Simple N-Sulfinyl-Based Chiral Sulfur–Olefin Ligands for Rhodium-Catalyzed Asymmetric 1,4-Additions

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.

Synthesis of Isothiazoles through N-Propargylsulfinylamide: TFA-Promoted Sulfinyl Group-Involved Intramolecular Cyclization

A new reactivity mode of tert-butanesulfinamide has been developed, which proceeds through C-S and O-S bond cleavage of N-propargyl tert-butanesulfinylamide allowing rapid assembly of poly functionalized isothiazoles. This intramolecular cyclization reaction could be conducted under mild and convenient conditions and tolerates several fluoroalkyl and substituted phenyl groups with good chemical yields. This reaction not only represents a new reactivity of tert-butanesulfinamide but also provides an easy strategy for the synthesis of isothiazoles.

Increasing the Functional Group Diversity in Helical β-Peptoids: Achievement of Solvent- and pH-Dependent Folding

We report the synthesis of a series of bis-functionalized β-peptoid oligomers of the hexamer length. This was achieved by synthesizing and incorporating protected amino- or azido-functionalized chiral building blocks into precursor oligomers by a trimer segment coupling strategy. The resulting hexamers were readily elaborated to provide target compounds displaying amino groups, carboxy groups, hydroxy groups, or triazolo-pyridines, which should enable metal ion binding. Analysis of the novel hexamers by circular dichroism (CD) spectroscopy and ¹H-¹³C heteronuclear single quantum coherence nuclear magnetic resonance (HSQC NMR) spectroscopy revealed robust helical folding propensity in acetonitrile. CD analysis showed a solvent-dependent degree of helical content in the structural ensembles when adding different ratios of protic solvents including an aqueous buffer. These studies were enabled by a substantial increase in solubility compared to previously analyzed β-peptoid oligomers. This also allowed for the investigation of the effect of pH on the folding propensity of the amino- and carboxy-functionalized oligomers, respectively. Interestingly, we could show a reversible effect of sequentially adding acid and base, resulting in a switching between compositions of folded ensembles with varying helical content. We envision that the present discoveries can form the basis for the development of functional peptidomimetic materials responsive to external stimuli.

Enantioselective C,P-Palladacycle-Catalyzed Arylation of Imines

Chiral diarylmethylamines are of great interest because of their prevalence in biological and pharmaceutical sciences. Herein, we report a C,P-palladacycle-catalyzed enantioselective synthesis of chiral diarylmethylamines via asymmetric arylation of N-protected imines with arylboronic acids. The C,P-palladacycle showed high reactivity (up to 99% yield) and enantioselectivity (up to 99% ee) toward this arylation, enabling the tolerance of a wide range of functionalities, providing a convenient and efficient access to enantiomerically enriched diarylmethylamines. The absolute configuration of the product was well rationalized by the proposed stereochemical pathway and the catalytical cycle.

Steric Tuning of Sulfinamide/Sulfoxides as Chiral Ligands with C1, Pseudo-meso, and Pseudo-C2 Symmetries: Application in Rhodium(I)-Mediated Arylation

A new family of sulfinamide/sulfoxide derivatives was synthesized as chiral bidentate ligands by stereoselective additions of methylsulfinyl carbanions to N-tert-butylsulfinylimines. The new ligands, with C₁, pseudo-meso, and pseudo-C₂ symmetries, were successfully assayed in Rh-catalyzed additions of arylboronic acids to activated ketones. The sterically dissymmetric C₁ ligand (R_S,S_C,R_S)-N-[1-(phenylsulfinyl)-3-methylbut-2-yl] tert-butylsulfinamide turned out to be the optimal one, allowing the 1,4-additions of diverse arylboronic acids, on different α,β-unsaturated cyclic ketones with high chemical yields and enantioselectivities up to >99% ee.

Electronic and Steric Tuning of an Atropisomeric Disulfoxide Ligand Motif and Its Use in the Rh(I)-Catalyzed Addition Reactions of Boronic Acids to a Wide Range of Acceptors

A novel chiral disulfoxide ligand pair bearing fluorine atoms at the 6 and 6' position of its atropisomeric backbone, ( M, S, S)- and ( P, S, S)- p-Tol-6F-BIPHESO, was synthesized. Complexation to a rhodium(I) precursor gave rise to μ-Cl- and μ-OH-bridged rhodium dimer complexes incorporating the new ( M, S, S)- p-Tol-6F-BIPHESO ligand, while its sibling ( P, S, S)- p-Tol-6F-BIPHESO was not complexed efficiently to the rhodium precursor. The performance of this disulfoxide ligand [( M, S, S)- p-Tol-6F-BIPHESO] in catalysis was tested in both 1,4- and 1,2-addition reactions of arylboronic acids. We show that addition to both cyclic and acyclic enones as well as N-tosylarylimines proceeds with high yields and high enantioselectivities to give the corresponding products. The synthesis of enantiomerically pure p-Tol-6F-BIPHESO is straightforward and inexpensive which, together with the high catalytic performance and wide substrate scope for these addition reactions, makes it a very attractive alternative to more classical chiral ligand entities.

Chiral Sulfinyl-Based Olefin Ligands for Rhodium-Catalysed Asymmetric Conjugate Addition of Arylboronic Acids to Cyanoalkenes

A rhodium/sulfinyl-based olefin ligand-catalysed asymmetric conjugate addition of arylboronic acids to cyanoalkenes without activation groups has been developed, where p-tolylsulfinyl-functionalised olefin ligands have been shown to be effective and with moderate enantioselectivities. This is the first example of applying chiral sulfinyl-based olefin ligands in the catalytic asymmetric addition to cyanoalkenes.

Chiral ligands designed in China

Asymmetric catalysis has become an indispensable and productive field within the Chinese organic chemistry society. The design of chiral ligands is one of the most prominent research areas in this field. Since the late 1990s, Chinese organic chemists have developed numerous chiral ligands possessing novel chiral skeletons and design concepts. Some of these ligands have been widely adopted and can be regarded as ‘privileged ligand’, which have shown excellent performance in many asymmetric catalytic reactions. In this review, we provide an overview of the chiral ligands designed by Chinese scientists with the aim of promoting the development of this area in China and with the hope of encouraging more scientists across the world to use these ligands when designing asymmetric reactions.

Arylative Intramolecular Allylation of Ketones with 1,3-Enynes Enabled by Catalytic Alkenyl-to-Allyl 1,4-Rhodium(I) Migration

Alkenyl‐to‐allyl 1,4‐rhodium(I) migration enables the generation of nucleophilic allylrhodium(I) species by remote C−H activation. This new mode of reactivity was employed in the diastereoselective reaction of arylboron reagents with substrates containing a 1,3‐enyne tethered to a ketone, to give products containing three contiguous stereocenters. The products can be obtained in high enantioselectivities using a chiral sulfur‐alkene ligand.

Chiral Organosulfur Ligands/Catalysts with a Stereogenic Sulfur Atom: Applications in Asymmetric Synthesis

Asymmetric synthesis, in which chiral organocatalysts or metal complexes with chiral ligands are used, has become the most valuable methodology for the preparation of enantiomerically pure organic compounds. Among such catalysts/ligands, a growing number constitute various organosulfur compounds. This Review provides comprehensive and critical information on the plethora of sulfur-based chiral ligands and organocatalysts used in asymmetric synthesis, which have been published within the last 15 years. However, it is confined to the presentation of only those chiral catalysts/ligands that possess a stereogenic sulfur atom and includes sulfoxides, sulfinamides, N-sulfinyl ureas, sulfoximines, and some related S-chiral derivatives.

Two phenyls are better than one or three: synthesis and application of terminal olefin-oxazoline (TOlefOx) ligands

A novel terminal olefin-oxazoline ligand was introduced into rhodium-catalyzed asymmetric conjugate addition of arylboronic acids to enones and gave excellent enantioselectivities. The two phenyls proved better than one or three in ligand evaluations.

Pseudo enantiomeric mixed S/P ligands derived from carbohydrates for the 1,4-addition of phenyl boronic acid to cyclohexenone

Mixed S/P ligands 3 and 10, good catalyst precursors for the Rh(i)-catalysed 1,4-addition of phenylboronic acid to cyclohexenone behave as enantiomers even though both derived from d-sugars.

Facile synthesis of enantioenriched phenol-sulfoxides and their aluminum complexes

Chiral phenolic p-tolylsulfoxides, t-butylsulfoxides and well-defined aluminum complexes were prepared by short synthetic routes from readily available starting materials.

The emergence of sulfoxides as efficient ligands in transition metal catalysis

Sulfoxides are capable of forming stable complexes with transition metals and there have been many comprehensive studies into their binding properties. However, the use of sulfoxides, particularly chiral sulfoxides, as ligands in transition metal catalysis is rather less well developed. This review aims to describe these catalytic studies and covers new developments that are showing very promising results and that have led to a renewed interest in this field.

Recent applications of chiral N-tert-butanesulfinyl imines, chiral diene ligands and chiral sulfur–olefin ligands in asymmetric synthesis

The highly efficient asymmetric reactions of chiral sulfinyl auxiliary, diene ligands and sulfur–olefin ligands are presented.

Enantioselective synthesis of monofluorinated allylic compounds: Pd-catalyzed asymmetric allylations of dimethyl 2-fluoromalonate using new N-sulfinyl-based ligands

Using both new chiralS,N-ligand and dimethyl 2-fluoromalonate, Pd-catalyzed asymmetric allylic substitutions of allylic acetates with dimethyl 2-fluoromalonate were accomplished. This method produced monofluorinated allylation products in up to high yield with excellent enantioselectivity.

Simple sulfur–olefins as new promising chiral ligands for asymmetric catalysis

Recent progress on exploring sulfur-containing hybrid olefins as new elegant ligands for enantioselective catalytic processes is surveyed.

Ru-catalyzed 1,4-addition of arylboronic acids to acrylic acid derivatives in the presence of phenols

[Ru(p-cymene)Cl2]2-catalyzed 1,4-addition reactions between arylboronic acids and butyl acrylate and acrylamide in the presence of phenols were investigated, good to excellent yields were obtained. The addition of phenols remarkably promoted the protonolysis and inhibited the β-H elimination of the 1,4-addition intermediates, and also efficiently suppressed the protonolysis of arylboronic acids.