N-Heterocyclic Carbene-Catalyzed Silyl Enol Ether Formation

The Silicon-Hydrogen Exchange Reaction: A Catalytic σ-Bond Metathesis Approach to the Enantioselective Synthesis of Enol Silanes

The use of chiral enol silanes in fundamental transformations such as Mukaiyama aldol, Michael, and Mannich reactions as well as Saegusa-Ito dehydrogenations has enabled the chemical synthesis of enantiopure natural products and valuable pharmaceuticals. However, accessing these intermediates in high enantiopurity has generally required the use of either stoichiometric chiral precursors or stoichiometric chiral reagents. We now describe a catalytic approach in which strongly acidic and confined imidodiphosphorimidates (IDPi) catalyze highly enantioselective interconversions of ketones and enol silanes. These "silicon-hydrogen exchange reactions" enable access to enantiopure enol silanes via tautomerizing σ-bond metatheses, either in a deprotosilylative desymmetrization of ketones with allyl silanes as the silicon source or in a protodesilylative kinetic resolution of racemic enol silanes with a carboxylic acid as the silyl acceptor.

Exploring bis-(amino)cyclopropenylidene as a non-covalent Brønsted base catalyst in conjugate addition reactions

The catalytic application of bis-(amino)cyclopropenylidene, as a non-covalent Brønsted base, in the 1,4- and 1,6-conjugate addition of carbon nucleophiles to enones andp-quinone methides, respectively, is described.

The importance of N-heterocyclic carbene basicity in organocatalysis

This review highlights the importance of N-heterocyclic carbene (NHC) basicity for transformations in which NHCs are used as catalysts.

N-Heterocyclic Carbene-Catalysed Mukaiyama–Michael Reaction and Mukaiyama Aldol/Mukaiyama–Michael Three-Component Coupling Reaction

An N-heterocyclic carbene-catalysed Mukaiyama–Michael addition between several trimethylsilyl (TMS) enol ethers and chalcone derivatives has been discovered. In addition, a related reaction cascade involving dehydrative Mukaiyama aldol followed by a Mukaiyama–Michael addition process has been developed. The later reaction can also be achieved as a three-component coupling reaction. The enantioselective variant of these reactions has been examined with homochiral catalysts. Though these catalysts were active, they fail to achieve enantioinduction.

Temperature-controlled Mukaiyama aldol reaction of cyclododecanone (CDD) with aromatic aldehydes promoted by TMSCl via the (TMS)3Si intermediate generated in situ

A temperature controlled chemo-, regio- and diastereoselective synthesis of enones and Mukaiyama aldol reaction have been developed using sterically hindered CDD with organosilane as a catalyst.

N-Heterocyclic carbene catalysed 1,6-hydrophosphonylation of p-quinone methides and fuchsones: an atom economical route to unsymmetrical diaryl- and triarylmethyl phosphonates

A direct method for the synthesis of diaryl- and triarylmethyl phosphonates through 1,6-hydrophosphonylation ofp-quinone methides and fuchsones using NHC as a Brønsted base catalyst is described.

Expedient access to unsymmetrical triarylmethanes through N-heterocyclic carbene catalysed 1,6-conjugate addition of 2-naphthols to para-quinone methides

A direct and atom-economical protocol for the synthesis of unsymmetrical triarylmethanes through 1,6-conjugate addition of 2-naphthols topara-quinone methides using N-heterocyclic carbene as a Brønsted base catalyst is described.

Reactivity of an NHC-stabilized silylene towards ketones. Formation of silicon bis-enolates vs. bis-silylation of the CO bond

The reaction of NHC-stabilized silylaminosilylene 1 with enolizable ketones rapidly led to the regio- and stereoselective formation of silicon bis-enolates in excellent yields under mild conditions.

Nickel catalyzed dealkoxylative Csp2–Csp3 cross coupling reactions – stereospecific synthesis of allylsilanes from enol ethers

The application of cyclic and acyclic enol ethers as electrophiles in cross coupling reactions offers new possibilities for the preparation of functional compounds.

Changing the reaction pathway by NHC/Brønsted base cooperative catalysis: highly stereoselective synthesis of multifunctional benzo[a]fluoren-11-ones from the dimerization of 2-(aroylvinyl)arylaldehydes

The unprecedented NHC/Brønsted base-cocatalyzed dimerization reaction of 2-(aroylvinyl)arylaldehydes was reported. In the presence of a triazole carbene catalyst alone, no reaction of 2-(aroylvinyl)arylaldehydes was observed. However, the combination of triazole carbene and 4-methoxyphenolate efficiently catalyzed the dimerization of 2-(aroylvinyl)arylaldehydes to proceed through a benzoin-Michael-Michael reaction cascade, producing 6-aroyl-5-(aroylmethyl)-11a-hydroxybenzo[a]fluoren-11-ones as the sole diastereomers in good yields.

Acyl anion free N-heterocyclic carbene organocatalysis

Reaction discovery using N-heterocyclic carbene organocatalysis has been dominated by the chemistry of acyl anion equivalents. Recent studies demonstrate that NHCs are far more diverse catalysts, with a variety of reactions discovered that proceed without acyl anion equivalent formation. In this tutorial review selected examples of acyl anion free NHC catalysis using carbonyl compounds are presented.

N-Heterocyclic carbenes (NHCs) as organocatalysts and structural components in metal-free polymer synthesis

The chemistry of N-heterocyclic carbenes (NHCs) has witnessed tremendous development in the past two decades: NHCs have not only become versatile ligands for transition metals, but have also emerged as powerful organic catalysts in molecular chemistry and, more recently, in metal-free polymer synthesis. To understand the success of NHCs, this review first presents the electronic properties of NHCs, their main synthetic methods, their handling, and their reactivity. Their ability to activate key functional groups (e.g. aldehydes, esters, heterocycles, silyl ketene acetals, alcohols) is then discussed in the context of molecular chemistry. Focus has been placed on the activation of substrates finding analogies with monomers (e.g. bis-aldehydes, multi-isocyanates, cyclic esters, epoxides, N-carboxyanhydrides, etc.) and/or initiators (e.g. hydroxy- or trimethylsilyl-containing reagents) employed in such "organopolymerisation" reactions utilizing NHCs. A variety of metal-free polymers, including aliphatic polyesters and polyethers, poly(α-peptoid)s, poly(meth)acrylates, polyurethanes, or polysiloxanes can be obtained in this way. The last section covers the use of NHCs as structural components of the polymer chain. Indeed, NHC-based photoinitiators, chain transfer agents or functionalizing agents, as well as bifunctional NHC monomer substrates, can also serve for metal-free polymer synthesis.

One-pot synthesis of acenaphtho[1,2-b]pyrroles in the presence of supported ionic liquid catalyst

Eleven new 8-[(alkyl or aryl)amino]-9-(alkyl or aryl)acenaphtho[1,2-b]pyrroles were synthesized by one-pot reaction of (acenaphthylen-1-yloxy)trimethylsilane, alkyl or aryl aldehyde, ammonium acetate, and aryl or alkyl isocyanide in refluxing DMF.

N-Heterocyclic Carbene-Promoted Rauhut]Currier Reactions between Vinyl Sulfones and α,β-Unsaturated Aldehydes

N-heterocyclic carbenes (NHCs) promote the addition of 1,1-bis(phenylsulfonyl)ethylene to the α-position of α,β-unsaturated aldehydes. The proposed reaction pathway for this Rauhut-Currier-type reaction includes an unusual conjugate addition of an NHC to a bis-vinyl sulfone.

N-heterocyclic carbene-catalyzed rearrangements of vinyl sulfones

N-heterocyclic carbenes catalyze the rearrangement of 1,1-bis(arylsulfonyl)ethylene to the corresponding trans-1,2-bis(phenylsulfonyl) under mild conditions. Tandem rearrangement/cycloadditions have been developed to capitalize on this new process and generate highly substituted isoxazolines and additional heterocyclic compounds. Preliminary mechanistic studies support a new conjugate addition/Umpolung process involving the ejection and subsequent unusual re-addition of a sulfinate ion.

N-Heterocyclic Carbenes as Organic Catalysts

The synthetic utility of azolium salts as catalysts was thought to be limited to the generation of acyl anion equivalents for use in benzoin and Stetter reactions. The discovery, in 2004, of new catalysts, substrates, and reaction manifolds has ignited a new generation of reactions that fall under the general rubrik of N-heterocyclic carbene (NHC) catalyzed reactions. These powerful new processes include the catalytic generation of activated carboxylates for α-functionalized aldehydes, enantioselective annulations via catalytically generated ester enolate equivalents, and the NHC-catalyzed generation of formal homoenolate equivalents. The history of these new reactions and an overview of the reactions, their substrate scope, and mechanistic pathways are summarized in this Chapter.

N-heterocyclic carbene-induced zwitterionic ring-opening polymerization of ethylene oxide and direct synthesis of alpha,omega-difunctionalized poly(ethylene oxide)s and poly(ethylene oxide)-b-poly(epsilon-caprolactone) block copolymers

An N-heterocyclic carbene (NHC), namely, 1,3-bis-(diisopropyl)imidazol-2-ylidene (1), was demonstrated to bring about the metal-free ring-opening polymerization of ethylene oxide at 50 degrees C in dimethyl sulfoxide, in absence of any other reagents. Poly(ethylene oxide) (PEO) of polydispersities <1.2 and molar masses perfectly matching the [monomer]/[(1)] ratio could thus be obtained in quantitative yields, attesting to the controlled/living character of such carbene-initiated polymerizations. It is argued that (1) adds to ethylene oxide to form a zwitterionic species, namely 1,3-bis-(diisopropyl)imidazol-2-ylidinium alkoxide, that further propagates by a zwitterionic ring-opening polymerization (ZROP) mechanism. Through an appropriate choice of terminating agent NuH or NuSiMe(3) at the completion of the polymerization, a variety of end-functionalized PEO chains could be generated. In particular, alpha,omega-bis(hydroxy)-telechelic PEO, alpha-benzyl,omega-hydroxy, and alpha-azido,omega-hydroxy-difunctionalized PEOs were synthesized by NHC (1)-initiated ZROP, using H(2)O, PhCH(2)OH, and N(3)SiMe(3) as terminating agent, respectively. Characterization of these alpha,omega-difunctionalized PEOs by techniques such as (1)H NMR spectroscopy, MALDI-TOF spectrometry, and size exclusion chromatography confirmed the quantitative introduction of functional groups at both alpha and omega positions of the PEO chains and the formation of very narrow molar mass polymers. Finally, the synthesis of a poly(ethylene oxide)-b-poly(epsilon-caprolactone) diblock copolymer by sequential ZROP of the corresponding monomers was successfully achieved using (1) as organic initiator without isolation of the PEO block intermediate.