Dewar Benzene Research Articles

4]Paracyclophane: electronic absorption spectrum and trapping by alcohols

[4]Paracyclophane (2) is generated photochemically from 1,4-tetramethylene Dewar benzene; its electronic absorption spectrum and trapping reaction by alcohols are reported.

Zur stabilisierung von [5]paracyclophanen durch substitution am benzolring

A relatively efficient route to substituted [5]paracyclophanes 11 is described. Diels Alder reaction of the pentamethylenecyclobutacfienes 3 and 4, prepared in situ, with acetylenes E-C≡C-E (5, E = electron withdrawing group) gives a mixture of Dewar benzenes, from which the 1,4-bridged isomer 7 is conveniently isolated after aromatization of the other isomers. Irradiation (254 nm) of 7 at 220 K establishes a photo equilibrium between 7 and 11 with 2–15% of the latter, depending on E. The aromatic character of 11 is derived from the UV- and 1H-NMR spectra; at 220 K, two conformational isomers 11' and 11 can be discerned. Although most 11 are thermally more stable than the parent compound [5]paracyclophane, they slowly decompose in solution at room temperature.

ChemInform Abstract: PE Spectra of Dewar Benzenes, Bridged by a Cyclohexadiene or a Butadiene Unit.

AbstractPE spectra of (III) and (VIII)‐(XI) indicate a considerable interaction in (X) between the Cyclohexadiene moiety and the bicyclic hexadiene unit in contrast to a small interaction in (IX) between the butadiene and the Dewar benzene moieties.

Surface photochemistry: on the mechanism of the semiconductor photoinduced valence isomerization of hexamethyl Dewar benzene to hexamethylbenzene

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSurface photochemistry: on the mechanism of the semiconductor photoinduced valence isomerization of hexamethyl Dewar benzene to hexamethylbenzeneHussain Al-Ekabi and Paul De MayoCite this: J. Phys. Chem. 1986, 90, 17, 4075–4080Publication Date (Print):August 1, 1986Publication History Published online1 May 2002Published inissue 1 August 1986https://doi.org/10.1021/j100408a048RIGHTS & PERMISSIONSArticle Views118Altmetric-Citations39LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (760 KB) Get e-Alerts

PE Spectra of Dewar Benzenes, Bridged by a Cyclohexadiene or a Butadiene Unit

AbstractThe electronic structure of [4.2.2]propella‐2,4,7,9‐tetraene (1), 2,5‐etheno[4.2.2]propella‐3,7,9‐triene (2), and of several hydrogenated derivatives of 2 has been investigated using He(I) PE spectroscopy and MO calculations. It is found that in 2 the 1,4‐cyclohexadiene moiety interacts considerably with the bicyclo[2.2.0]hexadiene unit, while in 1 the interaction between the butadiene fragment and the Dewar benzene unit is minute. The syntheses of 3–5 are reported.

ChemInform Abstract: Small Rings. Part 59. Unusual Preparation as well as Crystal and Molecular Structure of Trimethyl 3,4,5‐Tri‐tert‐butylprismane‐1,2,6‐tricarboxylate.

AbstractIn the search for a suitable analogue of tetra‐tert‐butyltetrahedrane or hexa‐tert‐butylprismane the Dewar benzene (I) is used for valence isomerization.

ChemInform Abstract: Theoretical Study on the Stability of Hexafluoro Dewar Benzene

Abstract Ab initio calculations were carried out on benzene, Dewar benzene and their perfluoro derivatives. Full geometry optimization was performed for the above molecules, and vibrational analysis was also carried out for the Dewar isomers. The mechanism of stabilization by fluorine substitution is discussed. It is suggested that the vibrational relaxation of the resultant molecule is important for the preventation of the decomposition of a molecule with such a highly strained carbon skeleton.

(Z)‐[6]Paracycloph‐3‐ene

The smallest isoiable paracyclophane, the title compound 1, was obtained by thermal valence isomerization of the Dewar benzene isomer 2. The 1H-NMR and, in particular, the UV spectra indicate that the benzene ring in 1 deviates much more strongly from planarity than in [6]paracyclophane.

Theoretical study on the stability of hexafluoro dewar benzene

Ab initio calculations were carried out on benzene, Dewar benzene and their perfluoro derivatives. Full geometry optimization was performed for the above molecules, and vibrational analysis was also carried out for the Dewar isomers. The mechanism of stabilization by fluorine substitution is discussed. It is suggested that the vibrational relaxation of the resultant molecule is important for the preventation of the decomposition of a molecule with such a highly strained carbon skeleton.

On the generalized approach to the structure count

A generalized approach to the structure count of n-radical-m-cation systems based on the properties of the acyclic polynomial is presented. The mathematical proof for the expression relating the structure count to the coefficients of the acyclic polynomial is given. The connection between the structure count of biradical structures and the number of Dewar structures is discussed and tested on some examples.

7,8-dimethoxycarbonyl-10,11-dimethyl[5]paracyclophane

The title compound ( 1) was obtained in 13% yield by UV irradiation of the corresponding Dewar benzene ( 2); it is considerably more stable than [5]paracyclophane itself.

Synthesis and silver‐ion‐catalyzed reactions of 3,3′‐ethylenebicyclopropenyl

AbstractThe title compound 2 was obtained from 1,2‐dimethylenecyclobutane (1) by double addition of dibromocarbene to give the tetrabromide 9, followed by tin hydride reduction to the dibromide 10 and potassium tert‐butoxide elimination of HBr to furnish 2. Silver‐ion‐catalyzed reaction of 2 did not furnish the expected 1,2‐ or 1,4‐ethylene Dewar benzenes 3 and 4, respectively. Instead, in CDCl3 or dioxane, polymeric material was obtained, while in methanol or methanol‐O‐d1, opening of one or both cyclopropene rings and addition of methanol gave 11 or 12E/Z, respectively, depending upon the reaction conditions. The two latter compounds provide valuable information about intermediate 13 in the silver‐ion‐catalyzed rearrangement of bicyclopropenyls to Dewar benzenes. The mechanism of this reaction is discussed.

Syntheses with cyclobutadienes -15 . Prismane / dewar benzene isomerization- x-ray crystal structure of tert-butyl 3,4,5-tri-tert-butyl-2,6-bis(trifluoromethyl)prismane-1-carboxylate

The heavily substituted Dewar benzene derivatives 3 and 5 merely yield the carboxylic acids 7 and 9 by loss of isobutene when heated to 150–200 °C; isomerization to the benzoic acid 8 is not at all observed. Prismane derivative 4 , which is obtained from 5 photochemically, behaves analogously at 150°C, but the primarily formed acid 10 rearranges to the isomeric Dewar benzenecarboxylic acids 9 and 11 on prolonged heating at 150°C. The Dewar benzenecarboxylic esters 12 and 13 , obtained from 9 and 11 by esterification with diazomethane, are transformed into prismane 15 photochemically.

The dynamics of low-lying excited states of benzene: the biradicaloid structure of the S2 state

An ab initio SCF–Cl optimized biradicaloid structure of the S2 state of benzene shows Cs symmetry and allows a mechanism for the photochemical conversion into Dewar benzene to be proposed.

Oligomerization of phenylacetylene on fe‐catalysts

AbstractUsing a low activity system as a catalyst gives additional opportunities for study of polymerization mechanism. We made an attempt using phenylacetylene as a monomer to synthesize, separate, and identify oligomers of this alkyne on melting iron catalyst—“naked” metal. Four components with the lowest molecular weight were identified as: (compound 1) isomers of 1,2‐diphenylcyclobutadiene, (compound 2) 1,3‐diphenylcyclobutadiene, (compound 3) 1,4‐diphenylbuten‐1‐yne‐3, and (compound 4) triphenyl substituted Dewar benzene. It was shown, that in the presence of iron catalysts the phenylacetylene oligomerization proceeds as a linear polymerization through a 2 + 2 cycloaddition.

A Periodic Table for Polycyclic Aromatic Hydrocarbons. Part V. 1-Factorable, 2-Factorable, and Dewar Graph Structures Associated with Benzenoid Hydrocarbons

Abstract Further properties of a Formula Periodic Table for Benzenoid PAH6s are presented. Whether the number of components of 1-factor and 2-factor subgraphs of benzenoid hydrocarbons are even or odd and the sign of the aN coefficient of the characteristic polynomial is negative or positive are prescribed by this table. A method for computing the number of Dewar structures from the aac N-2 coefficient of the acrylic polynomial is reviewed. It is shown that a stronger correlation of resonance energy and In aac N-2 of In KNc exists than for In K where K = SC (structure count) aac N. It is conjectured that ethene is subspectral to all benzenoid PAH6 structures belonging to the Nc = 0(mod 4) row series of the formula periodic table for benzenoid PAH6s. Previous research from this laboratory has evolved a formula periodic table for PAH6s which for the first time unified formula/structure relationships of benzenoid hydrocarbons into a systematic framework [1]. Herein additional correlations of this periodic table are presented. Also, structural relationships of the coefficients of the acyclic and characteristic polynomials within the frame of this table are reviewed.

Photochemistry of Halogenated benzene derivatives. part V1. Photolytic reductive Dechlorination and Isomerization of Tetrachlorobenzenes in Acetonitrile‐water mixtures

Photochemical reactions of tetrachlorobenzenes (Cl4—Bzs) such as 1,2,3,4‐tetrachloro‐benzene (1,2,3,4—Cl4—Bz) (1), 1,2,3,5—Cl4—Bz (2) and 1,2,4,5—Cl4—Bz (3) in sCH3CN—H2O in the absence and presence of acetone as sensitizer at wavelengths ≥285 nm were examined. Special attention was paid to the identification and quantification of the photoproducts arising through the apparent reductive dechlorination and isomerization of the starting chlorobenzenes. Reductive dechlorination of 1–3 appears to sbe the main photochemical pathway during both sensitized and non‐sensitized photolyses. In the generation of 1,4—Cl2—Bz, the major product of 1, the formation of the intermediate π‐tetrachlorobenzene from 1 in S1 state is proposed. Since both sensitized and non‐sensitized photolyses of tetrachlorobenzenes frequently yield valence‐isomers of chlorinated benzenes, it is suggested that tetrachloro(Dewar benzenes) yielded from the starting tetrachlorobenzenes in the T1 state may be key intermediates in the eventual formation of photoisomerized tetrachlorobenzes.

The Effect of Structural Changes on the Polarizability and Second Hyperpolarizability in Some Benzene Isomers and Their Anions

The polarizability and second hyperpolarizability of fulvene, dimethylene-cyclobutene, trimethylene- cyclopropane, bicyclopentadiene-propane, bicyclopentadiene-propene, Dewar benzene, benzvalene and several of their anions are computed, employing a CNDO coupled Hartree-Fock procedure. The interaction of the lone pair on the anionic center with double and single bonds, the HOMO-LUMO gap, and substituent effects are employed to illuminate the structurepolarization relationship. An approximate, computationally found formula for the second hyperpolarizability is proposed.

A comparative investigation of 1,4‐pentamethylene and 1,4‐hexamethylene Dewar benzene: Evidence for the intermediate formation of [5]paracyclophane

AbstractThe synthesis of the title compounds 1a and 1b is described. Starting from the corresponding 1,2‐dimethylenecycloalkanes 6, the compounds 1 were obtained in four steps, viz. addition of dichlorocarbene, reduction with triphenyltin hydride, treatment with potassium tert‐butoxide and silver‐ion‐catalyzed rearrangement. In the last step, the 1,2‐isomers 11 of 1 were also formed, and their thermal rearrangement to the benzocycloalkenes 4 is briefly described. Compound 1b rearranged to its aromatic isomer [6]paracyclophane (2b) both thermally (60°C in solution, 100‐460°C flow pyrolysis) and under silver‐ion catalysis at room temperature; in this latter reaction, the initially formed 2b was gradually further isomerized to 4b. At higher temperatures, 2b rearranged to the spirotrienes 3b and finally fragmented to give p‐ethylstyrene (17). From 1a, the spirotriene 3a and benzocycloheptene (4a) were obtained by thermolysis and by silver‐ion catalysis, respectively. The mechanism of these reactions is discussed and it is concluded that [5]paracyclophane (2a) is a transient intermediate in the reaction of 1a.

An MNDO SCF MO investigation of some structural isomers of the perfluoro diazines (pyrimidine, pyrazine, and pyridazine) of relevance to their plasma polymerization

AbstractExtensive MNDO SCF MO calculations were made to determine the heats of formation of the ground state of geometry‐optimized perfluorodiazabenzenes (pyridazine, pyrimidine, and pyrazine) and some of their structural isomers (Dewar benzene, benzvalene, prismane, fulvene, and hexadienyne). The heats of formation of the cation, singlet, and triplet states, were derived for the ground‐state geometry of 1,3‐diaza isomers to gain some insight into the excited state manifolds. These results were used to interpret various experimental observations that relate to the plasma polymerization of the diazines. In particular, the interconversion of the parent heteroaromatic, the relative energies of interconversion and polymerization, and the possibility of the elimination of a small stable molecule were considered. Comparison is made with results reported for the tetrafluorobenzenes.