Calcium(II)- and Triflimide-Catalyzed Intramolecular Hydroacyloxylation of Unactivated Alkenes in Hexafluoroisopropanol

Electrophilic (Hetero)Arene C-H Nitration by Ferric Nitrate Nonahydrate in Hexafluoroisopropanol

In this article, we provide an extended substrate scope and more detailed mechanistic studies of an operationally simple and generally applicable arene nitration that we previously reported. This method utilizes safe and inexpensive Fe(NO3)3•9H2O as the nitro source in easily recyclable HFIP and obviates the need for corrosive acids (HNO3 + H2SO4). As a result, we speculated that it could serve as an effective substitute for the traditional mixed acid approach under most scenarios due to its operational simplicity. A general guidance for the application of this method was provided.

Hexafluoroisopropanol Solvent Effects on Enantioselectivity of Dirhodium Tetracarboxylate-Catalyzed Cyclopropanation

In recent years, additives that modulate both reactivity and selectivity in rhodium-catalyzed reactions of aryldiazoacetates have become increasingly prominent. 1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) has been shown to have a profound effect on rhodium carbene reactivity and selectivity, especially on enabling carbene cyclopropanation in the presence of various nucleophilic poisons. HFIP also has a variable influence on the enantioselectivity of the reactions catalyzed by chiral dirhodium tetracarboxylates, and this study examines the fundamental properties of the rhodium carbene/HFIP system through experimentation, density functional theory (DFT), and molecular dynamics (MD) simulations. These studies revealed that the C4-symmetric bowl-shaped catalysts, which have been previously considered to be relatively rigid, experience far greater flexibility in this hydrogen bonding media, resulting in distortion of the bowl-shaped catalysts. These studies explain why even though a majority of the catalysts have a drop in enantioselectivity in HFIP, some catalysts, such as Rh2(TCPTAD)4, lead to a switch in enantioselectivity, whereas others, such as Rh2(NTTL)4, lead to a considerably enhanced enantioselectivity.

Dual Au/Ag Catalyzed Regiospecific Intramolecular Hydroacyloxylation and Hydroalkoxylation of Unactivated Geminal-Substituted Olefins

A mild, regiospecific Gold-Silver bimetallic catalytic system has been devised for the intramolecular hydroacyloxylation and hydroetherification of alkenoic acids and alcohols. This method exhibits precise specificity for the geminal substituted olefinic center and facilitates the synthesis of substituted phthalide and hydroisocoumarin derivatives. This method has been effectively applied for late-state functionalization to produce bioactive natural products such as rumphellaone A, mycophenolate, and (-)-ambrox. The successful gram-scale synthesis of the anticonvulsant, hypnotic drug (±)-ethyl phenyl butyro lactone (EPBL), (±)-Boivinianin A and the ability to synthesize challenging spiro and bicyclic lactone underscores the synthetic potential of this methodology. Mechanistic insights into gold-silver catalyzed lactonization of olefins have also been discussed.

Pd-catalyzed Markovnikov selective oxidative amination of 4-pentenoic acid

Pd-catalyzed regioselective amination of unactivated alkene remains a challenge and is of great interest. Herein, a palladium-catalyzed and ligand-controlled strategy for the Markovnikov selective oxidative amination of 4-pentenoic acid has been described. The protocol effectively reverses the carboxylic acid-directed anti-Markovnikov selectivity in oxidative amination of 4-pentenoic acid, successfully constructing γ-ketoamide derivatives.

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.

Difunctionalization Processes Enabled by Hexafluoroisopropanol

In the past 5 years, hexafluoroisopropanol (HFIP) has been used as a unique solvent or additive to enable challenging transformations through substrate activation and stabilization of reactive intermediates. In this Review, we aim at describing difunctionalization processes which were unlocked when HFIP was involved. Specifically, we focus on cyclizations and additions to alkenes, alkynes, epoxides, and carbonyls that introduce a wide range of functional groups of interest.

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.

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.

Copper-Catalyzed Bifunctionalization/Annulation of Unactivated Alkene with Alkyl Bromides

β-Lactam is a ubiquitous scaffold in bioactive compounds and pharmaceuticals. Herein, we disclose a streamlined method for the construction of β-lactams starting from unactivated alkenes and alkyl bromides via a Cu-catalyzed inter-/intramolecular carboamidation. This reaction proceeded smoothly under mild reaction conditions and exhibited a broad substrate scope and various functional groups. This protocol is not only compatible with 1, 2, and 3° alkyl bromides but also suitable for α-bromo nitrile as well as various benzyl bromides. The mechanism exploration indicated that sequential radical addition/reductive elimination was involved.

Visible-Light-Enabled Lanthanum-Mediated Intramolecular Epoxy-Ring Opening/Dehydrogenative Lactonization

Catalytic C(sp3)-H functionalization has afforded great opportunities to prepare organic substances, facilitating the derivatization of complex drugs and natural molecules. This letter describes an efficient and practical protocol for lanthanum-catalyzed continuous epoxy-ring opening and oxidative dehydrogenative lactonization under visible-light irradiation. Notably, the lanthanum catalyst also acts as a photocatalyst while acting as a Lewis acid in this reaction; therefore, no additional photocatalyst is required. We can conveniently prepare a series of diverse isochromanones with oxygen-containing spirocyclic structural units under a balloon-oxygen atmosphere at room temperature. Mechanistic studies and control experiments reveal that the in situ-generated lanthanum bromide should be crucial in the reaction.

Ab Initio Metadynamics Simulations of Hexafluoroisopropanol Solvent Effects: Synergistic Role of Solvent H-Bonding Networks and Solvent-Solute C-H/π Interactions

The solvent effects in Friedel-Crafts cycloalkylation of epoxides and Cope rearrangement of aldimines were investigated by using ab initio molecular dynamics simulations. Explicit molecular treatments were applied for both reactants and solvents. The reaction mechanisms were elucidated via free energy calculations based on metadynamics simulations. The results reveal that both reactions proceed in a concerted fashion. Key solvent-substrate interactions are identified from the structures of transition states with explicit solvent molecules. The remarkable promotion effect of hexafluoroisopropanol solvent is ascribed to the synergistic effect of H-bonding networks and C-H/π interactions with substrates.

Calcium-catalysed synthesis of amines through imine hydrosilylation: an experimental and theoretical study

A method to reduce aldimines through hydrosilylation is reported. The catalytic system involves calcium triflimide (Ca(NTf₂)₂) and potassium hexafluorophosphate (KPF₆) which have been shown to act in a synergistic manner. The expected amines are obtained in fair to very high yields (40-99%) under mild conditions (room temperature in most cases). To illustrate the potential of this method, a bioactive molecule with antifungal properties was prepared on the gram scale and in high yield in environmentally friendly 2-methyltetrahydrofuran. Moreover, it is shown in this example that the imine can be prepared in situ from the aldehyde and the amine without isolating the imine. The mechanism involved has been explored experimentally and through DFT calculations, and the results are in accordance with an electrophilic activation of the silane by the calcium catalyst.

Re2O7/HReO4 Mediated Intramolecular Hydroacyloxylation of Unactivated Alkenes: A Dual Hydrogen-Bonding Effect

This publication describes the application of Re₂O₇ in hexafluoroisopropanol (HFIP) for the activation of inert as well as electronically deactivated olefins to facilitate a challenging intramolecular hydroacyloxylation reaction. Both HFIP and an internal carboxy group have been proven to be crucial for the successful implementation of this transformation; these are proposed to assist the formation and stabilization of the key cationic intermediate via hydrogen-bonding interactions with perrhenate anion (ReO₄^-).

Substrate-Dependent Selectivity in Sc(OTf)3-Catalyzed Cyclization of Alkenoic Acids and N-Protected Alkenamides

Five- and six-membered ring lactones and lactams are ubiquitous frameworks in various natural and synthetic molecules and are key building blocks in organic synthesis. Catalytic addition of an O-H or N-H bond across an unactivated C–C double bond is an appealing approach to rapidly access such highly valuable N- and O-containing skeletons in a waste-free and 100% atom efficient process. Herein, we report, for the first time, the efficient and high-yield cyclization of δ/ε-alkenoic acids and N-protected δ-alkenamides catalyzedby practical and easily accessible Lewis acid scandium(III) triflate under thermal and microwave conditions. The selectivity outcome of the reaction of δ/ε-alkenoic acids was dependent on the substitution patterns of the backbone chain and alkene moiety, leading to the exclusive formation of either the corresponding γ/δ-lactones via an O-selective cyclization or the Friedel–Crafts-type product by C-selective cyclization. An uncommon and rarely disclosed O-selective cyclization occurred preferentially or exclusively when N-protected δ-alkenamides were engaged in the reaction. The atom selectivity of the cyclization was unambiguously confirmed by single crystal X-ray crystallography.

Unified Approach to Diverse Fused Fragments via Catalytic Dehydrative Cyclization

A range of highly functionalized polycyclic fragments have been synthesized, employing a catalytic dehydrative cyclization. A range of nucleophiles are shown to be successful, with the reaction producing numerous high value motifs.

Triflamides and Triflimides: Synthesis and Applications

Among the variety of sulfonamides, triflamides (CF3SO2NHR, TfNHR) occupy a special position in organic chemistry. Triflamides are widely used as reagents, efficient catalysts or additives in numerous reactions. The reasons for the widespread use of these compounds are their high NH-acidity, lipophilicity, catalytic activity and specific chemical properties. Their strong electron-withdrawing properties and low nucleophilicity, combined with their high NH-acidity, makes it possible to use triflamides in a vast variety of organic reactions. This review is devoted to the synthesis and use of N-trifluoromethanesulfonyl derivatives in organic chemistry, medicine, biochemistry, catalysis and agriculture. Part of the work is a review of areas and examples of the use of bis(trifluoromethanesulfonyl)imide (triflimide, (CF3SO2)2NH, Tf2NH). Being one of the strongest NH-acids, triflimide, and especially its salts, are widely used as catalysts in cycloaddition reactions, Friedel-Crafts reactions, condensation reactions, heterocyclization and many others. Triflamides act as a source of nitrogen in C-amination (sulfonamidation) reactions, the products of which are useful building blocks in organic synthesis, catalysts and ligands in metal complex catalysis, and have found applications in medicine. The addition reactions of triflamide in the presence of oxidizing agents to alkenes and dienes are considered separately.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Transition-Metal-Free Regioselective Intermolecular Hydroamination of Conjugated 1,3-Dienes with Heterocyclic Amines

The unique property of hexafluoroisopropanol (HFIP) enables the regioselective hydroamination of 1,3-dienes with nitrogen heterocycles in a Markovnikov manner in the presence of catalytic Brønsted acid. This transition-metal-free intermolecular hydroamination protocol is achieved under mild reaction conditions. The aggregation by HFIP and Brønsted acid helps to activate the terminal double bond regioselectively. Following the protonation of diene, the C-N bond formation is accomplished upon the involvement of heterocyclic amines.

Direct Assembly of Phthalides via Calcium(II)-Catalyzed Cascade ortho-C-Alkenylation/Hydroacyloxylation of 3-Aminobenzoic Acids with Alkynes in Hexafluoroisopropanol

By virtue of a calcium(II)/hexafluoroisopropanol cocatalytic system, the efficient and practical coupling of 3-aminobenzoic acids with alkynes has been realized, giving direct and regioselective access to the phthalide framework with good substrate/functional group compatibility. Mechanistic studies identified 3-amino-2-vinylbenzoic acid species as the active intermediate, thereby revealing an ortho-C-alkenylation/hydroacyloxylation cascade for this transformation.

Studies on the Origin of the Stabilizing Effects of Fluorinated Alcohols and Weakly Coordinated Fluorine-Containing Anions on Cationic Reaction Intermediates

Many synthetic methods that use fluorinated alcohols as solvents have been reported, and the fluorinated alcohols have been found to be crucial to the success of these methods. In addition, there have been reports indicating that adding a weakly coordinated fluorine-containing anion, such as BF₄^-, PF₆^-, or SbF₆^-, to fluorinated alcohols can improve yields. The boosting effect of fluorinated alcohols is attributed mainly to hydrogen bond activation. A few studies have suggested that the very polar fluorinated alcohols can stabilize cationic reaction intermediates. However, how they do so and why weakly coordinated fluorine-containing anions improve yields have not been studied in depth. Here, we used quaternary ammonium cations, a quaternary phosphonium cation, and a triaryl-substituted carbocation as models for short-lived cationic intermediates and studied the possible interactions of these cations with fluorinated alcohols and BF₄^-, PF₆^-, or SbF₆^-. On the basis of the results, we propose that the C-F dipoles of fluorinated alcohols and the E-F dipoles (where E is B, P, or Sb) of weakly coordinated fluorine-containing anions stabilized these cations by intermolecular charge-dipole interactions. We deduced that in the same fashion the C-F and E-F dipoles can thermodynamically stabilize cationic reaction intermediates.

Direct activation of alcohols via perrhenate ester formation for an intramolecular dehydrative Friedel–Crafts reaction

A general and highly efficient intramolecular dehydrative Friedel–Crafts reactions via Re2O7 mediated hydroxyl group activation is described for the syntheses of tetrahydronaphthalene, tetrahydroquinoline, tetrahydroisoquinoline, chromane, and isochromane derivatives.

DFT Analysis into the Calcium(II)-Catalyzed Coupling of Alcohols With Vinylboronic Acids: Cooperativity of Two Different Lewis Acids and Counterion Effects

The mechanism of the calcium-catalyzed coupling of alcohols with vinylboronic acids has been analyzed by means of density functional theory computations. This study reveals that the calcium and boron Lewis acids associate to form a superelectrophile able to promote a pericyclic group transfer reaction with allyl alcohols. With other alcohols, the two Lewis acids act synergistically to activate the OH functionality and trigger a S_Ni reaction pathway. These two mechanisms are affected by the nature of the counterions, which has been rationalized by electronic and steric factors.

Earth-Abundant 3d Transition Metal Catalysts for Hydroalkoxylation and Hydroamination of Unactivated Alkenes

This review summarizes the most noteworthy achievements in the field of C–O and C–N bond formation by hydroalkoxylation and hydroamination reactions on unactivated alkenes (including 1,2- and 1,3-dienes) promoted by earth-abundant 3d transition metal catalysts based on manganese, iron, cobalt, nickel, copper and zinc. The relevant literature from 2012 until early 2021 has been covered.

Modular Synthesis of 9,10-Dihydroacridines through an ortho-C Alkenylation/Hydroarylation Sequence between Anilines and Aryl Alkynes in Hexafluoroisopropanol

9,10-Dihydroacridines are frequently encountered as key scaffolds in OLEDs. However, accessing those compounds from feedstock precursors typically requires multiple steps. Herein, a modular one-pot synthesis of 9,10-dihydroacridine frameworks is achieved through a reaction sequence featuring a selective ortho-C alkenylation of diarylamines with aryl alkynes followed by an intramolecular hydroarylation of the olefin formed as an intermediate. This transformation was accomplished by virtue of the combination of hexafluoroisopropanol and triflimide as a catalyst that triggers the whole process.

Exploiting hexafluoroisopropanol (HFIP) in Lewis and Brønsted acid-catalyzed reactions

Hexafluoroisopropanol (HFIP) is a solvent with unique properties that has recently gained attention for promoting a wide range of challenging chemical reactions. It was initially believed that HFIP was almost exclusively involved in the stabilization of cationic intermediates, owing to its high polarity and low nucleophilicity. However, in many cases, the mechanism of action of HFIP appears to be more complex. Recent findings reveal that many Lewis and Brønsted acid-catalyzed transformations conducted in HFIP additionally involve cooperation between the catalyst and HFIP hydrogen-bond clusters, akin to Lewis- or Brønsted acid-assisted-Brønsted acid catalysis. This feature article showcases the remarkable versatility of HFIP in Lewis and Brønsted acid-catalyzed reactions, with an emphasis on examples yielding mechanistic insight.

Calcium-Catalyzed Stereoselective Tandem [4+2] and [3+2] Annulation Reaction for the Synthesis of Dihydropyrrolo[1,2-a]quinolines

Calcium-catalyzed highly facile one-pot, A⁴ annulation of aldehyde, amine, alkene, and alkyne to form fused 4,5-dihydropyrrolo[1,2-a]quinolines with exclusive syn diastereoselectivity is reported. This selectivity arises from an inverse electron demand [4+2] aza-Diels-Alder cycloaddition, and the adduct further undergoes a formal [3+2] cyclization with activated alkynes. This diversity-oriented protocol is highly general and furnishes the dihydropyrrolo[1,2-a]quinoline derivatives with a broad substrate scope in good to excellent yields.