Recent Progress toward the Construction of Axially Chiral Molecules Catalyzed by an N-heterocyclic Carbene

Enantioselective Synthesis of δ-Carbolinones via N-Heterocyclic Carbene Catalysis

An enantioselective synthesis of δ-carbolinones was accomplished through N-heterocyclic carbene-catalyzed formal [3+3] annulation of 3-aminoindoles with 2-bromoenals. This protocol offers a rapid and efficient synthetic approach for accessing a wide range of enantioenriched structurally interesting δ-carbolinones under mild reaction conditions.

Construction of N-Polyheterocycles by N-Heterocyclic Carbene Catalysis via a Regioselective Intramolecular Radical Addition/Cyclization Cascade

N-Polyheterocycles are rapidly accessed by N-heterocyclic carbene (NHC) catalysis through regioselective sequential radical addition/cyclization in the absence of any metals or oxidants. The transformation occurs under mild conditions and enjoys a wide substrate scope with excellent functional group compatibility. Furthermore, a gram-scale synthesis is also conducted. Preliminary mechanistic studies reveal the potential involvement of an NHC radical cation intermediate.

N-Heterocyclic Carbene-Catalyzed Atroposelective Synthesis of Pyrrolo[3,4-b]pyridines with Configurationally Stable C-N Axial Chirality

The first atroposelective synthesis of pyrrolo[3,4-b]pyridines catalyzed by N-heterocyclic carbene has been achieved. A wide range of chiral atropisomers of pyrrolo[3,4-b]pyridines were obtained in high yields with excellent enantioselectivities (96-99% enantiomeric excess). The experimental results and density functional theory calculations showed that the C-N axial chirality of the product had high thermal stability.

Transition Metal-Catalyzed Biaryl Atropisomer Synthesis via a Torsional Strain Promoted Ring-Opening Reaction

ConspectusArising from the restricted rotation of a single bond caused by steric or electronic effects, atropisomerism is one of the few fundamental categories for molecules to manifest their three-dimensional characters into which axially chiral biaryl compounds fall. Despite the widespread occurrence of axially chiral skeletons in natural products, bioactive molecules, and chiral ligands/organocatalysts, catalytic asymmetric methods for the synthesis of these structures still lag behind demand. Major challenges for the preparation of these chiral biaryls include accessing highly sterically hindered variants while controlling the stereoselectivity. A couple of useful strategies have emerged for the direct asymmetric synthesis of these molecules in the last two decades.Recently, we have engaged in catalytic asymmetric synthesis of biaryl atropisomers via transition metal catalysis, including asymmetric ring-openings of dibenzo cyclic compounds. During these studies, we serendipitously discovered that the two substituents adjacent to the axis cause these dibenzo cyclic molecules to be distorted to minimize steric repulsion. The distorted compounds display higher reactivity in the ring-opening reactions than the non-distorted molecules. In other words, torsional strain can promote a ring-opening reaction. On the basis of this concept, we have successfully realized the catalytic asymmetric ring-opening reaction of cyclic diaryliodoniums, dibenzo silanes, and 9H-fluoren-9-ols, which delivered several differently substituted ortho tetra-substituted biaryl atropisomers in high enantioselectivity. The torsional strain not only activates the substrates toward ring-opening under mild conditions but also changes the chemoselectivity of bond-breaking events. In the palladium-catalyzed carboxylation of S-aryl dibenzothiophenium, the torsional strain inversed the bond selectivity from exocyclic C-S bond cleavage to the ring-opening reaction.In this Account, we summarize our studies on copper-, rhodium-, or palladium-catalyzed asymmetric ring-opening reactions of dibenzo cyclic compounds as a useful collection of methods for the straightforward preparation of ortho tetra-substituted biaryl atropisomers with high enantiopurity on the basis of the above-mentioned torsional strain-promoted ring-opening coupling strategy. In the last part, the torsional strain energies are also discussed with the aid of density functional theory (DFT) calculations.

N-Heterocyclic Carbene-Catalyzed Enantioselective Synthesis of Carbocycle-Fused Uracils via Formal [4 + 2] Annulation/Lactone Formation/Decarboxylation Cascade

An unprecedented organocatalytic asymmetric construction of novel six-membered carbocycle fused uracils has been demonstrated by the reaction of the remotely enolizable 6-methyluracil-5-carbaldehydes with 2-bromoenals. The reaction involves an N-heterocyclic carbene-catalyzed formal [4 + 2] annulation of o-quinodimethane (oQDM) dienolates with α,β-unsaturated acylazoliums, followed by a cascade lactone formation/decarboxylation process. This protocol unlocks a new platform to access enantioenriched carbocycle-fused uracils.

Organocatalytic enantioselective construction of axially chiral (1H)-isochromen-1-imines

Heterocycloalkenyl atropisomers, derived from biaryl atropisomers and axially chiral styrenes, have emerged as a new class of nonbiaryl C-C atropisomers due to the benefit in improving the pharmacological activity and structural diversity. This paper proposes an intramolecular annulation strategy for constructing the heterocycloalkenyl atropisomers (1H)-isochromen-1-imines by organocatalysis. Various heterocycloalkenyl atropisomers (1H)-isochromen-1-imines were prepared in good to excellent yields with excellent enantioselectivity (up to 98% ee), and could be easily converted to atropisomeric lactones isocoumarins.

Organocatalytic Enantioselective Construction of Spiroketal Lactones Bearing Axial and Central Chirality via an Asymmetric Domino Reaction

The catalytic asymmetric synthesis of chiral compounds with multiple stereogenic elements via a single catalytic process is challenging. This paper proposes a domino asymmetric electrophilic halocyclization strategy for constructing heterocycloalkenyl atropisomeric spiroketal lactones. A single catalyst was utilized to realize two independent stereodetermining steps. Various spiroketal lactones containing both chiral axes and chiral centers were prepared in excellent yields with excellent enantioselectivity and diastereoselective (up to 99% ee and >20:1 dr).

Synthesis of Axially Chiral Aldehydes by N-Heterocyclic-Carbene-Catalyzed Desymmetrization Followed by Kinetic Resolution

Axially chiral aldehydes have received increasing attention in enantioselective catalysis. However, only very few catalytic methods have been developed to construct structurally diverse axially chiral aldehydes. We herein describe an NHC-catalyzed atroposelective esterification of biaryl dialdehydes as a general and practical strategy for the construction of axially chiral aldehydes. Mechanistic studies indicate that coupling proceeds through a novel combination of NHC-catalyzed desymmetrization of the dialdehydes and kinetic resolution. This protocol features excellent enantioselectivity, mild conditions, good functional-group tolerance, and applicability to late-stage functionalization and provides a modular platform for the synthesis of axially chiral aldehydes and their derivatives.

N-Heterocyclic-Carbene-Catalyzed C-H Acylation via Radical Relay

A method of N-fluorocarboxamide-directed N-heterocyclic-carbene (NHC)-catalyzed benzylic C-H acylation with aldehydes via the hydrogen atom transfer strategy is disclosed. This transformation involves a sequence of single-electron transfer, 1,5-hydrogen atom transfer, and radical cross-coupling steps. This method offers facile access to various highly functionalized ketones and exhibits good chemical yields and functional group tolerance.

Catalytic asymmetric synthesis of 3,4'-indole-pyrazole derivatives featuring axially chiral bis-pentatomic heteroaryls

The atroposelective synthesis of bis-pentatomic heteroaryl systems is challenging due to the low rotation barrier and configurational instability of the 5,5-ring system. 3,4'-Indole-pyrazole is a bis-pentatomic heteroaryl scaffold existing in...

Mechanism and origin of selectivities for NHC-catalyzed synthesis of axially chiral benzothiophene/benzofuran-fused biaryls

By performing density functional theory (DFT) calculations, we investigated and identified the fundamental pathway for N-heterocyclic carbene (NHC)-catalyzed synthesis of axially chiral benzothiophene-fused biaryl using enal and 2-benzyl-benzothiophene-3-carbaldehyde, which includes...

Highly enantioselective δ-protonation and formal [3 + 3] annulation promoted by N-heterocyclic carbene

A chiral NHC catalyst reacts with α,β-γ,δ-diunsaturated aldehydes to generate an extended Breslow intermediate. Upon δ-protonation and tautomerization, the resulting α,β-unsaturated acyl azolium undergoes [3 + 3] annulation with enamines to afford various dihydropyridinones.

Synthesis of axially chiral N-aryl benzimidazoles via chiral phosphoric acid catalyzed enantioselective oxidative aromatization

Hantzsch ester and benzothiazoline have been extensively used as H2 donors in enantioselective transfer hydrogenation of C=N bond during the past several decades. However, the corresponding Hantzsch pyridine and benzothiazoles...

An NHC-Catalyzed [3+2] Cyclization of β-Disubstituted Enals with Benzoyl Cyanides

The NHC-catalyzed asymmetric [3+2] cyclization of benzoyl cyanides to homoenolate generated in-situ from enals was reported. This methodology leads to the efficient construction of a series of chiral cyclic compounds...

N-Heterocyclic Carbene-Catalyzed Enantioselective Dearomatizing Annulation of Benzoxazoles with Enals

The first N-heterocyclic carbene-catalyzed enantioselective dearomatizing annulation of benzoxazoles with α,β-unsaturated aldehydes has been achieved. The reaction was found to be compatible with a wide range of benzoxazoles and the...

The N-heterocyclic carbene-catalyzed [3 + 2] annulation of isoindigos with enals: the enantioselective construction of three contiguous stereogenic centers

An NHC-catalyzed enantioselective [3 + 2] annulation of enals and isoindigo is introduced as an efficient strategy for the construction of dimeric spirocyclic bisindoline alkaloid derivatives with moderate yields and good enantioselectivities.