Palladium Supported on Zinc Oxide Nanoparticles as Efficient Heterogeneous Catalyst for SuzukiMiyaura and Hiyama Reactions under Normal Laboratory Conditions

Nanoscale Pd supported on ZnO was prepared by a facile coprecipitation method. Pd/ZnO Nanoparticles were characterized by using XRD, TEM, SEM, XPS, BET specific surface area measurement, and thermogravimetric analysis. This catalyst was used as novel and excellent heterogeneous catalyst for ligand‐free C? C bond‐formation particularly in the synthesis of unsymmetrical biaryls by Suzuki? Miyaura and Hiyama cross‐coupling reactions under air atmosphere without use of any Ar or N₂ flow. The catalyst can be recovered and recycled several times without marked loss of activity.     

N‐Heterocyclic carbene/phosphite synergistically assisted Pd/C‐catalyzed Suzuki coupling of aryl chlorides

An N‐heterocyclic carbene and phosphite synergistically enhanced Pd/C catalyst system has been developed for Suzuki coupling of aryl chlorides and aryl boronic acids from commercially available Pd/C with sterically demanding N,N′‐bis(2,6‐diisopropylphenyl)imidazolylidene and trimethylphosphite. A remarkable increase in catalytic activity of Pd/C was observed when used along with 1 equiv. N,N′‐bis(2,6‐diisopropylphenyl)imidazolium chloride and 2 equiv. phosphite with respect to palladium in appropriate solvents that were found to play a crucial role in Pd/C‐NHC‐P(OR)₃‐catalyzed Suzuki coupling. A dramatic ortho‐substitution effect of carbonyl and nitrile groups in aryl chlorides was observed and explained by a modified quasi‐heterogeneous catalysis mechanism. The Pd/C catalyst could be easily recovered from reaction mixtures by simple filtration and only low palladium contamination was detected in the biparyl products. A practical process for the synthesis of 4‐biphenylcarbonitrile has therefore been developed using the N‐heterocyclic carbene/phosphite‐assisted Pd/C‐catalyzed Suzuki coupling. Copyright © 2013 John Wiley & Sons, Ltd.     

Palladium‐Catalyzed Direct C2 Arylation of N‐Substituted Indoles with 1‐Aryltriazenes

A novel and efficient palladium‐catalyzed C2 arylation of N‐substituted indoles with 1‐aryltriazenes for the synthesis of 2‐arylindoles was developed. In the presence of BF₃ ? OEt₂ and palladium(II) acetate (Pd(OAc)₂), N‐substituted indoles reacted with 1‐aryltriazenes in N,N‐dimethylacetamide (DMAC) to afford the corresponding aryl–indole‐type products in good to excellent yields.     

Synthesis of Asymmetrically Substituted Terpyridines by Palladium‐Catalyzed Direct C?H Arylation of Pyridine N‐Oxides

The synthesis of asymmetrically substituted 2,2′:6′,2′′‐terpyridines is reported. First, palladium‐catalyzed C? H arylation of pyridine N‐oxides with substituted bromopyridines gave 2,2′‐bipyridine N‐oxides, which were further arylated in a second step to form 2,2′:6′,2′′‐terpyridine N‐oxides. Yields of up to 77 % were obtained with N‐oxides bearing an electron‐withdrawing ethoxycarbonyl substituent in the 4‐position. Pd(OAc)₂ with either P(tBu)₃ or P(o‐tolyl)₃ was used as the catalyst. Cyclometalated complexes derived from Pd(OAc)₂ and these phosphines were also effective. K₃PO₄ as the base gave better results than K₂CO₃. Subsequent deoxygenation with H₂ and Pd/C as the catalyst gave the asymmetrically substituted 2,2′:6′,2′′‐terpyridines in near quantitative yield. This reaction sequence significantly reduces the number of steps required in comparison with known cross‐coupling methods and therefore allows convenient and scalable access to substituted terpyridines.     

Reactivity of ortho‐Substituted Aryl–Palladium Complexes towards Carbodiimides,Isothiocyanates, Nitriles,and Cyanamides

Complexes [Pd(C₆H₃XH‐2‐R′‐5)Y(N^N)] (X=O, NH; Y=Br, I; R′=H, NO₂; N^N=N,N,N′,N′‐tetramethylethylenediamine (tmeda), 2,2′‐bipyridine (bpy), 4,4′‐di‐tert‐butyl‐2,2′‐bipyridine (dtbbpy)) react with RN?C?E (E=NR, S) or RC≡N (R=alkyl, aryl, NR′′₂) and TlOTf (OTf=CF₃SO₃) to give, respectively, 1) products of the insertion of the C?E group into the C? Pd bond, protonation of the N atom, and coordination of X to Pd, [Pd{κ²‐X,E‐(XC₆H₃{EC(NHR)}‐2‐R′‐4)}(N^N)]OTf or [Pd(κ²‐X,N‐{ZC₆H₃(NH?CR)‐2‐R′‐4})(N^N)]OTf, or products of the coordination of carbodiimides and OH addition, [Pd{κ²‐C,N‐(C₆H₄{OC(NR)}NHR‐2)}(bpy)]OTf; or 2) products of the insertion of the C≡N group to Pd and N‐protonation, [Pd(κ²‐X,N‐{XC₆H₃(NH?CR)‐2‐R′‐4})(N^N)]OTf.     

Pd‐Catalyzed Ring Assembly by Vinylation and Intramolecular Heck Coupling: A Versatile Strategy Towards Functionalized Azadibenzocyclooctynes

A new modular approach based on Pd‐catalyzed C? C bond formation is presented for the assembly of a benzannulated azocine scaffold, the key intermediate in the synthesis of functionalized azadibenzocyclooctynes (aza‐DIBOs). The intramolecular ring‐closing Heck coupling was investigated by variation of the C? X bond. The reaction rate is limited by the initial oxidative addition step and the regiochemistry strongly depends on the auxiliary phosphine. Under optimized conditions, the 8‐endo regioisomer was obtained in 71 % yield over two steps (with no protecting group chemistry) or in one pot, inclusive of C? N bond formation. The practical generation of the octyne triple bond of a prototypical N‐benzoyl aza‐DIBO, without the need for chromatographic purification, is also described. The structural features, including those of the ring‐strained cyclic octyne, were elucidated by NMR spectroscopy and X‐ray crystallographic analysis. The high reactivity of the N‐benzoyl aza‐DIBO synthesized is demonstrated in a strain‐promoted azide–alkyne cycloaddition reaction with an alkyl azide (k=0.38 M ^?1 s^?1).     

A [4+1] Cyclative Capture Approach to 3H‐Indole‐N‐oxides at Room Temperature by Rhodium(III)‐Catalyzed CH Activation

The rhodium(III)‐catalyzed [3+2] C? H cyclization of aniline derivatives and internal alkynes represents a useful contribution to straightforward synthesis of indoles. However, there is no report on the more challenging synthesis of pharmaceutically important N‐hydroxyindoles and 3H‐indole‐N‐oxides. Reported herein is the first rhodium(III)‐catalyzed [4+1] C? H oxidative cyclization of nitrones with diazo compounds to access 3H‐indole‐N‐oxides. More significantly, this reaction proceeds at room temperature and has been extended to the synthesis of N‐hydroxyindoles and N‐hydroxyindolines.     

Copper‐Catalyzed Aerobic Oxidative C?H Functionalization of Substituted Pyridines: Synthesis of Imidazopyridine Derivatives

A novel, efficient, and practical method for the synthesis of imidazopyridine derivatives has been developed through the copper‐catalyzed aerobic oxidative C? H functionalization of substituted pyridines with N‐(alkylidene)‐4H‐1,2,4‐triazol‐4‐amines. The procedure occurs by cleavage of the N? N bond in the N‐(alkylidene)‐4H‐1,2,4‐triazol‐4‐amines and activation of an aryl C? H bond in the substituted pyridines. This is the first example of the preparation of imidazopyridine derivatives by using pyridines as the substrates by transition‐metal‐catalyzed C? H functionalization. This method should provide a novel and efficient strategy for the synthesis of other nitrogen heterocycles.     

Diversity‐Oriented Synthesis of Substituted Benzo[b]thiophenes and Their Hetero‐Fused Analogues through Palladium‐Catalyzed Oxidative CH Functionalization/Intramolecular Arylthiolation

An efficient, high yielding route to multisubstituted benzo[b]thiophenes has been developed through palladium‐catalyzed intramolecular oxidative C?H functionalization–arylthiolation of enethiolate salts of α‐aryl‐β‐(het)aryl/alkyl‐β‐mercaptoacrylonitriles/acrylates or acrylophenones. The overall strategy involves a one‐pot, two‐step process in which enethiolate salts [generated in situ through base‐mediated condensation of substituted arylacetonitriles, deoxybenzoins, or arylacetates with (het)aryl (or alkyl) dithioates] are subjected to intramolecular C?H functionalization–arylthiolation under the influence of a palladium acetate (or palladium chloride)/cupric acetate catalytic system and tetrabutylammonium bromide as additive in N,N‐dimethylformamide (DMF) as solvent. In a few cases, the yields of benzo[b]thiophenes were better in a two‐step process by employing the corresponding enethiols as substrates. In a few examples, Pd(OAc)₂ (or PdCl₂) catalyst in the presence of oxygen was found to be more efficient than cupric acetate as reoxidant, furnishing benzothiophenes in improved yields by avoiding formation of side products. The method is compatible with a diverse range of substituents on the aryl ring as well as on the 2‐ and 3‐positions of the benzothiophene scaffold. The protocol could also be extended to the synthesis of a raloxifene precursor and a tubulin polymerization inhibitor in good yields. The versatility of this newly developed method was further demonstrated by elaborating it for the synthesis of substituted thieno‐fused heterocycles such as thieno[2,3‐b]thiophenes, thieno[2,3‐b]indoles, thieno[3,2‐c]pyrazole, and thieno[2,3‐b]pyridines in high yields. A probable mechanism involving intramolecular electrophilic arylthiolation via either a Pd‐S adduct or palladacycle intermediate has been proposed on the basis of experimental studies.     

Synthesis of bis‐N‐alkyl imidazolium salts and their palladium(0)(NHC)(η2‐MA)2 complexes

New N‐Alkyl‐substituted imidazolium salts as well as a series of their corresponding [Pd(NHC)(MA)₂] complexes have been obtained by three routes in good yield. The previously reported synthesis for the analogous N‐aryl substituted [Pd(NHC)(MA)₂] complexes has been improved. The N‐alkyl‐substituted [Pd(NHC)(MA)₂] complexes are thermally more labile than their N‐aryl counterparts. Catalytic transfer semi‐hydrogenation of phenylpropyne resulted in good to excellent chemo‐ and stereo‐ selectivity conversion into (Z)‐phenylpropene. The size of the alkyl substituents correlates with the rate of hydrogenation in the sense that more bulky substituents give rise to faster transfer hydrogenation rates. Copyright © 2012 John Wiley & Sons, Ltd.     

Palladium‐Catalyzed Decarboxylative ortho‐C(sp2)−H Aroylation of N‐Sulfoximine Benzamides at Room Temperature

A palladium‐catalyzed method for the decarboxylative ortho C?H acylation of N‐sulfoximine benzamides is developed at room temperature. The catalytic method enables easy access to various functionalized 2‐aroylaromatic carboxylic acid derivatives in good isolated yields. Based on our mechanistic studies, a Pd(II)/Pd(IV) catalytic cycle that involves aroyl radical intermediate is proposed for the reaction.     

Ruthenium–Porphyrin‐Catalyzed Diastereoselective Intramolecular Alkyl Carbene Insertion into CH Bonds of Alkyl Diazomethanes Generated In Situ from N‐Tosylhydrazones

With a ruthenium–porphyrin catalyst, alkyl diazomethanes generated in situ from N‐tosylhydrazones efficiently underwent intramolecular C(sp³)? H insertion of an alkyl carbene to give substituted tetrahydrofurans and pyrrolidines in up to 99 % yield and with up to 99:1 cis selectivity. The reaction displays good tolerance of many functionalities, and the procedure is simple without the need for slow addition with a syringe pump. From a synthetic point of view, the C? H insertion of N‐tosylhydrazones can be viewed as reductive coupling between a C?O bond and a C? H bond to form a new C? C bond, since N‐tosylhydrazones can be readily prepared from carbonyl compounds. This reaction was successfully applied in a concise synthesis of (±)‐pseudoheliotridane.     

Photoluminescent and Liquid‐Crystalline Properties of Donor–Acceptor‐Type 2,5‐Diarylthiazoles

The synthesis of donor–acceptor‐type 2,5‐diarylthiazoles that bear electron‐donating N,N‐dialkylamine and electron‐withdrawing cyano groups at the 2‐ and 5‐position, respectively, were carried out with transition‐metal‐catalyzed C? H arylation reactions developed by us. The compounds were synthesized by the C? H arylation of unsubstituted thiazole at the 2‐position with a palladium/copper catalyst in the presence of tetrabutylammonium fluoride (TBAF) as an activator. Further C? H arylation of the 2‐arylated thiazole at the 5‐position was carried out by the palladium‐catalyzed reaction in the presence of silver(I) fluoride to afford the donor–acceptor‐type 2,5‐diarylthiazoles with N,N‐dialkylamine groups of different chain lengths. The UV/Vis absorption, photoluminescence, and electrochemical behavior were similar regardless of chain length, whereas liquid‐crystalline behavior and thermal characteristics were found to be dependent on the alkyl‐chain length. The compounds with N,N‐diethylamine or N‐butyl‐N‐methyl groups showed a stable liquid‐crystalline phase over a wide temperature range as well as higher stability to thermal decomposition.     

Efficient Bottom‐Up Preparation of Graphene Nanoribbons by Mild Suzuki–Miyaura Polymerization of Simple Triaryl Monomers

Herein an efficient bottom‐up solution‐phase synthesis of N=9 armchair graphene nanoribbons (GNRs) is described. Catalyzed by Pd(PtBu₃)₂, Suzuki–Miyaura polymerization of a simple and readily available triaryl monomer provides a novel GNR precursor with a high molecular weight and excellent solubility. Upon cyclodehydrogenation, the resulting GNR exhibits semiconducting properties with an approximately 1.1 eV band gap (LUMO: ?3.52 eV; HOMO: ?4.66 eV) as characterized by UV/Vis‐NIR spectroscopy and cyclic voltammetry.     

A Highly Versatile Catalyst System for the Cross‐Coupling of Aryl Chlorides and Amines

The syntheses of 2‐(di‐tert‐butylphosphino)‐N,N‐dimethylaniline ( L1 , 71 %) and 2‐(di‐1‐adamantylphosphino)‐N,N‐dimethylaniline ( L2 , 74 %), and their application in Buchwald–Hartwig amination, are reported. In combination with [Pd(allyl)Cl]₂ or [Pd(cinnamyl)Cl]₂, these structurally simple and air‐stable P,N ligands enable the cross‐coupling of aryl and heteroaryl chlorides, including those bearing as substituents enolizable ketones, ethers, esters, carboxylic acids, phenols, alcohols, olefins, amides, and halogens, to a diverse range of amine and related substrates that includes primary alkyl‐ and arylamines, cyclic and acyclic secondary amines, N? H imines, hydrazones, lithium amide, and ammonia. In many cases, the reactions can be performed at low catalyst loadings (0.5–0.02 mol % Pd) with excellent functional group tolerance and chemoselectivity. Examples of cross‐coupling reactions involving 1,4‐bromochlorobenzene and iodobenzene are also reported. Under similar conditions, inferior catalytic performance was achieved when using Pd(OAc)₂, PdCl₂, [PdCl₂(cod)] (cod=1,5‐cyclooctadiene), [PdCl₂(MeCN)₂], or [Pd₂(dba)₃] (dba=dibenzylideneacetone) in combination with L1 or L2 , or by use of [Pd(allyl)Cl]₂ or [Pd(cinnamyl)Cl]₂ with variants of L1 and L2 bearing less basic or less sterically demanding substituents on phosphorus or lacking an ortho‐dimethylamino fragment. Given current limitations associated with established ligand classes with regard to maintaining high activity across the diverse possible range of C? N coupling applications, L1 and L2 represent unusually versatile ligand systems for the cross‐coupling of aryl chlorides and amines.     

Synthesis and polymerizability of CN monomers

The synthesis and polymerizability of imine C?N monomers is surveyed. The investigated imines were either far more reactive than similarly substituted C?C or C?O monomers, or too stable to polymerize. Imines with electron‐attracting substituents on N favor polymerization by anionic mechanism, but led only to low molecular weight polymers. Imines with a donor substituent on N, such as N‐arylmethyleneimines, polymerized by cationic or anionic mechanism. 1‐ and 2‐Aza‐1,3‐butadienes were also rather unstable and polymerized to oligomers. The symmetrically substituted 2,3‐diaza‐1,3‐butadienes could be purified and polymerized successfully using anionic initiators, resulting in both 1,4‐ and 1,2‐structures in the polymer backbone, depending on the substituents. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Cavitand‐Based Pd‐Pyridyl Coordination Capsules: Guest‐Induced Homo‐ or Heterocapsule Selection and Applications of Homocapsules to the Protection of a Photosensitive Guest and Chiral Capsule Formation

A 2 : 4 mixture of tetrakis[4‐(4‐pyridyl)phenyl]cavitand ( 1 ) or tetrakis[4‐(4‐pyridyl)phenylethynyl]cavitand ( 2 ) and Pd(dppp)(OTf)₂ self‐assembles into a homocapsule { 1 ₂ ? [Pd(dppp)]₄}⁸⁺ ? (TfO^?)₈ ( C1 ) or { 2 ₂ ? [Pd(dppp)]₄}⁸⁺ ? (TfO^?)₈ ( C2 ), respectively, through Pd?Npy coordination bonds. A 1 : 1 : 4 mixture of 1 , 2 , and Pd(dppp)(OTf)₂ produced a mixture of homocapsules C1 , C2 , and a heterocapsule { 1 ? 2 ? [Pd(dppp)]₄}⁸⁺ ? (TfO^?)₈ ( C3 ) in a 1 : 1 : 0.98 mole ratio. Selective formation (self‐sorting) of homocapsules C1 and C2 or heterocapsule C3 was controlled by guest‐induced encapsulation under thermodynamic control. Applications of Pd?Npy coordination capsules with the use of 1 were demonstrated. Capsule C1 serves as a guard nanocontainer for trans‐4,4′‐diacetoxyazobenzene to protect against the trans‐to‐cis photoisomerization by encapsulation. A chiral capsule { 1 ₂ ? [Pd((R)‐BINAP)]₄}⁸⁺ ? (TfO^?)₈ ( C5 ) was also constructed. Capsule C5 induces supramolecular chirality with respect to prochiral 2,2′‐bis(alkoxycarbonyl)‐4,4′‐bis(1‐propynyl)biphenyls by diastereomeric encapsulation through the asymmetric suppression of rotation around the axis of the prochiral biphenyl moiety.     

Pd‐ and Cu‐Catalyzed Stereo‐ and Regiocontrolled Decarboxylative/CH Fluoroalkenylation of Heteroarenes

Pd/Cu‐catalyzed decarboxylative/direct C?H alkenylations of heteroarenes with α‐fluoroacrylic acid is reported. This method offers step‐economical and stereocontrolled access to valuable heteroarylated monofluoroalkenes as both Z and E isomers, which are known to be useful in the synthesis of fluorinated biomolecules.     

Generation and Rearrangement of N,O‐Dialkenylhydroxylamines for the Synthesis of 2‐Aminotetrahydrofurans

A new diastereoselective route to 2‐aminotetrahydrofurans has been developed from N,O‐dialkenylhydroxylamines. These intermediates undergo a spontaneous C?C bond‐forming [3,3]‐sigmatropic rearrangement followed by a C?O bond‐forming cyclization. A copper‐catalyzed N‐alkenylation of an N‐Boc‐hydroxylamine with alkenyl iodides, and a base‐promoted addition of the resulting N‐hydroxyenamines to an electron‐deficient allene, provide modular access to these novel rearrangement precursors. The scope of this de novo synthesis of simple nucleoside analogues has been explored to reveal trends in diastereoselectivity and reactivity. In addition, a base‐promoted ring‐opening and Mannich reaction has been discovered to covert 2‐aminotetrahydrofurans to cyclopentyl β‐aminoacid derivatives or cyclopentenones.     

Advancements in the Synthesis and Applications of Cationic N‐Heterocycles through Transition Metal‐Catalyzed C−H Activation

Cationic N‐heterocycles are an important class of organic compounds largely present in natural and bioactive molecules. They are widely used as fluorescent dyes for biological studies, as well as in spectroscopic and microscopic methods. These compounds are key intermediates in many natural and pharmaceutical syntheses. They are also a potential candidate for organic light‐emitting diodes (OLEDs). Because of these useful applications, the development of new methods for the synthesis of cationic N‐heterocycles has received a lot of attention. In particular, many C?H activation methodologies that realize high step‐ and atom‐economies toward these compounds have been developed. In this review, recent advancements in the synthesis and applications of cationic N‐heterocycles through C?H activation reactions are summarized. The new C?H activation reactions described in this review are preferred over their classical analogs.