Nickel-catalyzed enantioselective vinylation of aryl 2-azaallyl anions

Integrating Catalysis and Analysis: Accelerated Asymmetric Reaction Development With a Multitasking Reactivity Umpolung/Chiroptical Reporter Auxiliary

High-throughput asymmetric reaction development of an organocatalytic Michael addition is achieved by using a readily available isatin molecule as an amine protecting group, reagent for reactivity Umpolung, and as a chiroptical yield and enantioselectivity reporter. The use of UV and circular dichroism inductions allows on-the-fly, chromatography-free parallel screening of a hundred reaction conditions to identify a catalytic procedure that converts a primary amine to either enantiomer of an a-branched chiral product in excellent yields and ee's. The isatin is recyclable and can be quantitatively recovered if desired. This case study shows how chiroptical high-throughput screening with a multitasking reagent can increase method development speed and reduce cost and waste compared to traditional reaction optimization workflows by streamlining the analysis of small-scale reaction setups and eliminating time-consuming work-up protocols.

Nickel-Catalyzed Umpolung Difluoroalkylation of Imines Enables General Access to β-Difluoroalkylated Amines

Fluoroalkylated amines play a pivotal role in medicinal chemistry, yet the general and efficient synthesis of β-difluoroalkylated amines remains elusive. Here, we developed a nickel-catalyzed umpolung strategy that enables the difluoroalkylation of 2-azaallyl anions generated from aliphatic and aromatic imines, effectively overcoming the previous limitations. By inverting the polarity of imines, this strategy allows for the coupling of a variety of readily accessible difluoroalkyl bromides and iodides. This approach is characterized by its high efficiency, broad substrate scope, high functional group tolerance, and ease of synthesis. The rapid modification of bioactive molecules by the efficient synthesis of difluorinated analogs of key amine moieties present in bioactive molecules, including amphetamine, using the current approach shows the promising potential of this protocol in advancing drug discovery and development.

Visible-light-driven net-1,2-hydrogen atom transfer of amidyl radicals to access β-amido ketone derivatives

Hydrogen atom transfer (HAT) processes provide an important strategy for selective C-H functionalization. Compared with the popularity of 1,5-HAT processes, however, net-1,2-HAT reactions have been reported less frequently. Herein, we report a unique visible-light-mediated net-1,2-HAT of amidyl radicals for the synthesis of β-amido ketone derivatives. Single-electron transfer (SET) to N-aryloxy amides generates nitrogen-centered radicals (N˙), which undergo a rare net-1,2-HAT to form carbon-centered radicals (C˙). The C-centered radicals are then captured by silyl enol ethers on the way to β-amido ketones. A series of β-amido ketone derivatives (33 examples, up to 97% yield) were prepared with good functional group tolerance demonstrating the synthetic utility of this method. Mechanistic studies, including EPR, radical trapping experiments, deuterium labeling and KIE measurements, suggest an intramolecular radical net-1,2-HAT pathway.

Enantioselective Synthesis of Aminals Via Nickel-Catalyzed Hydroamination of 2-Azadienes with Indoles and N-Heterocycles

New methods for the enantioselective synthesis of N-alkylated indoles and their derivatives are of great interest because indoles are pivotal structural elements in biologically active molecules and natural products. They are also versatile intermediates in organic synthesis. Among well-established asymmetric hydroamination methods, the asymmetric hydroamination with indole-based substrates is a formidable challenge. This observation is likely due to the reduced nucleophilicity of the indole nitrogen. Herein, a unique nickel-catalyzed enantio- and branched-selective hydroamination of 2-azadienes with indoles and structurally related N-heterocycles is reported for the generation of enantioenriched N,N-aminals. Salient features of this reaction include good yields, mild reaction conditions, high enantioselectivities, and broad substrate scope (60 examples, up to 96% yield and 99% ee). The significance of this approach with indoles and other N-heterocycles is demonstrated through structural modification of natural products and drug molecules and the preparation of enantioenriched N-alkylated indole core structures. Mechanistic studies reveal that olefin insertion into a Ni-H bond in the hydroamination is the enantio-determining step and oxidative addition of the N-H bond may be the turnover-limiting step.

Efficient construction of functionalized pyrroloindolines through cascade radical cyclization/intermolecular coupling

Pyrroloindolines are important structural units in nature and the pharmaceutical industry, however, most approaches to such structures involve transition-metal or photoredox catalysts. Herein, we describe the first tandem SET/radical cyclization/intermolecular coupling between 2-azaallyl anions and indole acetamides. This method enables the transition-metal-free synthesis of C3a-substituted pyrroloindolines under mild and convenient conditions. The synthetic utility of this transformation is demonstrated by the construction of an array of C3a-methylamine pyrroloindolines with good functional group tolerance and yields. Gram-scale sequential one-pot synthesis and hydrolysis reactions demonstrate the potential synthetic utility and scalability of this approach.

Radical C(sp3)-S Coupling for the Synthesis of α-Amino Sulfides

A unique transition-metal-free radical thiolation of 2-azaallyl anions has been developed. Easily accessible thiosulfonates and 2-azaallyls undergo the tandem process of single-electron transfer and radical-radical coupling to construct C(sp³)-S bonds. This robust protocol enables a mild and chemoselective coupling between 2-azaallyl anions and thiosulfonates to access α-amino sulfides in 50-92% yields (25 examples). The scalability of this protocol was demonstrated by telescopic gram-scale experiments. Mechanistic studies provide significant evidence for this radical thiolation reaction.

Synthesis of 1-pyrroline derivatives via cyclization of terminal alkynes with 2-azaallyls

An efficient transition-metal-free cyclization reaction to prepare 1-pyrroline derivatives bearing various functional groups is described. In this method, a simple combination of a base and a solvent allows the cyclization reaction of terminal alkynes and 2-azaallyls to be carried out efficiently under mild and metal-free conditions. This cyclization reaction will provide an efficient method for the synthesis of medicinally relevant polysubstituted and multifunctionalized pyrrolines.

α-Branched amines through radical coupling with 2-azaallyl anions, redox active esters and alkenes

α-Branched amines are fundamental building blocks in a variety of natural products and pharmaceuticals. Herein is reported a unique cascade reaction that enables the preparation of α-branched amines bearing aryl or alkyl groups at the β- or γ-positions. The cascade is initiated by reduction of redox active esters to alkyl radicals. The resulting alkyl radicals are trapped by styrene derivatives, leading to benzylic radicals. The persistent 2-azaallyl radicals and benzylic radicals are proposed to undergo a radical-radical coupling leading to functionalized amine products. Evidence is provided that the role of the nickel catalyst is to promote formation of the alkyl radical from the redox active ester and not promote the C-C bond formation. The synthetic method introduced herein tolerates a variety of imines and redox active esters, allowing for efficient construction of amine building blocks.

Super-Electron-Donor 2-Azaallyl Anions Enable Construction of Isoquinolines

Herein is introduced the application of "super-electron-donor"(SED) 2-azaallyl anions in a tandem reduction/radical cyclization/radical coupling/aromatization protocol that enables the rapid construction of isoquinolines. The value of this transition-metal-free method is highlighted by the wide range of isoquinoline ethyl amines prepared with good functional group tolerance and yields. An operationally simple gram scale synthesis is also conducted, confirming the scalability.