Formation of highly stabilized intramolecular dimer radical cation and pi-complex of [3n]cyclophanes (n = 3, 5, 6) during pulse radiolysis

Formation of radical cation and charge-transfer complex of [3n]cyclophanes (n = 3, 5, 6) was investigated by transient absorption spectroscopy during pulse radiolysis. Radical cations of [3n]cyclophanes showed the charge resonance band around 700 nm which exhibited a blue-shift as the number of trimethylene bridges increased, indicating formation of highly stabilized intramolecular dimer radical cation of [3n]cyclophanes. The absorption peak of the charge-transfer complex with chlorine atom also showed the shift in accord with the oxidation potential of [3n]cyclophanes.     

Synthesis of [m.n]Cyclophanes: Regiochemistry Transfer from Vinyl Halides to Cyclophanes via Fischer Carbene Complexes

The double benzannulation of bis‐carbene complexes of chromium with α,ω‐diynes generates [m.n]cyclophanes in which all three rings are generated in a single reaction. This triple annulation process is very flexible allowing for the construction of symmetrical [n.n]cyclophanes and unsymmetrical [m.n]cyclophanes as well as isomers in which the two benzene rings are both meta bridged or both para bridged, and isomers that contain both meta and para bridges. The connectivity patterns of the bridges in the cyclophanes can be controlled by regioselectivity transfer from the bis‐vinyl carbene complexes in which the substitution pattern of the vinyl groups in the carbene complexes dictate the connectivity pattern in the [m.n]cyclophanes. This synthesis of [n.n]cyclophanes is quite flexible with regard to ring size and can be used with tether lengths ranging from n=2 to n=16 and thus to ring sizes with up to 40 member rings. The only limitation to regioselectivity transfer from the carbene complexes to the [m.n]cyclophanes was found in the synthesis of para,para‐cyclophanes with four carbon tethers for which the loss of fidelity occurred with the unexpected formation of meta,para‐cyclophanes.     

Modular synthesis of pi-acceptor cyclophanes derived from 1,4,5,8-naphthalenetetracarboxylic diimide and 1,5-dinitronaphthalene

Three neutral cyclophanes were synthesized, and their association with indole, an aromatic pi-donor, was studied. The cyclophanes were designed to contain a rigid, hydrophobic binding cavity with 1,4,5,8-naphthalenetetracarboxylic diimide or 1,5-dinitronaphthalene as the pi-acceptor. Two of the cyclophanes also contain a (S)-(valine-leucine-alanine) tripeptide unit to provide chiral hydrogen bonding interactions with guest molecules. Despite the fact that these cyclophanes contain a hydrophobic binding cavity of appropriate dimensions, their association with indole is very weak. In the case of cyclophanes derived from 1,5-dinitronaphthalene, steric interactions force the nitro groups out of the plane of the naphthalene ring, diminishing their effectiveness as pi-acceptors. A simple UV--visible titrimetric method, using N,N,N',N'-tetramethyl-1,4-phenylenediamine (TMPD) as a pi-donor, was used to rank the pi-acceptor strength of these and other aromatic units. These titrations show that 1,4,5,8-naphthalenetetracarboxylic diimide and 1,5-dinitronaphthalene derivatives are weaker pi-acceptors than viologens, which make good pi-acceptor cyclophanes. Methyl viologen is in turn a weaker pi-acceptor than anthaquinone disulfonate, suggesting that the latter may serve as a useful building block for pi-accepting cyclophane hosts.     

Synthesis and guest-binding study of polytopic multi(cyclophane) hosts

Novel bis(cyclophanes) bearing glucosides and pentakis(cyclophanes) bearing glucosides were prepared as water-soluble hosts by connecting two or five macrocyclic skeletons, respectively. The guest-binding affinities of the present bis(cyclophanes) and pentakis(cyclophanes) toward a hydrophobic dye, 6-p-toluidinonaphthalene-2-sulfonate, were enhanced 13- and 1200-fold, respectively, relative to that by a corresponding monocyclic cyclophane, reflecting multivalency effects in macrocycles.     

Self-complementary Tetracationic Cyclophanes Containing 4,4′-Bipyridinium and 2,5-Dimethoxy-1,4-xylyl Subunits

4,4′-Bipyridinium-based tetracationic cyclophanes containing a 2,5-dimethoxy-1,4-xylyl unit were synthesized by using either m-terphenyl building blocks or incorporating a 4-hydroxy benzyl spacer between the complementary subunits. The cyclophanes show weak intramolecular charge-transfer (CT) bands in the visible region and one of the cyclophanes formed a green-coloured CT complex with ferrocene with an association constant (K _a) of 6.3?M^?1. The electrochemical parameters obtained for the cyclophanes indicate that all the redox processes are reversible.     

Azolium-linked cyclophanes: effects of structure, solvent, and counteranions on solution conformation behavior

This paper describes the synthesis, structural characterization, and solution behavior of some xylyl-linked imidazolium and benzimidazolium cyclophanes decorated with alkyl or alkoxy groups. The addition of alkyl/alkoxy chains to the cyclophanes allows for studies in chlorinated solvents, whereas previous solution studies of azolium cyclophanes have generally required highly polar solvents. The azolium cyclophanes may exist in a syn/syn conformation (azolium rings mutually syn, arene rings mutually syn) or a syn/anti conformation (azolium rings mutually syn, arene rings mutually anti). The preferred conformation is significantly affected by (i) binding of bromide (ion pairing) to the protons on the imidazolium or benzimidazolium rings, which occurs in solutions of bromide salts of the cyclophanes in chlorinated solvents, and (ii) the addition of alkoxy groups to the benzimidazolium cyclophanes. These structural modifications have also led to cyclophanes that adopt conformations not previously identified for similar azolium cyclophane analogues. Detailed (1)H NMR studies for one cyclophane identified binding of bromide at two independent sites within the cyclophane.     

Palladium-Catalyzed Enantioselective C−H Olefination to Access Planar-Chiral Cyclophanes by Dynamic Kinetic Resolution

Planar-chiral cyclophanes have received increasing attention for drug discovery and catalyst design. However, the catalytically asymmetric synthesis of planar-chiral cyclophanes has been a longstanding challenge. We describe the first Pd(II)-catalyzed enantioselective C−H olefination of prochiral cyclophanes. The low rotational barrier of less hindered benzene ring in the substrates allows the reaction to proceed through a dynamic kinetic resolution. This approach exhibits broad substrate scope, providing the planar-chiral cyclophanes in high yields (up to 99 %) with excellent enantioselectivities (up to >99 % ee). The ansa chain length scope studies reveal that the chirality of the cyclophanes arises from the bond rotation constraint of the benzene ring around the macrocycle plane, rather than the C−N axis. The C−H activation approach is also applicable to the late-stage modification of bioactive molecules and pharmaceuticals.     

Preparation of syn and anti cyclophanes having oligothiophene units and their spectral properties

Syn and anti cyclophanes consisting of oligothiophene units as a component have been synthesized for the first time. Correlation between the cyclophane structure and fluorescence spectral properties has been examined. Emission from intramolecular excimer-formation is confirmed for the syn cyclophanes and the mobile cyclophanes, but not for the anti cyclophanes.     

Benzidine/Quinoidal‐Benzidine‐Linked,Superbenzene‐Based π‐Conjugated Chiral Macrocycles and Cyclophanes

Synthesis of fully conjugated cyclophanes containing large‐size polycyclic aromatics is challenging. Now, three benzidine‐linked, hexa‐peri‐hexabenzocoronene (superbenzene)‐based ortho‐, para‐, and meta‐cyclophanes are synthesized through intermolecular Yamamoto coupling reaction of structurally pre‐organized precursors. Subsequent oxidative dehydrogenation gave the corresponding quinoidal benzidine‐linked cyclophanes. Their geometries were confirmed by X‐ray crystallographic analysis and their electronic properties were investigated by electronic absorption, cyclic voltammetry, and DFT calculations. The quinoidal benzidine‐linked cyclophanes show thermally populated paramagnetic activity with a relatively large singlet‐triplet energy gap. Two enantiomers for the ortho‐cyclophanes ( 1‐NH and 1‐N ) were isolated and their chiral figure‐of‐eight macrocyclic structures were identified. The cage‐like cyclophanes 2‐NH and 3‐NH with concave surface can selectively encapsulate fullerene C₇₀.     

Synthesis of a new class of electron rich chiral cyclophanes with large cavities

BINOL based electron rich chiral cyclophanes possessing large cavities have been synthesized starting from m-terphenyl dibromide and methyl m-/p-cinnamate. The presence of double bonds in the chiral cyclophanes makes them electron rich as revealed by C-T complexation studies of such cyclophanes donors with guests like TCNE, TCNQ and DDQ.     

咪唑环番仿生新体系研究进展

仿生化学是当今化学的挑战性领域之一。咪唑环番是构筑仿生体系的优良基体,但迄今研究工作极少。本文简要评述了咪唑的选择性反应和咪唑衍生物的选择性合成、新型咪唑环番和咪唑鎓环番的设计合成及仿生功能研究的一些新进展。     

Synthesis of meta- and paracyclophanes containing unsaturated amino acid residues

[7.7], [8.8], [9.9] and [13.13] Paracyclophanes and [9.9] and [13.13] metacyclophanes containing two unsaturated amino acid residues have been synthesised. An X-ray crystallographic study of three of the paracyclophanes and molecular modelling of two paracyclophanes and two metacyclophanes revealed two main structural types. The ‘staggered’ structure appears to be favoured by longer hydrocarbon chains, whilst the ‘barrel’ structure appears to be more accessible to compounds containing shorter hydrocarbon chains. The [9.9] paracyclophane has been hydrogenated and deprotected to give a saturated amino acid, and an alternative approach to key aldehydes is reported.     

Synthesis of [n]- and [n.n]cyclophanes by using Suzuki-Miyaura coupling

Reaction of the bis-9-BBN adduct of several dienes with 1,3-dibromobenzene via Suzuki coupling leads to a series of [n]metacyclophanes ranging in size from 10 to 17 atom members. In each case, two carbon-carbon bonds are formed in one reaction vessel. However, when the bis-9-BBN adduct of 1,5-hexadiene is coupled with a variety of aryl dihalides, larger [n.n]cyclophanes were formed in preference to the [n]cyclophanes. Four carbon-carbon bonds are formed in this instance. Single-crystal X-ray analyses of these [n.n]cyclophanes reveal interestingly shaped molecules with large cavities.     

Transannular distance dependence of stabilization energy of the intramolecular dimer radical cation of cyclophanes

The intramolecular dimer radical cation and charge-transfer complex of various cyclophanes were investigated by using pulse radiolysis measurements. The charge resonance band due to the dimer radical cation of cyclophanes appeared in the near-IR region, which showed a blue-shift as the distance between the two benzene rings of cyclophane decreased. The stabilization energy of the dimer radical cation, which was estimated from the peak position of the charge resonance band, was explained by the exchange interaction, while the substituent effect was small. The absorption peak of the charge-transfer complex with chlorine atom also showed the shift in accordance with the oxidation potential of cyclophanes.     

Cyclophanes: Simulation of the formation enthalpy by quantum-chemical methods

Comparison of experimental values of gas-phase formation enthalpy of cyclophanes with the results of semiempirical quantum-chemical simulation has revealed good linear correlation with the PM-3 data. Using the derived equation, formation enthalpies of 30 cyclophanes have been predicted.     

Polycationic Redox-Active Cyclophanes with Integrated Electron-Rich Diboron Units

Cationic cyclophanes are widely used in a variety of applications in supramolecular chemistry and materials science. In this work the authors systematically study the integration of electron-rich diboron units with B^II atoms into polycationic cyclophanes with viologen-like electron-acceptor units. They also report a first hexacationic cage-compound in which three diboron units connect two tris(4-pyridyl)triazine acceptor units. Moreover, di- and tetracationic open-structure compounds, in which one diboron unit connects two bispyridyl groups, were synthesized and the properties compared to those of the corresponding closed structures (cyclophanes). The combination of diboron electron-donor units and bi- or oligopyridyl electron-acceptor units leads to intriguing optical and redox properties.     

Benzidine/Quinoidal-Benzidine-Linked,Superbenzene-Based π-Conjugated Chiral Macrocycles and Cyclophanes

Synthesis of fully conjugated cyclophanes containing large-size polycyclic aromatics is challenging. Now, three benzidine-linked, hexa-peri-hexabenzocoronene (superbenzene)-based ortho-, para-, and meta-cyclophanes are synthesized through intermolecular Yamamoto coupling reaction of structurally pre-organized precursors. Subsequent oxidative dehydrogenation gave the corresponding quinoidal benzidine-linked cyclophanes. Their geometries were confirmed by X-ray crystallographic analysis and their electronic properties were investigated by electronic absorption, cyclic voltammetry, and DFT calculations. The quinoidal benzidine-linked cyclophanes show thermally populated paramagnetic activity with a relatively large singlet-triplet energy gap. Two enantiomers for the ortho-cyclophanes ( 1-NH and 1-N ) were isolated and their chiral figure-of-eight macrocyclic structures were identified. The cage-like cyclophanes 2-NH and 3-NH with concave surface can selectively encapsulate fullerene C₇₀.     

Hydrogen-bond-assisted pi-stacking of shape-persistent cyclophanes

The pi-stacking interaction between shape-persistent cyclophanes works cooperatively with multiple hydrogen bonding sites to form cyclophane dimers. These findings considerably broaden the applicability of pi-stacking interactions as a driving force in self-assembly chemistry. A gel formation effect was also noticed in one of the cyclophanes.     

1,x-elimination reactions: extending the limits of a classical organic reaction

Alpha,omega-dibromo derivatives in which the two terminal carbon atom are separated by an unsaturated spacer unit ("pi spacer") undergo 1,x-elimination reactions (with x=6, 8, 10, and 14), using Mori's reagent (nBu3SnSiMe3/CsF). The resulting cumulenic intermediates cyclodimerize in a subsequent step yielding novel macrocyclic acetylenic and bridged aromatic compounds (cyclophanes). Thus 1,6-eliminations were carried out with dibromide 17 to yield 1,3,7,9-cyclododecatetrayne (20) and with benzylbromide 24 to provide cyclophanes 26 and 27. By 1,8-eliminations the 16-membered macrocycle 33 could be prepared from enediyne 31, the benzannelated 1,5-cyclooctadiyne 41 from dibromide 38, and a mixture of cyclophanes 45 and 46 from the precursor 43. 1,10-Eliminations were carried out successfully with dibromides 47, 50, and 53 yielding the corresponding unsaturated cyclophanes ("cyclophynes") 49, 52, and 55. The influence of the solvent on the cyclodimerization 47-->49 was investigated, with acetonitrile providing the highest yields. The heterophanes 59 a and b were obtained by 1,10-elimination of the precursor dibromides 57 a and b, and in an elimination experiment involving a 1:1 mixture of the dibromides 50 and 57 b the "mixed dimer" 60 was isolated, besides the homodimers 52 and 59 b. The method reached its limits with the 1,14-elimination of 68, 70, and 74 providing the cyclophanes 69, 71, and 75 in varying amounts. Two final debrominations with 76 and 77, which in principle could undergo 1,16- and 1,20-eliminations reactions, respectively, failed. The structures of the new cyclophanes 49, 50, 59 a, and 59 b were established by X-ray structural analysis; all other structure assignments rest on the usual spectroscopic and analytical data.     

Electron-Rich,Lewis Acidic Diborane Meets N-Heterocyclic Aromatics: Formation and Electron Transfer in Cyclophane Boranes

Herein reported are the reactions of an electron-rich, Lewis acidic diborane with N-heterocyclic aromatics to give first members of an unprecedented family of highly charged cationic cyclophanes with diboranyl units. Tetracationic cyclophanes with 4,4‘-bipyridine/ 1,2-bis(4-pyridyl)ethylene and diboranyl units were synthesized and their redox chemistry was studied. Cyclisation of two diboranyl and two pyrazine units is accompanied by electron transfer from the diboranyl unit to the pyrazine. Our results pave the way for the integration of redox-active diboranyl units into cyclophanes and supramolecular structures.