Phosphine-Catalyzed Sequential [3+3]/Aza-6π-Electrocyclization Reaction of Cross-Conjugated Azatrienes and δ-Sulfonamido-Allenoates

An unprecedented sequential [3+3]/aza-6π-electrocyclization between cross-conjugated azatrienes and δ-sulfonamido-allenoates, catalyzed by phosphine, has been developed, which provides efficient and facile access to highly functionalized tetrahydroisoquinoline derivatives. The products can be easily transformed into (dihydro)isoquinolines and their fused polycyclic compounds. The reactivity of both azatrienes and δ-sulfonamido-allenoates in this text, acting as a five-atom unit, is unique in the phosphine-catalyzed annulations of allenoates.

NHC-catalyzed enantioselective synthesis of β-trifluoromethyl-β-hydroxyamides

The N-heterocyclic carbene (NHC)-catalyzed formal [2 + 2] cycloaddition between α-aroyloxyaldehydes and trifluoroacetophenones, followed by ring opening with an amine or a reducing agent is described. The resulting β-trifluoromethyl-β-hydroxyamide and alcohol products are produced with reasonable diastereocontrol (typically ≈70:30 dr) and excellent enantioselectivity, and they can be isolated in moderate to good yield as a single diastereoisomer.

New Developments of the Principle of Vinylogy as Applied to π-Extended Enolate-Type Donor Systems

The principle of vinylogy states that the electronic effects of a functional group in a molecule are possibly transmitted to a distal position through interposed conjugated multiple bonds. As an emblematic case, the nucleophilic character of a π-extended enolate-type chain system may be relayed from the legitimate α-site to the vinylogous γ, ε, ..., ω remote carbon sites along the chain, provided that suitable HOMO-raising strategies are adopted to transform the unsaturated pronucleophilic precursors into the reactive polyenolate species. On the other hand, when "unnatural" carbonyl ipso-sites are activated as nucleophiles (umpolung), vinylogation extends the nucleophilic character to "unnatural" β, δ, ... remote sites. Merging the principle of vinylogy with activation modalities and concepts such as iminium ion/enamine organocatalysis, NHC-organocatalysis, cooperative organo/metal catalysis, bifunctional organocatalysis, dicyanoalkylidene activation, and organocascade reactions represents an impressive step forward for all vinylogous transformations. This review article celebrates this evolutionary progress, by collecting, comparing, and critically describing the achievements made over the nine year period 2010-2018, in the generation of vinylogous enolate-type donor substrates and their use in chemical synthesis.

Phosphine-Catalyzed [4+1] Cycloadditions of Allenes with Methyl Ketimines, Enamines, and a Primary Amine

Unprecedented phosphine-catalyzed [4+1] cycloadditions of allenyl imides have been discovered using various N-based substrates including methyl ketimines, enamines, and a primary amine. These transformations provide a one-pot access to cyclopentenoyl enamines and imines, or (chiral) γ-lactams through two geminal C-C bond or two C-N bond formations, respectively. Several P-based key intermediates including a 1,4-(bis)electrophilic α,β-unsaturated ketenyl phosphonium species have been detected by ³¹ P NMR and HRMS analyses, which shed light on the postulated catalytic cycle. The synthetic utility of this new chemistry has been demonstrated through a gram-scaling up of the catalytic reaction as well as regioselective hydrogenation and double condensation to form cyclopentanoyl enamines and fused pyrazole building blocks, respectively.

Kinetic resolution of β-ketoesters with quaternary stereocenters via a carbene-catalyzed benzoin reaction

Chiral β-ketoesters bearing fully substituted carbon centers are important building blocks in organic synthesis. Mono-substituted ketoesters have been widely used to synthesize the above compounds through asymmetric additions or substitutions. The limitations of these protocols mainly exist in the substrate scopes, and α-methyl or α-fluoro-substituted β-ketoesters or acetyl acetates are frequently used owing to their relatively higher reactivity. To break through this limitation, we employed N-heterocyclic carbene-catalyzed kinetic resolution to achieve the access to enantioenriched β-ketoesters with quaternary stereocenters. Sterically more bulky groups such as benzyl, allyl, phenyl and cyclopropyl groups are all tolerated using this method.

Kinetic resolution of 2,2-disubstituted-1,3-diketones via carbene catalysis

Organocatalytic kinetic resolution of 1,3-diketones with central quaternary stereocenters was achieved for the first time. The resolution proceeds via two basic modes, and the inherent principles between the different combinations of ketone groups and the resolution patterns were also disclosed.

Enantioselective N-Heterocyclic Carbene-Catalyzed Synthesis of Spirocyclic Oxindole-benzofuroazepinones

Herein, we report an enantioselective synthesis of azepinones via the N-heterocyclic carbene (NHC) catalyzed [3+4] annulation reaction of isatin-derived enals and aurone-derived azadienes. The corresponding spirocyclic oxindole-benzofuroazepinones were obtained in good yields, with excellent diastereo- and enantioselectivities. The resulted azepinones were evaluated for their in vitro cytotoxic activities against six human tumor cell lines, with two compounds showing significant inhibitory activity comparable with that of cisplatin.

Theoretical study on the mechanism and enantioselectivity of NHC-catalyzed intramolecular SN2′ nucleophilic substitution: what are the roles of NHC and DBU?

A possible catalytic mechanism was proposed and studied in very detail by using the DFT method for a recently reported enantioselective intramolecular S_N2′ substitution of aldehydes with trisubstituted allylic bromides.

Construction of Fused Pyrrolidines and β-Lactones by Carbene-Catalyzed C-N, C-C, and C-O Bond Formations

A carbene-catalyzed intermolecular C-N bond formation, which initiates a highly selective cascade reaction for the synthesis of pyrrolidine fused β-lactones, is disclosed. The nitrogen-containing bicyclic β-lactone products are obtained with good yields and excellent stereoselectivities. Synthetic transformations of the reaction products into useful functional molecules, such as amino catalysts, can be efficiently realized under mild reaction conditions. Mechanistically, this study provides insights into modulating the reactivities of heteroatoms, such as nitrogen atoms, in challenging carbene-catalyzed asymmetric carbon-heteroatom bond-forming reactions.

N-Heterocyclic carbene-catalyzed stereoselective construction of olefinic carbon–sulfur bonds via cross-coupling reaction of gem-difluoroalkenes and thiols

A novel organocatalytic olefinic carbon–sulfur bond forming reaction was developed.

A case study of proton shuttling in palladium catalysis

Thanks to mechanistic studies, the catalytic performance of SCS indenediide Pd pincer complexes has been spectacularly enhanced using catechol additives as proton shuttles.

The mechanism of alkynoic acid cycloisomerization with SCS indenediide Pd pincer complexes has been investigated experimentally and computationally. These studies confirmed the cooperation between the Pd center and the backbone of the pincer ligand, and revealed the involvement of a second molecule of substrate. It acts as a proton shuttle in the activation of the acid, it directs the nucleophilic attack of the carboxylic acid on the π-coordinated alkyne and it relays the protonolysis of the resulting vinyl Pd species. A variety of H-bond donors have been evaluated as external additives, and polyols featuring proximal hydroxyl groups, in particular catechol derivatives, led to significant catalytic enhancement. The impact of 4-nitrocatechol and 1,2,3-benzenetriol is particularly striking on challenging substrates such as internal 4- and 5-alkynoic acids. Endo/exo selectivities up to 7.3/1 and 60-fold increase in reactivity were achieved.

N-Heterocyclic carbene-triggered transition-metal-free synthesis of 2,3-disubstituted benzofuran derivatives

An NHC-triggered, transition-metal-free strategy for the efficient synthesis of 2,3-disubstituted benzofuran derivatives in moderate to excellent yields from aryl or alkyl aldehydes and o-quinone methides has been developed.

DFT perspective toward [3 + 2] annulation reaction of enals with α-ketoamides through NHC and Brønsted acid cooperative catalysis: mechanism, stereoselectivity, and role of NHC

The DFT perspective toward the [3 + 2] annulation reaction through NHC and Brønsted acid cooperative catalysis has been investigated.

A DFT study on NHC-catalyzed intramolecular aldehyde–ketone crossed-benzoin reaction: mechanism, regioselectivity, stereoselectivity, and role of NHC

A computational study on NHC-catalyzed intramolecular aldehyde–ketone crossed-benzoin reaction has been performed using a DFT method.

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.).

An NHC-catalyzed in situ activation strategy to β-functionalize saturated carboxylic acid: an enantioselective formal [3+2] annulation for spirocyclic oxindolo-γ-butyrolactones

An in situ NHC-catalyzed activation strategy to β-functionalize saturated carboxylic acid was developed. This asymmetric formal [3+2] annulation could deliver spirocyclic oxindolo-γ-butyrolactones from saturated carboxylic acid and isatin in good yields with high to excellent enantioselectivities. The easy availability of the starting materials, direct installation of functional units at unreactive carbon atom and the convergent assembly make this protocol attractive in the field of organic synthesis.

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.

A quantum mechanical study of the mechanism and stereoselectivity of the N-heterocyclic carbene catalyzed [4 + 2] annulation reaction of enals with azodicarboxylates

The mechanism and stereoselectivity of the NHC-catalyzed [4 + 2] annulation reaction of enals with azodicarboxylates have been investigated using the DFT method.

Stereoselective synthesis of ε-lactones or spiro-heterocycles through NHC-catalyzed annulation: divergent reactivity by catalyst control

NHC-catalyzed divergent annulation of enals with heterocyclic enones was developed to produce benzofuran/indole-containing ε-lactones or spiro-heterocycles in a highly diastereo- and enantioselective fashion. The chemo-selectivity controlled by the chiral catalyst backbone is particularly noteworthy.