N‐Bromosuccinimide‐Mediated Radical Cyclization of 3‐Arylallyl Azides: Synthesis of 3‐Substituted Quinolines

Visible light irradiation of N‐bromosuccinimide serves as an effective means to convert methyl 2‐(azidomethyl)‐3‐arylpropenoates and 2‐(azidomethyl)‐3‐arylacrylonitriles to the corresponding iminyl radicals via α‐hydrogen abstraction and subsequent extrusion of dinitrogen. Thus formed iminyl radicals then undergo intramolecular ortho attack on the aryl ring, affording methyl quinoline‐3‐carboxylates and quinoline‐3‐carbonitriles respectively.

     

Regioselective Synthesis of β‐Aryl Enaminones and 1,4,5‐ Trisubstituted 1,2,3‐Triazoles from Chalcones and Benzyl Azides

A highly regioselective synthesis of β‐aryl enaminones and 1,4,5‐trisubstituted 1,2,3‐triazoles from chalcones and benzyl azides based on reaction solvent selection is reported. In the presence of a catalytic amount of Ce(OTf)₃, reactions of chalcones with benzyl azides in DMF at 100 °C afforded densely substituted Z‐β‐aryl enaminones in good to excellent yields, whereas treatment of chalcones with benzyl azides in toluene at 100 °C selectively produced 1,4,5‐trisubstituted 1,2,3‐triazoles in excellent yields.

     

Synthesis of α‐Diazo Carbonyl Compounds with the Shelf‐Stable Diazo Transfer Reagent Nonafluorobutanesulfonyl Azide

Nonafluorobutanesulfonyl azide is a shelf‐stable, cost‐effective and general diazo transfer reagent for the efficient synthesis of α‐diazo carbonyl compounds in excellent yields and in very short reaction times, under mild conditions. The diazo products can be readily isolated in pure form after a simple aqueous extractive work‐up that avoids chromatographic purification in most cases. Because of its high efficiency and wide substrate scope, shelf‐stability, relatively low cost, and ease of product purification, nonafluorobutanesulfonyl azide offers an advantageous alternative to other commonly used diazo transfer reagents.     

Manganese(II)‐Mediated Domino Annulation Reaction of Vinyl Azides and 4‐Hydroxycoumarin: A Stereoselective Synthesis of Spirobenzofuranone‐lactams

A manganese(II) acetate‐catalyzed domino reaction of vinyl azides and 4‐hydroxycoumarin has been developed for the synthesis of polyfunctionalized spirofuranone‐lactams. A wide range of vinyl azides are capable of providing the desired spirofuranone‐lactams in good to excellent yields. The reaction was achieved via thermal decomposition of vinyl azides to 2H‐azirines, followed by an intramolecular nucleophilic attack and stereoselective cyclization. The mild reaction conditions and easy operation make this reaction advantageous for the synthesis of spirofuranone‐lactams.

     

Visible Light‐Mediated Metal‐Free Synthesis of Vinyl Sulfones from Aryl Sulfinates

Visible light and eosin Y catalyze the synthesis of vinyl sulfones from aryl sulfinates and alkenes by a photoredox process. The reaction scope is broad in aryl sulfinates and alkenes and the general and simple procedure provides a metal‐free alternative for the synthesis of synthetically valuable vinyl sulfones.

     

tert‐Butyl Hydroperoxide and Tetrabutylammonium Iodide‐Promoted Free Radical Cyclization of α‐Imino‐N‐arylamides and α‐Azido‐N‐arylamides

The oxidizing system of tert‐butyl hydroperoxide (TBHP) and tetrabutylammonium iodide (TBAI) is capable of generating α‐(arylaminocarbonyl)iminyl radicals from ethyl 2‐(N‐arylcarbamoyl)‐2‐iminoacetates. These iminyl radicals preferably undergo intramolecular ipso attack on the benzene ring to give azaspirocyclohexadienyl radicals, which are readily captured by molecular oxygen under an oxygen atmosphere to yield azaspirocyclohexadienones. In the absence of oxygen, the reaction affords quinoxalin‐2‐one products. This oxidizing system is also effective to convert α‐aryl‐α‐azido‐N‐arylamides to the corresponding iminyl radicals under basic conditions (sodium tert‐butoxide, t‐BuONa), and the subsequent cyclization of these iminyl radicals results in the formation of azaspirocyclohexadienone products in high yields under an oxygen atmosphere. Plausible mechanisms are proposed to rationalize the experimental results, and factors influencing the reactions are discussed.

     

Enantioselective Protonation by Aza‐Michael Reaction between Pyrazoles and α‐Substituted Vinyl Ketones

An enantioselective protonation by means of chiral scandium complex‐catalyzed aza‐Michael reaction was realized. A series of α‐aryl‐substituted vinyl ketones reacted with pyrazoles smoothly, affording the corresponding enantiomerically enriched pyrazole derivatives with excellent results (up to 99% yield, 94% ee). Water and hydrogen chloride were found to accelerate the protonation process.

     

Synthesis of Ag/g‐C3N4 Composite as Highly Efficient Visible‐Light Photocatalyst for Oxidative Amidation of Aromatic Aldehydes

In this contribution, an Ag/g‐C₃N₄ nanocomposite was synthesized and utilized as highly efficient and green photocatalyst for organic reactions under visible light irradiation. A layered, porous g‐C₃N₄ was synthesized following a modified solvothermal‐roasting process by using melamine and cyanuric chloride as precursor. Silver nanoparticles (NPs) were well anchored on g‐C₃N₄ nanosheets, which were prepared by a facile impregnation–roasting method. The inexpensive, stable g‐C₃N₄ coupled with the localized surface plasmon resonance (LSPR) effect of Ag NPs exhibited high photocatalytic activities toward aerobic oxidative amidation of aromatic aldehydes under visible light irradiation. Good to excellent yields were achieved for various substrates under the light of a 25 W compact fluorescent light (CFL) bulb in air. The operationally easy procedure provides an economical, green, and mild alternative for the formation of amide bonds.

     

Improved Radical Approach to N‐Unsubstituted Indol‐2‐one and Dihydro‐2‐quinolinone Compounds Bearing Spirocyclic Cyclohexanone/Cyclohexadienone Rings

N‐Unsubstituted indol‐2‐one and 3,4‐dihydro‐2‐quinolinone compounds bearing spirocyclic cyclohexanone/cyclohexadienone rings can be accessed through a novel, improved method entailing the key radical spirocyclization of easily accessible N‐methoxymethyl (MOM)‐protected 2‐iodoanilides derived from 4‐azidobenzoic and 2‐(4‐azidophenyl)acetic acid.     

Photo‐Induced atom transfer radical polymerization with nanosized α‐Fe2O3 as photoinitiator

Photo‐induced atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) was achieved in poly(ethylene glycol)‐400 with nanosized α‐Fe₂O₃ as photoinitiator. Well‐defined poly(methyl methacrylate) (PMMA) was synthesized in conjunction with ethyl 2‐bromoisobutyrate (EBiB) as ATRP initiator and FeCl₃·6H₂O/Triphenylphosphine (PPh₃) as complex catalyst. The photo‐induced polymerization of MMA proceeded in a controlled/living fashion. The polymerization followed first‐order kinetics. The obtained PMMA had moderately controlled number‐average molecular weights in accordance with the theoretical number‐average molecular weights, as well as narrow molecular weight distributions (M_w/M_n). In addition, the polymerization could be well controlled by periodic light‐on–off processes. The resulting PMMA was characterized by ¹H nuclear magnetic resonance and gel permeation chromatography. The brominated PMMA was used further as macroinitiator in the chain‐extension with MMA to verify the living nature of photo‐induced ATRP of MMA. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42389.     

Efficient Synthesis of 2‐Fluoromethylated Quinolines via Copper‐Catalyzed Alkynylation and Cyclization of Fluorinated Imidoyl Iodides

2‐Fluoromethylated quinolines were synthesized through the reaction of N‐aryl‐fluorinated imidoyl iodides with terminal alkynes in good yields by the catalysis of copper(I) iodide (CuI) alone.     

Organoselenium‐Catalyzed Baeyer–Villiger Oxidation of α,β‐Unsaturated Ketones by Hydrogen Peroxide to Access Vinyl Esters

By carefully screening the organoselenium pre‐catalysts and optimizing the reaction conditions, simple dibenzyl diselenide was found to be the best pre‐catalyst for Baeyer–Villiger oxidation of (E)‐α,β‐unsaturated ketones with the green oxidant hydrogen peroxide at room temperature. The organoselenium catalyst used in this reaction could be recycled and reused several times. This new method was suitable not only for methyl unsaturated ketones, but also for alkyl and aryl unsaturated ketones. Therefore, it provided a direct, mild, practical, highly functional group‐tolerant process for the chemoselective preparation of the versatile (E)‐vinyl esters from the readily available (E)‐α,β‐unsaturated ketones. A possible mechanism was also proposed to rationalize the activity of the organoselenium catalyst in the presence of hydrogen peroxide in this Baeyer–Villiger oxidation reaction.

     

Iron‐Based Imidazolium Salts as Versatile Catalysts for the Synthesis of Quinolines and 2‐ and 4‐Allylanilines by Allylic Substitution of Alcohols

Readily available iron(III)‐based imidazolium salts have proven to be very versatile catalysts for the allylic substitution reaction of alcohols with anilines, allowing the synthesis of quinolines, 2‐allylanilines and 4‐allylanilines just by modulating the reaction conditions. Noteworthy, the formation of quinoline derivatives proceeds by ortho‐allylation of the corresponding aniline and subsequent oxidative cyclization mediated by atmospheric oxygen. The reaction using anilines as nucleophiles is selective to the C‐alkylation versus the N‐alkylation, under any reaction conditions studied.

     

Mechanistic Study of the Reaction of Thiol‐Containing Enzymes with α,β‐Unsaturated Carbonyl Substrates by Computation and Chemoassays

We investigated the reactions between substituted α,β‐unsaturated carbonyl compounds (Michael systems) and thiols by computations as well as chemoassays. The results give insight into variations in the underlying mechanisms as a function of the substitution pattern. This is of interest for the mechanisms of inhibition of the SARS coronavirus main protease (SARS‐CoV M^pro) by etacrynic acid derivatives as well as for the excess toxicity of substituted α,β‐unsaturated carbonyl compounds. This study compares possible reaction courses including 1,4‐addition followed by a ketonization step, and underscores the importance of a base‐catalyzed step for the reactivity of thiol groups in enzymes. Phenyl and methyl substituents at the Michael system decrease the reactivity of the electrophilic compound, but chlorophenyl substituents partly recover the reactivity. Computations also indicate that electron‐pushing substituents lead to a change in the reaction mechanism. The conformation of the Michael system is also found to significantly influence reactivity: the s‐cis conformation leads to higher reactivity than the s‐trans conformation. The computed data explain the trends in measured inhibition potencies of substituted α,β‐unsaturated carbonyl compounds and of reaction rates in chemical assays. They also indicate that the reversibility of inhibition does not stand in contrast to the formation of a new covalent bond between inhibitor and protease.     

Synthesis of polar vinyl monomer–olefin copolymers by α‐diimine nickel catalyst

Vinyl acetate, methyl methacrylate, acrylonitrile and methyl vinyl ketone were investigated for co‐ and terpolymerization with ethylene and ethylene–propylene. Precursor [bis(N,N ′‐dimesitylimino)acenaphthene]dibromonickel, activated by methylaluminoxane was used as a catalyst system and trialkylaluminium was employed to block the polar groups for these polymerizations. Polymerization activities of the order of magnitude of 10⁶ in the case of vinyl acetate and methyl methacrylate, and 10⁵ in the case of acrylonitrile were achieved. Microanalysis and GPC of acrylonitrile copolymers found about 17 units of acrylonitrile per polymer chain. Copolymers with very different properties from the parent homopolymers were obtained in all cases except that of methyl vinyl ketone. © 2001 Society of Chemical Industry     

Preparation and Use of Polystyryl‐DABCOF2: An Efficient Recoverable and Reusable Catalyst for β‐Azidation of α,β‐Unsaturated Ketones in Water

Novel solid fluorides were prepared to optimize the β‐azidation of α,β‐unsaturated ketones. The higher loading of these catalysts compared to that of commercially available fluorides has allowed the use of a smaller mass of catalyst helping the mixing of the reaction mixture. Porous polymeric supports have proved to be more efficient in the presence of water as reaction medium. Water has played a crucial role showing a beneficial effect on the reactivity by improving dispersion of the reaction mixture and also by avoiding organic fouling caused by the retention of the reaction mixture within the polymeric matrix. This has facilitated the recovery of the products from the catalyst. The protocol reported has allowed a significant reduction in the organic solvent required for the complete recovery of the pure product whilst leaving the catalyst clean and reusable. E‐factors are in the range of 5.9–10.5 and therefore ca. 3 times smaller than previous procedures operating under solvent‐free conditions. To further improve the efficiency of our approach we have developed a protocol operating in a continuous‐flow manner that has allowed us to achieve an E‐factor of 1.7–1.9, with a reduction of ca. 80% of the corresponding batch conditions. The continuous‐flow protocol has allowed us to minimize the use of trimethylsilyl azide making the recovery and reuse of water and catalyst 5f very efficient and simple. Finally, a novel reduction system using palladium on alumina (5 mol%) and equimolar amount of formic acid has been used in the presence of 1 equivalent of di‐tert‐butyl pyrocarbonate to set a multistep protocol operating in continuous‐flow conditions for the preparation of two representative N‐Boc‐β‐amino ketones starting from the corresponding enones with E‐factors of 3.2 and 2.7, respectively.     

Synthesis of 2‐(1‐Alkoxyvinyl)anilines by Palladium/Norbornene‐Catalyzed Amination Followed by Termination with Vinyl Ethers

A palladium/norbornene‐catalyzed ortho‐amination and ipso vinyl ether termination reaction of iodoarenes is reported. The benzyl vinyl ether serves as an efficient alternative carbonyl source in palladium/norbornene catalysis for the final ipso termination reaction to give [1‐(benzyloxy)vinyl]arenes, which readily undergo hydrolysis to deliver methyl ketones under aqueous acidic conditions. The final Heck termination reaction with vinyl ethers has high branched/linear selectivity. This reaction tolerates a range of iodoarene and O‐benzoylhydroxylamine substrates, and it provides a convenient way to prepare o‐acetylanilines. The synthetic utility of [1‐(benzyloxy)vinyl]arenes and the corresponding ketones is briefly investigated.

     

Highly Efficient Synthesis of New α‐Arylamino‐α′‐chloropropan‐2‐ones via Oxidative Hydrolysis of Vinyl Chlorides Promoted by Calcium Hypochlorite

The oxidative hydrolysis of different trifluoroacetyl‐protected N‐(2‐chloroallyl)anilines, promoted by calcium hypochlorite, is able to yield several not previously described α‐arylamino‐α′‐chloropropan‐2‐ones, very valuable building blocks that are useful as precursors of several drugs, in excellent yields and short reaction times. The main requirement of the reaction for avoiding the undesired aromatic chlorination (N‐protection) is effectively solved by the use of the easily formed and removed N‐trifluoroacetyl group. Thus, it is possible to perform the oxidative hydrolysis‐deprotection step using a one‐pot strategy, obtaining quantitative yields in very short reaction times.     

Enantioselective Synthesis of α‐Trifluoromethyl Arylmethylamines by Ruthenium‐Catalyzed Transfer Hydrogenation Reaction

A simple combination of dichloro(para‐cymene)ruthenium(II) dimer, a chiral amino alcohol and isopropyl alcohol allowed for in‐situ generation of the bifunctional catalyst responsible for the transfer hydrogenation reaction of trifluoromethyl ketimines in excellent yields with high enantioselectivities (up to 93% ee). Herein, we describe the optimization, scope, limitations, and applications of the method.