Diarylmethane synthesis through Re2O7-catalyzed bimolecular dehydrative Friedel-Crafts reactions

Metal-Free Switchable Chemo- and Regioselective Alkylation of Oxindoles Using Secondary Alcohols

In this study, we have disclosed N-alkylation and C-alkylation reactions of 2-oxindoles with secondary alcohols. Interestingly, these chemoselective reactions are tunable by changing the reaction conditions. Utilization of protic solvent and Brønsted acid catalyst afforded C-alkylation, whereas, aprotic solvent and Lewis acid catalyst afforded N-alkylation of 2-oxindoles in good to excellent yields. Regioselectivity is achieved by protecting the N-center of the oxindole and C5 alkylated product is furnished exclusively. This protocol is notable because it demonstrates functionalization at the C7 position of oxindole without the need for any directing group at the N-center. Further, a new protocol has been reported for C-H oxygenation at the benzylic position of one of the C5 alkylated derivative.

Alcohols as Substrates in Transition-Metal-Catalyzed Arylation, Alkylation, and Related Reactions

Alcohols are abundant and attractive feedstock molecules for organic synthesis. Many methods for their functionalization require them to first be converted into a more activated derivative, while recent years have seen a vast increase in the number of complexity-building transformations that directly harness unprotected alcohols. This Review discusses how transition metal catalysis can be used toward this goal. These transformations are broadly classified into three categories. Deoxygenative functionalizations, representing derivatization of the C-O bond, enable the alcohol to act as a leaving group toward the formation of new C-C bonds. Etherifications, characterized by derivatization of the O-H bond, represent classical reactivity that has been modernized to include mild reaction conditions, diverse reaction partners, and high selectivities. Lastly, chain functionalization reactions are described, wherein the alcohol group acts as a mediator in formal C-H functionalization reactions of the alkyl backbone. Each of these three classes of transformation will be discussed in context of intermolecular arylation, alkylation, and related reactions, illustrating how catalysis can enable alcohols to be directly harnessed for organic synthesis.

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.

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.

A Fenton Approach to Aromatic Radical Cations and Diarylmethane Synthesis

Manipulating carbon-centered radicals to add to electron-deficient systems is a well-precedented process. By coupling the Fe(II)-mediated Fenton reaction with the Fe(III)-mediated single-electron oxidation of anisolic compounds, we demonstrate how electron-rich carbon-centered radicals can react with electron-rich arenes through a radical-polar cascade pathway. This bioinspired approach produces diarylmethane derivatives from simple unfunctionalized precursors.

Synthesis of 1-Aryltetralins via Re2O7/HReO4 Mediated Intramolecular Hydroarylations

Here, we describe highly efficient intramolecular hydroarylations mediated by Re2O7/HReO4. Styrene derivatives of different electronic properties have been activated to effect a challenging intramolecular hydroarylation for the facile access to various substituted 1-aryltetralin structures. This method is characterized by mild reaction conditions, broad substrate scope, high chemical yields, and 100% atom economy. The potential synthetic application of this methodology was exemplified by the efficient total synthesis of an isoCA-4 analogue.

Ring-closing C-O/C-O metathesis of ethers with primary aliphatic alcohols

In canonical organic chemistry textbooks, the widely adopted mechanism for the classic transetherifications between ethers and alcohols starts with the activation of the ether in order to weaken the C-O bond, followed by the nucleophilic attack by the alcohol hydroxy group, resulting in a net C-O/O-H σ-bond metathesis. In this manuscript, our experimental and computational investigation of a Re₂O₇ mediated ring-closing transetherification challenges the fundamental tenets of the traditional transetherification mechanism. Instead of ether activation, the alternative activation of the hydroxy group followed by nucleophilic attack of ether is realized by commercially available Re₂O₇ through the formation of perrhenate ester intermediate in hexafluoroisopropanol (HFIP), which results in an unusual C-O/C-O σ-bond metathesis. Due to the preference for the activation of alcohol rather than ether, this intramolecular transetherification reaction is therefore suitable for substrates bearing multiple ether moieties, unparalleled by any previous methods.

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.

Temperature-Assisted Generation of Arylmethyl Radicals from Bis(arylmethyl)tin Dichlorides: Efficient Reagents for C s p 3 ${{{\bf C}}_{{{\bf s p}}^{3}}}$ - C s p 2 ${{{\bf C}}_{{{\bf s p}}^{2}}}$ Bond-Forming Reactions

The oxidative-addition reaction between an arylmethyl chloride (RCH₂ Cl; R=1-C₁₀ H₇ , 2,4,6-Me₃ C₆ H₂ , 4-MeC₆ H₄ , 3-MeC₆ H₄ , C₆ H₅ , 4-ClC₆ H₄ ) and tin powder in boiling toluene produces bis(arylmethyl)tin dichlorides, [(RCH₂ )₂ SnCl₂ ] in good yields. At 160 °C in mesitylene bis(1-naphthylmethyl)tin dichloride undergoes Sn-C homolytic cleavage to generate two 1-naphthylmethyl radicals (1-C₁₀ H₇ CH₂ ⋅) which were trapped by TEMPO (C₉ H₈ NO⋅). Subsequently, the radicals (RCH₂ ⋅) produced in this manner were utilized for efficient substitution reactions with electron-rich arenes (R'H; R'=2,4,6-Me₃ C₆ H₂ , 1,2,4,5-Me₄ C₆ H, 1,2,3,4,5-Me₅ C₆ ) to obtain a variety of unsymmetrical diarylmethanes (RR'CH₂ ). The addition of one equivalent of iodine (I₂ ) to the reaction mixture resulted in a significant increase in the yields of coupled products.

Cooperative Friedel-Crafts Alkylation of Electron-Deficient Arenes via Catalyst Activation with Hexafluoroisopropanol

A Friedel-Crafts alkylation of electron-deficient arenes with aldehydes through ''catalyst activation'' is presented. Through hydrogen bonding interactions, the solvent 1,1,1,3,3,3, -hexafluoroisopropanol (HFIP) interacted with the added Brønsted acid catalyst pTSA•H₂ O, increasing its acidity. This activated catalyst enabled the Friedel-Crafts alkylation of electron-neutral as well as electron-deficient arenes. Strongly electron withdrawing arenes including arenes with multiple halogen atoms, NO₂ , CHO, CO₂ R, and CN, groups acted as efficient nucleophiles in this reaction. DFT studies reveal multiple roles of solvent HFIP viz; increasing the Brønsted acidity of the catalyst pTSA•H₂ O, and stabilization of the transition states through a concerted pathway enabling the challenging reaction.

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^-).

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.

Synergistic Brønsted/Lewis acid catalyzed aromatic alkylation with unactivated tertiary alcohols or di-tert-butylperoxide to synthesize quaternary carbon centers

Dual Brønsted/Lewis acid catalysis involving environmentally benign, readily accessible protic acid and iron promotes site-selective tert-butylation of electron-rich arenes using di-tert-butylperoxide. This transformation inspired the development of a synergistic Brønsted/Lewis acid catalyzed aromatic alkylation that fills a gap in the Friedel–Crafts reaction literature by employing unactivated tertiary alcohols as alkylating agents, leading to new quaternary carbon centers. Corroborated by DFT calculations, the Lewis acid serves a role in enhancing the acidity of the Brønsted acid. The use of non-allylic, non-benzylic, and non-propargylic tertiary alcohols represents an underexplored area in Friedel–Crafts reactivity.

Dual Brønsted/Lewis acid catalysis involving environmentally benign, readily accessible protic acid and iron promotes site-selective tert-alkylation of arenes using di-tert-butylperoxide and tertiary alcohols.

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.

Solvent Effects and Mechanistic Studies for Re2O7-Catalyzed Allylative Annulation Reactions

The sequence of allylic alcohol transposition, carbonyl group trapping, oxocarbenium ion formation, and nucleophilic addition results in the formation of a ring while serving as a fragment-coupling and stereocenter-generating reaction. Successful applications of these processes require a balancing of the kinetics of numerous productive and unproductive steps. This work describes the manner in which solvent changes can be used to expand the scope and change the stereochemical outcomes of these processes. Mechanistic studies provide greater insight into the nuances of the transformations and the reactive species that are generated.

Dehydrative Re2O7-Catalyzed Approach to Dihydropyran Synthesis

Monoallylic 1,3- and 1,5-diols undergo Re₂O₇-mediated ionization to form allylic cations that engage in cyclization reactions to form dihydropyran products. The reactions give the 2,6-trans-stereoisomer as the major products as a result of minimizing steric interactions in a boat-like transition state. The results of these studies are consistent with cationic intermediates, with an intriguing observation of stereochemical retention in one example.

Dictating Thermodynamic Control through Tethering: Applications to Stereoselective Bis-Spiroketal Synthesis

Approaches to stereocontrol that invoke thermodynamic control fail when two or more potential products are energetically similar, but rational structural perturbations can be employed to break the energetic degeneracy and provide selective transformations. This manuscript illustrates that tethering is an effective approach for the stereoselective construction of bis-spiroketals with thermodynamically similar stereoisomers, providing a new approach to set remote stereocenters and prepare complex structures that have not previously been accessed stereoselectively.

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.

Re2O7-Catalyzed Approach to Spirocyclic Ether Formation from Acyclic Precursors: Observation of Remote Stereoinduction

Ketones that are flanked by an allylic alcohol and an alkene isomerize to spirocyclic ethers in the presence of Re2O7 through allylic alcohol transposition, oxocarbenium ion formation, and Prins cyclization. These processes provide significant increases in molecular complexity, with multiple stereocenters being set relative to a stereocenter in the substrate. Stereoselectivity arises from the initial reversible steps being more rapid than the final step, thereby allowing for thermodynamically controlled stereochemical equilibration prior to product formation.

Two-component boronic acid catalysis for increased reactivity in challenging Friedel–Crafts alkylations with deactivated benzylic alcohols

The combination of a boronic acid catalyst with perfluoropinacol as a co-catalyst improves the scope of Friedel–Crafts benzylations of arenes with electronically deactivated primary and secondary benzylic alcohols.