Synthesis of 1,4-Diketones from β-Oxo Esters and Enol Acetates by ­Cerium-Catalyzed Oxidative Umpo­lung Reaction

Silver-Catalyzed Radical Umpolung Cross-Coupling of Silyl Enol Ethers with Activated Methylene Compounds: Access to Diverse Tricarbonyl Derivatives

A silver-catalyzed protocol for the intermolecular radical umpolung cross-coupling protocol of silyl enol ethers with activated methylene compounds is disclosed. The protocol exhibits excellent functional group tolerance, enabling the expedient preparation of a variety of tricarbonyl compounds. Preliminary mechanistic investigations suggest that the reaction proceeds through a process involving free radicals in which silver oxide has a dual role, acting as both a catalyst and a base.

Electrochemical Synthesis of α-Thiocyanated/Methoxylated Ketones Using Enol Acetates

We have developed the synthesis of α-substituted ketone compounds with enol acetates in an electrochemical way. By using cheap NH4SCN and MeOH as the radical sources, a series of valuable α-thiocyanates/methoxy ketones were synthesized under mild electrolysis conditions in acceptable yields with diverse functional group compatibility. Additionally, the scale-up experiment and synthetic transformations reveal potential applications in organic synthesis.

Hypervalent Iodine(III) Reagents with Transferable Primary Amines: Structure and Reactivity on the Electrophilic α-Amination of Stabilized Enolates

A new family of hypervalent iodine reagents containing transferable primary amine groups is described. Benziodoxolone-based reagents were synthesized on the gram-scale through operationally simple reactions in up to quantitative yields. These bench-stable solids were characterized by X-ray analysis and successfully employed in the α-amination of indanone-based β-ketoesters in up to 83% yield. Mechanistic studies indicate a substitution mechanism involving an electrophilic amine.

Renaissance of Homogeneous Cerium Catalysts with Unique Ce(IV/III) Couple: Redox-Mediated Organic Transformations Involving Homolysis of Ce(IV)-Ligand Covalent Bonds

Recent advances in the catalytic application of cerium complexes were achieved through controlling the Ce(IV/III) redox couple. Although Ce(IV) complexes have been extensively investigated as stoichiometric oxidants in organic synthesis on the basis of their highly positive redox potentials, these complexes can be used as catalysts, not only by introducing supporting ligands around the coordination sphere of cerium, but also by taking advantage of the photoresponsive properties of Ce(IV) and Ce(III) species. Cerium is highly abundant, comparable to that of some first-row transition metals such as copper, nickel, and zinc. Cerium complexes are new and promising homogeneous catalyst candidates for a variety of organic transformations under mild reaction conditions. They are typically used to activate dioxygen to oxidize organic compounds and applied for organic radical generation using the photoresponsive character of Ce(IV) carboxylates and alkoxides as well as electronic transition of Ce(III), in which homolysis of Ce(IV)-ligand covalent bonds is an important step for the overall catalytic cycle. In this Perspective, we first review the early discovery of Ce(OAc)₄-mediated oxidative transformations to emphasize the importance of Ce(IV)-OAc bond homolysis in various C-C bond-forming reactions and its relation to recent developments. We then focus on the fundamental importance of Ce(IV) reactivity involving thermal and photoassisted homolysis of the Ce(IV)-ligand covalent bond and the developments regarding Ce(IV/III) redox changes in catalytic reactions together with our recent findings on cerium-based catalysis.

Improved and Flexible Synthetic Access to the Spiroindole Backbone of Cebranopadol

By changing the dimethylamino to a nitro group, a novel synthetic access to the spirocyclic opioid analgesic cebranopadol was developed that is much more efficient compared with the established route. On the basis of the α-acidity of α-nitrotoluene, the two-fold Michael addition to acrylate gave an acyclic precursor compound, which was easily transformed by Dieckmann condensation and decarboxylation to the cyclohexanone derivative needed for the annulation of the indole ring by an oxa-Pictet-Spengler reaction. As an additional benefit, the reduction of the nitro group furnished an amine, which could be late-stage-diversified to carboxamides, sulfonamides, ureas, and N-alkyl congeners. The transformation of the nitro group at the spirocyclic scaffold to the dimethylamino function of the actual title compound was achieved in one step with zinc/formic acid/formaldehyde in 83% yield.

Electrochemical-Induced Ring Transformation of Cyclic α-(ortho-Iodophenyl)-β-oxoesters

Cyclic α-(ortho-iodophenyl)-β-oxoesters were converted in a ring-expanding transformation to furnish benzannulated cycloalkanone carboxylic esters. The reaction sequence started by electrochemical reduction of the iodoarene moiety. In a mechanistic rationale, the resulting carbanionic species was adding to the carbonyl group under formation of a strained, tricyclic benzocyclobutene intermediate, which underwent carbon-carbon bond cleavage and rearrangement of the carbon skeleton by retro-aldol reaction. The scope of the reaction sequence was investigated by converting cyclic oxoesters with different ring sizes yielding benzocycloheptanone, -nonanone and -decanone derivatives in moderate to good yields. Furthermore, acyclic starting materials and cyclic compounds carrying additional substituents on the iodophenyl ring were submitted to this reaction sequence. The starting materials for this transformation are straightforwardly obtained by conversion of β-oxoesters with phenyliodobis(trifluoroacetate).

Oxidative trifluoromethylselenolation of 1,3-dicarbonyls with [Me4N][SeCF3]

The first oxidative trifluoromethylchalcogenation of 1,3-dicarbonyls with [Me₄N][XCF₃] (X = Se and S) and an oxidant is described.

Gold(I) Complexes with Ferrocenylphosphino Sulfonate Ligands: Synthesis and Application in the Catalytic Addition of Carboxylic Acids to Internal Alkynes in Water

The synthesis and characterization of novel gold(I) complexes containing hydrophilic ferrocenylphosphino sulfonate ligands, i.e., compounds [AuCl{(η5-C5H3PR2(SO3iPr))Fe(η5-C5H5)}] (R = Ph (2a), p-Tol (2b), Cy (2c)), are presented, including a single-crystal X-ray diffraction study on 2a. Complexes 2a–c were checked as catalysts for the intermolecular addition of carboxylic acids to nonactivated internal alkynes using water as a green reaction medium. The best results in terms of activity were obtained with 2a in combination with AgOAc, which was able to promote the selective anti addition of a variety of aromatic, aliphatic, and α,β-unsaturated carboxylic acids to both symmetrical and unsymmetrical internal alkynes at 60 °C, employing metal loadings of only 2 mol %.

Electrochemical, Manganese-Assisted Carbon-Carbon Bond Formation between β-Keto Esters and Silyl Enol Ethers

The electrochemical carbon-carbon bond formation process between β-keto esters and silyl enol ethers was investigated utilizing manganese salts. The tricarbonyl compounds were generated in moderate to good yields under neutral conditions. Control experiments revealed that an electro-generated base at the cathode is important. Electroanalytical measurements with a Mn(TPA) complex suggested that the oxidation of the silyl enol ether is the first step in the oxidation process initiated by a corresponding Mn(IV) species.

Synthesis of Annulated Indolines by Reductive Fischer Indolization

Cyclic β-oxoesters and arylhydrazine derivatives were converted at ambient temperature under modified Fischer indolization conditions to furnish annulated indolines with quaternary bridgehead carbon center. Reaction conditions were Brønsted acidic (with CF₃ CO₂ H), but also reductive (with Et₃ SiH). The latter reduced intermediate iminium ions under formation of the 2,3-dihydroindole product constitutions. Racemic products (13 examples) were obtained as single diastereoisomers with relative cis-configuration, which was proved in two cases by single-crystal X-ray structure analysis. Conversion of 4-bromo-1-indanone-2-carboxylate and 4-bromophenylhydrazine gave indeno[1,2-b]indolines with either 1-bromo, 8-bromo, or 1,8-dibromo substitution, which were further diversified by Suzuki coupling reactions with several arylboronic acids (nine examples).

Formation of δ-Lactones with anti-Baeyer-Villiger Regiochemistry: Investigations into the Mechanism of the Cerium-Catalyzed Aerobic Coupling of β-Oxoesters with Enol Acetates

The cerium-catalyzed, aerobic coupling of β-oxoesters with enol acetates and dioxygen yields δ-lactones with a 1,4-diketone moiety. In contrast to the Baeyer-Villiger oxidation (BVO), where the higher substituted residue migrates; in the case of this oxidative C-C coupling reaction, the less substituted alkyl residue undergoes a 1,2-shift. An endoperoxidic oxycarbenium ion comparable to the Criegee intermediate in the BVO is proposed as a reaction intermediate and submitted to conformational analysis by computational methods. As a result, the inverse regiochemistry is explained by a primary stereoelectronic effect. A Hammett analysis using different donor- and acceptor-substituted enol esters provides support for the oxycarbenium ion being the crucial intermediate in the rate determining step of the conversion. An overall mechanism is suggested with a radical chain reaction for the formation of endoperoxides from β-oxoesters, enol acetates and dioxygen with a cerium(IV) species as initiating reagent.

Cerium-catalyzed, oxidative synthesis of annulated, tetrasubstituted dihydrofuran-derivatives

Densely functionalized, annulated dihydrofuran derivatives are prepared from β-oxoesters and silylenolethers by a cerium-catalyzed aerobic oxidation.

Formation of δ-Lactones by Cerium-Catalyzed, Baeyer-Villiger-Type Coupling of β-Oxoesters, Enol Acetates, and Dioxygen

Formation of δ-lactones is observed when cyclopentanone-2-carboxylates are converted in a cerium-catalyzed reaction with α-aryl vinyl acetates under oxidative conditions. The products of this transformation possess a 1,4-dicarbonyl constitution together with a quaternary carbon center. Atmospheric oxygen is the oxidant in this process, which can be regarded as ideal from economic and ecological points of view. Further advantages of this new C-C coupling and oxidation reaction are its operational simplicity and the application of nontoxic and inexpensive CeCl3·7 H2O as precatalyst. This so far unprecedented reaction is proposed to proceed via 1,2-dioxane derivatives, which decompose under formation of an oxycarbenium cation in a Baeyer-Villiger-type pathway. This mechanistic picture is supported by the observation that electron-rich (donor substituted or heteroaromatic) enol esters give higher yields than electron deficient congeners. Apart from 1,4-diketones and α-hydroxylated β-oxoesters formed as byproducts, the yields of δ-lactones range from moderate to good (up to 74%).