Enantio- and Diastereoselective, Lewis Base Catalyzed, Cascade Sulfenoacetalization of Alkenyl Aldehydes

Asymmetric Synthesis with Organoselenium Compounds - The Past Twelve Years

The discovery and synthetic applications of novel organoselenium compounds and their reactions proceeded rapidly during the past fifty years and such processes are now carried out routinely in many laboratories. At the same time, the growing demand for new enantioselective processes provided new challenges. The convergence of selenium chemistry and asymmetric synthesis led to key developments in the 1970s, although the majority of early work was based on stoichiometric processes. More recently, greater emphasis has been placed on greener catalytic variations, along with the discovery of novel reactions and a deeper understanding of their mechanisms. The present review covers the literature in this field from 2010 to early 2023 and encompasses asymmetric reactions mediated by chiral selenium-based reagents, auxiliaries, and especially, catalysts. Protocols based on achiral selenium compounds in conjunction with other species of chiral catalysts, as well as reactions that are controlled by chiral substrates, are also included.

Chalcogen bonding catalysis

Catalysts play a major role in chemical synthesis, and catalysis is considered to be a green and economic process. Catalysis is dominated by covalent interactions between the catalyst and substrate. The design of non-covalent catalysts came into limelight only recently. Hydrogen bonding (HB) catalysts are well established among non-covalent catalysts, including asymmetric HB catalysts. Though halogen bonding (XB) catalysis and its asymmetric version are gaining admiration, non-covalent chalcogen bonding catalysis (ChB) is in the budding stage. This tutorial review will focus on the recently evolved chalcogen bonding catalysis and emphasis will be given to the chalcogen bonding of chiral molecules. Since successful enantioselective chalcogen bonding catalysis is yet to be reported, this review will focus on the basics of non-covalent bonding catalysis, chalcogen bonding catalysis, chiral chalcogenide synthesis, rigidification of transition states by ChB, stabilization of cations by chiral chalcogens, details of unsuccessful asymmetric chalcogen bonding catalysis, enantioseparation of racemic molecules using ChB, and the existence of ChB in chiral biomolecules.

Aldehydes as O-Nucleophiles in Cobalt Hydride Hydrogen Atom Transfer Catalysis: Overriding the Innate Somophilicity

In metal hydride-catalyzed alkene hydrofunctionalization reactions via hydrogen atom transfer, simple carbonyl groups have been well-recognized as good somophiles at the carbon for C-C bond formation. Here we report an alternative pathway exploring the carbonyl as an O-nucleophile to make new C-O bonds during the CoH-catalyzed oxidative cyclization of alkenyl aldehydes. This reaction provides a rapid, mild, modular, and stereoselective (up to >20:1) entry to saturated O-heterocycles via nucleophilic trapping of an in situ-formed oxocarbenium intermediate. The key to overriding the carbonyl's innate somophilicity was found to be promoting the formation of organocobalt species and suppressing the radical exchange.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Stereocontrolled Synthesis of Fluorinated Isochromans via Iodine(I)/Iodine(III) Catalysis

The success of saturated, fluorinated heterocycles in contemporary drug discovery provides a stimulus for creative endeavor in main group catalysis. Motivated by the ubiquity of isochromans across the bioactive small molecule spectrum, the prominence of the anomeric effect in regulating conformation, and the metabolic lability of the benzylic position, iodine(I)/iodine(III) catalysis has been leveraged for the stereocontrolled generation of selectively fluorinated analogs. To augment the current arsenal of fluorocyclization reactions involving carboxylic acid derivatives, the reaction of readily accessible 2-vinyl benzaldehydes is disclosed (up to >95 : 05 d.r. and 97 : 03 e.r.). Key stereoelectronic interactions manifest themselves in the X-ray crystal structures of the products, thereby validating the [CH2 -CHF] fragment as a stereoelectronic mimic of the [O-CH(OR)] acetal motif.

Copper-Catalyzed Enantioselective Oxysulfenylation of Alkenols: Synthesis of Arylthiomethyl-Substituted Cyclic Ethers

Saturated heterocycles containing oxygen and sulfur are found in biologically significant molecules. The enantioselective oxysulfenylation of alkenols provides a straightforward synthesis route. To date, organocatalytic methods have dominated this approach. Herein, a complementary approach via copper catalysis is presented. This exoselective method provides enantioenriched arylthiomethyl-substituted tetrahydrofurans, phthalans, isochromans, and morpholines from acyclic alkenols. This method provides the largest scope to date for the exocyclization mode, and with generally high enantioselectivity. The enantioselectivity of this copper-catalyzed oxysulfenylation is rationalized by a proposed mechanism involving alkene oxycupration followed by C─S bond formation via radical-mediated atom transfer.

Enantioselective Inter- and Intramolecular Sulfenofunctionalization of Unactivated Cyclic and (Z)-Alkenes

A method for the enantioselective, Lewis base-catalyzed sulfenofunctionalization of cyclic and (Z)-alkenes is reported. The intermediate thiiranium ion generated in the presence of a selenophosphoramide catalyst is intercepted by a variety of nucleophiles. A diverse array of inter- and intramolecular functionalizations proceed in high yield and good to high enantioselectivity (86:14-98:2 er). Prior experimental and computational studies indicated such enantiotopic face discrimination to be poor; however, the results disclosed herein remediate the previous findings. Control experiments were performed to investigate the different behavior of (Z)-alkenes and their more established (E)-counterparts.

Directed, nickel-catalyzed 1,2-alkylsulfenylation of alkenyl carbonyl compounds

We report a regioselective, nickel-catalyzed syn-1,2-carbosulfenylation of non-conjugated alkenyl carbonyl compounds with alkyl/arylzinc nucleophiles and tailored N-S electrophiles. This method allows the simultaneous installation of a variety of C(sp³) and S(Ar) (or Se(Ar)) groups onto unactivated alkenes, which complements previously developed 1,2-carbosulfenylation methodology in which only C(sp²) nucleophiles are compatible. A bidentate directing auxiliary controls regioselectivity, promotes high syn-stereoselectivity with a variety of E- and Z-internal alkenes, and enables the use of an array of electrophilic sulfenyl (and seleno) electrophiles. Among compatible electrophiles, those with N-alkyl-benzamide leaving groups were found to be especially effective, as determined through comprehensive structure-reactivity mapping.

Electrophilic Sulfur Reagent Design Enables Directed syn-Carbosulfenylation of Unactivated Alkenes

A multi-component approach to structurally complex organosulfur products is described via the nickel-catalyzed 1,2-carbosulfenylation of unactivated alkenes with organoboron nucleophiles and tailored organosulfur electrophiles. The key to the development of this transformation is the identification of a modular N-alkyl-N-(arylsulfenyl)arenesulfonamide family of sulfur electrophiles. Tuning the electronic and steric properties of the leaving group in these reagents controls pathway selectivity, favoring three-component coupling and suppressing side reactions, as examined via computational studies. The unique syn-stereoselectivity differs from traditional electrophilic sulfenyl transfer processes involving a thiiranium ion intermediate and arises from the directed arylnickel(I) migratory insertion mechanism, as elucidated through reaction kinetics and control experiments. Reactivity and regioselectivity are facilitated by a collection of monodentate, weakly coordinating native directing groups, including sulfonamides, alcohols, amines, amides, and azaheterocycles.

Chiral Selenide/Achiral Sulfonic Acid Cocatalyzed Atroposelective Sulfenylation of Biaryl Phenols via a Desymmetrization/Kinetic Resolution Sequence

Enantioselective synthesis of axially chiral sulfur-containing biaryl derivatives through the electrophilic sulfenylation of biaryl phenols has been achieved for the first time. This catalytic asymmetric system, which involves sequential desymmetrization and kinetic resolution, is enabled by a combination of a novel 3,3'-disubstituted BINOL-derived selenide catalyst and an achiral sulfonic acid. Control experiments and computational studies suggest that multiple noncovalent interactions between the cocatalysts and substrate, especially a network of hydrogen bond interactions, play a crucial role in determining the enantioselectivity and reactivity.

Catalytic enantioselective oxysulfenylation of o-vinylanilides

Tf₂NH-assisted BINAM-derived thiophosphoramide catalysis has been accomplished for the enantioselective oxysulfenylation of o-vinylanilides with N-(aryl/alkylthio)imides. The developed reaction offers access to diverse substituted aryl/alkylthio tethered 3,1-benzoxazines in excellent yields and enantiomeric ratios. Furthermore, synthetic applications of benzoxazines and aryl/alkylthio moieties and a transition state model for the observed enantioselectivity are also discussed.

Lewis Base Catalyzed, Sulfenium Ion Initiated Enantioselective, Spiroketalization Cascade

A Lewis base catalyzed, enantioselective sulfenocyclization of alkenes to afford [6,6]spiroketals has been developed. The method uses a chiral Lewis base catalyst with an electrophilic sulfur source to generate enantioenriched thiiranium ion with alkenes. Upon formation, the thiiranium ion is subsequently captured in a cascade-type reaction, wherein a ketone oxygen serves as the nucleophile to open the thiiranium ion and an alcohol provides the secondary cyclization to form biorelevant spiroketals. A variety of electron-rich and electron-neutral E-substituted styrenes form the desired spiroketals in good yields with excellent enantio- and diastereoselectivities. Alkyl-substituted and terminal alkenes participate in the cascade reaction, but with a limited scope compared to the styrenyl substrates. This method allows for rapid formation of highly substituted spiroketals in good yield and excellent enantioselectivity.

Catalytic base-controlled regiodivergent heteronucleophilic hydrofunctionalization of β,γ-unsaturated amides

A general catalytic base-controlled regiodivergent nucleophilic hydrofunctionalization of both terminal and internal β,γ-unsaturated amides has been reported. The atom-economical addition of various S/P-based nucleophiles was also exclusively chemoselective. More than 60 branched or linear hetero-substituted aliphatic amides were synthesized from common starting materials under transition-metal-free conditions. Preliminary mechanistic studies are consistent with our proposed divergent catalytic cycles.

Brønsted acid-catalyzed dynamic kinetic resolution of in situ formed acyclic N,O-hemiaminals: cascade synthesis of chiral cyclic N,O-aminals

A H2O controlled dynamic kinetic resolution was involved in a Brønsted acid-catalyzed acyclic N,O-hemiaminal formation/oxa-Michael reaction cascade, leading to highly enantioenriched cis-2,6-disubstituted tetrahydropyrans bearing an exo amide group.

Catalytic, Enantioselective Sulfenofunctionalization of Alkenes: Development and Recent Advances

The last decade has witnessed a burgeoning of new methods for the enantioselective vicinal difunctionalization of alkenes initiated by electrophilic sulfenyl group transfer. The addition of sulfenium ions to alkenes results in the generation of chiral, non-racemic thiiranium ions. These highly reactive intermediates are susceptible to attack by a myriad of nucleophiles in a stereospecific ring-opening event to afford anti 1,2-sulfenofunctionalized products. The practical application of sulfenium ion transfer has been enabled by advances in the field of Lewis base catalysis. This Review will chronicle the initial discovery and characterization of thiiranium ion intermediates followed by the determination of their configurational stability and the challenges of developing enantioselective variants. Once the framework for the reactivity and stability of thiiranium ions has been established, a critical analysis of pioneering studies will be presented. Finally, a comprehensive discussion of modern synthetic applications will be categorized around the type of nucleophile employed for sulfenofunctionalization.

Chalcogenide-Catalyzed Intermolecular Electrophilic Thio- and Halofunctionalization of gem-Difluoroalkenes: Construction of Diverse Difluoroalkyl Sulfides and Halides

Thio- and halodifluoromethylated compounds are an important class of compounds in medicinal chemistry and organic synthesis. Herein, we report a facile method for the construction of these compounds via chalcogenide-catalyzed intermolecular electrophilic thio- and halofunctionalization of gem-difluoroalkenes. Simple treatment of gem-difluoroalkenes with electrophilic sulfur/halogen reagents and various O- or N-nucleophiles affords diverse multifunctionalized thio- and halodifluoromethylated compounds. This reaction features a relatively broad substrate scope, good functional group tolerance, and mild reaction conditions.

Organocatalytic Asymmetric Tandem Cyclization/Michael Addition via Oxazol-5(2H)-One Formation: Access to Perfluoroalkyl-Containing N,O-Acetal Derivatives

Herein, we report a convenient organocatalytic asymmetric tandem cyclization/Michael addition protocol for the synthesis of diastereomerically pure and highly enantioenriched perfluoroalkyl-containing N,O-acetal derivatives starting from racemic N-perfluoroacyl amino acids under mild conditions. This efficient atom economic reaction leads to highly enantioselective and diastereoselective construction of N,O-acetal derivatives containing oxazolone and perfluoroalkyl moieties containing vicinal quaternary and tertiary stereocenters (up to 97% yield, up to 96% ee, and up to >20:1 dr).

Enantioselective Synthesis of γ-Lactams by Lewis Base Catalyzed Sulfenoamidation of Alkenes

A method for the catalytic, enantioselective, intramolecular 1,2-sulfenoamidation of alkenes is described. Lewis base activation of a suitable sulfur electrophile generates an enantioenriched, thiiranium ion intermediate from a β,γ-unsaturated sulfonyl carboxamide. This intermediate is subsequently intercepted by the sulfonamide nitrogen resulting in cyclization to form γ-lactams. Electron-poor alkenes required the use of a new selenophosphoramidate Lewis base catalyst. Subsequent manipulations of the products harness the latent reactivity of both the amide and thioether functionality.

Enantioselective Construction of Axially Chiral Amino Sulfide Vinyl Arenes by Chiral Sulfide-Catalyzed Electrophilic Carbothiolation of Alkynes

The enantioselective construction of axially chiral compounds by electrophilic carbothiolation of alkynes is disclosed for the first time. This enantioselective transformation is enabled by the use of a Ts-protected bifunctional sulfide catalyst and Ms-protected ortho-alkynylaryl amines (Ts=tosyl; Ms=mesyl). Both electrophilic arylthiolating and electrophilic trifluoromethylthiolating reagents are suitable for this reaction. The obtained products of axially chiral vinyl-aryl amino sulfides can be easily converted into biaryl amino sulfides, biaryl amino sulfoxides, biaryl amines, vinyl-aryl amines, and other valuable difunctionalized compounds.

Lewis Base/Brønsted Acid Co-Catalyzed Asymmetric Thiolation of Alkenes with Acid-Controlled Divergent Regioselectivity

A divergent strategy for the facile preparation of various enantioenriched phenylthio-substituted lactones was developed based on Lewis base/Brønsted acid co-catalyzed thiolation of homoallylic acids. The acid-controlled regiodivergent cyclization (6-endo vs. 5-exo) and acid-mediated stereoselective rearrangement of phenylthio-substituted lactones were explored. Experimental and computational studies were performed to clarify the origins of the regioselectivity and enantioselectivity. The calculation results suggest that C-O and C-S bond formation might occur simultaneously, without formation of a commonly supposed catalyst-coordinated thiiranium ion intermediate and the potential π-π stacking between substrate and SPh as an important factor in the enantio-determining step. Finally, this methodology was applied in the rapid syntheses of the bioactive natural products (+)-ricciocarpin A and (R)-dodecan-4-olide.