极具潜在应用价值的催化剂——N-杂环卡宾金属配合物

N杂环卡宾的反应性能较高,与周期表中几乎所有的金属都能发生反应形成稳定的配合物.主要阐述了N杂环卡宾的结构与类型,其金属配合物的合成方法及在化学反应中的催化作用和应用前景.     

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。     

N-杂环卡宾的反应及应用

自从1991年Arduengo第一次分离得到稳定的游离N-杂环卡宾以后,N-杂环卡宾金属络合物的研究在近几年来得到了迅速的发展。N-杂环卡宾的反应性能较高,它们与周期表中几乎所有的元素都能发生反应。N-杂环卡宾金属络合物对许多反应有催化作用,它们是一类有潜在应用价值的催化剂。本文对近年来相关的研究成果进行了综述。     

Replacement of N-heterocyclic carbenes by N-heterocyclic silylenes at a Pd(0) center: Experiment and theory

Via NMR-spectroscopy the relative reactivity of N-heterocyclic silylenes (NHSi) and carbenes (NHC) was studied. Reaction of sterically crowded bis-N-heterocyclic Pd(0) carbene complexes with free N-heterocyclic silylenes led to complete displacement of the N-heterocyclic carbene, which is unexpected knowing that usually a silylene is a weaker bound ligand compared to a carbene. High-level DFT calculations on a small model system and the experimentally used complexes confirm the experimental findings and indicate that steric interactions play an important role in the substitution reaction.     

A stable N-heterocyclic carbene with a diboron backbone

The synthesis of a novel five-membered inorganic ring, a stable N-heterocyclic carbene with a diboron backbone, is reported. A pentacarbonyltungsten complex containing the new carbene is also described. Spectroscopic evidence indicates that the sterically encumbered carbene is a better sigma-donor than classical N-heterocyclic analogues, demonstrating the significant influence of the inorganic backbone on the coordinating properties of the carbon ligand. Crystal structures have been determined for an iminium precursor, the free carbene, and the tungsten complex.     

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.     

Ruthenium(II)-NHC-catalyzed (NHC = perhydrobenzimidazol-2-ylidene) alkylation of amines using the hydrogen borrowing methodology under solvent-free conditions

Transition Metal Chemistry - New ruthenium(II) complexes with N-heterocyclic carbene ligand were synthesized by transmetalation reactions between silver(I) N-heterocyclic carbene complexes and...     

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.     

氮杂环卡宾催化二氧化碳羧化反应的研究进展

二氧化碳是一种来源丰富的可再生资源,科研工作者一直致力于开发能够高效转化二氧化碳的催化体系。氮杂环卡宾在有机化学中是一类非常重要的催化剂,利用氮杂环卡宾-过渡金属配合物催化实现二氧化碳的高效化学转化受到了人们的广泛关注。本文主要根据氮杂环卡宾-过渡金属配合物进行分类,总结归纳了近年来氮杂环卡宾-过渡金属配合物催化二氧化碳羧化反应的研究进展。     

Protonation of Homoenolate Equivalents Generated by N-Heterocyclic Carbenes

Homoenolate equivalents are generated by Lewis basic N-heterocyclic carbene catalysts and then protonated to generate efficiently saturated esters from unsaturated aldehydes. This reactivity is extended to the generation of β-acylvinyl anions from alkynyl aldehydes. The asymmetric protonation of a homoenolate equivalent generated from a β,β-disubstituted aldehyde can be accomplished with a chiral N-heterocyclic carbene.     

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.     

An efficient synthesis of achiral and chiral 1,2,4-triazolium salts: bench stable precursors for N-heterocyclic carbenes

[reaction: see text] The promising utility of triazolyl N-heterocyclic carbene catalysts in umpolung aldehyde chemistry requires a straightforward reliable synthesis from readily available materials. Herein, we describe the synthesis of a variety of triazolyl N-heterocyclic carbene precursors. The reactions commence from commercially available amino acids and proceed in 44-68% overall yields. The N-heterocyclic salts are air-stable crystalline solids that can be stored with no special precaution and can generate the active catalyst when treated with an appropriate base.     

Carbon monoxide-promoted carbene insertion into the aryl substituent of an N-heterocyclic carbene ligand: Buchner reaction in a ruthenium carbene complex

In the presence of carbon monoxide, ruthenium carbenes give a net insertion/ring expansion (Buchner reaction) into one of the aromatic rings of the N-heterocyclic carbene ligand. In alkene metathesis applications, the N-heterocyclic carbene ligand is both robust and typically inert to reactions with the metal-bound carbene. This unique reaction is completely regioselective. The complexes obtained through ring expansion were fully characterized in the solid state using X-ray crystallography and in solution using NMR and IR spectroscopy.     

Access to macrocyclic endocyclic and exocyclic allylic alcohols by nickel-catalyzed reductive cyclization of ynals

Ni-catalyzed reductive macrocyclizations of ynals are reported. Disubstituted alkynes afford either endocyclic or exocyclic allylic alcohols depending on the ligand. Phosphine ligands favor the formation of endocyclic olefins, whereas N-heterocyclic carbene ligands favor the formation of exocyclic olefins. Terminal alkynes provide 1,2-disubstituted olefins with N-heterocyclic carbene ligands.     

Dual electron uptake by simultaneous iron and ligand reduction in an N-heterocyclic carbene substituted [FeFe] hydrogenase model compound

An N-heterocyclic carbene containing [FeFe]H(2)ase model complex, whose X-ray structure displays an apical carbene, shows an unexpected two-electron reduction to be involved in its electrocatalytic dihydrogen production. Density functional calculations show, in addition to a one-electron Fe-Fe reduction, that the aryl-substituted N-heterocyclic carbene can accept a second electron more readily than the Fe-Fe manifold. The juxtaposition of these two one-electron reductions resembles the [FeFe]H(2)ase active site with an FeFe di-iron unit joined to the electroactive 4Fe4S cluster.     

A stable four-membered N-heterocyclic carbene

The synthesis of the first four-membered N-heterocyclic carbene is described. Depending on the substituents on the nitrogen atoms, it is possible to characterize at room temperature the carbene dimer or the free carbene. Crystallographic analyses are provided for these carbene species.     

Function of an N-heterocyclic carbene ligand based on concept of chiral mimetic

Function of a new N-heterocyclic carbene ligand based on the concept of a chiral mimetic is described. With (4R,5R)-4,5-diphenyl-1,3-dialkyl-4,5-dihydro-3H-imidazol-1-ium tetrafluoroborates as N-heterocyclic carbene precursors, Rh-catalyzed enantioselective arylation (up to 27% ee) of aromatic aldehydes with arylboronic acids and Pd-catalyzed enantioselective intramolecular alpha-arylation (up to 66% ee) of N-(2-bromoaryl)-N-alkyl-2-arylpropanamides are investigated.     

Synthesis and structural characterization of a silver complex of a mixed-donor N-heterocyclic carbene linked cyclophane

The synthesis of a dicationic imidazolium-linked cyclophane and a dimeric silver-N-heterocyclic carbene complex, that is the first silver complex with a N-heterocyclic carbene ligand involved in a pi-bonding interaction, is reported.     

N-heterocyclic carbene catalyzed reaction of phthalaldehydes: controllable stereoselective synthesis of polyhydroxylated spiro- and fused indenones dictated by the structure of NHC catalysts

The N-heterocyclic carbene catalyzed stereoselective dimerization reactions of phthalaldehydes produced polyhydroxylated spiro- or fused indenones. The reaction pathways were dictated by the structures of NHC catalysts. Under the catalysis of a imidazole carbene, phthalaldehydes produced dihydroxylspiro[indene-2,1'-isobenzofuran]-3-ones in good to excellent yields, whereas a triazole carbene catalyzed reaction of phthalaldehydes afforded fully cis-trihydroxylindeno[2,1-a]inden-5-ones in high yields. This work not only provides a highly efficient method for the construction of valuable polyhydroxyl substituted indene derivatives that are not easily assembled by other synthetic means but also reflects the versatility of organocatalysis using N-heterocyclic carbenes.     

Ruthenium alkylidenes: modulation of a new class of catalysts for controlled radical polymerization of vinyl monomers

Air-stable and readily available ruthenium benzylidene complexes of the general type [RuCl2(=CHPh)(L)(L')] (L, L' = PCy3 and/or N-heterocyclic carbene) constitute a new class of catalyst precursors for atom-transfer radical polymerization (ATRP) of methyl methacrylate and styrene, and provide an unprecedented example for the involvement of ruthenium alkylidenes in radical reactions. They promote the polymerization of various monomers with good to excellent yields, and in a controlled way with methyl methacrylate and styrene. Variations of their basic structural motif provide insights into the essential parameters responsible for catalytic activity. The ligands L (PCy3 and/or N-heterocyclic carbene) turned out to play a particularly important role in determining the rate of the polymerizations. A similarly pronounced influence is exerted by the substituents on the N-heterocyclic carbene. Our results indicate that the catalysts decompose quickly under ATRP conditions, and polymerizations are mediated by both [RuCl2(=CHPh)(L)(L')] complexes and ruthenium species bereft of the benzylidene moiety, through a pathway in which both tricyclohexylphosphane and/or N-heterocyclic carbene ligands remain bound to the metal center. Polymerization of n-butyl acrylate and vinyl acetate is not controlled and most probably takes place through a redox-initiated free-radical process.