Diels-Alder Reactions of [2.2] Paracyclophan-1-ene and [2.2] Paracyclophane-1,9-diene with 3,6-Disubstituted 1,2,4,5-Tetrazines

[2.2] Paracyclophan-1-ene ( 1 ) and [2.2] paracyclophane-1,9-diene ( 6 ) apparently act as dienophiles with inverse electron demand and smoothly react with dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate ( 2a ) at room temperature forming dihydropyridazine adducts, which are dehydrogenated to the pyridazino-anellated [2.2] paracyclophanes 5a and 8a , respectively. The molecular structure of 5a is determined by X-ray crystal-structure analysis. Under more rigorous conditions, phenyl-substituted derivatives 5b and 8b are obtained from 1 and 6 , respectively, with 3,6-diphenyl-1,2,4,5-tetrazine. Compounds 1 and 6 are less reactive dienophiles than other strained cyclic olefins as shown by kinetic measurements.     

Vapor deposition polymerization of heteroatom-bridged [2.2]paracyclophanes: 1,9-dithia[2.2]paracyclophane and 1,9-dithia[2.2]paracyclophane-1,1,9,9-tetroxide

1,9-Dithia[2.2]paracyclophane-1,1,9,9-tetroxide ( 3 ) was synthesized as white needles in a high yield from 1,9-dithia[2.2]paracyclophane ( 2 ) by oxidation with m-chloroperbenzoic acid, and its molecular structure was determined with single-crystal X-ray diffraction analysis. Vapor deposition polymerizations of 2 and 3 gave amorphous and brittle polymer films along with considerable amounts of nonpolymeric byproducts. A polymer film from 2 was a copolymer of p-(phenylene-methylenesulfide) with p-(phenylene-methylene) units, and a polymer film from 3 was a homopolymer of p-(phenylene-methylene) units with head-to-tail, head-to-head, and tail-to-tail placements. The elimination of sulfur atoms in 2 and sulfone units in 3 took place during their pyrolysis reactions. Plausible mechanisms for vapor deposition polymerizations of both cyclophanes are proposed. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1892–1900, 2001     

Synthesis and ring-opening metathesis of tetraalkoxy-substituted [2.2]paracyclophane-1,9-dienes

Tetraalkoxy-substituted [2.2]paracyclophane-1,9-dienes can be prepared in three steps from dithia[3.3]paracyclophanes. A mixture of pseudo-geminal and pseudo-ortho diastereomers is produced and the pure compounds can be separated by fractional crystallization. The solid state structures of these diastereomers reveal strongly distorted aromatic rings consistent with high levels of ring strain. Reaction of these diastereomers with the second generation Grubbs catalyst shows that only the pseudo-geminal isomer can be ring opened to give cis,trans-distrylbenzenes. The origin of this selectivity is discussed and the photoisomerization of the as-formed cis,trans-product to the all trans isomer is demonstrated.     

Cyclopolymerization of (E,E) - [6.2]paracyclophane-1,5-diene

Polymerization of (E,E)-[6.2]paracyclophane-1,5-diene proceeds by an intra–intermolecular mechanism to give a polymer containing a [3.2]paracyclophane in the repeat unit. Polymerization occurs with either free radical or cationic initiation; anionic initiation was unsuccessful. Cationic polymerization is favored and appears to proceed through a stabilized carbonium ion intermediate. Spectroscopic and model compound studies are consistent with the proposed polymer structure. Thermal analyses of the polymer indicate a complex thermooxidative behavior in the presence of oxygen, while depolymerization occurs above about 400°C in an inert atmosphere.     

Resolution of (±)-[2.2]paracyclophane-4,12-dicarboxylic acid

The resolution of (±)-[2.2]paracyclophane-4,12-dicarboxylic acid (±)-1 has been realized through the diastereomeric esters of (1S)-hydroxymethyl-4,7,7-trimethyl-2-oxabicyclo-[2.2.1]heptan-3-one, simply separated by flash chromatography and hydrolyzed with ^tBuOK/H₂O. (R)-(−)-1 and (S)-(+)-1 were obtained in high enantiomeric excesses (>97%) while the determinations of the absolute configurations of (R)-1 and (S)-1 were carried out by X-ray diffraction.     

The Radical Anions of [2.2]Paracyclophane-1,9-diene and Some of its Derivatives. An Unexpected Conformational Interconversion

The radical anions of [2.2]paracyclophane-1,9-diene ( 2 ) and its 1,10,12,13,15,16-hexadeuterio derivative 2 -D₆, as well as those of 4,5,7,8-tetramethyl[2.2]paracyclophane-1,9-diene ( 3 ) and its 12,13,15,16-tetradeuterio derivative 3 -D₄, have been studied by ESR spectroscopy. The coupling constants for 2 ^?· at 178 K are 0.422 mT for four equivalent olefinic protons and 0.046 and 0.020 mT, each for a set of four equivalent aromatic protons. This hyperfine pattern is consistent with either benzene ring bearing two pairs of equivalent protons and it points to a lowering of the anticipated D_2h symmetry. The ESR spectra of 2 ^?· are strongly temperature dependent, due to modulation of the two coupling constants of 0.046 and 0.020 mT; these have opposite signs and average to 0.013 mT at 273 K. The experimental findings are interpreted in terms of a transition state of D_2h symmetry, 33 kJ/mol above two interconverting equivalent conformations of lower symmetry. Several pieces of evidence suggest that this symmetry is D₂, i.e., the benzene rings in 2 ^?· are twisted in opposite directions about the vertical axis. Temperature dependence of the ESR spectra, resulting from modulation of the hyperfine interactions with the aromatic protons, is also observed for 2 -D₆^?· and 3 ^?·. In the case of 3 ^?·, the olefinic protons are, as expected, only equivalent in pairs, the pertinent coupling constants being 0.560 and 0.325 mT. Upon standing at low temperatures, 2 ^?· and 3 ^?· gradually convert into the radical anions of [2.2]paracyclophane ( 1 ) and its 4,5,7,8-tetramethyl derivative, respectively. At higher temperatures, cleavage of one bridging chain in 2 ^?· also occurs, with the formation of the radical anion of (E)-4,4′-dimethylstilbene ( 7 ). Both reactions of 2 ^?· must involve the transient radical anion of [2.2]paracyclophane-1-ene ( 4 ) as proved by the observation of the spectra of 1 ^?· and 7 ^?· with 4 as the starting material.     

Thermal degradation of poly((E,E)-[6.2]paracyclophane-1,5-diene)

Thermal degradation of poly((E,E)-[6.2]paracyclophane-1,5-diene) is studied in inert and oxidative environments by using thermogravimetric analysis, pyrolysis GC/MS, pyrolysis GC/FT-IR, and variable temperature-diffuse reflectance infrared spectroscopy (VT-DRIFTS). Thermal degradation in helium begins by depolymerization yielding a volatile product capable of abstracting hydrogens from the polymer residue. Multiple hydrogen abstractions result in a variety of volatile species containing benzyl-benzyl bonds. In the presence of oxygen, polymer decomposition is dictated by reactions of peroxy and hydroperoxy radicals. At low temperatures, oxygenated species are the primary products. At higher temperatures, increased unsaturation is detected in the polymer residue. In both inert and oxidative environments, the strain associated with alignment of paracyclophane aromatic rings is lost during the initial stages of thermal degradation. © 1992 John Wiley & Sons, Inc.     

Dihalogencarben-cycloadditionen an 1-silacyclohexa-2.4-diene 4(5)-sila-bicyclo[4.1.0]-hept-2-ene

4R-1-silacyclohexa-2.4-dienes react with dichlorocarbene and dibromocarbene - prepared by phase-transfer catalysis - to give the cycloadducts at the 2.3- or 4.5- double bond, depending on the nature of R.Über Cycloadditionen von Carbenen an ungesättigte Sila- und Germa-heterocyclen liegen bislang nur wenige Untersuchungen vor.Nach D. Seyferth u. Mitarb. [1] werden bei der Reaktion von 1.1.3.4-Tetramethyl-1-silacyclopenten-3 mit Dichlorcarben (thermolytisch aus C₆H₅HgCCl₂Br) offenkettige Produkte erhalten, die allerdings auf eine vorangegangene Cycloaddition von CCl₂ schließen lassen.Bei der analogen Umsetzung von 1-Germacyclopentenen-3 [1] erhält man als Folgeprodukte der primär gebildeten Cyclopropanaddukte sowohl Germacyclohexadiene-2.4 - als Ergebnis einer Ringerweiterung - als auch Ringöffnungsprodukte. Die Cyclopropanderivate konnten von D. Seyferth u. Mitarb. [2] erst bei Einhaltung sehr milder Reaktionsbedingungen isoliert werden.     

Extended cooperative electronic effects in poly((E,E)-[6.2]paracyclophane-1,5-diene)

Fluorescence spectroscopy of poly((E,E)-[6.2]paracyclophane-1,5-diene) shows the existence of several emission maxima which appear at wavelengths longer than that of the [3.2]paracyclophane repeat-unit excimer. These maxima appear to be emission bands due to “extended excimer” fluorescence from multiples of electronically interacting repeat units. Oxidation of the polymer by exposure to iodine vapor results in a material which exhibits a strong, asymmetric electron-spin-resonance spectrum with g = ca. 2.00 and a DC conductivity of 5 X 10^—4 S-cm^—1. These results are interpreted by a model in which segments of interacting radical cation salts occur pendant to and along the polymer chains but are of random length and orientation in the bulk polymer. Similar results were obtained for a structurally related polymer containing [3.3]paracyclophane rings.