Nucleophilic N-Heterocyclic Carbene Research Articles

Insights into the Factors Controlling the Origin of Activation Barriers in Group 13 Analogues of the Four-Membered N-Heterocyclic Carbenes.

The mechanisms of C–H bond insertion and alkene cycloaddition were investigated theoretically using five model systems: group 13 analogues of the four-membered nucleophilic N-heterocyclic carbenes (NHCs) (1E; E = group 13 element). The theoretical findings indicate that, except for 1B with H2C=CH2, these four-membered NHCs undergo insertion and [1 + 2] cycloaddition reactions with difficulty because their activation barriers are quite high (31 kcal/mol). The theoretically confirmed chemical inertness of the four-membered NHCs 1Ga and 1In might explain why they have been experimentally detected at room temperature. Additionally, our theoretical observations indicate that the reactivity of these four-membered NHCs featuring a central group 13 element follows the order 1B ≫ 1Al > 1Ga > 1In > 1Tl. The theoretical examination suggests that the smaller the atomic radius of the central group 13 element in the four-membered NHC analogue is, the larger the aromaticity of this carbenic molecule is, the higher the basicity of this carbenic molecule in nature is, the larger its nucleophilic attack on other oncoming molecules is, the smaller the barrier heights of its C–H bond insertion and [1 + 2] cycloaddition reactions will be, the higher its exothermicities for these products will be, and thus, the greater its reactivity will be. Moreover, the present theoretical findings reveal that the reactivity of 1B is governed by its highest occupied molecular orbital, a nonbonding sp2 lone pair orbital. In contrast, the reactivity of the four heavier 1E′ (E′ = Al, Ga, In, and Tl) molecules is mainly determined by their lowest unoccupied molecular orbital, a vacant p−π orbital. The conclusions gained from this study allow many predictions to be made.

Enantioselective N-Heterocyclic Carbene Catalyzed Bis(enoate) Rauhut-Currier Reaction.

While the enantioselective Rauhut-Currier reaction is established with bis(enone) substrates, it is yet to be reported with less electrophilic bis(enoate) substrates. By exploiting high-nucleophilicity N-heterocyclic carbenes, it is possible to achieve Rauhut-Currier reactions with these substrates. The reaction is demonstrated with a range of intramolecular reactions (20 examples) and six esterification/RC reaction cascades, which all proceed with high enantioselectivity (most >93:7 er).

Catalytic Asymmetric Stetter Reaction Onto Vinylphosphine Oxides and Vinylphosphonates

An intramolecular Stetter reaction of vinylphosphine oxides and vinylphosphonates has been developed. Treatment of an aldehyde with a nucleophilic N-heterocyclic carbene catalyst allows for addition of an acyl anion equivalent into a vinylphosphine oxide or vinylphosphonate Michael acceptor in yields up to 99% and ee values up to 96%.

Highly Efficient Trialkylsilylcyanation of Aldehydes, Ketones and Imines Catalyzed by a Nucleophilic N‐Heterocyclic Carbene.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Cyano‐Phosphorylation of Aldehydes Catalyzed by a Nucleophilic N‐Heterocyclic Carbene.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Construction of Tetrasubstituted Carbon by an Organocatalyst: Cyanation Reaction of Ketones and Ketimines Catalyzed by a Nucleophilic N-Heterocyclic Carbene

A method for cyanation reaction of ketones and ketimines having lower reactivity than aldehydes and aldimines with TMSCN in the presence of N-heterocyclic carbene prepared from 1,3-bis(2,4,6-trimethylphenyl)imidazolium chloride and potassium tert-butoxide, as a nucleophilic organocatalyst, is described. These cyanations of ketones and ketimines afford the corresponding products in good yields under mild reaction conditions.

Highly efficient trialkylsilylcyanation of aldehydes, ketones and imines catalyzed by a nucleophilic N-heterocyclic carbene

The synthetic utility of N-heterocyclic carbenes was demonstrated by the trialkylsilylcyanation of aldehydes, ketones and imines. In the presence of a catalytic amount of 3a, the reactions with Me 3SiCN proceeded smoothly to give the corresponding cyanohydrin trimethylsilyl ethers or amino nitrile derivatives in good to excellent yields.

N-Heterocyclic carbenes: Effective organic catalysts for living polymerization

An organocatalytic approach to living and condensation polymerization using N-heterocyclic carbenes as nucleophilic catalysts is detailed. The N-heterocyclic carbene catalyst platform is extremely versatile with the nature of the substituents having a pronounced effect upon catalyst stability and activity towards different substrates. Rapid screening of libraries of catalysts provided a basic understanding of catalyst structure (sterics, electronics, etc.) as it influences the polymerization rate, control, substrate and range of molecular weights. ROP from an immiscible ionic liquid (precatalyst reservoir) and a THF solution of monomer and initiator is presented as a simplified method of carbene formation. In situ activation of the ionic liquid generated a nucleophilic N-heterocyclic carbene, which migrates to the organic phase effecting ROP. Other simplified methods of generating carbene thermally from carbene adducts are also presented as polymerization catalysts.

Cyano-Phosphorylation of Aldehydes Catalyzed by a Nucleophilic N-Heterocyclic Carbene

The first method for cyano-phosphorylation of aldehydes with diethyl cyanophosphonate in the presence of N-heterocyclic carbene prepared from 1,3-bis(2,4,6-trimethylphenyl)imidazolium chloride and KOt-Bu, as a nucleophilic catalyst, is described.

Ruthenium Complexes Bearing N-Heterocyclic Carbene (NHC) Ligands

A review of recently published aspects on ruthenium complexes with nucleophilic N-heterocyclic carbene (NHC) ligands is presented here, in continuation of a paper published in the July issue of this Journal, that covered initial work and subsequent main developments in this chemistry. In Part II selected applications of these complexes as pre-catalysts in metathesis reactions are highlighted. Particular attention is paid to metathesis in room temperature ionic liquids as the solvents, asymmetric catalysis, and in situ generated, very active catalytic systems based on NHC-platinum group metals. Most NHC-platinum group metal complexes are useful as highly active and selective pre-catalysts for fundamental chemical transformations.

Ruthenium Complexes Bearing N-Heterocyclic Carbene (NHC) Ligands

The vast family of ruthenium complexes with nucleophilic N-heterocyclic carbene (NHC) ligands is selectively discussed as an improved alternative to their parent diphosphane ruthenium counterparts previously applied in olefin metathesis reactions. The survey covers work done to-date on the ruthenium alkylidene, vinylidene, allenylidene and indenylidene complexes bearing imidazolin-2-ylidene and dihydroimidazolin-2-ylidene carbenes, as well as a number of immobilised ruthenium complexes having these types of ligands. Examples of chiral compounds complete the spectrum of NHC ruthenium complexes. Synthetic methods, catalytic properties and application profile in ring-closing metathesis, cross metathesis, ring-opening metathesis and metathesis polymerisation are highlighted.

Transesterification/Acylation Reactions Catalyzed by Molecular Catalysts

AbstractFor Abstract see ChemInform Abstract in Full Text.

Further Studies on Palladium‐Catalyzed Bismetallative Cyclization of Enynes in the Presence of Bu3SnSiMe3.

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10 Carbenes: reactivity and catalysis

Since the first report of a stable carbene by Arduengo in 1991, the use of N-heterocyclic carbenes (NHC) in organic synthesis has increased dramatically. This is principally due to their unique properties. NHC are known to form strong bonds to late transition metals since they are excellent σ-donors and possess (in general) minimal π-back donation capabilities. In view of this, NHCs have rapidly been used as a new class of ligand, capable of mimicking the well-known phosphines without the classical problems associated with phosphine use. The recent use of NHC is not restricted to catalysis using late transition metals, but they are also used as organic catalysts or as reactants in multi-component reactions, as is described in later sections of this report. Studies have shown that nucleophilic N-heterocyclic carbenes represent a very versatile class of ligands due to their interesting steric and electronic properties. These ligands have several advantages over the commonly utilized phosphines: (i) stabilizing effect, (ii) high thermal stability, (iii) resistance to dissociation from the metal center. Therefore an excess of the ligand is not required in order to prevent aggregation of the catalyst to yield bulk metal. As a consequence, an increasing number of catalytic reactions make use of nucleophilic carbenes as catalyst modifiers. Specific examples include the use of metal–carbene complexes in hydrosilylation, Ru-catalysed furan synthesis and olefin metathesis. In this report, attention will be focused on the latest highlights in the field of use of carbenes. Undoubtedly, NHC ligands are currently important in a wide range of reactions and the reader will find here a summary of current applications.

Further studies on palladium-catalyzed bismetallative cyclization of enynes in the presence of Bu 3SnSiMe 3

Bismetallative cyclization of enynes with Bu 3SnSiMe 3 catalyzed by Pd(0) complex was realized for the first time, which gives cyclized products containing a vinylsilane moiety and a homoallyltin moiety in good yield. In this cyclization, Pd 2(dba) 3·CHCl 3 or Pd(OH) 2 on charcoal is effective as a Pd(0) catalyst and the addition of a phosphine ligand increased the formation of alkyne bismetallation by-product. On the other hand, it was found that a nucleophilic N-heterocyclic carbene could be utilized as a ligand for this cyclization. The utility of the cyclized products obtained from this cyclization in synthetic organic chemistry have been proven by transformation into cyclopropanol derivatives.

Pd(0)‐Catalyzed Bismetallative Cyclization of Enynes in the Presence of Bu3SnSiMe3 Using N‐Heterocyclic Carbene as a Ligand.

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Pd‐Catalyzed Allylic Substitution Using Nucleophilic N‐Heterocyclic Carbene as a Ligand.

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Pd-catalyzed allylic substitution using nucleophilic N-heterocyclic carbene as a ligand.

A nucleophilic N-heterocyclic carbene has been successfully used in a Pd(0)-catalyzed allylic substitution for the first time. It was found that allylic substitution with a soft nucleophile using a Pd-carbene catalyst proceeds via retention of configuration, the stereochemical reaction pathway being the same as that of the reaction using a Pd-phosphine complex. [reaction--see text]

Suzuki—Miyaura Cross‐Coupling Reactions Mediated by Palladium/Imidazolium Salt Systems.

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Catalytic Cross‐Coupling Reactions Mediated by Palladium/Nucleophilic Carbene Systems.

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