Visible-Light-Induced Selective Defluoroborylation of Polyfluoroarenes,gem-Difluoroalkenes,and Trifluoromethylalkenes

Fluorinated organoboranes serve as versatile synthetic precursors for the preparation of value-added fluorinated organic compounds. Recent progress has been mainly focused on the transition-metal catalyzed defluoroborylation. Herein, we report a photocatalytic defluoroborylation platform through direct B−H activation of N-heterocyclic carbene boranes, through the synergistic merger of a photoredox catalyst and a hydrogen atom transfer catalyst. This atom-economic and operationally simple protocol has enabled defluoroborylation of an extremely broad scope of multifluorinated substrates including polyfluoroarenes, gem-difluoroalkenes, and trifluoromethylalkenes in a highly selective fashion. Intriguingly, the defluoroborylation protocol can be transition-metal free, and the regioselectivity obtained is complementary to the reported transition-metal-catalysis in many cases.     

NBS‐Mediated Aziridination between Styrenes and Amides under Transition Metal‐Free Conditions

An efficient and simple protocol for N‐bromosuccinimide (NBS)‐mediated styrenes aziridination using amides as the nitrenoid source has been developed. This aziridination affords desired products in moderate to good yields without using transition metal catalyst under very mild reaction condition.     

Transition‐Metal‐Free Ring‐Opening Silylation of Indoles and Benzofurans with (Diphenyl‐tert‐butylsilyl)lithium

A practical method is presented for ring opening various indoles and benzofurans with concomitant stereoselective silylation using readily generated (diphenyl‐tert ‐butylsilyl)lithium to afford ortho ‐β‐silylvinylanilines or ‐phenols. Dearomatization of the heteroarene core proceeds in the absence of any transition‐metal catalyst through addition of a silyl anion and a subsequent stereoselective β‐elimination. DFT calculations provide insight into the mechanism. Functionalizing C−X bond cleavage of heteroarenes is rare and generally requires transition‐metal catalysts.     

Visible‐Light‐Mediated Metal‐Free Hydrosilylation of Alkenes through Selective Hydrogen Atom Transfer for Si−H Activation

Although there has been significant progress in the development of transition‐metal‐catalyzed hydrosilylations of alkenes over the past several decades, metal‐free hydrosilylation is still rare and highly desirable. Herein, we report a convenient visible‐light‐driven metal‐free hydrosilylation of both electron‐deficient and electron‐rich alkenes that proceeds through selective hydrogen atom transfer for Si−H activation. The synergistic combination of the organophotoredox catalyst 4CzIPN with quinuclidin‐3‐yl acetate enabled the hydrosilylation of electron‐deficient alkenes by selective Si−H activation while the hydrosilylation of electron‐rich alkenes was achieved by merging photoredox and polarity‐reversal catalysis.     

Regioselective synthesis of 1,4‐disubstituted triazoles using bis[(L)prolinato‐N,O]Zn complex as an efficient catalyst in water as a sole solvent

A concise, convenient and mild route for one‐pot regioselective synthesis of N‐aryl‐ and N‐alkyltriazoles in water as a sole solvent is reported. The methodology involves a three‐component reaction comprising aryl/alkyl‐alkyne, sodium azide and aryl/alkyl/allyl halide catalyzed by zinc(II) L ‐prolinate. Prominent features of our protocol are incorporation of transition metal catalyst other than copper, water as the reaction medium, recyclability of catalyst and avoidance of hazardous aryl azide as a reactant. Copyright © 2011 John Wiley & Sons, Ltd.     

In situ Generated Ruthenium Catalyst Systems Bearing Diverse N‐Heterocyclic Carbene Precursors for Atom‐Economic Amide Synthesis from Alcohols and Amines

The transition‐metal‐catalyzed direct synthesis of amides from alcohols and amines is herein demonstrated as a highly environmentally benign and atom‐economic process. Among various catalyst systems, in situ generated N‐heterocyclic carbene (NHC)‐based ruthenium (Ru) halide catalyst systems have been proven to be active for this transformation. However, these existing catalyst systems usually require an additional ligand to achieve satisfactory results. In this work, through extensive screening of a diverse variety of NHC precursors, we discovered an active in situ catalyst system for efficient amide synthesis without any additional ligand. Notably, this catalyst system was found to be insensitive to the electronic effects of the substrates, and various electron‐deficient substrates, which were not highly reactive with our previous catalyst systems, could be employed to afford the corresponding amides efficiently. Furthermore, mechanistic investigations were performed to provide a rationale for the high activity of the optimized catalyst system. NMR‐scale reactions indicated that the rapid formation of a Ru hydride intermediate (signal at δ=?7.8 ppm in the ¹H NMR spectrum) after the addition of the alcohol substrate should be pivotal in establishing the high catalyst activity. Besides, HRMS analysis provided possible structures of the in situ generated catalyst system.     

Glycerol: A Promising Benign Solvent for Catalyst‐free One‐pot Multi‐component Synthesis of 1H‐Pyrozolo[1,2‐b]phthalazine‐5,10‐diones and 2H‐Indazolo[2,1‐b]phthalazine‐triones Under Controlled Microwaves Irradiation

A simple green and efficient one‐pot multi‐component synthesis of 1H‐pyrozolo[1,2‐b]phthalazine‐5,10‐diones and 2H‐indazolo[2,1‐b]phthalazine‐triones has been developed utilizing one‐pot multi‐component reaction of aromatic aldehydes, active methylene reagents, phthalic anhydride, and hydrazine hydrate or alternatively phthalhydrazide in glycerol without catalyst under controlled microwave heating. The current synthetic protocol offers several advantages such as excellent yields, high EcoScale and atom economy, simple working up reactants and products, and the absence of hazardous catalysts or solvents.     

Convenient Synthesis of 1H‐Indol‐1‐yl Boronates via Palladium(0)‐Catalyzed Borylation of Bromo‐1H‐indoles with ‘Pinacolborane’

An atom‐economic Pd⁰‐catalyzed synthesis of a series of pinacol‐type indolylboronates 3 from the corresponding bromoindole substrates 2 and pinacolborane (pinBH) as borylating agent was elaborated. The optimal catalyst system consisted of a 1 : 2 mixture of [Pd(OAc)₂] and the ortho‐substituted biphenylphosphine ligand L‐3 (Scheme 4, Table). Our synthetic protocol was applied to the fast, preparative‐scale synthesis of 1‐substituted indolylboronates 3a – h in the presence of different functional groups, and at a catalyst load of only 1 mol‐% of Pd.     

Palladium‐Catalyzed Regio‐ and Stereoselective Chlorothiolation of Terminal Alkynes with Sulfenyl Chlorides

Chlorothiolation of terminal alkynes with sulfenyl chlorides yields anti‐adducts without transition‐metal catalysts. In sharp contrast, transition‐metal‐catalyzed chlorothiolation has not been developed to date, possibly because organosulfur compounds can poison catalyst. Herein, the regio‐ and stereoselective palladium‐catalyzed chlorothiolation of terminal alkynes with sulfenyl chlorides is described. syn‐Chlorothiolation offers a complementary synthetic route to chloroalkenyl sulfides. 2‐Chloroalkenyl sulfides can easily be transformed into various sulfur‐containing products, most of which are often found in natural products and pharmaceuticals.     

Sequential Norrish Type II Photoelimination and Intramolecular Aldol Cyclization of α‐Diketones: Synthesis of Polyhydroxylated Cyclopentitols by Ring Contraction of Hexopyranose Carbohydrate Derivatives

The excitation of the innermost carbonyl of nono‐2,3‐diulose derivatives by irradiation with visible‐light initiates a sequential Norrish type II photoelimination and aldol cyclization process that finally gives polyfunctionalized cyclopentitols. The rearrangement has been confirmed by the isolation of stable acyclic photoenol intermediates that can be independently cyclized by a thermal 5‐(enolexo)‐exo‐trig uncatalyzed aldol reaction with high diastereoselectivity. In this last step, the large deuterium kinetic isotope effect found for the 1,5‐hydrogen atom transfer seems to indicate that the aldol reaction runs through a concerted pericyclic mechanism. Owing to the ready availability of pyranose sugars of various configurations, this protocol has been used to study the influence of pyranose ring‐substituents on the diastereoselectivity of the aldol cyclization reaction. In contrast with other pyranose ring contraction methodologies no transition‐metal reagents are needed and the sequential rearrangement occurs simply by using visible light and moderate heating (0 to 60 °C).     

Sodium Formate‐Catalyzed One‐Pot Synthesis of Benzopyranopyrimidines and 4‐Thio‐substituted 4H‐Chromenes via Multicomponent Reaction at Room Temperature

A simple, straightforward and highly efficient multicomponent one‐pot synthesis of a series of pharmaceutically interesting benzopyranopyrimidine and 4H‐chromene derivatives has been developed on the basis of low‐cost and environment‐friendly sodium formate catalyst via tandem reactions of salicylic aldehydes, malononitrile, and cyclic secondary amines in ethanol at room temperature. Nature of nucleophile used in this reaction directs the course of the reaction; cyclic secondary amines result in the formation of benzopyrano[2,3‐d]pyrimidines, whereas thiophenol furnish corresponding 4‐thio‐subtituted 4H‐chromenes under the same reaction conditions. High atom‐economy, good yields, eco‐friendly, and mild reaction conditions are some of the important features of this protocol.     

One‐pot Two‐step Reaction for Synthesis of Poly‐substituted Pyrano[3,2‐c]pyridones and Spiro[indoline‐3,4′‐pyrano[3,2‐c]pyridine]‐2,5′(6′H)‐diones in Water

An efficient four‐component approach for the synthesis poly‐substituted pyrano[3,2‐c]pyridones and spiro[indoline‐3,4′‐pyrano[3,2‐c]pyridine]‐2,5′(6′H)‐diones in water has been established. During the reaction, the products were readily achieved through one‐pot two‐step reaction using solid acid as catalyst. The advantages of atom and step economy, the recyclability of heterogeneous solid acid catalyst, easy workup procedure, and the wide scope of substrates make the reaction a powerful tool for assembling pyrano[3,2‐c]pyridone skeletons of chemical and medical interest.     

Ionic‐Liquid‐Grafted Rigid Poly(p‐Phenylene) Microspheres: Efficient Heterogeneous Media for Metal Scavenging and Catalysis

Novel guanidinium ionic liquid‐grafted rigid poly(p‐phenylene) (PPPIL) microspheres have been developed for metal scavenging and catalysis. The noble‐metal nanoparticles supported on the microspheres surface can be used as efficient heterogeneous catalysts. The combination of nanoparticles and ionic liquid fragments on the microsphere surfaces enhance the activity and durability of the catalyst. The PPPIL ? Pd⁰ catalyst has been tested in the Suzuki cross‐coupling reaction, and exhibits much higher catalytic activity than Pd catalysts supported on porous polymer matrices. The PPPIL ? Pd⁰ catalyst can be recycled at least for nine runs without any significant loss of activity. The present approach may, therefore, have potential applications in transition‐metal‐nanocatalyzed reactions.     

[N‐(2‐Hydroxy­ethyl)ethyl­enediamine‐κ3N,N′,O]‐cis‐bis(isothio­cyanato‐κN)copper(II)

In the novel transition metal isothio­cyanate complex of N‐(2‐hydroxy­ethyl)ethyl­enediamine (hydet‐en) with copper, [Cu(NCS)₂(C₄H₁₂N₂O)], the Cu atom lies in a distorted square‐pyramidal environment, coordinated by four N atoms in the basal plane and an apical O atom. The hydet‐en ligand is N,N,O‐tridentate, in contrast to the disposition in previously studied complexes, while the isothio­cyanate ions act as N‐atom donor ligands. The monomeric units are linked to one another by hydrogen bonds.     

Influence of the catalyst on monomer insertion in the syndiospecific copolymerization of styrene and para‐methylstyrene

The effect of the kind of transition‐metal catalyst on the extent of comonomer insertion in the syndiospecific complex‐coordinative copolymerization of styrene and para‐methylstyrene has been investigated. The results for the influence of the polymerization conditions have shown that there is no real difference between solution copolymerization in toluene and solvent‐free styrene copolymerization in bulk, with respect to the reactivity ratio for para‐methylstyrene (r₂), under comparable conditions in the presence of methylaluminoxane and triisobutylaluminum and at low polymerization conversions. All the investigated catalysts lead to a preferred incorporation of para‐methylstyrene into the polymer chain in comparison with styrene and over the whole range of monomer compositions. The increasing capability of the different catalysts to provide copolymers with enhanced para‐methylstyrene concentrations can be summarized by the increasing r₂ values for the copolymerization in bulk as follows: η⁵‐pentamethylcyclopentadienyl titanium trichloride < η⁵‐octahydrofluorenyl titanium trimethoxide < η⁵‐octahydrofluorenyl titanium tristrifluoroacetate < η⁵‐cyclopentadienyl titanium(N,N‐dicyclohexylamido)dichloride < η⁵‐cyclopentadienyl titanium trichloride. For a correlation between the catalyst structure and the comonomer insertion, the catalysts can be described by electronic effects (electrostatic charge of the transition‐metal atom) and steric effects (minimum structural cone angle). The results show that the steric properties of the transition‐metal complexes have the most important effect on the insertion of para‐methylstyrene into the copolymer. If the minimum structural cone angle of the ligand of the transition‐metal catalyst decreases, the incorporation of the comonomer para‐methylstyrene increases significantly. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2061–2067, 2005     

Preparation of N‐n‐Octyl‐D‐glucamine by the catalytic hydrogenation with palladium complex of MgO‐supported melamine‐formaldehyde polymer

The palladium complex of MgO‐supported melamine‐formaldehyde polymer catalyst was prepared and characterized by X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS). The preparation of N‐n‐octyl‐D ‐glucamine was investigated by using this complex as the catalyst. It was found that the palladium complex of MgO‐supported melamine‐formaldehyde polymer has a good catalytic activity for the hydrogenation of n‐octylamine with D ‐glucose to produce N‐n‐octyl‐D ‐glucamine. The effects of additive, solvent, temperature, hydrogen pressure, Pd content in the catalyst and the amount of catalyst on the preparation of N‐n‐octyl‐D ‐glucamine have all been studied. Under the optimum experimental conditions—D ‐glucose, 37.2 mmol; n‐octylamine, 31 mmol; triethylamine, 1.0 ml; ethanol, 60 ml; temperature, 333 K; hydrogen pressure, 1.5 MPa; the amount of the catalyst (Pd content 3.55%, N/Pd molar ratio 12), 0.7 g—the highest yield of N‐n‐octyl‐D ‐glucamine (57.6%) was obtained. XRD results show that melamine‐formaldehyde polymer changed the structure of MgO, and XPS results suggest that coordination bonds were formed between the hexatomic ring and metal atom, and palladium particles were immobilized on the polymer. Copyright © 2004 John Wiley & Sons, Ltd.     

Pd/Light‐Accelerated Atom‐Transfer Carbonylation of Alkyl Iodides: Applications in Multicomponent Coupling Processes Leading to Functionalized Carboxylic Acid Derivatives

The atom‐transfer carbonylation reaction of various alkyl iodides thereby leading to carboxylic acid esters was effectively accelerated by the addition of transition‐metal catalysts under photoirradiation conditions. By using a combined Pd/hν reaction system, vicinal C‐functionalization of alkenes was attained in which α‐substituted iodoalkanes, alkenes, carbon monoxide, and alcohols were coupled to give functionalized esters. When alkenyl alcohols were used as acceptor alkenes, three‐component coupling reactions, which were accompanied by intramolecular esterification, proceeded to give lactones. Pd‐dimer complex [Pd₂(CNMe)₆][PF₆]₂, which is known to undergo homolysis under photoirradiation conditions, worked quite well as a catalyst in these three‐ or four‐component coupling reactions. In this metal/radical hybrid system, both Pd radicals and acyl radicals are key players and a stereochemical study confirmed the carbonylation step proceeded through a radical carbonylation mechanism.     

Visible‐Light‐Catalyzed Direct Benzylic C(sp3)–H Amination Reaction by Cross‐Dehydrogenative Coupling

A conceptually new and synthetically valuable cross‐dehydrogenative benzylic C(sp³)–H amination reaction is reported by visible‐light photoredox catalysis. This protocol employs DCA (9,10‐dicyanoanthracene) as a visible‐light‐absorbing photoredox catalyst and an amide as the nitrogen source without the need of either a transition metal or an external oxidant.