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.     

5，10，15，20-四 [4-(4''-乙基哌嗪基)苯基]卟啉的合成及其与DNA的相互作用

基于卟啉对癌细胞的特殊亲和作用和哌嗪化合物的抗肿瘤、抗病毒作用，设计并合成了具有哌嗪结构的新型卟啉化合物5，10，15，20-四[4-(4'-乙基哌嗪基)苯基]卟啉（TEPPPH₂），其结构经UV-Vis, 元素分析，¹H NMR等手段证明。采用UV-Vis光谱和荧光光谱研究了TEPPPH₂和小牛胸腺DNA 的相互作用模式和结合机理。实验发现，TEPPPH₂能嵌入到DNA的碱基对中，1个小牛胸腺DNA分子对TEPPPH₂分子的最大结合数n约为88，结合常数为8.4×10⁶mol•L^-1 。TEPPPH₂与DNA的结合数和结合常数大于已知的四（4-N-甲基吡啶基）卟啉和Ca/sal-his、Ni/sal–aln型席夫碱抗癌药物。     

Synthesis and Structure–Activity Correlation Studies of Secondary‐ and Tertiary‐Amine‐Based Glutathione Peroxidase Mimics

In this study, a series of secondary‐ and tertiary‐amino‐substituted diaryl diselenides were synthesized and studied for their glutathione peroxidase (GPx) like antioxidant activities with H₂O₂, cumene hydroperoxide, or tBuOOH as substrates and with PhSH or glutathione (GSH) as thiol cosubstrates. This study reveals that replacement of the tert‐amino groups in benzylamine‐based diselenides by sec‐amino moieties drastically enhances the catalytic activities in both the aromatic thiol (PhSH) and GSH assay systems. Particularly, the N‐propyl‐ and N‐isopropylamino‐substituted diselenides are 8–18 times more active than the corresponding N,N‐dipropyl‐ and N,N‐diisopropylamine‐based compounds in all three peroxide systems when GSH is used as the thiol cosubstrate. Although the catalytic mechanism of sec‐amino‐substituted diselenides is similar to that of the tert‐amine‐based compounds, differences in the stability and reactivity of some of the key intermediates account for the differences in the GPx‐like activities. It is observed that the sec‐amino groups are better than the tert‐amino moieties for generating the catalytically active selenols. This is due to the absence of any significant thiol‐exchange reactions in the selenenyl sulfides derived from sec‐amine‐based diselenides. Furthermore, the seleninic acids (RSeO₂H) derived from the sec‐amine‐based compounds are more stable toward further reactions with peroxides than their tert‐amine‐based analogues.     

N‐heterocyclic carbene–palladium complexes for Suzuki–Miyaura coupling reaction with benzyl chloride and aromatic boronic acid leading to diarylmethanes

A family of N‐heterocyclic carbene–palladium(II)–N,N‐dimethylbenzylamine complexes ((NHC)LPdCl₂; L = N,N‐dimethylbenzylamine) were synthesized as well as characterized using single‐crystal X‐ray diffraction and spectroscopic data. These complexes exhibited higher catalytic activities for the Suzuki reaction of benzyl chlorides to afford diarylmethanes under milder conditions than other efficient (NHC)LPdCl₂ complexes. Using the optimum conditions, the expected coupling products were obtained in moderate to high yields. All reactions were carried out in air and all starting materials were used as supplied without purification.     

Catalytic Synthesis of 8‐Membered Ring Compounds via Cobalt(III)‐Carbene Radicals

The metalloradical activation of o‐aryl aldehydes with tosylhydrazide and a cobalt(II) porphyrin catalyst produces cobalt(III)‐carbene radical intermediates, providing a new and powerful strategy for the synthesis of medium‐sized ring structures. Herein we make use of the intrinsic radical‐type reactivity of cobalt(III)‐carbene radical intermediates in the [Co^II(TPP)]‐catalyzed (TPP=tetraphenylporphyrin) synthesis of two types of 8‐membered ring compounds; novel dibenzocyclooctenes and unprecedented monobenzocyclooctadienes. The method was successfully applied to afford a variety of 8‐membered ring compounds in good yields and with excellent substituent tolerance. Density functional theory (DFT) calculations and experimental results suggest that the reactions proceed via hydrogen atom transfer from the bis‐allylic/benzallylic C?H bond to the carbene radical, followed by two divergent processes for ring‐closure to the two different types of 8‐membered ring products. While the dibenzocyclooctenes are most likely formed by dissociation of o‐quinodimethanes (o‐QDMs) which undergo a non‐catalyzed 8π‐cyclization, DFT calculations suggest that ring‐closure to the monobenzocyclooctadienes involves a radical‐rebound step in the coordination sphere of cobalt. The latter mechanism implies that unprecedented enantioselective ring‐closure reactions to chiral monobenzocyclooctadienes should be possible, as was confirmed for reactions mediated by a chiral cobalt‐porphyrin catalyst.     

Porphyrin‐based polymer‐supported palladium as an excellent and recyclable catalyst for Suzuki–Miyaura coupling reaction in water

A porphyrin‐based polymer with high surface area was synthesized using 5,10,15,20‐tetraphenylporphyrin through a one‐pot Friedel–Crafts alkylation reaction. Pd(II) was successfully supported on this polymer. This strategy provides an easy approach to produce highly stable Pd–porphyrin‐based polymer. The resulting Pd catalyst was characterized using Fourier transform infrared and X‐ray photoelectron spectroscopies, thermogravimetric analysis, scanning and transmission electron microscopies and N₂ adsorption–desorption measurements. This porphyrin‐based polymer‐supported Pd was used as a heterogeneous catalyst for Suzuki–Miyaura coupling reaction in water. The results demonstrated that this Pd catalyst indeed exhibited excellent catalytic activity and recycling performance in water, even for inactive aryl chloride substrate. A new heterogeneous strategy for catalyzing the Suzuki–Miyaura reaction in water is provided.     

NHC‐Containing Manganese(I) Electrocatalysts for the Two‐Electron Reduction of CO2

The synthesis and characterization of the first catalytic manganese N‐heterocyclic carbene complexes are reported: MnBr(N‐methyl‐N′‐2‐pyridylbenzimidazol‐2‐ylidine)(CO)₃ and MnBr(N‐methyl‐N′‐2‐pyridylimidazol‐2‐ylidine)(CO)₃. Both new species mediate the reduction of CO₂ to CO following two‐electron reduction of the Mn^I center, as observed with preparative scale electrolysis and verified with ¹³CO₂. The two‐electron reduction of these species occurs at a single potential, rather than in two sequential steps separated by hundreds of millivolts, as is the case for previously reported MnBr(2,2′‐bipyridine)(CO)₃. Catalytic current enhancement is observed at voltages similar to MnBr(2,2′‐bipyridine)(CO)₃.     

Hirshfeld and DFT analysis of the N‐heterocyclic carbene proligand methylenebis(N‐butylimidazolium) as the acetonitrile‐solvated diiodide salt

N‐Heterocyclic carbene (NHC) based systems are usually exploited in the exploration of catalytic mechanisms and processes in organocatalysis, and homo‐ and heterogeneous catalysis. However, their molecular structures have not received adequate attention. The NHC proligand methylenebis(N‐butylimidazolium) has been synthesized as the acetonitrile solvate of the diiodide salt, C₁₅H₂₆N₄²⁺·2I⁻·CH₃CN [1,1′‐methylenebis(3‐butylimidazolium) diiodide acetonitrile monosolvate], and fully characterized. An interesting cation–anion connection pattern has been identified in the crystal lattice, in which three iodide anions interact simultaneously with the cisoid‐oriented cation. A Hirshfeld surface analysis reveals the predominance of hydrogen bonding over anion–π interactions. This particular arrangement is observed in different methylene‐bridged bis(imidazolium) cations bearing chloride or bromide counter‐anions. Density functional theory (DFT) calculations with acetonitrile as solvent reproduce the geometry of the title cation.     

Potential N‐Heterocyclic Carbene Precursors in the Palladium‐Catalyzed Heck Reaction

A novel 1‐(cyclobutylmethyl)‐substi‐tuted imidazolidinium/benzimidazolium salts as N‐heterocyclic carbene (NHC) precursors were successfully synthesized and characterized by ¹H NMR, ¹³C NMR, IR, and elemental analysis techniques. These compounds were easily prepared from the reaction of N‐alkyl imidazoline/N‐alkyl benzimidazole with bromomethylcyclobutane in high yields. The in situ formed catalytic system derived from the NHC precursor and Pd(OAc)₂ was used in the Heck reaction between aryl halides and styrene with potassium hydroxide in water. The corresponding Heck products were obtained in good yields. © 2012 Wiley Periodicals, Inc. Heteroatom Chem 24:77–83, 2013; View this article online at wileyonlinelibrary.com . DOI 10.1002/hc.21065     

Phosphorus‐Stabilized Titanium Carbene Complexes: Synthesis,Reactivity and DFT Studies

The synthesis of two novel titanium carbene complexes from the bis(thiophosphinoyl)methanediide geminal dianion 1 (SCS^2?) is described. Dianion 1 reacts cleanly with 0.5 equivalents of [TiCl₄(thf)₂] to afford the bis‐carbene complex [(SCS)₂Ti] ( 2 ) in 86 % yield. The mono‐carbene complex [(SCS)TiCl₂(thf)] ( 3 ) can also be obtained by using an excess of [TiCl₄(thf)₂]. The structures of 2 and 3 are confirmed by X‐ray crystallography. A strong nucleophilic reactivity towards various electrophiles (ketones and aldehydes) is observed. The reaction of 3 with N,N′‐dicyclohexylcarbodiimide (DCC) and phenyl isocyanate leads to the formation of two novel diphosphinoketenimines 8 a and 8 b . The bis‐titanium guanidinate complex 9 is trapped as the by‐product of the reaction with DCC. The X‐ray crystal structures of 8 a and 9 are presented. The mechanism of the reaction between complex 3 and DCC is rationalized by DFT studies.     

A ruthenium(II)–porphyrin–carbene complex with a weakly bonded methanol ligand

The title di­phenyl­carbene porphyrin complex (di­phenyl­carbenyl‐κC)(methanol‐κO)(5,10,15,20‐tetra‐p‐tolyl­por­phy­rin­ato‐κ⁴N)ruthenium(II) methanol solvate, [Ru­(C₁₃H₁₀)(C₄₈H₃₆N₄)(CH₄O)]·CH₄O, has a six‐coordinate Ru atom with a methanol mol­ecule as the second axial ligand. The carbene fragment is slightly distorted from an ideal sp² configuration, with a C(phenyl)—C(carbene)—C(phenyl) angle of 112.2 (3)°. The Ru—C bond length of 1.845 (3) Å is comparable with other carbene complexes. The two phenyl rings of the carbene ligand are perpendicular to the carbene plane. Methanol solvate mol­ecules link the methanol ligands of adjacent porphyrin complexes via hydrogen bonds.     

A Porphyrin‐Based Porous rtl Metal–Organic Framework as an Efficient Catalyst for the Cycloaddition of CO2 to Epoxides

A porous rtl metal–organic framework (MOF) [Mn₅L(H₂O)₆?(DMA)₂]?5DMA?4C₂H₅OH ( 1? Mn) (H₁₀L=5,10,15,20‐tetra(4‐(3,5‐dicarboxylphenoxy)phenyl)porphyrin; DMA=N,N′‐dimethylacetamide) was synthesized by employing a new porphyrin‐based octacarboxylic acid ligand. 1? Mn exhibits high Mn^II density in the porous framework, providing it great Lewis‐acid heterogeneous catalytic capability for the cycloaddition of CO₂ with epoxides. Strikingly, 1? Mn features excellent catalytic activity to the cycloaddition of CO₂ to epoxides, with a remarkable initial turnover frequency 400 per mole of catalyst per hour at 20 atm. As‐synthesized 1? Mn also exhibits size selectivity to different epoxide substrates on account of their steric hindrance. The high catalytic activity, size selectivity, and stability toward the epoxides on catalytic cycloaddition of CO₂ make 1? Mn a promising heterogeneous catalyst for fixation and utilization of CO₂.     

Divergent CH Insertion–Cyclization Cascades of N‐Allyl Ynamides

Gold carbene reactivity patterns were accessed by ynamide insertion into a C(sp³)? H bond. A substantial increase in molecular complexity occurred through the cascade polycyclization of N‐allyl ynamides to form fused nitrogen‐heterocycle scaffolds. Exquisite selectivity was observed despite several competing pathways in an efficient gold‐catalyzed synthesis of densely functionalized C(sp³)‐rich polycycles and a copper‐catalyzed synthesis of fused pyridine derivatives. The respective gold–keteniminium and ketenimine activation pathways have been explored through a structure–reactivity study, and isotopic labeling identified turnover‐limiting C? H bond‐cleavage in both processes.     

Synthesis of palladium complexes derived from imidazolidin‐2‐ylidene ligands and used for catalytic amination reactions

N‐Aryl amination and the Buchwald–Hartwig reaction are of great synthetic and industrial interest and scientists accept their usefulness and versatility for obtaining arylamines. In this study Ag–N‐heterocyclic carbene complexes were used as transmetallation reagents for the synthesis of Pd–N‐heterocyclic carbene complexes. The new Pd–N‐heterocyclic carbene complexes were characterized using elemental analysis and ¹H NMR, ¹³C NMR and infrared spectroscopies. The crystal structure of one, namely dichlorobis[1,3‐bis(2‐methylbenzyl)imidazolidin‐2‐yliden]palladium(II), is presented. The activity of the Pd(II) complexes in the coupling reaction of anilines or amines with bromobenzene was investigated. These complexes exhibited high catalytic activities in the direct synthesis of triarylamines and secondary amines in a single step. Copyright © 2016 John Wiley & Sons, Ltd.     

Voltammetric Behavior of Osmium‐Labeled DNA at Mercury Meniscus‐Modified Solid Amalgam Electrodes. Detecting DNA Hybridization

The complex of osmium tetroxide with 2,2′‐bipyridine has been utilized as a probe of DNA structure and an electroactive marker of DNA in DNA hybridization sensors. It produces several voltammetric signals, the most negative of them has been observed only at mercury electrodes. This signal is of catalytic nature affording a high sensitivity of DNA determination. The catalytic current due to evolution of hydrogen in voltammetry of DNA modified by complex of osmium tetroxide with 2,2′‐bipyridine (DNA‐Os,bipy) was studied. Solid amalgam electrodes (modified with mercury menisci) of silver (m‐AgSAE), copper (m‐CuSAE), gold, and of combined bismuth and silver, were used as possible substitutes for mercury electrodes. Besides the hanging mercury drop electrode (HMDE), the catalytic current was observed only on m‐AgSAE and m‐CuSAE. Electrodes of gold and bismuth amalgams did not give the catalytic current. The detection limit of DNA‐Os,bipy on HMDE was 0.1 ng mL^?1 (RSD=2.3 %, N=11), and on m‐AgSAE 0.2 ng mL^?1 (RSD=3.1%, N=11). The m‐AgSAE was successfully applied as a detection electrode in double‐surface DNA hybridization experiments offering highly specific discrimination between complementary (target) and nonspecific DNAs, as well as determination of the length of a repetitive DNA sequence. The m‐AgSAE has proved a convenient alternative to the HMDE or carbon electrodes used for similar purposes in previous work.     

Expedient Mechanosynthesis of N,N‐Dialkyl Imidazoliums and Silver(I)–Carbene Complexes in a Ball‐Mill

The absence of solvent, associated with intensive mechanical agitation, allowed the first mechanosynthesis of high‐value silver(I)–carbene complexes and the corresponding N,N‐dialkylimidazolium precursors. This procedure gave outstanding results in terms of yield and reaction time, when compared to solution‐based conditions previously described in literature, and was generalized to unprecedented compounds. Silver(I)–carbene complexes could either be obtained from N,N‐dialkylimidazolium salts or directly from imidazole and alkyl halides in a one‐pot two‐step procedure without isolating the imidazolium intermediate. Additionally, an efficient one‐pot three‐step sequence, including imidazole alkylation, silver metalation, and transmetalation is reported.     

Copper‐Catalyzed Direct Sulfoximination of Heteroaromatic N‐Oxides by Dual C−H/N−H Dehydrogenative Cross‐Coupling

A dual C?H/N?H dehydrogenative coupling of quinoline‐type N‐oxides with sulfoximines that leads to N‐(hetero)arylsulfoximines in high yields has been realized by using a catalytic amount of CuBr in air. The method does not require any additional ligand, base, reactivity modifier or oxidant and provides a practical route towards a series of sulfoximidoyl‐functionalized quinolines and derivatives.     

Novel (N‐heterocyclic carbene)Pd(pyridine)Br2 complexes for carbonylative Sonogashira coupling reactions: Catalytic efficiency and scope for arylalkynes,alkylalkynes and dialkynes

New N,N′‐substituted imidazolium salts and their corresponding dibromidopyridine–palladium(II) complexes were successfully synthesized and characterized. Reactions of palladium bromide with the newly synthesized N,N′‐substituted imidazolium bromides ( 2a and 2b ) in pyridine afforded the corresponding new N‐heterocyclic carbene pyridine palladium(II) complexes ( 3a and 3b ) in high yields. Their single‐crystal X‐ray structures show a distorted square planar geometry with the carbene and pyridine ligands in trans position. Both complexes show a high catalytic activity in carbonylative Sonogashira coupling reactions of aryl iodides and aryl diiodides with arylalkynes, alkylalkynes and dialkynes.     

Comparing Grubbs‐, Werner‐, and Hofmann‐Type (Carbene)ruthenium Complexes: The Key Role of Pre‐Equilibria for Olefin Metathesis

A gas‐phase comparison of intrinsic olefin metathesis rates for (carbene)ruthenium complexes by means of electrospray‐ionization tandem mass spectrometry reveals a reversal of the reactivity trends observed in solution. The solution‐phase ordering of reactivity is accordingly attributed to a more favorable pre‐equilibrium, producing the metathesis‐active species in the case of the Hofmann‐ and Werner‐type complexes relative to those of the Grubbs type.     

Copper–Carbene Intermediates in the Copper‐Catalyzed Functionalization of OH Bonds

Copper–carbene [Tp^xCu?C(Ph)(CO₂Et)] and copper–diazo adducts [Tp^xCu{η¹‐N₂C(Ph)(CO₂Et)}] have been detected and characterized in the context of the catalytic functionalization of O?H bonds through carbene insertion by using N₂?C(Ph)(CO₂Et) as the carbene source. These are the first examples of these type of complexes in which the copper center bears a tridentate ligand and displays a tetrahedral geometry. The relevance of these complexes in the catalytic cycle has been assessed by NMR spectroscopy, and kinetic studies have demonstrated that the N‐bound diazo adduct is a dormant species and is not en route to the formation of the copper–carbene intermediate.