HFIP in Organic Synthesis

Hydro-phosphorothiolation of Styrene and Cyclopropane with S-Hydrogen Phosphorothioates under Ambient Conditions

A metal-free hexafluoroisopropanol-mediated hydro-phosphorothiolation of styrenes and donor-acceptor cyclopropanes with S-hydrogen phosphorothioates in a Markovnikov fashion has been developed under ambient reaction conditions to afford a library of S-alkyl phosphorothioates. Notably, this strategy provides a simple and efficient way to produce biologically significant kitazin and iprobenfos derivatives. Mechanistic studies disclose that the reaction proceeds through a carbocation intermediate.

Visible light-induced reductive aza-6π electrocyclization access to phenanthridines

Visible light-induced aza-6π electrocyclization was developed for the synthesis of aza-arenes from nitroarenes with diverse aldehydes. This protocol allows the reduction of nitroarenes by B2nep2 and subsequent 6π-electrocyclization of the in situ formed imine under visible light. An array of 6- and multi-substituted phenanthridines were constructed in moderate to good yields under purple LEDs at room temperature. A wide scope of substrates with diverse functional groups were well tolerated. In addition, the synthetic utility of this methodology was further demonstrated in the late-stage functionalization of celecoxib.

Understanding the Regiodivergence between Hydroarylation and Trifluoromethylarylation of 1,3-Dienes Using Anilines in HFIP

Conjugated dienes (1,3-dienes) are versatile and valuable chemical feedstocks that can be used as two-carbon or four-carbon synthons with vast applications across the chemical industry. However, the main challenge for their productive incorporation in synthetic routes is their chemo-, regio-, and stereoselective functionalization. Herein, we introduce a unified strategy for the 1,2-hydroarylation and 1,4-trifluoromethylarylation of 1,3-dienes using anilines in hexafluoroisopropanol. DFT calculations point toward a kinetically controlled process in both transformations, particularly in the trifluoromethylarylation, to explain the regiodivergent outcome. In addition, we perform an extensive program of functionalization and diversification of the products obtained, including hydrogenation, oxidation, cyclizations, or cross-coupling reactions, that allows access to a library of high-value species in a straightforward manner.

Metal-Free Selective Synthesis of α,β-Unsaturated Aldehydes from Alkenes and Formaldehyde Catalyzed by Dimethylamine

α,β-Unsaturated aldehydes are important building blocks for the synthesis of a wide range of chemicals, including polymers. The synthesis of these molecules from cheap feedstocks such as alkenes remains a scientific challenge, mainly due to the low reactivity of alkenes. Here we report a selective and metal-free access to α,β-unsaturated aldehydes from alkenes with formaldehyde. This reaction is catalyzed by dimethylamine and affords α,β-unsaturated aldehydes in yields of up to 80 %. By combining Density Functional Theory (DFT) calculations and experiments, we elucidate the reaction mechanism which is based on a cascade of hydride transfer, hydrolysis and aldolization reactions. The reaction can be performed under very mild conditions (30-50 °C), in a theoretically 100 % carbon-economical fashion, with water as the only by-product. The reaction was successfully applied to non-activated linear 1-alkenes, thus opening an access to industrially relevant α,β-unsaturated aldehydes from cheap and widely abundant chemicals at large scale.

HFIP-Promoted Divergent Cycloadditions of Difluoroenoxysilanes with 2-Indolylmethanols: Synthesis of Fluoro 2H-Pyrano[3,4-b]indoles and gem-Difluoro Cyclopenta[b]indoles

An oxa-6π-electrocyclization of difluoroenoxysilanes with diaryl 2-indolylmethanols has been developed. In addition, a rarely reported C3-nucleophilic [3+2] cycloaddition of difluoroenoxysilanes with dialkyl 2-indolylmethanols has been disclosed. This divergent cycloaddition approach affording readily available difluoroenoxysilanes as three-atom and C2 synthons provides rapid access to fluoro 2H-pyrano[3,4-b]indoles and gem-difluoro cyclopenta[b]indoles in good to excellent yields with good functional group tolerance. The metal-free and mild conditions using only HFIP as the solvent without any external acid catalyst illuminate practical and environmentally benign advantages.

Regio- and Stereoselective Hexafluoroisopropoxylation and Trifluoroethoxylation of Allenamides

Incorporating fluorinated moieties into organic molecules is an attractive strategy to enhance drug-like properties. Herein, we have developed a simple and self-promoted protocol for hexafluoroisopropoxylation and trifluoroethoxylation of allenamides with fluorinated alcohols such as HFIP and TFE. The reaction provided the fluoroalkoxylated products in a regio- and stereoselective manner in good to moderate yields under mild conditions.

Controlling the regioselectivity of the bromolactonization reaction in HFIP

The halolactonization reaction provides rapid access to densely functionalized lactones from unsaturated carboxylic acids. The endo/exo regioselectivity of this cyclization reaction is primarily determined by the electronic stabilization of alkene substituents, thus making it inherently dependent on substrate structures. Therefore this method often affords one type of halolactone regioisomer only. Herein, we introduce a simple and efficient method for regioselectivity-switchable bromolactonization reactions mediated by HFIP solvent. Two sets of reaction conditions were developed, each forming endo-products or exo-products in excellent regioselectivity. A combination of computational and experimental mechanistic studies not only confirmed the crucial role of HFIP, but also revealed the formation of endo-products under kinetic control and exo-products under thermodynamic control. This study paves the way for future work on the use of perfluorinated solvents to dictate reaction outcomes in organic synthesis.

Cu(OTf)2/HFIP catalyzed regioselective cycloisomerization of indole-C3-functionalized alkynols to carbazoles

We report here a simple and atom economic cycloisomerization reaction of indole-tethered alkynols for constructing diverse carbazoles using Cu(OTf)2/HFIP as the excellent promoter system. The reaction proceeds through a one-pot, domino process of spiro cyclization and 1,2-migration followed by aromatization to deliver carbazoles.

Visible-Light-Driven Decarboxylative Coupling of 2H-Indazoles with α-Keto Acids without Photocatalysts and Oxidants

An efficient synthesis of functionalized 3-acyl-2H-indazoles via visible-light-induced self-catalyzed energy transfer was developed. This method utilized a self-catalyzed energy transfer process between 2H-indazoles and α-keto acids, offering advantages like absence of photosensitizers, metal catalysts, and strong oxidants, broad substrate compatibility, and operational simplicity under mild conditions.

Acetate Assistance in Regioselective Hydroamination of Allenamides: A Combined Experimental and Density Functional Theory Study

A key factor in the development of selective nucleophilic addition to allenamides is controlling the reactivity of electrophilic intermediates, which is generally achieved using an electrophilic activator via conjugated iminium intermediates. In this combined experimental and computational study, we show that a general and highly chemoselective hydroamination of allenamides can be accomplished using a combination of 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and NaOAc. Experimental mechanistic studies revealed that HFIP mediates proton transfer to activate the allenamide, while the acetate additive significantly contributes to N-selective interception. This strategy enables a general hydroamination of allenamides without the use of metals. We demonstrated that various functionalized 1,3-diamines could be readily synthesized and diversified into value-added structural motifs. Detailed mechanistic investigations using the density functional theory revealed the role of NaOAc in the formation of reactive electrophilic intermediates, which ultimately governed the selective formation of 1,3-diamine products. Critically, calculations of the potential energy surface around the proton-transfer transition state revealed that two different reactive electrophilic intermediates were formed when NaOAc was added.

Synthesis of N-H Aziridines from Unactivated Olefins Using Hydroxylamine-O-Sulfonic Acids as Aminating Agent

Herein, we presented a practical methodology for the intermolecular aziridination of alkenes, using HOSA as the aminating agent, alongside pyridine or piperidine as the base, within HFIP solvent system. Notably, this approach showcases excellent reactivity, especially with nonactivated alkenes, and facilitates the transformation of various alkenes substrates, including mono-, di-, tri, and tetra-substituted alkenes, into aziridines with moderate to excellent yield. This method presents a promising avenue for synthesizing aziridines from a wide range of alkenes, featuring the benefits of straightforward operation, mild reaction conditions, extensive substrate compatibility, and scalability.

Hydrogen bonding-promoted tunable approach for access to aza-bicyclo-[3.3.0]octanes and cyclopenta[b] pyrroles

A unified strategy is disclosed that builds on successfully engaging the aniline nitrogen of 1,3-amphoteric γ-aminocyclopentenone for a tandem annulation with electron-poor alkynes, solely assisted by the H-bonding network of HFIP. This metal-free mild strategy provides access to medicinally relevant aza-bicyclo-octanes en route to another important scaffold: cyclopenta[b]pyrrole.

Late-Stage C-H Nitration of Unactivated Arenes by Fe(NO3)3·9H2O in Hexafluoroisopropanol

Operationally simple and generally applicable arene nitration with cheap and easily accessible chemicals has been a long-sought transformation in the synthetic organic community. In this work, we realized this goal with nontoxic and inexpensive Fe(NO3)3·9H2O as the nitro source and easily recyclable solvent hexafluoroisopropanol as the promotor via a network of hydrogen-bonding interactions. As a result of the relative mildness and high reliability of this protocol, late-stage nitration of various highly functionalized natural products and commercially available drugs was realized.

Mild Method for Deprotection of the N-Benzyloxycarbonyl (N-Cbz) Group by the Combination of AlCl3 and HFIP

Herein, we report our findings on the novel ability of aluminum chloride (AlCl3) in fluorinated solvent 1,1,1,3,3,3-hexafluoroisopropanol [HFIP] to selectively deprotect the N-benzyloxycarbonyl group (N-Cbz). The salient features of this method are good functional group tolerance including other reducible groups, cost-effectiveness, easy-to-handle, safe protocol, amenable to scale-up, high yields, and ambient temperature reactions. The methodology would serve as an excellent alternative to the use of pyrophoric hydrogen gas and metal catalyst reagents that pose severe safety and environmental concerns. The most notable feature of this methodology is the orthogonal deprotection of the N-Cbz group in the presence of O- and N-Bn protecting groups, hence, expanding the scope for designing synthetic routes to target compounds requiring multiple functional group transformations.

HFIP-Mediated Desulfinative Friedel-Crafts Cyclobutenylation Reaction

In 1,1,1,3,3,3-hexafluoroisopropyl alcohol (HFIP), gem-bis(triflyl)cyclobutenes, which can be prepared by the (2+2) cycloaddition reaction of Tf2C=CH2 with alkynes, underwent desulfination to generate the corresponding cyclobutenyl cation. This unique reactivity was successfully applied to the Friedel-Crafts type cyclobutenylation reaction of several (hetero)aromatic compounds.

HFIP-Mediated Multicomponent Reactions: Synthesis of Pyrazole-Linked Thiazole Derivatives

The development of one-pot, atom, and step-economic new methods avoiding metal, harsh reaction conditions, and toxic reagents for the synthesis of medicinally important hybrid molecules bearing more than one bioactive moieties is currently one of the hot topics in organic synthesis. Herein, we report a green and efficient room temperature multicomponent reaction for the synthesis of novel pyrazole-linked thiazoles involving a one-pot C-C, C-N, and C-S bond-forming process from the reaction of aryl glyoxal, aryl thioamide, and pyrazolones in 1,1,1,3,3,3-hexafluoroisopropanol, a hydrogen bond donating reaction medium. A set of diverse hybrid molecules bearing thiazole and pyrazole moieties were prepared in good to excellent yields by using this method. This methodology can also be extended to prepare thiazole-linked barbiturates as well as imidazole-linked pyrazoles. All the products were fully characterized by spectroscopic techniques. The notable features of this protocol are room temperature, metal as well as additive-free reaction conditions, use of recyclable solvent, water as the byproduct, wide substrate scope, operational simplicity, easy purification, applicability for gram-scale synthesis, high atom economy, and the presence of two bioactive pyrazole and thiazole moieties in the products.

Desulfurative Functionalization of β-Acyl Allylic Sulfides with N-H Free Indoles Highly Regioselective at C3 and N1 Positions: Rapid Access to α-Branched Enones

A regiodivergent allylation of 1H-indoles highly selectively at the C3 and N1 positions with β-acyl allylic sulfides through desulfurative C-C/C-N bond-forming reactions has been developed under mild conditions. Notably, the remarkable site-selective switch can be achieved by a delicate choice of solvents and bases. This cost-efficient method displays a broad substrate scope, good functional compatibility, and excellent site-selectivity, thus offering a divergent synthesis of indole substituted α-branched enones, which possess diverse potential opportunities for further applications and derivatization.

SOMOphilic alkyne vs radical-polar crossover approaches: The full story of the azido-alkynylation of alkenes

We report the detailed background for the discovery and development of the synthesis of homopropargylic azides by the azido-alkynylation of alkenes. Initially, a strategy involving SOMOphilic alkynes was adopted, but only resulted in a 29% yield of the desired product. By switching to a radical-polar crossover approach and after optimization, a high yield (72%) of the homopropargylic azide was reached. Full insights are given about the factors that were essential for the success of the optimization process.

Rhodium-Catalyzed Asymmetric Hydrogenation and Transfer Hydrogenation of 1,3-Dipolar Nitrones

Owing to their distinctive 1,3-dipolar structure, the catalytic asymmetric hydrogenation of nitrones to hydroxylamines has been a formidable and longstanding challenge, characterized by intricate enantiocontrol and susceptibility to N-O bond cleavage. In this study, the asymmetric hydrogenation and transfer hydrogenation of nitrones were accomplished with a tethered TsDPEN-derived cyclopentadienyl rhodium(III) catalyst (TsDPEN: p-toluenesulfonyl-1,2-diphenylethylene-1,2-diamine), the reaction proceeds via a novel 7-membered cyclic transition state, producing chiral hydroxylamines with up to 99 % yield and >99 % ee. The practical viability of this methodology was underscored by gram-scale catalytic reactions and subsequent transformations. Furthermore, mechanistic investigations and DFT calculations were also conducted to elucidate the origin of enantioselectivity.

Controllable 1,3-Bis-Functionalization of 2-Nitroglycals with High Regioselectivity and Stereoselectivity Enabled by a H-Bond Catalyst

The selective modification of carbohydrates is significant for producing their unnatural analogues for drug discovery. C1-functionalization (glycosylation) and C1,C2-difunctionalization of carbohydrates have been well developed. In contrast, C3-functionalization or C1,C3-difunctionalization of carbohydrates remains rare. Herein, we report such processes that efficiently and stereoselectively modify carbohydrates. Specifically, we found that trifluoroethanol (TFE) could promote 1,3-bis-indolylation/pyrrolylation of 2-nitroglycals generated carbohydrate derivatives in up to 93% yield at room temperature; slightly reducing the temperature could install two different indoles at the C1- and C3-positions. Switching TFE to a bifunctional amino thiourea catalyst leads to the generation of C3 monosubstituted carbohydrates, which could also be used to construct 1,3-di-C-functionalized carbohydrates. This approach produced a range of challenging sugar derivatives (over 80 examples) with controllable and high stereoselectivity (single isomer for over 90% of the examples). The potential applications of the reaction were demonstrated by a set of transformations including the synthesis of bridged large-ring molecules and gram scale reactions. Biological activities evaluation demonstrated that three compounds exhibit a potent inhibitory effect on human cancer cells T24, HCT116, AGS, and MKN-45 with IC50 ranged from 0.695 to 3.548 μM.

An Intermolecular Hydroarylation of Unactivated Arylcyclopropane via Re2O7/HFIP-Mediated Ring Opening

In this paper, we describe a Re2O7-mediated ring-opening arylation of unactivated arylcyclopropane because of its functionalization with various arenes via Friedel-Crafts-type reactivity. This protocol allows facile access to functionalized 1,1-diaryl alkanes and is characterized by a broad substrate scope, mild reaction conditions, high efficiency, and high atom economy. Both density functional theory calculations and deuterium labeling experiments were carried out to justify the indispensable role of HFIP in this transformation and pointed to Re2O7-mediated ring opening being the rate-determining step.

Hexafluoroisopropanol as a Bioconjugation Medium of Ultrafast, Tryptophan-Selective Catalysis

The past decade has seen a remarkable growth in the number of bioconjugation techniques in chemistry, biology, material science, and biomedical fields. A core design element in bioconjugation technology is a chemical reaction that can form a covalent bond between the protein of interest and the labeling reagent. Achieving chemoselective protein bioconjugation in aqueous media is challenging, especially for generally less reactive amino acid residues, such as tryptophan. We present here the development of tryptophan-selective bioconjugation methods through ultrafast Lewis acid-catalyzed reactions in hexafluoroisopropanol (HFIP). Structure-reactivity relationship studies have revealed a combination of thiophene and ethanol moieties to give a suitable labeling reagent for this bioconjugation process, which enables modification of peptides and proteins in an extremely rapid reaction unencumbered by noticeable side reactions. The capability of the labeling method also facilitated radiofluorination application as well as antibody functionalization. Enhancement of an α-helix by HFIP leads to its compatibility with a certain protein, and this report also demonstrates a further stabilization strategy achieved by the addition of an ionic liquid to the HFIP medium. The nonaqueous bioconjugation approaches allow access to numerous chemical reactions that are unavailable in traditional aqueous processes and will further advance the chemistry of proteins.

Facile Friedel-Crafts alkylation of arenes under solvent-free conditions

The Friedel-Crafts alkylation of arenes is an important part of electrophilic aromatic substitution reactions. However, the reactivity of arenes is weakened by electron-withdrawing substituents, leading to limited substrate scopes and applications. Herein, we developed an efficient HOTf-promoted Friedel-Crafts alkylation reaction of broad arenes with α-aryl-α-diazoesters under metal-free and solvent-free conditions.

Unlocking Structurally Nontraditional Naphthyridine-Based Electron-Transporting Materials with C-H Activation-Annulation

The inherent benefits of C-H activation have given rise to innovative approaches in designing organic optoelectronic molecules that depart from conventional methods. While theoretical calculations have suggested the suitability of the 2,6-naphthyridine scaffold for electron transport materials (ETMs) in organic light-emitting diodes (OLEDs), the existing synthetic methodologies have proven to be insufficient for the construction of multiple arylated and fully aryl-substituted molecules. Herein, we present a solution for the synthesis of 2,6-naphthyridine derivatives, with the rhodium-catalyzed consecutive C-H activation-annulation process of fumaric acid with alkynes standing as the pivotal step within this strategy. The ETMs, purposefully designed and synthesized based on the 2,6-naphthyridine framework, exhibit an impressively high glass-transition temperature (Tg) of 282 °C and high electron mobility (μe), setting a new benchmark for ETMs in OLEDs with a μe exceeding 10-2 cm2 V-1 s-1. These materials prove to be versatile ETM candidates suitable for red, green, and blue phosphorescent OLED devices.

Protective role of chlorogenic acid in preserving cytochrome-c stability against HFIP-induced molten globule state at physiological pH

Numerous neurodegenerative disorders are characterized by protein misfolding and aggregation. The mechanism of protein aggregation is intricate, and it is very challenging to study at cellular level. Inhibition of protein aggregation by interfering with its pathway is one of the ways to prevent neurodegenerative diseases. In the present work, we have evaluated the protective effect of a polyphenol compound chlorogenic acid (CGA) on the native and molten globule state of horse heart cytochrome c (cyt c). A molten globule state of this heme protein was achieved in the presence of fluorinated alcohol 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) at physiological pH, as studied by UV-Vis absorption, circular dichroism, intrinsic and ANS fluorescence. We found that at 50 % (v/v) HFIP, the native cyt c transformed into a molten globule state. The same techniques were also used to analyze the protective effect of CGA on the molten globule state of cyt c, and the results show that the CGA prevented the molten globular state and retained the protein close to the native state at 1:1 protein:CGA sub molar ratio. Molecular dynamics study also revealed that CGA retains the stability of cyt c in HFIP medium by preserving it in an intermediate state close to native conformation.

Synthesis of Phenol-Pyridinium Salts Enabled by Tandem Electron Donor-Acceptor Complexation and Iridium Photocatalysis

Herein, we describe a dual photocatalytic system to synthesize phenol-pyridinium salts using visible light. Utilizing both electron donor-acceptor (EDA) complex and iridium(III) photocatalytic cycles, the C-N cross-coupling of unprotected phenols and pyridines proceeds in the presence of oxygen to furnish pyridinium salts. Photocatalytic generation of phenoxyl radical cations also enabled a nucleophilic aromatic substitution (SNAr) of a fluorophenol with an electron-poor pyridine. Spectroscopic experiments were conducted to probe the mechanism and reaction selectivity. The unique reactivity of these phenol-pyridinium salts were displayed in several derivatization reactions, providing rapid access to a diverse chemical space.

Synthesis of Fluorine-Containing Multisubstituted Oxa-Spiro[4,5]cyclohexadienones via a Fluorinated Alcohol-Catalyzed One-Pot Sequential Cascade Strategy

In recent years, the application of fluorinated alcohols as solvents, cosolvents, or additives has become important in modern organic synthesis. However, their potential as efficient catalysts in organic synthesis has not been well-explored. In this article, we report on the development of a one-pot sequential cascade reaction of p-quinone methides with difluoroenoxysilanes using hexafluoroisopropanol as catalyst. This reaction allows for the preparation of fluorinated multisubstituted oxa-spiro[4,5]cyclohexadienones. By using 50 mol % 1,1,1,3,3,3-Hexafluoroisopropanol (HFIP), the reaction proceeds smoothly to yield 1,6-conjugated products, which are then subjected to oxidative dearomatization/hemiacetalization using PhI(OAc)2. The overall process affords moderate to high yields and excellent diastereoselectivities.

Biological Evaluation of Isosteric Applicability of 1,3-Substituted Cuneanes as m-Substituted Benzenes Enabled by Selective Isomerization of 1,4-Substituted Cubanes

We herein evaluate a biological applicability of 1,3-substituted cuneanes as an isostere of m-substituted benzenes based on its structural similarity. An investigation of a method to obtain 1,3-substituted cuneanes by selective isomerization of 1,4-substituted cubanes enables this attempt by giving a key synthetic step to obtain a cuneane analogs of pharmaceuticals having m-substituted benzene moiety. Biological evaluation of the synthesized analogs and in silico study of the obtained result revealed a potential usage of cuneane skeleton in medicinal chemistry.

Selective Hydrofunctionalization of N-Allenyl Derivatives with Heteronucleophiles Catalyzed by Brønsted Acids

In this study, we present a novel and environmentally sustainable protocol for the γ-hydrofunctionalization of N-allenyl compounds using various heteronucleophiles catalyzed solely by simple Brønsted acids. The method displays remarkable attributes, highlighting its sustainability, efficiency, regio- and stereoselectivity, as well as its versatile applicability to diverse heteroatom-containing enamides. Notably, our approach eliminates the need for metal catalysts and toxic solvents, representing a significant advancement in greener chemistry practices. We demonstrate the broad scope of our protocol by successfully scaling up reactions to gram-scale syntheses, underscoring its robustness for potential industrial implementation. The resulting γ-heterosubstituted enamides offer new possibilities for further synthetic transformations, yielding highly functionalized compounds with diverse applications. Mechanistic investigations reveal the pivotal role of CSA as a catalyst, enabling alcohol addition via a covalent activation mode.

Electrohydrogenation of Nitriles with Amines by Cobalt Catalysis

Catalytic hydrogenation of nitriles represents an efficient and sustainable one-step synthesis of valuable bulk and fine chemicals. We report herein a molecular cobalt electrocatalyst for selective hydrogenative coupling of nitriles with amines using protons as the hydrogen source. The key to success for this reductive reaction is the use of an electrocatalytic approach for efficient cobalt-hydride generation through a sequence of cathodic reduction and protonation. As only electrons (e- ) and protons (H+ ) as the redox equivalent and hydrogen source, this general electrohydrogenation protocol is showcased by highly selective and straightforward synthesis of various functionalized and structurally diverse amines, as well as deuterium isotope labeling applications. Mechanistic studies reveal that the electrogenerated cobalt-hydride transfer to nitrile process is the rate-determining step.

Synthesis of mesoionic triazolones via a formal [3+2] cycloaddition between 4-phenyl-1,2,4-triazoline-3,5-dione and alkynes

1,2,4-Triazoline-3,5-diones (TADs) are versatile reagents and have found widespread adoption in chemical science. Despite their remarkable reactivity toward a wide array of unsaturated hydrocarbons, the chemical reaction between TADs and alkynes has remained largely unexplored. Herein, we demonstrate that 1,1,1,3,3,3-hexafluoro-2-propanol facilitates the unusual [3+2] cycloaddition between 4-phenyl-1,2,4-triazoline-3,5-dione and alkynes, resulting in the formation of unprecedented mesoionic triazolones. Moreover, the structural properties of the resulting triazolone have been investigated by employing X-ray diffraction analysis and Density Functional Theory (DFT) calculations.

Route to Functionalized Tetrahydrobenzo[d]azepines via Re2O7-Mediated Intramolecular Friedel-Crafts Reaction

We describe a Re2O7-mediated intramolecular dehydrative Friedel-Crafts reaction for the efficient synthesis of various benzo-fused heterocycles such as benzazepines and benzazocines. This process is characterized by a broad substrate scope, mild reaction conditions, high efficiency, and high atom economy. The potential application of this methodology was exemplified by the facile preparation of a NMDA antagonist as well as a key intermediate en route to SKF 38393.

Total Synthesis of Alanense A through an Intramolecular Friedel-Crafts Alkylation

The first total synthesis of cadinane sesquiterpenoid alanense A, in which an intramolecular dehydrative Friedel-Crafts alkylation of 2,5-diaryl-2-pentanol is incorporated as a key step, has been achieved. The combinatorial use of p-TsOH·H2O as a catalyst and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) as a solvent provides 1,1-disubstituted tetrahydronaphthalene in 97% yield. It was also found that the combination of p-TsOH and HFIP is effective for the removal of phenolic MOM ether.

Dehydrogenative Electrochemical Synthesis of N-Aryl-3,4-Dihydroquinolin-2-ones by Iodine(III)-Mediated Coupling Reaction

Electrochemically generated hypervalent iodine(III) species are powerful reagents for oxidative C-N coupling reactions, providing access to valuable N-heterocycles. A new electrocatalytic hypervalent iodine(III)-mediated in-cell synthesis of 1H-N-aryl-3,4-dihydroquinolin-2-ones by dehydrogenative C-N bond formation is presented. Catalytic amounts of the redox mediator, a low supporting electrolyte concentration and recycling of the solvent used make this method a sustainable alternative to electrochemical ex-cell or conventional approaches. Furthermore, inexpensive, readily available electrode materials and a simple galvanostatic set-up are applied. The broad functional group tolerance could be demonstrated by synthesizing 23 examples in yields up to 96 %, with one reaction being performed on a 10-fold higher scale. Based on the obtained results a sound reaction mechanism could be proposed.

Acid-Promoted Cyclization of α-Azidobenzyl Ketones through C═N Bond Formation: Synthesis of 6-Substituted Quinoline Derivatives

An acid-promoted cyclization of α-azidobenzyl ketones has been developed for the synthesis of 6-substituted quinoline derivatives. A variety of synthetically useful 6-OTf or -OMs quinoline derivatives were obtained in moderate to good yields. The reaction proceeds via C═N bond formation without organophosphine, providing convenient access to structurally interesting and synthetically important 6-substituted quinoline derivatives in moderate to good yields. A mechanistic perspective that is different from the traditional intramolecular Schmidt reaction has been proposed.

Concise and Free-Metal Access to Lactone-Annelated Pyrrolo[2,1-a]isoquinoline Derivatives via a 1,2-Rearrangement Step

Here, An efficient approach to obtaining previously unknown furo[2',3':2,3]pyrrolo[2,1-a]isoquinoline derivatives from readily available 1-R-1-ethynyl-2-vinylisoquinolines is described. The reaction features a simple procedure, occurs in hexaflouroisopropanol and does not require elevated temperatures. It has been found that the addition of glacial acetic acid significantly increases the yields of the target spirolactone products. Using trifluoroethanol instead of hexaflouroisopropanol results in the formation of pyrido[2,1-a]isoquinolines.

Electrochemical Fluorination Functionalization of gem-Difluoroalkenes with CsF as a Fluorine Source: Access to Fluoroalkyl Building Blocks

Using easily handled CsF as a fluorine source, an electrochemically metal-free protocol for chemo- and regioselective synthesis of various types of long-chain perfluoroalkyl aromatics with gem-difluoroalkene as a substrate and an alcohol or azole as an additional nucleophile was developed. The eletrochemical transformation could tolerate several functional groups, such as halogens, cyanos, benzyls, and heterocycles, and is amenable to gram-scale. The application of this electrochemical method in radiofluorination was also tested.

Palladium-Catalyzed Electrooxidative Double C-H Arylation

The electrochemical transition metal-catalyzed cross-dehydrogenative reaction has emerged as a promising platform to achieve a sustainable and atom-economic organic synthesis that avoids hazardous oxidants and minimizes undesired byproducts and circuitous functional group operations. However, a poor mechanistic understanding still prevents the widespread adoption of this strategy. In this regard, we herein present an electrochemical palladium-catalyzed oxidative coupling strategy to access biaryls in the absence of a stoichiometric chemical oxidant. The robust palladaelectrocatalysis considerably suppresses the occurrence of homocoupling and oxygenation, being compatible even with electron-deficient arenes. Late-stage functionalization and Boscalid precursor synthesis further highlighted the practical importance of our electrolysis. Remarkably, mechanistic studies including the evaluation of the reaction order of each component by variable time normalization analysis (VTNA) and initial rate analysis, H/D exchange experiment, kinetic isotope effect, and stoichiometric organometallic experiments provided strong support for the involvement of transmetalation between two organopalladium complexes in the turnover limiting step. Therefore, matching the concentrations or lifetimes of two distinct organopalladium intermediates is revealed to be a pivot to the success of electrooxidative catalysis. Moreover, the presence of cationic copper(II) seems to contribute to the stabilization of the palladium(0) catalyst instead of playing a role in the oxidation of the catalyst.

Brønsted Acid-Catalyzed Synthesis of 4-Functionalized Tetrahydrocarbazol-1-ones from 1,4-Dicarbonylindole Derivatives

A p-toluenesulfonic acid-catalyzed cascade reaction is reported for the synthesis of 4-functionalized tetrahydrocarbazolones via the reaction of 4-(indol-2-yl)-4-oxobutanal derivatives with a variety of nucleophiles in acetonitrile or hexafluoroisopropanol. After the initial intramolecular Friedel-Crafts hydroxyalkylation, the 3-indolylmethanol intermediate is subsequently activated and reacted with the external nucleophile. The reaction conditions are crucial to avoid alternative reaction pathways, allowing direct substitution reaction with thiols, (hetero)arenes, alkenes, or sulfinates. The procedure features high overall yields to access a diverse family of compounds bearing the tetrahydrocarbazole core.

DFT Study on the Mechanism of the Palladium-Catalyzed [3 + 2] Annulation of Aromatic Amides with Maleimides via Benzylic and meta-C-H Bond Activation: Role of the External Ligand Ac-Gly-OH

The mechanism of the Ac-Gly-OH-assisted palladium-catalyzed [3 + 2] annulation of aromatic amides with maleimides is investigated using density functional theory calculations. The results show that the reaction undergoes the sequential steps of N-H bond deprotonation, first benzylic C-H bond activation, maleimide insertion, second meta-C-H bond activation, reductive elimination, and oxidation. The external ligand Ac-Gly-OH acts as the internal base for hydrogen abstraction in the first benzylic C-H bond activation. The maleimide insertion step is found to be the rate-determining step. Based on the nearly same energetic span of the two pathways to generate the enantio products, the computational results are consistent with the experimental observation that the terminal [3 + 2] annulation products are racemic when using an achiral ligand. These calculation results disclose the detailed reaction mechanism and shed light on some experimental ambiguities.

Total Synthesis of Calanthoside, a Potential Hair Growth Stimulant: A Facile Synthetic Approach via One-Pot S- and O-Glucosidic Bond Formation

The first total synthesis of calanthoside (1), which exhibits potent proliferative activity against human hair follicle dermal papilla cells, has been achieved in seven steps with an overall yield of 43% on a gram scale starting from anthranilic acid (11). The synthetic strategy features a one-pot process involving thioglucoside bond formation via nucleophilic substitution reaction and enol-glucosylation for building the S-,O-bisdesmoside structure of 1. Moreover, the one-pot reaction showed broad substrate adaptability to several sugar donors other than d-glucose, thus affording S,O-bisglycoside intermediates in ∼84% yield.

Chiral NHC Ligands for Enantioselective Gold(I) Catalysis Under Aerobic Conditions: the Importance of Conformational Flexibility and Steric Hindrance of NHC Ligand on Reactivity

Gold(I) catalysis has been recognized as a valuable tool for the unique transformation of multiple carbon-carbon bonds. Enantioselective π-catalysis based on gold(I) complexes is, however, still underdeveloped due to lack of privileged ligands. Herein, we present an accessible method to a new family of stable yet catalytically active chiral NHC-Au(I)-Cl complexes. The key to preserving a simultaneous fine balance between reactivity and stability in this newly developed family appears to be sterically hindered, but conformationally flexible NHC ligands. These could be easily accessed on a multigram scale by merging sterically hindered anilines with commercially available amino alcohols and amines via a four-steps synthetic sequence without the need for chromatographic purification. Further investigations of the catalytic activity of NHC-Au-Cl complexes identified the OH functionality incorporated into the NHC core as crucial for the level of enantioselectivity as well as the TsO- anion responsible for the activation of NHC-Au(I)-Cl. Finally, NMR studies and X-ray investigations revealed for the first time that the widely accepted ion metathesis (NHC-Au-Cl to NHC-Au-OSO2 R) responsible for the activation of NHC-Au-Cl complexes does not take place (or it is very slow) in commonly used MeNO2 in contrast to DCM.

Efficient synthesis of glycosylated imidazo[1,2-a]pyridines via solvent catalysed Groebke-Blackburn-Bienayme reaction

A solvent catalysed and metal catalyst-free Groebke-Blackburn-Bienayame three component reaction (GBB-3CR) has been developed for the synthesis of 2-(β-D-glycal-1-yl)-3-N-alkylamino-1-azaindolizines and 2-alkyl/aryl/heteroaryl-3-N-alkylamino-1-azaindolizines. The modified GBB reaction protocol is highly efficient, versatile, atom economic and has been performed in hexafluoroisopropanol (HFIP) without any added catalyst. The GBB-3CR showed high tolerance for a large no of substrates in term of aldehydes, differently substituted 2-aminopyridines and isocyanides without being affected by the presence of electron donating and electron withdrawing substituents at either aldehydes or 2-aminopyridines.

Metal-free synthesis of difluoro/trifluoromethyl carbinol-containing chromones via tandem cyclization of o-hydroxyaryl enaminones

We herein propose a HFIP-promoted tandem cyclization reaction for the synthesis of difluoro/trifluoromethyl carbinol-containing chromones from o-hydroxyphenyl enaminones at room temperature. This protocol provides a facile and efficient approach to access diverse difluoro/trifluoromethylated carbinols in good to excellent yields. In addition, gram-scale and synthetic derivatization experiments have also been performed.

Transition-metal-free and additive-free intermolecular hydroarylation of alkenes with indoles in hexafluoroisopropanol

Hydroarylation of alkenes is one of the most straightforward and atom-economical strategy for the construction of multi-aryl-substituted alkanes, but systematic studies have been limited to transition metal catalysis. Here we report a hexafluoroisopropanol (HFIP)-promoted hydroarylation of alkenes with indoles without the presence of transition metal catalysts or any additive. HFIP was the only reagent used in this work, and could be easily removed via evaporation, and recovered via distillation in industry settings. This reaction was shown to provide an efficient, clean and operationally simple procedure with a remarkable substrate scope and versatile transformations, delivering a variety of multi-aryl alkanes incorporating the indole motif. In preliminary studies, several of these products showed biologically activity against cells from an array of human cancer cell lines. A mechanistic study was also carried out and suggested that the quinone methide might be the key intermediate. And in contrast to the conclusions of a previous report, the current work suggested that protonation by HFIP might not be the rate-determining step.

Nickel-Catalyzed Domino Reaction of α-Aryloxyacetonitriles with Arylboronic Acids: Synthesis of 2-Aroylbenzo[b]furans

We disclose the first catalytic domino reaction of α-(2-formylaryloxy)acetonitriles with arylboronic acids, yielding a range of 2-aroylbenzo[b]furans with yields of up to 93%. Ni(acac)2 serves as an effective dual catalyst. The protocol is also applicable to α-(2-acetylphenoxy)acetonitrile, giving rise to 3-methyl-2-aroylbenzo[b]furans. This domino process is efficient, additive-free, and compatible with a variety of aryl boronic acids, including those with CF3, NO2, CN, and CO2Me groups. Mechanistic studies highlight the dual activation facilitated by the nickel catalyst.

1,1,1,3,3,3-Hexafluoro-2-Propanol-Promoted Friedel-Crafts Reaction: Metal-Free Synthesis of C3-Difluoromethyl Carbinol-Containing Imidazo[1,2-a]pyridines at Room Temperature

A facile and efficient method has been developed for the synthesis of C3-difluoromethyl carbinol-containing imidazo[1,2-a]pyridines at room temperature via the HFIP-promoted Friedel-Crafts reaction of difluoroacetaldehyde ethyl hemiacetal and imidazo[1,2-a]pyridines. This strategy could be applied to the direct C(sp2)-H hydroxydifluoromethylation of imidazo[1,2-a]pyridines and afford a series of novel difluoromethylated carbinols in good to satisfactory yields with 29 examples. Furthermore, gram-scale and synthetic transformation experiments have also been achieved, demonstrating its potential applicable value in organic synthesis. This green protocol has several advantages, including being transition metal- and oxidant-free, being carried out at room temperature, having high efficiency, and having a wide substrate scope.

Trifluoromethylarylation of alkenes using anilines

Nitrogen containing compounds, such as anilines, are some of the most widespread and useful chemical species, although their high and unselective reactivity has prevented their incorporation into many interesting transformations, such as the functionalization of alkenes. Herein we report a method that allows the trifluoromethylarylation of alkenes using anilines, for the first time, with no need for additives, transition metals, photocatalysts or an excess of reagents. An in-depth mechanistic study reveals the key role of hexafluoroisopropanol (HFIP) as a unique solvent, establishing a hydrogen bonding network with aniline and trifluoromethyl reagent, that is responsible for the altered reactivity and exquisite selectivity. This work uncovers a new mode of reactivity that involves the use of abundant anilines as a non-prefunctionalized aromatic source and the simultaneous activation of trifluoromethyl hypervalent iodine reagent.

Iron-Catalyzed Synthesis of Peroxylpyrrolo[2,1-a]isoquinolines through Oxidative Dearomatization

A mild late-stage modification of pyrrolo[2,1-a]isoquinolines was established through iron-catalyzed oxidative dearomatization and peroxidation. Peroxylated pyrroloisoquinolines have been prepared readily with hydroperoxide in low to good yields (up to 72%) at room temperature. Interestingly, the treatment of fully aromatized pyrrolo[1,2-a]quinolines under the current reaction system resulted in the formation of ring-opening products.

Photoinduced NiH Catalysis with Trialkylamines for the Stereodivergent Transfer Semi-Hydrogenation of Alkynes

We report a selectivity-switchable nickel hydride-catalyzed methodology that enables the stereocontrolled semi-reduction of internal alkynes to E- or Z-alkenes under very mild conditions. The proposed transfer semi-hydrogenation process involves the use of a dual nickel/photoredox catalytic system and triethylamine, not only as a sacrificial reductant, but also as a source of hydrogen atoms. Mechanistic studies revealed a pathway involving photo-induced generation of nickel hydride, syn-hydronickelation of alkyne, and alkenylnickel isomerization as key steps. Remarkably, mechanistic experiments indicate that the control of the stereoselectivity is not ensuing from a post-reduction alkene photoisomerization under our conditions. Instead, we demonstrate that the stereoselectivity of the reaction is dependent on the rate of a final protonolysis step which can be tuned by adjusting the pKa of an alcohol additive.