Konfigurations- und konformationsisomere paratrope,rotationsdynamische Tetraepoxy[32]annulene(6.2.6.2) und diatrope Tetraoxa[30]porphyrin(6.2.6.2)-Dikationen : Nachweis eines Tetraepoxy[31]annulen(6.2.6.2)-Radikalkations

Configurational and Conformational Isomeric Paratopic, Rotational Dynamics Tetraepoxy[30]annulenes(6.2.6.2) and Diatropic Tetraoxa[30]porphyrin(6.2.6.2) Dications: Detection of a Tetraepoxy[31]annulene(6.2.6.2)Radical Cation The synthesis of tetraepoxy[32]annulenes(6.2.6.2) ( 4 ) by a cyclizing twofold Wittig reaction of (E,E,E)-5,5′-(hexa-1,3,5-triene-1,6-diyl)bis[furan-2-carbaldehyde] ( 6 ) and the corresponding bis-phosphonium salt 7 is described (Scheme 1). Contrary to the configuration of the educts, the obtained annulenes 4a and 4b are (Z,E,E,E,Z,E,E,E)- and (E,Z,E,E,E,Z,E,E)-configurated, respectively. The ¹H-NMR spectra establish the paratropic, antiaromatic character of 4 . The annulenes 4 are highly dynamic systems, the (E)-ethenediyl bridges rotate around the adjacent σ-bonds, these rotations are frozen at −80°. The McMurry condensation of dialdehyde 6 yields the (E,E,Z,E,E,E,Z)-4,5-dihydrotetraepoxy[32]annulene(6.2.6.2) ( 13a ), where the configuration of the dialdehyde 6 – beside the hydrogenated double bond – is retained. As result of an intramolecular McMurry reaction of 6 , (Z,E,Z,Z)-dioxa[16]annulene(6.2) 14 is formed. By oxidation of the [32]annulenes(6.2.6.2) 4a and 4b , a mixture of the four stereoisomeric tetraoxa[30]porphyrin(6.2.6.2) dications 5a / 5a ′/ 5b / 5c is obtained; the configuration of the isomers is determined by COSY, NOESY, and NOE experiments. The Δδ values (26.81, 25.83, and 21.11 ppm) underline the diatropic, aromatic character of the dications 5 , the Soret bands are shifted bathochromically to 550 nm, and the Q-bands are in the NIR region (896 – 1039 nm). The dihydroannulene 13a is dehydrogenated by p-chloroanil (tetrachloro-1,4-benzoquinone) to give the annulenes 4a and 4b , its oxidation with DDQ (=4,5-dichloro-3,6-dioxocyclohexa-1,4-diene-1,2-dicarbonitrile) results in the same mixture of dications 5 . Entirely different results are obtained by reaction of the dihydroannulene 13a with DDQ. Here, the (E,E,E,Z,E,E,E,Z) tetraoxa[30]porphyrin(6.2.6.2) dication 5c – formed only in traces from 4a / 4b – is the main product. Beside 5c , a by-product (3%) can be isolated, which turns out (ESR, conductivity) to be the (E,E,E,Z,E,E,E,Z)-tetraoxa[31]porphyrin(6.2.6.2) radical cation 16 , obviously the intermediate in the oxidation sequence of the annulene to the dication. This result leads to the conclusion that the reaction of the dihydro compound 13a with p-chloroanil and DDQ follows different reaction mechanisms. For all isolated stereoisomeric tetraepoxy annulenes and tetraoxaporphyrin dications, the ΔH_f values are calculated by the semiempiric AM1 method. The results are in agreement with the experimental observations. All data confirm the antiaromaticity of the tetraepoxy[32]annulenes(6.2.6.2) 4 and the aromaticity of the tetraoxa[30]porphyrin(6.2.6.2) dications.     

Correlated flipping of four benzene rings in substituted [2.2.2.2]Paracyclophanetetraenes

Dynamic NMR spectroscopy and molecular mechanics calculations show that the flipping of the four benzene binge in certain substituted [2.2.2.2]paracyclophanetetraenes occurs in a correlated fashion.     

Temperature dependent chemical shift change in 4-substituted[2.2.2.2]paracyclophanes

Temperature dependent NMR spectra of 4-substituted tetrakis[2.2.2.2]paracyclophane (4°-PCP) were measured at low temperature. The high field shift at low temperature was remarkable for the protons meta and para to an electron withdrawing and/or bulky substituent. While the chemical shift change was very small for the aromatic protons of 4°-PCP bearing a not so bulky and electron donating substituent. From these observations it is concluded that the rotation or vibration of a benzene bearing an electron withdrawing and/or bulky substituent around C₂-C₃-C₆-C₉ axis was increasingly frozen as the temperature decreased to a more stable “lateral” (or half “lateral”, the intermediary state between “face” and “lateral”⁴) conformation, in an interesting contrast to the favored face conformation of unsubstituted 4°-PCP at low temperature as well as at room temperature.     

Superheteroaromaten mit Furan‐Bausteinen: Isomere antiaromatische Tetraepoxy[36]annulene(6.4.6.4) und aromatische Tetraoxa[34]porphyrin(6.4.6.4)‐Dikationen

Superheteroaromatic Systems with Furan Building Blocks: Isomeric Antiaromatic Tetraepoxy[36]annulenes(6.4.6.4) and Aromatic Tetraoxa[34]porphyrin(6.4.6.4) Dications The title compounds are available by a twofold cyclizing Wittig reaction of (all‐E)‐3,3′‐(hexa‐1,3,5‐triene‐1,6‐diyldifuran‐5,2‐diyl)bis[prop‐2‐enal] ( 4 ) with (all‐E)‐(hexa‐1,3,5‐triene‐1,6‐diyl)bis(furan‐5,2‐diylmethylene)bis[triphenylphosphonium] dibromide ( 7 ). Two conformational isomers 2a / 2a ′ of (Z,E,E,E,E,Z,E,E,E,E)‐tetraepoxy[36]annulene(6.4.6.4) are obtained. The oxidation of 2a / 2a ′ yields two (E,E,Z,E,E,E,E,Z,E,E)‐tetraoxa[34]porphyrin(6.4.6.4) dications 3a / 3a ′, which are conformers, too. The oxidation of 2a / 2a ′ is accompanied by the isomerization of four ethen‐1,2‐diyl bridges. The reduction of the dications 3a / 3a ′ leads to the new (E,E,Z,E,E,E,E,Z,E,E)‐tetraepoxy[36]annulene(6.4.6.4) ( 2b ) and (E,E,E,Z,E,E,E,E,Z,E)‐tetraepoxy[36]annulene(6.4.6.4) ( 2c ). In 2b as well as in 2c , both 1,3‐butadiene‐1,4‐diyl bridges are rotating until −90°. The Δδ values, i.e., the maximum δ difference of the `inner' and `outer' perimeter protons of 3a / 3a ′ (26.62 and 25.32 ppm) are of the same size as the Δδ value of the tetramethyl[34]porphyrin(5.5.5.5) dication ( 1 ; Δδ=25.3 ppm); therefore, they might be called `superheteroaromatic' too. The Δδ values of the tetraepoxy[36]annulenes(6.4.6.4) ( 2a – c ; Δδ=2.3 – 3.3 ppm) establish that they are still paratropic; they represent the most expanded antiaromatic systems yet known.     

Preparations and properties of pseudo-geminal ketones of tris-[2.2.2]- and tetrakis[2.2.2.2]parazylylene

Pseudo-geminal bridged ketones of tris[2.2.2]paraxylylene, 1, and of tetrakis[2.2.2.2]paraxylylene, 2, were prepared and their spectral properties were investigated in comparison with those of dibenzosuberone, 3. The CO stretching band of 1 was observed at abnormally high frequency region (by ca. 50 cm^?1) compared with that of 3, while that of 2 was at a position similar to that of 3. The electronic spectrum of 1 showed that 1 has enormous loss of conjugation due to lack of coplanarity between CO and aromatics. The chemical shift of H⁰, ortho to CO, of 1 also supported this structural characteristic.     

The [4.4.4.4]Fenestranes and[2.2.2.2]paddlane. Prospects for the realization of planar tetracoordinate carbon?

Semi-empirical (MINDO/3 and UNDO) MO calculations on highly strained planar tetracoordinate carbon candidates indicate the central carbons in $\text{cis}$- [4.4.4.4] fenestrane ($\text{1}$) to have pyramidal ($\text{1a}$) and in trans-[4.4.4.]fenestrane ($\text{2}$) to have distorted tetrahedral ($\text{2a}$) geometries. In [2.2.2.2]paddlane ($\text{3}$), the two central carbons are pentacoordinate. Each is nearly coplanar with four carbon neighbours; additionally, the two bridgehead carbons are connected by a single bond ($\text{3a}$).     

The Radical Anions of [2.2.2.2]Paracyclophane-1,9,17,25-tetraene and Some of its Heteroaromatic Analogues

The radical anions of [2.2.2.2]paracyclophane- 1,9,17,25-tetraene (I), [2] (2, 5)-furano [2]paracyclo [2] (2,5)furano [2]paracyclophane-1,8, 16,23-tetraene (II), [2]-(2,5)thiopheno [2]paracyclo [2] (2,5)thiopheno [2]paracyclophane-1,8,16,23-tetraene (III) and [2.2.2.2](2,5)thiophenophane-1,8,15,22-tetraene (IV) have been studied by ESR. and ENDOR. spectroscopy. The assignment of the proton coupling constants, a_Hμ is to a large extent based on investigations of deuteriated derivatives. These investigations impressively demonstrate the potential of ENDOR. spectroscopy as an analytical tool. The Arrhenius activation energies, E_a, for the rotation of phenylene fragments about the bonds linking them with the ethylenic parts in I ? and II ? are 36±6 and 28±4 kJ/mol, respectively. The value a_Hμ of the olefinic protons in I? appears substantially smaller than expected for the corresponding planar radical anion. The hyperfine data for II ?, III ? and IV ? are consistent with the conformations which should minimize the deviations of the macrocyclic π-systems from planarity. In the case of II ?, tight ion pairs are formed by the radical anion and its counter-ion, K ⊕, in DME , owing to the strong association of the alkali metal cation with one of the furan moieties. An analogous interaction of K ⊕ with a thiophene moiety in III ? must be weaker, since no effects of ion pairing on the ESR. and ENDOR. spectra have been observed for this radical anion.     

1H NMR spectral properties and conformational preferences of some 1,2,9,10,17,18,25,26-octathia[2.2.2.2]-metacyclophanes

The stereochemistry in solution of three octathia[2.2.2.2]metacyclophanes has been investigated by means of ¹H NMR spectroscopy. The results lead to the conclusion that the saddle or crown conformation is preferred for these compounds, depending on the substitution pattern of the constituent aromatic rings.     

Hydrogen bond in the excited state of [2.2.2.2](1,2,4,5)cyclophane quinhydrones

Intra- and intermolecular hydrogen bond was studied in electron-donor-acceptor complexes in the excited state. It was shown that low-frequency shifts of electron-transfer bands are correlated with the strength of the hydrogen bond in the ground electron state. A satisfactory agreement was obtained between the results of quantum-chemical calculations of [2.2.2.2](1,2,4,5)cyclophane bisquinones and their reduced forms, [2.2.2.2] (1,2,4,5)cyclophane quinhydrones, with experimental data, and unusual physicochemical properties of quinhydrones are noted.