The 50 most-cited articles in Organic Letters, 2017

A new protocol for photochemical perfluoroalkylation reactions using perfluoroalkyl iodide, amine additive, and THF solvent is reported. This protocol does not require a photoredox catalyst and proceeds at ambient temperature with irradiation from a compact fluorescent lamp, low-intensity UV lamp, or sunlight. This protocol can be applied to the synthesis of perfluoroalkyl-substituted phenanthridines as well as effect the iodo-perfluoroalkylation of alkenes/alkynes and the C-H perfluoroalkylation of electron-rich arenes and heteroarenes. This C-H perfluoroalkylation reaction offers a unique method for site-selective labeling of oligopeptides at the tryptophan residue.

The conventional methods for preparing alkyl boronates often necessitate anhydrous and demanding reaction conditions. Herein, a new, operationally simple decarboxylative borylation reaction of readily available aliphatic acid derivatives under additive-free visible-light photoredox conditions in nonanhydrous solvents has been described. Primary and secondary alkyl boronates or tetrafluoroborates with various functional groups were prepared accordingly. A catalytic cycle involving alkyl radical reaction with base-activated diboron species has been proposed.

An ionic cobalt-PNP complex is developed for the efficient α-alkylation of ketones with primary alcohols for the first time. A broad range of ketone and alcohol substrates were employed, leading to the isolation of alkylated ketones with yields up to 98%. The method was successfully applied to the greener synthesis of quinoline derivatives while using 2-aminobenzyl alcohol as an alkylating reagent.

Catalytic asymmetric electrochemical C-H functionalization of simple ketones has been developed. The transformation is realized by the combination of electrochemical oxidation and chiral primary amine catalysis. This metal- and oxidant-free method furnishes diverse C1-alkylated tetrahydroisoquinolines in high yields and with excellent enantioselectivities under very mild conditions.

Sulfoxonium ylides act as an efficient carbene precursor in rhodium(III)-catalyzed C-H acylmethlyation of a variety of arenes assisted by different chelating groups, and both aryl- and alkyl-substituted β-carbonyl sulfoxonium ylides are applicable. The system proceeded under redox-neutral conditions with a broad scope, high efficiency, and functional group tolerance.

Bodipy-anthracene dyads with two chromophores assuming orthogonal geometry to enhance the spin-orbital charge-transfer intersystem crossing (SOCT-ISC) were prepared. The photosensitizers show strong absorption of visible light, efficient triplet-state formation (quantum yield 90%), and a long-lived triplet state (85 μs). The dipole moment orientation exerts significant effect on the ISC efficiency. It is also the first time that photosensitizers based on SOCT-ISC were used for triplet-triplet annihilation upconversion. The upconversion quantum yield is up to 15.8%.

A method of synthesizing 3-arylquinoxalin-2(1H)-ones using diaryliodonium tetrafluoroborates under mild conditions is described. This protocol has a wide substrate scope and enables direct C-H functionalization. The synthetic potential of this coupling was explored using a range of readily accessible diaryliodonium salts and quinoxalin-2(1H)-ones.

An enantioselective Ni-catalyzed cross-coupling of N-hydroxyphthalimide esters with vinyl bromides is reported. The reaction proceeds under mild conditions and uses tetrakis(N,N-dimethylamino)ethylene as a terminal organic reductant. Good functional group tolerance is demonstrated, with over 20 examples of reactions that proceed with >90% ee.

Here we describe the development of a method for the Pd-catalyzed electrochemical acetoxylation of C–H bonds. The oxidation step of the catalytic cycle is probed through cyclic voltammetry and bulk electrolysis studies of a preformed palladacycle of 8-methylquinoline. A catalytic system for C–H acetoxylation is then developed and optimized with respect to the cell configuration, rate of oxidation, and chemistry at the counter electrode. This transformation is then applied to substrates containing various directing groups and to the acetoxylation of both C(sp 2 )–H and C(sp 3 )–H bonds.

A very simple and inexpensive catalytic system based on abundant manganese as transition metal and on an inexpensive phosphine-free bidendate ligand, 2-(aminomethyl)pyridine, has been developed for the reduction of a large variety of carbonyl derivatives with 2-propanol as hydrogen donor. Remarkably, the reaction proceeds at room temperature with low catalyst loading (down to 0.1 mol %) and exhibits a good tolerance toward functional groups. High TON (2000) and TOF (3600 h –1 ) were obtained.

Abstract A broadly applicable copper catalyst for photoredox transformations of organic halides is reported. Upon visible light irradiation in the presence of catalytic amounts of [(DPEphos)(bcp)Cu]PF6 and an amine, a range of unactivated aryl and alkyl halides were shown to be smoothly activated through a rare Cu(I)/Cu(I)*/Cu(0) catalytic cycle. This complex efficiently catalyzes a series of radical processes, including reductions, cyclizations, and direct arylation of arenes.

Nonplanar, electronically destabilized amides have emerged as powerful intermediates in organic synthesis. We report a highly selective method for transamidation of common secondary amides under mild, metal-free conditions that relies on transient N-selective functionalization to weaken amidic resonance. The combination of rational modification of the amide bond with nucleophilic addition mechanism, and the thermodynamic collapse of the resultant tetrahedral intermediate constitutes a two-step procedure to accomplish a challenging transamidation of secondary amides under mild conditions.

A constant-current electrochemical method for reducing catalytic nickel complexes in sp 2 -sp 3 cross-electrophile coupling reactions has been developed. The electrochemical reduction provides reliable nickel catalyst activation and turnover and offers a tunable parameter for reaction optimization, in contrast to more standard activated metal powder reductants. The electrochemical reactions give yields (i.e., 51–86%) and selectivities as high or superior to those using metal powder reductants and provide access to a wider substrate scope.

The first ferrous chloride catalyzed boration/β-fluorine elimination of trifluoromethyl alkenes is described. Thus, a full range of gem-difluoroallylboronates were obtained in high yield under mild conditions. As an important fluorinated building block, gem-difluoroallylboronate can be readily converted into diverse difluoro-substituted species.

An electrooxidative direct arylsulfonlylation of ynones with sulfinic acids via a radical tandem cyclization strategy has been developed for the construction of sulfonated indenones under oxidant-free conditions. This method provides a simple and efficient approach to prepare various sulfonylindenones in good to excellent yields, demonstrating the tremendous prospect of utilizing electrocatalysis in oxidative coupling. Notably, this reaction could be easily scaled up with good efficiency.

A practical, electrochemical method is developed for the direct dehydrogenative lactonization of C(sp 2 /sp 3 )–H bonds under external oxidant- and metal-free conditions, delivering diverse lactones, including coumarin derivatives with excellent regioselectivity. The scalable nature of this newly developed electrochemical process was demonstrated on a 40 g scale following an operationally simple protocol. The remote lactonization of C(sp 3 )–H bonds would constitute an important synthetic advance toward electrochemical C–O bond formation.

The first general method is reported for transamidation of secondary carboxamides catalyzed by Pd-NHC (NHC = N-heterocyclic carbene) complexes. Commercially available, air- and moisture-stable (NHC)Pd(R-allyl)Cl complexes can effect C-N cross-coupling of a wide range of N-Boc and N-Ts amides, obtained by selective amide N-functionalization, with non-nucleophilic anilines and sterically hindered amines in very good yields. The first use of versatile Pd-NHC complexes as catalysts is represented for transition-metal-catalyzed C(acyl)-N amination of amides by N-C activation.

An electrochemical C–H acylation of electron-deficient N -heteroarenes with α-keto acids is reported. This first electrochemical Minisci acylation reaction proceeded using NH 4 I as a redox catalyst. A broad N -heteroarene scope and high functional group tolerance are observed. Selective monoacylation of N -heteroarenes is achieved via control of acyl radical at a low concentration. The results of cyclic voltammetry and control experiments disclose that the electrogenerated I 2 is likely the active species to initiate the oxidative decarboxylation of carboxylate anion via an acyl hypoiodite intermediate. The electrochemical Minisci acylation provides a straightforward approach for the late-stage functionalization of pharmacophores.

A class of N -(naphthalen-1-yl)- N ′-alkyl oxalamides have been proven to be powerful ligands, making a coupling reaction of (hetero)aryl iodides with primary amines proceed at 50 °C with only 0.01 mol % of Cu 2 O and ligand as well as a coupling reaction of (hetero)aryl bromides with primary amines and ammonia at 80 °C with only 0.1 mol % of Cu 2 O and ligand. A wide range of coupling partners work well under these conditions, thereby providing an easy to operate method for preparing (hetero)aryl amines.

Direct and efficient synthesis of 1-naphthols has been realized via Rh(III)-catalyzed C-H activation of sulfoxonium ylides and subsequent annulation with alkynes, where the sulfoxonium ylide functioned as a new traceless bifunctional directing group. This reaction occurred under redox-neutral conditions with a broad substrate scope.

A novel and efficient method for the synthesis of ( Z )-fluorinated alkenylboronic acid pinacol esters via Cu-catalyzed stereoselective borylation of gem -difluoroalkenes using bis(pinacolato)diboron (B 2 pin 2 ) as the boron source with the assistance of NaO t Bu and Xantphos at room temperature was developed.

A straightforward protocol for diarylannulated sulfone construction is efficiently established via SO 2 /I exchange of iodonium(III) salts. Readily available inorganic Na 2 S 2 O 5 was served as a safe and convenient SO 2 surrogate. Diverse functionalized diarylannulated sulfones were smoothly achieved in good to excellent yields with great functional group compatibility. Organic light emitting diodes (OLEDs) material molecules were subsequently established via this method in gram scale. The unsymmetrical conjugated systems with donor-acceptor groups and π-conjugation bridges motifs, which substantially communicate electron mobility in semiconductor material molecules, were successfully afforded under the facile conditions of the exchange strategy.

A direct C-H cyanoalkylation of heteroaromatic N-oxides and quinones with cyclobutanone oximes is reported. This redox-neutral, operationally simple cyanoalkylation reaction is successfully amenable to a wide range of heteroaromatic N-oxides, quinones, and cyclobutanone oximes. A novel catalytic system consisting of a nickel source proved crucial for cleavage of the C-C bond of cyclobutanone oximes and for selective C-C bond formation over β-hydride elimination. Mechanistic studies suggest that a radical intermediate might be involved in this transformation.

Palladium-catalyzed arene C(sp 2 )–H acetoxylation has emerged as a powerful tool to construct a carbon–oxygen (C–O) bond. However, the requirement of stoichiometric chemical oxidants for this transformation possesses a significant disadvantage. To solve this fundamental problem, we now report an anodic oxidation strategy to achieve arene C(sp 2 )–H acetoxylation.

In this letter, an efficient method for the photolytic generation of difluoromethyl radicals from [bis(difluoroacetoxy)iodo]benzene reagents is described. The present approach enables the introduction of difluoromethyl groups into various heteroarenes under mild conditions in the absence of any additional reagents or catalysts.

A new type of Cp*Ir complex bearing a functional 2,2′-bibenzimidazole ligand was designed, synthesized, and found to be a highly effective and general catalyst for the N -methylation of a variety of amines with methanol in the presence of a weak base (0.3 equiv of Cs 2 CO 3 ).

Radical alkynyltrifluoromethylation of alkenes with actylenic triflones has been achieved. This radical chain reaction is initiated by a catalytic amount of an electron-donor-acceptor complex composed of Togni's reagent and N-methylmorpholine. This transformation proceeds under exceptionally mild and operationally simple conditions. A variety of alkenes are compatible in this protocol including aliphatic alkenes, vinyl ethers, enecarbamates, styrenes, and even acrylates, providing diverse β- trifluoromethyl alkynes in good to excellent yields.

)-H difluoroalkylations and perfluoroalkylations of alkenes and (hetero)arenes with commercially available fluoroalkyl halides has been developed with a copper-amine catalyst system. This method is characterized by high yields, mild reaction conditions, low-cost catalyst, broad substrate scope, and excellent functional group compatibility, therefore providing a convenient synthetic strategy toward various difluoroalkyl- and perfluoroalkyl-substituted alkenes and (hetero)arenes.

A photocatalytic cascade cyclization of 1,7-enynes with acyl chlorides has been established. This method offers an operationally simple access to diverse fused pyran derivatives with a broad substrate scope in high yields from simple acyl chlorides via an acyl radical intermediate.

This Letter describes the development of a catalytic decarbonylative C-S coupling reaction that transforms thioesters into thioethers. Both Pd- and Ni-based catalysts are developed and applied to the construction of diaryl, aryl alkyl, and heterocycle-containing thioethers.

An effective synthesis of chiral atropoisomeric biaryl phosphine–olefin compounds via palladium-catalyzed enantioselective C–H olefination has been developed for the first time. The reactions are operationally simple, tolerate wide functional groups, and have a good ee value. Notably, P(O)R 2 not only acts as the directing group to direct C–H activation in order to make a useful ligand but also serves to facilitate composition of the product in a useful manner in this transformation.

Efficient cobalt(III)-catalyzed (hetero)aryl and alkenyl C–H hydroarylations of maleimides and maleate esters have been achieved under remarkably mild reaction conditions. Thus, the versatile cobalt(III) catalyst [Cp*CoI 2 (CO)] showed excellent atom−and step−economy as well as high chemo- and site-selectivity providing expedient access to pharmacologically useful succinimides with wide functional group tolerance.

A method for the coupling of aryl chlorides and thiophenols using an air-stable nickel(0) catalyst is described. This thioetherification procedure can be effectively applied to a range of electronically diverse aryl/heteroaryl chlorides without more expensive metal catalysts such as palladium, iridium, or ruthenium. This investigation also illustrates both, a variety of thiol coupling partners and, in certain cases, the use of Cs 2 CO 3 .

A convenient protocol for the synthesis of sulfonylated benzofurans via Ag-catalyzed oxidative cyclization has been established. Chemically stable and easily available sodium sulfinates were used as the sulfonylation reagents and building block for the heterocycle construction. With this novel strategy, various benzofurans bearing dual functional groups could be obtained in good yields with high chemo- and regioselectivities under mild conditions.

A microwave-assisted, Cp*Co III -catalyzed direct C–H activation/double C–N bond formation reaction of simple N H-sulfoximines with 1,4,2-dioxazol-5-ones to produce diverse thiadiazine-1-oxides is reported. The reaction tolerates a broad range of functional groups under external oxidant-free conditions and only releases CO 2 and H 2 O as the sole byproducts. The preliminary mechanistic studies revealed an electrophilic metalation pathway is likely involved in the reaction.

A phosphorylation of alkenyl and aryl C-O bonds at room temperature via photoredox/nickel dual catalysis is reported. By starting from easily available and inexpensive sulfonates, a variety of important alkenyl phosphonates and aryl phosphine oxides are generated in moderate to excellent yields. This method features mild reaction conditions, high selectivity, good functional group tolerance, wide substrate scope, and easy scalability.

The first example of oxidant-free cobalt-catalyzed synthesis of five-membered spirocycles is reported from benzimidates and maleimides utilizing nitrobenzene as promoter. In contrast to previously known cobalt-catalyzed oxidative C-H functionalization reactions, this transformation occurs efficiently in the absence of oxidant and is accompanied by liberation of hydrogen. The spiro-lactams were readily achieved by the hydrolysis of as-prepared spirocyclic compounds. The Cp*Rh(III) catalyst shows poor reactivity.

The manganese-catalyzed addition of C-2 position of indoles to maleimides has been achieved under additive-free conditions. The manganese catalyst exhibits excellent chemo- and regioselectivity, good functional group compatibility, and high catalytic efficiency. The substrate scope can also be extended to maleates, ethyl acrylate, 1,4-dihydro-1,4-epoxynaphthalene, pyrroles, and 2-phenylpyridine, which further demonstrates the universality of this straightforward approach.

A new procedure for oxidative coupling of aromatic and acrylic acids with alkynes has been developed using abundant, nontoxic, and air stable Cp*Co(III) catalyst. The coupling involves initial cyclometalation via weak chelation-assisted C-H bond activation followed by alkyne coordination, insertion, and reductive elimination leading to diverse isocoumarins (α-pyranones) in good yields under mild conditions.

A catalytic asymmetric [4 + 2] cycloaddition of ortho-quinone methides (o-QMs) is described. With the readily available vinyl sulfides as the key 2π partner and a properly chosen chiral phosphoric acid catalyst, the reaction proceeded under mild conditions to form the corresponding adduct with high enantio- and diastereoselectivity. Owning to the easy removal and conversion of the sulfenyl group in the product, the present process provides indirect access to generally substituted chromanes previously lacking easy access. Mechanistically, the reaction is also a new demonstration of the rarely achieved sole activation of o-QM for asymmetric control.

Bioassay-guided metabolomic analyses led to the characterization of four new 20-membered glycosylated polyketide macrolactams, macrotermycins A-D, from a termite-associated actinomycete, Amycolatopsis sp. M39. M39's sequenced genome revealed the macrotermycin's putative biosynthetic gene cluster. Macrotermycins A and C had antibacterial activity against human-pathogenic Staphylococcus aureus and, of greater ecological relevance, they also had selective antifungal activity against a fungal parasite of the termite fungal garden.

A highly efficient Co-based catalytic system, composed of a commercially available Co salt, a tetradentate phosphine ligand P(CH 2 CH 2 PPh 2 ) 3 (PP 3 ), and a base (denoted as [Co]/PP 3 /base), is developed for the methylation of C(sp 3 )–H and C(sp 2 )-H bonds using methanol as a methylating reagent. The Co(BF 4 ) 2 ·6H 2 O/PP 3 /K 2 CO 3 catalytic system showed high catalytic activity for the methylation of C–H bonds in aryl alkyl ketones, aryl acetonitriles, and indoles, with wide substrate scope and good functional group tolerance, and methyl-substituted products were obtained in good to excellent yields at 100 °C. This cheap, readily available, and highly efficient Co-based catalytic system may have promising applications in methylation reaction using methanol.

Triarylsulfonium, alkyl- and fluoroalkyl(diaryl)sulfonium, and aryl(dialkyl)sulfonium triflates are successfully used as a new family of cross-coupling participants in the Sonogashira reaction as aryldiazonium, diaryliodonium, and tetraphenylphosphonium salts. It was found that terminal alkynes reacted mildly with triarylsulfonium or (2,2,2-trifluoroethyl)diphenylsulfonium triflate at room temperature under Pd- and Cu-cocatalysis to give the corresponding arylalkynes in up to >99% yield. This protocol represents the first use of arylsulfonium salts as cross-coupling partners in the Pd/Cu-catalyzed Sonogashira reaction.

Cobalt(III) and rhodium(III) catalysts exhibited complementary scope in C-H amidation of aryl enaminones. The amidation reactions proceeded with broad scope under the assistance of a weakly coordinating and bifunctional enaminone directing group. The electrophilicity of the enaminone group can be further utilized in subsequent hydrolysis-cyclization reactions to afford NH 4-quinolones in telescoping reactions.

Zaragozic acid A (1) is a potent cholesterol lowering, polyketide natural product made by various filamentous fungi. The reconstitution of enzymes responsible for the initial steps of the biosynthetic pathway of 1 is accomplished using an engineered fungal heterologous host. These initial steps feature the priming of a benzoic acid starter unit onto a highly reducing polyketide synthase (HRPKS), followed by oxaloacetate extension and product release to generate a tricarboxylic acid containing product 2. The reconstitution studies demonstrated that only three enzymes, HRPKS, citrate synthase, and hydrolase, are needed in A. nidulans to produce the structurally complex product.

An electrophilic, imide-based, visible-light-promoted photoredox/Ni-catalyzed cross-coupling reaction for the synthesis of aliphatic ketones has been developed. This protocol proceeds through N-C(O) bond activation, made possible through the lower activation energy for metal insertion into this bond due to delocalization of the lone pair of electrons on the nitrogen by electron-withdrawing groups. The operationally simple and mild cross-coupling reaction is performed at ambient temperature and exhibits tolerance for a variety of functional groups.

Lewis acid catalyzed [3 + 2]-cycloaddition reaction of donor-acceptor cyclopropanes with vinyl azides has been developed to obtain diastereomerically enriched azidocyclopentane derivatives. In addition, thermal chemoselective ring expansion of azidocyclopentanes to tetrahydropyridine derivatives and further diastereospecific reduction to a substituted piperidine derivative, with an excellent yield, was also achieved.

Decarboxylative borylation of aryl and alkenyl carboxylic acids with bis(pinacolato)diboron was achieved through N-hydroxyphthalimide esters using tert-butyl isonicotinate as a catalyst under base-free conditions. A variety of aryl carboxylic acids possessing different functional groups and electronic properties can be smoothly converted to aryl boronate esters, including those that are difficult to decarboxylate under transition-metal catalysis, offering a new method enabling use of carboxylic acid as building blocks in organic synthesis. Mechanistic analysis suggests the reaction proceeds through coupling of a transient aryl radical generated by radical decarboxylation with a pyridine-stabilized persistent boryl radical. Activation of redox active esters may proceed via an intramolecular single-electron-transfer (SET) process through a pyridine-diboron-phthalimide adduct and accounts for the base-free reaction conditions.

A novel strategy for the asymmetric construction of functionalized 4-aryl-3,4-dihydrocoumarins and 4-aryl-3,4-dihydroquinolin-2-ones via an intramolecular vinylogous Rauhut-Currier reaction of para-quinone methides (p-QMs) under the bifunctional catalysis of chiral amine-phosphine is described. This intramolecular mode for the catalytic enantioselective 1,6-conjugate addition of p-QMs has been explored for the first time, delivering two types of synthetically important heterocycles in high yields and enantioselectivites.

An efficient strategy toward 4-arylquinolines and 4-arylpyrimidines from readily available precursors is described. Oxidative annulation promoted by K 2 S 2 O 8 involving anilines, aryl ketones, and DMSO as a methine (═CH−) equivalent leads to 4-arylquinolines via a cascade that entails generation of a sulfenium ion, subsequent C–N and C–C bond formations, and cyclization. The application of this strategy to the activation of acetophenone–formamide conjugates toward the synthesis of 4-arylpyrimidines is also described.