Theoretical study of the thermal interconversion mechanism between the norbornadiene and quadricyclane radical cations

Geometry optimizations at the UHF/6-31G^* and UMP2/6-31G^* levels of theory were performed to find the transition state in the interconversion between norbornadiene (N) and quadricyclane (Q) radical cations. Two transition structures, TS^+·₁ and TS^+·₂, were obtained which have C₁ and C₂ symmetry, respectively. Vibrational analysis at the UHF and UMP2 levels of theory and IRC calculation showed that TS^+·₁ is the true transition state connecting N^+· and Q^+·, while TS^+·₂ is a second order saddle point.     

Cycloaddition reaction of quadricyclane and fluoroolefins

Reaction of quadricyclane (1) with fluoroolefins of different structure results in stereoselective formation of polyfluorinated exo-tricyclo[4.2.1.0^2,5]non-7-enes. The reaction of a mixture of trans/cis CF₃CFCFCF₃ with 1 is stereoselective and the resulting cycloadducts 7a, b preserve the original alkene stereochemistry. The relative rate constants of cycloaddition of a series of fluoroolefins to 1 under pseudo first-order conditions measured by kinetic NMR at 109 °C provide a kinetic scale of reactivities of the fluoroolefins in this reaction.These relative rate constants correlate well with the number of fluoroalkyl groups connected to the double bond, reaching a maximum for the tri-substituted olefin: CF₃CFCF₂:CF₃CFCFCF₃:(CF₃)₂CC(CF₃)₂:(CF₃)₂CCFC₂F₅ = 1:1.2-1.9:4:138.     

The Bonding Structure of Quadricyclanylidene Derivatives

The crystal structures of bisquadricyclanylidene 1 , 7‐quadricyclanylidenenorbornadienes 2 and 3 are solved by X‐ray diffraction analyses. The bond lengths of the cyclopropyl moieties of 1 , 2 and 3 are compared with several other quadricyclanylidene derivatives, and the differences are analyzed by computational models. The results showed that the variation of bond lengths in this series of compounds is related to the electronic nature of substituents.     

Formation, structure, and reactivities of 1:1 adducts between quadricyclane and (η-cyclopentadienyl)(1,2-diphenyl- or -dimethoxycarbonyl-1,2-ethenedithiolato)rhodium(III)

The rhodiadithiolene complexes [Rh(Cp)(S₂C₂Z₂)] (Z=Ph (1a) and COOMe (1b)) reacted with quadricyclane (Q) to give 1:1 adducts [Rh(Cp)(S₂C₂Z₂) (C₇H₈)] (Z=Ph (2a) and COOMe (2b)) in which Rh and S of the complexes are bridged by C(7) (bridge carbons) and C(5) (edge carbons) of norbornene (C₇H₈), respectively. The structure of the adduct 2a was re-investigated and determined by X-ray structural analysis. The rhodiadithiolene complexes and those adducts showed the catalytic activities for the thermal isomerization from Q to norbornadiene (NBD). Adduct 2a photochemically dissociated to give the original complex 1a and NBD upon irradiation with a high-pressure mercury lamp. Skeletal rearrangements of the hydrocarbon moiety were confirmed in the formation of these adducts and in their photo-dissociation, according to deuterium labeling experiments.     

Thermal decomposition of high-energy density materials at high pressure and temperature

High-energy density materials (HEDMs) are being investigated for use as propellants in rocket, air-breathing, and combined-cycle applications. These types of materials may be attractive alternatives to conventional propellants because of their high heat of combustion, density, and high strain energy. Because advanced propulsion systems may operate at very high pressure and temperature (>25 atm and temperatures exceeding 500 °C), the thermal decomposition of individual HEDMs is of interest to future fuel system designers. A laboratory-scale flow reactor was used to subject small amounts (approximately 1 ml) of deoxygenated HEDM to controlled conditions of temperature and residence-time-at-temperature at constant pressure (34 atm) in the liquid or supercritical phase. The reactor was 316 stainless steel HPLC tubing. Using an in-line analytical system, as well as off-line chromatographic analysis of products, the thermal stability of the parent material, as well as the thermal fragmentation products of each HEDM was measured. Some of the candidate materials tested (dimethyl-2-azidoethylamine (DAMEZ), quadricyclane, and bicyclopropylidene (BCP)) showed only marginal thermal stability with major decomposition occurring before 400 °C (3 s residence time). Other candidate materials (JP-10, RP-1, RG-1, RJ-6, and RJ-7) showed excellent thermal stability: little decomposition even at 600 °C. Results show the pyrolytic stability of candidate materials relative to each other, and provided insights to the mechanisms of thermal decomposition for specific fuel candidates.     

Synthèse et propriétés photochimiques de modèles imides et amides contenant des structures norbornadiènes pentaméthylés

Four pentamethylated norbardiene (NBD) models containing imide, bis-imide and diamides were synthesized in the first step of this work with a good yield and characterized by IR, ¹HNMR and ¹³CNMR spectroscopies. These models were used for study, by UV-Visible spectrophotometry, of the photochemical isomerisation under solar radiations of NBD residues, which they contain to quadricyclane (QC). Acquired results show, for all these models, that more than 70 % of NBD are transformed into QC after only 30 minutes of solar radiations and following a first order rate.     

Reactions of quadricyclane with fluorinated nitrogen-containing compounds. Synthesis of 3-aza-4-perfluoroalkyl-tricyclo[4.2.1.0]non-3,7-dienes

The cycloaddition reactions of quadricyclane (1) and polyfluorinated imines and nitriles were studied. Both (CF₃)₂CNH and (CF₃)₂CN(2-FC₆H₄) were found to have low reactivity towards 1, giving the corresponding [2 + 2 + 2] cycloadducts in a low yield. C₂F₅NCFCF₃ however, reacts with 1 rapidly, giving a mixture of two isomeric cycloadducts in a high yield. Perfluoroalkyl nitriles R_fCN (R_f = CF₃, C₂F₅, n-C₃F₇) were found to have surprisingly high reactivity to 1 producing exo-3-aza-4-(fluoroalkyl)-tricyclo[4.2.1.0^2,5]non-3,7-dienes in 56-81% yields at elevated temperature. Exo-3-aza-4-(perfluoroalkyl)-tricyclo[4.2.1.0^2,5]non-3,7-dienes rapidly react with CF₃Si(CH₃)₃ in the presence of CsF catalyst. The reaction results in addition of CF₃Si(CH₃)₃ across the CN bond of the azadienes with selective formation of only one stereoisomer of exo-3-aza-3-(trimethylsilyl)-4,4-bis(perfluoroalkyl)-tricyclo[4.2.1.0^2,5]non-7-enes. Silyl group in this compounds can be removed either by the action of tetrabutylammonium fluoride hydrate, leading to the formation of the corresponding amine after hydrolysis, or by reaction with HCl resulting in the formation of the corresponding amine hydrochloride.     

Quadricyclane—thermal cycloaddition to polyfluorinated carbonyl compounds: A simple synthesis of polyfluorinated 3-oxatricyclo[4.2.1.0]non-7-enes

Quadricyclane (1) readily undergoes [2+2+2] cycloaddition reactions with electron-deficient fluorinated carbonyl compounds to give polyfluorinated 3-oxatricyclo[4.2.1.0^2,5]non-7-enes in high yields. Hexfluoroacetone, trifluoroacetyl chloride, methyl trifluoropyruvate, α-(fluorosulfonyl)difluoroacetyl fluoride, and bis(trifluoromethyl)ketene all react rapidly with 1. Trifluoracetyl fluoride although less reactive, slowly interacts with 1 at ambient temperature. 1,1,1-Trifluoroacetone, trifluoroacetophenone, carbonyl fluoride, and CF₃C(O)OC₆F₅ require higher temperatures (60-90 °C) for reaction, and ethyl trifluoroacetate is unreactive at 90 °C. Heating 1 with the ethyl hemiacetal of trifluoroacetaldehyde gives the corresponding cycloadduct of CF₃C(O)H in 44% yield.The oxetane product from hexafluoroacetone is remarkably stable to both acids and bases, whereas the oxetanes with α-F or Cl leaving groups are sensitive to acid-catalyzed rearrangement.     

Zn- and La-modified TiO2 photocatalysts for the isomerization of norbornadiene to quadricyclane

Zn-doped and La/Zn co-doped TiO₂ nanoparticles were prepared by sol–gel method and utilized as the photocatalysts for the isomerization of norbornadiene to quadricyclane that has significant potential for solar energy storage and high-energy fuel synthesis. For Zn-doped samples, Zn ions do not enter the TiO₂ lattice, but distribute on the particle surface in the form of ZnO crystallites. These crystallites inhibit the agglomeration, growth and anatase-to-rutile phase transformation of TiO₂. The prepared particles contain considerable amount of surface-bound OHs, especially for 1%Zn/TiO₂. A red shift in the optical absorption is observed due to the electron transfer between TiO₂ and ZnO. In the photocatalytic isomerization reaction, Zn-doped TiO₂ exhibits higher activity than homogenous sensitizer like Ethyl Michler's Keton, and 1%Zn/TiO₂ produces the highest yield of quadricyclane. To further enhance the activity, 1%Zn/TiO₂ was co-doped with La. La₂O₃ crystallites also distribute on the surface of TiO₂, similar to the case of ZnO. The particle size is reduced to <7 nm but the surface-bound OHs are decreased to some degree. There is a significant blue shift in the optical absorption with a sharply increased absorbance in the UV region due to the quantum-size effect. 5%La–1%Zn/TiO₂ and 3%La–1%Zn/TiO₂ exhibit higher activity compared with 1%Zn/TiO₂, but higher or lower content of La is detrimental to the reaction. It is concluded that doping Zn can significantly increase the surface-bound OHs, whereas doping La reduces the particle to quantum-size at the expense of surface-bound OHs. A good compromise between the two factors eventually provides a high activity. The isomerization reaction over semiconductors is proposed to proceed through an exciplex (charge-transfer) intermediate.