Coupling Of Phenols Research Articles

Asymmetric Oxidative Coupling of Phenols and Hydroxycarbazoles.

The first examples of asymmetric oxidative coupling of simple phenols and 2-hydroxycarbazoles are outlined. Generation of a more vanadium catalyst by ligand design and by addition of an exogenous Brønsted or Lewis acid was found to be key to coupling the more oxidatively resistant phenols. The resultant vanadium complex is both more Lewis acidic and more strongly oxidizing. Good to excellent levels of enantioselectivity could be obtained, and simple trituration readily provided the products with ≥95% ee.

Total Synthesis of (+)-Gracilamine Based on an Oxidative Phenolic Coupling Reaction and Determination of Its Absolute Configuration.

The enantioselective total synthesis of (+)-gracilamine (1) is described. The strategy features a diastereoselective phenolic coupling reaction followed by a regioselective intramolecular aza-Michael reaction to construct the ABCE ring system. The configuration at C3a in 1 was controlled by the stereocenter at C9a, which was selectively generated (91 % ee) by an organocatalytic enantioselective aza-Friedel-Crafts reaction developed by our research group. This synthesis revealed that the absolute configuration of (+)-gracilamine is 3aR, 4S, 5S, 6R, 7aS, 8R, 9aS.

Total Synthesis of (+)‐Gracilamine Based on an Oxidative Phenolic Coupling Reaction and Determination of Its Absolute Configuration

AbstractThe enantioselective total synthesis of (+)‐gracilamine (1) is described. The strategy features a diastereoselective phenolic coupling reaction followed by a regioselective intramolecular aza‐Michael reaction to construct the ABCE ring system. The configuration at C3a in 1 was controlled by the stereocenter at C9a, which was selectively generated (91 % ee) by an organocatalytic enantioselective aza‐Friedel–Crafts reaction developed by our research group. This synthesis revealed that the absolute configuration of (+)‐gracilamine is 3aR, 4S, 5S, 6R, 7aS, 8R, 9aS.

Bioinspired Total Synthesis of (-)-Vescalin: A Nonahydroxytriphenoylated C-Glucosidic Ellagitannin.

The first total synthesis of the 2,3,5-O-(S,R)-nonahydroxytriphenoylated (NHTP) C-glucosidic ellagitannin (-)-vescalin was accomplished through a series of transformations mimicking the sequence of events leading to its biogenesis. The key steps of this synthesis encompass a Wittig-mediated ring opening of a glucopyranosic hemiacetal, a C-glucosidation event through a phenolic aldol-type reaction, and a Wynberg-Feringa-Yamada-type oxidative phenolic coupling, which forged the NHTP unit of (-)-vescalin.

Bioinspired Total Synthesis of (−)‐Vescalin: A Nonahydroxytriphenoylated C‐Glucosidic Ellagitannin

AbstractThe first total synthesis of the 2,3,5‐O‐(S,R)‐nonahydroxytriphenoylated (NHTP) C‐glucosidic ellagitannin (−)‐vescalin was accomplished through a series of transformations mimicking the sequence of events leading to its biogenesis. The key steps of this synthesis encompass a Wittig‐mediated ring opening of a glucopyranosic hemiacetal, a C‐glucosidation event through a phenolic aldol‐type reaction, and a Wynberg–Feringa–Yamada‐type oxidative phenolic coupling, which forged the NHTP unit of (−)‐vescalin.

Meso-Tetraphenylporphyrin Iron Chloride Catalyzed Selective Oxidative Cross-Coupling of Phenols.

A novel catalytic system for oxidative cross-coupling of readily oxidized phenols with poor nucleophilic phenolic partners based on an iron meso-tetraphenylporphyrin chloride (Fe[TPP]Cl) complex in 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) was developed. The unique chemoselectivity of this reaction is attributed to the coupling between a liberated phenoxyl radical with an iron-ligated phenolic coupling partner. The conditions are scalable for preparing a long list of unsymmetrical biphenols assembled from a less reactive phenolic unit substituted with alkyl or halide groups.

Towards semisynthetic natural compounds with a biaryl axis: Oxidative phenol coupling in Aspergillus niger

Regio- and stereoselective phenol coupling is difficult to achieve using synthetic strategies. However, in nature, cytochrome P450 enzyme-mediated routes are employed to achieve complete axial stereo- and regiocontrol in the biosynthesis of compounds with potent bioactivity. Here, we report a synthetic biology approach whereby the bicoumarin metabolic pathway in Aspergillus niger was specifically tailored towards the formation of new coupling products. This strategy represents a manipulation of the bicoumarin pathway in A. niger via interchange of the phenol-coupling biocatalyst and could be applied to other components of the pathway to access a variety of atropisomeric natural product derivatives.

Bismuthanes as Hemilabile Donors in an O2 -Activating Palladium(0) Complex.

A xanthene-based bismuthane/phosphane chelating ligand has been accessed that has enabled the synthesis of a palladium(0) bismuthane complex. The bismuthane donor proved to be hemilabile as it switched to a dangling position upon addition of O2 that gave a palladium(II) peroxide complex. Unlike the corresponding 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos) palladium peroxide, the bismuth analogue could be employed for catalytic phosphane oxidation and oxidative phenol coupling.

Copper ferrite superparamagnetic nanoparticles as a heterogeneous catalyst for directed phenol/formamide coupling

The copper ferrite-catalyzed, directed coupling of ortho-arylated phenols and dialkylformamides in the presence of a peroxide oxidant is described. Acyclic and cyclic amides were compatible with the reaction conditions. The copper ferrite catalyst is heterogeneous since substantial leaching was not detected and re-use of the catalyst for 9 consecutive reactions proceeded without a significant decrease in yield. To the best of our knowledge, this transformation has not been previously performed under heterogeneous catalysis conditions.

First total synthesis of quiquesetinerviusin A

The first total synthesis of the dihydrobenzofuran neolignan quiquesetinerviusin A and its related structure have been described. Phenolic coupling is the key step to constructing the dihydrobenzofuran skeleton with vanillin as the raw material. The hydroxyl group was protected with dihydropyran (DHP) and the ester group was reduced with diisobutylaluminium hydride (DIBAL-H) in order to obtain the crucial intermediate diol, which was then condensed with an acid ligand to give the desired compounds following removal of the protecting groups.

Inside Cover: Synthesis of ortho‐Azophenols by Formal Dehydrogenative Coupling of Phenols and Hydrazines or Hydrazides (Chem. Eur. J. 36/2017)

Inside Cover: Synthesis of <i>ortho</i>‐Azophenols by Formal Dehydrogenative Coupling of Phenols and Hydrazines or Hydrazides (Chem. Eur. J. 36/2017)

A Bioinspired Catalytic Aerobic Functionalization of Phenols: Regioselective Construction of Aromatic C–N and C–O Bonds

We report a bioinspired approach for the regioselective construction of both aromatic C–O and C–N bonds through the dehydrogenative coupling of phenols and aliphatic amines. Mechanistically, the process hinges on the ortho-oxygenation of phenols to ortho-quinones followed by aromatic C–N bond formation through a condensation/redox isomerization cascade. Overall, the process enables the regioselective formation of aromatic C–O and C–N bonds directly from C–H bonds under mild reaction conditions.

Bio-inspired oxidative phenolic coupling: Total synthesis of the diarylether heptanoid (±)-pterocarine

The diaryletherheptanoid natural product, pterocarine, is expeditiously synthesized using a bioinspired intramolecular oxidative phenolic coupling of acerogenin G. The cyclization precursor is prepared from a simple cinnamic acid derivative in three high yielding synthetic operations. The key oxidative coupling is inspired by biosynthetic hypotheses; however, the oxidative coupling proceeds with concomitant hydroxylation of the diphenyl ether motif.

Synthesis of ortho-Azophenols by Formal Dehydrogenative Coupling of Phenols and Hydrazines or Hydrazides.

Azophenols are important chromophores and reagents in organic synthesis, with applications as pigments and molecular switches. Here, we describe a catalytic aerobic process that couples phenols and hydrazines or hydrazides for their synthesis. The key aromatic C-N bond is formed by condensation between the hydrazine or hydrazide and an ortho-quinone, which triggers a redox-isomerization to install the azo-functionality. Notable features include rapid access to highly functionalized azophenols with a range of electronic configurations, including "push-pull" systems, conditions that employ simple, unactivated substrates, occurrence at room temperature using an earth-abundant and commercially available copper catalyst, and production of water as the only stoichiometric byproduct.

Unified Synthesis of 1,2-Oxy-aminoarenes via a Bio-inspired Phenol-Amine Coupling

Summary 1,2-Aminophenols and their related nitrogen-containing heterocycles are ubiquitous in medicine, agriculture, and materials science. Here, we describe a bio-inspired and unified approach for their synthesis through a direct, catalytic aerobic coupling of phenols and amines. Key to the formation of the aromatic C–N bond is a facile condensation between the amine and an ortho -quinone, which triggers redox isomerization to an ortho -iminophenol before a series of downstream events. We discuss the factors that govern regioselectivity during aerobic ortho -oxygenation of the phenol and condensation of the amine, and we demonstrate synthetic utility by preparing a structurally diverse family of immunosuppressive heterocycles and the indole alkaloid cephalandole A. Our conditions involve a wide range of simple, un-activated substrates, occur at room temperature with an earth-abundant copper-catalyst and a commercially available diamine ligand, and produce water as the only stoichiometric by-product.

Nature Inspires an Aerobic Coupling of Phenol and Amine

In this issue of Chem , Lumb and colleagues report on an effective and simple catalytic system of copper and diamine ligands for the bio-inspired and unified synthesis of 1,2-aminophenols through direct aerobic phenol-amine coupling. Using this method, they prepared a structurally diverse family of 1,2-aminophenols and their derived heterocycles, including immunosuppressive benzoxazoles and the indole alkaloid cephalandole A.

Cu2O/SiC as efficient catalyst for Ullmann coupling of phenols with aryl halides

A Cu2O/SiC heterogeneous catalyst was prepared via a two-step liquid-phase method using diethylene glycol as both the solvent and the reducing agent. The catalyst was characterized using X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and H2 temperature-programmed reduction. All the results indicate that Cu is present on the SiC support primarily as Cu2O. The SEM and TEM results show that cubic Cu2O nanoparticles are uniformly dispersed on the β-SiC surface. The reaction conditions, namely the temperature, reaction time, and amounts of base and catalyst used, for the Ullmann-type C–O cross-coupling reaction were optimized. A model reaction was performed using iodobenzene (14.0 mmol) and phenol (14.0 mmol) with Cu2O/SiC (5 wt% Cu) as the catalyst, Cs2CO3 (1.0 equiv.) as the base, and tetrahydrofuran as the solvent at 150 °C for 3 h; a yield of 97% was obtained and the turnover frequency (TOF) was 1136 h−1. The Cu2O/SiC catalyst has a broad substrate scope and can be used in Ullmann-type C–O cross-coupling reactions of aryl halides and phenols bearing a variety of different substituents. The catalyst also showed high activity in the Ullmann-type C–S cross-coupling of thiophenol with iodobenzene and substituted iodobenzenes; a TOF of 1186 h−1 was achieved. The recyclability of the Cu2O/SiC catalyst in the O-arylation of phenol with iodobenzene was investigated under the optimum conditions. The yield decreased from 97% to 64% after five cycles. The main reason for the decrease in the catalyst activity is loss of the active component, i.e., Cu2O.

Regioselective synthesis of C3 alkylated and arylated benzothiophenes

Benzothiophenes are heterocyclic constituents of important molecules relevant to society, including those with the potential to meet modern medical challenges. The construction of molecules would be vastly more efficient if carbon–hydrogen bonds, found in all organic molecules, can be directly converted into carbon–carbon bonds. In the case of elaborating benzothiophenes, functionalization of carbon–hydrogen bonds at carbon-number 3 (C3) is markedly more demanding than at C2 due to issues of regioselectivity (C3 versus C2), and the requirement of high temperatures, precious metals and the installation of superfluous directing groups. Herein, we demonstrate that synthetically unexplored but readily accessible benzothiophene S-oxides serve as novel precursors for C3-functionalized benzothiophenes. Employing an interrupted Pummerer reaction to capture and then deliver phenol and silane coupling partners, we have discovered a directing group-free method that delivers C3-arylated and -alkylated benzothiophenes with complete regioselectivity, under metal-free and mild conditions.

Visible Light Copper Photoredox-Catalyzed Aerobic Oxidative Coupling of Phenols and Terminal Alkynes: Regioselective Synthesis of Functionalized Ketones via C≡C Triple Bond Cleavage.

Direct oxidative coupling of phenols and terminal alkynes was achieved at room temperature by a visible-light-mediated copper-catalyzed photoredox process. This method allows regioselective synthesis of hydroxyl-functionalized aryl and alkyl ketones from simple phenols and phenylacetylene via C≡C triple bond cleavage. 47 examples were presented. From a synthetic perspective, this protocol offers an efficient synthetic route for the preparation of pharmaceutical drugs, such as pitofenone and fenofibrate.

Solvent-controlled selective synthesis of biphenols and quinones via oxidative coupling of phenols.

A regioselective synthesis of unsymmetrical and symmetrical biphenols and binaphthols via oxidative coupling of phenols or naphthols in the presence of K2S2O8 in CF3COOH under ambient conditions is described. Interestingly, the 1 : 1 ratio of H2O and CH3CN solvent mixtures at 80 °C instead of CF3COOH provided substituted unsymmetrical quinones. A gram-scale synthesis of biphenols and binaphthols was demonstrated.