Organocatalytic Asymmetric Synthesis of α-Oxetanyl and α-Azetidinyl Tertiary Alkyl Fluorides and Chlorides

Organocatalytic atroposelective fluorooxindole addition to coumarin Michael acceptors

Organocatalytic atropisomeric synthesis with fluorinated oxindoles and 4-halo-3-nitrocoumarins gives congested structures displaying a Csp2-Csp3 chirality axis and an adjacent tetrasubstituted stereogenic carbon center with good yields, up to 97% ee and 41 : 1 dr. The scalable dehalogenative C-C bond formation is achieved under mild conditions with a commercially available urea catalyst.

Enantioselective Conjugate Addition of Aldehydes to Oxetane- and Azetidine-Containing Nitroolefins: An Entry to Spirocyclic Pyrrolidines

Enantioselective amine-catalyzed conjugate additions of aldehydes to oxetane- and azetidine-containing nitroolefins afford γ-nitroaldehydes as key building blocks en route to spirocyclic oxetane/azetidine-pyrrolidines. The study provided insights into the stability and reactivity of these β,β-disubstituted nitroolefins and enabled the enantioselective synthesis of chiral oxetanes and azetidines in moderate-to-high yields and enantioselectivities. This approach expands synthetic access to medicinally relevant scaffolds and broadens the scope of enantioselective organocatalytic transformations.

Advances in the Synthesis of Cyclopropylamines

Cyclopropylamines are an important subclass of substituted cyclopropanes that combine the unique electronic and steric properties of cyclopropanes with the presence of a donor nitrogen atom. In addition to their presence in a diverse array of biologically active compounds, cyclopropylamines are utilized as important synthetic intermediates, particularly in ring-opening or cycloaddition reactions. Consequently, the synthesis of these compounds has constituted a significant research topic, as evidenced by the abundant published synthetic methods. In addition to the widely used Curtius rearrangement, classical cyclopropanation methods have been adapted to integrate a nitrogen function (Simmons-Smith reaction, metal-catalyzed reaction of diazo compounds on olefins, Michael-initiated ring-closure reactions) with significant advances in enantioselective synthesis. More recently, specific methods have been developed for the preparation of the aminocyclopropane moiety (Kulinkovich reactions applied to amides and nitriles, addition to cyclopropenes, metal-catalyzed reactions involving C-H functionalization, ...). The topic of this review is to present the different methods for the preparation of cyclopropylamine derivatives, with the aim of covering the methodological advances as best as possible, highlighting their scope, their stereochemical aspects and future trends.

Catalytic Asymmetric Mannich-Type Reaction of α-Haloacetonitriles

An asymmetric Mannich-type addition of aldimines and haloacetonitriles is reported here, yielding halogenated aminonitriles with excellent stereoselectivity, facilitated by a pincer Ni(II) complex as a catalyst. Haloacetonitriles are recognized as reactive electrophiles, and the possibility of their use as a pronucleophile has been almost neglected for many years. The resulting adduct can be readily converted into various valuable derivatives, including chiral aziridines, starting from chlorinated compounds.

Organocatalyzed Asymmetric Michael Addition of 3-Fluorooxindole to Vinylidene Bisphosphonates

Both the 3-fluorooxindole and germinal bisphosphonate structural motifs are prevalent in bioactive molecules because of their associated biological activities. We describe an approach to accessing 3,3-disubstituted 3-fluorooxindoles bearing a geminal bisphosphate fragment through a highly enantioselective Michael addition reaction between 3-fluorooxindoles and vinylidene bisphosphonates. These reactions are catalyzed by a commercially available cinchona alkaloid catalyst, have a broad substrate scope concerning 3-fluorooxindoles, and provide the corresponding addition products in a yield of up to 95% with an enantiomeric excess of up to 95%. A reasonable reaction pathway to explain the observed stereochemistry is also proposed.

Organocatalytic Asymmetric Conjugate Addition of Fluorooxindoles to Quinone Methides

Fluorooxindoles undergo asymmetric Michael addition to para-quinone methides under phase-transfer conditions with 10 mol% of a readily available cinchona alkaloid ammonium catalyst. This reaction affords sterically encumbered, multifunctional fluorinated organic compounds displaying two adjacent chirality centers with high yields, ee's and dr's.

Enantioseparation and racemization of 3-fluorooxindoles

Fluorinated oxindoles are frequently used building blocks in asymmetric synthesis and represent an important scaffold found in a variety of biologically relevant compounds. While it is understood that incorporation of fluorine atoms into organic molecules can improve their pharmacological properties, the impact on the configurational stability of chiral organofluorines is still underexplored. In this study, semipreparative HPLC enantioseparations of five oxindoles were carried out, and the resulting enantiomerically enriched solutions were used to investigate base promoted racemization kinetics at room temperature. It was found that incorporation of fluorine at the chiral center increases the configurational stability, while substitutions on the aromatic ring and at the lactam moiety also have significant effects on the rate of racemization, which generally follows reversible first-order reaction kinetics.

Recent Advances in Greener Asymmetric Organocatalysis Using Bio-Based Solvents

Efficient synthetic methods that avoid the extensive use of hazardous reagents and solvents, as well as harsh reaction conditions, have become paramount in the field of organic synthesis. Organocatalysis is notably one of the best tools in building chemical bonds between carbons and carbon-heteroatoms; however, most examples still employ toxic volatile organic solvents. Although a portfolio of greener solvents is now commercially available, only ethyl alcohol, ethyl acetate, 2-methyltetrahydrofuran, supercritical carbon dioxide, ethyl lactate, and diethyl carbonate have been explored with chiral organocatalysts. In this review, the application of these bio-based solvents in asymmetric organocatalytic methods reported in the last decade is discussed, highlighting the proposed mechanism pathway for the transformations.

Selective Csp3-F Bond Functionalization with Lithium Iodide

A highly efficient method for C-F bond functionalization of a broad variety of activated and unactivated aliphatic substrates with inexpensive lithium iodide is presented. Primary, secondary, tertiary, benzylic, propargylic and α-functionalized alkyl fluorides react in chlorinated or aromatic solvents at room temperature or upon heating to the corresponding iodides which are isolated in 91-99% yield. The reaction is selective for aliphatic monofluorides and can be coupled with in situ nucleophilic iodide replacements to install carbon-carbon, carbon-nitrogen and carbon-sulfur bonds with high yields. Alkyl difluorides, trifluorides, even in activated benzylic positions, are inert under the same conditions and aryl fluoride bonds are also tolerated.

Catalytic Asymmetric Allylic Alkylation with Arylfluoroacetonitriles

A palladium(phosphinoxazoline) catalyzed method for asymmetric allylic alkylation of α-aryl-α-fluoroacetonitriles is introduced. This reaction achieves C-C bond formation and incorporation of two adjacent chirality centers with moderate to good yields, high enantioselectivities, and up to 15:1 dr.

Enantioselective Synthesis of Bispiro[indanedione-oxindole-cyclopropane]s through Organocatalytic [2+1] Cycloaddition

A series of compounds featuring a novel bispiro[indanedione-oxindole-cyclopropane] moiety have been synthesized through a squaramide-catalyzed [2+1] cycloaddition reaction. The tandem Michael-alkylation reaction of 2-arylidene-1,3-indanediones with 3-bromooxindoles furnished the cycloadducts in high yields with excellent diastereo- and enantioselectivities. The ammonium ylide in the catalytic process, as a key intermediate, was revealed by the high-resolution mass spectrometry study.

Synthesis of functionalized 3,2′-pyrrolidinyl spirooxindoles via domino 1,6-addition/annulation reactions of para-quinone methides and 3-chlorooxindoles

A highly efficient diastereoselective [4 + 1] cycloaddition of ortho-tosylaminophenyl-substituted p-QMs with 3-chlorooxindoles has been developed to afford 3,2′-pyrrolidinyl spirooxindoles.

Synthesis of oxindoles bearing a stereogenic 3-fluorinated carbon center from 3-fluorooxindoles

3,3-Disubstituted oxindoles bearing a stereogenic 3-fluorinated carbon center are privileged structural motifs present in many bioactive molecules. The straightforward functionalization of 3-fluorooxindoles constitutes a powerful method for the synthesis of 3-fully substituted 3-fluorooxindoles, taking advantage of the ease of preparation of 3-fluorooxindoles with different substitution patterns and the atom efficiency of chemical reactions. In the past decade, many papers have appeared on the synthesis of 3-fully substituted 3-fluorooxindoles from 3-fluorooxindoles. Importantly, many asymmetric catalytic methods have been developed for the enantioselective synthesis of these valuable compounds. This review summarizes the achievements in this area, and overviews synthetic opportunities that still exist.

Palladium and Nickel Catalyzed Suzuki Cross-Coupling with Alkyl Fluorides

Suzuki cross-coupling of benzylic and unactivated aliphatic fluorides with aryl- and alkenylboronic acids has been achieved via mechanistically distinct Pd and Ni catalyzed pathways that outperform competing protodeboronation, β-hydride elimination, and homocoupling processes. The utility is demonstrated with more than 20 examples including heterocyclic structures, 1,1-disubstituted and trans-1,2-disubstituted alkenes, and by the incorporation of acetonitrile into functionalized (hetero)arenes.

Enantioseparation and racemization of α-aryl-α-fluoroacetonitriles

The 2-Aryl-2-fluoroacetonitriles have garnered increasing interest as versatile building blocks in asymmetric synthesis. However, the configurational stability of these organofluorines is poorly understood and analytical methods that can be used to differentiate between their enantiomers remain underdeveloped. In this study, baseline high performance liquid chromatography (HPLC) enantioseparation of ten 2-aryl-2-fluoroacetonitriles was achieved by screening frequently used chiral stationary phases. While Chiralcel OD, Chiralpak AD, and Chiralpak AS proved to be most broadly useful, preparative separation of the enantiomers of 2-(2-naphthyl)-2-fluoroacetonitrile was possible on Chiralcel OJ. This enabled racemization studies at various temperatures and in the presence of organic bases which showed that this compound is configurationally stable under neutral conditions upon heating to 130°C for 6 h but undergoes complete racemization within 10 h in the presence of stoichiometric amounts of a guanidine base at room temperature. The racemization is likely to proceed via formation of an achiral keteniminate intermediate and obeys reversible first-order reaction kinetics with a half-life time of 87.7 min in ethanolic hexanes at 23.2°C. Racemization is significantly slower and occurs with a half-life time of 23.1 h at 22.4°C when the guanidine is replaced with a weaker amidine base.

Catalytic Asymmetric Allylic Amination with Isatins, Sulfonamides, Imides, Amines, and N-Heterocycles

A generally useful palladium-catalyzed method for the asymmetric allylic amination with a large variety of isatins, sulfonamides, imides, amines, and N-heterocycles is introduced. A single protocol with a readily available catalyst accomplishes this reaction at room temperature with high yields and enantioselectivities often exceeding 90%, which is demonstrated with 31 examples.

Application of E1cB Elimination in Asymmetric Organocatalytic Cascade Reactions To Construct Polyheterocyclic Compounds

By introducing a carbon functionality at 2-position of chromane, the formal asymmetric functionalization of the 3-position of 2-substituted chromane has been realized via a highly chemo-, regio-, and stereoselective organocatalytic cascade reaction in a sequential one-pot manner involving an E1cB mechanism governed ring-opening process. Critical to our success was the design of a chiral dipeptide-based bifunctional acid-base organocatalyst, which exhibited high catalytic activity at low catalyst loading (1-0.1 mol %), leading to biologically interesting polyheterocyclic compounds.

Enantioselective Mannich Reactions of 3-Fluorooxindoles with Cyclic N-Sulfamidate Aldimines

Both the 3-fluorooxindole and cyclic sulfamidate frameworks are important in medicinal chemistry owing to their associated biological activities. We report an approach accessing 3-fully substituted 3-fluorooxindoles, containing a benzo-fused sulfamidate subunit through highly enantioselective Mannich-type reactions between 3-fluorooxindoles and cyclic benzo-fused N-sulfamidate aldimines. These reactions are promoted by a commercially available cinchona alkaloid catalyst, accommodate a broad substrate scope, and deliver the desired products in a yield of up to 99% with an enantiomeric excess of up to 94%. A plausible reaction pathway is also presented.

Catalytic Enantioselective Ynamide Additions to Isatins: Concise Access to Oxindole Alkaloids

The highly enantioselective addition of terminal ynamides to a variety of isatins, catalyzed by a bisoxazolidine copper complex under mild, base-free reaction conditions, is described. The reaction is broad in scope, scalable, applicable to unprotected isatins, and provides efficient access to 3-hydroxyoxindoles carrying a tetrasubstituted chiral center with excellent yields and enantioselectivities. Synthetically versatile, multifunctional 3-hydroxyindolinones are obtained by hydration, partial hydrogenation, or hydroxyacyloxylation of the ynamide moiety at room temperature and exhaustive hydrogenation followed by reductive detosylation and spontaneous cyclization affords cinchonamidine alkaloids.

Organocatalytic asymmetric syntheses of 3-fluorooxindoles containing vicinal fluoroamine motifs

An organocatalytic Mannich reaction of 3-fluorooxindoles has been developed. Using a commercially available cinchona alkaloid catalyst, a wide range of 3-fluorooxindoles was successfully reacted with N-sulfonyl aldimines to give biologically important 3-fluorooxindoles containing vicinal fluoroamine motifs with high efficiency and good enantioselectivity. This protocol uses readily available reactants and cheap organocatalysts, and it is operationally simple.

Catalytic Asymmetric Mannich Reaction of α-Fluoronitriles with Ketimines: Enantioselective and Diastereodivergent Construction of Vicinal Tetrasubstituted Stereocenters

Diastereodivergent and enantioselective conversion of isatin ketimines to α-fluoro-β-aminonitriles with vicinal tetrasubstituted stereocenters is achieved by a chiral copper complex/guanidine base catalyzed Mannich reaction with proper choice of the bisphosphine ligand. The reaction is broad in scope, scalable, and provides efficient access to a series of 3-aminoindolinones exhibiting a quaternary carbon-fluorine stereocenter with high yields and stereoselectivities. Selective transformations of the Mannich reaction products into multifunctional 3-aminooxindoles without erosion of enantiomeric and diastereomeric purity highlight the synthetic utility.