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

Abstract

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.

Preparation and Synthetic Applications of [2.2]Paracyclophane Trifluoroborates: An Efficient and Convenient Route to Nucleophilic [2.2]Paracyclophane Cross‐Coupling Building Blocks

“…Besides this fundamental curiosity, PCP has received increasing attention for its wide applications as a planar chiral ligands in asymmetric synthesis, purely carbon and hydrogen based optical materials, molecular junctions and as a rigid backbone in light‐emitting diode TADF emitters …”

“…This has been described by Cram et al as "bent and battered". [1] Besides this fundamental curiosity, PCP has received increasing attention for its wide applications as a planar chiral ligands in asymmetric synthesis, [2] purely carbon and hydrogen based optical materials, [3] molecular junctions [4] and as a rigid backbone in light-emitting diode TADF emitters. [5] In the last couple of years, transition-metal mediated crosscoupling reactions have dramatically changed the face of modern paracyclophane chemistry.…”

Transannular Hydrogen Bonding in Planar‐Chiral [2.2]Paracyclophane‐Bisamides

“…Crystallization by slow evaporation from MeOH yielded colorless plates suitable for single crystal X-ray diffraction. 1 1(12),4,6,10,13,15-hexaene-5,11-dicarboxamide; (�)-1b). This compound was synthesized following the same method as (�)-1a using oxalyl chloride (0.10 mL, 1.2 mmol), (�)-4 (72 mg, 0.24 mmol), hexylamine (0.13 mL, 1.0 mmol), and DIPEA (0.13 mL, 1.0 mmol) in CH 2 Cl 2 (12 mL -1(12),4,6,10,13,15 .…”

“…Paracyclophane ([2.2]pCp) is a classically interesting hydrocarbon that has fostered a better understanding of chromophore interactions, aromaticity, and stereochemistry for well over a half-century. [1] Its utility comes from the proximity and non-planarity of its 'bent and battered benzene rings' [2][3][4] and rigid, structurally defined skeleton. The limited mobility and rotation of the bridging atoms and benzene 'decks' can lead to planar chirality in certain substituted [2.2]pCps, while the two phenyl rings are electronically coupled both through-bond [σ(bridge) -π(deck)] and through-space (transannular π -π) to perturb the molecule's chemical, electronic, and optical…”

Soft Matter Technology at KIT: Chemical Perspective from Nanoarchitectures to Microstructures

“…[39,40] A great advantage of the CVD process regarding PCP derivatives is the ability of substrate-independent coating and the avoidance of solvents or catalysts (Figure 3a). Synthetic variations of PCP led to a wide variety of (a)chiral PCP derivatives enabling the CVD process of phane-based structures, suitable for (opto)electronics and biomedical devices.…”