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

Abstract [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.

1. Introduction and History 1.1. PCP:F rom Synthetic Curiosity to Inspiring Functions [2.2]Paracyclophane was discovered by Brown and Farthing in 1949 by the gas-phase pyrolysis of para-xylene under low pressure. [1] Twoyears later, Cram and Steinberg reported the first synthesis of this novel and intriguing compound by intramolecular cyclization. [2] Since then, this rather unusual "bent and battered" [3] strained organic scaffold has sparked numerous investigations and received attention because of its intriguing structural and electronic properties.These result in unique physical and chemical behavior,a ccompanied by aesthetically pleasing structures. [4] [2.2]Paracyclophane (1) consists of two cofacially stacked, strongly interacting benzene rings (decks) with an average ring-to-ring distance of 3.09 .T his is far less than the standard van der Waals distance of 3.40 observed between layers in graphite. [5] The phenyl rings in PCP are stacked cofacially in proximity,h eld together by two ethylene "bridges" (2.83 )a tt he bridgehead carbon atoms in a para orientation (see molecular structure in Figure 1, top). [6] Thestacking of the two benzene rings leads to as train energy of about 31 kcal per mole for PCP, [7] and causes ad istortion so that the two benzene rings are forced to bend out from planarity at the bridgehead carbon atom by 12.68 8 out of the benzene plane.This provides the basis for distortion abnormalities from aromatic planarity and for correlation between the properties and unusual reactivity behavior. In larger [n.n]cyclophanes,s uch as [3.3]paracyclophanes and [4.4]paracyclophanes, [8] the longer propyl or butyl bridges between the two benzene rings allow the decks to be further apart (3.3 avg.) and, therefore,less strained (ca. 12 kcal mol À1 ), while [6.6]paracyclophane is nearly strain-free (2 kcal mol À1 ), comparable to an open-chain compound. [3] Thestrain energy,conformations,and rotational barriers of the [2.2]-, [3.3]-, and [4.4]paracyclophanes predicted by DFT studies are in close agreement with experiment. [9] Thep ioneering research on [2.2]paracyclophane chemistry has been predominantly mechanistic in nature,d emon-strating structure/reactivity relationships,a nd understanding the unusual physical/chemical properties of PCP. In particular,t he development of synthetic methods reported by the groups of Hopf, [10] de Meijere, [11] Longone, [12a] Misumi, [12b] and others have greatly actuated the formation of various molecular stacks such as ring-fused (2), bridge-extended (3), and quadruple stacks (4)a sw ell as other chemical topologies to examine molecular strain and transannular p-p interactions. [13] [2.2]Paracyclophane chemistry has evolved from functional molecules to functional materials and from synthetic curiosity to emerging applications in asymmetric synthesis,e nergy materials, p-stacked polymers,and functional parylene coatings (polymer made by polymerization of PCP induced by vapor-phase pyrolysis). [14] Numerous material applications dealing with planar chirality and through-space conjugation have been the subject of excellent reviews. [15] With such alarge body of work, it is impossible to cover every aspect here.The aim of this Minireview is to describe most of the recent advances and some landmark results that are particularly appealing for chemists,m aterial scientists,a nd engineers aiming to work in the areas of 1) cyclophane chemistry,2 )design and development of new ligand and catalyst systems,3 )asymmetric synthesis,a nd 4) advanced polymer materials.

Structure-Property Relationships of PCPs:F rom Asymmetric Synthesis to Materials Applications
Using carefully chosen reaction parameters and transformation steps,the PCP core allows different substituents to [2.2]Paracyclophane (PCP) is aprevalent scaffold that is widely utilized in asymmetric synthesis, p-stacked polymers,energy materials, and functional parylene coatings that finds broad applications in bioand materials science.Int he last few years, [ 2.2]paracyclophane chemistry has progressed tremendously,enabling the fine-tuning of its structural and functional properties.T his 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 pstacked conjugated materials by transition-metal-catalyzed crosscoupling reactions.Current synthetic challenges,limitations,aswell as future researchdirections and new avenues for advancing cyclophane chemistry are also summarized.
be positioned regioselectively at either only one ( Figure 2; 5-9)o rb oth benzene rings (Figure 2; 10-17). Functional moieties can be incorporated directly onto the benzene rings (such monosubstituted PCPs possess planar chirality) or the ethylene bridges,s uch as in PCP 18,w hich lead to centrally chiral compounds.T he substitution pattern of PCPs heavily influences the nature of the compound and even minor changes can alter its properties significantly.
Particular descriptors are employed to designate the relationship of the two substituents in disubstituted PCP derivatives.F or disubstitution on one phenyl ring, the conventional prefixes of ortho-, para-, and meta-are used, while disubstitution on both benzene rings gives rise to pseudo-geminal,p seudo-meta,p seudo-para,a nd pseudoortho prefixes (Figure 3).
Appropriatelyf unctionalizedd erivativeso fP CP have founda mple applications as oneo ft he most successful, versatile, andc ommonlyu sedc lasses of planar chiral ligands or chiral catalysts forstereoselective syntheses, second only to ferrocene-baseds ystems (e.g.t he JosiPhos family). The applicationo fP CP as ap lanarc hirall igand hasb een extensivelyi nvestigated by theg roupso fR ossena nd Pye, [16] Rozenberg, [17] Rowlands, [18] Bolm, [19] Paradies, [20] andBräse. [21] Our research groups have devoted significant efforts over the past two decades to the development of new and generally useful classes of enantiomerically pure mono-and disubstituted [2.2]paracyclophane-based planar chiral ligands and catalysts.T hey have been successfully employed for various synthetically important stereocontrolled and enantioselective transformations,f or example,t he addition of alkyl, aryl, alkynyl, and alkenyl zinc reagents to aromatic and aliphatic aldehydes and imines. [21] In this Minireview,w ec onfine our discussion only to the molecular design and specific structural features of some chiral representatives,based on the nature of the chirality,donor atoms,and their denticity,such as di-, tri-, and tetradentate N,O-[2.2]paracyclophane ligands (19)(20)(21).
Some prominent examples of planar chiral PCP ligands are the P, P-ligand PhanePhos (22), mixed P, N-ligands containing pyridine or quinoline (23), as well as N,N-ligands such as bisoxazoline (24), all of which have shown remarkable performance in asymmetric catalysis (Figure 4). Ad etailed description of their preparations and diverse applications as ligands or catalysts in aw ide range of asymmetric syntheses can be found in the literature. [22] Thefunction-inspired design of through-space-conjugated molecular assemblies based on [2.2]paracyclophane and their

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Minireviews 2158 www.angewandte.org numerous applications has been ap rominent objective in diverse areas of materials research. To engineer discrete p-stacks of aromatic assemblies,t he groups of Chujo, [23] Bazan, [24] Collard, [25] and others [26] have employed specific PCP precursors with the aim of tuning the photophysical, optoelectronic,a nd electrochemical features of substantial chemical and industrial importance ( Figure 5). Rigidity, stability,p lanarity,t olerance to moisture,a nd p-stacking of the PCP core fulfill the critical requirements for organic electronic materials.

Difunctionalization of PCP at One or Both Benzene Rings: Reactivity/Selectivity
Disubstituted PCPs are accessible starting from monosubstituted PCP.T he mechanisms of the regioselective electrophilic aromatic substitution reactions of PCPs and their interconversions were investigated by Cram and Reich. [3] As trong directing effect by the first substituent on PCP derivatives was observed, which is known as the transannular directive effect. [39] Thep seudo-geminal regioisomers are more easily accessible and can be efficiently prepared selectively through the electrophilic aromatic substitution of monosubstituted [2.2]paracyclophane derivatives.Inthe iron-catalyzed bromination of 1,modest regioselectivity is observed, with pseudopara-( 26 %) and pseudo-ortho-dibromides (16 %) as the major isolated products accompanied by lesser amounts of the pseudo-meta-(6%)and para-dibromides (5 %). [33,34] From ageometric point of view,the two appending moieties on the [2.2]paracyclophanes can be held parallel to each other, that is,p seudo-geminal (44), antiparallel in pseudo-para (45)a nd para arrangements (48), and finally two variants of V-shaped geometries are possible in pseudo-ortho (46)a nd ortho (47; 608 8), meta (49), and pseudo-meta arrangements (50;1 208 8)a s depicted in Scheme 2. Thet ransannular-directed regioselective bromination to pseudo-geminal PCP derivatives was first reported by Reich and Cram for various carbonyl-substituted PCP derivatives (44), such as carboxylic acid, methyl ester, acetyl derivatives,a nd nitro-substituted derivatives. [34] By employing at hermal isomerization procedure,p seudo-meta isomers (50)c an be accessed from the readily available pseudo-geminal isomer (44). Thep roximity of the two substituents within the pseudo-geminal substitution pattern of isomer 44 drives the equilibrium towards the thermodynamically more favored pseudo-meta isomers (50)asaresult of steric repulsion. [34] Thus,abroad set of architectural building blocks with different geometrical arrangements, Thet hermal isomerization proceeds by homolytic cleavage of atwo-carbon bridge to form ad iradical, rotation, and subsequent ring closure.T he isomerization of pseudo-para products into their isomeric pseudo-ortho derivatives was investigated in detail by the groups of Rozenberg [46] and Hopf. [41] Braddock et al. developed cleaner reaction conditions (180 8 8C, 6min, in DMF) for ahigh-yielding synthesis of up to 38 %f or the pseudo-ortho-dihydroxy[2.2]paracylophane,w ith simple separation of the two enantiomers by precipitation. [42] Am icrowave-assisted method for the isomerization of pseudo-para-dibromo[2.2]paracyclophane into the corresponding pseudo-ortho-dibromo[2.2]paracyclophane in DMF has also proven useful. [43] Recently, regioselective direct pseudo-ortho-metalation, ortho-halogenation, and para-selective acetylation through nonconventional functionalization strategies involving activation of carbon-hydrogen bonds have been reported. This is discussed in the upcoming section on nonconventional functionalization strategies for PCP.

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Chemie sired substituents on the aromatic rings in the appropriate configuration, followed by desulfurization. [49] This route has the advantage that heterocycles can be incorporated into the skeleton. TheD iels-Alder cycloaddition of 1,2,4,5-hexatetraenes with symmetrically or unsymmetrically substituted acetylene to yield the corresponding tetrasubstituted [2.2]paracyclophane derivatives has also been reported. [50]

Conventional versus Nonconventional Functionalization Strategies of [2.2]Paracyclophane
Synthetic strategies for the functionalization of aP CP molecule mostly rely on electrophilic substitution reactions or certain transformations via preinstalled functional groups. Despite the significant advances in metal-catalyzed C À H bond functionalization, the direct functionalization of [2.2]paracyclophane has remained largely unexplored. The nonconventional functionalization approach is atom-economic with aminimal number of steps,skips prefunctionalization, and can, therefore,m inimize tedious synthetic efforts.A s ac onsequence of the chemically similar nature of the CÀH bonds in PCP molecules,s ite selectivity is one of the main challenges in direct functionalization. Thef irst palladiumcatalyzed direct C À Hb ond acetoxylation of [2.2]paracyclophanes 66 was reported by Bolm and co-workers,a nd was suitable for obtaining various ortho-substituted hydroxy-[2.2]paracyclophane derivatives 68 via cyclophane-based palladacycles 67 using 1-5 mol %p alladium(II) acetate in combination with iodobenzene diacetate as an oxidant. [51] The direct ortho-selective acetoxylation can be effectively directed by aldoxime ethers,k etoxime ethers,a nd esters,w ith 2pyridyl and pyrazole acting as active directing groups on the [2.2]paracyclophane (Scheme 5, top). Although oximes proved to be excellent ortho-directing groups for Pd-catalyzed ortho-CÀHactivation, numerous other common directing groups were inactive,either because they were incompatible under the reaction conditions or too bulky (Scheme 5).
Following the pioneering studies on the Pd-catalyzed ortho-acetoxylation of PCP,aPd-catalyzed selective orthobromination/iodination procedure employing [2.2]paracyclophane derivative 66 b was developed. This method provided swifter access to 4,5-disubstituted [2.2]paracyclophanes via ortho-functionalized intermediates 69 a,b.H owever, a20mol %catalyst loading is required (Scheme 5, bottom). [52] Thed escribed ortho-selectivity of the C À Hb ond functionalization in aP CP backbone was achieved with an Omethyloxime directing group.T he synthetic value of this procedure was further demonstrated by exemplary conversions of the carbaldehyde and halogen groups.
Subsequently,Bolm and co-workers extended the scope of the regioselective ortho-C À Hf unctionalization to acidic conditions to synthesize ap lanar chiral (1,4)carbazolophane 71 by oxidative cyclization under aerated conditions starting from N-phenylamino[2.2]paracyclophane 70 obtained by Hartwig-Buchwald CÀNc ross-coupling of the respective anilines and 4-bromo[2.2]paracyclophane (Scheme 6A). [53] In contrast to the ortho-acetoxylation to form 68,asignificantly increased catalyst loading of 20 mol %was used to obtain 71 in 62 %y ield. It is noteworthy that the oxidative cyclization does not occur with the N-methylated derivative.A lthough the reaction conditions are rather harsh, this procedure is reported to be successful for awide range of electron-rich and -poor aniline derivatives,w hich give access to the very interesting class of carbazolophanes.T hey have the potential to replace the ubiquitously used carbazole group in anumber of material science applications,w here the increased steric bulk or planar chirality are of key interest. [28a] Recently,t he para-C À Hf unctionalization of phenylamino-and acetamido-substituted PCPs (72 a-d)m ediated by phenyliodide diacetate (PIDA) as an oxidant (Scheme 6B) was reported. [54] Va rious nucleophiles such as acetate,f ormate,m ethanolate,e thanolate,a nd bromide could be successfully positioned at the para-position (73 a-d). Insight into the mechanism was gained when an excess of PIDAw as added, which give ab enzoquinimine 74 in 54 %y ield. The presence of an oxidized ketone intermediate is strongly supported. In asimilar manner,benzoquinone 78 was already reported by Cram and Day in 1966 and is ac onvenient intermediate, [55] as both the enantiopure precursors 75 are readily available and the benzoquinone 78 can be easily converted into the para-bistriflate 79,animportant synthon in cross-coupling reactions (Scheme 7). [56] Efforts in C À Hf unctionalization have been successful because directing groups on PCP change the reactivity of the nearby CÀHb ond. However,n ew methods in CÀHf unctionalization (without directing groups) to install active functionalities sequentially either at one or both benzene rings of the [2.2]paracyclophane backbone were only discovered recently by Yu and coworkers. [57]
Racemic 4-formyl[2.2]paracyclophane (80)c an be easily enantioenriched by fractional crystallization of the diastereomeric mixture of the Schiff base derivative 81 with (R)-aphenylethylamine.T he enantiomerically and diastereomerically pure imine is easily hydrolyzed under SiO 2 /acidic conditions to afford (S P )-aldehyde 80 (Scheme 8). The enantiomeric excess can be conveniently monitored by 1 HNMR spectroscopy of the imine hydrogen atom. This procedure has proven efficient and convenient for an orthohydroxyformyl PCP derivative,akey building block and chiral ligand for asymmetric catalysis. [59] Recently,Benedetti, Micouin, and co-workers reported an efficient kinetic resolution procedure involving asymmetric transfer hydrogenation (Scheme 8B), which gives the desired key intermediate on ag ram scale. [62] This method can be used for the kinetic resolution and desymmetrization of difunctionalized PCP derivatives bearing an aldehyde functionality. [63] Rowlands and Seacome reported am ethod for the preparation of monosubstituted chiral sulfoxides 86 using readily available chiral sulfinic ester derivatives,s uch as toluenesulfinate 84 [64] and thiosulfinate 85, [65] as suitable derivatizing agents for the PCP core (Scheme 9). In contrast to the unstable Schiff bases,the diastereomeric sulfoxides 86 can be separated by column chromatography on at en gram scale, [65] with > 99 % ee in the case of 86 b. [64] Thereafter,t he sulfoxide group is cleaved by n-BuLi to obtain an enantiopure lithiated PCP intermediate that can be quenched with an umber of nucleophiles or can be derivatized to ac hiral thiol. Essentially,this is apromising method to access awide range of precursors in an enantiopure way.

[2.2]Paracyclophanes as Modular Building Blocks: Application-Based Design Considerations of p-Stacked Conjugated Polymers, Macrocycles, and Devices
Tr ansition-metal-catalyzed reactions for the formation of carbon-carbon bonds have become an essential tool in

Functionalized [2.2]Paracyclophanes as Modular Building Blocks in p-Stacked Conjugated Polymers
In the last couple of years,m ilder, broader,a nd more efficient transition-metal catalysts have dramatically changed the face of modern paracyclophane chemistry,a nd an ew dimension has been opened for the exploration of the PCP scaffold towards novel p-stacked conjugated polymers.
[2.2]Paracyclophanes containing halides and pseudohalides represent versatile modular molecular building blocks in the design and development of hole-transporting materials [77] and helically structured chiral macrocycles. [78] These building blocks can be incorporated into numerous p-stacked conjugated polymers to introduce the innate physical/chemical properties of PCP such as their planar chirality and layered structure. [79] As Figure 6i llustrates,t he Suzuki-Miyaura, Sonogashira-Hagihara, and Mizoroki-Heck reactions are the most common Pd-catalyzed coupling routes for the assembly of p-stacked conjugated polymers containing iodo-, bromo-, vinyl-, ethynyl-, and formyl-substituted [2.2]paracyclophanes as key components in their skeleton. [80] Structurally different PCP derivatives such as pseudo-geminal- ( 44), pseudo-para- ( 45), and pseudo-ortho-dibromo- [2.2]paracyclophane (46)a llow the construction of various p-stacked conformations such as linear, zig-zag, and fully stacked structures.D ifferent p-systems,s uch as donor (fluorene) and acceptor (2,1,3-benzothiadiazole) segments can be alternately incorporated as co-monomers to tune the energy levels and charge-transfer properties in the resulting pstacked polymer system. [29] In as imilar way,c harge-transfer polymers consisting of [2.2]paracyclophane-based dithiophenes,c arbazoles,t hieno [3,4-b]pyrazine,a nd ferrocenyls (Figure 6, bottom) in the main chain have been prepared using palladium-catalyzed synthetic procedures. [79] Thee lectronic and optical properties of the polymer backbone can also be altered through the external co-monomers (Figure 6A-G).
Hopf and co-workers have reported as eries of diverse ethynyl [2.2]paracyclophanes obtained through aPd-catalyzed Sonogashira-Hagihara cross-coupling reaction of their corresponding brominated and/or formylated precursors. [81] These carbon-rich acetylene-tagged cyclophanes can be employed as new building blocks in copper-catalyzed alkyne-azide click (CuAAC) reactions as well as multifold Sonogashira-Hagihara cross-coupling reactions to design and build complex extended molecular scaffolds.

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Chemie bridged bis-imidazole dimer is composed of two photochromic units and is an ideal scaffold to study stepwise twophoton-gated photochemical phenomena ( Figure 7C). The two imidazole rings are constrained and restrict the diffusion of the radical, hence the rate of the thermal back reactions can be tuned on time scales from sub-microseconds to hundreds of milliseconds. [87] Upon absorption of the first photon by 98, as hort-living biradical species is generated and as econd photon absorption results in at etraradical species which undergoes ar apid reaction to the long-lived quinoid 99.

Chemical Vapor Deposition to Functional Surface Coatings
[2.2]Paracyclophanes and their functionalized derivatives are well-established precursors for the formation of poly-(para-xylylene) polymers (parylenes). As first described by Gorham, [88] [2.2]paracyclophane can be cracked homolytically at the ethylene bridges at high temperatures,w hich generates 1,4-quinodimethanes (para-xylylene;F igure 8A). After deposition of these reactive intermediates from the gas phase,asubstrate-independent polymerization occurs at the interface.
This process is named chemical vapor deposition (CVD). Although the strain and chirality of the PCP is lost, one of the advantages of parylene coatings generated from (functional) PCP monomers is the absence of any side products.T herefore,t his CVD process has found ample application in the coating of interesting biological and optoelectronic devices. [89] As numerous functional groups are stable under the furnace conditions,t he transfer of functional groups from the [2.2]paracyclophane monomer to an interface is possible.T o date,c ompelling results have been obtained with functionalized CVD coatings,e specially with regard to three-dimensional polymer nanostructures and bio-interface engineering. [90] As functionalized PCP derivatives generate reactive parylene coatings with active tunable functional groups at the interface,ageneric surface-engineering procedure becomes available.F urthermore,m icrostructuring by sequential CVD with patterning and nanolithography techniques has been developed ( Figure 8B). [91] Thes urface-deposited functional groups are readily accessible for post-deposition surface functionalization, for example,b yo rthogonal "click" reactions of terminal alkynes with biomolecules to generate devices on anano-to micrometer scale. [92] Recently,CVD was reported within liquid-crystal (LC) droplets.A ni ntriguing shape control of the resulting nanofibers was observed to be dependent on the anisotropy of the liquid crystals (Figure 8C). It is believed that the functional 1,4-quinodimethane biradicals diffuse within the liquid crystals and follow the lattice structure of the respective LC template.F or example, cholesteric, porous polymeric structures were reported when appropriate LC templates were used. [93] This finding opens an ew platform for functional polymer nanostructures,a s chirality can be templated to well-ordered 3D soft-matter architectures and various functional groups can be introduced for post-functionalization to accomplish sensing,filtration, or catalytic applications.  , with an extraordinarily high binding affinity of K a > 10 12 m À1 in water. [94] In their study,t he methylated 4-pyridyl [2.2]paracyclophane derivative 100 (synthesized by Suzuki-Miyaura cross-coupling employing (rac)-4-bromo[2.2]paracyclophane and 4-pyridylboronic acid as cross-coupling partners), was used as acompeting indicator for the drug memantine (101), which exhibits alarge Stokes shift when bound in the cavity of CB[8] (Scheme 11). An indicator displacement assay was constructed, which was able to determine the concentration of this commercially available Alzheimer drug in blood serum in ap hysiologically relevant sub-to low micromolar concentration range.

Conclusion and Outlook
The [ 2.2]paracyclophane scaffold is celebrating its 70th birthday and has been investigated for decades because of its unusual chemical and stereochemical features as well as applications within catalysis and materials.H owever,i ts till holds many surprises.D espite the impressive progress, synthetic challenges still remain and it is still sometimes hampered by low-yielding functionalization methods.Significant advances in metal-catalyzed CÀHbond functionalization have been made,but the direct functionalization of [2.2]paracyclophane has scarcely been studied. New methods of C À H functionalization at the PCP backbone with excellent selectivity and improved reactivity to install ab road range of functionalities are very much desired goals,but are yet to be discovered. Investigation of the PCP derivatives as chiral ligands in asymmetric catalysis has been one of the most active areas so far;however, this scaffold with its key features of ar igid, chiral, and stable building block is emerging in other fields of research as it possesses manifold applications that are to be explored. Among them are,for example,use as achiral drug derivative,athrough-space light/energy-harvesting material, aC VD coating precursor,i ns upramolecular host-guest assays,and most noteworthy as abuilding block in materials with asophisticated three-dimensional architecture. Selective functionalization at specific positions of the PCP backbone,which allows for the incorporation of avast range of substitutents,i sp articularly important from as ynthetic point of view and with regard to materials perspectives.  Designing chiral functional PCPs for CVD may open an ew dimension in the development of helically twisted nanofibers and thin films.PCPs as functional molecules are now evolving toward functional materials of significant topological complexity.
All this fascinating research is continuously bottle-necked by the challenging chemistry of the [2.2]paracyclophane, which demands for convergence between synthesis and engineering.D espite remaining challenges,h owever,i tc an be anticipated that focusing on [2.2]paracyclophanes will stimulate further research and will be scientifically rewarding in countless ways.W ea re looking forward to exciting new applications being uncovered in the near future.