Hafnium Triflate as an Efficient Catalyst for Direct Friedel–Crafts Reactions of Chromene Hemiacetals

The direct Friedel–Crafts reaction of chromene hemiacetals with indoles, furans and sterically hindered anilines has been accomplished with high selectivity and excellent yields in the presence of a catalytic amount of hafnium triflate [Hf(OTf)₄, 0.1 mol%, 0–40 °C]. The mild conditions tolerate various sensitive functional and protecting groups, and the products were confirmed unambiguously from their spectra and by single‐crystal X‐ray analysis. This direct Friedel–Crafts reaction of chromene hemiacetals should inspire and encourage the consideration of hafnium triflate in the development of mild reaction conditions for the efficient derivatization of hemiacetal‐containing compounds.     

In situ compatibilization of polystyrene/polyolefin elastomer blends by the Friedel–Crafts alkylation reaction

Blends of polystyrene (PS) with polyolefin elastomer (POE) were prepared by a reactive extrusion method. In order to increase the compatibility of the two blending components, a Lewis acid catalyst, aluminium chloride (AlCl₃), was adopted to initiate the Friedel–Crafts alkylation reaction. Fourier‐transform infrared (FTIR) spectra of the PS/POE/AlCl₃ blends extracted with butanone verified the graft structure between the PS and POE. Because the in situ generated PS‐graft‐POE copolymers acted as compatibilizers, the mechanical properties of PS/POE blends were greatly improved. For example, after compatibilization, the Charpy impact strength of an 80/20 (wt%) PS/POE blend was increased from 6.29 to 8.50 kJ m^?2. Scanning electron microscopy (SEM) showed that the size of the droplets decreased from 9–10 µm to less than 2 µm with the addition of AlCl₃. Gel permeation chromatography (GPC) showed competition between the grafting reaction and the degradation of blending components in the presence of AlCl₃. Copyright © 2005 Society of Chemical Industry     

Efficient Synthesis of 9‐Tosylaminofluorene Derivatives by Boron Trifluoride Etherate‐Catalyzed Aza‐Friedel–Crafts Reaction of in situ Generated N‐Tosylbenzaldimines

An efficient and expeditious boron trifluoride etherate (BF₃⋅Et₂O) catalyzed one‐pot reaction for the synthesis of N‐tosyl‐9‐aminofluorenes and anthracene derivatives from in situ generated N‐tosylbenzaldimines via an aza‐Friedal–Crafts reaction has been developed. The catalytic reaction shows high substrate tolerance with excellent yields.     

A Copper(II) Triflate‐Catalyzed Tandem Friedel–Crafts Alkylation/Cyclization Process towards Dihydroindenes

A one‐pot synthesis of dihydroindenes from substituted benzenes and haloalkenes was developed. The reaction proceeded via a copper(II) triflate [Cu(OTf)₂]‐catalyzed tandem Friedel–Crafts alkylation/cyclization process with high efficiency under relatively mild conditions.     

Triarylaminium Salt‐Initiated Aerobic Double Friedel–Crafts Reaction of Glycine Derivatives with Indoles

A novel and efficient stable radical cation triarylaminium salt‐catalyzed aerobic double Friedel–Crafts alkylation reaction of glycine derivatives with indoles has been developed. The reaction was performed in the absence of any other additives under mild conditions and only requires an air atmosphere (or oxygen, 1 atm) as co‐oxidant.     

Enantioselective Organocatalytic Synthesis of Arylglycines via Friedel–Crafts Alkylation of Arenes with a Glyoxylate Imine

The enantioselective organocatalytic synthesis of arylglycines has been developed employing 1 mol% of an enantiopure N‐triflyl phosphoramide Brønsted acid as organocatalyst. Various differently substituted phenylglycine derivatives can be synthesized in good to excellent yields and enantiomeric excesses based on a Friedel–Crafts alkylation of electron‐rich arenes with a glyoxylate imine. A novel protocol for the deprotection of the N‐tert‐butylsulfonyl (Bus) group has also been developed.     

Highly Enantioselective Zinc‐Catalyzed Friedel–Crafts Alkylation of Indoles with Ethyl Trifluoropyruvate

A highly enantioselective Friedel–Crafts alkylation of indoles with ethyl trifluoropyruvate has been developed using N,N′‐dioxide‐zinc(II) complexes. Both enantiomers of the desired adducts were obtained by the use of enantiomeric ligands in excellent results (up to 99% yield and 98% ee) within 0.5 h under mild conditions. On the basis of the experimental results, a proposed working model was proposed to explain the origin of the asymmetric induction.     

Asymmetric Friedel–Crafts Reaction of Indoles with Ethyl Trifluoropyruvate Using a Copper(I)‐Bisoxazolidine Catalyst

Bisoxazolidine 1 is an effective ligand in the copper(I)‐catalyzed Friedel–Crafts reaction of alkyl trifluoropyruvates and indoles. A range of ethyl 2‐(3′‐indolyl)‐3,3,3‐trifluoro‐2‐hydroxypropanoates was produced in up to 99% yield and 94% ee within 30 min to 4 h. The effect of temperature on conversion and enantioselectivity proved to be substrate specific and was optimized individually. Of particular interest is that this method tolerates the presence of substituents in various positions in the indole ring. Yields ranging from 90–97% and ee values between 90 and 94% were obtained at optimized temperatures with substrates carrying substituents in position 1 or 7.     

Enantioselective Construction of Cyclic Indolyl α‐Amino Esters via a Friedel–Crafts Alkylation Reaction

An enantioselective Friedel–Crafts alkylation reaction of indoles with cyclic N‐sulfonyl ketimino esters was developed. Under the optimized conditions using a chiral copper(II) triflate‐bisoxazoline complex as the catalyst, a range of N‐sulfonyl ketimino ester derivatives and indoles reacted smoothly to afford indole‐containing chiral cyclic α‐amino esters bearing tetrasubstituted α‐stereogenic centers [3‐ethoxycarbonyl‐3‐(3‐indolyl)‐2,3‐dihydrobenzo[d]isothiazole 1,1‐dioxides] in excellent yields and with high enantioselectivities (up to 99% ee). Pyrrole and N,N‐dimethylaniline were also investigated as aromatic substrates to afford the corresponding products with good results. An asymmetric induction model was then proposed on the basis of the observed absolute configuration of the product 3‐ethoxycarbonyl‐3‐(5‐bromo‐3‐indolyl)‐2,3‐dihydrobenzo[d]isothiazole 1,1‐dioxide. Synthetic transformations to convert the products into cyclic chiral N‐sulfonamido alcohols and the deprotection of the sulfonamides were performed. This study provides an efficient approach to chiral α‐tetrasubstituted indolic α‐amino acids as potential building blocks for peptides and biologically active molecules.

     

Facile Creation of 3‐Indolyl‐3‐hydroxy‐2‐oxindoles by an Organocatalytic Enantioselective Friedel–Crafts Reaction of Indoles with Isatins

The first direct enantioselective Friedel–Crafts reaction of indoles with isatins has been developed. The process is catalyzed by simple cupreine under mild reaction conditions and affords synthetically and biologically interesting, chiral 3‐indolyl‐3‐hydroxy‐2‐oxindoles in good yields (68–97%) and with high enantioselectivities (76–91%).     

Polyolefin/polystyrene in situ compatibilization using Friedel–Crafts alkylation

Polyolefin/polystyrene (PS) blends are difficult to compatibilize using in situ reactive compatibilization techniques, because neither of these polymers has any functional groups that one can use in the formation of a copolymer from these polymer components. In this study, the Friedel–Crafts alkylation was realized in a polyethylene/PS (PE/PS) melt blend, which resulted in improved compatibility between PE and PS. A number of Lewis acid compounds were tested as catalysts, among which the AlCl₃ was the most efficient. It was found in this study that the presence of a cocatalyst, such as a cationically polymerizable monomer or a halogenated alkane, significantly enhances the formation of PE-g-PS copolymer. The effects of blending parameters, such as temperature and blending time, on the in situ copolymer formation were investigated. The mechanical properties of compatibilized PE/PS blends were improved considerably. Such an in situ compatibilization technique has potential in the recycling of mixed polymer wastes. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 1385–1393, 1997     

Highly Enantioselective Friedel–Crafts Reaction of Indole with Alkylidenemalonates Catalyzed by Heteroarylidene Malonate‐Derived Bis(oxazoline) Copper(II) Complexes

A series of cheap and easily accessible heteroarylidenemalonate‐derived bis(oxazoline) ligands 1 and 2 were synthesized and their copper(II) complexes were applied to the catalytic Friedel–Crafts reaction between indoles and diethyl alkylidenemalonates, Excellent asymmetric enantioselectivities were afforded for the S‐enantiomer (up to >99% ee) in isobutyl alcohol, and the R‐enantiomer (up to 96.5% ee) in dichloromethane.     

Catalytic Friedel–Crafts Acylation and Benzoylation of Aromatic Compounds Using Activated Hematite as a Novel Heterogeneous Catalyst

Catalytic Friedel–Crafts acylation of benzene and unactivated benzenes such as chlorobenzene and nitrobenzene have been successfully carried out using activated hematite (α‐Fe₂O₃) as a new, heterogeneous and green catalyst. Sonication of neat α‐Fe₂O₃ in a water bath under air atmosphere at room temperature followed by heating at 200 °C, dramatically increase the activity of α‐Fe₂O₃. With the catalyst loading as low as 5.0 mol%, a wide variety of benzene derivatives were easily converted into the corresponding acylated products in a clean and high‐yielding acylation reaction. It was found that the activated α‐Fe₂O₃ could be efficiently recycled and reused several times by simple washing with ethyl acetate, this cannot be attained with most of the traditional catalysts.     

Novel Enantiocomplementary C2‐Symmetric Chiral Bis(imidazoline) Ligands: Highly Enantioselective Friedel–Crafts Alkylation of Indoles with Ethyl 3,3,3‐Trifluoropyruvate

Enantioselective Friedel–Crafts alkylations of a variety of indoles with ethyl 3,3,3‐trifluoropyruvate catalyzed by novel chiral m‐phenylenebis(imidazoline)‐copper(II) complexes or the bis(imidazoline)‐achiral acid combination afforded products with high enantioselectivity. Both enantiomers of indole derivatives can be prepared with high enantioselectivities by tuning the N‐substituents of the imidazoline.     

Synthesis of carvacrol by Friedel–Crafts alkylation of o‐cresol with isopropanol using superacidic catalyst UDCaT‐5

BACKGROUND: Alkylation of o‐cresol with propylene or isopropyl alcohol (IPA) over solid acid catalysts results in the formation of carvacrol, which finds potential applications in the synthesis of fine chemicals, intermediates, specialty chemicals, flavours and fragrances. RESULTS: The present work covers evaluation of novel mesoporous superacidic catalysts namely UDCaT‐4, UDCaT‐5 and UDCaT‐6 in the greener synthesis of carvacrol. The catalysts are modified versions of zirconia showing high catalytic activity, stability and reusability. The catalytic activity increases in the following order: UDCaT‐5 > UDCaT‐4 > UDCaT‐6 > sulfated zirconia. The process was optimized and a mathematical model developed to describe the reaction pathway in liquid phase. Carvacrol could be efficiently obtained with a selectivity up to 82% at an isopropanol conversion of 98% after 2 h over UDCaT‐5 at 180 °C. CONCLUSION: The reaction is free from any external mass transfer as well as intraparticle diffusion limitations and is intrinsically kinetically controlled. An overall second‐order kinetic equation was used to fit the experimental data. The activation energy was found to be 19.2 kcal/mol. The reaction was carried out without any solvent in order to make the process cleaner and greener. Copyright © 2009 Society of Chemical Industry     

Chiral Phosphoric Acid‐Catalyzed Asymmetric Aza‐Friedel–Crafts Reaction of Indoles with Cyclic N‐Acylketimines: Enantioselective Synthesis of Trifluoromethyldihydroquinazolines

An enantioselective aza‐Friedel–Crafts (F‐C) reaction of indoles with cyclic N‐acylketimines has been developed. By using chiral phosphoric acid catalysts, a wide range of enantioenriched trifluoromethyldihydroquinazolines was obtained in excellent yields (up to 98%) with good to high enantioselectivities (up to 99% ee).

     

Effect of Friedel–Crafts reaction on the thermal stability and flammability of high‐density polyethylene/brominated polystyrene/graphene nanoplatelet composites

Brominated flame‐retarded high‐density polyethylene (HDPE) composites containing graphene nanoplatelets (GNPs) were prepared via melt blending. A Lewis acid catalyst, anhydrous aluminium chloride (AlCl₃), was added to initiate Friedel–Crafts reaction for promoting the dispersion of the GNPs in the polymer matrix. Transmission electron microscopy images and Raman spectroscopy revealed that the GNPs were partly unfolded and the domains became smaller in the presence of AlCl₃. Limiting oxygen index and microscale combustion calorimetry showed that the incorporation of AlCl₃ into HDPE reduced flammability and slowed down the heat release rate. Thermogravimetric analysis and char residue measurements proved that a uniform dispersion of GNPs was crucial for forming a continuous and compact carbon layer, thus isolating the underlying materials from flame and preventing heat transfer. Rheological and mechanical tests indicated that interfacial adhesion between polymer chains and GNPs was enhanced. © 2014 Society of Chemical Industry     

Asymmetric Brønsted Acid‐Catalyzed Friedel–Crafts Reactions of Indoles with Cyclic Imines ‐ Efficient Generation of Nitrogen‐Substituted Quaternary Carbon Centers

A new enantioselective Brønsted acid‐catalyzed Friedel–Crafts reaction of indole with cyclic imines has been develeoped. This organocatalytic reaction provides for the first time optically active indolindolinone derivatives in high yields and with excellent enantioselectivities (up to 91% ee) under mild reaction conditions.