Dewar Benzene Research Articles

Ultrafast [2 + 2]-cycloaddition in norbornadiene.

Excitation of norbornadiene (bicyclo[2.2.1]hepta-2,5-diene) at 200 nm populates two states in parallel, the second pi(pi*) state and a Rydberg state. We monitored both populations by transient nonresonant ionization. From the pi(pi*) state the molecule relaxes in consecutive steps with time constants 5, 31 and 55 fs down to the ground-state surface, whereas the Rydberg population merges to the other path on the pi(pi*) surface within 420 fs. The relaxation steps are discussed in terms of conical intersections (CoIns) between different surfaces Information on them is inferred from known spectroscopy and, for the last CoIn, from published calculations on Dewar benzene-->prismane conversion and on ethylene photodimerization for which norbornadiene with its two nonconjugated double bonds is a model. The calculation predicts symmetry breaking for this CoIn, the two ethylenes forming a rhombus Although this distortion is hindered in norbornadiene by ring strain, this CoIn seems easily accessible as indicated by the short time (<55 fs) found for passing through it.

Synthesis and characterization of [46]paracyclophanedodecayne derivative

[46]paracyclophanedodecayne derivative 1 having a novel belt-shaped fully conjugated π-system was photochemically generated from the corresponding hexakis(Dewar benzene) precursor 7 as a highly strained but isolable compound.

Dewar benzene as a protecting group? Demonstration of photolithographic crystallization.

The concept of Dewar benzene as a supramolecular protecting group for solid-state aryl-aryl interactions is reported. Photoisomerization of 1,4,5,6-tetramethyl-bicyclo[2.2.0]hexa-2,5-diene-2,3-dicarboxylic acid dimethyl ester (1) to the corresponding benzene isomer proceeds with rapid crystal formation. Herein this property is applied to the photolithographic patterning of crystal domains on a surface. [structure: see text]

Hexafluorobenzene photochemistry: wellspring of fluorocarbon structures.

Ultraviolet irradiation of hexafluorobenzene in the vapor phase gives exclusively its Dewar valence isomer, from which a wealth of highly fluorinated compounds has been derived. Photoexcited in solution, hexafluorobenzene is capable of cycloaddition with a variety of unsaturated molecules. Like the Dewar benzene, resulting adducts have served as intermediates en route to many new fluorocarbons and fluorocarbon derivatives.

Kinetic Stabilization of the [1.1]Paracyclophane System: Isolation and X-ray Structural Analysis of a [1.1]Paracyclophane Derivative and Its Interconversion with the Transannular Adduct

Intentionally designed kinetic stabilization of the [1.1]paracyclophane skeleton has been achieved by multiple substitution of the aromatic rings with trimethylsilylmethyl and N,N-dimethylcarbamoyl groups, which serve to shield the proximate bridgehead carbon atoms sterically from access by other reagents. The bis(Dewar benzene) precursor (1 a) has been prepared in essentially the same manner as previous derivatives—starting from the photocycloaddition of 1,4-bis(trimethylsilyl)-2-butyne to octahydroindacene-1,5-dione—except for a few critical modifications described in the text. Substituted [1.1]paracyclophane (2 a), photochemically generated from the precursor, is indefinitely stable at 50 °C and suffers decomposition only by 8 % after 2 h at 100 °C in degassed n-decane, demonstrating its greatly improved kinetic stability compared to previous [1.1]paracyclophanes. Since 2 a undergoes efficient photochemical transformation into the transannular addition product 3 a, irradiation of 1 a tends to produce a mixture of products consisting mainly of 3 a. Compound 3 a, however, reverts thermally to 2 a in a process of half life 40 min at 55 °C; the activation parameters for this process are ΔH≠=21.1±0.8 kcal mol−1 and ΔS≠=−10.5±2.6 cal K−1 mol−1. Thus, on heating 3 a in benzene and cooling the resultant solution, 2 a is obtained as orange-red crystals. X-ray crystallographic analysis of 2 a reveals benzene rings bent to the highest degree ever reported for a paracyclophane, with their face-to-face arrangement in unusually close proximity. The shortest nonbonding interatomic distance is 2.376 Å; less than the sum of the van der Waals radii by more than 1.0 Å. The generation of related substituted [1.1]paracyclophanes and their kinetic stabilities are also reported.

Kinetic stabilization of the [1.1]paracyclophane system: isolation and X-ray structural analysis of a [1.1]paracyclophane derivative and its interconversion with the transannular adduct.

Intentionally designed kinetic stabilization of the [1.1]paracyclophane skeleton has been achieved by multiple substitution of the aromatic rings with trimethylsilylmethyl and N,N-dimethyl-carbamoyl groups, which serve to shield the proximate bridgehead carbon atoms sterically from access by other reagents. The bis(Dewar benzene) precursor (1a) has been prepared in essentially the same manner as previous derivatives--starting from the photocycloaddition of 1,4-bis(trimethylsilyl)-2-butyne to octa-hydroindacene-1,5-dione--except for a few critical modifications described in the text. Substituted [1.1]paracyclophane (2a), photochemically generated from the precursor, is indefinitely stable at 50 degrees C and suffers decomposition only by 8 % after 2 h at 100 degrees C in degassed n-decane, demonstrating its greatly improved kinetic stability compared to previous [1.1]paracyclophanes. Since 2a undergoes efficient photochemical transformation into the transannular addition product 3a, irradiation of la tends to produce a mixture of products consisting mainly of 3a. Compound 3a, however, reverts thermally to 2a in a process of half life 40 min at 55 degrees C; the activation parameters for this process are deltaH(not equal to)= 21.1 +/- 0.8 kcalmol(-1) and deltaS(not equal to) = -10.5 +/- 2.6 cal K(-1)mol(-1). Thus, on heating 3a in benzene and cooling the resultant solution, 2a is obtained as orange-red crystals. X-ray crystallographic analysis of 2a reveals benzene rings bent to the highest degree ever reported for a paracyclophane, with their face-to-face arrangement in unusually close proximity. The shortest nonbonding interatomic distance is 2.376 A; less than the sum of the van der Waals radii by more than 1.0A. The generation of related substituted [1.1]paracyclophanes and their kinetic stabilities are also reported.

Syntheses of Acetylenic Oligophenylene Macrocycles Based on a Novel Dewar Benzene Building Block Approach.

A general synthetic approach to strained p-phenylene-based acetylenic macrocycles is described. A key feature in this approach is exploitation of Dewar benzene as an angular p-phenylene synthon. Thus, 1,4-acetal-bridged 2,5-dichloro(Dewar benzene) 5, prepared in four steps from dimethyl acetylenedicarboxylate and 1,2-dichloroethylene, is applied as such a building block in the syntheses of strained macrocycles 13 and anti-20. For the synthesis of 13, m-phenylene units are used as spacers and modified Eglington-Glaser coupling is applied for the macrocyclization step. For the synthesis of anti-20, on the other hand, o-phenylene units are used as spacers and Sonogashira coupling is applied for the macrocyclization step. Macrocycles 13 and anti-20 are characterized crystallographically, and their strained nature is reflected mainly in the deviation of the acetylene units from linearity; the C&tbd1;C-C angles range from 168.7(3) degrees to 179.9(3) degrees in 13 and from 168.0(5) degrees to 171.4(4) degrees in anti-20. Macrocycle 13 shows unique conformational property, namely, the p-phenylene units arranged in parallel in the rectangular framework rotate freely about the long axes, as evidenced by the (1)H NMR studies. Macrocycle anti-20 exhibits a Stokes shift of 179 nm, which is exceptionally large for phenylacetylene macrocycles, presumably owing to the characteristic stacking structure.

Natural resonance theory. I. General formalism

The updated code calculates \documentclass{article}\pagestyle{empty}\begin{document}$D(\underline{\mathrm{W}})$\end{document} in accord with the description in the natural bond orbital (NBO) program manual and the original NRT article. The revision has been sent to commercial providers of the NBO algorithm. Users of the stand-alone NBO program should make the following changes: Old: CALL MULTI(FW,A(I35),A(I33), A(I18),IRESET,LNAO,NREF) New: CALL MULTI(FW,A(I35),A(I33), A(I18),IRESET,LNAO,NREF,NNAO), Old: SUBROUTINE MULTI(FW,GAMMAW,WGT, SCR,IRESET,LVAL,NREF) New: SUBROUTINE MULTI(FW,GAMMAW,WGT, SCR,IRESET,LVAL,NREF,NVAL), Old: SUBROUTINE GETDW(DW,GAMMAW,WGT, LVAL,NREF) New: SUBROUTINE GETDW(DW,GAMMAW,WGT, LVAL,NREF,NVAL), and add the NVAL variable to the end of all CALL GETDW commands in subroutine MULTI. In subroutine GETDW the following changes were made: The two Kekulé, 1, and three Dewar, 2, resonance forms were specified as reference structure for benzene using the NRTSTR keyword. The results for the old and new \documentclass{article}\pagestyle{empty}\begin{document}$D(\underline{\mathrm{W}})$\end{document} routines are summarized in Table I. The new routine finds the Dewar structures to be weighted less than before, and there is a corresponding increase in CC bond order of about 2%. For naphthalene only the three Kekulé structures, 3–5, were used as NRTSTR defined reference structures. (The addition of the 42 singly, doubly, and triply excited Dewar-type structures did not change these results.) The percentage weighting of the structures changed very little with the new routine. Structure 3 went from 28.62 to 29.05%, while structures 4 and 5 both went from 17.94 to 17.42%. The \documentclass{article}\pagestyle{empty}\begin{document}$D(\underline{\mathrm{W}})$\end{document} increased from 0.01612 to 0.01848. The bond order changes are shown in Figure 1. The largest percentage change in bond order between the two methods is 2%. A comparison of NRT calculated bond orders with the old \documentclass{article}\pagestyle{empty}\begin{document}$D(\underline{\mathrm{W}})$\end{document} routine (italics) and the new \documentclass{article}\pagestyle{empty}\begin{document}$D(\underline{\mathrm{W}})$\end{document} routine (bold). Structures 6 and 7 were used as the NRTSTR reference structures. The results are shown in Table I. The percent change in calculated bond order is less than 1%. Boron trifluoride is an example of a severe disagreement between the old and new routines. Structures 8 and 9 were used as the NRTSTR reference structures, which forced an unsymmetrical description of the bonding. The results are shown in Table I. The old \documentclass{article}\pagestyle{empty}\begin{document}$D(\underline{\mathrm{W}})$\end{document} routine used a large percentage of the covalent structure, 9, while the new \documentclass{article}\pagestyle{empty}\begin{document}$D(\underline{\mathrm{W}})$\end{document} routine uses a negligible amount. The large change in BF3 bond order is offset somewhat by finding the average BF bond order, but the error is still on the order of 10%. In summary, we found and corrected an error in the NRT code that affects the reference structure weighting procedures. In most cases the differences between the incorrect and correct code result in small errors, usually on the order of 1–2%. In some cases, such as BF3, the errors can be as large as 10%. The corrected code has been distributed to NBO software distributors, and the stand-alone NBO program can be fixed with the replacement of one small subroutine.

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.

Regioselective and diastereoselective dimethyldioxirane epoxidation of substituted norbornenes and hexamethyl Dewar benzene

Various substituted norbornenes, such as endo-dicyclopentadiene 1, exo-dicyclopentadiene 5, 5-methylenebicyclo[2.2.1]hept-2-ene 8 and hexamethyl Dewar benzene 10 were transformed into the corresponding mono- and bis-epoxides by epoxidation with dimethyldioxirane (as an acetone solution).

First Dewar benzene approach to acetylenic oligophenylene macrocycles: synthesis and structure of a molecular rectangle bearing two spindles

A novel Dewar benzene approach to the construction of oligophenylene macrocycles has been developed by introducing Dewar benzene 1 as a building block and applied to the synthesis of a molecular rectangle 8, whose structure was characterized crystallographically.

The Structures and Energies of Phosphaalkyne Trimers, (HCP) 3

(CH)3P3 structures have been investigated by ab initio (MP4SDTQ/6-31+G**//MP2(fu)/6-31G*) and DFT (B3LYP/6-311+G**) methods. The framework is the main factor determining the realtive energy of isomers; the substitution pattern is less important. The thermodynamic stability order of heteroatom-substituted benzenes > benzvalenes > Dewar benzenes > prismanes > bicyclopropenyls parallels the situation in the (CH)6 hydrocarbons. The 3 HCP 1,2,3-triphosphabenzene trimerization energy is only –84 kcal mol–1; half as large as that for the 3 acetylene benzene conversion (–164 kcal mol–1). Head-to-tail additions are favored for the [4+2] cycloadditions of HCP to 1,2- and 1,3-diphosphete (to give triphospha Dewar benzenes); the computed activation barriers are low: ΔH*298 = 2.2 and 0.8 kcal mol–1, respectively, at MP4/6-31+G**//MP2(fu)/6-31G* (2.0 and 1.5 kcal mol–1 at MC-QDPT2/6-31G*//CAS-SCF(6,6)/6-31G*). Mono- as well as ortho-, meta-, and para-diphosphabenzenes and the triphosphabenzenes are found to be as aromatic as benzene according to geometric and energetic criteria. NICS calculations (a magnetic criterion) suggest only slightly reduced aromaticity.

Disrotatory versus conrotatory electrocyclic ring opening of Dewar benzene: the conrotatory pathway is preferred and does not involve trans-benzene

For the electrocyclic ring opening of Dewar benzene ( 2) into benzene ( 1), both a disrotatory and conrotatory pathway with distinct transition states, TS1 and TS2, respectively, were found at the CASSCF(10,10)/6-311G** level of theory. The importance of the CASSCF(10,10) active space for the proper description of TS1 and TS2 was illustrated by similar calculations using a smaller active space, viz. CASSCF(2,2), CASSCF(4,4) and CASSCF(6,6). Although TS2 represents a true saddle point, TS1 appears to be a higher order saddle point at these levels of theory. Single-point multi-reference SDCI (MRCI) calculations at the CASSCF(10,10)/6-311G** geometries and natural orbitals were performed at all stationary points to obtain more reliable total energies. In contrast to common belief, TS2 lies below TS1 (by 6.62 kcal/mol), i.e. the conrotatory process is favored. Moreover, CASSCF(10,10)/6-311G** Intrinsic Reaction Coordinate (IRC) calculations show that upon conrotatory electrocyclic ring opening 2 does not give the extraordinarily strained trans-benzene ( 3), i.e. cis, cis, trans-cyclohexa-1,3,5-triene. The conversion of 3 into 1 and vice versa is a distinct process on the C 6H 6 potential energy surface.

Synthesis and reaction of 1,4-acetal-bridged 2,3,5,6-tetramethylidene-bicyclo[2.2.0]hexane: the first Diels–Alder route to Dewar benzenes

The 1,4-acetal-bridged derivative of hitherto unknown 2,3,5,6-tetramethylidenebicyclo[2.2.0]hexane, which produces the Dewar benzene skeleton through two successive Diels–Alder reactions with dienophiles, has been synthesized in four steps from dimethyl acetylenedicarboxylate and 1,4-dichlorobut-2-ene.

Aromaticity of bent benzene rings: A VBSCF study

The effect of ring deformation on aromaticity has been studied for bent benzene molecules in which two carbon atoms have been bent out of plane, resulting in a boat conformation. Valence-bond self-consistent field (VBSCF) calculations have been performed on these molecules to obtain insight into the aromaticity of bent benzenes. Results for total energy, structure energies, weights, and orbital overlaps show that the molecule keeps its aromatic nature up to 55°. After 55° a transition to Dewar benzene occurs. The valence-bond model, by showing the weights of both Dewar and Kekulé structures, is an excellent tool to study deformed benzene. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 74: 213–221, 1999

Ab initio calculations of the heats of formation for (CH) 6 isomers

The heats of formation for the five (CH) 6 isomers have been calculated using both the atomization and isodesmic bond separation schemes at the following six theoretical levels: G2(MP2,SVP), G2(MP2), G2(MP3), G2, CBS-Q, and CBS-APNO. It is found that, with the isodesmic scheme, all four G2-based methods give Δ H f values that are within a few kJ mol −1 of each other. In particular, the G2(MP2,SVP) and G2 yield the most accurate Δ H f values for benzene, the only isomer whose listed experimental data are suitable for comparison with ab initio results. For the CBS-Q and CBS-APNO levels, the isodesmic scheme gives worse results than the atomization scheme, but the results from the two levels are more consistent in the isodesmic scheme. These results suggest that a combination of either the G2(MP2,SVP) or G2 model with the isodesmic scheme should afford accurate thermochemical data for relatively large hydrocarbon compounds. At the G2 level, using the isodesmic bond separation scheme, we obtain the following Δ H f0 values (in kJ mol −1) for the five (CH) 6 isomers: 100.5 for benzene (compared to the experimental value of 100.4±1), 415.5 for Dewar benzene, 567.2 for prismane, 397.5 for benzvalene, and 593.6 for 3,3′-bicyclopropenyl. At the same level and with the same scheme, the calculated Δ H f298 values (in kJ mol −1) are: 82.0 for benzene (compared to the experimental value of 82.9±0.3), 397.1 for Dewar benzene, 547.0 for prismane, 378.1 for benzvalene, and 578.8 for 3,3′-bicyclopropenyl.

Direct Characterization of Hexamethyl(Dewar Benzene) Radical Cation by Electronic Absorption Spectroscopy

The electronic absorption spectrum of the hexamethyl(Dewar benzene) radical cation (1• +) is presented for the first time. Partial stabilization of 1• + in organic glasses at cryogenic temperatures is explained in terms of matrix rigidity, which can inhibit the isomerization process. Annealing of the matrix leads to fast isomerization of 1• + to hexamethylbenzene radical cation (2• +) and, at high concentrations of 1, formation of the hexamethylbenzene dimer radical cation can be observed.

Ab initio calculations on the isomerization of the benzene radical cation to the Dewar benzene structure and on possible pathways for the formation of C 4H 4 fragment ions from the benzene radical cation

Ab initio calculations of possible fragmentation pathways for the formation of C 4H 4 fragment ions from the benzene radical cation show that the dissociation of lowest energy leads to the methylene cyclopropene radical cation. The calculated barriers for formation of the other classical C 4H 4 ion structures, vinyl acetylene, cyclobutadiene, and butatriene, are about 10 kcal mol −1 higher and almost identical. The fact that, in fragmentations of C 6H 6 precursors, besides the methylene cycloprene ion, only the vinyl acetylene ion is observed, should be ascribed to the fact that formation of this ion structure is much more direct than a fragmentation to the cyclobutadiene or the butatriene ion structure.

New Phosphorus Heterocycles by Rearrangement of a [1,4‐Bis(trimethylsilyl) η8‐cyclooctatetraene‐1,3,5‐triphospha‐7‐hafnanorbornadiene Complex

Abstract[1,4‐Bis(trimethylsilyl)‐n8‐cyclooctatetraene]‐2, 4, 6‐tri‐tert‐butyl‐1, 3, 5‐triphospha‐7‐hafnanorbornadiene (3) rearranges nearly quantitatively to the corresponding 3,5,6‐tri‐tert‐butyl‐1,2,4‐triphospha‐7‐hafnanorbornadiene complex 5 upon heating at 70°C. Treatment of complex 5 with trimethylphosphane at 50°C induces the displacement of di‐tert‐butylacetylene to give the new (n4‐triphosphacyclobutadiene)hafnium complex 7, the crystalstructure of which has been determined by X‐ray analysis. From complex 5 3,5,6,‐tri‐tert‐butyl‐1,2,4‐triphospha Dewar benzene (6) can be synthesized by a redox reaction with hexachloroethane.

1.1]Paracyclophane. Photochemical Generation from the Corresponding Bis(Dewar benzene) Derivative and Theoretical Study of Its Structure and Strain Energy

The first generation of [1.1]paracyclophane (1a) and its bis(methoxycarbonyl) derivative (1b) from the corresponding bis(Dewar benzene) precursors, 3a and 3b, has been investigated. Irradiation of 3a in a glassy mixture of ether−isopentane−ethanol at 77 K leads to the formation of species exhibiting absorption extending to 450 nm, which readily undergoes secondary photolysis to give an isomer showing λmax at 244 nm. On the basis of these UV/vis spectral observations and the accompanying 1H NMR measurement, the structures of 1a and the corresponding transannular [4 + 4] adduct (21a) are assigned to the initial and the secondary products, respectively. Compound 3b undergoes similar successive phototransformation into 1b and 21b. [1.1]Paracyclophanes, 1a and 1b, and their photoisomers, 21a and 21b, are sufficiently stable to permit the measurement of 1H NMR spectra at low temperature, but are consumed fairly rapidly in solution at ambient temperature, defying their isolation. The results of geometrical optim...