Novel Axially Chiral Ligand-Enabled Copper-Catalyzed Asymmetric Oxidative Coupling of 2-Naphthols for the Synthesis of 6,6′-Disubstituted BINOLs

Electrochemical Cross-Coupling between N-(4-Hydroxyphenyl)-sulfonamides and 2-Naphthols: Synthesis of 2,2'-Bis(arenol)s

We herein present an electrochemical method for the dehydrogenative cross-coupling of N-(4-hydroxyphenyl)-sulfonamides and 2-naphthols. This transformation provides a direct and scalable approach to a wide range of C1-symmetric 2,2'-bis(arenol)s with moderate to high yields under mild conditions. Preliminary attempts with the asymmetric variant of this reaction were also performed with ≤55% ee for the synthesis of 2,2'-bis(arenol)s. Control experiments were conducted to propose a plausible mechanism for the reaction.

Switching the regioselectivity of acid-catalyzed reactions of arylnaphtho[2,1-b]furans via a [1,2]-aryl shift

The [1,2]-aryl shift reaction was used to synthesize naphtho[2,1-b]furans as promising fluorescent scaffolds for organic electronics. The target compounds are furan analogues of phenanthrene formally accessed by isosteric replacement of the CHCH moiety with an oxygen atom. The straightforward and robust approach involving a [1,2]-aryl shift as a key step provides easy access to a wide range of naphtho[2,1-b]furans with the possibility of late-stage functionalization. Efficient switching of the regioselectivity of acid-catalyzed reactions of arylnaphtho[2,1-b]furans via a [1,2]-aryl shift has been demonstrated. A one-pot protocol involving sequential intramolecular condensation/[1,2]-aryl shift/intermolecular oxidative aromatic coupling to provide access to binaphtho[2,1-b]furan analogues of BINOL was developed. The advantage of these compounds lies in the strong variation in chemical properties and spectral performance depending on the nature and position of the aryl substituent, which facilitates the synthesis of compounds with desired spectral characteristics and opens up prospects for their further use in electronics, biotechnologies and organic synthesis.

An Axially Chiral Quinoline-2-Carboxylic Acid-Cu Catalyst for Enantioselective Synthesis of C2- and C1-Symmetric BINOLs

The in situ dimeric coordination of two chiral ligands bearing quinoline-2-carboxylic acid units and substituted BINOL backbones with a copper ion generates a new chiral catalyst for oxidative homo- and cross-coupling of various 2-naphthols, enabling enantioselective synthesis of a broad range of highly useful diversely substituted C2- and C1-symmetric BINOLs in up to 96% yield with good to excellent enantioselectivities (up to 98:2 e.r.).

Cu(I)-Catalyzed Atropselective Heterobiaryl Coupling Employing Umpoled Indoles

An enantioselective Cu(I)-catalyzed coupling of N-carboxyindoles with various 2-naphthols and phenols for the synthesis of axially chiral arylindoles has been developed. Our mechanistic studies, bolstered by experimental evidence and DFT calculations, reveal a novel closed-shell mechanism involving outer-sphere attack of N-carboxyindoles on the Cu-bound naphthols. This mechanism allows for unprecedented diversity of 2-naphthols and phenols in C-H arylation. Enantiocontrol is achieved through center-to-axis chirality transfer via a key dearomatized naphthol intermediate, which prevents undesired epimerization of the C-C axis.

Chiral α-Aminophosphonates as Ligands in Copper-Catalyzed Asymmetric Oxidative Coupling of 2-Naphthols

Chiral α-aminophosphonates with adjacent carbon and phosphonate stereogenic centers have been employed as ligands in the copper-catalyzed oxidative coupling of 2-naphthols, resulting in the production of chiral BINOLs in favorable yields and moderate to good enantiomeric excess. This represents the first application of chiral P-based ligands to enable such a transformation. The synthesis of these chiral α-aminophosphonate ligands offers a significant advantage over approaches that typically necessitate elaborate synthetic processes for chiral ligand production.

Decarboxylative Iodination and Suzuki-Miyaura Coupling Reactions to Access Chiral 3,3'-Diaryl-1,1'-bi-2-naphthols

An efficient synthesis of the enantiomerically pure 3,3'-bis-arylated BINOLs is accomplished through decarboxylative iodination of the dimethyl ether derivative of BINOL-3,3'-dicarboxylic acid followed by Suzuki-Miyaura coupling using a one-pot protocol. The decarboxylative iodination is effected with the dimethyl ether derivative of BINOL-3,3'-dicarboxylic acid using iodine as a terminal oxidant and the cheaply available K3PO4 as a base under neat conditions. This protocol facilitated the introduction of the aryl group at the 3,3'-position on the binaphthyl system using aryl boronic acid through a palladium-catalyzed Suzuki-Miyaura coupling reaction.

Advances in the Asymmetric Synthesis of BINOL Derivatives

BINOL derivatives have shown relevant biological activities and are important chiral ligands and catalysts. Due to these properties, their asymmetric synthesis has attracted the interest of the scientific community. In this work, we present an overview of the most efficient methods to obtain chiral BINOLs, highlighting the use of metal complexes and organocatalysts as well as kinetic resolution. Further derivatizations of BINOLs are also discussed.

Atroposelective Synthesis of C-C Axially Chiral Compounds via Mono- and Dinuclear Vanadium Catalysis

Axially chiral compounds with rotationally constrained σ-bonds that exhibit atropisomerism are lucrative synthetic targets because of their ubiquity in functional materials and natural products. The metal complex-catalyzed enantioselective fabrication of axially chiral scaffolds has been widely investigated, and thus far, considerable progress has been made. Over the past two decades, we have developed a highly efficient strategy for constructing axially chiral biarenol derivatives using chiral mono- and dinuclear vanadium complexes. These complexes are readily prepared from vanadium(IV) salts and Schiff base ligands (generated from the condensation of (S)-tert-leucine and di- or monoformyl-(R)-1,1'-bi-2-naphthol (BINOL) derivatives) under O₂ and act as highly active catalysts for highly stereoselective C-C bond formation. In particular, the vanadium complex-catalyzed enantioselective oxidative coupling of 2-naphthols 1 under oxygen or in air, which is a green oxidant, affords the desired axially chiral molecules in high yields and high stereoselectivity (up to quantitative yield and 97% ee), along with water as the sole coproduct. This coupling reaction tolerated various functional groups (such as halogens, alkoxys, and boryls) and avoided overoxidation of coupling products.The key feature of dinuclear vanadium(V) catalysts such as (R_a,S,S)-5a is an outstanding mode of the homocoupling reaction, in which a single molecule of the catalyst activates two molecules of the starting material (e.g., 2-naphthols) simultaneously. With this "dual activation" mechanism, the oxidative coupling promoted by the dinuclear catalyst proceeds in an intramolecular manner. The homocoupling rate using 5 mol % of the dinuclear vanadium(V) complex (R_a,S,S)-5a was measured to be 111 times faster than that of the mononuclear vanadium(IV) complex (S)-4a bearing a half motif of the dinuclear vanadium complex.In the case of the heterocoupling reaction utilizing two different kinds of arenol derivatives, only a starting arenol having lower oxidation potential seems to be activated by the mononuclear vanadium complex. The reaction rate of the heterocoupling using either mono- or dinuclear vanadium complexes showed no difference to give the coupling product in high yields but with a different enantioselective manner; chiral mononuclear vanadium(V) complexes showed better enantioselectivites than that of the dinuclear vanadium(V) complexes. A competing heterocoupling study and a linear correlation between the ee of the mononucaler vanadium catalyst and ee of the heterocoupling suggested that the heterocoupling involves an intermolecular radical-anion coupling pathway.In this Account, we summarize the recent advances in vanadium-catalyzed coupling reactions that produced important chiral molecules, such as biresorcinols, polycyclic biphenols, oxa[9]helicenes, bihydroxycarbazoles, and C₁-symmetrical biarenols, and their coupling reaction mechanisms. By pursuing vanadium catalysis, we believe numerous additional transformations as well as a renewed interest in catalytic and chemo-, regio-, and enantioselective aryl-aryl bond constructions will be manifested.

Catalytic Oxidative Coupling of Phenols and Related Compounds

Phenols and their derivatives are the elementary building blocks for several classes of complex molecules that play essential roles in biological systems. Nature has devised methods to selectively couple phenolic compounds, and many efforts have been undertaken by chemists to mimic such coupling processes. A range of mechanisms can be involved and with well-studied catalysts, reaction outcomes in phenol-phenol oxidative coupling reactions can be predicted with a good level of fidelity. However, reactions with catalysts that have not been studied or that do not behave similarly to known catalysts can be hard to predict and control. This Perspective provides an overview of catalytic methods for the oxidative coupling of phenols, focusing on the last 10 years, and summarizes current challenges.