Methoden der Reaktivitätsumpolung

Preparation of Functionalized Amides Using Dicarbamoylzincs

We report a new convenient preparation of dicarbamoylzincs of type (R¹R²NCO)₂Zn by the treatment of ZnCl₂ and formamides R¹R²NCHO with LiTMP in THF (15 °C, 15 min) or by the reaction of formamides R¹R²NCHO with TMP₂Zn (25 °C, 16 h). This second method tolerates sensitive groups such as an ester, ketone or nitro function. Reaction of these dicarbamoylzincs with allylic, benzylic, aryl, alkenyl bromides, acid chlorides, aldehydes or enones provided various polyfunctional amides in 47–97 % yields. ¹³C NMR characterization of these new carbamoylzinc derivatives is reported.

NHC Catalysis for Umpolung Pyridinium Alkylation via Deoxy‐Breslow Intermediates

Umpolung N‐heterocyclic carbene (NHC) catalysis of non‐aldehyde substrates offers new pathways for C−C bond formation, but has proven challenging to develop in terms of viable substrate classes. Here, we demonstrate that pyridinium ions can undergo NHC addition and subsequent intramolecular C−C bond formation through a deoxy‐Breslow intermediate. The alkylation demonstrates, for the first time, that deoxy‐Breslow intermediates are viable for catalytic umpolung of areniums.

NHC Catalysis for Umpolung Pyridinium Alkylation via Deoxy-Breslow Intermediates

Umpolung N-heterocyclic carbene (NHC) catalysis of non-aldehyde substrates offers new pathways for C-C bond formation, but has proven challenging to develop in terms of viable substrate classes. Here, we demonstrate that pyridinium ions can undergo NHC addition and subsequent intramolecular C-C bond formation through a deoxy-Breslow intermediate. The alkylation demonstrates, for the first time, that deoxy-Breslow intermediates are viable for catalytic umpolung of areniums.

Deoxygenative Cross-Coupling of Aromatic Amides with Polyfluoroarenes

Considering the ubiquitous nature and ready synthesis of amides, and the great significance of organofluorine-containing species, the cross-coupling of amides and polyfluoroarenes, leading to new carbon-carbon bond-forming methodologies, would find useful applications in synthesis, late-stage functionalization, and rapid generation of molecular diversity. Herein, we present a novel synthesis of α-polyfluoroaryl amines via Sm/SmI₂ -mediated deoxygenative cross-coupling of aromatic amides with polyfluoroarenes through direct C-H functionalization. The structural and functional diversity of these readily available precursors provides a versatile and flexible strategy for the streamlined synthesis of α-polyfluoroaryl amines. Combining experimental and theoretical studies, a novel plausible mechanism of the α-aminocarbene-mediated C-H insertion has been revealed, which may stimulate future work for the development of novel methods in amine synthesis.

Excited-State Copper Catalysis for the Synthesis of Heterocycles

Heterocycles are one of the largest groups of organic moieties with significant medicinal, chemical, and industrial applications. Herein, we report the discovery and development of visible-light-induced, synergistic excited-state copper catalysis using a combination of Cu(IPr)I as a catalyst and rac-BINAP as a ligand, which produces more than 10 distinct classes of heterocycles. The reaction tolerates a broad array of functional groups and complex molecular scaffolds, including derivatives of peptides, natural products, and marketed drugs. Preliminary mechanistic investigation suggests in situ generations of [Cu(BINAP)₂ ]⁺ and [Cu(IPr)₂ ]⁺ catalysts that work cooperatively under visible-light irradiation to facilitate catalytic carbo-aroylation of unactivated alkenes, affording a wide range of useful heterocycles.

A masked form of an O-borylated Breslow intermediate for the diastereoselective FLP-type activation of aldehydes

Breslow intermediates are very often elusive species whose application in Frustrated Lewis Pair chemistry is unprecedented. We describe herein the use of a masked form of an O-Borylated Breslow (OBB) intermediate that performs FLP-type activation of the carbonyl function of five different benzaldehyde derivatives with complete diastereoselectivity. The resulting compounds are characterised in solution by NMR spectroscopy (compounds 4 - 8 ) and in solid state by X-Ray diffraction analysis (compounds 4 - 6 ). A combined kinetic and theoretical investigation reveals the associative nature of the rate determining step and suggests that the OBB intermediate part is never released during the whole process.

π-Lewis-Base-Catalyzed Asymmetric Vinylogous Umpolung Reactions of Cyclopentadienones and Tropone

We disclose that the carbonates of 4-hydroxy-2-cyclopentenones can form π-allylpalladium-based 1,2-carbodipoles, which isomerize to interesting η² -Pd⁰ -cyclopentadienone complexes. Compared with the labile parent cyclopentadienone, the HOMO energy of the related η² -complex was significantly raised via the back-bonding of Pd⁰ as a π-Lewis base, rendering the uncoordinated C=C bond an electron-richer dienophile in inverse-electron-demand aza-Diels-Alder-type reactions with diverse 1-azadienes. The vinylogous (aza)Morita-Baylis-Hillman or cross Rauhut-Currier addition to (imine)carbonyls or activated alkenes, respectively, was also realized to afford chiral [4+2] or [2+2] cycloadducts, respectively, after trapping the re-generated π-allylpalladium species. New C₁ -symmetric ligands from simple chiral sources were developed, exhibiting high stereoselectivity even with racemic substrates via an unusual dynamic kinetic resolution process. Besides, tropone could be similarly activated by a Pd⁰ complex.

Recent Progress in Enolonium Chemistry under Metal-Free Conditions

Umpolung approach through inversion of the polarity of conventional enolates, has opened up an unprecedented opportunity in the cross-coupling via alkylation. The enolonium equivalents can be accessed either by hypervalent iodine reagents, activation/oxidation of amides, or the oxidation of alkynes. Under umpolung conditions, highly basic conditions required for classical enolate chemistry can be avoided, and they can couple with unmodified nucleophiles such as heteroatom donors and electron-rich arenes.

Merging Electron Transfer with 1,2-Metalate Rearrangement: Deoxygenative Arylation of Aromatic Amides with Arylboronic Esters

Amides are essentially inert carboxyl derivatives in many types of chemical transformations. In particular, deoxygenative C-C bond formation of amides to synthetically important amines is a long-standing challenge for synthetic chemists due to the inertness of the resonance-stabilized amide C=O bond. Herein, it is disclosed that by merging electron-transfer-induced activation with 1,2-metalate rearrangement, a wide range of aromatic amides react smoothly with arylboron reagents, affording a series of biologically relevant diarylmethylamines as deoxygenative C-C bond cross-coupling products. With its simplicity and versatility, this reaction shows great promise in the synthesis of amines from amides, which may open up new avenues in retrosynthetic planning and find widespread use in academia and industry.

Generation of Diazomethyl Radicals by Hydrogen Atom Abstraction and Their Cycloaddition with Alkenes

A general catalytic methodology for the synthesis of pyrazolines from α-diazo compounds and conjugated alkenes is reported. The direct hydrogen atom transfer (HAT) process of α-diazo compounds promoted by the tert-butylperoxy radical generates electrophilic diazomethyl radicals, thereby reversing the reactivity of the carbon atom attached with the diazo group. The regiocontrolled addition of diazomethyl radicals to carbon-carbon double bonds followed by intramolecular ring closure on the terminal diazo nitrogen and tautomerization affords a diverse set of pyrazolines in good yields with excellent regioselectivity. This strategy overcomes the limitations of electron-deficient alkenes in traditional dipolar [3+2]-cycloaddition of α-diazo compounds with alkenes. Furthermore, the straightforward formation of the diazomethyl radicals provides umpolung reactivity, thus opening new opportunities for the versatile transformations of diazo compounds.

Enantioselective α-Etherification of Branched Aldehydes via an Oxidative Umpolung Strategy

Saturated carbonyl compounds are, via their enolate analogues, inherently nucleophilic at the α-position. In the presence of a benzoquinone oxidant, the polarity of the α-position of racemic α-branched aldehydes is inverted, allowing for an enantioselective etherification using readily available oxygen-based nucleophiles and an amino acid-derived primary amine catalyst. A survey of benzoquinone oxidants identified p-fluoranil and DDQ as suitable reaction partners. p-Fluoranil enables the preparation of α-aryloxylated aldehydes using phenol nucleophiles in up to 91 % ee, following either a one-step or a two-step, one-pot protocol. DDQ allows for a more general etherification protocol in combination with a broader range of alcohol nucleophiles with enantioselectivities up to 95 % ee. Control experiments and isolation of a key quinol intermediate supports a mechanism proceeding via an S_N 2 dynamic-kinetic resolution. These studies provide the basis for an aminocatalytic umpolung concept that allows for the asymmetric construction of tertiary ethers in the α-position of aldehydes.

Catalytic Generation of Carbanions through Carbonyl Umpolung

Carbonyl Umpolung is a powerful strategy in organic chemistry to construct complex molecules. Over the last few years, versatile catalytic approaches for the generation of acyl anion equivalents from carbonyl compounds have been developed, but methods to obtain alkyl carbanions from carbonyl compounds in a catalytic fashion are still at an early stage. This Minireview summarizes recent progress in the generation of alkyl carbanions through catalytic carbonyl Umpolung. Two different catalytic approaches can be utilized to enable the generation of alkyl carbanions from carbonyl compounds: the catalytic Wolff–Kishner reaction and the catalytic single‐electron reduction of carbonyl compounds and imines. We discuss the reaction scope, mechanistic insights, and synthetic applications of the methods as well as potential future developments.

The Reactivity of Isomeric Nitrenium Lewis Acids with Phosphines, Carbenes, and Phosphide

Alkylation of spiro[fluorene‐9,3’‐indazole] at N(1) and N(2) with tBuCl affords the nitrenium cations [C₆H₄N₂(tBu)C(C₁₂H₈)][BF₄], 1 and 2, respectively. Compound 1 converts to 2 over the temperature range 303–323 K with a free energy barrier of 28±5 kcal mol⁻¹. Reaction of 1 with PMe₃ afforded the N‐bound phosphine adduct [C₆H₄N(tBu)N(PMe₃)C(C₁₂H₈)]BF₄] 3. However, phosphines attack 2 at the para‐carbon atom of the aryl group with concurrent cleavage of N(2)−C(1) bond and proton migration to C(1) affording [(R₃P)C₆H₃NN(tBu)CH(C₁₂H₈)][BF₄] (R=Me 4, nBu 5). Analogous reactions of 1 and 2 with the carbene SIMes prompt attack at the para‐carbon with concurrent loss of H^. affording the radical cation salts [(SIMes)C₆H₃N(tBu)NC(C₁₂H₈)^.][BF₄] 6 and [(SIMes)C₆H₃NN(tBu)C(C₁₂H₈)^.][BF₄] 7, whereas reaction of 2 with BAC gives the Lewis acid‐base adduct, [C₆H₄N(BAC)N(tBu)C(C₁₂H₈)][BF₄] 8. Finally, reactions of 1 and 2 with KPPh₂ result in electron transfer affording (PPh₂)₂ and the persistent radicals C₆H₄N(tBu)NC(C₁₂H₈)^. and C₆H₄NN(tBu)C(C₁₂H₈)^.. The detailed reaction mechanisms are also explored by extensive DFT calculations.

Umpolung of donor–acceptor cyclopropanes via N-heterocyclic carbene organic catalysis

A carbene-catalyzed formal umpolung of donor–acceptor (D–A) cyclopropanes is disclosed with chiral spirocyclic lactones bearing multiple functional groups afforded with excellent enantio- and diastereoselectivities.

Photocatalytic Reductive Radical-Polar Crossover for a Base-Free Corey-Seebach Reaction

A metal-free generation of carbanion nucleophiles is of prime importance in organic synthesis. Herein we report a photocatalytic approach to the Corey-Seebach reaction. The presented method operates under mild redox-neutral and base-free conditions giving the desired product with high functional group tolerance. The reaction is enabled by the combination of photo- and hydrogen atom transfer (HAT) catalysis. This catalytic merger allows a C-H to carbanion activation by the abstraction of a hydrogen atom followed by radical reduction. The generated nucleophilic intermediate is then capable of adding to carbonyl electrophiles. The obtained dithiane can be easily converted to the valuable α-hydroxy carbonyl in a subsequent step. The proposed reaction mechanism is supported by emission quenching, radical-radical homocoupling and deuterium labeling studies as well as by calculated redox-potentials and bond strengths.

An α-Cyclopropanation of Carbonyl Derivatives by Oxidative Umpolung

The reactivity of iodine(III) reagents towards nucleophiles is often associated with umpolung and cationic mechanisms. Herein, we report a general process converting a range of ketone derivatives into α-cyclopropanated ketones by oxidative umpolung. Mechanistic investigation and careful characterization of side products revealed that the reaction follows an unexpected pathway and suggests the intermediacy of non-classical carbocations.

Access to Vinyl Ethers and Ketones with Hypervalent Iodine Reagents as Oxy-Allyl Cation Synthetic Equivalents

We report an Umpolung strategy of enol ethers to generate oxy-allyl cation equivalents based on the use of hypervalent iodine reagents. Under mild basic conditions, the addition of nucleophiles to aryloxy-substituted vinylbenziodoxolone (VBX) reagents, easily available in two steps from silyl alkynes, resulted in the stereoselective formation of substituted aryl enol ethers. The reaction was most efficient with phenols as nucleophiles, but preliminary results were also achieved for C- and N- nucleophiles. In absence of external nucleophiles, the 2-iodobenzoate group of the reagent was transferred. The obtained aryl enol ethers could then be transformed into α-difunctionalized ketones by oxidation. The described "allyl cation"-like reactivity contrast with the well-established "vinyl-cation" behavior of alkenyl iodonium salts.

Direct Umpolung Morita-Baylis-Hillman like α-Functionalization of Enones via Enolonium Species

Herein we report on the umpolung of Morita-Baylis-Hillman type intermediates and application to the α-functionalization of enone C-H bonds. This reaction gives direct access to α-chloro-enones, 1,2-diketones and α-tosyloxy-enones. The latter are important intermediates for cross-coupling reaction and, to the best of our knowledge, cannot be made in a single step from enones in any other way. The proposed mechanism is supported by spectroscopic studies. The key initial step involves conjugate attack of an amine (DABCO or pyridine), likely assisted by hypervalent iodine acting as a Lewis acid leading to formation of an electrophilic β-ammonium-enolonium species. Nucleophilic attack by acetate, tosylate, or chloride anion is followed by base induced elimination of the ammonium species to give the noted products. Hydrolysis of α-acetoxy-enones lead to formation of 1,2-diketones. The α-tosyl-enones participate in Negishi coupling reactions under standard conditions.

B-B Bond Nucleophilicity in a Tetraaryl μ-Hydridodiborane(4) Anion

The tetraaryl μ‐hydridodiborane(4) anion [2H]⁻ possesses nucleophilic B−B and B−H bonds. Treatment of K[2H] with the electrophilic 9‐H‐9‐borafluorene (HBFlu) furnishes the B₃ cluster K[3], with a triangular boron core linked through two BHB two‐electron, three‐center bonds and one electron‐precise B−B bond, reminiscent of the prominent [B₃H₈]⁻ anion. Upon heating or prolonged stirring at room temperature, K[3] rearranges to a slightly more stable isomer K[3 a]. The reaction of M[2H] (M⁺=Li⁺, K⁺) with MeI or Me₃SiCl leads to equimolar amounts of 9‐R‐9‐borafluorene and HBFlu (R=Me or Me₃Si). Thus, [2H]⁻ behaves as a masked [:BFlu]⁻ nucleophile. The HBFlu by‐product was used in situ to establish a tandem substitution‐hydroboration reaction: a 1:1 mixture of M[2H] and allyl bromide gave the 1,3‐propylene‐linked ditopic 9‐borafluorene 5 as sole product. M[2H] also participates in unprecedented [4+1] cycloadditions with dienes to furnish dialkyl diaryl spiroborates, M[R₂BFlu].

One Amine-3 Tasks: Reductive Coupling of Imines with Olefins in Batch and Flow

Owing to their wide range of biological properties, γ-aminobutyric acid derivatives (GABA) have been extensively studied and found noteworthy industrial applications. However, atom-economical and efficient processes for their production are scarce and would greatly benefit from further investigations. Herein, we demonstrate that an iridium-based photocatalyst promotes the direct reductive cross-coupling of imines with olefins upon irradiation with visible light to give GABA derivatives in good yields and selectivities. We also stress the enabling triple role of tributylamine additive in this process, discuss the advantages of strategies based on proton-coupled electron transfer (PCET) and demonstrate the scale-up of this reaction in continuous flow.

Asymmetric Synthesis of 1,4-Dicarbonyl Compounds from Aldehydes by Hydrogen Atom Transfer Photocatalysis and Chiral Lewis Acid Catalysis

Enantioenriched 1,4-dicarbonyl compounds are versatile synthons in natural product and pharmaceutical drug synthesis. We herein report a mild pathway for the efficient enantioselective synthesis of these compounds directly from aldehydes through synergistic cooperation between a neutral eosin Y hydrogen atom transfer photocatalyst and a chiral rhodium Lewis acid catalyst. This method is distinguished by its operational simplicity, abundant feedstocks, atom economy, and ability to generate products in high yields (up to 99 %) and high enantioselectivity (up to 99 % ee).

Catalytic Enantioselective Synthesis of 1,4-Keto-Alkenylboronate Esters and 1,4-Dicarbonyls

A catalytic enantioselective method for the synthesis of 1,4-keto-alkenylboronate esters by a rhodium-catalyzed conjugate addition pathway is disclosed. A variety of novel, bench-stable alkenyl gem-diboronate esters are synthesized. These easily accessible reagents react smoothly with a collection of cyclic α,β-unsaturated ketones, generating a new C-C bond and stereocenter. Products are isolated in up to 99 % yield with greater than 20:1 E/Z and greater than 99:1 e.r. Mechanistic studies show the site-selectivity of transmetalation and reactivity is ligand dependent. The utility of the approach is highlighted by gram-scale synthesis of enantioenriched cyclic 1,4-diketones, and stereoselective transformations of the products by hydrogenation, allylation, and isomerization.