Thiourea-catalysed ring opening of episulfonium ions with indole derivatives by means of stabilizing non-covalent interactions

Catalysis of an SN2 pathway by geometric preorganization

Bimolecular nucleophilic substitution (SN2) mechanisms occupy a central place in the historical development and teaching of the field of organic chemistry1. Despite the importance of SN2 pathways in synthesis, catalytic control of ionic SN2 pathways is rare and notably uncommon even in biocatalysis2,3, reflecting the fact that any electrostatic interaction between a catalyst and the reacting ion pair necessarily stabilizes its charge and, by extension, reduces polar reactivity. Nucleophilic halogenase enzymes navigate this tradeoff by desolvating and positioning the halide nucleophile precisely on the SN2 trajectory, using geometric preorganization to compensate for the attenuation of nucleophilicity4. Here we show that a small-molecule (646 Da) hydrogen-bond-donor catalyst accelerates the SN2 step of an enantioselective Michaelis-Arbuzov reaction by recapitulating the geometric preorganization principle used by enzymes. Mechanistic and computational investigations show that the hydrogen-bond donor diminishes the reactivity of the chloride nucleophile yet accelerates the rate-determining dealkylation step by reorganizing both the phosphonium cation and the chloride anion into a geometry that is primed to enter the SN2 transition state. This new enantioselective Arbuzov reaction affords highly enantioselective access to an array of H-phosphinates, which are in turn versatile P-stereogenic building blocks amenable to myriad derivatizations. This work constitutes, to our knowledge, the first demonstration of catalytic enantiocontrol of the phosphonium dealkylation step, establishing a new platform for the synthesis of P-stereogenic compounds.

Polarizability matters in enantio-selection

The prevalence of chirality, or, handedness in biological world is a fundamental phenomenon and a characteristic hallmark of life. Thus, understanding the origin of enantio-selection, i.e., the sense and magnitude of asymmetric induction, has been a long-pursued goal in asymmetric catalysis. Herein, we demonstrated a polarizability-derived electronic effect that was shown to be capable of rationalizing a broad range of stereochemical observations made in the field of asymmetric catalysis. This effect provided a consistent enantio-control model for the prediction of major enantiomers formed in a ruthenium-catalyzed asymmetric transfer hydrogenations of ketones. Direct and quantitative linear free energy relationships between substrates' local polarizabilities and observed enantio-selectivity were also revealed in three widely known asymmetric catalytic systems covering both reductions and oxidations. This broadly applicable polarizability-based electronic effect, in conjunction with conventional wisdom mainly leveraging on steric effect considerations, should aid rational design of enantio-selective processes for better production of chiral substances.

Benchtop Nickel Catalysis Invigorated by Electron-Deficient Diene Ligands

ConspectusDue to the rarity of precious metals like palladium, nickel catalysis is becoming an increasingly important player in organic synthesis, especially for the formation of bonds with sp3-hybridized carbon centers. Traditionally, catalytic processes involving active Ni(0) species have relied on Ni(COD)2 or in situ reduction of Ni(II) salts. However, Ni(COD)2 is an air- and temperature-sensitive material that requires use in an inert-atmosphere glovebox, and in situ reduction protocols of Ni(II) salts using metallic or organometallic reductants add additional complications to reaction development.This Account chronicles the development of air-stable Ni(0) precursors as replacements for Ni(COD)2 or in situ reduction. Based on Schrauzer's seminal discovery of Ni(COD)(DQ) as an air-stable zerovalent organonickel complex, our research laboratories at Scripps Research and Bristol Myers Squibb have developed a class of precatalysts based on the Ni(COD)(EDD) (EDD = electron-deficient diene) framework, relying on the steric and electronic properties of the supporting diene to render the metal center stable to air, moisture, and even silica gel but reactive to ligand substitution and redox changes.The stable Ni(0) complexes can be accessed through ligand exchange with Ni(COD)2, through reduction of Ni(acac)2 using DIBAL-H, or electrochemically via cathodic reduction of Ni(acac)2 to Ni(COD)2, followed by addition of an EDD ligand in one pot. As a toolkit, the complexes demonstrate reactivity that is equivalent or enhanced compared to Ni(COD)2, catalyzing C-C and C-N cross-couplings, Miyaura borylations, C-H activations, and other transformations. Since the initial report on Ni(COD)(DQ), its reactivity in C(sp2)-CN activation, metallophotoredox, and electric field-induced cross-coupling have also been demonstrated.By incorporating the precatalyst toolkit into reaction discovery campaigns, our laboratories have been able to perform C(sp3)-S(alkyl) couplings and metallonitrenoid carboamination, both of which represent challenging transformations that were inaccessible with traditional phosphine, nitrogen, or electron-deficient olefin ligands. Computational and experimental studies demonstrate how the quinone ligands are hemilabile, adopting η1(O)-bound geometries to relieve steric strain or stabilize transition states and intermediates; redox-active, able to transiently oxidize the metal center; and electron-withdrawing or -donating, depending on metal oxidation state and coordination geometry. These studies show how the ligands enable key steps in catalysis beyond imparting air-stability.Since our report documenting the catalytic activity of Ni(COD)(DQ), many other laboratories have also observed unique reactivity with this precatalyst. Ni(COD)(DQ) was found to offer superior reactivity to Ni(COD)2 in C-N cross coupling to form N,N-diaryl sulfonamides and in preparation of biaryls from aryl halides and benzene through a Ni-mediated, base-assisted homolytic aromatic substitution.

Role of Noncovalent Interactions in Inducing High Enantioselectivity in an Alcohol Reductive Deoxygenation Reaction Involving a Planar Carbocationic Intermediate

The involvement of planar carbocation intermediates is generally considered undesirable in asymmetric catalysis due to the difficulty in gaining facial control and their intrinsic stability issues. Recently, suitably designed chiral catalyst(s) have enabled a guided approach of nucleophiles to one of the prochiral faces of carbocations affording high enantiocontrol. Herein, we present the vital mechanistic insights from our comprehensive density functional theory (B3LYP-D3) study on a chiral Ir-phosphoramidite-catalyzed asymmetric reductive deoxygenation of racemic tertiary α-substituted allenylic alcohols. The catalytic transformation relies on the synergistic action of a phosphoramidite-modified Ir catalyst and Bi(OTf)₃, first leading to the formation of an Ir-π-allenyl carbocation intermediate through a turn-over-determining S_N1 ionization, followed by a face-selective hydride transfer from a Hantzsch ester analogue to yield an enantioenriched product. Bi(OTf)₃ was found to promote a significant number of ionic interactions as well as noncovalent interactions (NCIs) with the catalyst and the substrates (allenylic alcohol and Hantzsch ester), thus providing access to a lower energy route as compared to the pathways devoid of Bi(OTf)₃. In the nucleophilic addition, the chiral induction was found to depend on the number and efficacy of such key NCIs. The curious case of reversal of enantioselectivity, when the α-substituent of the allenyl alcohol is changed from methyl to cyclopropyl, was identified to originate from a change in mechanism from an enantioconvergent pathway (α-methyl) to a dynamic kinetic asymmetric transformation (α-cyclopropyl). These molecular insights could lead to newer strategies to tame tertiary carbocations in enantioselective reactions using suitable combinations of catalysts and additives.

Exploring Structure-Function Relationships of Aryl Pyrrolidine-Based Hydrogen-Bond Donors in Asymmetric Catalysis Using Data-Driven Techniques

Hydrogen bond-based organocatalysts rely on networks of attractive noncovalent interactions (NCIs) to impart enantioselectivity. As a specific example, aryl pyrrolidine substituted urea, thiourea, and squaramide organocatalysts function cooperatively through hydrogen bonding and difficult-to-predict NCIs as a function of the reaction partners. To uncover the synergistic effect of the structural components of this catalyst class, we applied data science tools to study various model reactions using a derivatized, aryl pyrrolidine-based, hydrogen-bond donor (HBD) catalyst library. Through a combination of experimentally collected data and data mined from previous reports, statistical models were constructed, illuminating the general features necessary for high enantioselectivity. A distinct dependence on the identity of the electrophilic reaction partner and HBD catalyst is observed, suggesting that a general interaction is conserved throughout the reactions analyzed. The resulting models also demonstrate predictive capability by the successful improvement of a previously reported reaction using out-of-sample reaction components. Overall, this study highlights the power of data science in exploring mechanistic hypotheses in asymmetric HBD catalysis and provides a prediction platform applicable in future reaction optimization.

Evidence for Oxonium Ions in Ethereal "Hydrogen Chloride"

Although solutions of hydrogen chloride in ethereal solvents like diethyl ether or dioxane are commonly employed in the laboratory, the solution structure of these reagents has yet to be firmly established. Here, we analyze solutions of ethereal hydrogen chloride or deuterium chloride in toluene, in dichloromethane, or in the absence of a co-solvent by in situ infrared spectroscopy. The resulting spectra are inconsistent with free HCl or often-proposed 1:1 HCl-ether complexes but closely match the predicted spectra of oxonium ions generated via protonation of diethyl ether. Molecular dynamics simulation of the oxonium chloride complexes provides evidence for an outer-sphere contact ion pair. These results suggest new approaches for tuning the acidity of strong Brønsted acids in organic solvents and demonstrate the importance of conducting spectroscopic measurements under reaction-relevant conditions.

Strategies That Utilize Ion Pairing Interactions to Exert Selectivity Control in the Functionalization of C-H Bonds

Electrostatic attraction between two groups of opposite charge, typically known as ion-pairing, offers unique opportunities for the design of systems to enable selectivity control in chemical reactions. Catalysis using noncovalent interactions is an established and vibrant research area, but it is noticeable that hydrogen bonding interactions are still the main interaction of choice in system design. Opposite charges experience the powerful force of Coulombic attraction and have the ability to exert fundamental influence on the outcome of reactions that involve charged reagents, intermediates or catalysts. In this Perspective, we will examine how ion-pairing interactions have been used to control selectivity in C-H bond functionalization processes. This broad class of reactions provides an interesting and thought-provoking lens through which to examine the application of ion-pairing design strategies because it is one that encompasses great mechanistic diversity, poses significant selectivity challenges, and perhaps most importantly is of immense interest to synthetic chemists in both industry and academia. We survey reactions that proceed via radical and ionic mechanisms alongside those that involve transition metal catalysis and will deal with control of site-selectivity and enantioselectivity. We anticipate that as this emerging area develops, it will become an ever-more important design strategy for selectivity control.

Heterocyclization of Bis(2-chloroprop-2-en-1-yl)sulfide in Hydrazine Hydrate–KOH: Synthesis of Thiophene and Pyrrole Derivatives

The article is devoted to heterocyclization of bis(2-chloroprop-2-en-1-yl)sulfide which proceeds in hydrazine hydrate–alkali medium and leads to formation of thiophene and pyrrole derivatives: previously described 4,5,9,10-tetrahydrocycloocta[1,2-c;5,8-c’]dithiophene, as well as unknown hydrazone of 5-methylidene-3-methyldihydrothiophen-2-one and 1-amino-2-(propynylsulfanylpropenylsulfanyl)-3,5-dimethylpyrrole. Tentative mechanisms for the formation of the heterocyclic products are discussed. Obtained hydrazone of 5-methylidene-3-methyldihydrothiophen-2-one was used for the synthesis of a range of azine derivatives and in oxidation process with SeO₂. The found reactions open up expedient approaches to the formation of various hardly accessible thiophene and pyrrole compounds from 2,3-dichloropropene and elemental sulfur as starting reagents.

Chloride-Mediated Alkene Activation Drives Enantioselective Thiourea and Hydrogen Chloride Co-Catalyzed Prins Cyclizations

The mechanism of chiral hydrogen-bond donor (HBD) and hydrogen chloride (HCl) co-catalyzed Prins cyclizations was analyzed through a combination of experimental and computational methods and revealed to involve an unexpected and previously unrecognized mode of alkene activation. Kinetic and spectroscopic studies support the participation of a catalytically active HCl·HBD complex that displays reduced Brønsted acidity relative to HCl alone. Nevertheless, rate acceleration relative to the HCl-catalyzed background reaction as well as high levels of enantioselectivity are achieved. This inverse Brønsted correlation is ascribed to chloride-mediated substrate activation in the rate-limiting and enantiodetermining cyclization transition state. Density functional theory (DFT) calculations, distortion-interaction analysis, and quasiclassical dynamics simulations support a stepwise mechanism in which rate acceleration and enantioselectivity are achieved through the precise positioning of the chloride anion within the active site of the chiral thiourea to enhance the nucleophilicity of the alkene and provide transition-state stabilization through local electric field effects. This mode of selective catalysis through anion positioning likely has general implications for the design of enantioselective Brønsted acid-catalyzed reactions involving π-nucleophiles.

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.

Hydrogen Bonding Phase-Transfer Catalysis with Alkali Metal Fluorides and Beyond

Phase-transfer catalysis (PTC) is one of the most powerful catalytic manifolds for asymmetric synthesis. Chiral cationic or anionic PTC strategies have enabled a variety of transformations, yet studies on the use of insoluble inorganic salts as nucleophiles for the synthesis of enantioenriched molecules have remained elusive. A long-standing challenge is the development of methods for asymmetric carbon-fluorine bond formation from readily available and cost-effective alkali metal fluorides. In this Perspective, we describe how H-bond donors can provide a solution through fluoride binding. We use examples, primarily from our own research, to discuss how hydrogen bonding interactions impact fluoride reactivity and the role of H-bond donors as phase-transfer catalysts to bring solid-phase alkali metal fluorides in solution. These studies led to hydrogen bonding phase-transfer catalysis (HB-PTC), a new concept in PTC, originally crafted for alkali metal fluorides but offering opportunities beyond enantioselective fluorination. Looking ahead, the unlimited options that one can consider to diversify the H-bond donor, the inorganic salt, and the electrophile, herald a new era in phase-transfer catalysis. Whether abundant inorganic salts of lattice energy significantly higher than those studied to date could be considered as nucleophiles, e.g., CaF₂, remains an open question, with solutions that may be found through synergistic PTC catalysis or beyond PTC.

Comparative Analysis of Sulfonium-π, Ammonium-π, and Sulfur-π Interactions and Relevance to SAM-Dependent Methyltransferases

We report the measurement and analysis of sulfonium-π, thioether-π, and ammonium-π interactions in a β-hairpin peptide model system, coupled with computational investigation and PDB analysis. These studies indicated that the sulfonium-π interaction is the strongest and that polarizability contributes to the stronger interaction with sulfonium relative to ammonium. Computational studies demonstrate that differences in solvation of the trimethylsulfonium versus the trimethylammonium group also contribute to the stronger sulfonium-π interaction. In comparing sulfonium-π versus sulfur-π interactions in proteins, analysis of SAM- and SAH-bound enzymes in the PDB suggests that aromatic residues are enriched in close proximity to the sulfur of both SAM and SAH, but the populations of aromatic interactions of the two cofactors are not significantly different, with the exception of the Me-π interactions in SAM, which are the most prevalent interaction in SAM but are not possible for SAH. This suggests that the weaker interaction energies due to loss of the cation-π interaction in going from SAM to SAH may contribute to turnover of the cofactor.

Tunable and Cooperative Catalysis for Enantioselective Pictet-Spengler Reaction with Varied Nitrogen-Containing Heterocyclic Carboxaldehydes

Herein we report an organocatalytic enantioselective functionalization of heterocyclic carboxaldehydes via the Pictet-Spengler reaction. Through careful pairing of novel squaramide and Brønsted acid catalysts, our method tolerates a breadth of heterocycles, enabling preparation of a series of heterocycle conjugated β-(tetrahydro)carbolines in good yield and enantioselectivity. Careful selection of carboxylic acid co-catalyst is essential for toleration of a variety of regioisomeric heterocycles. Utility is demonstrated via the three-step stereoselective preparation of pyridine-containing analogues of potent selective estrogen receptor downregulator and U.S. FDA approved drug Tadalafil.

Organocatalyst-controlled site-selective arene C-H functionalization

Over the past three decades, organocatalysis has emerged as a powerful catalysis platform and has gradually been incorporated into the routine synthetic toolbox to obtain chiral molecules. However, its application in the site- and enantioselective functionalization of inactive aryl C-H bonds remains in its infancy. Here, we present an organocatalyst-controlled para-selective arene C-H functionalization strategy that addresses this issue, which remains an enduring challenge in arene functionalization chemistry. By emulating enzyme catalysis, the chiral phosphoric acid catalyst offers an ideal chiral environment for stereoinduction, and the projecting substituents give control of chemo- and site-selectivity. Various types of nucleophile are compatible with this method, affording more than 100 para-selective adducts with stereodefined carbon centres or axes in viable molecular contexts. This protocol is expected to provide a general strategy for para-selective functionalization of arene C-H bonds in a controlled manner.

Enantioconvergent Substitution Reactions of Racemic Electrophiles by Organocatalysis

Over the past decades, the development of enantioselective catalysis using organocatalysts has evolved into an active research field and a number of enantioselective transformations have been established. However, despite their being a highly desirable process for the synthesis of organic molecules in an enantioenriched form, the enantioconvergent substitution reactions of racemic electrophiles using organocatalysts still present several challenges. Although intrinsic difficulties in the catalytic stereocontrol abound due to the initial chiral information of racemic electrophiles, in recent years, mechanistically diverse enantioconvergent processes have been intensively investigated in organocatalysis. This Minireview focuses on recent achievements in the development of enantioconvergent substitution reactions of racemic electrophiles using organocatalysts. The contents are classified on the basis of the mechanistic types of enantioconvergent processes.

A Case Study in Catalyst Generality: Simultaneous, Highly-Enantioselective Brønsted- and Lewis-Acid Mechanisms in Hydrogen-Bond-Donor Catalyzed Oxetane Openings

Generality in asymmetric catalysis can be manifested in dramatic and valuable ways, such as high enantioselectivity across a wide assortment of substrates in a given reaction (broad substrate scope) or as applicability of a given chiral framework across a variety of mechanistically distinct reactions (privileged catalysts). Reactions and catalysts that display such generality hold special utility, because they can be applied broadly and sometimes even predictably in new applications. Despite the great value of such systems, the factors that underlie generality are not well understood. Here, we report a detailed investigation of an asymmetric hydrogen-bond-donor catalyzed oxetane opening with TMSBr that is shown to possess unexpected mechanistic generality. Careful analysis of the role of adventitious protic impurities revealed the participation of competing pathways involving addition of either TMSBr or HBr in the enantiodetermining, ring-opening event. The optimal catalyst induces high enantioselectivity in both pathways, thereby achieving precise stereocontrol in fundamentally different mechanisms under the same conditions and with the same chiral framework. The basis for that generality is analyzed using a combination of experimental and computational methods, which indicate that proximally localized catalyst components cooperatively stabilize and precisely orient dipolar enantiodetermining transition states in both pathways. Generality across different mechanisms is rarely considered in catalyst discovery efforts, but we suggest that it may play a role in the identification of so-called privileged catalysts.

Enantioselective, Catalytic Multicomponent Synthesis of Homoallylic Amines Enabled by Hydrogen-Bonding and Dispersive Interactions

We report a one-step catalytic, enantioselective method for the preparation of homoallylic N-Boc amines directly from acetals. Reactive iminium ion intermediates are generated in situ through the combination of an acetal, a chiral thiourea catalyst, trialkylsilyl triflate, and N-Boc carbamate and are subsequently trapped by a variety of allylsilane nucleophiles. No homoallylic ether byproducts are detected, consistent with allylation of the iminium intermediate being highly favored over allylation of the intermediate oxocarbenium ion. Acetals derived from aromatic aldehydes possessing a variety of functional groups and substitution patterns yield homoallylic amines with excellent levels of enantiomeric enrichment. Experimental and computational data are consistent with an anchoring hydrogen-bond interaction between the protioiminium ion and the amide of the catalyst in the enantiodetermining transition state, and with stereodifferentiation achieved through specific noncovalent interactions (NCIs) with the catalyst pyrenyl moiety. Evidence is provided that the key NCI in the major pathway is a π-stacking interaction, contrasting with the cation-π interactions invoked in previously studied reactions promoted by the same family of aryl-pyrrolidino-H-bond-donor catalysts.

Enantio- and Diastereoselective Nucleophilic Addition of N-tert-Butylhydrazones to Isoquinolinium Ions through Anion-Binding Catalysis

A highly enantio- and diastereoselective thiourea-catalyzed dearomatization of isoquinolines employing N-tert-butylhydrazones as neutral α-azo carbanions and masked acyl anion equivalents has been developed. Experimental and computational data supports the generation of highly ordered complexes wherein the chloride behaves as a template for the catalyst, the hydrazone reagent, and the isoquinolinium cation, providing excellent stereocontrol in the formation of two contiguous stereogenic centers. The ensuing selective and high-yielding transformations provide appealing dihydroisoquinoline derivatives.

The Size-Accelerated Kinetic Resolution of Secondary Alcohols

The factors responsible for the kinetic resolution of alcohols by chiral pyridine derivatives have been elucidated by measurements of relative rates for a set of substrates with systematically growing aromatic side chains using accurate competitive linear regression analysis. Increasing the side chain size from phenyl to pyrenyl results in a rate acceleration of more than 40 for the major enantiomer. Based on this observation a new catalyst with increased steric bulk has been designed that gives enantioselectivity values of up to s=250. Extensive conformational analysis of the relevant transition states indicates that alcohol attack to the more crowded side of the acyl-catalyst intermediate is favoured due to stabilizing CH-π-stacking interactions. Experimental and theoretical results imply that enantioselectivity enhancements result from accelerating the transformation of the major enantiomer through attractive non-covalent interactions (NCIs) rather than retarding the transformation of the minor isomer through repulsive steric forces.

Dynamic covalent bond constrained ureas for multimode fluorescence switching, thermally induced emission, and chemical signaling cascades

A combination of organic ureas and dynamic covalent chemistry was demonstrated for multistate switching, thermally induced fluorescence, and signaling cascades.

Carbosulfenylation of Alkenes with Organozinc Reagents and Dimethyl(methylthio)sulfonium Trifluoromethanesulfonate

The electrophilic alkylthiolation of alkenes, initiated by dimethyl(methylthio)sulfonium salts and the subsequent addition of various heteronucleophilies has been well-established. Regarding the use of carbon nucleophiles, however, only carefully designed sp-type carbon sources have been successfully applied. We herein present our findings on the methylthiolation of alkenes with dimethyl(methylthio)sulfonium trifluoromethanesulfonate, followed by carbon-carbon bond formation in the presence of organozinc reagents, thus achieving a catalyst-free protocol toward to the carbosulfenylation of alkenes.

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.

Chiral Brønsted Acid Catalyzed Enantioconvergent Propargylic Substitution Reaction of Racemic Secondary Propargylic Alcohols with Thiols

Despite the significant progress of the enantioselective reaction using chiral catalysts, the enantioselective nucleophilic substitution reaction at the chiral sp³ -hybridized carbon atom of a racemic electrophile has not been largely explored. Herein, we report the enantioconvergent propargylic substitution reaction of racemic propargylic alcohols with thiols using chiral bis-phosphoric acid as the chiral Brønsted acid catalyst. The substitution products were formed in high yields with high enantioselectivities in most cases. The cation-stabilizing effect of the sulfur functional group introduced at the alkynyl terminus is the key to achieving the efficient enantioconvergent process, in which chiral information originating from not only the racemic stereogenic center but also the formed contact ion pair is completely eliminated from the present system.

Rh(II)/Ag(I)-Cocatalyzed Three-Component Reaction via SN1/SN1'-Type Trapping of Oxonium Ylide with the Nicholas Intermediate

A Rh(II)/Ag(I)-cocatalyzed three-component reaction of propargylic alcohol-Co₂(CO)₆ complexes with diazo compounds and benzyl alcohols is described, which represents the first trapping process of oxonium ylides with carbocations via the S_N1/S_N1'-type pathway. This transformation provides an efficient approach for construction of dicobalt hexacarbonyl-complexed 3,3-disubstituted oxindoles. Further derivatization of the product, initiated by the deprivation of the dicobalt species, gives the 3,3-disubstituted oxindoles with the ene alkynyl group and the privileged spirooxindole-vinyldihydropyran structure.

Enantioselective Tail-to-Head Cyclizations Catalyzed by Dual-Hydrogen-Bond Donors

Chiral urea derivatives are shown to catalyze enantioselective tail-to-head cyclization reactions of neryl chloride analogues. Experimental data are consistent with a mechanism in which π-participation by the nucleophilic olefin facilitates chloride ionization and thereby circumvents simple elimination pathways. Kinetic and computational studies support a cooperative mode of catalysis wherein two molecules of the urea catalyst engage the substrate and induce enantioselectivity through selective transition state stabilization.

Enantioselective Catalysis of an Anionic Oxy-Cope Rearrangement Enabled by Synergistic Ion Binding

Charge-accelerated rearrangements present interesting challenges to enantioselective catalysis, due in large part to the competing requirements for maximizing reactivity (ion-pair separation) and stereochemical communication. Herein, we describe application of a synergistic ion-binding strategy to catalyze the anionic oxy-Cope rearrangement of a symmetric bis-styrenyl allyl alcohol in up to 75:25 e.r. Structure-reactivity-selectivity relationship studies, including linear free-energy-relationship analyses, with bifunctional urea catalysts indicate that H-bonding and cation-binding interactions act cooperatively to promote the chemo- and enantioselective [3,3]-rearrangement. Implications for catalyst designs applicable to other transformations involving oxyanionic intermediates are discussed.

Nickel-Catalyzed Asymmetric Friedel-Crafts Propargylation of 3-Substituted Indoles with Propargylic Carbonates Bearing an Internal Alkyne Group

The nickel-catalyzed highly enantioselective Friedel-Crafts propargylation of 3-substituted indoles with propargylic carbonates bearing an internal alkyne group was developed. A wide array of the propargylic carbonates as well as 3-substituted indoles can be applicable to the asymmetric nickel catalysis, providing the corresponding chiral C-3 propargylated indolenine derivatives bearing two vicinal chiral centers in up to 89% yield with up to >99% ee and 94:6 dr (24 examples).

Mechanistic Insight toward Understanding the Role of Charge in Thiourea Organocatalysis

Pyranylation and glycosylation are pivotal for accessing a myriad of natural products, pharmaceuticals, and drug candidates. Catalytic approaches for enabling these transformations are of utmost importance and integral to advancing this area of synthesis. In exploring this chemical space, a combined experimental and computational mechanistic study of pyranylation and 2-deoxygalactosylation catalyzed by a cationic thiourea organocatalyst is reported. To this end, a thiourea-cyclopropenium organocatalyst was employed as a model system in combination with an arsenal of mechanistic techniques, including ¹³C kinetic isotope effect experiments, deuterated labeling studies, variable-temperature ¹H NMR spectroscopy, and density functional theory calculations. From these studies, two distinct reaction pathways were identified for this transformation corresponding to either dual hydrogen bond (H-bond) activation or Brønsted acid catalysis. The former involving thiourea orchestrated bifurcated hydrogen bonding proceeded in an asynchronous concerted fashion. In contrast, the latter stepwise mechanism involving Brønsted acid catalysis hinged upon the formation of an oxocarbenium intermediate accompanied by subsequent alcohol addition.

Enantioselective Synthesis of α-Allyl Amino Esters via Hydrogen-Bond-Donor Catalysis

We report a chiral-squaramide-catalyzed enantio- and diastereoselective synthesis of α-allyl amino esters. The optimized protocol provides access to N-carbamoyl-protected amino esters via nucleophilic allylation of readily accessible α-chloro glycinates. A variety of useful α-allyl amino esters were prepared, including crotylated products bearing vicinal stereocenters that are inaccessible through enolate alkylation, with high enantioselectivity (up to 97% ee) and diastereoselectivity (>10:1). The reactions display first-order kinetic dependence on both the α-chloro glycinate and the nucleophile, consistent with rate-limiting C-C bond formation. Computational analysis of the uncatalyzed reaction predicts an energetically inaccessible iminium intermediate, and a lower energy concerted SN2 mechanism.