共查询到20条相似文献,搜索用时 62 毫秒
1.
Dr. Joseph F. DeJesus Lindy M. Sherman Darius J. Yohannan Jeffrey C. Becca Shelby L. Strausser Leonhard F. P. Karger Dr. Lasse Jensen Dr. David M. Jenkins Dr. Jon P. Camden 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(19):7655-7660
The remarkable resilience of N-heterocyclic carbene (NHC) gold bonds has quickly made NHCs the ligand of choice when functionalizing gold surfaces. Despite rapid progress using deposition from free or CO2-protected NHCs, synthetic challenges hinder the functionalization of NHC surfaces with protic functional groups, such as alcohols and amines, particularly on larger nanoparticles. Here, we synthesize NHC-functionalized gold surfaces from gold(I) NHC complexes and aqueous nanoparticles without the need for additional reagents, enabling otherwise difficult functional groups to be appended to the carbene. The resilience of the NHC−Au bond allows for multi-step post-synthetic modification. Beginning with the nitro-NHC, we form an amine-NHC terminated surface, which further undergoes amide coupling with carboxylic acids. The simplicity of this approach, its compatibility with aqueous nanoparticle solutions, and its ability to yield protic functionality, greatly expands the potential of NHC-functionalized noble metal surfaces. 相似文献
2.
Versatile Micropatterns of N‐Heterocyclic Carbenes on Gold Surfaces: Increased Thermal and Pattern Stability with Enhanced Conductivity 下载免费PDF全文
D. Thao Nguyen Matthias Freitag Martin Körsgen Sebastian Lamping Dr. Andreas Rühling Dr. Andreas H. Schäfer Martin H. Siekman Prof. Heinrich F. Arlinghaus Prof. Wilfred G. van der Wiel Prof. Frank Glorius Prof. Bart Jan Ravoo 《Angewandte Chemie (International ed. in English)》2018,57(35):11465-11469
Patterned monolayers of N‐heterocyclic carbenes (NHCs) on gold surfaces were obtained by microcontact printing of NHC–CO2 adducts and NHC(H)[HCO3] salts. The NHC‐modified areas showed an increased conductivity compared to unmodified gold surface areas. Furthermore, the remaining surface areas could be modified with a second, azide‐functionalized carbene, facilitating further applications and post‐printing modifications. Thorough elucidation by a variety of analytical methods offers comprehensive evidence for the viability of the methodology reported here. The protocol enables facile access to versatile, microstructured NHC‐modified gold surfaces with highly stable patterns, enhanced conductivity, and the option for further modification. 相似文献
3.
Expanded‐Ring N‐Heterocyclic Carbenes Efficiently Stabilize Gold(I) Cations,Leading to High Activity in π‐Acid‐Catalyzed Cyclizations 下载免费PDF全文
Oleg S. Morozov Andrey V. Lunchev Alexander A. Bush Aleksandr A. Tukov Andrey F. Asachenko Victor N. Khrustalev Sergey S. Zalesskiy Valentine P. Ananikov Mikhail S. Nechaev 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(20):6162-6170
A series of six‐ and seven‐membered expanded‐ring N‐heterocyclic carbene (er‐NHC) gold(I) complexes has been synthesized using different synthetic approaches. Complexes with weakly coordinating anions [(er‐NHC)AuX] (X?=BF4?, NTf2?, OTf?) were generated in solution. According to their 13C NMR spectra, the ionic character of the complexes increases in the order X?=Cl?<NTf2?<OTf?<BF4?. Additional factors for stabilization of the cationic complexes are expansion of the NHC ring and the attachment of bulky substituents at the nitrogen atoms. These er‐NHCs are bulkier ligands and stronger electron donors than conventional NHCs as well as phosphines and sulfides and provide more stabilization of [(L)Au+] cations. A comparative study has been carried out of the catalytic activities of five‐, six‐, and seven‐membered carbene complexes [(NHC)AuX], [(Ph3P)AuX], [(Me2S)AuX], and inorganic compounds of gold in model reactions of indole and benzofuran synthesis. It was found that increased ionic character of the complexes was correlated with increased catalytic activity in the cyclization reactions. As a result, we developed an unprecedentedly active monoligand cationic [(THD‐Dipp)Au]BF4 (1,3‐bis(2,6‐diisopropylphenyl)‐3,4,5,6‐tetrahydrodiazepin‐2‐ylidene gold(I) tetrafluoroborate) catalyst bearing seven‐membered‐ring carbene and bulky Dipp substituents. Quantitative yields of cyclized products were attained in several minutes at room temperature at 1 mol % catalyst loadings. The experimental observations were rationalized and fully supported by DFT calculations. 相似文献
4.
Unsymmetrical Pincer‐Type Ruthenium Complex Containing β‐Protic Pyrazole and N‐Heterocyclic Carbene Arms: Comparison of Brønsted Acidity of NH Groups in Second Coordination Sphere 下载免费PDF全文
Tatsuro Toda Prof. Dr. Shigeki Kuwata Prof. Dr. Takao Ikariya 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(31):9539-9542
A reaction of a 2‐(imidazol‐1‐yl)methyl‐6‐(pyrazol‐3‐yl)pyridine with [RuCl2(PPh3)3] resulted in tautomerization of the imidazole unit to afford the unsymmetrical pincer‐type ruthenium complex 2 containing a protic pyrazole and N‐heterocyclic carbene (NHC) arms. Deprotonation of 2 with one equivalent of a base led to the formation of the NHC–pyrazolato complex 3 , indicating that the protic NHC arm is less acidic. When 2 was treated with two equivalents of a base under H2 or in 2‐propanol, the hydrido complex 4 containing protic NHC and pyrazolato groups was obtained through metal–ligand cooperation. 相似文献
5.
Protic N‐Heterocyclic Carbene Versus Pyrazole: Rigorous Comparison of Proton‐ and Electron‐Donating Abilities in a Pincer‐Type Framework 下载免费PDF全文
Tatsuro Toda Akihiro Yoshinari Prof. Dr. Takao Ikariya Prof. Dr. Shigeki Kuwata 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(46):16675-16683
Evaluation of the acidity of proton‐responsive ligands such as protic N‐heterocyclic carbenes (NHCs) bearing an NH‐wingtip provides a key to understanding the metal–ligand cooperation in enzymatic and artificial catalysis. Here, we design a CNN pincer‐type ruthenium complex 2 bearing protic NHC and isoelectronic pyrazole units in a symmetrical skeleton, to compare their acidities and electron‐donating abilities. The synthesis is achieved by direct C?H metalation of 2‐(imidazol‐1‐yl)‐6‐(pyrazol‐3‐yl)pyridine with [RuCl2(PPh3)3]. 15N‐Labeling experiments confirm that deprotonation of 2 occurs first at the pyrazole side, indicating clearly that the protic pyrazole is more acidic than the NHC group. The electrochemical measurements as well as derivatization to carbonyl complexes demonstrate that the protic NHC is more electron‐donating than pyrazole in both protonated and deprotonated forms. 相似文献
6.
Dr. Matthew J. Fuchter 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(41):12286-12294
An overview of the recent bibliography concerning the N‐heterocyclic carbene (NHC)‐mediated activation of tetravalent silicon compounds is presented. Diverse reactions are discussed, such as the NHC‐mediated addition of silyl pronucleophiles to a variety of electrophiles, NHC‐promoted organic and inorganic polymerisation and the reduction of CO2 by hydrosilanes as facilitated by NHCs. The review concludes with a discussion of the current knowledge regarding the role of Lewis acid–base NHC–Si interactions in the mechanistic course of these reactions. 相似文献
7.
The bonding strength of N‐heterocyclic carbene (NHC) ligands to a neutral AuCl test moiety are compared to that of several phosphanes and other ligands. Of the ligands studied, the NHCs clearly form the strongest bonds to AuCl. A simplified triangular CN2 model is also introduced for the NHCs. 相似文献
8.
Densely functionalized cyclopentenones are useful synthetic intermediates. We report herein a new method to synthesize this important class of compounds through a highly enantioselective (98?→?99% ee) triene cyclization that is cocatalyzed by acetic acid and a chiral N-heterocyclic carbene (NHC). We discovered that acetic acid not only could coexist with NHCs but also could greatly stabilize the active catalyst, which enables a long-lived catalyst with high reactivity and selectivity. 相似文献
9.
Andreas Kolmer Lukas Kaltschnee Volker Schmidts Lars H. Peeck Herbert Plenio Christina M. Thiele 《Magnetic resonance in chemistry : MRC》2013,51(11):695-700
There has been much debate about the σ‐donor and π‐acceptor properties of N‐heterocyclic carbenes (NHCs). While a lot of synthetic modifications have been performed with the goal of optimizing properties of the catalyst to tune reactivity in various transformations (e.g. metathesis), direct methods to characterize σ‐donor and π‐acceptor properties are still few. We believe that dynamic NMR spectroscopy can improve understanding of this aspect. Thus, we investigated the intramolecular dynamics of metathesis precatalysts bearing two NHCs. We chose four systems with one identical NHC ligand (N,N′‐Bis(2,4,6‐trimethylphenyl)‐imidazolinylidene (SIMes) in all four cases) and NHCewg ligands bearing four different electron‐withdrawing groups (ewg). Both rotational barriers of the respective Ru‐NHC‐bonds change significantly when the electron density of one of the NHCs (NHCewg) is modified. Although it is certainly not possible to fully dissect σ‐donor and π‐acceptor portions of the bonding situations in the respective Ru‐NHC‐bond via dynamic NMR spectroscopy, our studies nevertheless show that the analysis of the rotation around the Ru‐SIMes‐bond can be used as a spectroscopic parameter complementary to cyclic voltammetry. Surprisingly, we observed that the rotation around the Ru‐NHCewg‐bond shows the same trend as the initiation rate of a ring‐closing metathesis of the four investigated bis‐NHC‐complexes. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
10.
Zikri Altun Erdi A. Bleda Carl Trindle Jason Wang 《International journal of quantum chemistry》2014,114(10):675-687
N‐heterocyclic carbenes (NHCs) based on imidazole‐2‐ylidene ( 1 ) or the saturated imidazolidine‐2‐ylidene ( 2 ) scaffolds are long‐lived singlet carbenes. Both benefit from inductive stabilization of the sigma lone pair on carbon by neighboring N atoms and delocalization of the N pi lone pairs into the nominally vacant p‐pi atomic orbital at the carbene carbon. With thermochemical schemes G4 and CBS‐QB3, we estimate the relative thermodynamic stabilization of smaller ring carbenes and acyclic species which may share the keys to NHC stability. These include four‐membered ring systems incorporating the carbene center, two trivalent N centers, and either a boron or a phosphorus atom to complete the ring. Amino‐substituted cyclopropenylidenes have been reported but three‐membered rings containing the carbene center and two N atoms are not known. Our calculations suggest that amino‐substituted cyclopropenylidenes are comparable in stability to the four‐membered NHCs but that diazacyclopropanylidenes would be substantially less effectively stabilized. Concluding the series are acyclic carbenes with and without neighboring N atoms and a series of “two‐membered ring” azapropadienenylidene cations of form :C?N?W with W = an electron‐withdrawing agent. We have studied W = NO2, CH2(+), CF2(+), and (CN)2C(+). Although these systems display a degree of stabilization and carbene‐like electronic structure, the stability of the NHCs is unsurpassed. © 2014 Wiley Periodicals, Inc. 相似文献
11.
Adduct Formation,B−H Activation and Ring Expansion at Room Temperature from Reactions of HBcat with NHCs 下载免费PDF全文
Sabrina Würtemberger‐Pietsch Heidi Schneider Prof. Dr. Todd B. Marder Prof. Dr. Udo Radius 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(37):13032-13036
We report the reactions of catecholborane (HBcat; 1 ) with unsaturated and saturated NHCs as well as CAACMe. Mono‐NHC adducts of the type HBcat?NHC (NHC=nPr2Im, iPr2Im, iPr2ImMe, and Dipp2Im) were obtained by stoichiometric reactions of HBcat with the unsaturated NHCs. The reaction of CAACMe with HBcat yielded the B?H activated product CAACMe(H)Bcat via insertion of the carbine‐carbon atom into the B?H bond. The saturated NHC Dipp2SIm reacted in a 2:2 ratio yielding an NHC ring‐expanded product at room temperature forming a six‐membered ?B?C=N?C=C?N? ring via C?N bond cleavage and further migration of the hydrides from two HBcat molecules to the former carbene‐carbon atom. 相似文献
12.
Dr. Javier Izquierdo Dr. Gerri E. Hutson Daniel T. Cohen Prof. Karl A. Scheidt 《Angewandte Chemie (International ed. in English)》2012,51(47):11686-11698
N‐Heterocyclic carbene (NHC) catalyzed transformations have emerged as powerful tactics for the construction of complex molecules. Since Stetter’s report in 1975 of the total synthesis of cis‐jasmon and dihydrojasmon by using carbene catalysis, the use of NHCs in total synthesis has grown rapidly, particularly over the last decade. This renaissance is undoubtedly due to the recent developments in NHC‐catalyzed reactions, including new benzoin, Stetter, homoenolate, and aroylation processes. These transformations employ typical as well as Umpolung types of bond disconnections and have served as the key step in several new total syntheses. This Minireview highlights these reports and captures the excitement and emerging synthetic utility of carbene catalysis in total synthesis. 相似文献
13.
Reaching for two new stable ambiphilic quinoline‐derived N‐heterocyclic carbenes at DFT level 下载免费PDF全文
Seyed Majid Musavi Javad Amani Parva Noruzi 《International journal of quantum chemistry》2015,115(4):224-230
A detailed investigation on the thermodynamic and kinetic stability of four carbenic tautomers of quinoline 1 , including quinoline‐2‐ylidene 2 , quinoline‐3‐ylidene 3 , quinoline‐4‐ylidene 4 , and 3,4‐dihydroquinoline‐4‐ylidene 5 , reveals that singlet planar six‐membered ring N‐heterocyclic carbenes (NHCs) 2 and 4 have less stability than Arduengo type NHC but seems to have enough conceivably for reaching at B3LYP/aug‐cc‐pVTZ//B3LYP/6–31+G* and B3LYP/6–311++G**//B3LYP/6–31+G* levels. All these six‐membered NHCs are extremely ambiphilic with the more nucleophilic and electrophilic characters compared to the Arduengo type one. The aromaticity of singlet 2 and 4 is a significant contributor to their stability which is confirmed through their Nucleus‐independent chemical shift(1)zz values. Finally, among 2–5 , the normal NHC 2 is thermodynamically preferred but the remote NHC 4 is kinetically proffered over the other isomeric carbenes. The effects of different N‐ or C‐substituted NHCs of 2 are studied using appropriate isodesmic reactions. The trimethylsilyl substituent exhibits slightly larger carbene stabilization in quinoline‐derived NHCs than the pyridine analogue. © 2014 Wiley Periodicals, Inc. 相似文献
14.
Xing‐Xing Gou Tong Liu Yao‐Yu Wang Ying‐Feng Han 《Angewandte Chemie (International ed. in English)》2020,59(38):16683-16689
Controlling the size and surface functionalization of nanoparticles (NPs) can lead to improved properties and applicability. Herein, we demonstrate the efficiency of the metal‐carbene template approach (MCTA) to synthesize highly robust and soluble three‐dimensional polyimidazolium cages (PICs) of different sizes, each bearing numerous imidazolium groups, and use these as templates to synthesize and stabilize catalytically active, cavity‐hosted, dispersed poly‐N‐heterocyclic carbene (NHC)‐anchored gold NPs. Owing to the stabilization of the NHC ligands and the effective confinement of the cage cavities, the as‐prepared poly‐NHC‐shell‐encapsulated AuNPs displayed promising stability towards heat, pH, and chemical regents. Most notably, all the Au@PCCs (PCC=polycarbene cage) exhibited excellent catalytic activities in various chemical reactions, together with high stability and durability. 相似文献
15.
Alex J. Veinot Dr. Abrar Al-Rashed Dr. J. Daniel Padmos Ishwar Singh Dianne S. Lee Dr. Mina R. Narouz Dr. Paul A. Lummis Prof. Dr. Christopher J. Baddeley Prof. Dr. Cathleen M. Crudden Prof. Dr. J. Hugh Horton 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(50):11431-11434
Benzimidazolium hydrogen carbonate salts have been shown to act as N-heterocyclic carbene precursors, which can remove oxide from copper oxide surfaces and functionalize the resulting metallic surfaces in a single pot. Both the surfaces and the etching products were fully characterized by spectroscopic methods. Analysis of surfaces before and after NHC treatment by X-ray photoelectron spectroscopy demonstrates the complete removal of copper(II) oxide. By using 13C-labelling, we determined that the products of this transformation include a cyclic urea, a ring-opened formamide and a bis-carbene copper(I) complex. These results illustrate the potential of NHCs to functionalize a much broader class of metals, including those prone to oxidation, greatly facilitating the preparation of NHC-based films on metals other than gold. 相似文献
16.
Lei Zhang Zichao Wei Srinivas Thanneeru Michael Meng Megan Kruzyk Gaël Ung Ben Liu Jie He 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(44):15981-15987
The stability of metal nanocatalysts for electrocatalytic CO2 reduction is of key importance for practical application. We report the use of two polymeric N‐heterocyclic carbenes (NHC) (polydentate and monodentate) to stabilize metal nanocatalysts (Au and Pd) for efficient CO2 electroreduction. Compared with other conventional ligands including thiols and amines, metal–carbene bonds that are stable under reductive potentials prevent the nanoclustering of nanoparticles. Au nanocatalysts modified by polymeric NHC ligands show an activity retention of 86 % after CO2 reduction at ?0.9 V for 11 h, while it is less than 10 % for unmodified Au. We demonstrate that the hydrophobicity of polymer ligands and the enriched surface electron density of metal NPs through σ‐donation of NHCs substantially improve the selectivity for CO2 reduction over proton. 相似文献
17.
Facile Syntheses of N‐Heterocyclic Carbene Precursors through I2‐ or NIS‐Promoted Amidiniumation of N‐Alkenyl Formamidines 下载免费PDF全文
We have developed I2‐ or N‐iodosuccinimide (NIS)‐mediated amidiniumation of N‐alkenyl formamidines for the syntheses of cyclic formamidinium salts, some of which could be directly used as N‐heterocyclic carbene (NHC) precursors. Treatment of iodine‐containing formamidinium salts with Al2O3 led to the formation of cyclic formamidinium salts with an unsaturated backbone. A rhodium(I) complex ligated by a representative NHC was prepared by the reaction of [Rh(cod)Cl]2 (cod=1,5‐cyclooctadiene) with the free carbene obtained in situ from deprotonation of the corresponding formamidinium salts. The NHCs prepared in situ can also react with S8 to afford the corresponding thiones. 相似文献
18.
David H. Ringger Ilia J. Kobylianskii Dr. Daniel Serra Prof. Dr. Peter Chen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(44):14270-14281
The gas‐phase bond‐dissociation energies of a SO2–imidazolylidene leaving group of three gold(I) benzyl imidazolium sulfone complexes are reported (E0=46.6±1.7, 49.6±1.7, and 48.9±2.1 kcal mol?1). Although these energies are similar to each other, they are reproducibly distinguishable. The energy‐resolved collision‐induced dissociation experiments of the three [L]–gold(I) (L=ligand) carbene precursor complexes were performed by using a modified tandem mass spectrometer. The measurements quantitatively describe the structural and electronic effects a p‐methoxy substituent on the benzyl fragment, and trans [NHC] and [P] gold ligands, have towards gold carbene formation. Evidence for the formation of the electrophilic gold carbene in solution was obtained through the stoichiometric and catalytic cyclopropanation of olefins under thermal conditions. The observed cyclopropane yields are dependent on the rate of gold carbene formation, which in turn is influenced by the ligand and substituent. The donation of electron density to the carbene carbon by the p‐methoxy benzyl substituent and [NHC] ligand stabilizes the gold carbene intermediate and lowers the dissociation barrier. Through the careful comparison of gas‐phase and solution chemistry, the results suggest that even gas‐phase leaving‐group bond‐dissociation energy differences of 2–3 kcal mol?1 enormously affect the rate of gold carbene formation in solution, especially when there are competing reactions. The thermal decay of the gold carbene precursor complex was observed to follow first‐order kinetics, whereas cyclopropanation was found to follow pseudo‐first‐order kinetics. Density‐functional‐theory calculations at the M06‐L and BP86‐D3 levels of theory were used to confirm the observed gas‐phase reactivity and model the measured bond‐dissociation energies. 相似文献
19.
Experimental and Theoretical Study of the Reactivity of Gold Nanoparticles Towards Benzimidazole‐2‐ylidene Ligands 下载免费PDF全文
Dr. María Rodríguez‐Castillo Gustavo Lugo‐Preciado Dr. Danielle Laurencin Dr. Frederik Tielens Dr. Arie van der Lee Dr. Sébastien Clément Dr. Yannick Guari Dr. José M. López‐de‐Luzuriaga Dr. Miguel Monge Dr. Françoise Remacle Dr. Sébastien Richeter 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(30):10446-10458
The reactivity of benzimidazol‐2‐ylidenes with respect to gold nanoparticles (AuNPs) has been investigated using a combined experimental and computational approach. First, the grafting of benzimidazol‐2‐ylidenes bearing benzyl groups on the nitrogen atoms is described, and comparisons are made with structurally similar N‐heterocyclic carbenes (NHCs) bearing other N‐groups. Similar reactivity was observed for all NHCs, with 1) the erosion of the AuNPs under the effect of the NHC and 2) the formation of bis(NHC) gold complexes. DFT calculations were performed to investigate the modes of grafting of such ligands, to determine adsorption energies, and to rationalize the spectroscopic data. Two types of computational models were developed to describe the grafting onto large or small AuNPs, with either periodic or cluster‐type DFT calculations. Calculations of NMR parameters were performed on some of these models, and discussed in light of the experimental data. 相似文献
20.
Guangrong Meng Lazaros Kakalis Steven P. Nolan Michal Szostak 《Tetrahedron letters》2019,60(4):378-381
The σ-donor properties of NHC ligands (NHC?=?N-heterocyclic carbene) are crucial in controlling their interaction with transition metals, and as a consequence, to determine the selectivity and reactivity of NHCs in transition-metal-catalysis. Herein, we report a simple NMR method for estimating the σ-donor properties of NHC ligands based on a straightforward 1H NMR measurement of ligand precursors. We present evaluation of σ-donating properties for a range of NHC ligands varied by structure and electronics that are relevant to transition-metal-catalysis. We expect that the simple measurement of σ-donating properties of NHCs, together with the known methods for evaluating sterics and π-backbonding, will enhance the understanding of the properties of NHCs in transition-metal-catalysis. 相似文献