Chiral Brønsted Acid-Catalyzed Formal α-Vinylation of Cyclopentanones for the Enantioselective Construction of Quaternary Carbon Centers

Asymmetric Construction of Ethenyl-Substituted Acyclic Quaternary Stereocenters at the α-Position of Carbonyl Surrogates via Stereoselective Sulfonylvinylation-Reductive Desulfonylation

N-Sulfinyl metalloenamines, derived from geometry-defined β,β-disubstituted enesulfinamides, undergo conjugate addition-elimination reaction with β-tosyl nitroolefin to afford α-sulfonylvinylated ketimines with high stereocontrol. Further desulfonylation using sodium naphthalenide gives carbonyl surrogates bearing a less-accessible acyclic quaternary α-stereocenters substituted with an ethenyl group and two sterically and electronically similar groups (e.g., methyl and ethyl). Synthetic application of the described protocol was demonstrated by enantioselective synthesis of (S)-bakuchiol.

Stereoelectronic Effect in the Reaction of α-Methylene Lactones with Tertiary Phosphines and Its Application in Organocatalysis

The kinetic data indicate that the addition of tertiary phosphines to α-methylene lactones in acetic acid is strongly accelerated in comparison to the reactions of related open-chain esters. Six-membered α-methylene-δ-valerolactone exhibited a more pronounced rate increase than five-membered α-methylene-γ-butyrolactone. The use of α-methylene-γ-butyrolactam as a nitrogen analogue of α-methylene-γ-butyrolactone resulted in a total loss of the reaction acceleration. The observed reactivities were rationalized by DFT calculations at the RwB97XD/6-31+G(d,p) level of theory, showing that the intramolecular interaction between phosphonium and enolate oxygen centers provided by the locked s-cis-geometry of the heterocycles plays an important role in the stabilization of intermediate zwitterions. The reactivity is also controlled by the conformational flexibility of the heterocycle. The geometries of five-membered and, especially, six-membered lactone cycles are slightly changed upon the nucleophilic attack of phosphine, leading to the stabilizing stereoelectronic effect by the Ρ···Ο interaction. The addition of phosphine to α-methylene-γ-butyrolactam significantly distorts the initial geometry of the heterocycle, making the nucleophilic attack unfavorable. The application of the stereoelectronic effect to enhance the efficiency of the phosphine-catalyzed Michael and Pudovik reactions of α-methylene lactones was demonstrated.

Phosphorus-Involved Wagner-Meerwein Rearrangement of Phosphiranes: An Entry to Four-Membered Phosphacycles

Phosphenium ions [R₂P]⁺ are important and highly reactive dicoordinate phosphorus species. Herein, we report a rearrangement of the carbocation into the phosphenium cation driven by ring strain. This phosphorus-involved Wagner-Meerwein rearrangement pathway converted the 1-acylphosphirane complex into phosphetane and 1,2-dihydrophosphete derivatives depending on the reaction temperature. The generation of the intermediate phosphenium cation was identified by the intramolecular reaction with ether, which also disclosed its strong Lewis acidity. This work expands the boundary of the phosphorus-carbon analogy.

Chiral Calcium Phosphate-Catalyzed Enantioselective Amination of 3-Aryl-2-oxindoles with Dibenzyl Azodicarboxylate

A chiral calcium phosphate-catalyzed enantioselective amination of 2-oxindoles with dibenzyl azodicarboxylate has been developed, affording the products in consistently high yields and excellent enantioselectivity. This synthetic method features low catalyst loading and a high catalytic efficiency. Moreover, the practical value of this process is well demonstrated by a scale-up experiment and a trial of catalyst recovery and reuse.

Enantioselective [2+2] Cycloaddition of 1,2-Dihydroquinolines with 3-Olefinic Oxindoles via Brønsted Acid Catalysis

Two complementary regiodivergent Brønsted acid-catalyzed atom-economic [2+2] cycloaddition and ene reaction of 1,2-dihydroquinolines with 3-olefinic oxindoles are reported. In the presence of a chiral phosphoramide catalyst, the [2+2] cycloaddition affords...

Kinetic Resolution of Racemic Tertiary Allylic Alcohols through SN2’ Reaction Using a Chiral Bisphosphoric Acid/Silver(I) Salt Co-catalyst System

A highly efficient kinetic resolution (KR) of racemic tertiary allylic alcohols was achieved through an intramolecular allylic substitution reaction using a co-catalyst system composed of chiral bisphosphoric acid and silver carbonate. This reaction afforded enantioenriched diene monoepoxides along with the recovery of tertiary allylic alcohols in a highly enantioselective manner, realizing an extremely high s-factor in most cases. The present method provides a new access to enantioenriched tertiary allylic alcohols, multifunctional compounds that are applicable for further synthetic manipulations.

A highly efficient KR of racemic tertiary allylic alcohols was developed through the intramolecular S_N2′ reaction using the chiral bisphosphoric acid/silver carbonate co-catalyst system, affording cis-epoxides and recovered alcohols in a high s-factor.

Enantioselective synthesis of chiral tetrasubstituted allenes: harnessing electrostatic and noncovalent interactions in a bifunctional activation model for N-triflylphosphoramide catalysis

DFT calculation reveals that the oxygen activation model is preferred than the nitrogen activation model due to the preferred chiral electrostatic environment.

Chiral Calcium Phosphate Catalyzed Enantioselective Amination of 3-Aryl-2-benzofuranones

A 4-tert-butyl-phenyl substituted (R)-[H₈]-BINOL chiral calcium phosphate catalyzed enantioselective amination of 3-aryl-2-benzofuranones with dibenzyl azodicarboxylate is described. The catalyst loading of the reaction is 1 mol %. This transformation is facile and has a high degree atom economy, which gave products with good yields and high enantioselectivities (79% to 99%). This reaction has excellent ee and a broad substrate scope with mild reaction conditions.

Enantioselective Intramolecular Allylic Substitution via Synergistic Palladium/Chiral Phosphoric Acid Catalysis: Insight into Stereoinduction through Statistical Modeling

The mode of asymmetric induction in an enantioselective intramolecular allylic substitution reaction catalyzed by a combination of palladium and a chiral phosphoric acid was investigated by a combined experimental and statistical modeling approach. Experiments to probe nonlinear effects, the reactivity of deuterium-labeled substrates, and control experiments revealed that nucleophilic attack to the π-allylpalladium intermediate is the enantio-determining step, in which the chiral phosphate anion is involved in stereoinduction. Using multivariable linear regression analysis, we determined that multiple noncovalent interactions with the chiral environment of the phosphate anion are integral to enantiocontrol in the transition state. The synthetic protocol to form chiral pyrrolidines was further applied to the asymmetric construction of C-O bonds at fully substituted carbon centers in the synthesis of chiral 2,2-disubstituted benzomorpholines.

Chiral strong Brønsted acid-catalyzed enantioselective addition reaction of simple olefins with ethyl glyoxylate

An enantioselective intermolecular addition reaction of 1,1,2-trisubstituted and 1,2- disubstituted simple olefins with ethyl glyoxylate was developed using F₁₀BINOL-derived N-sulfonyl phosphoramide as a chiral strong Brønsted acid catalyst.

Asymmetric synthesis of cyclopentanones through dual Lewis acid-catalysed [3+2]-cycloaddition of donor–acceptor cyclopropanes with ketenes

A dual Lewis acid system promotes the formal [3+2]-cycloaddition of enantioenriched donor–acceptor cyclopropanes with ketenes to afford cyclopentanones.