Increased efficiency in cross-metathesis reactions of sterically hindered olefins

Efficiency in olefin cross-metathesis reactions is affected upon reducing the steric bulk of N-heterocyclic carbene ligands of ruthenium-based catalysts. For the formation of disubstituted olefins containing one or more allylic substituents, the catalyst bearing N-tolyl groups is more efficient than the corresponding N-mesityl catalyst. In contrast, the formation of trisubstituted olefins is more efficient using the N-mesityl-containing catalyst. A hypothesis to explain this dichotomy is described.     

Highly efficient ruthenium catalysts for the formation of tetrasubstituted olefins via ring-closing metathesis

[reaction: see text] A series of ruthenium-based metathesis catalysts with N-heterocyclic carbene (NHC) ligands have been prepared in which the N-aryl groups have been changed from mesityl to mono-ortho-substituted phenyl (e.g., tolyl). These new catalysts offer an exceptional increase in activity for the formation of tetrasubstituted olefins via ring-closing metathesis (RCM), while maintaining high levels of activity in ring-closing metathesis (RCM) reactions that generate di- and trisubstituted olefins.     

N-heterocyclic carbene catalyzed umpolung of Michael acceptors for intermolecular reactions

leahciM! The N-heterocyclic carbene catalyzed umpolung of Michael acceptors proceeds through the formation of a deoxy-Breslow intermediate (see scheme; EWG=electron-withdrawing group). This nucleophilic species can react with other Michael acceptors in an intermolecular fashion, thereby resulting in the formation of homo- or heterodimeric olefins. This "Michael umpolung" should become a valuable method for the formation of densely functionalized olefins.     

Efficient synthesis of benzofuranones: N-heterocyclic carbene (NHC)/base-catalyzed hydroacylation-Stetter-rearrangement cascade

A N-heterocyclic carbene/base-catalyzed cascade reaction leading to the formation of functionalized benzofuranones is reported. The reaction proceeds via an intramolecular hydroacylation of unactivated alkynes followed by an intermolecular Stetter reaction and a base-catalyzed chromanone to benzofuranone rearrangement.     

N-杂环卡宾配体在聚合反应中的研究进展

N-杂环卡宾配体的优异催化性能引起了人们的广泛关注, 已成功应用于多种烯烃聚合反应。本文结合N-杂环卡宾的相关研究报道,首先简要介绍N-杂环卡宾,随后重点介绍N-杂环卡宾在开环易位聚合、烯烃配位聚合和原子转移自由基聚合等聚合反应的应用。在此基础上,指出了今后N-杂环卡宾配体在聚合反应研究的发展方向。     

Enantioselective synthesis of [7]helicene: dramatic effects of olefin additives and aromatic solvents in asymmetric olefin metathesis

The asymmetric synthesis of [7]helicene was accomplished in good ee (80%) by kinetic resolution by means of asymmetric olefin metathesis. Three key factors contributed to the success of the kinetic resolution: the use of new Ru-based olefin metathesis catalysts bearing C1-symmetric N-heterocyclic carbene ligands, simple olefins as additives to control the nature of the propagating alkylidene and hexafluorobenzene as a solvent.     

Mesoionic oxides: facile access from triazolium salts or triazolylidene copper precursors, and catalytic relevance

Reaction of CsOH with triazolium salts affords mesoionic compounds containing an exocyclic oxygen; the same product is obtained by reaction of the corresponding Cu(I) triazolylidenes with CsOH and represents an unusual reactivity pattern of N-heterocyclic carbene precursors that has implications for carbene copper-catalyzed reactions.     

Microwave-assisted regioselective olefinations of cyclic mono- and di-ketones with a stabilized phosphorus ylide

A number of cyclic mono- and di-ketones underwent regioselective olefination with (carbethoxyethylidene)triphenylphospharane under controlled microwave heating. The Wittig reaction of 4-substituted cyclohexanones or 1,2- and 1,4-cyclohexanediones with the ylide at 190 °C for 20 min in MeCN afforded the exocyclic olefins in >94:6 isomer ratios. On the other hand, the same reactions carried out at 230 °C for 20 min in the presence of 20 mol % DBU furnished the endocyclic olefins in >83:17 isomer ratios. The base-mediated isomerization of the exocyclic olefins into the endocyclic isomers was primarily driven by thermodynamic stability of the products and the effect of ring structures on deconjugation was examined.     

Influence of Bis-NHC Ligand on Platinum-Catalyzed Hydrosilylation of Internal Alkynes

Synthesis and characterization of a new bifunctional N-heterocyclic carbene precursor with sterically expanded substituents is described. The obtained NHC is employed in the catalytic system for hydrosilylation of internal alkynes providing a fully selective and effective process in mild conditions under air. Comparison of the performance of the newly developed catalytic system with its analogs based on monofunctional NHC ligands proves that the presence of two metal atoms and the proper design of the steric environment around the metal center ensure a definitely greater activity in the hydrosilylation process.     

The pivotal role of symmetry in the ruthenium-catalyzed ring-closing metathesis of olefins

The synthesis of Ru-based precatalysts with N-heterocyclic carbene (NHC) ligands bearing syn- and anti-methyl groups on the NHC backbone and aryl N-substituents with differing steric bulk was carried out. The catalytic behavior of the monophospine Ru precatalysts (7a, 7b, 8a, and 8b) was compared to the corresponding family of phosphine-free catalysts (9a, 9b, 10a and 10b) in the ring-closing metathesis (RCM) of olefins. These catalysts showed high efficiency in RCM reactions and the syn-isomers 7a and 9a, in particular, proved to be among the most active catalysts in the formation of tetrasubstituted olefins through RCM. DFT studies on the entire RCM catalytic cycle of hindered olefins were performed to rationalize the different behaviors of catalysts with syn- and anti-methyl groups on the NHC backbone. Theoretical results not only disclosed how NHC symmetry influences the overall activity of the catalyst, but also gave relevant and more general indications on the crucial steps of the RCM of olefins.     

What difference one double bond makes: Electronic structure of saturated and unsaturated n-heterocyclic carbene ligands in Grubbs 2nd generation-type catalysts

N-heterocyclic carbene (NHC) ligands are a versatile and useful class of ligands that have enjoyed much success over the past few decades in organometallic chemistry. This fact is exemplified most convincingly in Grubbs 2nd generation olefin metathesis catalysts. We explore the electronic impact of the NHC-ligand by decoupling electronic and steric effects through simplified model N-heterocyclic carbenes. Saturated and unsaturated N-heterocyclic carbene ligands give rise to fundamentally different frontier orbitals in these catalysts, suggesting a need to classify them as two electronically distinct ligand classes.     

Grubbs Metathesis Enabled by a Light-Driven gem-Hydrogenation of Internal Alkynes

[(NHC)(cymene)RuCl₂] (NHC=N-heterocyclic carbene) complexes instigate a light-driven gem-hydrogenation of internal alkynes with concomitant formation of discrete Grubbs-type ruthenium carbene species. This unorthodox reactivity mode is harnessed in the form of a “hydrogenative metathesis” reaction, which converts an enyne substrate into a cyclic alkene. The intervention of ruthenium carbenes formed in the actual gem-hydrogenation step was proven by the isolation and crystallographic characterization of a rather unusual representative of this series carrying an unconfined alkyl group on a disubstituted carbene center.     

N-杂环卡宾金属配合物在酮硅氢加成反应中的新进展

N-杂环卡宾是一类新型催化剂和配体, 在有机化学中得到了极大的重视. N-杂环卡宾金属配合物的研究在近几年来得到迅速的发展,总结了酮硅氢加成反应中N-杂环卡宾金属配合物催化剂的应用新进展.     

温和条件下金属双氮杂环卡宾配合物催化CO2直接转化为炔酸的高效均相羧化反应

近年来,催化CO2合成精细化学品的研究备受关注。本研究在温和条件下利用金属双氮杂环卡宾催化剂实现CO2与末端炔烃的直接羧化反应,并提出合理的催化机理。首先,合成制备了铜基、银基两种金属双氮杂环卡宾催化剂,实验证明银双氮杂环卡宾配合物具有较好催化活性。通过改变环境条件和底物种类,对反应条件及催化剂底物适应性进行了探究,利用核磁共振谱仪表征产物分子结构并计算直接羧化反应的催化产率。结果表明,适宜催化条件为:1.2eq Cs2CO3作为添加剂、1大气压、室温、无水溶剂和1(mmol)%催化剂用量。银基金属催化剂活性较铜基催化剂高并具有广泛的底物适应性,对苯乙炔的催化产率高达93%;对乙炔气体同样具有良好的催化活性。此类催化剂具有优良的催化活性,能催化合成丙炔酸等重要医药中间体,在工业应用上具有极大潜力和广阔前景。     

Endocyclic vs exocyclic olefin formation from 4-piperidones via the wittig reaction

The reaction of 1-methyl-4-piperidone and 1-benzoyl-4-piperidone with triethyl phosphono-acetate in the presence of excess base yields both the endocyclic and exocyclic olefins. However, treatment of 1-methyl-4-piperidone with diethyl cyanomethylphosphonate, even in the presence of excess base, yielded only the exocyclic olefin. Factors affecting isomer distribution during the course of the reaction are presented. The isolation of an exocyclic tetrahydropyridine from the sodium borohydride reduction of the methiodide of 4-pyridylacetonitrile is reported.     

以N-杂环卡宾为配体的金属络合物催化有机合成的反应

综述了近几年来以N-杂环卡宾为配体的金属络合物催化有机合成的反应。     

Catalytic boracarboxylation of alkynes with diborane and carbon dioxide by an N-heterocyclic carbene copper catalyst

By the use of an N-heterocyclic carbene copper(I) complex as a catalyst, the boracarboxylation of various alkynes (e.g., diaryl alkynes, aryl/alkyl alkynes, and phenylacetylene) with a diborane compound and carbon dioxide has been achieved for the first time, affording the α,β-unsaturated β-boralactone derivatives regio- and stereoselectively via a borylcupration/carboxylation cascade. Some important reaction intermediates were isolated and structurally characterized to clarify the reaction mechanism.     

Ruthenium-catalyzed ring-closing metathesis to form tetrasubstituted olefins

[structure: see text]. Increased efficiency for ring-closing metathesis to form tetrasubstituted olefins using N-heterocyclic carbene ligated ruthenium catalysts was achieved by reducing the size of the substituents at the ortho positions of the N-bound aryl rings.     

Alkoxydiaminophosphine Ligands as Surrogates of NHCs in Copper Catalysis

A family of phosphine ligands containing a five-membered ring similar to the popular N-heterocyclic carbene ligands and an alkoxy third substituent has been developed. These alkoxydiaminophosphine ligands (ADAP) can be generated in one pot and reacted with a copper(I) source leading to the high yield isolation of complexes [(ADAP)CuX]₂ (X=Cl, Br). The dinuclear nature of these compounds has been established by means of X-ray studies and DOSY experiments. A screening of the catalytic properties of these complexes toward carbene-transfer reactions from diazocompounds to C−H bonds (alkane, arene), olefins or N−H bonds, as well as in CuAAC or nitrene transfer reactions have shown a performance at least similar, if not better, than their (NHC)CuCl analogues, opening a new window in copper catalysis with these readily tunable ADAP ligands.     

A sub-stoichiometric tungsten-mediated Pauson-Khand reaction: Scope and limitations

Terminal alkynes do not partake in sequential [2+2+1]/benzoannulation reactions with Fischer carbene complexes and highly reactive and sterically hindered olefins; instead, they undergo a Fischer carbene complex-catalyzed Pauson-Khand reaction (PKR). This result has allowed the development, for the first time, of a tungsten-catalyzed PKR, although of limited scope.