Chirality Transfer from the Oxidative Dearomatization of Axially Chiral Binols with Oxone under Mild Conditions

Regioselective Substitution of BINOL

1,1'-Bi-2-naphthol (BINOL) has been extensively used as the chirality source in the fields of molecular recognition, asymmetric synthesis, and materials science. The direct electrophilic substitution at the aromatic rings of the optically active BINOL has been developed as one of the most convenient strategies to structurally modify BINOL for diverse applications. High regioselectivity has been achieved for the reaction of BINOL with electrophiles. Depending upon the reaction conditions and substitution patterns, various functional groups can be introduced to the specific positions, such as the 6-, 5-, 4-, and 3-positions, of BINOL. Ortho-lithiation at the 3-position directed by the functional groups at the 2-position of BINOL have been extensively used to prepare the 3- and 3,3'-substituted BINOLs. The use of transition metal-catalyzed C-H activation has also been explored to functionalize BINOL at the 3-, 4-, 5-, 6-, and 7-positions. These regioselective substitutions of BINOL have allowed the construction of tremendous amount of BINOL derivatives with fascinating structures and properties as reviewed in this article. Examples for the applications of the optically active BINOLs with varying substitutions in asymmetric catalysis, molecular recognition, chiral sensing and materials are also provided.

Electrochemical Dearomatizing Methoxylation of Phenols and Naphthols: Synthetic and Computational Studies

The electrochemical oxidative dearomatizing methoxylation of phenols and naphthols was developed. It provides an alternative route for the preparation of methoxycyclohexadienones, important and versatile synthetic intermediates, that eliminates the need for stoichiometric high-energy chemical oxidants and generates hydrogen as a sole by-product. The reaction proceeds in a simple constant current mode, in an undivided cell, and it employs standardized instrumentation. A collection of methoxycyclohexadienones derived from various 2,4,6-tri-substituted phenols and 1-substituted-2-naphthols was obtained in moderate to excellent yields. These include a complex derivative of estrone, as well as methoxylated dearomatized 1,1'-bi-2-naphthols (BINOLs). The mechanism of the reaction was subject to profound investigations using density functional theory calculations. In particular, the reactivity of two key intermediates, phenoxyl radical and phenoxenium ion, was carefully examined. The obtained results shed light on the pathway leading to the desired product and rationalize experimentally observed selectivities regarding a side benzylic methoxylation and the preference for the functionalization at the para over the ortho position. They also uncover the structure-selectivity relationship, inversely correlating the steric bulk of the substrate with its propensity to undergo the side-reaction. Moreover, the loss of stereochemical information from enantiopure BINOL substrates during the reaction is rationalized by the computations.

13-8-13-Membered Tricyclic Ladder-Type Siloxanes Hybridized with BINOLs: Synthesis, Characterization, and Fluorescence Sensing of Fluorides

Developing fluorescent chemosensors with sensitivity and high specificity for recognizing fluorides is still challenging. Herein, four innovative compounds based on 13-8-13-membered tricyclic ladder-type siloxanes hybridized with BINOLs (abbreviated as TLS-BINOLs) were prepared through the B(C6F5)3-catalyzed Piers-Rubinsztajn reaction. The well-defined ladder-type structure of the TLS-BINOLs was determined by X-ray crystallographic analysis. Additionally, the fluorescent sensing ability of the TLS-BINOLs toward anions was studied. Our finding revealed that all four ladder-type compounds (TLS-BINOLs) exhibited high specificity in recognizing fluorides through fluoride-triggered structural decomposition. The detection limits for fluorides were determined to be 0.37, 0.35, 0.39, and 0.48 μM for the respective TLS-BINOLs. The nonemissive product induced by the fluorides was also determined using single-crystal X-ray diffraction analysis.

C-O Coupling/[4+2] Cycloaddition Tandem Reactions via Oxidative Dearomatization of BINOLs: Access to Bridged Polycyclic Compounds

Intramolecular C-H activation/C-O coupling, dearomatization, and [4+2] cycloaddition of BINOL units have been well developed in a one-pot approach with maleimide derivatives as the dienophiles. This tandem catalytic system generates a variety of functionalized bridged polycyclic products in a step-economical manner, which greatly enriches the modification methods and strategies for the BINOL skeletons.

Oxidation of BINOLs by Hypervalent Iodine Reagents: Facile Synthesis of Xanthenes and Lactones

Xanthene derivatives have broad applications in medicines, fluorescent probes, dyes, food additives, etc. Therefore, much attention was focused on developing the synthetic methods to prepare these compounds. Binaphthyl‐based xanthene derivatives were prepared through the oxidation of BINOLs promoted by the hypervalent iodine reagent iodosylbenzene (PhIO). Nine‐membered lactones were obtained through a similar oxidative reaction when iodoxybenzene (PhIO₂) was used. Additionally, one‐pot reactions of BINOLs, PhIO and nucleophiles such as alcohols and amines were also investigated to provide alkoxylated products and amides in good to excellent yields.

Dearomatization reaction of β-naphthols with disulfurating reagents

p-TsOH-catalyzed intermolecular dearomatization reactions of β-naphthols with disulfurating reagents were developed. Various β-naphthalenones bearing a quaternary carbon stereogenic center were obtained smoothly in good to excellent yields with high chemoselectivity in the presence of 5 mol% p-TsOH. This reaction features mild reaction conditions and excellent functional group tolerance.

Regioselective benzoyloxylative dearomatization of naphthols by benzoyl peroxide under catalyst-free conditions

A direct regioselective benzoyloxylative dearomatization of both α- and β-naphthols by benzoyl peroxide under an air atmosphere, and radical inhibitor- and catalyst-free conditions at room temperature is described. The methodology provides a new efficient strategy for the construction of α-ketol derivatives bearing an oxo-quaternary carbon center from naphthols with good to excellent yields.