Enantioselective anti-Dihalogenation of Electron-Deficient Olefin: A Triplet Halo-Radical Pylon Intermediate

Asymmetric Synthesis of Optically Active Pyrazolidines or Pyrazoline Derivatives via Ni(II)-Bipyridine-N,N'-dioxide Complexes

Easily obtainable and efficient chiral C2-symmetric bipyridine-N,N'-dioxide ligands with Ni(OTf)2 were developed for application in catalyzing [3 + 2] cycloaddition reactions to synthesize optically active fused pyrazolidines or pyrazoline derivatives featuring three contiguous stereogenic centers by employing azomethine imines and α,β-unsaturated 2-acyl imidazoles, affording the corresponding adducts with the opposite configuration compared to previous synthetic products in 80-98% yields with 28-99% ee and >20:1 dr. In addition, subsequent amplification experiments and derivative transformations of the product further demonstrated the efficient catalytic performance of the catalyst Ni(II)-bipyridine-N,N'-dioxide complexes and the practicality of this synthesis methodology.

Stereospecific syn-dichlorination of allylic amines enabled by identification of a superior stereo-directing group

Alteration of a well-established reaction mechanism for access to different molecular structures is an inherently intriguing research subject. In that context, syn-stereospecific alkene dihalogenation draws attention as a long-standing problem in synthetic organic chemistry. The simplest approach would be the incorporation of an additional stereo-inverting step within the traditional anti-dihalogenation process. Surprisingly, this seemingly trivial idea turned out challenging, and no suitable stereo-directing group was known before our work. Herein, we describe a highly efficient syn-dichlorination of N-protected allylic amines through the anchimeric assistant phenomenon that has been inapplicable to alkene dihalogenation. Upon rational identification of a superior stereo-director, 1,8-naphthalimide, our practical reaction conditions with mild and convenient dichlorinating reagents can accommodate the formerly unemployable aryl alkenes in excellent yields (>95%) and stereospecificity (>50:1). DFT calculation suggests a concerted internal trapping mechanism without a discrete carbocationic species, which accounts for the conservation of the stereochemical integrity.

Late-Stage Halogenation of Complex Substrates with Readily Available Halogenating Reagents

ConspectusLate-stage halogenation, targeting specific positions in complex substrates, has gained significant attention due to its potential for diversifying and functionalizing complex molecules such as natural products and pharmaceutical intermediates. Utilizing readily available halogenating reagents, such as hydrogen halides (HX), N-halosuccinimides (NXS), and dichloroethane (DCE) reagents for late-stage halogenation shows great promise for expanding the toolbox of synthetic chemists. However, the reactivity of haleniums (X+, X = Cl, Br, I) can be significantly hindered by the presence of various functional groups such as hydroxyl, amine, amide, or carboxylic acid groups. The developed methods of late-stage halogenation often rely on specialized activating reagents and conditions. Recently, our group (among others) has put great efforts into addressing these challenges and unlocking the potential of these readily available HX, NXS, and DCE reagents in complex molecule halogenation. Developing new methodologies, catalyst systems, and reaction conditions further enhanced their utility, enabling the efficient and selective halogenation of intricate substrates.With the long-term goal of achieving selective halogenation of complex molecules, we summarize herein three complementary research topics in our group: (1) Efficient oxidative halogenations: Taking inspiration from naturally occurring enzyme-catalyzed oxidative halogenation reactions, we focused on developing cost-effective oxidative halogenation reactions. We found the combination of dimethyl sulfoxide (DMSO) and HX (X = Cl, Br, I) efficient for the oxidative halogenation of aromatic compounds and alkenes. Additionally, we developed electrochemical oxidative halogenation using DCE as a practical chlorinating reagent for chlorination of (hetero)arenes. (2) Halenium reagent activation: Direct electrophilic halogenation using halenium reagents is a reliable method for obtaining organohalides. However, compared to highly reactive reagents, the common and readily available NXS and dihalodimethylhydantoin (DXDMH) demonstrate relatively lower reactivity. Therefore, we focused on developing oxygen-centered Lewis base catalysts such as DMSO, 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) and nitromethane to activate NXS or DXDMH, enabling selective halogenation of bioactive substrates. (3) Halogenation of inert substrates: Some substrates, such as electron-poor arenes and pyridines, are inert toward electrophilic functionalization reactions. We devised several strategies to enhance the reactivity of these molecules. These strategies, characterized by mild reaction conditions, the ready availability and stability of catalysts and reagents, and excellent tolerance for various functional groups, have emerged as versatile protocols for the late-stage aromatic halogenation of drugs, natural products, and peptides. By harnessing the versatility and selectivity of these catalysts and methodologies, synthetic chemists can unlock new possibilities in the synthesis of halogenated compounds, paving the way for the development of novel functional materials and biologically active molecules.

Small-Molecule-Based Strategy for Mitigating Deactivation of Chiral Lewis Acid Catalysis

Chiral Lewis acid catalysts are widely used in organic synthesis due to their diverse applications. However, their high Lewis acidity makes them susceptible to deactivation by basic Lewis reagents and water. Here, we present a novel strategy for mitigating this deactivation using small molecules. By incorporating weakly coordinating anions into the secondary coordination sphere of the metal center, we designed a highly reusable chiral Lewis acid complex. This complex exhibits excellent thermal stability and allows for the use of electron-poor nucleophiles in the reactions. Spectroscopic and titration studies confirmed the robustness of the optimized complex. This work provides valuable insights for overcoming the limitations of chiral Lewis acids in Lewis basic environments, expanding their potential for chemical synthesis.

Chemoselective Synthesis of 3-Bromomethyloxindoles via Visible-Light-Induced Radical Cascade Bromocyclization of Alkenes

A novel visible-light-induced radical cascade bromocyclization of N-arylacrylamides has been accomplished. This reaction overcomes the overbromination at the benzene rings suffered in traditional electrophilic reactions, thus enabling the first highly chemoselective synthesis of valuable 3-bromomethyloxindoles. The combination of pyridine and anhydrous medium is identified as the key factor for the high chemoselectivity in the current photoreaction system, which might work by suppressing the in situ generation of low-concentration Br2 from N-bromosuccinimide. Moreover, the mild reaction conditions ensure the generation of a wide range of the new desired products with excellent functional group tolerance.

Bipyridine-N,N'-dioxides Catalysts: Design, Synthesis, and Application in Asymmetric Synthesis of 1H-Pyrazolo[3,4-b]pyridine Analogues

A novel type of highly efficient chiral C2-symmetric bipyridine-N,N'-dioxides ligand application in catalyzing Michael addition/Cyclization of 5-aminopyrazoles with α,β-unsaturated 2-acyl imidazoles has been developed, affording the corresponding adducts in 85-97% yield with up to 99% enantioselectivity under mild conditions with a lower catalyst loading and broad scope. Remarkably, this protocol exhibits advantages in terms of reactivity and enantioselectivity, giving the fact that as low as 2.2 mol % of L1 and 2.0 mol % of Ni(OTf)2 can promote the title reaction on gram scale to afford the desired product with excellent enantioselectivity.

Stereospecific syn-dihalogenations and regiodivergent syn-interhalogenation of alkenes via vicinal double electrophilic activation strategy

Whereas the conventional anti-dihalogenation of alkenes is a valuable synthetic tool with highly predictable stereospecificity, the restricted reaction mechanism makes it challenging to alter the diastereochemical course into the complementary syn-dihalogenation process. Only a few notable achievements were made recently by inverting one of the stereocenters after anti-addition using a carefully designed reagent system. Here, we report a conceptually distinctive strategy for the simultaneous double electrophilic activation of the two alkene carbons from the same side. Then, the resulting vicinal leaving groups can be displaced iteratively by nucleophilic halides to complete the syn-dihalogenation. For this purpose, thianthrenium dication is employed, and all possible combinations of chlorine and bromine are added onto internal alkenes successfully, particularly resulting in the syn-dibromination and the regiodivergent syn-bromochlorination.

Bimetallic tandem catalysis-enabled enantioselective cycloisomerization/carbonyl-ene reaction for construction of 5-oxazoylmethyl α-silyl alcohol

A bimetallic tandem catalysis-enabled enantioselective cycloisomerization/carbonyl-ene reaction was developed. The reaction proceeded well with a broad range of N-propargylamides and acylsilanes, affording the target chiral 5-oxazoylmethyl α-silyl alcohols in up to 95% yield and 99% ee under mild conditions. Importantly, this facile protocol was available for the late-stage modification of several bioactive molecules. Based on the mechanistic study and control experiments, a possible catalytic cycle and transition state are proposed to elucidate the reaction process and enantioinduction.

Asymmetric Synthesis of α-Methylene-γ-Butyrolactones via Tandem Allylboration/Lactonization: a Kinetic Resolution Process

The α-methylene-γ-butyrolactone motif is a widely encountered unit in many natural products and pharmaceutical compounds. Herein, a practical and efficient synthesis of α-methylene-γ-butyrolactones from readily available allylic boronates and benzaldehyde derivatives was developed with chiral N,N'-dioxide/AlIII complex as the catalyst. The key success of this transformation was the kinetic resolution of allylboration intermediate via asymmetric lactonization. This protocol enabled to assemble all of four stereoisomers from the same set of starting materials upon variable lactonization. Taking advantage of the current method as the key step, catalytic asymmetric total synthesis of eupomatilones 2, 5, and 6 was accomplished. Control experiments were carried out to probe into the tandem reaction as well as the origin of stereoselectivities.