Dewar Benzene Research Articles

Density functional theory study of vibrational spectra, 6: assignment of fundamental vibrational frequencies of benzene isomers: Dewar benzene, benzvalene, trimethylenecyclopropane, prismane, and 3,4-dimethylenecyclobutene

Density functional theory study of vibrational spectra, 6: assignment of fundamental vibrational frequencies of benzene isomers: Dewar benzene, benzvalene, trimethylenecyclopropane, prismane, and 3,4-dimethylenecyclobutene

Density functional theory study of vibrational spectra, 6: assignment of fundamental vibrational frequencies of benzene isomers: Dewar benzene, benzvalene, trimethylenecyclopropane, prismane, and 3,4-dimethylenecyclobutene

The molecular structures and vibrational spectra of Dewar benzene, benzvalene, trimethylenecyclopropane, prismane and 3,4-dimethylenecyclobutene have been investigated by density functional theory using Becke's exchange with Lee—Yang—Parr's correlation functionals and the 6–31G∗ basis set. Both the calculated structural parameters and vibrational frequencies are in good agreement with available experimental data. On the basis of the agreement between the calculated and experimental results, assignments of fundamental vibrational frequencies of Dewar benzene, benzvalene, and trimethylenecyclopropane were examined and some reassignments are proposed. The calculations also predict prominent IR and Raman spectral features of prismane and 3,4-dimethylenecyclobutene, which can assist experimental identification of these compounds and the assignment of observed spectral features when they are available.

Density Functional Study of the Hexamethyl-(Dewar Benzene) Radical Cation and Some Related Compounds.

A theoretical study has been undertaken of the hexamethyl(Dewar benzene) radical cation (both 2B2 and 2A1 states in C2v symmetry) as well as of two related ions, the pentamethylbenzene and the pentamethylbenzyl trifluoroacetate radical cations, which have been suggested as alternative assignments of the EPR spectrum attributed to the hexamethyl(Dewar benzene) cation. The geometries were optimized at the HF and LDA levels of theory using a standard 6-31G basis set. Single point calculations of the hyperfine coupling constants (hfcc) were then performed at the more accurate MP2, B3PW91, B3LYP and PWP86 levels using larger basis sets. The theoretical results of the present study support the assignment of 2B2 as the ground state of the hexamethyl(Dewar benzene) cation, yielding hydrogen atom hfcc’s in very good agreement with experiment.

Synthesis and antimalarial activity of heteroatom-containing bicyclic endoperoxides

Mechanism-based design and short syntheses involving novel Tebbe methylenations of a-heteroatom-substituted ketones led to preparation of heteroatom-containing bicyclic endoperoxides 4- 6. The crucial final photo-oxygenative cyclization step succeeded only when the intermediate 1,6-dienes carried anisyl but not phenyl substituents. Distinguishing between endoperoxide and cyclobutane cyclization products was achieved reliably by 13C NMR spectroscopy. Antimalarial testing of endoperoxides 4- 6 in vitro showed them to have only weak activities (IC 50 = 500-1100nM). Ferrous bromide-induced reductions of sulfonamide endoperoxides 4, although forming the expected hydroxylated ether and ring-contracted products 7 and 8, caused virtually no rearrangement of hexamethyl Dewar benzene; therefore, the intermediacy of any oxidatively damaging hich-valent iron-oxo intermediate appears unlikely.

Valence points and symmetry views of the structures of carbon clusters, fullerenes and metallofullerenes

Planar Schlegel diagrams of fullerene cages for the reaction mechanisms between open clusters and between metals and fullerenes are used to determine the qualitative symmetry properties. The detailed Schlegel diagrams help to illustrate the subgroup symmetries of the cages that contain localized π bondings with Dewar structure bondings plus free radical π electrons, positive and negative ions, metal locations, hydrogen additions etc. Cluster interactions include C 36H 12, C 36H 18, C 24H 12, C 20H 20, C 20H 10. Free radical π-electrons are shown inside C 88(D 3), C 82(C 3v), C 74(D 3h), C 60(I h), C 36(D 3h), C 34(C 3v) along with their endohedral interaction, with single metals Sc, Y, La and Lu having three top electrons s 2d. Similarly, free-radical π-electrons are shown inside C 28(T d) and C 56(T d) along with their interaction with single metals Ti, Zr, Hf and Th having four-top electrons s 2d 2 (plus metal U having f 3d). The extra case of Ti@C 28H 4 (C 3v or D 2d) is also considered. For dimetal interactions, we consider C 84(T d) yielding Sc 2@C 84 and Lu 2@C 84 with subgroup symmetry D 2d and C 80( I h) yielding Lu 2@C 80 with subgroup symmetry D 3d. For trimetal interactions, we consider C 82(C 3v) yielding Sc 3C 82 and La 3@C 82 with subgroup symmetry C 3v and C 88(D 3) yielding La 3@C 88 with subgroup symmetry C 3. For exohedral interactions, we consider C 30(D 5h) yielding LaC 29 + and C 34(C 3v) yielding LaC 33 + with subgroup symmetry C s.

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.

Reactions of Cyclic Diynes with Acceptor‐Substituted Alkynes: Syntheses of Doubly Bridged Dewar Benzenes

AbstractThe reaction of the AlCl3 complex of cyclic diynes (mainly a 14‐membered ring) with dimethyl acetylenedicarboxylate (DMAD) or bis(tert‐butylsulfonyl)acetylene (BTSA) and related compounds was investigated. The reaction of the AlCl3 complex of cyclotetradeca‐1,8‐diyne (1, n = 5) with DMAD affords in 60% yield two isomers in a ratio of 7:3. The isomers were identified as the 1,4‐ and 2,3‐bridged Dewar benzene derivative A and the 1,2‐ and 3,4‐bridged Dewar benzene derivative B. Acetylenes with sterically more demanding groups such as BTSA or bis(diphenylphosphanyl)acetylene P,P′‐dioxide (BDPA) yield only the isomer A. The X‐ray structures of the cycloadduct between 1 (n = 5 and 6) and BTSA, as well as of 2,3‐bis(tert‐butylsulfonyl)‐1,4,5,6‐tetramethylbicyclo[2.2.0]hexa‐2,5‐diene (31) are described. The mechanism of the cycloadditions is discussed.

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).

Electrocyclic Ring Opening Modes of Dewar Benzenes: Ab Initio Predictions for Möbius Benzene and trans-Dewar Benzene as New C6H6 Isomers

Ab initio calculations have been performed on the two modes of electrocyclic ring opening of Dewar benzene (1), as well as related structures. Both CASSCF and MP2 optimizations predict the existence of Mobius benzene (3, cis,cis,trans-1,3,5-cyclohexatriene) in a very shallow minimum ca. 100 kcal/mol above benzene (2). This structure has alternating single and double bonds, with one fully trans double bond. The barrier for π bond rotation (3 → 2) is predicted to be less than 3 kcal/mol. Stationary points have been located at the TCSCF/6-31G* level for both conrotatory and disrotatory ring opening of 1. The disrotatory path proceeds through a second-order saddle point, which appears to be a general characteristic of symmetry-forbidden reactions. At every level of calculation, conrotatory ring opening of 1 proceeds through a barrier that is 1.5−5.1 kcal/mol lower than disrotation. The predicted conrotatory enthalpic barrier of 24−28 kcal/mol agrees well with the experimental value of 25.1 ± 2. Intrinsic reac...

Structure, Strain Energy, and Magnetic Susceptibility of [4]Paracyclophane and the Activation Energy for Its Interconversion with 1,4-Tetramethylene Dewar Benzene

Structure, Strain Energy, and Magnetic Susceptibility of [4]Paracyclophane and the Activation Energy for Its Interconversion with 1,4-Tetramethylene Dewar Benzene

Reactions of (2,4,6-Tri-t-butyl)thiobenzaldehyde with Diazo Compounds. Synthesis and Reactions of Sterically Congested Thiiranes

Abstract Sterically congested thiiranes having a 2,4,6-tri-t-butylphenyl group were synthesized by the reactions of (2,4,6-tri-t-butyl)thiobenzaldehyde with sterically hindered diazo compounds. Thermal desulfurization of the most congested thiirane, 2,2-di-t-butyl-3-(2,4,6-tri-t-butylphenyl)thiirane (15c), did not proceed even upon using highly reactive reagents, such as hexamethylphosphorous triamide or organolithiums. The photo-reaction of 15c did not give the corresponding styrene, but afforded several products containing 2,4,6-tri-t-butylbenzo[b]thiophene and Dewar benzenes, 2,2-di-t-butyl-3-(3,4,6-tri-t-butylbicyclo[2.2.0]hexa-2,5-dien-2-yl)thiirane and 2-t-butyl-3,3-dimethyl-1-(1,3,5-tri-t-butylbicyclo[2.2.0]hexa-2,5-diene-2-yl)-1-butene (24), the latter of which is the first example for a vinyl-substituted Dewar benzene. Compound 24 has a unique reactivity, giving a rearrangement product, 2-t-butyl-3,3-dimethyl-1-(1,3,5-tri-t-butylbicyclo[2.2.0]hexa-2,5-diene-2-yl)-1-butene on thermolysis.

The effect of pressure on the trimerization and Diels‐Alder reaction of cyanoacetylene. Synthesis and reactivity of 2,3,5‐tricyanobicyclo[2.2.0]hexa‐2,5‐diene („tricyano Dewar benzene”︁)

AbstractThe trimerization of cyanoacetylene (1a) and the Diels‐Alder reaction of 1a with 1,3‐cyclohexadiene (2) show a powerful pressure‐induced acceleration which allows the reaction temperature to be reduced from 160°C at 1 bar to 40°C at 12 kbar or from 100deg;C at 1 bar to room temperature at 7 kbar. At high pressure thermally unstable intermediates like the tricyano Dewar benzene 12 generated in the trimerization of 1a or the primary adduct 3a formed in the Diels‐Alder reaction of 1a with 2 were isolated.

Determination of the reaction enthalpy and kinetic parameters for the isomerization of bridged Dewar benzenes to 8,9-disubstituted [6]paracyclophanes by differential scanning calorimetry

In solution the reaction enthalpy for the thermal isomerization of the Dewar isomers 2a and 2b to their corresponding 8,9-disubstituted [6]paracyclophanes 1a and 1b was determined as −128±13 kJ·mol −1 and −146±15 kJ·mol −1, while the activation energies were found to be 101±10 kJ·mol −1 and 117±12 kJ·mol −1 respectively. An improved calorimetric technique was employed using samples in sealed NMR tubes.

Formation and characterization of the radical cation of pentamethylbenzyl trifluoroacetate from the oxidation of hexamethyl (Dewar benzene) by thallium(III) trifluoroacetate in trifluoroacetic acid—a slow and complex reaction

The reaction between hexamethyl (Dewar benzene)(HMD) and TlIII trifluoroacetate (TlIII) in trifluoroacetic acid has been investigated in detail. The first step involves a slow acid-catalysed conversion of HMD into hexamethylbenzene (HMB) which is then oxidized by TlIII to pentamethylbenzyl trifluoroacetate in a process which overall is about 15 times faster than the HMD → HMB reaction. In the formal one-electron transfer reaction between HMB and TlIII, the corresponding radical cation, HMB˙+, appears in significant concentration and with a lifetime which makes it easy to monitor by EPR spectroscopy. It is shown that the true lifetime of HMB˙+ in trifluoroacetic acid is shorter by a factor of ≈0.03 (22 °C) or ≈0.005 (–11 °C) than the ‘decay’ lifetime recorded during an experiment in which it is generated by the reaction between HMB and TlIII under otherwise identical conditions. Thus the ‘decay’ rate constant of HMB˙+ is actually a reflection of its slow rate of formation from HMB-TlIII. The fact that the formal one-electron transfer reaction between HMB and TlIII exhibits a significant kinetic isotope effect of both substrate and solvent type, indicates that this apparently simple step must be complex.Pentamethylbenzyl trifluoroacetate, as well as the corresponding acetate, alcohol, methyl ether or chloride, exhibits a characteristic 13 × 8 line EPR spectrum when irradiated in trifluoroacetic acid with TlIII trifluoroacetate at –11 °C. The similarity of this spectrum to the previously described, less well-resolved 13 line EPR spectrum from the oxidation of HMD in matrices at low temperatures or on a solid substrate, is profound. The fact that HMD on oxidation by TlIII in trifluoroacetic acid gives the 13 × 8 line spectrum of pentamethylbenzyl trifluoroacetate, indicates that the transformation of HMD into a suitable pentamethylbenzyl derivative might be the origin of the 13-line EPR spectrum recorded by the matrix technique.

The radical cation from hexamethyl(Dewar benzene): derived from a substitution product of hexamethylbenzene?

Photolysis of thallium(III) trifluoroacetate and pentamethylbenzyl acetate or pentamethylbenzyl alcohol in neat trifluoroacetic acid at –11 °C produces a well-resolved 13 × 8-line EPR spectrum of what probably originates from pentamethylbenzyl trifluoroacetate radical cation; the same spectrum could be generated by similar treatment of hexamethyl(Dewar benzene).

Ab initio studies of the thermal decomposition of azaaromatics: free radical versus intramolecular mechanism

The energetics of the key reactions in the thermal decomposition of the azaaromatic molecules pyridine, pyrimidine, pyrazine and pyridazine have been investigated using ab initio quantum-chemical techniques. Two distinct mechanisms are considered: (a) intramolecular rearrangement where a bicyclic, Dewar isomer is formed as an intermediate, followed by its fission, producing acetylene and hydrogen cyanide, and (b) the formation of azyl radicals in a CH bond fission initiation reaction followed by decomposition as well as hydrogen abstraction reactions. The calculated energetics of the free radical mechanism are found to be consistent with the results of earlier experimental studies. The estimates of the critical energies associated with the intramolecular Dewar mechanism are, however, such as to rule it out as a viable alternative. A detailed ab initio study of the benzene → Dewar benzene rearrangement is also reported.

The oxidation of tetrasubstituted alkenes by cytochrome P450.

Hexamethyl(Dewar benzene) (HMDB) was selected to probe the intermediacy of an alkene radical cation during cytochrome P450 (P450) catalyzed epoxidation. P450 catalyzed the allylic oxidation of HMDB exclusively; no rearranged products (indicative of a radical cation intermediate) nor epoxide was detected. In addition, P450 exclusively catalyzed the allylic oxidation of 1,2-dimethylcyclohexene (DMCH) and 1,2,4,5-tetramethyl-1,4- cyclohexadiene. The double bonds of HMDB and DMCH were readily epoxidized by (tetraphenylporphyrinato)iron-(III) chloride with minor production of allylic alcohols. The rates of alkene epoxidation by model systems tend to increase with increasing alkene substitution, but it is apparent that P450 has additional mechanistic options or constraints that depress the rate of epoxidation with increasing alkene substitution. The failure of P450 to epoxidize tetrasubstituted alkenes illustrates a failure of P450 model systems to truly mimic the chemistry of the P450 iron-oxo species.

Boron-Nitrogen Analogues of Cyclobutadiene, Benzene and Cyclooctatetraene

Abstract Iminoboranes RB [tbnd] NR′, isoelectronic with alkynes, cyclooligomerize to give B-N rings (RBNR′)n that are either analogues of cyclobutadienes (n = 2) or benzenes or Dewar benzenes (n = 3) or cyclooctatetraenes (n = 4), depending on the ligand set R/R′ and on conducting the oligomerization either thermally or catalytically. Cyclodimers and cyclotetramers may either undergo reversible interconversions or cyclotetramers are formed irreversibly from cyclodimers. The (4+2) cycloaddition of cyclodimers and iminoboranes yields cyclotrimers. Cyclotrimers may thermally be converted into cyclodimers, and poly(iminoboranes), isoelectronic with polyalkynes, can thermally be depolymerized into cyclotrimers. nido-Cluster derivatives of N2B4H6, arachno-cluster derivatives of N2B3H7, four-membered rings N2B2R3R′, or five-membered rings N2B3R4R′ are isolated, when three-membered rings NB2R3, isoelectronic with cyclopropenyl cations, either dimerize or add aminoboranes H2B=NR′2 or insert nitrenes R′N (from R′N3) or insert iminoboranes R′B [tbnd] NR into the B-B bond, respectively.

The heavier group 15 analogues of benzene, cyclobutadiene and their valence isomers (M 6 and M 4, M = P, As, Sb and Bi)

The structures and stability of the benzene, Dewar benzene, benzvalene and prismane structures of M 6 (M = P, As, Sb and Bi) are investigated by means of ab initio molecular orbital calculations. The prismane structure becomes the most stable as M becomes heavier. An interesting finding is that the heavier group 15 atoms are much less reluctant to form planar benzene-like structures than the heavier group 14 atoms. The structures and stability of the cyclobutadiene and tetrahedrane structures of M 4 are also investigated.