B(C6F5)3/Chiral Phosphoric Acid Promoted Asymmetric C-3 gem-Difluoroalkylation of Quinoxalin-2-ones with Difluoroenoxysilanes

The asymmetric Mannich-type reaction of quinoxalin-2-ones with difluoroenoxysilanes has been developed for the synthesis of chiral gem-difluoroalkylated quinoxalin-2-ones. The reaction worked in the presence of chiral phosphoric acid CPA 1 and B(C6F5)3 in THF at room temperature. The reaction exhibited a good substrate scope furnishing the products in good yields (up to 97%) with up to 96% ee.

Cobalt-catalyzed decarboxylative difluoroalkylation of nitrophenylacetic acid salts

The selective installation of fluorine-containing groups into biologically relevant molecules has been used as a common strategy for the development of pharmaceutically active molecules. However, the selective incorporation of gem-difluoromethylene groups next to sterically demanding secondary and tertiary alkyl groups remains a challenge. Herein, we report the first cobalt-catalyzed regioselective difluoroalkylation of carboxylic acid salts. The reaction allows for the facile construction of various difluoroalkylated products in good yields tolerating a wide range of functionalities on either reaction partner. The potential of the method is illustrated by the late-stage functionalization of molecules of biological relevance. Mechanistic studies support the in situ formation of a cobalt(i) species and the intermediacy of difluoroalkyl radicals, thus suggesting a Co(i)/Co(ii)/Co(iii) catalytic cycle.

Chiral gem-difluoroalkyl reagents: gem-difluoroalkyl propargylic borons and gem-difluoroalkyl α-allenols

Chiral fluorinated reagents provide new opportunities for the discovery of drugs and functional materials because the introduction of a fluorinated group significantly alters a molecule's physicochemical properties. Chiral gem-difluoroalkyl fragments (R-CF2-C*) are key motifs in many drugs. However, the scarcity of synthetic methods and types of chiral gem-difluoroalkyl reagents limits the applications of these compounds. Herein, we report two types of chiral gem-difluoroalkyl reagents chiral gem-difluoroalkyl propargylic borons and gem-difluoroalkyl α-allenols and their synthesis by means of methods involving rhodium-catalyzed enantioselective B-H bond insertion reactions of carbenes and Lewis acid-promoted allenylation reactions. The mild, operationally simple method features a broad substrate scope and good functional group tolerance. These two types of reagents contain easily transformable boron and alkynyl or allenyl moieties and thus might facilitate rapid modular construction of chiral molecules containing chiral gem-difluoroalkyl fragments and might provide new opportunities for the discovery of chiral gem-difluoroalkyl drugs and other functional molecules.

Iron-Catalyzed Alkoxyl Radical-Induced C-C Bond Cleavage/gem-Difluoroalkylation Cascade

An inexpensive iron-catalyzed alkoxyl radical-induced C-C bond cleavage/gem-difluoroalkylation cascade is presented. Regulated by the structure of alkoxyl radical precursors, fluorinated distal diketones were synthesized through a ring-opening strategy and difluoroalkylated medium-sized lactones and macrolactones were constructed via a ring-expansion strategy. Both protocols proceeded under mild and redox neutral conditions with a broad substrate scope and good functional group compatibility.

2-Oxindole and related heterocycles: synthetic methodologies for their natural products and related derivatives

Natural goods, medications, and pharmaceutically active substances all contain substituted oxindoles. Generally, the C-3 stereocenter of the substituents of oxindoles and their absolute arrangement have a substantial impact on the bioactivity of these substances. In this case, the desire for contemporary probe and drug-discovery programs for the synthesis of chiral compounds using desirable scaffolds with high structural diversity further drives research in this field. Also, the new synthetic techniques are generally simple to apply for the synthesis of other similar scaffolds. Herein, we review the distinct approaches for the synthesis of diverse useful oxindole scaffolds. Specifically, the research findings on the naturally existing 2-oxindole core and a variety of synthetic compounds having a 2-oxindole core are discussed. We present an overview of the construction of oxindole-based synthetic and natural products. In addition, the chemical reactivity of 2-oxindole and its related derivatives in the presence of chiral and achiral catalysts are thoroughly discussed. The data compiled herein provides broad information related to the bioactive product design, development, and applications of 2-oxindoles and the reported techniques will be helpful for the investigation of novel reactions in the future.

A Cu-BOX catalysed enantioselective Mukaiyama-aldol reaction with difluorinated silyl enol ethers and acylpyridine N-oxides

A Cu(II)/BOX complex catalyses the enantioselective addition of difluorinated silyl enol ethers to acylpyridine N-oxides. The reaction provides difluorinated chiral tertiary alcohols of great interest in medicinal chemistry. These compounds are obtained in moderate to excellent yields and with high enantioselectivities. The stereochemical outcome of the reaction has been explained by DFT calculations.

Enantio-, Diastereo- and Regioselective Synthesis of Chiral Cyclic and Acyclic gem-Difluoromethylenes by Palladium-Catalyzed [4+2] Cycloaddition

gem-Difluoromethylene moieties are attractive in medicinal chemistry due to their ability to mimic other more ubiquitous functional groups. Thus, effective asymmetric methods for their construction are highly desirable, especially for the industrial production of chiral drugs. Using a Pd-catalyzed asymmetric [4+2] cycloaddition between substituted-2-alkylidenetrimethylene carbonates and gem-difluoroalkyl ketones, we were able to easily access chiral 1,3-dioxanes that contain a tetrasubstituted difluoroalkyl stereogenic center in cyclic and acyclic skeletons. A novel phosphoramidite ligand, which contains a bulky 1,1-dinaphthylmethanamino moiety, was developed to provide the products in high yield with excellent enantio-, diastereo-, and regioselectivity. Strikingly, the gem-difluoro substitution pattern promotes the reaction, and pentafluoroethylketone, an α,α-difluorinated β-ketoester, and a β-ketosulfone are suitable substrates for this method.

Divergent Reaction of Activated Pyridines with α,α-Difluorinated gem-Diols: Regioselective Synthesis of gem-Difluorinated Dihydropyridines and Dihydropyridones

The nucleophilic reactivity of α,α-difluorinated gem-diols toward activated pyridinium salts has been capitalized in a highly regioselective fashion, offering biologically relevant 1,4-dihydropyridines and 3,4-dihydro-2-pyridones adorned with the valuable gem-difluoromethylene motif. The protocol is scalable as well as high yielding and accommodates a broad range of substrates and functional groups. Additionally, the synthesis of difluorinated oxa-azabicyclo[3.3.1]nonane frameworks has been showcased through product diversification.

Transaminases as suitable catalysts for the synthesis of enantiopure β,β-difluoroamines

Transaminases have shown the ability to catalyze the amination of a series of aliphatic and (hetero)aromatic α,α-difluorinated ketones with high stereoselectivity, thus providing the corresponding β,β-difluoroamines in high isolated yields (55-82%) and excellent enantiomeric excess (>99%). It was also observed that these activated substrates could be quantitatively transformed by employing a small molar excess of the amine donor since this amination process was thermodynamically favored. Selected transformations could be scaled up to 500 mg, showing the robustness of this methodology.

Direct C(sp3)−H Difluoromethylation via Radical-Radical Cross-Coupling by Visible-Light Photoredox Catalysis

Herein, the radical-radical cross-coupling strategy for direct difluoromethylation of C(sp3)−H bond is reported. This transformation was readily accomplished under transition metal-free photoredox catalysis in the presence of 3 mol% of...

Regio- and enantioselective umpolung gem-difluoroallylation of hydrazones via palladium catalysis enabled by N-heterocyclic carbene ligand

The enantioselective construction of C-CF₂R (R: alkyl or fluoroalkyl) bonds has attracted the attention of synthetic chemists because of the importance of chiral fluorinated compounds in life and materials sciences. Catalytic asymmetric fluoroalkylation has mainly been realized under organocatalysis and Lewis acid catalysis, with substrates limited to carbonyl compounds. Few examples using transition-metal catalysis exist, owing to side reactions including decomposition and isomerization of fluoroalkylating reagents. Herein we report umpolung asymmetric difluoroallylation of hydrazones with 3-bromo-3,3-difluoropropene (BDFP) under palladium catalysis. Difluoroallylation products having quaternary chiral carbon centers are afforded in good yields with high α/γ- and enantioselectivities. The usefulness of the reaction products is demonstrated and an inner-sphere mechanism of the reaction is proposed. The use of chiral N-heterocyclic carbene as ligand is the key for the selectivities as well as the productivity of the reaction.

Rapid incorporation of a difluoroacetate radical into para-quinone methides via radical 1,6-conjugate addition

Presented herein is a newly designed strategy that rapidly introduces ethyl difluoroacetate radicals through a dialkylzincs induced radical 1,6-conjugate addition pathway. Besides achieving high yields and excellent functional group compatibility, this protocol allowed the incorporation of a gem-difluoromethylene motif to be accomplished within minutes.

Stereoselective synthetic strategies of stereogenic carbon centers featuring a difluoromethyl group

The scope of this review is to summarize routine asymmetric synthetic methods which enable the effective and selective introduction of difluoromethyl groups into the desired compounds, providing a general introduction to this important research area.

Catalytic asymmetric synthesis of monofluoroalkenes and gem-difluoroalkenes: advances and perspectives

The latest achievements in the catalytic asymmetric synthesis of both monofluoro- and gem-difluoroalkenes are discussed.

Synthesis of Chiral gem-Difluoromethylene Compounds by Enantioselective Ethoxycarbonyldifluoromethylation of MBH Fluorides via Silicon-Assisted C-F Bond Activation

The enantioselective ethoxycarbonyldifluoromethylation of Morita-Baylis-Hillman (MBH) fluorides with Me₃SiCF₂CO₂Et under organocatalysis is described. Moderately functionalized chiral gem-difluoromethylene compounds with a stereogenic "C-CF₂-C*" unit were synthesized in high yields with high enantioselectivities. The initial C-F bond activation is assisted by the silicon atom via a dual S_N2'-S_N2' stepwise pathway. Dynamic kinetic resolution of the MBH-fluorides explained the high yields and high ee's of the products. The method was extended to the enantioselective introduction of "Het-CF₂" units.

Synthesis of Enantioenriched α,α-Difluoro-β-arylbutanoic Esters by Pd-Catalyzed Asymmetric Hydrogenation

Synthesis of optically active gem-difluorinated organic molecules attracts a great deal of interest due to their unique properties in pharmaceutical and agrochemical areas. Herein, a series of enantioenriched α,α-difluoro-β-arylbutanoic esters were prepared in high yields (83-99%) with moderate to excellent enantioselectivities (≤97:3 er) by palladium-catalyzed asymmetric hydrogenation.

Regio- and Enantioselective Bromocyclization of Difluoroalkenes as a Strategy to Access Tetrasubstituted Difluoromethylene-Containing Stereocenters

Difluoromethylene-containing compounds have attracted substantial research interest over the past decades for their ability to mimic biological functions of traditional functional groups while providing a wide variety of pharmacological benefits bestowed by the C-F bond. We report a novel strategy to access RCF2Br-containing heterocycles by regio- and enantioselective bromocyclization of difluoroalkenes enabled by chiral anion phase-transfer catalysis. The utility of this methodology was highlighted through a synthesis of an analogue of efavirenz, a drug used for treating HIV. Additionally, the synthetic versatility of the CF2Br intermediates was showcased through functionalization to a variety of enantioenriched α,α-difluoromethylene-containing products.

H-bond donor-directed switching of diastereoselectivity in the Michael addition of α-azido ketones to nitroolefins

The development of catalyst-controlled stereodivergent asymmetric catalysis is important for providing facile access to all stereoisomers of chiral products with multiple stereocenters from the same starting materials. Despite progress, new design strategies for diastereodivergent asymmetric catalysis are still highly desirable. Here we report the potency of H-bond donors as the governing factor to tune diastereoselectivity in a highly diastereoselective switchable enantioselective Michael addition of α-azido ketones to nitroolefins. While a newly developed bifunctional tertiary amine, phosphoramide, preferentially afforded syn-adducts, an analogous squaramide catalyst selectively gave anti-adducts. The resulting multifunctional tertiary azides can be converted to spiro-pyrrolidines with four continuous stereocenters in a one-pot operation. Mechanistic studies cast light on the control of diastereoselectivity by H-bond donors. While the squaramide-catalyzed reaction proceeded with a transition state with both squaramide N–H bonds binding to an enolate intermediate, an unprecedented model was proposed for the phosphoramide-mediated reaction wherein an amide N–H bond and an alkylammonium ion formed in situ interact with nitroolefins, with the enolate stabilized by nonclassical C–H⋯O hydrogen-bonding interactions.

We report the successful reversal of the diastereoselectivity in an unprecedented Michael addition of α-azido ketones to nitroolefins catalyzed by bifunctional tertiary amines, simply by varying the H-bond donor from phosphoramide to squaramide.

Synthesis of difluoroalkylated 2-azaspiro[4.5]decane derivatives via copper-catalyzed difluoroalkylation/dearomatization of N-benzylacrylamides

A copper-catalyzed synthesis of difluoroalkylated spiro-azacycles from N-benzylacrylamides is presented. The reaction involves the β-difluoroalkylation of acrylamide, 5-exo cyclization, and dearomatization.

Synthesis of gem-Difluoromethylene Containing Cycloalkenes via the Ring-Opening Reaction of gem-Difluorocyclopropanes and Subsequent RCM Reaction

The radical-type ring-opening reaction of gem-difluorocyclopropanes and subsequent regioselective monoepoxidation of the products were demonstrated. Introduction of a vinyl or allyl group to the epoxide produced the diene derivatives that were subjected to the ring closing metathesis reaction to furnish the gem-difluoromethylene containing cyclopentene, cycloheptene, and cyclooctene derivatives in good to excellent yields.

Transition-metal-free N-difluoromethylation of hydrazones with TMSCF2Br as the difluoromethylation reagent

A transition-metal-free N-difluoromethylation protocol is developed to synthesize a series of N-difluoromethyl hydrazones with TMSCF₂Br as the difluoromethylation reagent.

A Sc(OTf)3 catalyzed Mukaiyama–Mannich reaction of difluoroenoxysilanes with unactivated ketimines

The first example of a Mukaiyama–Mannich reaction of difluoroenoxysilanes with unactivated ketimines catalyzed by Sc(OTf)₃ is described.

Asymmetric synthesis of polycyclic 3-fluoroalkylproline derivatives by intramolecular azomethine ylide cycloaddition

The asymmetric intramolecular dipolar cycloaddition of azomethine ylides was developed for fluorinated dipolarophiles, being the RF group crucial for selectivity.

Catalytic selective mono- and difluoroalkylation using fluorinated silyl enol ethers

This feature article highlights our recent achievements in catalytic selective mono- and difluoroalkylation using fluorinated silyl enol ethers (FSEEs).

Dual Catalytic Switchable Divergent Synthesis: An Asymmetric Visible-Light Photocatalytic Approach to Fluorine-Containing γ-Keto Acid Frameworks

Herein, we describe a novel and efficient method for constructing a series of fluorine-containing γ-keto acid derivatives through combining visible-light photoredox catalysis and chiral Lewis acid catalysis. With this dual catalytic strategy, a variety of chiral γ-keto amides containing a gem-difluoroalkyl group and a series of fluorine-containing α,β-unsaturated-γ-keto esters were successfully constructed with high stereoselectivities, respectively. A series of experiments showed that the chemoselectivity of this process was highly dependent on the fluorine reagents besides the Lewis acid catalysts. This approach facilitates rapid access to γ-keto acid derivatives, an important class of precursors for pharmaceuticals, plasticizers, and various other additives.

Synthesis of difluoromethylated enynes by the reaction of α-(trifluoromethyl)styrenes with terminal alkynes

A novel and efficient method for the synthesis of difluoromethylated enynes by the reaction of α-(trifluoromethyl)styrenes with terminal alkynes with the assistance of NaOtBu was described. The mechanism of the reaction might involve the SN2' reaction of α-(trifluoromethyl)styrenes and a subsequent 1,3-H shift. Isomerization (E → Z) of 1-difluoromethyl-1,3-enynes in the presence of ZrCl4 was also developed.

Divergent Reactivity of gem-Difluoro-enolates toward Nitrogen Electrophiles: Unorthodox Nitroso Aldol Reaction for Rapid Synthesis of α-Ketoamides

An amination reaction of in situ generated gem-difluoro-enolates has been explored with electrophilic nitrogen sources. While their exposure to azodicarboxylates smoothly produced fluorinated α-amino ketones, reaction with nitrosoarenes (nitroso aldol reaction) furnished α-ketoamides in very high yields (up to 94%). The reaction is very fast (typically completed within 5 min) and scalable and tolerates various sensitive functional groups. Synthetic utility of this process was highlighted through the production of diverse nitrogen heterocycles and an orexin receptor antagonist.

Development of Synthetic Methodologies via Catalytic Enantioselective Synthesis of 3,3-Disubstituted Oxindoles

3,3-Disubstituted oxindoles are widely distributed in natural products, drugs, and pharmaceutically active compounds. The absolute configuration and the substituents on the fully substituted C3 stereocenter of the oxindole often significantly influence the biological activity. Therefore, tremendous efforts have made to develop catalytic enantioselective syntheses of this prominent structural motif. Research in this area is further fueled by the ever-increasing demand for modern probe- and drug-discovery programs for synthetic libraries of chiral compounds that are derived from privileged scaffolds with high structural diversity. Notably, the efficient construction of fully substituted C3 stereocenters of oxindole, tetrasubstituted or all-carbon quaternary, spirocyclic or not, also becomes a test ground for new synthetic methodologies. We have been engaged in developing efficient methods for diversity-oriented synthesis of chiral 3,3-disubstituted oxindoles from readily available starting materials. We have systematically developed catalytic enantioselective methods to prepare 3-substituted 3-hydroxyoxindoles, 3-aminooxindoles, and 3-thiooxindoles, quaternary oxindoles, and spirocyclic oxindoles. These protocols can be classified into six approaches: (1) enantioselective addition of nucleophiles to isatins or isatin ketimines; (2) unprotected 3-substituted oxindoles as nucleophiles; (3) functionalization of oxindole-derived tetrasubstituted alkenes; (4) desymmetrization of oxindole-based diynes; (5) spirocyclopropyl oxindoles as donor-acceptor (D-A) cyclopropanes; and (6) elaboration of diazooxindoles. By the use of these methods, chiral oxindoles with rich structural diversity are readily accessed with high to excellent enantioselectivity. Some methods have been used for the enantioselective formal or total synthesis of natural products, bioactive compounds, or their analogues. On the basis of these studies, we developed synthetic methodologies that have potential application. We designed phosphoramide-based bifunctional catalysts for the efficient construction of quaternary oxindoles: a cinchona-alkaloid-derived phosphoramide for the Michael addition of unprotected 3-substituted oxindoles to nitroolefins with broad substrate scope and a chiral 1,2-cyclohexanediamine-derived bifunctional phosphoramide for the activation of fluorinated enol silyl ethers for the addition to isatylidene malononitrile. The phosphoramide-based catalysts achieved better enantiofacial control than the analogous H-bond-donor-derived catalysts in these reactions, suggesting the potential of the former in new chiral catalyst development. We identified chiral Au(I) and Hg(II) catalysts for olefin cyclopropanation of diazooxindoles. We further disclosed the effective activation of spirocyclopropyl oxindoles by using electron-withdrawing N-protecting groups for enantioselective [3 + 3] cycloaddition, offering the promise of constructing a diverse range of spirocyclic oxindoles by the use of such monoactivated D-A cyclopropanes. We developed tandem sequences that allow the facile synthesis of 3,3-disubstituted oxindoles from simple starting materials in a one-pot operation, including a tandem Morita-Baylis-Hillman/bromination/[3 + 2] annulation sequence, a hydrogenation/ketimine formation/asymmetric 6π electrocyclization sequence, a C-H functionalization/Michael addition or amination sequence, and an aza-Wittig/Strecker sequence. We designed oxindole-based diynes to realize a highly enantioselective Cu-catalyzed alkyne-azide cycloaddition (CuAAC), outlining the desymmetrization of prochiral diynes as an effective strategy to exploit asymmetric CuAAC. This Account focuses on the synthetic methodologies developed in our group for the catalytic enantioselective synthesis of 3,3-disubstituted oxindoles and provides an overview of our research on the design, development, and applications of these methods that will provide useful insights for the exploration of new reactions.

Catalytic enantioselective synthesis of cyclopropanes featuring vicinal all-carbon quaternary stereocenters with a CH2F group; study of the influence of C–F⋯H–N interactions on reactivity

A highly diastereo- and enantioselective synthesis of cyclopropanes with adjacent all-carbon quaternary stereocenters featuring a CH₂F group is reported.

Neighboring phenolic group-activated gem-difluoroallylboration of imines for the catalyst-free synthesis of gem-difluorohomoallylamines

An unprecedented imine allylation with pinacol gem-difluoroallylborates for the synthesis of a wide range of racemic gem-difluorohomoallylamine derivatives under catalyst-free conditions was enabled by a neighboring phenolic group in an N-protecting group.

A convenient approach to difluoromethylated all-carbon quaternary centers via Ni(ii)-catalyzed enantioselective Michael addition

A Ni(ii)-catalyzed enantioselective Michael addition of 2-acetyl azarenes with β-difluoromethyl substituted nitroalkenes was successfully realized, which afforded chiral CF₂H-containing compounds in good enantioselectivities (up to 93% ee). This protocol provides a new convenient approach to all-carbon quaternary stereogenic centers featuring a CF₂H group.

Ni(ii)-catalyzed enantioselective Michael addition afforded compounds with all-carbon quaternary stereogenic centers featuring a CF₂H group in good enantioselectivities.

A general and efficient Lewis acid catalysed Mukaiyama-aldol reaction of difluoroenoxysilanes and ketones

We report a general and highly efficient Mukaiyama-aldol reaction of ketones and difluoroenoxysilanes. While the reaction of aryl ketones worked efficiently in the presence of Bi(OTf)₃, that of aliphatic ketones required the use of Sc(OTf)₃. In addition, Sc(OTf)₃ was capable of achieving excellent 1,2-selectivity in the corresponding reaction of α,β-unsaturated ketones. This method provides a facile access to differently substituted β-hydroxy α,α-difluoro ketones, versatile synthons for difluomethylated tertiary alcohols.

Silver-mediated oxidative C–H difluoromethylation of phenanthridines and 1,10-phenanthrolines

An efficient silver-mediated oxidative C–H difluoromethylation of phenanthridines and 1,10-phenanthrolines with TMSCF₂H was developed.