Highly Enantioselective Organocatalytic Oxidative Kinetic Resolution of Secondary Alcohols Using Chiral Alkoxyamines as Precatalysts: Catalyst Structure, Active Species, and Substrate Scope

Silver-Catalyzed Markovnikov Addition Hydrofunctionalization of Terminal Alkynes: Synthesis of N-Enoxyimides and Oximes

N-Enoxyimides are versatile and valuable synthetic synthons in modern organic synthesis and catalysis. Herein, a silver-catalyzed hydrooxyimidation of terminal alkynes that enables the synthesis of N-enoxyimides, has been demonstrated. The protocol features are simple, step- and atom-efficient, scalable, and exhibit a broad scope of functional group tolerance under mild conditions. A novel hydrolysis of the N-enoxyimides unexpectedly provides the corresponding aromatic oximes in 96% yield, showing the potential application of this strategy.

Catalytic Enantioselective Hydrogen Atom Abstraction Enables the Asymmetric Oxidation of Meso Diols

Desymmetrization of meso diols is an important strategy for the synthesis of chiral oxygen-containing building blocks. Oxidative desymmetrization is an important subclass, but existing methods are often constrained by the need for activated alcohol substrates. We disclose a conceptually distinct strategy toward oxidative diol desymmetrization that is enabled by catalytic enantioselective hydrogen atom abstraction. Following single electron oxidation of a cinchona alkaloid-derived catalyst, enantiodetermining hydrogen atom abstraction generates a desymmetrized ketyl radical intermediate which reacts with either DIAD or O2 before in situ elimination to form valuable hydroxyketone products. A range of cyclic and acyclic meso diols are competent, defining the absolute configuration of up to four stereocenters in a single operation. As well as providing rapid access to complex hydroxyketones, this work emphasizes the broad synthetic potential of harnessing hydrogen atom abstraction in an enantioselective manner.

Multiplicative enhancement of stereoenrichment by a single catalyst for deracemization of alcohols

Stereochemical enrichment of a racemic mixture by deracemization must overcome unfavorable entropic effects as well as the principle of microscopic reversibility; recently, photochemical reaction pathways unveiled by the energetic input of light have led to innovations toward this end, most often by ablation of a stereogenic C(sp3)-H bond. We report a photochemically driven deracemization protocol in which a single chiral catalyst effects two mechanistically different steps, C-C bond cleavage and C-C bond formation, to achieve multiplicative enhancement of stereoinduction, which leads to high levels of stereoselectivity. Ligand-to-metal charge transfer excitation of a titanium catalyst coordinated by a chiral phosphoric acid or bisoxazoline efficiently enriches racemic alcohols that feature adjacent and fully substituted stereogenic centers to enantiomeric ratios up to 99:1. Mechanistic investigations support a pathway of sequential radical-mediated bond scission and bond formation through a common prochiral intermediate and reveal that, although the overall stereoenrichment is high, the selectivity in each individual step is moderate.

Organic Synthesis Using Nitroxides

Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R1R2N-O•. The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R1 and R2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R1R2N═O+) or reduced to anions (R1R2N-O-), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C-O bonds, allowing for the thermal generation of C radicals through reversible C-O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

Generality-oriented optimization of enantioselective aminoxyl radical catalysis

Catalytic enantioselective methods that are generally applicable to a broad range of substrates are rare. We report a strategy for the oxidative desymmetrization of meso-diols predicated on a nontraditional catalyst optimization protocol by using a panel of screening substrates rather than a singular model substrate. Critical to this approach was rational modulation of a peptide sequence in the catalyst incorporating a distinct aminoxyl-based active residue. A general catalyst emerged, providing high selectivity in the delivery of enantioenriched lactones across a broad range of diols, while also achieving up to ~100,000 turnovers.

Copper- and Chiral Nitroxide-Catalyzed Oxidative Kinetic Resolution of Axially Chiral N-Arylpyrroles

A readily prepared C₂-symmetric, α-hydrogen-substituted chiral hydroxylamine serves as a precatalyst to generate a chiral nitroxide in situ. This chiral nitroxide catalyst in combination with a copper co-catalyst functions as an oxidant for an unprecedented enantioselective oxidative kinetic resolution (OKR) of racemic axially chiral N-arylpyrrole alcohols using atmospheric oxygen as an environmentally friendly terminal oxidant. The OKR process provides the axially chiral N-arylpyrroles in er up to 3.5:96.5 and with s factors up to 24.

Synthetic applications of hydride abstraction reactions by organic oxidants

Carbon-hydrogen bond functionalizations provide an attractive method for streamlining organic synthesis, and many strategies have been developed for conducting these transformations. Hydride-abstracting reactions have emerged as extremely effective methods for oxidative bond-forming processes due to their mild reaction conditions and high chemoselectivity. This review will predominantly focus on the mechanism, reaction development, natural product synthesis applications, approaches to catalysis, and use in enantioselective processes for hydride abstractions by quinone, oxoammonium ion, and carbocation oxidants. These are the most commonly employed hydride-abstracting agents, but recent efforts illustrate the potential for weaker ketone and triaryl borane oxidants, which will be covered at the end of the review.

Asymmetric hydrogenation of TIPS-protected oximes with chiral boranes

An enantioselective metal-free hydrogenation of TIPS-protected oximes has been successfully realized for the first time by using chiral borane catalysts derived from chiral dienes and Piers' borane. A variety of hydroxylamine derivatives were afforded in 84-99% yields with 33-68% ees.

Kinetic resolution of cyclic benzylic azides enabled by site- and enantioselective C(sp3)-H oxidation

Catalytic nonenzymatic kinetic resolution (KR) of racemates remains one of the most powerful tools to prepare enantiopure compounds, which dominantly relies on the manipulation of reactive functional groups. Moreover, catalytic KR of organic azides represents a formidable challenge due to the small size and instability of the azido group. Here, an effective KR of cyclic benzylic azides through site- and enantioselective C(sp³)-H oxidation is described. The manganese catalyzed oxidative KR reaction exhibits good functional group tolerance, and is applicable to a range of tetrahydroquinoline- and indoline-based organic azides with excellent site- and enantio-discrimination. Computational studies elucidate that the effective chiral recognition is derived from hydrogen bonding interaction between substrate and catalyst.

Methane Monooxygenase Mimic Asymmetric Oxidation: Self-Assembling μ-Hydroxo, Carboxylate-Bridged Diiron(III)-Catalyzed Enantioselective Dehydrogenation

Mimicking naturally occurring metalloenzymes to enrich the diversity of catalytic asymmetric oxidation reactions is a long-standing goal for modern chemistry. Toward this end, a range of methane monooxygenase (MMO) mimic chiral carboxylate-bridged (μ-hydroxo) diiron(III) dimer complexes using salan as basal ligand and sodium aryl carboxylate as additive have been designed and synthesized. The chiral diiron complexes exhibit efficient catalytic reactivity in dehydrogenative kinetic resolution of indolines using environmentally benign hydrogen peroxide as oxidant. In particular, complex C9 bearing sterically encumbered salan ligands and a 2-naphthoate bridge is identified as the optimal catalyst in terms of chiral recognition. Further investigation reveals that this MMO mimic chiral catalyst can be readily generated by self-assembly under the dehydrogenation conditions. The self-assembling catalytic system is applicable to a series of indolines with multiple stereocenters and diverse substituent patterns in high efficiency with a high level of chiral recognition (selectivity factor up to 153). Late-stage dehydrogenative kinetic resolution of bioactive molecules is further examined.

Catalytic Reduction of Oximes to Hydroxylamines: Current Methods, Challenges and Opportunities

Catalytic reduction of oximes represents a direct efficient approach to synthesize valuable hydroxylamine derivatives. However this transformation presents significant challenges: oximes are hard to reduce and, if reactive, reductive cleavage of the weak N-O bond often leads to primary amine side products. The first suitable systems involved the use of platinum-based heterogeneous catalysts with hydrogen as reductant and stoichiometric amounts of a strong Brønsted acid. More recently metal-free and transition-metal-based homogeneous catalysts have been developed, which display the highest turnovers (up to 4000). In the asymmetric variants, the E/Z-geometry of the oxime double bond affects significantly the stereoselectivity, sometimes requiring extra synthetic efforts in substrate preparation. This minireview provides an overview of the advances and limitations in catalytic oxime to hydroxylamine reduction. Emphasis is put on highlighting and comparing the practical aspects of the existing methods, such as their reaction conditions and substrate scope. Additionally, future directions for improving this young research area are suggested.

Iron-catalyzed regioselective synthesis of (E)-vinyl sulfones mediated by unprotected hydroxylamines

An Fe-catalyzed unprotected hydroxylamine mediated Heck-type coupling between sulfinic acids and alkenes furnished structurally important (E)-vinyl sulfones with moderate to good yields, high atom-economy and regioselectivity.

Efficiently Enantioselective Hydrogenation Photosynthesis of (R)-1-[3,5-Bis(trifluoromethyl)phenyl] ethanol over a CLEs-TiO2 Bioinorganic Hybrid Materials

Engineering of biological pathways with man-made materials provides inspiring blueprints for sustainable drug production. (R)-1-[3,5-Bis(trifluoromethyl)phenyl]ethanol [(R)-3,5-BTPE], as an important artificial chiral intermediate for complicated pharmaceutical drugs and biologically active molecules, is often synthesized through a hydrogenation reaction of 3,5-bis(trifluoromethyl)acetophenone (3,5-BTAP), in which enantioselectivity and sufficient active hydrogen are the key to restricting the reaction. In this work, a biohybrid photocatalytic hydrogenation system based on an artificial cross-linked enzymes (CLEs)-TiO₂-Cp*Rh(bpy) photoenzyme is developed through a bottom-up engineering strategy. Here, TiO₂ nanotubes in the presence of Cp*Rh(bpy) are used to transform NADP⁺ to NADPH during the formation of chiral alcohol intermediates from the catalytic reduction of a ketone substrate by alcohol dehydrogenase CLEs. Hydrogen and electrons, provided by water and photocatalytic systems, respectively, are transferred to reduce NADP⁺ to NADPH via [Cp*Rh(bpy)(H₂O)]²⁺. With the resulting NADPH, [(R)-3,5-BTPE] is synthesized using our efficient CLEs obtained from the cell lysate by nonstandard amino acid modification. Through this biohybrid photocatalytic system, the photoenzyme-catalyzed combined reductive synthesis of [(R)-3,5-BTPE] has a yield of 41.2% after reaction for 24 h and a very high enantiomeric excess value (>99.99%). In the case of reuse, this biohybrid system retained nearly 95% of its initial catalytic activity for synthesizing the above chiral alcohol. The excellent reusability of the CLEs and TiO₂ nanotubes hybrid catalytic materials highlights the environmental friendliness of (R)-3,5-BTPE production.

Iridium-Catalyzed Acid-Assisted Hydrogenation of Oximes to Hydroxylamines

We found that cyclometalated cyclopentadienyl iridium(III) complexes are uniquely efficient catalysts in homogeneous hydrogenation of oximes to hydroxylamine products. A stable iridium C,N-chelation is crucial, with alkoxy-substituted aryl ketimine ligands providing the best catalytic performance. Several Ir-complexes were mapped by X-ray crystal analysis in order to collect steric parameters that might guide a rational design of even more active catalysts. A broad range of oximes and oxime ethers were activated with stoichiometric amounts of methanesulfonic acid and reduced at room temperature, remarkably without cleavage of the fragile N-O bond. The exquisite functional group compatibility of our hydrogenation system was further demonstrated by additive tests. Experimental mechanistic investigations support an ionic hydrogenation platform, and suggest a role for the Brønsted acid beyond a proton source. Our studies provide deep understanding of this novel acidic hydrogenation and may facilitate its improvement and application to other challenging substrates.

Boron insertion into alkyl ether bonds via zinc/nickel tandem catalysis

Mild methods to cleave the carbon-oxygen (C-O) bond in alkyl ethers could simplify chemical syntheses through the elaboration of these robust, readily available precursors. Here we report that dibromoboranes react with alkyl ethers in the presence of a nickel catalyst and zinc reductant to insert boron into the C-O bond. Subsequent reactivity can effect oxygen-to-nitrogen substitution or one-carbon homologation of cyclic ethers and more broadly streamline preparation of bioactive compounds. Mechanistic studies reveal a cleavage-then-rebound pathway via zinc/nickel tandem catalysis.

Asymmetric Transfer Hydrogenation of α-Substituted-β-Keto Carbonitriles via Dynamic Kinetic Resolution

A catalytic protocol for the enantio- and diastereoselective reduction of α-substituted-β-keto carbonitriles is described. The reaction involves a DKR-ATH process with the simultaneous construction of β-hydroxy carbonitrile scaffolds with two contiguous stereogenic centers. A wide range of α-substituted-β-keto carbonitriles were obtained in high yields (94%-98%) and excellent enantio- and diastereoselectivities (up to >99% ee, up to >99:1 dr). The origin of the diastereoselectivity was also rationalized by DFT calculations. Furthermore, this methodology offers rapid access to the pharmaceutical intermediates of Ipenoxazone and Tapentadol.

Site- and Enantiodifferentiating C(sp3)-H Oxidation Enables Asymmetric Access to Structurally and Stereochemically Diverse Saturated Cyclic Ethers

A manganese-catalyzed site- and enantiodifferentiating oxidation of C(sp³)-H bonds in saturated cyclic ethers has been described. The mild and practical method is applicable to a range of tetrahydrofurans, tetrahydropyrans, and medium-sized cyclic ethers with multiple stereocenters and diverse substituent patterns in high efficiency with extremely efficient site- and enantiodiscrimination. Late-stage application in complex biological active molecules was further demonstrated. Mechanistic studies by combined experiments and computations elucidated the reaction mechanism and origins of stereoselectivity. The ability to employ ether substrates as the limiting reagent, together with a broad substrate scope, and a high level of chiral recognition, represent a valuable demonstration of the utility of asymmetric C(sp³)-H oxidation in complex molecule synthesis.

Oxidative Kinetic Resolution of Cyclic Benzylic Ethers

A manganese-catalyzed oxidative kinetic resolution of cyclic benzylic ethers through asymmetric C(sp³ )-H oxidation is reported. The practical approach is applicable to a wide range of 1,3-dihydroisobenzofurans bearing diverse functional groups and substituent patterns at the α position with extremely efficient enantiodiscrimination. The generality of the strategy was further demonstrated by efficient oxidative kinetic resolution of another type of five-membered cyclic benzylic ether, 2,3-dihydrobenzofurans, and six-membered 6H-benzo[c]chromenes. Direct late-stage oxidative kinetic resolution of bioactive molecules that are otherwise difficult to access was further explored.

Design, synthesis and biological evaluation of novel neuchromenin analogues as potential antifungal agents

In continuation of our program to discover new potential antifungal agents, thirty-two neuchromenin analogues were synthesized and characterized by the spectroscopic analysis. By using the mycelium growth rate method, the target compounds were evaluated systematically for antifungal activities in vitro against six plant pathogenic fungi, and structure-activity relationships (SAR) were derived. Compounds 6b-c, and 6l showed obvious inhibition activity on each of the fungi at 50 μg/mL. For the corresponding fungi, 7 of the compounds showed average inhibition rates of >80% at 50 μg/mL; especially, compounds 6b, 6d-e, and 6i-l displayed more potent antifungal activity against A. solani than that of thiabendazole (a positive control). Moreover, compound 6c also exhibited good activity against C. lunata with EC₅₀ values of 12.7 μg/mL, and the value was much superior to that of thiabendazole (EC₅₀ = 59.7 μg/mL). SAR analysis showed that the presence of conjugated structure, bearing a C=C bond conjugated to the C=O group, obviously decreased the activity; the type and position of the substituted R₅ significantly influenced the activity. Furthermore, the significantly bioactive compounds 6b-e, 6g, 6i and 6l showed very low toxicities against HL-7702, BEL-7402 and HCT-8 cells. Resistance development assay indicated that compounds 6b-e and 6l failed to induce the two tested strains of fungi to develop resistance. SEM analysis initially revealed that compound 6d may exert its antifungal effect by damaging fungal cell wall.

Iron-Catalyzed Aerobic Dehydrogenative Kinetic Resolution of Cyclic Secondary Amines

A nonenzymatic iron-catalyzed dehydrogenative kinetic resolution of cyclic secondary amines using air as an oxidant has been reported. The economical and practical method is applicable to a series of cyclic benzylic amines, including 5,6-dihydrophenanthridines and 1,2-dihydroquinolines, with diverse functional groups at the α position in high yields with excellent enantioselectivities. The direct dehydrogenative kinetic resolution of advanced intermediates of bioactive molecules that are difficult to access using existing catalytic asymmetric synthetic strategy was also demonstrated.

Direct N-O bond formation via oxidation of amines with benzoyl peroxide

Herein, we report a general and efficient method for direct N-O bond formation without undesirable C-N bond (amide) formation starting from commercially available amines and benzoyl peroxide. The oxidation of 1,2-diamines to furnish bis-(benzoyloxy)-1,2-diamines is reported for the first time. We found that a significant amount of water (BPO : water = 3 : 1) in combination with Cs2CO3 is necessary to achieve high selectivity and yield. The reaction conditions are applicable to a wide range of 1,2-diamine and 1,2-disubstituted-1,2-diamine substrates. Additionally this method is highly applicable to primary and secondary amines. Further, the present method can access chiral bis-hydroxamic acids and bis-hydroxyl amines in just two steps from 1,2-diamines. The reaction conditions are simple, mild and inert atmosphere free. The synthetic potential of this methodology is further demonstrated in the short synthesis of a chiral BHA ligand.

Burkholderia cepacia lipase immobilization for hydrolytic reactions and the kinetic resolution of the non-equimolar mixtures of isomeric alcohols

The major drawbacks of native lipase applications in processes occurring in water or in organic solvents include: difficulties in catalyst recycling, low activity and operational instability. The immobilization of Burkholderia cepacia lipase by adsorption or covalent binding onto 5 differently functionalized carriers (silica, acrylic, cellulose-based) was performed to overcome this problem. The optimization of the reaction preparation in water-rich media was based on the hydrolytic reactivity of the preparations, as well as the thermal, operational and storage stabilities. Aminated silica carrier, activated with glutaraldehyde, was determined to be the carrier of choice. Regarding processes in water-restricted media, carrier selection was based on reactivity after drying and five preparations were chosen for the resolution of a non-equimolar isomer mixture (85:15 ratio of R to S isomers), treating the kinetic resolution of ((+)-(S/R)-1-[(1S,5R)-6,6-dimethylbicyclo[3.1.0]hex-2-en-2-yl)]ethanol as a model. The resulting acetate of R configuration exhibits interesting sensory properties. The operational stability of the chosen catalysts was tested over 15 consecutive batch processes; the most beneficial results were obtained with lipase adsorbed on an acrylic carrier. Conversion increased gradually from 10 to 84% over the first five processes, which could be explained by the product sorption onto the carrier. Full kinetic resolution with maximal substrate conversion (approximately 84%) was achieved and remained stable during the next 10 runs, an excellent result, and thus, the proposed system might be regarded as an exceptionally attractive solution for the perfume and cosmetic industries.

Total Synthesis of Xanthoangelol B and Its Various Fragments: Toward Inhibition of Virulence Factor Production of Staphylococcus aureus

As an alternative strategy to fight antibiotic resistance, two-component systems (TCSs) have emerged as novel targets. Among TCSs, master virulence regulators that control the expression of multiple virulence factors are considered as excellent antivirulence targets. In Staphylococcus aureus, virulence factor expression is tightly regulated by a few master regulators, including the SaeRS TCS. In this study, we used a SaeRS GFP-reporter system to screen natural compound inhibitors of SaeRS, and identified xanthoangelol B 1, a prenylated chalcone from Angelica keiskei as a hit. We have synthesized 1 and its derivative PM-56 and shown that 1 and PM-56 both had excellent inhibitory potency against the SaeRS TCS, as demonstrated by various in vitro and in vivo experiments. As a mode of action, 1 and PM-56 were shown to bind directly to SaeS and inhibit its histidine kinase activity, which suggests a possibility of a broad spectrum inhibitor of histidine kinases.

Utilization of deep-sea microbial esterase PHE21 to generate chiral sec-butyl acetate through kinetic resolutions

We previously identified and characterized 1 novel deep-sea microbial esterase PHE21 and used PHE21 as a green biocatalyst to generate chiral ethyl (S)-3-hydroxybutyrate, 1 key chiral chemical, with high enantiomeric excess and yield through kinetic resolution. Herein, we further explored the potential of esterase PHE21 in the enantioselective preparation of secondary butanol, which was hard to be resolved by lipases/esterases. Despite the fact that chiral secondary butanols and their ester derivatives were hard to prepare, esterase PHE21 was used as a green biocatalyst in the generation of (S)-sec-butyl acetate through hydrolytic reactions and the enantiomeric excess, and the conversion of (S)-sec-butyl acetate reached 98% and 52%, respectively, after process optimization. Esterase PHE21 was also used to generate (R)-sec-butyl acetate through asymmetric transesterification reactions, and the enantiomeric excess and conversion of (R)-sec-butyl acetate reached 64% and 43%, respectively, after process optimization. Deep-sea microbial esterase PHE21 was characterized to be a useful biocatalyst in the kinetic resolution of secondary butanol and other valuable chiral secondary alcohols.

Manganese complex-catalyzed oxidation and oxidative kinetic resolution of secondary alcohols by hydrogen peroxide

The highly efficient catalytic oxidation and oxidative kinetic resolution (OKR) of secondary alcohols has been achieved using a synthetic manganese catalyst with low loading and hydrogen peroxide as an environmentally benign oxidant in the presence of a small amount of sulfuric acid as an additive. The product yields were high (up to 93%) for alcohol oxidation and the enantioselectivity was excellent (>90% ee) for the OKR of secondary alcohols. Mechanistic studies revealed that alcohol oxidation occurs via hydrogen atom (H-atom) abstraction from an α-CH bond of the alcohol substrate and a two-electron process by an electrophilic Mn-oxo species. Density functional theory calculations revealed the difference in reaction energy barriers for H-atom abstraction from the α-CH bonds of R- and S-enantiomers by a chiral high-valent manganese-oxo complex, supporting the experimental result from the OKR of secondary alcohols.

Total Synthesis of Rishirilide B by Organocatalytic Oxidative Kinetic Resolution: Revision of Absolute Configuration of (+)-Rishirilide B

Described herein is the enantioselective syntheses of (+)- and (-)-rishirilide B from the corresponding optically active β-substituted tetralones, which were obtained by oxidative kinetic resolution based on α-hydroxylation in the presence of a chiral guanidine-bisurea bifunctional organocatalyst. Benzylic oxidation of the tetralones at C1 followed by regioselective isomerization of the oxabenzonorbornadiene structure led to rishirilide B. Our findings lead to the revision of the previously proposed (2R,3R,4R) absolute configuration of (+)-rishirilide B to (2S,3S,4S).

The Cation-π Interaction in Small-Molecule Catalysis

Catalysis by small molecules (≤1000 Da, 10(-9)  m) that are capable of binding and activating substrates through attractive, noncovalent interactions has emerged as an important approach in organic and organometallic chemistry. While the canonical noncovalent interactions, including hydrogen bonding, ion pairing, and π stacking, have become mainstays of catalyst design, the cation-π interaction has been comparatively underutilized in this context since its discovery in the 1980s. However, like a hydrogen bond, the cation-π interaction exhibits a typical binding affinity of several kcal mol(-1) with substantial directionality. These properties render it attractive as a design element for the development of small-molecule catalysts, and in recent years, the catalysis community has begun to take advantage of these features, drawing inspiration from pioneering research in molecular recognition and structural biology. This Review surveys the burgeoning application of the cation-π interaction in catalysis.

Synthesis of Ketones and Esters from Heteroatom-Functionalized Alkenes by Cobalt-Mediated Hydrogen Atom Transfer

Cobalt bis(acetylacetonate) is shown to mediate hydrogen atom transfer to a broad range of functionalized alkenes; in situ oxidation of the resulting alkylradical intermediates, followed by hydrolysis, provides expedient access to ketones and esters. By modification of the alcohol solvent, different alkyl ester products may be obtained. The method is compatible with a number of functional groups including alkenyl halides, sulfides, triflates, and phosphonates and provides a mild and practical alternative to the Tamao-Fleming oxidation of vinylsilanes and the Arndt-Eistert homologation.