Catalytic activity of N-heterocyclic carbenes in ring opening polymerization of cyclic siloxanes

Taking advantage of the strong nucleophilic properties of N-heterocyclic carbene (NHC), the efficient catalytic ring opening polymerization (ROP) of cyclotetrasiloxane D₄ was achieved under mild conditions.     

Intramolecular cyclization reactions of unsaturated amino Fischer chromium carbenes forming indoles and quinolines

A thermally induced intramolecular annulation reaction of N-(2-alkenylphenyl)amino substituted Fischer chromium carbenes has been extensively examined. The carbene complexes were prepared in moderate to good yields by reaction of 2-aminostyrenes with intermediately formed acyloxy substituted carbenes. Upon heating, the thermally labile carbenes decomposed producing indoles and quinolines as the major products. The product distribution was found to be highly dependent on the substitution pattern and electronic properties of the starting material, and on the solvent used.     

一种阴离子功能化的氮杂环卡宾-锂配合物的合成与结构

氮杂环卡宾(NHC)作为有机膦的替代物在金属有机化学中的应用受到了愈来愈广泛的关注,由于其毒性小、给电子能力强、空间和电子效应很易通过改变氮原子上取代基进行调控,将这类配体用于后过渡金属催化剂,可大大改善这些催化剂在各类均相催化反应中的反应活性及选择性[1].     

Microwave-mediated intramolecular Diels-Alder cyclization of biodihydroxylated benzoic acid derivatives

The synthetic utility of biodihydroxylated benzoic acid derivatives for the construction of bridge bicyclo scaffolds was investigated. Biodihydroxylation of benzoic acid using Ralstonia eutropha B9 gave (1S,2R)-1,2-dihydroxycyclohexa-3,5-diene-1-carboxylic acid (DCD) in high optical purity (>95% ee). Protection of the intermediate and subsequent functional group transformation gave the required cyclization precursors in moderate to excellent overall yields. Subsequent intramolecular Diels-Alder cyclization of biodihydroxylated benzoic acid derivatives was carried out using either thermal or microwave conditions. Enantiomerically pure products with five chiral centers were obtained in 4-6 steps from achiral starting material.     

4,5-Bis(dialkylamino)-substituted imidazolium systems: facile access to N-heterocyclic carbenes with self-umpolung option

The first synthesis of 4,5-bis-(dimethylamino)-substituted imidazolium compounds was developed, which is based on the reaction of a 1,2-diamino-1,2-bis(phosphonio)ethene with lithiated formamidines. This represents the first application of this class of ethene derivatives for the preparation of heterocycles. These N-heterocyclic carbene (NHC) precursors show a remarkably reduced basicity and nucleophilicity of their NMe2 groups, which is due to the strong anomeric interactions of the latter with the imidazolium core. According to DFT calculations, these NHCs are capable of self-umpolung if sufficiently strong acceptor substituents are introduced at the carbene center. To test the self-umpolung capabilities of the NHCs, various substituents were attached to the carbene center and the obtained compounds were characterized by single-crystal X-ray analysis as well as quantum chemical computations. Strong acceptor substituents are required to induce self-umpolung, such as in the phosphonio-substituted derivative, for which partial self-umpolung was found. The N,N′-bis(4-dimethylaminophenyl)-substituted imidazolium compound represents a special case, as it incorporates as much as three two-step redox systems within the NHC framework. This will probably result in a high electronic flexibility of the corresponding nucleophilic carbenes, especially when they serve as ligands in transition metal complexes.     

Intramolecular Heck cyclization to the galanthamine-type alkaloids: total synthesis of (±)-lycoramine

A novel approach towards the construction of the galanthamine skeleton was demonstrated by the total synthesis of (±)-lycoramine. The key steps include a Pd-catalyzed intramolecular cyclization to form the seven-membered azepane ring and a spontaneous intramolecular Michael addition to afford the five-membered furan ring. This synthetic route has also been demonstrated to be useful for the preparation of novel derivatives with simplified galanthamine skeletons.     

N-杂环卡宾及其金属络合物的性质和合成方法

由于N-杂环卡宾结构的多样性，以及它们的金属络合物良好的稳定性和催化活性，近年来受到了人们的广泛关注。本文对N-杂环卡宾及其金属络合物的性质和合成方法进行了综述。参考文献30篇。     

Bis-ligated Ti and Zr complexes of chelating N-heterocyclic carbenes

In this communication we report the synthesis of novel titanium and zirconium complexes ligated by bidentate “salicylaldimine-like” N-heterocyclic carbenes (NHC). Double addition of the NHC chelate to either TiCl₄(thf)₂ or ZrCl₄ forms bis-ligated organometallic fragments with a distorted octahedral geometry. These complexes are rare examples of group IV transition-metal NHC adducts. Preliminary catalytic tests demonstrate that in the presence of methylaluminoxane (MAO) these complexes are useful initiators for the polymerization of ethylene and the copolymerization of ethylene with norbornene and 1-octene.     

Tuning the electronic properties of N-heterocyclic carbenes

The electron-donating properties of N-heterocyclic carbenes ([N,N'-bis(2,6-dimethylphenyl)imidazol]-2-ylidene and the respective dihydro ligands) with 4,4'-R-substituted aryl rings (4,4'-R=NEt2, OC(12)H(25), Me, H, Br, S(4-tolyl), SO(4-tolyl), SO2(4-tolyl)) were studied. Twelve new N-heterocyclic carbene (NHC) ligands were synthesized as well as the respective iridium complexes [IrCl(cod)(NHC)] and [IrCl(CO)2(NHC)]. Cyclic voltammetry (DeltaE1/2) and IR (nu (CO)) can be used to measure the electron-donating properties of the carbene ligands. Modifying the 4-positions with electron-withdrawing substituents (4-R=-SO(2)Ar, DeltaE1/2=+0.92 V) results in NHC ligands with virtually the same electron-donating capacity as a trialkylphosphine in [IrCl(cod)(PCy3)] (DeltaE1/2 =+0.95 V), while [IrCl(cod)(NHC)] complexes with 4-R=NEt2 (DeltaE1/2= +0.59 V) show drastically more cathodic redox potentials and significantly enhanced donating properties.     

A new approach to cyclic hydroxamic acids: Intramolecular cyclization of N-benzyloxy carbamates with carbon nucleophiles

N-Alkyl-N-benzyloxy carbamates, 2, undergo facile intramolecular cyclization with a variety of carbon nucleophiles to give functionalized five- and six-membered protected cyclic hydroxamic acids, 3, in good to excellent yields. This method can be extended to prepare seven-membered cyclic hydroxamic acids in moderate yields. The sulfone intermediates 3 from this study can be alkylated while the corresponding phosphonates have been shown to undergo HWE reaction. The α,β-unsaturated synthon, 8, prepared by thermal elimination of sulfoxide 3m, undergoes Michael addition with secondary amines. The usefulness of this approach to prepare polydentate chelators has been demonstrated by the synthesis of bis cyclic hydroxamic acids 12, 14, and 15.     

Electronic and ligating properties of carbocyclic carbenes: A theoretical investigation

Carbocyclic carbenes (CCCs) are a class of nucleophilic carbenes which are very similar to N-heterocyclic carbenes (NHCs) in terms of their reactivity, but they do not contain a stabilizing heteroatom in their cyclic ring system. In this study, 17 representative known CCCs and 34 newly designed CCCs are evaluated using quantum chemical methods, and the results are compared in terms of their stability, nucleophilicity, and proton affinity (PA) parameters. The results are divided on the basis of ring size of the known and reported CCCs. The stability, nucleophilicity, PA, complexation energy, and bond strength–related parameters were estimated using M06/6-311++G(d,p) method. The results indicated that the CCCs known in the literature are strong σ-electron donating species and have considerable π-accepting properties. This study led to the design and identification of a few new CCCs with dimethylamine and diaminomethynyl substituents which can be singlet stable and are substantially nucleophilic. © 2018 Wiley Periodicals, Inc.     

四种N-杂环卡宾钯配合物的电喷雾质谱研究

利用电喷雾质谱技术研究了四种N-杂环钯卡宾配合物,优化出较适宜检测的电喷雾质谱条件极性较高的溶剂(如乙腈),较低的离子源温度,样品浓度以1.0×10-4mol/L左右为宜,使用较低锥孔电压(5~35V).在此实验条件下,在全扫描电喷雾正离子谱中都会出现[M-I CH3CN] 及[M-I] 的质谱峰.通过源内碰撞诱导电离(CID)技术进一步分析样品在溶液中的性质.实验说明ESI不仅可作为一种分析工具,也可作为获得更多信息的一种方法,来研究有机金属基团在溶液中的性质.     

Chiral oxazolidinones as electrophiles: Intramolecular cyclization reactions with carbanions and preparation of functionalized lactams

The intramolecular cyclizations of oxazolidinones with carbanions adjacent to sulfones, sulfoxides, and phosphonates proceed in high yields to obtain functionalized γ and δ lactams. The chiral oxazolidinone precursors can be readily synthesized from commercial amino acids. The lactams from this study are useful synthetic intermediates, as demonstrated by the synthesis of a precursor for levetiracetam, an antiepileptic drug.     

Anionic N-heterocyclic carbenes by decarboxylation of sydnone-4-carboxylates

Unstable N-heterocyclic carbenes can be masked and stabilized as pseudo-cross-conjugated hetarenium-carboxylates which decarboxylate on warming. This study deals with the decarboxylation of carboxylates of mesoionic compounds to generate anionic N-heterocyclic carbenes. Lithium sydnone-4-carboxylates were therefore prepared via 4-bromosydnones by halogen-lithium exchange with nBuLi and subsequent treatment with carbon dioxide. Protonation gave the corresponding sydnone-4-carboxylic acids. Thermogravimetric measurements in addition to temperature dependent IR spectroscopy proved the decarboxylation of lithium sydnone-4-carboxylates and formation of the corresponding sydnone anions which can be represented as anionic N-heterocyclic carbenes. In DMSO-d6 solution, water favors the decarboxylation. Calculations have been performed to elucidate the mechanism of the decarboxylation in the absence and presence of water.     

N-heterocyclic carbenes as organocatalysts

Organocatalyzed reactions represent an attractive alternative to metal-catalyzed processes notably because of their lower cost and benign environmental impact in comparison to organometallic catalysis. In this context, N-heterocyclic carbenes (NHCs) have been studied for their ability to promote primarily the benzoin condensation. Lately, dramatic progress in understanding their intrinsic properties and in their synthesis have made them available to organic chemists. This has resulted in a tremendous increase of their scope and in a true explosion of the number of papers reporting NHC-catalyzed reactions. Here, we highlight the ever-increasing number of reactions that can be promoted by N-heterocyclic carbenes.     

N-heterocyclic carbenes mediated cyano-phosphorylation of ketones

Cyanation-O-protection reaction of ketones with different cyanide sources catalyzed by N-heterocyclic carbenes is reported. Under the catalysis of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, various ketones coupled with diethyl cyanophosphonate to produce cyanohydrin-O-phosphates with a quaternary carbon center in moderate to excellent yield. Furthermore, the reaction can be scaled-up easily and high yield maintained.     

A general and versatile approach to thermally generated N-heterocyclic carbenes

The synthesis of N-heterocyclic carbene (NHC) adducts by condensation of diamines with appropriately substituted benzaldehydes is described. This simplified approach provides the NHC adduct without first having to generate the carbene followed by its protection. These adducts undergo thermal deprotection to generate N-heterocyclic carbene in situ. Adduct decomposition temperatures were investigated as a function of catalyst structure by using thermal analysis and spectroscopic techniques. Importantly, unlike adducts derived from chloroform, the new pentafluorobenzene-based adducts are more readily prepared and are stable at room temperature. The utility of these adducts as organic catalyst precursors for living ring-opening polymerization (ROP) of lactide, transesterification reactions, and the synthesis of N-heterocyclic carbene ligated organometallic complexes is also described.