Highly regio- and stereoselective (3 + 2) annulation reaction of allenoates with 3-methyleneindolin-2-ones catalyzed by a planar chiral [2.2]paracyclophane-based bifunctional phosphine-phenol catalyst

A planar chiral [2.2]paracyclophane-based phosphine-phenol catalyst catalyzed the (3 + 2) annulation reaction of ethyl 2,3-butadienoate with 3-methyleneindolin-2-ones to produce 2,5-disubstituted cyclopentene-fused C3-spirooxindoles in high yields with high regio-, diastereo-, and enantioselectivities. This catalyst was suitable for reactions of not only benzylideneindolinones but also alkylideneindolinones, the chiral phosphine-catalyzed reactions of which have not yet been reported. Density functional theory calculations suggested that the formation of hydrogen bonds between the phenolic OH group of the catalyst and the allenoate carbonyl group, rather than between the OH group and the carbonyl group of indolinone, contributed to the formation of an efficient reaction space at the enantiodetermining step.

Enantiopure planar chiral [2.2]paracyclophanes: Synthesis and applications in asymmetric organocatalysis

This short review focuses on enantiopure planar chiral [2.2]paracyclophanes (pCps), a fascinating class of molecules that possess an unusual three-dimensional core and intriguing physicochemical properties. In the first part of the review, different synthetic strategies for preparing optically active pCps are described. Although classical resolution methods based on the synthesis and separation of diastereoisomeric products still dominate the field, recent advances involving the kinetic resolution of racemic compounds and the desymmetrization of meso derivatives open up new possibilities to access enantiopure key intermediates on synthetically useful scales. Due to their advantageous properties including high configurational and chemical stability, [2.2]paracyclophanes are increasingly employed in various research fields, ranging from stereoselective synthesis to material sciences. The applications of [2.2]paracyclophanes in asymmetric organocatalysis are described in the second part of the review. While historically enantiopure pCps have been mainly employed by organic chemists as chiral ligands in transition-metal catalysis, these compounds can also be used as efficient catalysts in metal-free reactions and may inspire the development of new transformations in the near future.

[2.2]Paracyclophane-Based Isothiourea-Catalyzed Highly Enantioselective α-Fluorination of Carboxylic Acids

Planar chiral [2.2]paracyclophane-based isothiourea catalysts have been prepared over a few simple steps in high yields. In the presence of these catalysts, highly efficient catalytic enantioselective fluorination of carboxylic acids has been accomplished, providing a broad range of optically active α-fluoroesters in high yield and excellent enantioselectivity.

Silylative aromatization of p-quinone methides under metal and solvent free conditions

A base-mediated silylation reaction leading to benzyl silanes has been developed. Under transition-metal and solvent free conditions, the silylation of a wide array of p-quinone methides is achieved using a Cs₂CO₃ catalyst in yields up to 96%. Carboxylation of the as-obtained organosilane with gaseous CO₂ provides a new synthetic protocol for the preparation of carboxylic acid.

A novel and efficient synthetic protocol is reported for the synthesis of benzyl silanes from readily available silylborane and p-quinone methides using 5% cesium carbonate under solvent-free conditions.

Activation of the Si–B interelement bond related to catalysis

Covering the past seven years, this review comprehensively summarises the latest progress in the preparation and application of Si–B reagents, including the discussion of relevant reaction mechanisms.

Synthesis of New Planar-Chiral Linked [2.2]Paracyclophanes-N-([2.2]-Paracyclophanylcarbamoyl)-4-([2.2]Paracyclophanylcarboxamide, [2.2]Paracyclophanyl-Substituted Triazolthiones and -Substituted Oxadiazoles

The manuscript describes the synthesis of new racemic and chiral linked paracyclophane assigned as N-5-(1,4(1,4)-dibenzenacyclohexaphane-1²-yl)carbamoyl)-5’-(1,4(1,4)-dibenzenacyclohexaphane-1²-yl)carboxamide. The procedure depends upon the reaction of 5-(1,4(1,4)-dibenzenacyclohexaphane-1²-yl)hydrazide with 5-(1,4(1,4)-dibenzenacyclohexaphane-1²-yl)isocyanate. To prepare the homochiral linked paracyclophane of a compound, the enantioselectivity of 5-(1,4(1,4)-dibenzenacyclohexaphane-1²-yl)carbaldehyde (enantiomeric purity 60% ee), was oxidized to the corresponding acid, which on chlorination, gave the corresponding acid chloride of [2.2]paracyclophane. Following up on the same procedure applied for the preparation of racemic-carbamoyl and purified by HPLC purification, we succeeded to obtain the target Sp-Sp-N-5-(1,4(1,4)-dibenzenacyclohexaphane-1²-yl)carbamoyl)-5’-(1,4(1,4)-dibenzenacyclohexaphane-1²-yl)carboxamide. Subjecting N-5-(1,4(1,4)-dibenzenacyclohexaphane-1²-yl)hydrazide to various isothiocyanates, the corresponding paracyclophanyl-acylthiosemicarbazides were obtained. The latter compounds were then cyclized to a new series of 5-(1,4(1,4)-dibenzenacyclohexaphane-1²-yl)-2,4-dihydro-3H-1,2,4-triazol-3-thiones. 5-(1,4(1,4)-Dibenzenacyclohexaphane-1²-yl)-1,3,4-oxadiazol-2-amines were also synthesized in good yields via internal cyclization of the same paracyclophanyl-acylthiosemicarbazides. NMR, IR, and mass spectra (HRMS) were used to elucidate the structure of the obtained products. The X-ray structure analysis was also used as an unambiguous tool to elucidate the structure of the products.

(Diacetoxyiodo)benzene-Mediated C-H Oxidation of Benzylic Acetals

A useful oxidation of C-H bond of benzylic acetals has been achieved. This method avoids the use of stoichiometric metals and is compatible with the presence of both electron-donating and electron-withdrawing substituents on the aromatic ring. Oxidation was carried out by rapid microwave irradiation of benzylic acetals with PhI(OAc)₂ as the oxidant. This led to the oxidation of acetals into 2-acetoxy-1,3-dioxolanes. Furthermore, this transformation protocol encompasses a wide range of valuable conversions of these useful synthons into different carboxylic acid derivatives.

Asymmetric Catalysis Using Aromatic Aldehydes as Chiral α-Alkoxyalkyl Anions

We have developed a new umpolung strategy for catalytically forming a chiral α-alkoxyalkyl anion from an aromatic aldehyde for use in asymmetric synthesis. The reaction between aromatic aldehydes and aryl or allyl electrophiles with a silylboronate utilizing a chiral copper-N-heterocyclic carbene catalyst and a palladium-bisphosphine catalyst in a synergistic manner occurred with high enantioselectivities to deliver the three-component coupling products, chiral silyl-protected secondary alcohol derivatives. Our method features the catalytic generation of enantioenriched chiral α-alkoxyalkylcopper(I) intermediates from aldehydes and their subsequent palladium-catalyzed stereospecific cross-coupling.

Suzuki Cross-Coupling of [2.2]Paracyclophane Trifluoroborates with Pyridyl and Pyrimidyl Building Blocks

We report a new Suzuki cross-coupling protocol for high yielding derivatization of [2.2]paracyclophane with pyridyl and pyrimidyl substituents. The [2.2]paracyclophane trifluoroborate salt presented herein is a bench stable, easily accessible, and convenient substitute to former cross-coupling substrates. This will be of very high interest for future paracyclophane derivatization endeavors.

Planar chiral [2.2]paracyclophanes: from synthetic curiosity to applications in asymmetric synthesis and materials

Planar chiral [2.2]paracyclophane-based ligands and employment of such enantiopure representative ligands to facilitate selective transformation of prochiral or racemic substances into enantiopure products are rarely explored compared to the complex chiral scaffolds such as ferrocenes. This tutorial discusses recent findings and inspiring progress in design, synthetic tunability and applications of planar chiral [2.2]paracyclophane systems as a practical class of catalysts for asymmetric synthesis. Here, we summarize a series of planar chiral [2.2]paracyclophanes that are becoming an important new tool-box in asymmetric synthesis, employed in a variety of synthetic venues such as new chiral ligands and catalysts for stereo-controlled and enantioselective addition of alkyl, alkenyl, alkynyl and aryl zinc reagents to aliphatic and aromatic aldehydes, ketones, imines and many more. Besides, planar chiral [2.2]paracyclophanes are useful synthons, from a material perspective, can be incorporated into conjugated polymeric systems for chiroptical and optoelectronic properties, find broad applications in bio- and materials science, for instance, gold-based cytostatics, surface-mounted chiral MOF thin films for selective adsorption or in functionalized parylene polymer coatings, to name a few. This is an up-to-date tutorial review, written exclusively on planar chiral [2.2]paracyclophane chemistry, covering key aspects of synthesis, structures, properties, applications and future directions of chiral polymeric assemblies and novel biomaterials built with [2.2]paracyclophanes.

Planar chiral [2.2]paracyclophane-based phosphine–phenols: use in enantioselective [3 + 2] annulations of allenoates and N-tosylimines

Planar chiral [2.2]paracyclophane-based phosphine–phenols efficiently catalyze the [3 + 2] annulation of N-tosylimines and allenoate.