Hf(IV)-catalyzed enantioselective epoxidation of N-alkenyl sulfonamides and N-tosyl imines

Ring-Expansion Reactions of Epoxy Amides and Enamides: Functionalized Azetidines, Dihydrofurans, Diazocanes, or Dioxa-3-azabicyclonon-4-enes?

Functionalized azetidines, 2,3-dihydrofurans, or the unorthodox dioxa-3-azabicyclonone-4-ene motifs are the products from transition metal-free reaction between N-oxiranylmethyl benzenesulfonamide and β-chloro-cinnamaldehyde, depending on whether one uses either NaI/K₂CO₃ or LiBr/K₂CO₃. These ring expansion reactions involve enamide (X-ray evidence) derived from N-oxiranylmethyl benzenesulfonamide and β-chloro-cinnamaldehyde as an intermediate. The N-oxiranylmethyl benzenesulfonamide itself upon heating gives readily separable and crystalline isomeric diazocanes that can be characterized by X-ray crystallography.

Catalytic Asymmetric 1,3-Dipolar Cycloaddition of α,β-Unsaturated Amide and Azomethine Imine

α,β-Unsaturated amides were incorporated as viable dipolarophiles in a catalytic asymmetric 1,3-dipolar cycloaddition of azomethine imines. The use of a 7-azaindoline auxiliary was essential to acquire sufficient reactivity with excellent diastereoselectivity, likely due to the chelating activation of the amide by the In(III)/bishydroxamic acid complex. Although the enantioselectivity remains unsatisfactory, this work is an important step toward the development of an asymmetric catalysis utilizing stable and low-reactive substrates.

A 5 + 1 Protic Acid Assisted Aza-Pummerer Approach for Synthesis of 4-Chloropiperidines from Homoallylic Amines

We report that HCl·DMPU induces the formation of (thiomethyl)methyl carbenium ion from DMSO under mild conditions. Homoallylic amines react with this electrophile to generate 4-chloropiperidines in good yields. The method applies to both aromatic and aliphatic amines. The use of HCl·DMPU as both non-nucleophilic base and chloride source constitutes an environmentally benign alternative for piperidine formation. The reaction has a broad substrate scope, and the conditions offer good chemical yields with high functional group tolerance and scalability.

Substrate-Directed Catalytic Selective Chemical Reactions

The development of highly efficient reactions at only the desired position is one of the most important subjects in organic chemistry. Most of the reactions in current organic chemistry are reagent- or catalyst-controlled reactions, and the regio- and stereoselectivity of the reactions are determined by the inherent nature of the reagent or catalyst. In sharp contrast, substrate-directed reaction determines the selectivity of the reactions by the functional group on the substrate and can strictly distinguish sterically and electronically similar multiple reaction sites in the substrate. In this Perspective, three topics of substrate-directed reaction are mainly reviewed: (1) directing group-assisted epoxidation of alkenes, (2) ring-opening reactions of epoxides by various nucleophiles, and (3) catalytic peptide synthesis. Our newly developed synthetic methods with new ligands including hydroxamic acid derived ligands realized not only highly efficient reactions but also pinpointed reactions at the expected position, demonstrating the substrate-directed reaction as a powerful method to achieve the desired regio- and stereoselective functionalization of molecules from different viewpoints of reagent- or catalyst-controlled reactions.

Catalyst-controlled site-selective asymmetric epoxidation of nerylamine and geranylamine derivatives

Novel catalysts for site- and enantioselective epoxidation of nerylamine and geranylamine derivatives have been developed. Although mCPBA oxidation took place selectively at the more electron-rich double bond to give the 6,7-epoxides, these catalysts provide the 2,3-epoxides in moderate to high enantioselectivity via the oxidation of the relatively electron-deficient double bond.

Catalytic asymmetric synthesis of CF3-substituted tertiary propargylic alcohols via direct aldol reaction of α-N3 amide

Organofluorine compounds are found in several important classes of chemicals, such as pharmaceuticals, agrochemicals, and functional materials. Chemists have been immensely interested in the development of methodologies for expeditious access to fluorine containing building blocks. In this study, we report a new method for the catalytic asymmetric synthesis of CF₃-substituted tertiary propargylic alcohols with two contiguous stereogenic centers via the direct aldol reaction of an α-N₃ amide to trifluoromethyl ketones. The key to the success of this method is the identification of a catalyst comprising Cu(ii)/chiral hydroxamic acid to promote the desired aldol reaction, constructing a tetrasubstituted carbon in a highly stereoselective fashion. Despite substantial prior advances in asymmetric catalysis, this class of catalysts has not been utilized for the formation of carbon-carbon bond-forming reactions. Our mechanistic study sheds light on the unique profile of this catalytic system, where the Cu(ii) complex plays a bifunctional role of serving as a Lewis acid and a Brønsted base. Furthermore, the densely functionalized aldol adducts undergo chemoselective transformations, affording a series of fluorine containing chiral building blocks with widespread application.

N-Sulfonyl α-imino ester-derived chiral oxaziridines: catalytic asymmetric synthesis and application as a modular chiral organic oxidant

A novel class of chiral N-sulfonyl oxaziridines is catalytically prepared and introduced for use as highly reactive and enantiospecific oxidants.

Theoretical insights into the reaction of Cp*(Cl)Hf(diene) with isonitriles

The reaction of Cp*(Cl)Hf(2,3-dimethylbutadiene) with isonitriles is theoretically investigated, and detailed elementary reactions and the substitution effects are examined.

Metal-Catalyzed Directed Regio- and Enantioselective Ring-Opening of Epoxides

Control of regio- and stereoselectivity of chemical reactions is the central theme in synthetic chemistry. Regioselective and enantiospecific ring opening of readily available enantioenriched epoxides precursors provides a straightforward access to diverse highly functionalized molecules which can serve as chiral building blocks for synthesis of biologically active compounds. However, prior to our research work, the scope of catalytic highly regioselective ring opening of epoxides is limited to structurally or electronically biased epoxides, such as terminal and aromatic epoxides. Regioselective ring-opening of epoxides with substituents on both sides demonstrating similar steric and electronic effects is still a formidable challenge for organic chemists. To address this challenge, our approach is to use the readily available functional moiety incorporated in an epoxide as a directing group to realize the regioselective nucleophilic attack on the oxirane ring. Alternatively, asymmetric ring-opening of epoxides can also provide the ring-opening products in highly enantioenriched form. However, excellent results are usually obtained in the case of the kinetic resolution of terminal epoxides or the desymmetrization of meso-epoxides. In these cases, the issue of regiocontrol of the ring-opening is circumvented or minimized. Based on our successful results of regioselective ring-opening of functionalized epoxides by implementing the directed ring-opening strategy, we also investigated the enantioselective ring-opening of internal epoxides bearing a functional moiety as directing group. This Account summarizes our research on metal-catalyzed directed ring-opening reactions of epoxides, which encompasses the following breakthroughs: (1) highly regioselective ring-opening of various substrates including epoxy allylic alcohols, epoxy homoallylic alcohols and epoxy allylic sulfonamides with a variety of N-, O-, and halide-nucleophiles catalyzed by W-, Mo-, or Ni-salt; (2) first kinetic resolutions of epoxy allylic alcohols, epoxy homoallylic alcohols, and epoxy allylic sulfonamides with various amines as nucleophiles, which were catalyzed by W-bishydroxamic acid (W-BHA), nickel-BINAM, and Gd-N,N'-dioxide catalytic system, respectively; (3) successful implementation of the strategy of combined asymmetric syntheses by the combination of the enantioselective epoxidation and the enantioselective ring-opening of 2,3-epoxy alcohols establishing a new entry to prepare amino alcohols in regio-, diastereo-, and enantiomerically pure form.

Enantioselective organocatalytic oxidation of ketimine

with promoted the enantioselective organocatalytic oxidation of to afford oxaziridines bearing a tetrasubstituted carbon stereogenic center in high yields with up to 87% ee.

Novel chiral Schiff base/Ti(iv) catalysts for the catalytic asymmetric epoxidation of N-alkenyl sulfonamides

A novel highly enantioselective Schiff base/Ti(iv) catalyst for the catalytic asymmetric epoxidation of a series of N-alkenyl sulfonamides under mild conditions was described.

Gadolinium-Catalyzed Regio- and Enantioselective Aminolysis of Aromatic trans-2,3-Epoxy Sulfonamides

The first enantioselective aminolysis of aromatic trans-2,3-epoxy sulfonamides has been accomplished, which was efficiently catalyzed by a Gd-N,N'-dioxide complex. Under the directing effect of the sulfonamide moiety the ring-opening reaction proceeded selectively at the C-3 position in a highly enantioselective manner furnishing various Ts- and SES-protected 3-amino-3-phenylpropan-2-olamines as products.

Asymmetric epoxidation of allylic alcohols catalyzed by vanadium-binaphthylbishydroxamic Acid complex

A vanadium-binaphthylbishydroxamic acid (BBHA) complex-catalyzed asymmetric epoxidation of allylic alcohols is described. The optically active binaphthyl-based ligands BBHA 2a and 2b were synthesized from (S)-1,1'-binaphthyl-2,2'-dicarboxylic acid and N-substituted-O-trimethylsilyl (TMS)-protected hydroxylamines via a one-pot, three-step procedure. The epoxidations of 2,3,3-trisubstituted allylic alcohols using the vanadium complex of 2a were easily performed in toluene with a TBHP water solution to afford (2R)-epoxy alcohols in good to excellent enantioselectivities.

Brønsted acid catalyzed phosphoramidic acid additions to alkenes: diastereo- and enantioselective halogenative cyclizations for the synthesis of C- and P-chiral phosphoramidates

The first highly diastereo- and enantioselective additions of a halogen and phosphoramidic acid to unactivated alkenes have been developed, catalyzed by a chiral Brønsted acid. A unique feature of these additions is the opportunity for stereocontrol at two noncontiguous chiral centers, carbon and phosphorus, leading to cyclic P-chiral phosphoramidates. In addition to their inherent value, the phosphoramidates are precursors to enantioenriched epoxy allylamines.

Tungsten-catalyzed regioselective and stereospecific ring opening of 2,3-epoxy alcohols and 2,3-epoxy sulfonamides

The first catalytic, highly C3-selective, stereosepecific ring-opening reaction of 2,3-epoxy alcohols and 2,3-epoxy sulfonamides has been accomplished. This process was efficiently promoted by W-salts, and the developed method was applicable to various epoxides with diverse N- and O-nucleophiles affording the products in good to excellent yields (up to 95%) and generally with high regioselectivities (C3:C2 up to >99:1).

Tungsten-catalyzed asymmetric epoxidation of allylic and homoallylic alcohols with hydrogen peroxide

A simple, efficient, and environmentally friendly asymmetric epoxidation of primary, secondary, tertiary allylic, and homoallylic alcohols has been accomplished. This process was promoted by a tungsten-bishydroxamic acid complex at room temperature with the use of aqueous 30% H2O2 as oxidant, yielding the products in 84-98% ee.

Synthesis of single-enantiomer bioactive molecules: a brief overview

Chiral-center enantiomers have been shown to differ significantly in biological activity, pharmacodynamics, pharmacokinetics and toxicity. New developments in the stereoselective organic synthesis have enriched the vast literature of synthetic methodologies applicable to access natural products as well as bioactive molecules. These compounds also include new drugs, drug candidates and reagents used to explore biological processes. The article reviews the synthesis of optically pure drugs, biologically active intermediates and amino alcohols by using different methods.

Catalytic asymmetric oxidation of N-sulfonyl imines with hydrogen peroxide-trichloroacetonitrile system

An efficient and highly enantioselective Payne-type oxidation of N-sulfonyl imines is developed. The reaction exhibits broad substrate generality and unique chemoselectivity based on the combined use of hydrogen peroxide and trichloroacetonitrile under the catalysis of P-spiro chiral triaminoiminophosphorane.