generation of N -acyloxy amidyl radicals via an inner-sphere electron-transfer process is described for the first time. With NaBr as the catalyst and electrolyte, the in situ generated amidyl radicals undergo intramolecular C(sp 2 /sp 3 )–H aminations to give lactams with unprecedented regio- and chemoselectivities. Moreover, the synthetic utility of current method is demonstrated by the synthesis of PJ34 and Phenaglaydon .

A general, Brønsted acid catalyzed method for the room temperature, nucleophilic ring opening of donor-acceptor cyclopropanes in fluorinated alcohol solvent, HFIP, is described. Salient features of this method include an expanded cyclopropane scope, including those bearing single keto-acceptor groups and those bearing electron-deficient aryl groups. Notably, the catalytic system proved amenable to a wide range of nucleophiles including arenes, indoles, azides, diketones, and alcohols.

Challenging couplings of hindered carboxylic acids with non-nucleophilic amines to form amide bonds can be accomplished in high yields, and in many cases, with complete retention of the adjacent stereogenic centers using the combination of N, N, N', N'-tetramethylchloroformamidinium hexafluorophosphate (TCFH) and N-methylimidazole (NMI). This method allows for in situ generation of highly reactive acyl imidazolium ions, which have been demonstrated to be intermediates in the reaction. The reagent delivers high reactivity similar to acid chlorides with the ease of use of modern uronium reagents.

The directly external oxidant-free oxytrifluoromethylation and aminotrifluoromethylation of alkenes have been developed through the constant current electrolysis synergizing with a Lewis-acid catalysis protocol. By using sodium trifluoromethanesulfinate as the trifluoromethyl source, the method enabled difunctionalization of C-C double bonds of a wide range of styrene derivatives.

A rhodium-catalyzed annulation between ethyl benzimidates and α- aroyl sulfur ylides was developed, affording a series of pyrano[4,3,2-ij]isoquinoline derivatives in moderate to good yields with good functional group compatibility. The procedure featured dual ortho-C-H functionalization and dual cyclization in one pot. The optoelectronic properties of those fused heteroarenes were tested by UV/vis and fluorescence spectrometers.

/L1 system is reported. This nickel-catalyzed reaction could be performed in gram scale and successfully applied in the synthesis of donepezil (Alzheimer's drug) and functionalization of steroid hormones and fatty acid derivatives. Synthesis of N-heterocycles, methylation of ketones, and one-pot double alkylation to bis-hetero aryl ketones using two different alcohols with a single catalyst broadens the scope of the catalytic protocol. Preliminary mechanistic studies using defined Ni-H species and deuterium-labeling experiments established the participation of the borrowing-hydrogen strategy.

With a dual organocatalytic system involving a chiral phosphoric acid and a dicyanopyrazine-derived chromophore (DPZ) photosensitizer and under the irradiation with visible light, an enantioselective Minisci-type addition of α-amino acid-derived redox-active esters (RAEs) to isoquinolines has been developed. A variety of prochiral α-aminoalkyl radicals generated from RAEs were successfully introduced on isoquinolines, providing a range of valuable α-isoquinoline-substituted chiral secondary amines in high yields with good to excellent enantioselectivities.

A unified strategy for intermolecular remote C(sp 3 )–H and C–C vinylation of O -acyl oximes with vinyl boronic acids has been achieved. This strategy is enabled by photoreductive generation of iminyl radicals from O -acyl oximes under irradiation by visible light. The translocated carbon-centered radicals, which are generated from the iminyl radicals through 1,5-hydrogen atom transfer or C–C cleavage, can be vinylated with vinyl boronic acids. This strategy opens up a new approach to remote functionalization via C(sp 3 )–H and C–C cleavage and provides an efficient and versatile solution to the synthesis of γ-vinylation of ketones and nitriles.

One hexanuclear Cu(I) cluster of 4,6-dimethylpyrimidine-2-thiolate efficiently catalyzes the dehydrogenative cross-coupling of secondary and primary alcohols to α-alkylated ketones with high selectivity. This transformation proceeds through a one-pot sequence of dehydrogenation of alcohols, condensation of aldehydes and ketones, hydrogenation of the resulting α,β-unsaturated ketones, and dehydrogenation of the α-alkylated alcohols to generate α-alkylated ketones. This catalytic system also displays high activity for the annulation reaction of secondary alcohols with γ-amino- and 2-aminobenzyl alcohols to yield pyridines and quinolines, respectively.

An iron(0) complex bearing a cyclopentadienone ligand catalyzed N-methylation and N-ethylation of aryl and aliphatic amines with methanol or ethanol in mild and basic conditions through a hydrogen autotransfer borrowing process is reported. A broad range of aromatic and aliphatic amines underwent mono- or dimethylation in high yields. DFT calculations suggest molecular hydrogen acts not only as a reducing agent but also as an additive to displace thermodynamic equilibria.

The design, synthesis, and application of [4-(acetylamino)phenyl]imidodisulfuryl difluoride (AISF), a shelf-stable, crystalline reagent for the synthesis of sulfur(VI) fluorides, is described. The utility of AISF is demonstrated in the synthesis of a diverse array of aryl fluorosulfates and sulfamoyl fluorides under mild conditions. Additionally, a single-step preparation of AISF was developed that installed the bis(fluorosulfonyl)imide group on acetanilide utilizing an oxidative C-H functionalization protocol.

This paper reports that cis-TPE dicycles emit strong fluorescence, while the gem dicycles show almost no emission in solution, demonstrating that the free rotation restriction of the double bond at the excited state is the key factor for AIE effects.

A general and efficient protocol for direct C-H alkylation and acylation of N-heterocycles, using readily accessible carboxylic acids as radical precursors under visible-light irradiation without a photocatalyst and an additional acid additive, has been developed. This protocol provides expedient access to substituted N-heterocycles under mild and metal-free conditions. Mechanistic experiments indicate that this reaction proceeds through a visible-light-initiated radical chain propagation mechanism.

A metal-free photoredox C-H alkylation of heteroaromatics from readily available carboxylic acids using an organic photocatalyst and hypervalent iodine reagents under blue LED light is reported. The developed methodology tolerates a broad range of functional groups and can be applied to the late-stage functionalization of drugs and drug-like molecules. The reaction mechanism was investigated with control experiments and photophysical experiments as well as DFT calculations.

Two catalyst systems are described, which together provide mild and general conditions for the Pd-catalyzed C-O cross-coupling of primary alcohols. For activated substrates, such as electron-deficient aryl halides, the commercially available ligand L2 promotes efficient coupling for a variety of alcohol nucleophiles. In the case of unactivated electrophiles, such as electron-rich aryl halides, the new ligand L8 was developed to improve these challenging C-O bond-forming reactions.

A nickel-catalyzed cross-coupling of benzylic pyridinium salts with arylboronic acids was developed. Coupled with chemoselective pyridinium formation, this method allows benzyl primary amines to be efficiently converted to di(hetero)arylmethanes. Excellent heteroaryl and functional group tolerance is observed, and a one-pot procedure enables benzylic amines to be converted to diarylmethanes directly.

A selective oxy-alkylation of allylamines with unactivated alkyl bromides and CO 2 via visible-light-driven palladium catalysis is reported. The commercially available Pd(PPh 3 ) 4 is used as the sole catalyst in this three-component reaction. A variety of tertiary, secondary, and primary alkyl bromides undergo reactions to generate important 2-oxazolidinones in high yields and selectivity. The mild reaction conditions, easy scalability, and facile derivatization of products provide great potential for application in organic synthesis and pharmaceutical chemistry.

A novel method to access 1,1-diarylalkanes from readily available, nonactivated alkyl bromides and aryl bromides via visible-light-driven nickel and iridium dual catalysis, wherein diisopropylamine ( i Pr 2 NH) is used as the terminal stoichiometric reductant, is reported. Both primary and secondary alkyl bromides can be successfully transformed into the migratory benzylic arylation products with good selectivity. Additionally, this method showcases tolerance toward a wide array of functional groups and the presence of bases.

Facile preparation of the α- C-vinyl and -aryl glycosides has been developed via mild Ni-catalyzed reductive vinylation and arylation of C1-glycosyl halides with vinyl/aryl halides. Good to high α-selectivities were achieved for C-glucosides, galactosides, maltoside, and mannosides, which were dictated by the employment of pyridine type ligands. As such, the present work represents unprecedented control for a high level of α-selectivity for C-vinyl-glucosides using cross-coupling approaches and offers hitherto optimal α-selective preparation of C-aryl glucosides via catalyst-controlled coupling strategies.

Abstract A method using calcium triflimide [Ca(NTf2)2] as a Lewis acid to activate sulfonyl fluorides toward nucleophilic addition with amines is described. The reaction converts a wide array of sterically and electronically diverse sulfonyl fluorides and amines into the corresponding sulfonamides in good yield.

A pair of chiral binaphthyl enantiomers ( S -/ R - 6 ) incorporating a tetraphenylethene (TPE) moiety as an aggregation-induced emission (AIE) active group exhibits bright yellow circularly polarized electroluminescence (CP-EL) emission with a remarkable g EL value, low turn-on voltage, and high brightness in the nondoped CP organic light emitting diodes (CP-OLEDs). This work provides a new strategy to develop doping-free CP-OLED materials.

A metal-free visible-light-promoted oxidative coupling between thiols and arylhydrazines has been developed to afford diaryl sulfides using a catalytic amount of rose bengal as photocatalyst under aerobic conditions. A library of unsymmetrical diaryl sulfides with broad functionalities was synthesized in good yields at room temperature. The present methodology is also applicable to benzo[ d]thiazole-2-thiols, benzo[ d]oxazole-2-thiol, 1 H-benzo[ d]imidazole-2-thiols, and 1 H-imidazole-2-thiol.

A novel and practical method for the selenosulfonation of alkynes with the insertion of sulfur dioxide has been developed. A series of β-(seleno)vinyl sulfones with high levels of regio- and stereoselectivity have been prepared. The key features of this reaction include a broad substrate scope, excellent functional-group tolerance, and amenability to scale-up synthesis. A plausible radical mechanism is proposed to illustrate this reaction.

A cascade radical cyclization of 2-isocyanoaryl thioethers with H -phosphorus oxides, organoboronic acids, or alkyl radical precursors has been efficiently developed, providing a novel and highly efficient methodology to structurally diverse C2-substituted benzothiazole derivatives with broad functional group tolerance and good yields. This cascade radical process achieves the first cycloaddition of an imidoyl radical from isocyanide to sulfur atom, rending C(sp 2 )–S bond formation.

A nickel-catalyzed cross-electrophile coupling reaction between benzyl alcohols and aryl halides has been developed using a homolytic C–O bond cleavage protocol that has recently been established. The treatment of a benzyl alcohol and aryl halide with a nickel catalyst and low-valent titanium reagent generated from TiCl 4 (lutidine) (lutidine = 2,6-lutidine) and manganese powder afforded the cross-coupled product in high yield. A mechanistic study indicated the intermediacy of the benzyl radicals that originate from the benzyl alcohols.

Contrary to the accepted convention, this work shows that Minisci-type C-H alkylation does not require any metal, photocatalyst, light, or prefunctionalization of the readily available and inexpensive carboxylic acids to proceed well under mild conditions. These mild conditions can be utilized for late-stage alkylations of complex molecules, including pharmaceutical compounds and light-sensitive compounds which degrade under photocatalytic conditions.

A resource saving protocol for the synthesis of 5-fluoromethyl-2-oxazolines by using electrochemistry has been realized. Thereby, a hypervalent iodine species I(III) is generated by anodic oxidation in the presence of Et 3 N·5HF and mediates the cyclization of N -allylcarboxamide to 5-fluoromethyl-2-oxazoline. This method allows application to various substrates furnishing the 2-oxazolines with yields up to 68%. The protocol is easy to conduct under constant current conditions offering a sustainable alternative over conventional reagent-based pathways.

An unprecedented N-heterocyclic carbene-catalyzed annulation of isatin-derived enals and o-hydroxyphenyl-substituted p-quinone methides as bifunctional reagents has been discovered. The new protocol involves a 1,6-addition of the homoenolate equivalent intermediates to the hydroxy donor-1,6-Michael acceptors and leads to spirocyclic oxindole-ε-lactones in high yields and very good stereoselectivities.

An iron-catalyzed redox-neutral, decarboxylative olefination of cycloketone oxime esters with α,β-unsaturated carboxylic acids has been developed. This reaction involves an iminyl radical mediated C-C bond cleavage/radical addition/decarboxylation cascade. This protocol is highlighted by its low-cost catalytic system and readily accessible starting materials, as well as broad substrate scope, thus providing facile access to structurally diverse cyano-containing alkenes.

A direct acyl radical addition to 2 H-indazoles has been achieved for the first time, where the less-aromatic quinonoid 2 H-indazoles readily accepted radical species to the C-3 position. Motivated by the lack of direct acylation strategy for 2 H-indazoles, the current method utilizes the radical acceptability of 2 H-indazoles, discovering an ambient temperature reaction to provide facile access to a diverse array of 3-acyl-2 H-indazoles with three points of structural diversification in 25%-83% yields.

A catalyst-free, photochemical oxidative annulation reaction between dialkylanilines and maleimides to generate tetrahydroquinolines is presented. The reaction is driven by the photochemical activity of an electron donor-acceptor (EDA) complex and has a broad substrate scope with the corresponding products isolated in good to excellent yields. Photochemical characterization of the EDA complex and a mechanistic rational is provided.

Enantioselective dearomative [3 + 2] cycloaddition reaction of 2-nitrobenzofurans with 3-isothiocyanato oxindoles was developed. The reaction employs a chiral bis(oxazoline)/Zn(OTf) 2 catalyst, allowing a practical, straightforward access to structurally diverse spirooxindoles containing a 2,3-dihydrobenzofuran motif and three contiguous stereocenters with excellent diastereo- and enantioselectivities. The synthetic potentials of the method have been demonstrated by the scale-up experiment and transformations of the products. The preliminary mechanism was investigated with experimental observations, nonlinear effects studies, and MS experiments.

A net redox-neutral method for the decarboxylative alkylation of heteroarenes using photoredox catalysis is reported. Additionally, this method features the use of simple, commercially available carboxylic acid derivatives as alkylating agents, enabling the facile alkylation of a variety of biologically relevant heterocyclic scaffolds under mild conditions.

A novel protocol for a significantly improved, practical, and chemoselective ammonia-free Birch reduction mediated by bench-stable sodium dispersions and recoverable 15-crown-5 ether is reported. A broad range of aromatic and heteroaromatic compounds is reduced with excellent yields.

A simple and efficient Cu-catalyzed decarboxylative/click reaction for the preparation of 1,4-disubstituted 5-arylselanyl-1,2,3-triazoles from propiolic acids, diselenides, and azides has been developed. The mechanistic study revealed that the intermolecular AAC reaction of an alkynyl selenium intermediate occurred. The resulting multisubstituted 5-seleno-1,2,3-triazoles were tested for in vitro anticancer activity by MTT assay, and compounds 4f, 4h, and 4p showed potent cancer cell-growth inhibition activities.

A chemodivergent copper-catalyzed ring opening of cycloketone oximes via radical-mediated C-C bond cleavage under redox-neutral conditions is described. This method allows the divergent synthesis of γ- and δ-acyloxylated, alkoxylated, and hydroxylated nitriles while avoiding the use of toxic cyanide reagents. Moreover, these reactions proceed under very mild conditions with good functional group tolerance. Notably, ring-opening reactions of the less-strained substrate cyclopentanone oxime also proceeded well under the established conditions.

)-H bond in tertiary β-enaminones is accomplished under catalyst-free conditions and ambient temperature by using aroyl peroxides as coupling partners. By means of a thermoinduced free-radical pathway, the present method enables facile and efficient synthesis of both acyloxylated chromones and enaminones.

Donor-acceptor cyclopropanes were reacted under Lewis acid catalysis with 3-thioxocyclobutanones as surrogates for disubstituted thioketenes. A broad scope of 2-substituted tetrahydrothiophenes with a semicyclic double bond was obtained under mild conditions with high functional group tolerance and in excellent yield. A sequence of a formal [3 + 2]-cycloaddition followed by the subsequent release of disubstituted ketene is postulated as the mechanism.

An efficient electrophilic persulfuration reaction leading to unsymmetrical disulfides and polysulfides has been developed. Various nucleophiles, including aryl boronic acids, β-keto esters, and thiols, can be used as substrates. The notable features of this method include very simple and practical conditions, general scope, and inexpensive copper catalysts.

New types of [2.2]paracyclophane derivatives, g -BNMe 2 -Cp and m -BNMe 2 -Cp, in which electron-donating NMe 2 and the electron-accepting BMes 2 are introduced at the pseudo -gem and pseudo -meta positions, were designed and synthesized. The efficient through-space charge transfer enables the intense fluorescence with thermally activated delayed fluorescence characteristics. The quantum yields are up to 0.72 and 0.39 in cyclohexane. In addition, no significant fluorescence quenching was observed in the solid state with fluorescence quantum yields of powder up to 0.53 and 0.33. Moreover, the enantiomerically pure forms of g -BNMe 2 -Cp exhibit strong CPL signals with g lum up to 4.24 × 10 –3 .

Photoredox-catalyzed decarboxylative alkylation of silyl enol ethers has been developed. Diverse functionalized aryl alkyl ketones were afforded in modest to good yields using N-(acyloxy)phthalimide as an easy access alkyl radical source under mild and operationally simple conditions. The excellent performance of drug molecules such as fenbufen and indomethacin and naturally occurring carboxylic acids such as stearic acid and dehydrocholic acid further demonstrated the practicability of the reaction.

This work describes a straightforward access to a large variety of aryl phosphonates by the simple combination of diaryliodonium salts with phosphites in the presence of a base and under visible-light illumination. The reaction proceeds smoothly, tolerates various functionalities, and was applied for the synthesis of pharmaceutically relevant compounds. Mechanistic investigations, including EPR, NMR, and DFT calculations, support the postulated reaction mechanism.

A reductive cross-coupling of gem -difluoroalkenes with diverse unactivated and heteroatom substituted olefins through a Fe-catalyzed hydrogen atom transfer (HAT) strategy is reported. Different from the previous HAT-type olefin cross-coupling reactions, the presence of a fluorine atom in the molecule results in a stereoselective β-F cleavage, leading to a C(sp 2 )–C(sp 3 ) bond formation. A wide variety of alkylated monofluoroalkenes were obtained in good efficiency with excellent Z selectivity under air- and water-tolerant reaction conditions. A similar defluorinative coupling reaction of monofluoroalkenes was also realized.

We report a system with ethylene-bridged flavinium salt 2b which catalyzes the aerobic oxidation of toluenes and benzyl alcohols with high oxidation potential ( E ox > +2.5 V vs SCE) to give the corresponding benzoic acids under visible light irradiation. This is caused by the high oxidizing power of excited 2b ( E ( 2b* ) = +2.67 V vs SCE) involved in photooxidation and by the accompanying dark organocatalytic oxygenation provided by the in situ formed flavin hydroperoxide 2b-OOH .