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.     

Persistance of the Cyclopnane Radical Anions and its Relation to Structure

Reactions of [2_N]cyclophanes (N = 2, ?6) with solvated electrons in 1,2-di-methoxyethane at 193 K have been studied by ESR. and ENDOR. spectroscopy. All but the two most highly bridged cyclophanes (N = 5 and 6) are reduced to paramagnetic species under these conditions. Whereas the radical anions of [2.2]-paracyclophane and [2₃](1,2,4)- and [2₄](l,2,4,5)cyclophanes are sufficiently persistent to be characterized by their hyperfine data, those of the remaining five cyclophanes undergo a rapid cyclization to the radical anions of 4,5,9,10-tetrahydropyrenes. These have been identified as the unsubstituted tetrahydropyrene (from [2.2]-metacyclophane and [2₃](l,2,3)cyclophane), the 2,7-dimethyl-derivative (from [2₃](1,3,5)- and [2₄](l,2,3,5)cyclophanes) and the 1,8-dimethyl-derivative (from (2₄l,2,3,4)cyclophane). The persistence of the cyclophane radical anions seems to depend on the numbers, n_meta and n_para, of the meta-and para-positions of the bridging ethano groups in the two benzene rings. The prerequisite for the radical anion to be persistent is n_meta?n_para.     

NMR studies of conformations and dynamic processes—1 : [24]cyclophanes with ethylene bridges

The conformations and dynamic processes in a series of relatively unstrained [2₄]paracyclophanes (with one, two or four -CH₂-CH₂-bridges) and some closely related compounds have been analysed. Their ¹NMR spectra have been recorded at low temperatures and the temperature dependence rationalised as being due to essentially two types of dynamic process-the torsional motion around the sp³-sp³ C-C bonds in the bridges, and the rotation around the sp²-sp³ C-C bonds adjacent to the benzene rings. The barriers to the former process are similar for the series of cyclophanes 1–6 and are due to steric and electronic interactions in the syn-oriented transition states. In cyclophanes 7–9, in which anti-orientations of the aromatic rings are possible, the barriers are lower. The latter process, involving the rotation of the benzene rings, becomes important at temperatures below 150 K and has not been further analysed.     

The Radical Cations of 4,5,7,13,15,16-Hexamethyl- and 4,5,7,8,12,13,15,16-Octamethyl [2.2]paracyclophane

4,5,7,13,15,16-Hexamethyl- (3) and 4,5,7,8,12,13,15,16-octamethyl[2.2]paracyclophane (4) have been oxidized to their radical cations in solution under relatively mild conditions. Substantial hyperfine splittings in the ESR. spectra of 3⊕. and 4⊕. arise from the methyl protons, whereas those from methylene protons are very small. This result indicates that the ethano bridges, unlike the methyl substituents, are rather ineffective in delocalizing the positive charge in 3⊕. and 4⊕. It is in line with the interpretation proposed previously to rationalize the gas-phase ionization potentials of multiply bridged [2_N]cyclophanes and methyl derivatives of [2.2]paracyclophane. The π-spin distributions in 3⊕. and 4⊕. are discussed in terms of a simple model in which the singly occupied orbitals are represented as the antibonding combinations of two benzene HOMO's.     

Dual Activation of Aromatic Diels–Alder Reactions

The unusually fast Diels–Alder reactions of [5]cyclophanes were analyzed by DFT at the BLYP-D3(BJ)/TZ2P level of theory. The computations were guided by an integrated activation-strain and Kohn–Sham molecular orbital analysis. It is revealed why both [5]metacyclophane and [5]paracyclophane exhibit a significant rate enhancement compared to their planar benzene analogue. The activation strain analyses revealed that the enhanced reactivity originates from 1) predistortion of the aromatic core resulting in a reduced activation strain of the aromatic diene, and/or 2) enhanced interaction with the dienophile through a distortion-controlled lowering of the HOMO–LUMO gap within the diene. Both of these physical mechanisms and thus the rate of Diels–Alder cycloaddition can be tuned through different modes of geometrical distortion (meta versus para bridging) and by heteroatom substitution in the aromatic ring. Judicious choice of the bridge and heteroatom in the aromatic core enables effective tuning of the aromatic Diels–Alder reactivity to achieve activation barriers as low as 2 kcal mol⁻¹, which is an impressive 35 kcal mol⁻¹ lower than that of benzene.     

NMR studies of conformations and dynamic processes—III : Cyclophanes with unsaturated bridges

Three cyclophanes, each displaying a different type of dynamic process, have been studied by NMR methods. The barriers to these processes are attributed mainly to the decrease in π-electron overlap between the benzene rings and adjacent double bonds which occurs in the transition state for each process. In [5₂] paracyclophanetetraene, two successive flippings of the benzene rings interconvert the two hydrogens in the methylene groups (Scheme 1). In tetramethyl [2₄] paracyclophanetetraene, the passage of one methyl group through the central cavity of the molecule interconverts two conformations of similar, but not equal, free energy (Scheme 2). In [2₆] orthoparacyclophanehexaene, the orthosubstituted rings change sides by passing through the centre of the cyclophane (Scheme 3).     

A computational study of [2.2]cyclophanes

A computational study of isomeric [2.2]cyclophanes, namely [2.2]paracyclophane 1, [2.2]metacyclophane 2, and [2.2]metaparacyclophane 3, has been carried out. For 1, geometry optimizations performed by various methods at different basis sets showed that MP2/6-31+G(d,p) and B3PW91/6-31+G(d,p) provide the best results in comparison to the X-ray data. Compound 1 has D(2) symmetry with distorted bridges. A conformational search was performed for [2.2]cyclophanes 2 and 3. Each cyclophane exists in two conformations which have different energies in the case of 3 but are degenerate in the case of 2. Relative energies and strain energies at the bridges follow the same order, indicating that the relief of bridge tension and repulsion between pi clouds are determining factors for the stability of [2.2]cyclophanes. Through a decomposition of strain energy, it can be concluded that both the rings or the bridges can absorb strain, but it depends on the conformer of butane that is considered in the calculation of SE(br). Changes in aromaticity of these compounds were evaluated by NICS and HOMA and were compared with benzene and xylenes dimers as models. Despite distortions from planarity and shortening and lengthening of the C-C bonds relative to the mean, the phenyl rings are aromatic. NICS suggests a concentration of electronic density between the rings as a result of bridging process. Computed MK, NPA, and GAPT charges were compared for the isomeric cyclophanes. The GIAO chemical shifts were calculated and indicate that 1 has a larger diamagnetic anisotropy than the other isomers.     

A novel three‐dimensional cadmium(II) coordination polymer incorporating 1,4‐bis­(1,2,4‐triazol‐1‐ylmethyl)­benzene and thio­cyanate

In the title complex, poly[cadmium(II)‐μ₂‐1,4‐bis­(1,2,4‐triazol‐1‐ylmeth­yl)benzene‐di‐μ₂‐thio­cyanato], [Cd(NCS)₂(C₁₂H₁₂N₆)]_n, the Cd^II atom lies on an inversion centre in a distorted octa­hedral environment. Four N atoms from the thio­cyanate and 1,4‐bis­(1,2,4‐triazol‐1‐ylmeth­yl)benzene (bbtz) ligands occupy the equatorial positions, and two S atoms from symmetry‐related thio­cyanate ligands occupy the axial positions. The benzene ring of the bbtz ligand lies about an inversion centre. Single thio­cyanate bridges link the Cd^II atoms into two‐dimensional sheets containing novel 16‐membered [Cd₄(μ‐NCS‐N:S)₄] rings. The bbtz ligands further link these two‐dimensional sheets into an unprecedented covalent three‐dimensional network for the cadmium–thio­cyanate system.     

Carbo‐biphenyls and Carbo‐terphenyls: Oligo(phenylene ethynylene) Ring Carbo‐mers

Ring carbo‐mers of oligo(phenylene ethynylene)s (OPEn, n=0–2), made of C₂‐catenated C₁₈ carbo‐benzene rings, have been synthesized and characterized by NMR and UV‐vis spectroscopy, crystallography and voltammetry. Analyses of crystal and DFT‐optimized structures show that the C₁₈ rings preserve their individual aromatic character according to structural and magnetic criteria (NICS indices). Carbo‐terphenyls (n=2) are reversibly reduced at ca. ?0.42 V/SCE, i.e. 0.41 V more readily than the corresponding carbo‐benzene (?0.83 V/SCE), thus revealing efficient inter‐ring π‐conjugation. An accurate linear fit of E_1/2^red1 vs. the DFT LUMO energy suggests a notably higher value (?0.30 V/SCE) for a carbo‐quaterphenyl congener (n=3). Increase with n of the effective π‐conjugation is also evidenced by a red shift of two of the three main visible light absorption bands, all being assigned to TDDFT‐calculated excited states, one of them restricting to a HOMO→LUMO main one‐electron transition.     

A silver-catalyzed Büchner reaction

A silver scorpionate complex, derived from the highly fluorinated [HB(3,5-(CF₃)₂Pz)₃]⁻, catalyzes the addition of ethyl diazoacetate to benzene rings, providing norcaradienes, which undergo electrocyclization to provide the corresponding cycloheptatriene. These reactions are surprisingly selective for addition to the aromatic moiety rather than C-H insertion.     

Enantioselective Synthesis of Polycyclic Aromatic Hydrocarbon (PAH)-Based Planar Chiral Bent Cyclophanes by Rhodium-Catalyzed [2+2+2] Cycloaddition

The enantioselective synthesis of polycyclic aromatic hydrocarbon (PAH)-based planar chiral cyclophanes was achieved for the first time by the rhodium-catalyzed intramolecular regio- and enantioselective [2+2+2] cycloaddition of tethered diyne-benzofulvenes followed by stepwise oxidative transformations. The thus synthesized planar chiral bent cyclophanes, that possess bent p-terphenyl- and 9-fluorenone-cores, were converted to 9-fluorenol-based ones with excellent ee values of >99 % by diastereoselective 1,2-reduction. These 9-fluorenol-based cyclophanes exhibited high fluorescence quantum yields, which were significantly higher than that of an acyclic reference molecule (78–82 % vs. 48 %). The bending effect on the chiroptical property was also examined, which revealed that the anisotropy factors (g_abs values) for electronic circular dichroism (ECD) of these 9-fluorenol-based planar chiral bent cyclophanes increase as the tether length becomes shorter.     

Assembly of a two‐dimensional layer structure with 1,4‐bis(1H‐benzimidazol‐1‐ylmethyl)benzene dihydrate via hydrogen bonds and π–π interactions

In the title compound, C₂₂H₁₈N₄·2H₂O, the organic fragment lies across a centre of inversion in the P2₁/n space group. The water molecules form C(2)‐type hydrogen‐bonded chains which are linked to the 1,4‐bis(1H‐benzimidazol‐1‐ylmethyl)benzene molecules through O—H...N hydrogen bonds, forming sheets reinforced by π–π stacking interactions between the aromatic rings within the layers.     

Divinyl aromatic compounds and Di(methacrylates) prepared by acid-catalyzed transformations of bis[4-(1-hydroxyethyl)phenyl]alkanes

The mechanism of formation of divinyl aromatic monomers including p,p′-divinyldiphenylmethane and 1,2-p,p′-divinyldiphenylethane and of their dimerization via terminal vinyl groups was studied. The factors affecting the structure, composition, and properties of thermosetting resins of a new class, Rolivsans, containing methacrylates of secondary aromatic alcohols (diols) and divinyl aromatic compounds with methylene bridges between benzene rings were examined.     

Synthesis of (Aza)n[3n]cyclophanes as host molecules

(Aza)_n[3ⁿ]cyclophanes were synthesized by the coupling reaction of p-toluenesulfonamide and bis(halomethyl) derivatives in the presence of a base (K₂CO₃, NaH etc.) using DMF, dioxane etc. as a solvent, in acceptable yields. Tetraaza macrocyclic compound (the dimer in Fig. 1) obtained by the coupling of 2,11-diaza[3.3]metacyclophane with 1,3-bis(bromomethyl)benzene gavea 1:1 adduct with benzene.Presented partly at the 42nd Autumn Annual Meeting of the Chemical Society of Japan, Sendai, September, 1980; Abstr. No. 1J17.     

Hexahelicenophanes

A reaction sequence was studied for the preparation of cyclophanes 12 , which contain 2,7‐bis(2‐phenylethenyl)naphthalene chromophores and polymethylenedioxy chains of different length. The irradiation of 12 in the presence of I₂ led then, by oxidative cyclization processes, to the hexahelicenophanes 13 , provided that the methylenedioxy chain of 12 is long enough (n = 8, 10). As competitive photoreaction, a twofold [2π+2π] cyclodimerization occurred, which furnished the belt cyclophanes 14 . The latter process is the only photoreaction for 12 with n=6. The hexahelicenophanes 13 have lower racemization barriers and longer spin? lattice relaxation times compared to those of hexahelicenes.     

catena‐Poly[[diisothio­cyanato­cad­mium(II)]‐bis­[μ‐1,3‐bis­(1,2,4‐triazol‐1‐ylmeth­yl)benzene‐κ2N4:N4′]]: a double‐chain coordination polymer

The coordination geometry of the Cd^II atom in the title complex, [Cd(NCS)₂(C₁₂H₁₂N₆)₂]_n or [Cd(NCS)₂(mbtz)₂]_n, where mbtz is 1,3‐bis­(1,2,4‐triazol‐1‐ylmeth­yl)benzene, is a distorted compressed octa­hedron in which the Cd^II atom lies on an inversion centre, coordinated by four N atoms from the triazole rings of four mbtz ligands and two N atoms from two monodentate NCS⁻ ligands. The structure is polymeric, with 24‐membered spiro‐fused rings extending along [100] and with the 24‐membered ring containing two inversion‐related mbtz mol­ecules.     

Two‐dimensional ZnII and one‐dimensional CoII coordination polymers based on benzene‐1,4‐dicarboxylate and pyridine ligands

Coordination polymers constructed from metal ions and organic ligands have attracted considerable attention owing to their diverse structural topologies and potential applications. Ligands containing carboxylate groups are among the most extensively studied because of their versatile coordination modes. Reactions of benzene‐1,4‐dicarboxylic acid (H₂BDC) and pyridine (py) with Zn^II or Co^II yielded two new coordination polymers, namely, poly[(μ₄‐benzene‐1,4‐dicarboxylato‐κ⁴O:O′:O′′:O′′′)(pyridine‐κN)zinc(II)], [Zn(C₈H₄O₂)(C₅H₅N)]_n, (I), and catena‐poly[aqua(μ₃‐benzene‐1,4‐dicarboxylato‐κ³O:O′:O′′)bis(pyridine‐κN)cobalt(II)], [Co(C₈H₄O₂)(C₅H₅N)₂(H₂O)]_n, (II). In compound (I), the Zn^II cation is five‐coordinated by four carboxylate O atoms from four BDC²⁻ ligands and one pyridine N atom in a distorted square‐pyramidal coordination geometry. Four carboxylate groups bridge two Zn^II ions to form centrosymmetric paddle‐wheel‐like Zn₂(μ₂‐COO)₄ units, which are linked by the benzene rings of the BDC²⁻ ligands to generate a two‐dimensional layered structure. The two‐dimensional layer is extended into a three‐dimensional supramolecular structure with the help of π–π stacking interactions between the aromatic rings. Compound (II) has a one‐dimensional double‐chain structure based on Co₂(μ₂‐COO)₂ units. The Co^II cations are bridged by BDC²⁻ ligands and are octahedrally coordinated by three carboxylate O atoms from three BDC²⁻ ligands, one water O atom and two pyridine N atoms. Interchain O—H…O hydrogen‐bonding interactions link these chains to form a three‐dimensional supramolecular architecture.     

Effects of Coordinating Solvents on the Supramolecular Assembling of Tectons arised from the Metallation of Sulfamethoxazole: Synthesis and Structural Characterization of Polymeric [Cd(sulfamethoxazolato)2(L)2]n {L = N,N‐dimethylformamide (DMF); Dimethyl Sulfoxide (DMSO)} and [Cd(sulfamethoxazolato)2(Py)2]n·n(Py)(Py =pyridine)

Cadmium acetate reacts with sulfamethoxazole (5‐methyl‐3‐isoxazolyl sulfanilamide) and with DMF / DMSO / pyridine to give the crystalline polymers [Cd(sulfamethoxazolato)₂(L₂)]_n {L = DMF ( 1 ), DMSO ( 2 )} and [Cd(sulfamethoxazolato)₂(Py)₂]_n·n(Py) ( 3 ). Complexes 1 , 2 and 3 confirm the tectonic character of the [Cd(sulfamethoxazolato)₂(L)₂] moieties and the remarkable ability of the {Cd(sulfamethoxazolato)₄} fragments to be non selectively stabilized by monodentate ligands. In the polymeric assemblies of the title complexes the cadmium(II) atoms are linked through sulfamethoxazolato anions which alternate in their coordination with the isoxazolic N‐atoms and the aromatic amino groups. The chains of vicinal rings build tunnels along the crystallographic c axis.     

Cadmium(II) iodide and thiocyanate complexes adopted by polycyclic 1,4‐bis(pyridazin‐4‐yl)benzene: interplay of coordination and π–π stacking interactions

New complexes containing the 1,4‐bis(pyridazin‐4‐yl)benzene ligand, namely diaquatetrakis[1,4‐bis(pyridazin‐4‐yl)benzene‐κN²]cadmium(II) hexaiodidodicadmate(II), [Cd(C₁₄H₁₀N₄)₄(H₂O)₂][Cd₂I₆], (I), and poly[[μ‐1,4‐bis(pyridazin‐4‐yl)benzene‐κ²N²:N^2′]bis(μ‐thiocyanato‐κ²N:S)cadmium(II)], [Cd(NCS)₂(C₁₄H₁₀N₄)]_n, (II), demonstrate the adaptability of the coordination geometries towards the demands of slipped π–π stacking interactions between the extended organic ligands. In (I), the discrete cationic [Cd—N = 2.408 (3) and 2.413 (3) Å] and anionic [Cd—I = 2.709 (2)–3.1201 (14) Å] entities are situated across centres of inversion. The cations associate via complementary O—H...N^2′ hydrogen bonding [O...N = 2.748 (4) and 2.765 (4) Å] and extensive triple π–π stacking interactions between pairs of pyridazine and phenylene rings [centroid–centroid distances (CCD) = 3.782 (4)–4.286 (3) Å] to yield two‐dimensional square nets. The [Cd₂I₆]²⁻ anions reside in channels generated by packing of successive nets. In (II), the Cd^II cation lies on a centre of inversion and the ligand is situated across a centre of inversion. A two‐dimensional coordination array is formed by crosslinking of linear [Cd(μ‐NCS)₂]_n chains [Cd—N = 2.3004 (14) Å and Cd—S = 2.7804 (5) Å] with N²:N^2′‐bidentate organic bridges [Cd—N = 2.3893 (12) Å], which generate π–π stacks by double‐slipped interactions between phenylene and pyridazine rings [CCD = 3.721 (2) Å].     

Influence of the Chemical Modification of Porphyrins on Their Coordination and Acid-Base Properties

For 3,7,13,17-tetramethyl-2,8,12,18-tetrabutylporphyrin, 5,15-diphenyl-3,7,13,17-tetramethyl-2,8,12,18-tetrabutylporphyrin, and the cyclophane dimer in which the monomeric porphyrin fragments are bound via meta positions of the benzene rings with two -O(CH₂)₃O- bridges, the kinetics of complexation with copper(II) acetate in acetonitrile and the base ionization in the presence of perchloric acid were studied. The reactivity of these compounds in complexation with the metal cation in acetonitrile correlates with their basicity.