Catalytic Enantioconvergent Decarb­oxylative Allylic Alkylation of Allyl Indol­enin-3-carboxylates

Tandem Dearomatization/Enantioselective Allylic Alkylation of Pyridines

Herein, we report a multistep one-pot reaction of substituted pyridines leading to N-protected tetrahydropyridines with outstanding enantioselectivity (up to 97% ee). An iridium(I)-catalyzed dearomative 1,2-hydrosilylation of pyridines enables the use of N-silyl enamines as a new type of nucleophile in a subsequent palladium-catalyzed asymmetric allylic alkylation. This telescoped process overcomes the intrinsic nucleophilic selectivity of pyridines to synthesize enantioenriched, C-3-substituted tetrahydropyridine products that have been otherwise challenging to access.

Palladium-catalyzed allylic alkylation dearomatization of β-naphthols and indoles with gem-difluorinated cyclopropanes

A palladium-catalyzed allylic alkylation dearomatization of β-naphthols and indoles with gem-difluorinated cyclopropanes has been developed for the first time.

Iridium-Catalyzed Enantioselective Intermolecular Indole C2-Allylation

The enantioselective intermolecular C2-allylation of 3-substituted indoles is reported for the first time. This directing group-free approach relies on a chiral Ir-(P, olefin) complex and Mg(ClO4 )2 Lewis acid catalyst system to promote allylic substitution, providing the C2-allylated products in typically high yields (40-99 %) and enantioselectivities (83-99 % ee) with excellent regiocontrol. Experimental studies and DFT calculations suggest that the reaction proceeds via direct C2-allylation, rather than C3-allylation followed by in situ migration. Steric congestion at the indole-C3 position and improved π-π stacking interactions have been identified as major contributors to the C2-selectivity.

Palladium(0)-Catalyzed Intermolecular Asymmetric Cascade Dearomatization Reaction of Indoles with Propargyl Carbonate

An intermolecular asymmetric cascade dearomatization reaction of indole derivatives with propargyl carbonate was developed. The challenges associated with both the chemoselectivity between the carbon and nitrogen nucleophile and the enantioselective control during the formation of an all-carbon quaternary stereogenic center were well addressed by a Pd catalytic system derived from the Feringa ligand. A series of enantioenriched multiply substituted fused indolenines were provided in good yields (71-86 %) with excellent enantioselectivity (91-96 % ee) and chemoselectivity (3/4>19:1 in most cases).

Palladium(0)-Catalyzed Intermolecular Asymmetric Allylic Dearomatization of Polycyclic Indoles

An intermolecular Pd-catalyzed allylic dearomatization reaction of polycyclic indoles with substituted allylic carbonates was realized in the presence of a newly synthesized chiral phosphoramidite ligand. Various polycyclic indoline and indolenine derivatives were successfully synthesized in excellent yields (up to 99%) with excellent enantioselectivity (up to 98% ee). The obtained products could undergo versatile transformations, increasing the application potential of the method in organic synthesis.

Chiral Phosphoric Acid Catalyzed Intramolecular Dearomative Michael Addition of Indoles to Enones

An enantioselective intramolecular dearomative Michael addition of indolyl enones is presented. In the presence of catalytic amount of chiral phosphoric acid, various enantioenriched spiro-indolenines bearing a quaternary stereogenic center were obtained with good yields and enantioselectivity (up to 97% ee) under mild reaction conditions.

Pd-catalyzed cascade allylic alkylation and dearomatization reactions of indoles with vinyloxirane

We have developed Pd-catalyzed intermolecular Friedel-Crafts-type allylic alkylation and allylic dearomatization reactions of substituted indoles bearing a nucleophilic group with vinyloxirane, providing an efficient method to synthesize structurally diverse tetrahydrocarboline and spiroindolenine derivatives under mild conditions.

Collective Synthesis of 3-Acylindoles, Indole-3-carboxylic Esters, Indole-3-sulfinic Acids, and 3-(Methylsulfonyl)indoles from Free (N-H) Indoles via Common N-Indolyl Triethylborate

A general and direct C3 functionalization of free (N-H) indoles with readily available electrophiles such as acid chlorides, chloroformates, thionyl chloride, and methylsulfonyl chloride via a common N-indolyl triethylborate intermediate is reported. The reaction proceeds smoothly under mild conditions in up to 93% yield. Indoles with substituents at the C2, C4, C5, C6, and C7 positions are well tolerated. The easy accessibility of a variety of important 3-acylindoles, indole-3-carboxylic esters, indole-3-sulfinic acids, and 3-(methylsulfonyl)indoles demonstrates the high degree of compatibility and practicability of this method.

Palladium(0)-Catalyzed Intermolecular Allylic Dearomatization of Indoles by a Formal [4+2] Cycloaddition Reaction

Bridged indoline derivatives were synthesized by an intermolecular Pd-catalyzed allylic dearomatization reaction of substituted indoles. The reaction between indoles and allyl carbonates bearing a nucleophilic alcohol side-chain proceeds in a cascade fashion, providing bridged indolines in excellent enantioselectivity.

Highly efficient synthesis and stereoselective migration reactions of chiral five-membered aza-spiroindolenines: scope and mechanistic understanding

An Ir-catalyzed asymmetric synthesis of five-membered aza-spiroindolenines is achieved. Based on the detailed investigation of the reaction patterns of the aryl iminium migration, a one-pot asymmetric allylic dearomatization/migration sequence from racemic indole derivatives is realized, affording enantioenriched Pictet-Spengler-type products bearing an additional allylic stereogenic center adjacent to the C3 position of the indole core.

Dearomatization of Indoles via Palladium-Catalyzed Allylic C-H Activation

The first Pd-catalyzed allylic dearomatization of substituted indoles triggered by C-H bond activation is reported. The presence of a catalytic amount of 2,5-DMBQ is proven to be a key factor for the high yield. This one-pot tandem allylic C-H activation/dearomatization sequence provides a straightforward access to 3,3-disubstituted indolines.

Iridium-Catalyzed Intramolecular Asymmetric Allylic Dearomatization Reaction of Pyridines, Pyrazines, Quinolines, and Isoquinolines

The first Ir-catalyzed intramolecular asymmetric allylic dearomatization reaction of pyridines, pyrazines, quinolines, and isoquinolines has been developed. Enabled by in situ formed chiral Ir-catalyst, the dearomatized products were isolated in high levels of yield (up to 99% yield) and enantioselectivity (up to 99% ee). It is worth noting that the Me-THQphos ligand is much more efficient than other tested ligands for the dearomatization of pyrazines and certain quinolines. Mechanistic studies of the dearomatization reaction were carried out, and the results suggest the feasibility of an alternative process which features the formation of a quinolinium as the key intermediate. The mechanistic findings render this reaction a yet unknown type in the chemistry of Reissert-type reactions. In addition, the utility of this method was showcased by a large-scale reaction and formal synthesis of (+)-gephyrotoxin.

Synthesis of pyrroloindolines and furoindolines via cascade dearomatization of indole derivatives with carbenium ion

Intermolecular cascade dearomatization of substituted indoles with benzodithiolylium tetrafluoroborate has been developed, providing C3 methyl-substituted pyrroloindolines and furoindolines.

Ir-catalyzed reverse prenylation of 3-substituted indoles: total synthesis of (+)-aszonalenin and (-)-brevicompanine B

The selective reverse prenylation of 3-substituted-1H-indoles at C3 is described. The iridium-catalyzed reaction proceeds with high branched to linear selectivity (>20:1) for a variety of indoles. In addition, a diastereoselective reverse prenylation of tryptophan methyl ester is disclosed, and its synthetic utility is demonstrated in the synthesis of (+)-aszonalenin and (-)-brevicompanine B.

Mechanistic insights into the Pd-catalyzed intermolecular asymmetric allylic dearomatization of multisubstituted pyrroles: understanding the remarkable regio- and enantioselectivity

In this article we report a comprehensive density functional theory study on the Pd-catalyzed intermolecular asymmetric allylic dearomatization reactions of multisubstituted pyrroles. The calculated results are in line with the previous experimental observations (J. Am. Chem. Soc. 2014, 136, 6590), and the remarkable regio- and enantioselectivity are well explained. Of all the potential nucleophilic sites around the multisubstituted pyrrole ring, the reaction always occurs at the position where the HOMO of the molecule distributes most significantly. In contrast to the common view on the enantioselectivity of the Pd-catalyzed asymmetric allylic substitution reactions, we find that the steric interaction between the nucleophile and the chiral ligand does not have the dominating effect on the enantioselectivity of the reaction. Instead, the interaction between the allyl moiety and the incoming nucleophile plays an important role in the enantioselectivity-determining process.

Transition-metal-catalyzed asymmetric allylic dearomatization reactions

Dearomatization reactions serve as powerful methods for the synthesis of highly functionalized, three-dimensional structures starting with simple planar aromatic compounds. Among processes of this type, catalytic asymmetric dearomatization (CADA) reactions are attractive owing to the large number of aromatic compounds that are readily available and the fact that they enable direct access to enantiopure polycycles and spirocycles, which frequently are key structural motifs in biologically active natural products and pharmaceuticals. However, as a consequence of their high stabilities, arenes only difficultly participate in dearomatization reactions that take place with high levels of enantioselectivity. Transition-metal-catalyzed asymmetric allylic substitution reactions have been demonstrated to be powerful methods for enantioselective formation of C-C and C-X (X = O, N, S, etc.) bonds. However, the scope of these processes has been explored mainly using soft carbon nucleophiles, some hard carbon nucleophiles such as enolates and preformed organometallic reagents, and heteroatom nucleophiles. Readily accessible aromatic compounds have been only rarely used directly as nucleophiles in these reactions. In this Account, we present the results of studies we have conducted aimed at the development of transition-metal-catalyzed asymmetric allylic dearomatization reactions. By utilizing this general process, we have devised methods for direct dearomatization of indoles, pyrroles, phenols, naphthols, pyridines, and pyrazines, which produce various highly functionalized structural motifs bearing all-carbon quaternary stereogenic centers in a straightforward manner. In mechanistic investigations of the dearomatization process, we found that the five-membered spiroindolenines serve as intermediates, which readily undergo stereospecific allylic migration to form corresponding tetrahydro-1H-carbazoles upon treatment with a catalytic amount of TsOH. It is worth noting that no notable loss of the enantiomeric excess of the spiroindolenine derivatives takes place during the rearrangement process as a consequence of the intervention of a "three-center-two-electron"-type transition state, a proposal that has gained support from the results of DFT calculations. Equally intriguing, upon tuning of the electronic nature of the tethers, pyrroles or indoles undergo unprecedented Ir or Ru catalyzed intramolecular allylic alkylation promoted dearomatization/migration reactions. The operation of this novel reaction pathway provides additional information leading to a greater mechanistic understanding of the transition-metal-catalyzed enantioselective intramolecular functionalizations of pyrroles and indoles. The combined results of this effort provide not only methods for the efficient synthesis of highly enantioenriched fused and spiro polycycles but also novel strategies in the field of asymmetric catalysis.