Ni(II)-catalyzed asymmetric alkenylations of ketimines

Organocatalytic Enantioselective Synthesis of [5.7]-Fused ε-Sultam N,O-Heterocycles via (3 + 2)-Annulation of Seven-Membered Cyclic N-Sulfonylimines with γ-Hydroxy-α,β-Unsaturated Ketones

A highly stereoselective protocol for the (3 + 2)-annulation of biphenyl-bridged seven-membered cyclic N-sulfonylimines with γ-hydroxy-α,β-unsaturated ketones was developed. The reactions afforded a wide range of chiral [5.7]-fused ε-sultams bearing N-adjacent 1,3-stereocenters in excellent yields (93-98% yields) and high enantio/diastereoselectivities (up to >99% ee, >20:1 d.r.) and two other examples with alkoxyl groups were obtained in 52-61% yields, 95% ee, and >20:1 d.r. by utilizing organocatalysis with quinine-derived squaramides.

Chiral Binaphthol-Catalyzed Enantioselective Conjugate Addition of Alkenyl and Arylboronic Acids to α,β-Unsaturated Cyclic N-Sulfonyl Ketimines

The chiral binaphthol-catalyzed enantioselective conjugate addition of alkenylboronic acids and heteroarylboronic acids to cyclic N-sulfonyl ketimines is reported, providing the 1,4-addition products in high yields and moderate to excellent enantioselectivities (up to >99% ee). This mild, scalable catalytic system exhibits high efficiency and broad substrate scopes. Additionally, arylboronic acids were viable nucleophiles under more forcing conditions.

Synergistically activating nucleophile strategy enabled organocatalytic asymmetric P-addition of cyclic imines

Herein, we present an attractive organocatalytic asymmetric addition of P-nucleophiles to five-membered cyclic N-sulfonyl imines facilitated by phosphonium salt catalysis, enabling the highly enantioselective synthesis of tri- and tetra-substituted cyclic phosphorus-containing benzosultams. With this protocol, various cyclic α-aminophosphonates were efficiently synthesized with high yields and exceptional enantioselectivities (up to >99% ee) under mild reaction conditions. The utility and practicality of this method were demonstrated through gram-scale reactions and straightforward elaborations. Notably, the success of this approach relies on the deliberate selection of a synergistic organocatalytic system, which helps circumvent foreseeable side effects while handling secondary phosphine oxides (SPOs). Systematic mechanistic studies, incorporating experiments and DFT calculations, have revealed the critical importance of judiciously selecting bifunctional phosphonium salt catalysts for effectively activating P-nucleophiles while stereoselectively controlling the P-attack process.

Base mediated aza-[2 + 1] annulation and regioselective aziridine ring-opening cascade: mild synthesis of functionalized β-amino ketones from cyclic N-sulfonyl aldimines and α-carbonyl sulfonium salts

Cyclic N-sulfonyl aldimines are well-known aza-[2C]-synthons for various [2 + n] annulation reactions. Herein we describe a novel base mediated [2 + 1] annulation and a regioselective aziridine ring-opening reaction cascade, which provides an efficient and distinct synthetic strategy from readily available cyclic N-sulfonyl aldimines and α-carbonyl sulfonium salts leading to β-amino ketone derivatives through the corresponding fused tri-substituted aziridines. This one-pot, two-step process involves formation of C-C and C-N bonds and subsequent cleavage of a C-N bond. The features of the developed reaction include the use of mild reaction conditions, broad substrate scope, and excellent yields. The synthetic utility of this approach was demonstrated by gram-scale operation and further product derivatizations.

Enantioselective synthesis of α-aryl α-hydrazino phosphonates

Catalysts generated in situ by the combination of pyridine-hydrazone N,N-ligands and Pd(TFA)2 have been applied to the addition of arylboronic acids to formylphosphonate-derived hydrazones, yielding α-aryl α-hydrazino phosphonates in excellent enantioselectivities (96 → 99% ee). Subsequent removal of the benzyloxycarbonyl (Cbz) N-protecting group afforded key building blocks en route to appealing artificial peptides, herbicides and antitumoral derivatives. Experimental and computational data support a stereochemical model based on aryl-palladium intermediates in which the phosphono hydrazone coordinates in its Z-configuration, maximizing the interactions between the substrate and the pyridine-hydrazone ligand.

Cobalt-Catalyzed Asymmetric Aza-Nozaki-Hiyama-Kishi (NHK) Reaction of α-Imino Esters with Alkenyl Halides

Chromium-catalyzed enantioselective Nozaki-Hiyama-Kishi (NHK) reaction represents one of the most powerful approaches for the formation of chiral carbon-heteroatom bond. However, the construction of sterically encumbered tetrasubstituted stereocenter through NHK reaction still posts a significant challenge. Herein, we disclose a cobalt-catalyzed aza-NHK reaction of ketimine with alkenyl halide to provide a convenient synthetic approach for the manufacture of enantioenriched tetrasubstituted α-vinylic amino acid. This protocol exhibits excellent functional group tolerance with excellent 99 % ee in most cases. Additionally, this asymmetric reductive method is also applicable to the aldimine to access the trisubstituted stereogenic centers.

DFT Mechanistic Insights into the Ni(II)-Catalyzed Enantioselective Arylative Cyclization of Tethered Allene-Ketones

Density functional theory (DFT) has provided a detailed mechanistic picture for the redox neutral nickel(II)-catalyzed arylative cyclization reactions of a tethered allene-ketone with arylboronic acids. A mechanistic rationale for the high diastereo- and enantioselectivity achieved experimentally at high reaction temperature was uncovered through modeling the reaction with a chiral ligand and the predicted stereochemical outcome corroborates with experimental results. An unprecedented mechanism for the base-free organoboron transmetalation was revealed and the regioselectivity of migratory insertion of tethered allene-ketones as well as the stability of the possible allylnickel isomers (σ-allyl vs π-allyl) were clarified. The multifaceted nature of the reaction is revealed with certain elementary steps preferring cationic compared to the neutral state.

Reductive Alkenylation of Ketimines via Hydride Transfer from Aldehydes

The first direct N-heterocyclic carbene (NHC)-catalyzed preparation of allylic amines bearing a quaternary center (α-tertiary amine) from isatin-derived ketimines in the presence of vinyl selenones and aldehydes is reported. This multicomponent reaction is expected to proceed via unprecedented in situ reduction of imines through a hydride transfer from the aldehydes.

The Origin of Stereoselectivity in the Hydrogenation of Oximes Catalyzed by Iridium Complexes: A DFT Mechanistic Study

Herein the reaction mechanism and the origin of stereoselectivity of asymmetric hydrogenation of oximes to hydroxylamines catalyzed by the cyclometalated iridium (III) complexes with chiral substituted single cyclopentadienyl ligands (Ir catalysts A1 and B1) under acidic condition were unveiled using DFT calculations. The catalytic cycle for this reaction consists of the dihydrogen activation step and the hydride transfer step. The calculated results indicate that the hydride transfer step is the chirality-determining step and the involvement of methanesulfonate anion (MsO^-) in this reaction is of importance in the asymmetric hydrogenation of oximes catalyzed by A1 and B1. The calculated energy barriers for the hydride transfer steps without an MsO^- anion are higher than those with an MsO^- anion. The differences in Gibbs free energies between TSA5-1fR/TSA5-1fS and TSB5-1fR/TSB5-1fS are 13.8/13.2 (ΔΔG^‡ = 0.6 kcal/mol) and 7.5/5.6 (ΔΔG^‡ = 1.9 kcal/mol) kcal/mol for the hydride transfer step of substrate protonated oximes with E configuration (E-2a-H⁺) with MsO^- anion to chiral hydroxylamines product R-3a/S-3a catalyzed by A1 and B1, respectively. According to the Curtin-Hammet principle, the major products are hydroxylamines S-3a for the reaction catalyzed by A1 and B1, which agrees well with the experimental results. This is due to the non-covalent interactions among the protonated substrate, MsO^- anion and catalytic species. The hydrogen bond could not only stabilize the catalytic species, but also change the preference of stereoselectivity of this reaction.

Water enables diastereodivergency in bispidine-based chiral amine-catalyzed asymmetric Mannich reaction of cyclic N-sulfonyl ketimines with ketones

Tuning diastereoselectivity is a great challenge in asymmetric catalysis for the inherent stereochemical bias of the substrates. Here, we report a diastereodivergent asymmetric Mannich reaction of cyclic N-sulfonyl ketimines with ketones catalyzed by a bispidine-based chiral amine catalyst, in which additional water switches the diastereoselectivity efficiently. Both chiral anti- and syn-benzosultams with potential anti-HIV-1 activity are obtained in excellent yields and good to excellent ee values. Control experiments and density functional theory (DFT) calculations were applied to study the diastereodivergent mechanism, which reveal that the diastereodivergent catalysis should be state-determined, and the water reverses the energies of states to realize the diastereodivergency. The findings are quite new and might inspire more diastereodivergent asymmetric synthesis.

A diastereodivergent asymmetric Mannich reaction of cyclic N-sulfonyl ketimines with ketones is realized by employing bispidine-based chiral amine as catalyst and additional water switching the diastereoselectivity.

Palladium-Catalyzed Asymmetric [3 + 2] Annulation of Vinylethylene Carbonates with Alkenes Installed on Cyclic N-Sulfonyl Imines: Highly Enantio- and Diastereoselective Construction of Chiral Tetrahydrofuran Scaffolds Bearing Three Vicinal and Quaternary Stereocenters

A multisubstituted tetrahydrofuran building block bearing three vicinal chiral carbon centers widely exists in a broad spectrum of bioactive natural products, and the development of efficient and convenient methods to establish this skeleton remains a challenging task. Herein, we have developed an efficient method for the construction of significant tetrahydrofuran scaffolds bearing three vicinal and α-quaternary chiral carbon stereocenters through Pd-catalyzed asymmetric [3 + 2] annulation of vinylethylene carbonates with alkenes installed on cyclic N-sulfonyl imines. A series of multisubstituted tetrahydrofuran derivatives are obtained in high efficiencies with excellent enantioselectivities and diastereoselectivities. Density functional theory (DFT) studies are accomplished to rationalize the stereocontrol of the annulation process and disclose that methanol could be applied to stabilize the reactive zwitterionic π-allylpalladium via the H-bond interaction.

Cu-catalyzed asymmetric addition of alcohols to β,γ-alkynyl-α-imino esters for the construction of linear chiral N,O-ketals

N,O-acetals are part of many synthetic intermediates and important skeletons of numerous natural products and pharmaceutical drugs. The most straightforward method of the synthesis of N,O-acetals is the enantioselective addition of O-nucleophiles to imines. However, using this method for the synthesis of linear chiral N,O-ketals still remains challenging due to the instability of raw materials under acidic or basic conditions. Herein, we developed a Cu-catalyzed asymmetric addition of alcohols to β,γ-alkynyl-α-imino esters under mild conditions, providing the corresponding linear chiral N,O-ketals with up to 96% ee. The method tolerates some variation in the β,γ-alkynyl-α-imino ester and alcohol scope, including some glucose and natural amino acid derivatives. Computational results indicate that the Boc group of the substrates assist in the extraction of hydrogen atoms from the alcohols to promote the addition reactions. These products could be synthesized on a gram-scale and can be used in several transformations. This asymmetric addition system provides an efficient, mild, gram-scale, and transition-metal-catalyzed synthesis of linear chiral N,O-ketals.

N,O-acetals are part of many synthetic intermediates and important skeletons of numerous natural products and pharmaceutical drugs. Here the authors show a Cu-catalyzed asymmetric addition of alcohols to β,γ-alkynyl-α-imino esters, providing the corresponding linear chiral N,O-ketals with up to 96% ee.

Pd(II)-Catalyzed Enantioselective Ring-Contraction for the Construction of 1,4-Benzoxazines

Enantioselective ring-contraction reactions have not been widely reported. We have developed an enantioselective ring contraction of 5,6-dihydro-2H-benzo[b][1,4]oxazocines, affording enantiomerically enriched 3,4-dihydro-2H-1,4-benzoxazine derivatives as single regioisomers. An acidic additive is necessary in order to obtain the products with good yields and enantiomeric ratios (up to 93% yield, 98:2 er). The reaction was successfully performed on a gram scale, and the products can be derivatized easily.

Enantio- and Diastereodivergent Synthesis of Spirocycles through Dual-Metal-Catalyzed [3+2] Annulation of 2-Vinyloxiranes with Nucleophilic Dipoles

The development of efficient and straightforward methods for obtaining all optically active isomers of structurally rigid spirocycles from readily available starting materials is of great value in drug discovery and chiral ligand development. However, the stereodivergent synthesis of spirocycles bearing multiple stereocenters remains an unsolved challenge owing to steric hindrance and ring strain. Herein, we report an enantio- and diastereodivergent synthesis of rigid spirocycles through dual-metal-catalyzed [3+2] annulation of oxy π-allyl metallic dipoles with less commonly employed nucleophilic dipoles (imino esters). A series of spiro compounds bearing a pyrroline and an olefin were easily synthesized in an enantio- and diastereodivergent manner (up to 19:1 dr, >99 % ee), which showed great promise as a new type of N-olefin ligand. Preliminary mechanistic studies were also carried out to understand the process of this bimetallic catalysis.

Nickel-Catalyzed Intramolecular Hydroalkenylation of Imines

A ligand-enabled nickel-catalyzed intramolecular hydroalkenylation of imines with unactivated alkenes has been developed. A variety of five- and six-membered cyclic allylic amines were synthesized in high yields. The use of both wide-bite-angle diphosphine ligand and Brønsted acid is crucial for realizing the reaction. Preliminary investigation of the asymmetric intramolecular hydroalkenylation of imines shows promising potential for the application of the method in the synthesis of enantio-enriched cyclic allylic amines.

Unmasking the reverse reactivity of cyclic N-sulfonyl ketimines: multifaceted applications in organic synthesis

The chemistry related to the exploration of cyclic N-sulfonyl ketimines and their derivatives has attracted significant attention in the last few decades because of their intriguing structures and properties. They serve broadly as reactive synthons in various reactions to create a diverse set of synthetically and biologically attractive molecules. Furthermore, these moieties, which possess multiple heteroatoms (N, O and S), display or can enhance many biological activities. In the case of synthetic reactions, chemists mainly focus on the chemical manipulation of the highly reactive prochiral CN bond of N-sulfonyl ketimines. Besides their traditional role as electrophiles, N-sulfonyl ketimines possess α-Csp³-H protons, and thus behave as potential carbonucleophiles, where they can undergo several C-X (X = C, N and O) bond-forming reactions with different types of electrophiles under various conditions to form a wide range of fascinating asymmetric and non-asymmetric versions of fused heterocycles, carbocycles, spiro-fused skeletons, pyridines, pyrroles, etc. Herein, we highlight the recent examples from our research work and others covering the scope of cyclic N-sulfonyl ketimines as useful carbonucleophiles. In addition, the detailed mechanistic studies of the above-mentioned reactions are also presented.

Highly Enantioselective Synthesis of Phosphorus-Containing ϵ-Benzosultams by Bifunctional Phosphonium Salt-Promoted Hydrophosphonylation

ϵ-Benzosultam derivatives are potential drug candidates with diverse biological activities. A series of chiral ϵ-benzosultams bearing phosphorus functionalities was synthesized by catalytic asymmetric hydrophosphonylation in the presence of a bifunctional phosphonium salt catalyst. The desired hydrophosphonylation products were obtained in good yields with high enantioselectivities, and scale-up reactions and further derivations were successfully accomplished. Some control experiments were also conducted to elucidate the plausible reaction mechanism of this chemical transformation.

Further Developments and Applications of Oxazoline-Containing Ligands in Asymmetric Catalysis

The chiral oxazoline motif is present in many ligands that have been extensively applied in a series of important metal-catalyzed enantioselective reactions. This Review aims to provide a comprehensive overview of the most significant applications of oxazoline-containing ligands reported in the literature starting from 2009 until the end of 2018. The ligands are classified not by the reaction to which their metal complexes have been applied but by the nature of the denticity, chirality, and donor atoms involved. As a result, the continued development of ligand architectural design from mono(oxazolines), to bis(oxazolines), to tris(oxazolines) and tetra(oxazolines) and variations thereof can be more easily monitored by the reader. In addition, the key transition states of selected asymmetric transformations will be given to illustrate the features that give rise to high levels of asymmetric induction. As a further aid to the reader, we summarize the majority of schemes with representative examples that highlight the variation in % yields and % ees for carefully selected substrates. This Review should be of particular interest to the experts in the field but also serve as a useful starting point to new researchers in this area. It is hoped that this Review will stimulate both the development/design of new ligands and their applications in novel metal-catalyzed asymmetric transformations.

Palladium/N,N'-Disulfonyl Bisimidazoline-Catalyzed Enantioselective Addition of Arylboronic Acids to Cyclic N-Sulfonyl Ketimines

The combination of Pd(TFA)₂ and an N,N'-disulfonyl bisimidazoline ligand shows high catalytic activity and excellent asymmetric induction in the addition of arylboronic acids to cyclic N-sulfonyl ketimines including benzo[d]isothiazole-1,1-dioxides, benzo[e][1,2,3]oxathiazine-2,2-dioxides, and 1,2,5-thiadiazole-1,1-dioxides, by which three types of chiral quaternary carbon-containing sultams with substantial substitution diversity were synthesized with high yields and excellent enantioselectivities (up to >99% ee). The current catalysis demonstrated a remarkable tolerance to oxygen and thus provided an operationally simple approach for constructing enantioenriched cyclic quaternary stereocenters.

Accelerated Reactivity Mechanism and Interpretable Machine Learning Model of N-Sulfonylimines toward Fast Multicomponent Reactions

We introduce chemical reactivity flowcharts to help chemists interpret reaction outcomes using statistically robust machine learning models trained on a small number of reactions. We developed fast N-sulfonylimine multicomponent reactions for understanding reactivity and to generate training data. Accelerated reactivity mechanisms were investigated using density functional theory. Intuitive chemical features learned by the model accurately predicted heterogeneous reactivity of N-sulfonylimine with different carboxylic acids. Validation of the predictions shows that reaction outcome interpretation is useful for human chemists.

Synthesis of chiral α-substituted α-amino acid and amine derivatives through Ni-catalyzed asymmetric hydrogenation

Highly efficient Ni-catalyzed asymmetric hydrogenation of cyclic N-sulfonyl ketimino esters was, for the first time, successfully developed, providing various chiral α-monosubstituted α-amino acid derivatives with excellent results (97-99% yields, 90 to >99% ee). Cyclic N-sulfonyl ketimines were also hydrogenated well to afford chiral amine derivatives with 98-99% yields and 97 to >99% ee. The gram-scale asymmetric hydrogenation was performed well with 85% yield and 99% ee using only 0.2 mol% catalyst.

Copper-Catalyzed Enantioselective Synthesis of Oxazolines from Aminotriols via Asymmetric Desymmetrization

A copper-catalyzed enantioselective transformation of tris(hydroxymethyl)aminomethane-derived aminotriols was developed to provide multisubstituted oxazolines with a tetrasubstituted carbon center. The present transformation consisted of sequential reactions involving mono-sulfonylation of aminotriols, subsequent intramolecular cyclization to afford prochiral oxazoline diols, and sulfonylative asymmetric desymmetrization of resultant oxazoline diols. In addition, the kinetic resolution process would be involved in the sulfonylative asymmetric desymmetrization step, which would amplify the enantiopurities of the desired products. Various aminotriols were tolerated in the present reaction, affording the desired oxazolines in good to high yields with excellent enantioselectivities. The synthetic utility of the present reaction was demonstrated by the transformation of the optically active oxazoline into a chiral α-tertiary amine motif.

The enantioselective construction of trifluoromethylated quaternary stereocenters via the Rh-catalyzed asymmetric dehydrated arylation of unprotected hemiaminals

An effective Rh/chiral diene catalyst system for the highly enantioselective synthesis of chiral benzosultams bearing trifluoromethylated quaternary carbon stereocenters has been developed.

A Co(ii)-catalyzed asymmetric ring opening reaction of spiro-epoxyoxindoles with allylboron

A Co(ii)/BOX-catalyzed asymmetric allylation of spiroepoxyoxindoles was developed, which is the first stereoconvergent allylation of epoxides, yielding chiral oxindoles bearing quaternary stereocenters.

A Scaffold-Diversity Synthesis of Biologically Intriguing Cyclic Sulfonamides

A "branching-folding" synthetic strategy that affords a range of diverse cyclic benzo-sulfonamide scaffolds is presented. Whereas different annulation reactions on common ketimine substrates build the branching phase of the scaffold synthesis, a common hydrogenative ring-expansion method, facilitated by an increase of the ring-strain during the branching phase, led to sulfonamides bearing medium-sized rings in a folding pathway. Cell painting assay was successfully employed to identify tubulin targeting sulfonamides as novel mitotic inhibitors.

Palladium-Catalyzed Enantioselective Alkenylation of Enelactams Using a Relay Heck Strategy

In this report, a palladium-catalyzed redox-relay Heck process to access optically active alkenylated α,β-unsaturated lactams is described. Under mild reaction conditions, electron-deficient alkenyl triflates and electron-rich alkenyl iodonium salts undergo enantioselective and site-selective coupling with enelactams to deliver the products in high yields and excellent enantioselectivities. Furthermore, the products allow facile access to natural products such as (+)-calvine and (+)-2-epicalvine in addition to the bioactive molecule aza-goniothalamin.

Facile synthesis of chiral ε-sultams via an organocatalytic aza-Friedel-Crafts reaction

Chiral ε-sultams, with their unique strain cyclic structure, are a type of molecule with important biological activities. A facile enantioselective aza-Friedel-Crafts reaction of seven-membered cyclic N-sulfonylimines with naphthols was developed with a cinchona alkaloid-based bifunctional organocatalyst, giving chiral ε-sultams with an enantiomeric excess of up to 92%.