Wen-Bo Liu

Enantioselective Construction of Spiroindolenines by Ir-Catalyzed Allylic Alkylation Reactions

With 2-methyl-1,2,3,4-tetrahydroquinoline-derived phosphoramidite ligand (R,R(a))-L(6), Ir-catalyzed intramolecular C-3 allylic alkylation of indoles has been realized to afford highly enantioenriched spiroindolenine derivatives in 92-98% yields with up to >99/1 dr and 97% ee.

Silylation of C–H bonds in aromatic heterocycles by an Earth-abundant metal catalyst

Heteroaromatic compounds containing carbon–silicon (C–Si) bonds are of great interest in the fields of organic electronics and photonics, drug discovery, nuclear medicine and complex molecule synthesis, because these compounds have very useful physicochemical properties. Many of the methods now used to construct heteroaromatic C–Si bonds involve stoichiometric reactions between heteroaryl organometallic species and silicon electrophiles or direct, transition-metal-catalysed intermolecular carbon–hydrogen (C–H) silylation using rhodium or iridium complexes in the presence of excess hydrogen acceptors. Both approaches are useful, but their limitations include functional group incompatibility, narrow scope of application, high cost and low availability of the catalysts, and unproven scalability. For this reason, a new and general catalytic approach to heteroaromatic C–Si bond construction that avoids such limitations is highly desirable. Here we report an example of cross-dehydrogenative heteroaromatic C–H functionalization catalysed by an Earth-abundant alkali metal species. We found that readily available and inexpensive potassium tert-butoxide catalyses the direct silylation of aromatic heterocycles with hydrosilanes, furnishing heteroarylsilanes in a single step. The silylation proceeds under mild conditions, in the absence of hydrogen acceptors, ligands or additives, and is scalable to greater than 100 grams under optionally solvent-free conditions. Substrate classes that are difficult to activate with precious metal catalysts are silylated in good yield and with excellent regioselectivity. The derived heteroarylsilane products readily engage in versatile transformations enabling new synthetic strategies for heteroaromatic elaboration, and are useful in their own right in pharmaceutical and materials science applications.

Iridium-Catalyzed Allylic Vinylation and Asymmetric Allylic Amination Reactions with o-Aminostyrenes

An Ir-catalyzed allylic vinylation reaction of allyl carbonates with o-aminostyrene derivatives has been realized, providing skipped (Z,E)-diene derivatives. With (E)-but-2-ene-1,4-diyl dimethyl dicarbonate as the substrate, an efficient enantioselective synthesis of 1-benzazepine derivatives via an Ir-catalyzed domino allylic vinylation/intramolecular allylic amination reaction has been developed. Mechanistic studies of the allylic vinylation reaction have been carried out, and the results suggest that the leaving group of the allylic precursor plays a key role in directing the reaction pathway. Screening of various allylic precursors showed that Ir-catalyzed reactions of allyl diethyl phosphates with o-aminostyrene derivatives proceed via an allylic amination pathway. A subsequent ring-closing metathesis (RCM) reaction of the amination products led to a series of enantiomerically enriched 1,2-dihydroquinoline derivatives. Their utility is indicated by an asymmetric total synthesis of (-)-angustureine.

Enantio-, diastereo-, and regioselective iridium-catalyzed asymmetric allylic alkylation of acyclic β-ketoesters.

The first regio-, diastereo-, and enantioselective allylic alkylation of acyclic β-ketoesters to form vicinal tertiary and all-carbon quaternary stereocenters is reported. Critical to the successful development of this method was the employment of iridium catalysis in concert with N-aryl-phosphoramidite ligands. Broad functional group tolerance is observed at the keto-, ester-, and α-positions of the nucleophile. Various transformations demonstrating the utility of this method for rapidly accessing complex enantioenriched compounds are reported.

Construction of Vicinal Tertiary and All-Carbon Quaternary Stereocenters via Ir-Catalyzed Regio-, Diastereo-, and Enantioselective Allylic Alkylation and Applications in Sequential Pd-Catalysis

Highly congested vicinal stereocenters comprised of tertiary and all-carbon quaternary centers were generated via Ir-catalyzed asymmetric allylic alkylation of β-ketoesters. These catalytic reactions proceed in excellent yields with a broad scope on either reaction partner and with outstanding regio-, diastereo-, and enantiocontrol. Implementation of a subsequent Pd-catalyzed alkylation affords dialkylated products with pinpoint stereochemical control of both chiral centers.

Iridium-Catalyzed Allylic Alkylation Reaction with N-Aryl Phosphoramidite Ligands: Scope and Mechanistic Studies

A series of N-aryl phosphoramidite ligands has been synthesized and applied to iridium-catalyzed allylic alkylation reactions, offering high regio- and enantioselectivities for a wide variety of substrates. These ligands feature the synthetic convenience and good tolerance of the ortho-substituted cinnamyl carbonates. Mechanistic studies, including DFT calculations and X-ray crystallographic analyses of the (π-allyl)-Ir complexes, reveal that the active iridacycle is formed via C(sp(2))-H bond activation.

Ir-Catalyzed Regio- and Enantioselective Friedel–Crafts-Type Allylic Alkylation of Indoles

Highly regio- and enantioselective Ir-catalyzed Friedel-Crafts type allylic alkylation of indoles have been realized using [Ir(COD)Cl]2/phosphoramidite ligand 1a, affording the branched products with up to >97/3 branched-linear ratio and 92% ee.

Iridium-Catalyzed Asymmetric Allylic Substitutions

Ir-catalyzed asymmetric allylic substitution reactions have been reviewed. This chapter discusses respectively the mechanistic investigation, reaction scope, and synthetic application of Ir-catalyzed allylic substitution reactions. The reaction scope is classified according to different types of nucleophiles such as C, N, O, and S.

Asymmetric dearomatization of pyrrolesviaIr-catalyzed allylic substitution reaction: enantioselective synthesis of spiro-2H-pyrroles

Asymmetric dearomatization of pyrroles has been accomplished by using Ir-catalyzed intramolecular asymmetric allylic alkylation reactions. Reactions of allylic carbonate tethered pyrroles in the presence of [Ir(cod)Cl]2 and a BINOL-derived phosphoramidite ligand lead to efficient generation of spiro-2H-pyrrole derivatives with up to 96% ee.

Asymmetric N‐Allylation of Indoles Through the Iridium‐Catalyzed Allylic Alkylation/Oxidation of Indolines

The synthesis of enantiopure indole derivatives is of significant importance because optically active indole moieties are a common occurrence in bioactive natural products and pharmaceuticals. To this end, enormous efforts have been devoted to the development of catalytic asymmetric reactions of indoles in the past decade. It has been well documented that the C3-position of an indole is the most reactive one among the three positions (N1, C2, C3) under the conditions of the Friedel–Crafts reaction. Recently, the enantioselective C2 alkylation of indoles was realized through variations of the Pictet–Spengler reaction, alkylation of 2-indolyl trifluoroborate salts, and alkylation/oxidation of 4,7-dihydroindoles. However, the enantioselective N-substitution of indoles has rarely been explored due to the weak acidity of the N H group, despite the fact that the products are privileged structural motifs in natural alkaloids and biologically active compounds (Figure 1).