Dewar Benzene Derivatives Research Articles

Dewar benzenoids in cyclophenacene nanobelts

Cyclophenacene molecules are carbon nanobelts and are the smallest possible hydrogen capped armchair carbon nanotubes. In this density functional theory study Dewar benzene derivatives within cyclophenacene isomers are examined for the [n]cyclophenacenes with n = 6–14 belt sizes (for even values of n). Cyclophenacenes containing Dewar benzenoids are found to have higher energy electronic singlet-triplet state transitions compared to the purely Kekulé benzenoid isomers of the same size. The [6]cyclophenacene isomer containing two maximally spaced Dewar benzenoids is found to have a similar energy to the exclusively Kekulé benzenoid isomer and the addition of solvent is predicted to stabilize the Kekulé form.

Synthesis of Substituted Linear Ter- and Quaterphenyls via Dewar Benzenes

Tetramethylcyclobutadiene-AlCl3 complex reacted with bisalkynes possessing phenylene and biphenylene spacer giving rise to mono- and bis-Dewar benzenes. The rearrangement of the bis- Dewar benzene derivatives under thermal conditions (150 °C) yielded the corresponding linear ter- and quaterphenyl compounds possessing substituted terminal benzene rings. A mixture of an alkynylbiphenyl and a terphenyl was also obtained by the reaction of tetramethylzirconacyclopentadiene with bisalkynylbenzene in 13% combined yield.

ChemInform Abstract: Phosphorus Compounds. Part 58. Spirocyclization of a Stable Phosphaalkyne with Aluminum Trichloride: Key Reaction for the Production of Triphospha Dewar Benzene Derivatives.

ChemInform Abstract: Phosphorus Compounds. Part 58. Spirocyclization of a Stable Phosphaalkyne with Aluminum Trichloride: Key Reaction for the Production of Triphospha Dewar Benzene Derivatives.

Rearrangement of Dewar Benzene Derivatives Studied by DFT

Skeletal rearrangement of a series of substituted Dewar benzenes to biaryl products was studied with DFT methods. Excellent agreement of calculated free energies of activation with experimental kinetic data was achieved. Two different transition states were proposed to be involved in the rearrangement. The reaction path was studied both by static intrinsic reaction coordinate (IRC) calculations and first-principles Car-Parrinello and Born-Oppenheimer molecular dynamics. All approaches confirmed the suggested reaction mechanism.

High‐Performance, Nondiffusive Crosslinked Polymers for Holographic Data Storage

High-performance crosslinked polymeric materials for 3D data storage are described. Their performance is based on photoisomerization of Dewar benzene derivatives, and they exhibit remarkable information storage properties including excellent photosensitivity, high data-storage capacity, good dimensional stability, and significant shelf- and archival-lifetimes.

Quantum Amplified Isomerization in Polymers Based on Triplet Chain Reactions

Photoinitiated triplet quantum amplified isomerizations (QAI) of substituted Dewar benzene derivatives in polymeric media are reported. The quantum efficiencies and the ultimate extents of reactant-to-product conversions increase significantly with the incorporation of appropriate co-sensitizers; compounds whose triplet energies are similar to or lower than that of the sensitizer and close to that of the reactant. These co-sensitizers serve to promote chain-propagating energy transfer processes and thereby increase the action sphere of photosensitization. Isomerization quantum yields increase, as predicted, with increasing concentrations of the reactants and the co-sensitizers. Chain amplifications as large as approximately 16 and extents of conversion that approach 100% have been achieved. Mechanistic schemes are proposed to account for the dynamics of the inherent energy transfer processes and provide a predictively useful model for the design of a new class of photoresponsive polymers based on changes in the refractive index of the materials.

Thermal isomerization of dewarbenzene derivatives

The rates of thermal conversion of seven simple dewarbenzene derivatives to their corresponding benzene isomers were measured. Relatively minor substituent changes were found to have profound effects; isomerization rates increased with the number and strength of electron-withdrawing moieties. Surprisingly, the rate of thermal isomerization for one derivative was the same in fluid solution as in a solid polymer matrix, suggesting that this reaction has a low volume of activation. Other preliminary experiments suggest that the putative intermediate has some polar character. These data may provide some insights into the reaction mechanism.

Highly Efficient Triplet Chain Isomerization of Dewar Benzenes: Adiabatic Rate Constants from Cage Kinetics

Quantum yields as high as 120 were achieved for triplet-sensitized photoisomerizations of several Dewar benzene reactants, R, to the corresponding benzene products, P. Considerable chain amplification is maintained even at high conversion. All relevant rate constants of this triplet chain reaction were extracted from laser flash photolysis plus steady-state photolysis experiments. The crucial rate constant ka for adiabatic isomerization of the triplet reactant to triplet product (R* --> P*) cannot be directly measured because it is so large relative to the bimolecular rate of R* formation via sensitization. However, ka was obtained indirectly using a cage/encounter complex model to analyze the competition between the dissociation of encounter pairs with the sensitizer, e.g., S/R* --> S + R*, and the in-cage processes, S/R* --> S/P* --> S*/P, in nonviscous and viscous solvents. These measurements yielded ka values of (approximately 4-9) x 10(9) s(-1), which suggests that only a small (approximately 3 kcal/mol) energy barrier exists along the potential energy surface from R* to P*. Steady-state data indicated that the chain-terminating rate constant R* --> R is negligibly small, an ideal condition for chain amplification. Triplet energy transfer from a series of sensitizers to the Dewar benzene derivatives shows a nonclassical falloff in rate constants with decreasing sensitizer triplet energy, suggesting energy transfer to thermally distorted configurations having lower singlet-triplet energy gaps. As a result of distorted geometries of R* and P*, the chain-propagating energy transfer from P* to R proceeds with a rate constant of only approximately 2 x 10(7) M(-1) s(-1), despite strong exothermicity. The isomerization reaction can release over 100 kcal/kcal of absorbed photons due to the high-energy content of the reactant together with the large chain length.

Photoinduced electron-transfer bicyclopropenyl-benzene rearrangements of 2,2',3,3'-tetraphenylbicyclopropenyls: a new mechanism via Dewar benzene.

[reaction: see text] The 9,10-dicyanoanthracene-sensitized photoreaction of 1-methyl- and 1,1'-dimethyl-2,2',3,3'-tetraphenylbicyclopropenyl gives the corresponding benzene and Dewar benzene derivatives, indicating that their photoinduced electron-transfer bicyclopropenyl-benzene rearrangements proceed via Dewar benzenes.

ChemInform Abstract: Preparation and Kinetic Stabilization of Highly Strained Paracyclophanes

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Preparation and Kinetic Stabilization of Highly Strained Paracyclophanes

Abstract In this Account we describe the preparation, structures, and properties of the most highly strained paracyclophanes known to date, i.e. [4]- and [1.1]paracyclophanes and their derivatives. The preparation of these cyclophanes has been accomplished through the photochemical valence isomerization of corresponding 1,4-bridged Dewar benzene derivatives. These strained paracyclophanes are prone to polymerize and are not stable enough to isolate: [4]paracyclophane is rapidly consumed even below -100 °C in fluid solution, though it remains intact indefinitely in a frozen organic glass at 77 K, while [1.1]paracyclophane is stable only below -20 °C in solution. By introducing substituents which sterically hinder access to the bridgehead carbon atoms by other reagents, however, one can remarkably stabilize these species kinetically so as to allow the measurement of 1H NMR spectrum of the former and the isolation of the latter as crystals. The 1H NMR spectrum of the [4]paracyclophane strongly support, in conjunction with theoretical calculations, the sustenance of considerable aromaticity despite the extreme bending of its benzene ring. [4]Paracyclophane is so strained that it is thermodynamically less stable than the corresponding Dewar isomer and the thermal conversion of the benzene form to the Dewar form is observed for the first time. A fully unsaturated derivative of [4]paracyclophane is also generated and confirmed to prefer the structure of 1,2,3,4-tetradehydro[4]paracyclophane rather than that of π-bond isomer, bicyclo[4.2.2]deca-1,3,5,7,9-pentaene. The stabilized [1.1]paracyclophane is found to undergo efficient photochemical transformation into a benzene p,p′-dimer structure, from which the former is thermally regenerated upon mild heating. Structural characteristic features of the [1.1]paracyclophane and the benzene p,p′-dimer are revealed by X-ray crystallography. Limits for experimentally accessible strained paracyclophane are also discussed.

A 2,3,5,6-tetramethylenebicyclo[2.2.0]hexane derivative: a novel bisdiene molecule that produces a Dewar benzene skeleton upon two successive Diels–Alder reactions with dienophiles

A new bisdiene molecule 1, the 1,4-acetal-bridged derivative of the hitherto unknown 2,3,5,6-tetramethylenebicyclo[2.2.0]hexane, is synthesized in four steps from dimethyl acetylenedicarboxylate and (E)-1,4-dichlorobut-2-ene. Reaction of 1 with dimethyl acetylenedicarboxylate or benzyne leads to the formation of the Dewar benzene derivatives 6 and 7, respectively, through two successive Diels–Alder additions of the dienophiles to 1. Dehydrogenation of the adducts 6 and 7 affords the corresponding 9,10-Dewar anthracene derivative 10 and 6,13-Dewar pentacene derivative 11, respectively. Compound 11 is characterized crystallographically. Reactions of 1,4-benzoquinone and N-phenylmaleimide with 1 are also reported.

Photochemical Behavior of Doubly Bridged Dewar Benzenes – Synthesis of Prismanes

AbstractIn this paper we report on the photochemistry of Dewar benzenes bridged at the 1,4‐ and 5,6‐positions and substituted at the 2,3‐positions. It was found that simple alkyl substitution leads to prismanes only to a minor extent. Introduction of phenyl groups into the 2‐ or 2,3‐positions gives the corresponding doubly bridged prismanes in yields up to 80% Pentamethylene‐bridged bis‐Dewar benzenes 19 rearrange upon irradiation to 22. Irradiation of the doubly bridged Dewar benzene derivatives with one ester group at the 5‐position does not yield isolable products. However, introduction of one tert‐butylsulfonyl group such as 30–33 gives almost quantitative yields of isolable prismanes if pentamethylene or hexamethylene bridges are used. The nonbridged species, 2‐tert‐butylsulfonyl‐1,4,5,6‐tetramethylbicyclo[2.2.0]hexa‐2,5‐diene (44), aromatizes upon irradiation into 1‐(tert‐butylsulfonyl)‐2,3,4,5‐tetramethylbenzene (45). In contrast to the pentamethylene‐bridged monosulfone 47, the corresponding disulfone 38 undergoes no photochemical reaction. This could be explained by the X‐ray structure, which reveals a strong interaction between one of the tert‐butyl groups and the pentamethylene chain bridging the double bond in 38.

Reactions with Twofold Bridged Sulfone‐Substituted Dewar Benzenes – Preparation of New Dewar Benzene Derivatives

AbstractThe twofold bridged sulfone‐substituted Dewar benzenes 15,16‐bis(tert‐butylsulfonyl)tetracyclo[7.5.2.01,9.02,8]hexadeca‐2(8),15‐diene (7a), 17,18‐bis(tert‐butylsulfonyl)tetracyclo[8.6.2.01,10.02,9]octadeca‐2(9),17‐diene (7b), 2,3‐bis(tert‐butylsulfonyl)‐1,4,5,6‐tetramethylbicyclo[2.2.0]hexa‐2,5‐diene (11), and 15‐(phenylsulfonyl)‐16‐(trimethylsilyl)tetracyclo[7.5.2.01,9.02,8]hexadeca‐2(8),15‐diene (13a) were treated with SmI2, methyllithium, phenyllithium, and methyl, phenyl, and benzyl Grignard reagents, respectively. In the case of the reduction with SmI2 one tert‐butylsulfonyl substituent (7a, 7b, 11) and the phenylsulfonyl group in 13a could be removed. Compound 7a reacts with methyllithium and phenyllithium with replacement of one RSO2 substituent by the alkyl or aryl group, respectively. With methyl and benzylmagnesium bromide both RSO2 groups in 7a could be replaced. The reaction of 7a with phenylmagnesium bromide leads to the mono‐ and disubstituted products and in addition to dimers of substituted phenyl Dewar benzenes. For the reaction with organolithium and Grignard compounds there are indications of a SET mechanism. The X‐ray structures of 15,16‐dimethyltetracyclo[7.5.2.01,9.02,8]hexadeca‐2(8),15‐diene (32a) and the corresponding dibenzyl derivative 834a) were investigated by using single crystals.

Organophosphorus Compounds, 112. Lewis Acid Mediated Spirocyclotrimerization of Kinetically Stabilized Phosphaalkynes – Key Step for the Selective Generation and Trapping of Triphospha Dewar Benzenes

AbstractIn the presence of Lewis acidic derivatives of group 13 elements, phosphaalkynes 7 undergo spirocyclotrimerization with incorporation of the corresponding Lewis acids to form 10a–f. Scope and limitations of this novel cyclooligomerization process are examined. Starting from the spirocyclotrimer 10a reaction with the Lewis base dimethyl sulfoxide, we have in hand for the first time a method for the selective generation of two isomeric triphospha Dewar benzene derivatives (18, 19). Both can be trapped efficiently by further reaction with the phosphaalkyne to furnish the two novel phosphaalkyne cyclotetramers 20 and 21, both still possessing a phosphorus–carbon double bond. In the case of 21, further functionalization of the phosphaalkene unit is possible by [3 + 2] cycloaddition with a nitrile oxide (→ 23).

Synthesis and Unusual Photochemical Reaction of Highly Congested 2,4,6-Tri-t-butylstyrene Episulfides

Abstract Title compounds were synthesized by the reaction of 2,4,6-tri-t-butylthiobenzaldehyde with some diazomethanes. The photolysis of the most sterically congested episulfide obtained from t-Bu2CN2 gave an unusual product resulting from 1,2-migration of t-butyl group instead of the corresponding styrene, a normal photo-product, the corresponding styrene, together with Dewar benzene and benzothiophene derivatives.

Spirocyclization of a Stable Phosphaalkyne with Aluminum Trichloride—Key Reaction for the Production of Triphospha Dewar Benzene Derivatives

Spirocyclization of a Stable Phosphaalkyne with Aluminum Trichloride—Key Reaction for the Production of Triphospha Dewar Benzene Derivatives

Stereochemistry of Dewar Benzenes – Molecular Structures Influenced by Electronic and Steric Effects

The molecular structures of the four Dewar benzene derivatives 4–7 were determined by means of X-ray diffraction. The central C1–C4 bonds are extremely long. The carboxylate groups on the bridgehead carbon atoms of 4, 5, and 7 are in optimal orientation for an electronic interaction with the central bonds having high p-character. Therefore the central bonds in 4, 5, und 7 (1.602–1.612 A) are even more elongated than in 6 (1.563 A). Repulsion between tert-butyl groups causes bond lengthening and structural deformations. The sp2-hybridized carbon atoms are pyramidalized. From structural comparison of Dewar benzene derivatives a strong correlation between the length of the central bond and the folding angle of the four-membered rings can be derived. A new rule has been established for the thermal stability of Dewar benzene derivatives.

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.

Synthesis of Dithiapropellanes with Annelated Benzene Rings–Precursors for Molecules with the Benzene‐Dewar Benzene Structure

Benzoannelated analogues of l,4-polymethylene-(Dewar)benzene derivatives, e.g. (3) and (4), are obtainable from the dithiapropellanes (1) and (2) (X, Y = S). (2) and (4) are chiral.