Efficient and Scalable Kinetic Resolution of Racemic 4-Formyl[2.2]paracyclophaneviaAsymmetric Transfer Hydrogenation

The hamburger-shape photocatalyst: thioxanthone-based chiral [2.2]paracyclophane for enantioselective visible-light photocatalysis of 3-methylquinoxalin-2(1H)-one and styrenes

A new thioxanthone-based photocatalyst with a [2.2]paracyclophane skeleton and planar chirality has been developed. The catalyst has been successfully applied in the visible light-mediated enantioselective aza Paternò-Büchi reactions of quinoxalinone and styrenes to produce azetidines. The structures of the catalyst derivatives were unequivocally determined by their single crystal X-ray crystallography analysis.

Kinetic resolution of substituted amido[2.2]paracyclophanes via asymmetric electrophilic amination

Planar chiral [2.2]paracyclophane derivatives are a type of structurally intriguing and practically useful chiral molecules, which have found a range of important applications in the field of asymmetric catalysis and material science. However, access to enantioenriched [2.2]paracyclophanes represents a longstanding challenge in organic synthesis due to their unique structures, which are still highly dependent on the chiral chromatography separation technique and classical chemical resolution strategy to date. In this work, we report on an efficient and versatile kinetic resolution protocol for various substituted amido[2.2]paracyclophanes, including those with pseudo-geminal, pseudo-ortho, pseudo-meta and pseudo-para disubstitutions, using chiral phosphoric acid (CPA)-catalyzed asymmetric amination reaction, which was also applicable to the enantioselective desymmetrization of an achiral diamido[2.2]paracyclophane. Detailed experimental studies shed light on a new reaction mechanism for the electrophilic aromatic C-H amination, which proceeded through sequential triazane formation and N[1,5]-rearrangement. The facile large-scale kinetic resolution reaction and diverse derivatizations of both the recovered chiral starting materials and the C-H amination products showcased the potential of this method.

Planar Chiral Analogues of PRODAN Based on a [2.2]Paracyclophane Scaffold: Synthesis and Photophysical Studies

We describe the synthesis and photophysical characterization of differently substituted planar chiral analogues of PRODAN based on a [2.2]paracyclophane scaffold. This experimental and theoretical study highlights that the (chir)optical properties of the new "phane" compounds, which incorporate an electron-withdrawing propionyl moiety and an electron-donating dimethylamino group at their para or pseudo-para positions, strongly depend on their substitution patterns. In particular, for this series of molecules, a more pronounced solvatochromism and clear chiroptical behaviors are observed when the two substituents are placed on the two rings of the pCp core in a non-"co-planar" arrangement (pseudo-para derivative). This observation may help design new pCp-based luminophores with finely tuned photophysical properties.

Diversity-Oriented Synthesis of [2.2]Paracyclophane-derived Fused Imidazo[1,2-a]heterocycles by Groebke-Blackburn-Bienaymé Reaction: Accessing Cyclophanyl Imidazole Ligands Library

This report describes the synthesis of a [2.2]paracyclophane‐derived annulated 3‐amino‐imidazole ligand library through a Groebke‐Blackburn‐Bienaymé three‐component reaction (GBB‐3CR) approach employing formyl‐cyclophanes in combination with diverse aliphatic and aromatic isocyanides and heteroaromatic amidines. The GBB‐3CR process gives access to skeletally‐diverse cyclophanyl imidazole ligands, namely 3‐amino‐imidazo[1,2‐a]pyridines and imidazo[1,2‐a]pyrazines. Additionally, a one‐pot protocol for the GBB‐3CR by an in situ generation of cyclophanyl isocyanide is demonstrated. The products were analyzed by detailed spectroscopic techniques, and the cyclophanyl imidazo[1,2‐a]pyridine was confirmed unambiguously by single‐crystal X‐Ray crystallography. The cyclophanyl imidazole ligands can be readily transformed to showcase their useful utility in preparing N,C‐palladacycles through regioselective ortho‐palladation.

Nickel-Catalyzed Asymmetric Hydrogenation for Kinetic Resolution of [2.2]Paracyclophane-Derived Cyclic N-Sulfonylimines

Nickel-catalyzed asymmetric hydrogenation for kinetic resolution of [2.2]paracyclophane-derived cyclic N-sulfonylimines was successfully developed. High selectivity factors were observed in most cases (s up to 89), providing the recovered materials and hydrogenation products in good yields with high levels of enantiopurity. The recovered materials and hydrogenation products are useful synthetic intermediates for the synthesis of planar chiral [2.2]paracyclophane-based compounds.

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.

Regioselective Functionalization of [2.2]Paracyclophanes: Recent Synthetic Progress and Perspectives

[2.2]Paracyclophane (PCP) is a prevalent scaffold that is widely utilized in asymmetric synthesis, π-stacked polymers, energy materials, and functional parylene coatings that finds broad applications in bio- and materials science. In the last few years, [2.2]paracyclophane chemistry has progressed tremendously, enabling the fine-tuning of its structural and functional properties. This Minireview highlights the most important recent synthetic developments in the selective functionalization of PCP that govern distinct features of planar chirality as well as chiroptical and optoelectronic properties. Special focus is given to the function-inspired design of [2.2]paracyclophane-based π-stacked conjugated materials by transition-metal-catalyzed cross-coupling reactions. Current synthetic challenges, limitations, as well as future research directions and new avenues for advancing cyclophane chemistry are also summarized.

Highly Enantioselective Asymmetric Transfer Hydrogenation: A Practical and Scalable Method To Efficiently Access Planar Chiral [2.2]Paracyclophanes

We report herein a general, practical method based on asymmetric transfer hydrogenation (ATH) to control the planar chirality of a range of substituted [2.2]paracyclophanes (pCps). Our strategy enabled us to perform both the kinetic resolution (KR) of racemic compounds and the desymmetrization of centrosymmetric meso derivatives on synthetically useful scales. High selectivities (up to 99% ee) and good yields (up to 48% for the KRs and 74% for the desymmetrization reactions) could be observed for several poly-substituted paracyclophanes, including a series of bromine-containing molecules. The optimized processes could be run up to the gram scale without any loss in the reaction efficiencies. Because of its broad applicability, the ATH approach appears to be the method of choice to access planar chiral pCps in their enantiopure form.

3D Coumarin Systems Based on [2.2]Paracyclophane: Synthesis, Spectroscopic Characterization, and Chiroptical Properties

In this article, we report the preparation of a series of [2.2]paracyclophane-fused coumarin systems through a simple and general procedure involving a transition-metal-catalyzed cyclization of aryl alkynoates as the key step. We also highlight the influence of the [2.2]paracyclophane (pCp) motif and its "phane" interactions on the spectroscopic properties of the newly synthesized fluorophores, which emit in the blue-green region of the visible spectrum (λ_em up to 560 nm) and show extremely large Stokes shifts (up to 230 nm). Finally, we demonstrate that our straightforward approach can easily be used to access optically active planar chiral 3D coumarins. Compared to previously described fluorescent paracyclophanes and other organic dyes, our compact heteroaromatic derivatives show promising chiroptical properties, both in term of circular dichroism ( g_abs ∼ 8 × 10 ^-3) and circularly polarized luminescence ( g_lum ∼ 5 × 10 ^-3), thus demonstrating a practical application of our synthetic method.

Synthesis of paracyclophanes with planar and central chirality: kinetic resolution of [2.2]paracyclophane aldimines via palladium-catalyzed addition of arylboronic acids

Kinetic resolution of [2.2]paracyclophane aldimines was achieved through a palladium-catalyzed enantioselective arylation, providing access to chiral paracyclophanes with planar and central chirality as well as the recovered chiral aldimines.

Synthesis and photophysical studies of through-space conjugated [2.2]paracyclophane-based naphthalene fluorophores

New three-dimensional compact organic fluorophores with tunable photophysical properties are easily accessible from [2.2]paracyclophane using an intramolecular dehydrogenative Diels–Alder reaction as a key step.