Chiral N-Heterocyclic Carbene Catalyzed Staudinger Reaction of Ketenes with Imines: Highly Enantioselective Synthesis of N-Boc β-Lactams

Stereo- and Chemodivergent NHC-Promoted Functionalisation of Arylalkylketenes with Chloral

Stereo- and chemodivergent enantioselective reaction pathways are observed upon treatment of alkylarylketenes and trichloroacetaldehyde (chloral) with N-heterocyclic carbenes, giving selectively either β-lactones (up to 88:12 dr, up to 94 % ee) or α-chloroesters (up to 94 % ee). Either 2-arylsubstitution or an α-branched iPr alkyl substituent within the ketene favours the chlorination pathway, allowing chloral to be used as an electrophilic chlorinating reagent in asymmetric catalysis.

Regio- and stereoselective synthesis of benzothiazolo-pyrimidinones via an NHC-catalyzed Mannich/lactamization domino reaction

An NHC-catalyzed regio- and stereoselective Mannich/lactamization domino reaction of N-(benzothiazolyl)imines with α-chloroaldehydes has been developed.

An NHC-catalyzed regio- and stereoselective Mannich/lactamization domino reaction of N-(benzothiazolyl)imines with α-chloroaldehydes has been developed. This new protocol provides a facile approach for the asymmetric synthesis of benzothiazolo-pyrimidinones and a pyrrolo[1,2-a]indolone in moderate to good yields (34–78%) and excellent stereoselectivities (87–99% ee, up to >20 : 1 d.r.).

Probing the Diastereoselectivity of Staudinger Reactions Catalyzed by N-Heterocyclic Carbenes

The reaction of ethylphenylketene with 1,3-dimesitylimidazol-2-ylidene (IMes) or 1,3-dimesitylimidazolin-2-ylidene (SIMes) afforded the corresponding azolium enolates in high yields. The two zwitterions were fully characterized by various analytical techniques. Their thermal stabilities were monitored by thermogravimetric analysis and the molecular structure of SIMes⋅EtPhCCO was determined by means of X-ray crystallography. A mechanism was proposed to account for the trans-diastereoselectivity observed in the [2+2] cycloaddition of ketenes and N-protected imines catalyzed by N-heterocyclic carbenes and an extensive catalytic screening was performed to test its validity. The steric bulk of the NHC catalyst markedly affected the cis/trans ratio of the model β-lactam product. The nature of the solvent used to carry out the Staudinger reaction also significantly influenced its diastereoselectivity. Conversely, the nature of the substituent on the N-sulfonated imine reagent and the reaction temperature were less critical parameters.

Carbene catalyzed umpolung of α,β-enals: a reactivity study of diamino dienols vs. azolium enolates, and the characterization of advanced reaction intermediates

Since their discovery by Bode and Glorius in 2004, N-heterocyclic carbene catalyzed conjugate umpolung reactions of α,β-enals have been postulated to involve the formation of diamino dienols ("homoenolates") and/or azolium enolates ("enolates"), typically followed by addition to electrophiles, e.g. Michael-acceptors. In this article, we provide evidence, for the first time, for the postulated individual and specific reactivity patterns of diamino dienols (γ-C-C-bond formation) vs. azolium enolates (β-C-C-bond formation). Our study is based on the pre-formation of well defined diamino dienols and azolium enolates, and the in situ NMR monitoring of their reactivities towards enone electrophiles. Additionally, reaction intermediates were isolated and characterized, inter alia by X-ray crystallography.

Highly enantioselective sulfa-Michael addition reactions using N-heterocyclic carbene as a non-covalent organocatalyst

We report the first asymmetric sulfa-Michael addition (SMA) reactions using a chiral N-heterocyclic carbene (NHC) as a non-covalent organocatalyst. We demonstrate that a triazolium salt derived NHC functions as an excellent Brønsted base to promote enantioselective carbon-sulfur bond formation. The reaction is applicable to a wide range of thiols and electrophilic olefins. Notably, quaternary chiral centers bearing both an S atom and a CF3 group were synthesized with excellent asymmetric control. Mechanistic studies suggest that the facial discrimination is likely to be guided by non-covalent interactions: hydrogen bonding and π-π stacking.

Enantioselective annulation of enals with 2-naphthols by triazolium salts derived from l-phenylalanine

A series of chiral triazolium salts have been synthesized from methyl l-phenylalaninate hydrochloride. The NHCs derived from this class of novel triazolium salts were found to be highly efficient catalysts in the annulation reaction of enals and 2-naphthols. These reactions proceeded with high chemoselectivity and wide substrate scope affording enantioenriched β-arylsplitomicins in good yields with up to 96% ee.

NHC-Catalyzed Asymmetric Synthesis of Functionalized Succinimides from Enals and α-Ketoamides

The efficient asymmetric synthesis of highly substituted succinimides from α,β-unsaturated aldehydes and α-ketoamides via NHC-catalyzed [3+2] cycloaddition has been developed. The new scalable protocol significantly expands the utility of NHC catalysis for the synthesis of heterocycles and provides easy access to assemble a wide range of succinimides from simple starting materials.

N-Heterocyclic Carbene-Catalyzed δ-Carbon LUMO Activation of Unsaturated Aldehydes

An N-heterocyclic carbene (NHC) catalyzed domino reaction triggered by a δ-LUMO activation of α,β-γ,δ-diunsaturated enal has been developed for the formal [4 + 2] construction of multisubstituted arenes and 3-ylidenephthalide. These two products, formed in a highly chemo- and regioselective manner, were obtained via different catalytic pathways due to a simple change of the substrate. The activation of the remote δ-carbon of unsaturated aldehydes expands the synthetic potentials of NHC organocatalysis.

N-Heterocyclic carbene-catalyzed [2+3] cyclocondensation of α-chloroaldehydes with azomethine imines

The N-heterocyclic carbene-catalyzed enantioselective [2+3] cyclocondensation of α-chloroaldehydes with azomethine imines was developed to give the corresponding pyrazolidinones in good yields with excellent enantioselectivities.

N-Heterocyclic carbene-catalyzed [3+3] cyclocondensation of bromoenals with aldimines: highly enantioselective synthesis of dihydropyridinones

The N-heterocyclic carbene-catalyzed [3+3] cyclocondensation of bromoenals and aldimines was developed, giving the corresponding dihydropyridinones in good yields with excellent enantioselectivities.

N-heterocyclic carbene-catalyzed cyclocondensation of 2-aryl carboxylic acids and enones: highly enantioselective synthesis of δ-lactones

The NHC-catalyzed [4 + 2] cyclocondensation of 2-aryl carboxylic acids with enones was developed, giving δ-lactones in good yields with high enantioselectivities.

NHC-catalyzed activation of α,β-unsaturated N-acyltriazoles: an easy access to dihydropyranones

Employing the NHC-catalyzed activation of α,β-unsaturated N-acyltriazoles an efficient enantioselective synthesis of dihydropyranones via a formal [3+3] cycloaddition with 1,3-dicarbonyl compounds is described.

N-heterocyclic carbene-catalysed pentafluorophenylation of aldehydes

N-heterocyclic carbenes have been utilized as highly efficient organocatalysts to catalyse multifluorophenylation of aldehydes with fluorinated aryltrimethylsilanes to afford the corresponding adducts in 49–99% yields.

N-heterocyclic carbene-catalyzed [4 + 2] cyclization of 2-bromo-2-enal with 3-alkylenyloxindoles: efficient assembly of spirocarbocyclic oxindole

A NHC-catalyzed [4 + 2] cyclization of 2-bromo-2-enal bearing γ-H with 3-alkylenyloxindoles under mild reaction conditions gives spirocarbocyclic oxindoles containing one quaternary carbon with high diastereoselectivies.

Asymmetric N-Heterocyclic Carbene Catalyzed Annulation of 2-Alkenylbenzothiazoles with α-Chloro Aldehydes

Diastereo- and enantioselective N-heterocyclic carbene catalyzed 1-azadiene Diels-Alder reactions of (E)-2-styrylbenzothiazoles with α-chloro aldehydes are reported. This annulation strategy provides an efficient access to medicinally important dihydrobenzothiazolopyridin-1-ones in good to excellent yields (44-97%) with very good to excellent stereoselectivities (up to 9:1 dr, 98% ee) and tolerates quite a range of substituents.

N-heterocyclic carbene-catalyzed highly enantioselective synthesis of substituted dihydropyranopyrazolones

Highly optically pure multisubstituted dihydropyrano[2,3-c]pyrazol-6-(1H)-ones were synthesized via the N-heterocyclic carbene-catalyzed enantioselective [4 + 2] annulation of α-chloroaldehydes and pyrazolone-derived oxodienes.

Asymmetric catalysis with N-heterocyclic carbenes as non-covalent chiral templates

N-heterocyclic carbenes are a class of persistent carbenes stabilized by adjacent heteroatoms that are part of a heterocycle. They play a central role in multiple enzymatic biosynthetic reactions that involve thiamine diphosphate. Inspired by this biocatalysis machinery, N-heterocyclic carbenes have emerged as one of the most versatile classes of organocatalysts for organic reactions. However, the asymmetric synthesis of carbon-carbon bonds through a non-covalent interaction mechanism has not been previously established for chiral carbenes. Here, we report an N-heterocylic carbene-catalysed, highly enantioselective process that uses weak hydrogen bonds to relay asymmetric bias. We find that catalytic amounts of hexafluoroisopropanol are the critical proton shuttle that facilitates hydrogen transfer to provide high-reaction rates and high enantioselectivity. We demonstrate that a successful asymmetric reaction of this type can be accomplished through a rational design that balances the pKa values of the substrate, the carbene precursor and the product.

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.

Functionalization of benzylic C(sp(3))-H bonds of heteroaryl aldehydes through N-heterocyclic carbene organocatalysis

Aryl aldehyde activation: Oxidative activation of 2-methylindole-3-carboxaldehyde (I) through N-heterocyclic carbene (NHC) organocatalysis generates heterocyclic ortho-quinodimethane (II) as a key intermediate. This intermediate then undergoes formal [4+2] cycloaddition with trifluoromethyl ketones or isatins to form polycyclic lactones containing a quaternary carbon center.

Highly enantioselective γ-amination by N-heterocyclic carbene catalyzed [4+2] annulation of oxidized enals and azodicarboxylates

γ-Amination: The title reaction was developed to give the corresponding dihydropyridazinones in good yields with excellent enantioselectivities (see scheme; Mes=2,4,6-trimethylphenyl). The annulation reaction worked well for γ-aryl, γ-alkyl, or γ-alkenyl oxidized enals. Highly enantiopure tetrahydropyridazinones and γ-amino acid derivatives could be easily prepared by chemical transformations of the resulting dihydropyridazinones.