Carbon-oxygen Bond Formation Research Articles

Triple Catalysis for the Tandem Transformation of Cyclopropyl Ketones to SCF3-Substituted Dihydrofurans.

In this study, we have developed an effective and general strategy for synthesizing various 4-[(trifluoromethyl)thio]-2,3-dihydrofuran derivatives with high regioselectivity from easily prepared cyclopropyl ketone under mild reaction conditions. By the combination of photoredox, copper, and Lewis acid catalysis into a triple catalytic system, this methodology facilitates the selective cleavage of the carbon-carbon bonds and the formation of new carbon-oxygen and carbon-sulfur bonds. In addition, to enhance the synthetic feasibility of this protocol, we demonstrate its broad applicability across a wide range of substrates and its scalability for large-scale synthesis.

Solar-Light-Driven Photocatalytic Oxidative Coupling of Phenol Derivatives over Bismuth-Based Porous Metal Halide Perovskites.

The selective oxidative coupling of phenol derivatives, involving carbon-carbon (C-C) and carbon-oxygen (C-O) bond formation, has emerged as a critical approach in the synthesis of natural products. However, achieving precise control over the selectivity in coupling reactions of unsubstituted phenols utilizing solar light as the driving force remains a big challenge. In this study, we report a series of porous Cs3Bi2X9 (X=Cl, Br, I) photocatalysts with tailored band gaps and compositions engineered for efficient solar-light-driven oxidative phenol coupling. Notably, p-Cs3Bi2Br9 exhibited about 73 % selectivity for C-C coupling, displaying a high formation rate of 47.3 μmol gcat -1 h-1 under solar radiation. Furthermore, this approach enables control of the site-selectivity for phenol derivatives on Cs3Bi2X9, enhancing C-C coupling. The distinctive porous structure and appropriate band-edge positions of Cs3Bi2Br9 facilitated efficient charge separation, and surface interaction/activation of phenolic hydroxyl groups, resulting in the kinetically preferred formation of C-C over C-O bond. Mechanistic insights into the reaction pathway, supported by comprehensive control experiments, unveiled the crucial role of interfacial charge transfers and Lewis acid Bi sites in stabilizing phenolic intermediates, thereby directing the regioselectivity of diradical couplings and resulting in the formation of unsymmetrical biphenols.

Exploring visible light for carbon–nitrogen and carbon–oxygen bond formationvianickel catalysis

In this review, we highlight the recent synthetic methodology development for the formation of C–N and C–O bondsviavisible-light-driven nickel catalysis, with in-depth discussions with reaction designs and mechanistic scenarios.

Palladium-Catalyzed Cascade 5-endo-dig Cyclization of Ynamides to Form 4-Alkynyloxazolones.

The selective synthesis of 4-alkynyloxazolones and their further applications as substrates to electrophile-promoted nucleophilic cyclization have been developed. The reaction of ynamides with terminal alkynes proceeded smoothly to give 4-alkynyloxazolones in the presence of a catalytic amount of palladium(II) acetate. The products were obtained with the sequential formation of new C-C and C-O bonds via a cascade procedure. The first step involved a carbon-oxygen bond formation, via a 5-endo-dig closure, which was confirmed by X-ray analyses of the crystalline sample. Subsequently, the reaction of 4-alkynyloxazolones with an electrophilic selenium source gave 3-phenylselanyl benzofuran derivatives via an electrophile-promoted nucleophilic cyclization.

Recyclable and reusable ionic liquid-supported azo precursors in Mitsunobu reactions.

A new type of azo precursor, ionic liquid-supported hydrazidecarboxylate, was synthesized and applied in Mitsunobu reactions. The developed reagent is recyclable during the reaction and reusable after recovery by the ionic liquids. The ionic liquid-based azo precursor in conjugation with PhI(OAc)2 has been proved to be useful in the formation of carbon-oxygen, carbon-nitrogen, and carbon-sulfur bonds.

Decarboxylative cross-nucleophile coupling via ligand-to-metal charge transfer photoexcitation of Cu(II) carboxylates.

Reactions that enable carbon-nitrogen, carbon-oxygen and carbon-carbon bond formation lie at the heart of synthetic chemistry. However, substrate prefunctionalization is often needed to effect such transformations without forcing reaction conditions. The development of direct coupling methods for abundant feedstock chemicals is therefore highly desirable for the rapid construction of complex molecular scaffolds. Here we report a copper-mediated, net-oxidative decarboxylative coupling of carboxylic acids with diverse nucleophiles under visible-light irradiation. Preliminary mechanistic studies suggest that the relevant chromophore in this reaction is a Cu(II) carboxylate species assembled in situ. We propose that visible-light excitation to a ligand-to-metal charge transfer (LMCT) state results in a radical decarboxylation process that initiates the oxidative cross-coupling. The reaction is applicable to a wide variety of coupling partners, including complex drug molecules, suggesting that this strategy for cross-nucleophile coupling would facilitate rapid compound library synthesis for the discovery of new pharmaceutical agents.

Organometallic Ni(ii), Ni(iii), and Ni(iv) complexes relevant to carbon–carbon and carbon–oxygen bond formation reactions

Herein, the pyridinophane tetradentate ligand PyNMe3 is used to isolate and structurally characterize well-defined organometallic Ni(ii) and Ni(iii) complexes bearing the cycloneophyl fragment, an alkyl/aryl C-donor ligand.

Carbon-oxygen bond formation via visible-light-induced O–H insertion between acylsilanes and oximes

Visible light promoted C–O bond formation reaction through O–H insertion of acylsilanes with oximes has been developed. The reaction occurred under mild reaction conditions (sole blue LED irradiation in the absence of any catalysts or additives) with a short reaction time and afforded the corresponding oxime ethers in moderate to good yields.

A Platform for Decarboxylative Couplings via Photoredox Catalysis: Direct Access to Carbocations from Carboxylic Acids for Carbon–Oxygen Bond Formation

A Platform for Decarboxylative Couplings via Photoredox Catalysis: Direct Access to Carbocations from Carboxylic Acids for Carbon–Oxygen Bond Formation

Formation of Carbon-Oxygen Bond Mediated by Hypervalent Iodine Reagents Under Metal-Free Conditions

Hypervalent iodine reagents are a class of non-metallic oxidants that have been widely used in the construction of several sorts of bond formations. This surging interest in hypervalent iodine reagents is essentially due to their very useful oxidizing properties, combined with their benign environmental character and commercial availability from the past few decades ago. Furthermore, these hypervalent iodine reagents have been used in the construction of many significant building blocks and privileged scaffolds of bioactive natural products. The purpose of writing this review article is to explore all the transformations in which carbon-oxygen bond formation occurs by using hypervalent iodine reagents under metal-free conditions.

Intrinsic kinetic model for oxidative dehydrogenation of ethane over MoVTeNb mixed metal oxides: A mechanistic approach

An intrinsic kinetic model for the selective production of ethene via oxidative dehydrogenation of ethane over the M1 phase of MoVTeNb mixed metal oxides is presented. Formation of acetic acid, carbon monoxide and carbon dioxide has been incorporated using a holistic reaction mechanism. The proposed model is based on two different oxygen sites, namely, lattice oxygen causing carbon-hydrogen bond cleavage and electrophilic surface oxygen responsible for the formation of carbon-oxygen bonds. It is found that carbon dioxide exclusively originates from decarboxylation of acetate species, while ethene selectively reacts to CO. Consumption and formation of all species are well predicted by the proposed model. Sensitivity analyses demonstrate the strong impact of the initial carbon-hydrogen cleavage on the net ethene production rate. Moreover, regeneration of lattice oxygen sites is found to become rate-determining at oxygen-lean conditions.

Facile protic hydration of acetonitrile to protonated acetamide at oxygen mediated by chloroauric acid: insights from experimental and calculations

Chemical transformations such as nitrile hydration or carbon–oxygen bond formation reactions under gentle conditions are important in the pharmaceutical industry because of the presence of potentially delicate functional groups. We present a non-catalytic hydration reaction of acetonitrile to the corresponding protonated acetamide gold(III) salt [CH3(OH)NH2]AuCl4 under ambient conditions in water using chloroauric acid H[AuCl4] for the first time. ATR-FTIR, Raman and 1H and 13C NMR spectroscopic data in addition to X-ray crystallography supported the isolation of protonated acetamide stabilized with [AuCl4]− anion. The protonation of N–C=O fragment of acetamide, O- versus N-protonation aptitude, was validated experimentally and theoretically. The X-ray crystal structure of the acetamide salt [CH3C(OH)NH2]AuCl4 in the triclinic Pī space group suggested the enolic form. However, the reaction of gold(III) trichloride AuCl3 with acetonitrile in water in the absence of a proton source formed the gold(I)/gold(III) salt [Au(CH3CN)2]AuCl4 without hydration as shown in X-ray structure in the monoclinic P21/c space group. Mapping of HOMO–LUMO energy gap using frontier molecular orbital theory and MESP surfaces of OH and NH conformers of acetamide from DFT calculations clearly shows subsequent changes in their profiles with the change in their protonation states. An energy gap of 56.4 kcal/mol in the optimized energies of OH and NH conformers of acetamide along with computed HOMO–LUMO energy difference represents the relative stability of OH conformer compared to NH conformer, thus leading to the conclusion that OH protonation site is more likely to exist in the acetamide structure as compared to the NH protonation state.

Dynamic stereoselective annulation via aldol-oxa-cyclization cascade reaction to afford spirooxindole pyran polycycles

Spiro polycyclic compounds bearing pyran ring systems are found in bioactive molecules, and we recently reported the construction of spirooxindole all-carbon polycycles. Here we show the development of catalytic stereoselective annulation reactions that afford spirooxindole pyran polycycles. Oxindole-derived spiro[4,5]decanes are reacted with arylglyoxal to construct a pyran ring via the formation of carbon-carbon and carbon-oxygen bonds through dynamic aldol-oxa-cyclization cascade reactions, leading to the formation of spirooxindole pyran polycycles bearing six stereogenic centers as single diastereomers. During the reaction, the starting material is isomerized to the diastereomer, and this is key to afford the product. Taking advantage of this isomerization, highly enantiomerically enriched single diastereomers of spirooxindole pyran polycycles are obtained. The reactions generating the spiro pyran polycycles show stereoselectivities distinct from those previously observed in the construction of all-carbon polycycles.

TBAF-Catalyzed O-Nucleophilic Cyclization of Enaminones: A Process for the Synthesis of Dihydroisobenzofuran Derivatives.

A novel methodology for the stereoselective synthesis of dihydroisobenzofuran derivatives is described in this paper. The procedure was realized by the bifunctional TBAF catalyzed selective O-nucleophilic cyclization of enaminone with intramolecular alkyne under mild and non-metal-mediated conditions. The results of control experiments suggested that the cation-π interaction and basicity, offered by TBAF, might be indispensable for the isomerization of enaminone and the formation of carbon-oxygen bond.

Catalytic, Enantioselective Synthesis of Cyclic Carbamates from Dialkyl Amines by CO2-Capture: Discovery, Development, and Mechanism.

Cyclic carbamates are a common feature of small-molecule therapeutics, offering a constrained hydrogen bond acceptor that is both polar and sterically small. Methods for their preparation most often focus first on amino alcohol synthesis and then reaction with phosgene or its equivalent. This report describes an enantioselective synthesis of cyclic carbamates in which carbon dioxide engages an unsaturated basic amine, facilitated by a bifunctional organocatalyst designed to stabilize a carbamic acid intermediate while activating it toward subsequent enantioselective carbon-oxygen bond formation. Six-membered cyclic carbamates are prepared in good yield with high levels of enantioselection, as constrained 1,3-amino alcohols featuring a chiral tertiary alcohol carbon. Spectroscopic analysis (NMR, DOSY) of various substrate-reagent combinations provides insight into the dominant species under the reaction conditions. Two peculiar requirements were identified to achieve highest consistency: a "Goldilocks" amount of water and the use of a noncrystalline form of the ligand. These atypical features of the final protocol notwithstanding, a diverse range of products could be prepared. Their functionalizations illustrate the versatility of the carbamates as precursors to enantioenriched small molecules.

Late-Stage Functionalization of Arylacetic Acids by Photoredox-Catalyzed Decarboxylative Carbon-Heteroatom Bond Formation.

The rapid transformation of pharmaceuticals and agrochemicals enables access to unexplored chemical space and thus has accelerated the discovery of novel bioactive molecules. Because arylacetic acids are regarded as key structures in bioactive compounds, new transformations of these structures could contribute to drug/agrochemical discovery and chemical biology. This work reports carbon-nitrogen and carbon-oxygen bond formation through the photoredox-catalyzed decarboxylation of arylacetic acids. The reaction shows good functional group compatibility without pre-activation of the nitrogen- or oxygen-based coupling partners. Under similar reaction conditions, carbon-chlorine bond formation was also feasible. This efficient derivatization of arylacetic acids makes it possible to synthesize pharmaceutical analogues and bioconjugates of pharmaceuticals and natural products.

Organocatalytic Electrochemical C–H Lactonization of Aromatic Carboxylic Acids

An electrochemical strategy has been developed for radical arene carbon–oxygen bond formation. This reaction utilizes DDQ as a redox mediator, with inexpensive glassy carbon electrodes to facilitate an intramolecular lactonization of biphenyl-2-carboxylic acid derivatives via aromatic carboxyl radical substitution to give 6H-benzo[c]chromen-6-ones.

Synthetic Studies toward the Tetrapetalones: Diastereoselective Construction of a Putative Intermediate.

A strategy toward tetrapetalones was explored including a site-selective ethylenation of the silyl enol ether A to afford a quaternary stereocenter that serves in a stereogenic capacity. Regio- and diastereoselective reactions were observed in conjunction with the oxidative formation of cation B, which included subsequent selective formation of either carbon-oxygen or carbon-carbon bonds at the δ or ζ position on the seven-membered ring. The fourth ring was formed using a Stetter reaction.

Transition-Metal-Catalyzed Utilization of Methanol as a C1 Source in Organic Synthesis.

Methanol is used as a common solvent, cost-effective reagent, and sustainable feedstock for value-added chemicals, pharmaceuticals, and materials. Among the various applications, the utilization of methanol as a C1 source for the formation of carbon-carbon, carbon-nitrogen, and carbon-oxygen bonds continues to be important in organic synthesis and drug discovery. In particular, the synthesis of C-, N-, and O-methylated products is of central interest because these motifs are found in a large number of natural products as well as fine and bulk chemicals. In this Minireview, we summarize the utilization of methanol as a C1 source in methylation, methoxylation, formylation, methoxycarbonylation, and oxidative methyl ester formation reactions.

Recent Developments in Cobalt Catalyzed Carbon–Carbon and Carbon–Heteroatom Bond Formation via C–H Bond Functionalization

Cobalt catalysts have evolved to be seen as versatile eco-compatible and economical catalysts in organic synthesis in recent years. Cobalt-catalyzed reactions are undoubtedly a classic in synthetic chemistry for the formation of carbon–carbon and carbon–heteroatom bonds. Another important aspect in this field is catalyst variants, such as low-valent and high-valent cobalt catalysts. This review summarizes the recent progress and synthetic utility of low-valent and high-valent cobalt catalysts towards C–H functionalization processes achieving C–C, C–O, C–N and C–B bond formation. Mechanistic insight is also discussed, with the goal of serving as a stepping stone for further development in this field. In addition, Csp3–H bond functionalization reactions provide many opportunities for novel synthesis approaches. 1 Introduction 2 Carbon–Carbon Bond Formation 2.1 Csp2–Csp3 Bond Formation 2.2 Csp2–Csp2 Bond Formation 3 Carbon–Nitrogen Bond Formation 4 Csp3–H Bond Functionalization 5 Carbon–Oxygen Bond Formation 6 Carbon–Boron Bond Formation 7 Conclusion