Kinetics and mechanism of the thermal reactions of endo- and exo-5-cyanobicyclo[2.2.2]oct-2-ene and methyl-substituted derivatives in the gas phase

The thermal reactions of endo- and exo-5-cyanobicyclo-[2.2.2]oct-2-ene and their trans- and cis-6-methyl-substituted derivatives have been investigated in the gas phase between 518 and 630 K. Each product decomposes by two parallel first-order retro-Diels-Alder reactions, a main one with formation of cyclohexa-1,3-diene and a minor one with elimination of ethene. Slight isomerizations are also observed. The kinetic results can be explained in terms of a biradical mechanism. The rate-determining step is shown to depend on the amount of resonance energy in the biradical. Heats of formation and entropies of the bicyclo[2.2.2]oct-2-enes studied are estimated.     

Kinetics of the pyrolyses of endo- and exo-5-methylbicyclo (2.2.2) oct-2-ene in the gas phase

The pyrolyses of endo- and exo-5-methylbicyclo (2.2.2) oct-2-ene (endo- and exo-MBO) have been studied between 608 and 679°K at pressures between 7 and 37 torr. These reactions correspond to parallel first-order eliminations of propene and ethylene: The rate constants (in sec^?1) for endo-MBO are given by and those for exo-MBO by Reaction mechanisms involving diradicals are shown to be compatible with the experimental results. The heats of formation and the entropies of endo and exo-MBO are estimated.     

Novel 2-phosphabicyclo[2.2.2]oct-5-ene derivatives and their use in phosphinylations

The [4 + 2] cycloaddition of the double-bond isomers ( A and B ) of dihydrophosphinine oxide 1 afforded novel phosphabicyclo[2.2.2]oct-5-ene derivates ( 2–4 ), formation of which was justified by PM3 semiempirical calculations. The compounds of dimer type ( 2–4 ) were utilized in the UV light-mediated fragmentation-related phosphinylation of nucleophiles, especially in that of alcohols. To explore the role of structural modifications on the fragmentation ability, disulfide 5 , phosphabicyclooctane 7 obtained by the hydrogenation of 2 , and the adduct of dihydrophosphinine oxide 1 with benzoquinone ( 7 ) were also synthesized and tested in fragmentation. © 2004 Wiley Periodicals, Inc. Heteroatom Chem 15:97–106, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10221     

Ionized bicyclo[2.2.2]oct-2-ene: a twisted olefinic radical cation showing vibronic coupling

The bicyclo[2.2.2]oct-2-ene radical cation (1(.+)) exhibits matrix ESR spectra that have two very different types of gamma-exo hydrogens (those hydrogens formally in a W-plan with the alkene pi bond), a(2H) about 16.9 G and a(2H) about 1.9 G, instead of the four equivalent hydrogens as would be the case in an untwisted C(2v) structure. Moreover, deuterium substitution showed that the vinyl ESR splitting is not resolved (and under about 3.5 G); this is also a result of the twist. Enantiomerization of the C(2) structures is rapid on the ESR timescale above 110 K (barrier estimated at 2.0 kcalmol(-1)). Density functional theory calculations estimate the twist angle at the double bond to be 11-12 degrees and the barrier as 1.2-2.0 kcalmol(-1). Single-configuration restricted Hartree-Fock (RHF) calculations at all levels that were tried give untwisted C(2v) structures for 1(.+), while RHF calculations that include configuration interactions (CI) demonstrate that this system undergoes twisting because of a pseudo Jahn-Teller effect (PJTE). Significantly, twisting does not occur until the sigma-orbital of the predicted symmetry is included in the CI active space. UHF calculations at all levels that include electron correlation (even semiempirical) predict twisting at the alkene pi bond because they allow the filled alpha and the beta hole of the SOMO to have different geometries. The 2,3-dimethylbicyclo[2.2.2]oct-2-ene radical cation (2(.+)) is twisted significantly less than 1(.+), but has a similar temperature for maximum line broadening. Neither the 2,3-dioxabicyclo[2.2.2]octane radical cation (3(.+)) nor its 2,3-dimethyl-2,3-diaza analogue (5(.+)) shows any evidence of twisting. Calculations show that the orbital energy gap between the SOMO and PJTE-active orbitals for 3(.+) is too large for significant PJTE stabilization to occur.     

Kinetics and mechanisms of the reaction of acetylene with cyclohexa-1,3-diene and the decomposition of bicyclo[2.2.2]-octa-2,5-diene in the gas phase

The reaction of acetylene (A) with cyclohexa-1,3-diene (CHD) has been studied between 450 and 592 K. The pressures of A ranged from 25 to 112 torr and those of CHD from 8 to 62 torr. The reaction yields only ethene (E) and benzene (B) instead of bicyclo[2.2.2]octa-2,5-diene (BOD), the product that is expected for a 1,4,1′,2′ addition of the Diels–Alder type. It is first order with respect to each reagent. The rate constant (in L/mol·s) is given by The thermal decomposition of BOD has also been studied. In the ranges of 354–435 K and 0.5–6 torr, the reaction is first order and results in the formation of equal amounts of B and E as the reaction of A with CHD does. Its rate constant (in s^?1) is given by The following consecutive reactions are proposed for the reaction between A and CHD: where BOD is the primary product that is too unstable to be detected. This implies that the rate constant k is equal to k_a. The reaction mechanisms and the strain energy in BOD are discussed.     

Gas phase kinetic and thermochemical data for endo- and exo-5-monosubstituted bicyclo[2.2.2]Oct-2-Enes

The thermal reactions of endo- and exo-5-Y bicyclo[2.2.2]oct-2-enes (NYBO and XYBO) where Y = methyl, ethyl, and isopropyl have been studied in the gas phase between 567 and 695 K. For both isomers they are parallel first-order retro-Diels-Alder reactions with elimination of ethene (E) and monosubstituted ethene (YE).The observed Arrhenius parameters are used to discuss the mechanism and to estimate the heats of formation and the entropies of NYBO and XYBO. Group values are proposed for estimation of these thermochemical data for compounds of the same type by means of the methods of Benson.     

Thermotropic copolyesters. II. Synthesis and characterization of copolyesters containing the bicyclo[2.2.2]oct-2-ene mesogenic unit

Random copolyesters based on 1,4-benzenedimethanol, trans-1,4-cyclohexanedimethanol, and 1,4-bicyclo[2.2.2]octanedimethanol as spacers in ratios of 2 : 3 of the mesogenic group, bicyclo[2.2.2]oct-2-ene-1,4-diol were prepared and characterized. The copolyesters containing the 1,4-benzenedimethanol moiety formed a more brightly colored birefringent fluid in the melt.     

Kinetics of the thermal gas phase isomerization of bicyclo[4.1.0]heptane

The kinetics of the gas-phase decomposition of bicyclo[4.1.0]heptane has been studied over the temperature range of 708–769 K at pressures between 1 and 17 torr. Isomerization to 1-methylcyclohex-1-ene, methylenecyclohexane, and cycloheptene accounts for 96–98% of the primary reaction products and occurs by first-order, homogeneous, nonradical processes.      

The average structures of 2,3-diazabicyclo[2.2.1]hept-2-ene and 2,3-diazabicyclo[2.2.2]oct-2-ene

The average molecular structures of 2,3-diazabicyclo[2.2.1]hept-2-ene and 2,3-diazabicyclo[2.2.2] oct-2-ene have been determined by electron diffraction in the gas phase. The structural parameters were obtained by applying a least squares analysis on the molecular scattering intensity functions. For 2,3-diazabicyclo[2.2.1]hept-2-ene, C_s symmetry was assumed in calculating the geometry of the molecule. The parameters thus determined are: N₃=N₂ = 1.221 Å, N₃- C₄ = 1.445 Å, C₄-C₅ = 1.538 Å, C-H_(ave.) = 1.112 Å, < C₁N₂N₃ = 116.3°, < N₃C₄C₅ = 105.2°, < C₁C₄C₅ = 71.5°, C₄-C₇ = 1.547 Å, C₅-C₆ = 1.530 Å, < C₁C₇C₄ = 108.0°. For 2,3-diazabicyclo[2.2.2]oct-2-ene, C_2vsymmetry was assumed. The geometrical parameters are: N₃ = N₂ = 1.243 Å, N₃-C₄ = 1.473 Å, C₄-C₅ = 1.550 Å, C₅-C₆ = 1.516 Å, C-H_(ave.) = 1.119 Å,< C₁N₂N₃ = 115.1°, < N₃C₄C₅ = 109.1°, < C₆C₁C₄ = 71.6°.     

Generation and reactions of two new functionalized tricyclo[3.3.0.0 3,7]oct-1(5)-ene derivatives

The generation of the new functionalized and highly pyramidalized alkenes, 3,7-(2,2'-biphenylene)tricyclo[3.3.0.0(3,7)]oct-1(5)-ene (20) and 3,7-sulfonyldioxytricyclo[3.3.0.0(3,7)]oct-1(5)-ene (39), and their trapping with 1,3-diphenylisobenzofuran and 11,12-dimethylene-9,10-dihydro-9,10-ethanoanthracene are described. While both alkenes 20 and 39 failed to give the expected cyclobutane or diene dimers, 20 was reacted with 3,7-dimethyltricyclo[3.3.0.0(3,7)]oct-1(5)-ene (1b) to give the cross-coupled product 4,5-(2,2')-biphenylene-10,11-dimethylpentacyclo[8.2.1.1(2,5).1(4,7).1(8,11)]hexadeca-1,7-diene (33). DFT calculations [B3LYP/6-31G(d)] on compound 20 gave important parameters of this pyramidalized alkene, such as the pyramidalization angle (61.7 degrees), the strain energy (72.9 kcal/mol), and the HOMO/LUMO gap (3.79 eV).     

Diels-Alder reactions of 4-triflyloxy-2,6,6-trimethyl-2,4-cyclohexadienone. An expedient methodology for the synthesis of bicyclo[2.2.2]oct-5-en-2-ones and bicyclo[2.2.2]octa-5,7-dien-2-ones

The synthesis of 4-triflyloxy-2,6,6-trimethyl-2,4-cyclohexadienone (13), bicyclo[2.2.2]octenones 1a-j and 15a-j, and bicyclo[2.2.2]octadienones 2a-f, 6a-d, and 11a-f is described. The 2,4-cyclohexadienones 4 and 13 were used for the first time as nondimerizing and easily accessible alternatives to 2,6,6-trimethyl-2,4-cyclohexadienone 12 in Diels-Alder reactions with acetylene derivatives 5a-d to prepare the adducts 6a-d and 11a-e in excellent yields. Compounds 11a-d were initially prepared by the alcoholysis of 6a-d to afford bicyclo[2.2.2]octene-2,5-diones 7a-dfollowed by treatment of 7a-d with N-phenyltriflimide in the presence of LHMDS at -78 degrees C. Diels-Alder reaction of 13 with an acetylene equivalent, phenyl vinyl sulfoxide, was also studied. A detailed study of the Diels-Alder reactions of various olefinic dienophiles 14a-j with 13 has been carried out to furnish cycloadducts 15a-j in high yields. Reductive removal of triflyloxy group of vinyl triflates 11a-f and 15a-j was performed in the presence of [Pd(PPh(3))(2)Cl(2)-Bu(3)N-HCO(2)H] to obtain the desired bicyclo[2.2.2]octadienones 2a-f and bicyclo[2.2.2]octenones 1a-j, respectively, in good overall yields.     

A highly regio- and stereoselective formation of bicyclo[4.2.0]oct-5-ene derivatives through thermal intramolecular [2 + 2] cycloaddition of allenes

Thermal [2 + 2] cycloaddition of allenes with an additional multiple bond is described. By simply heating the allenenes or allenynes having a three-atom tether in an appropriate solvent such as dioxane or DMF, the distal double bond of the allenic moiety regioselectively participates in the cycloaddition to form bicyclo[4.2.0]oct-5-ene derivatives in good to excellent yields. In all the reactions of allenenes, the olefin geometry was completely transferred to the cycloadducts. While the reaction of terminal allenes afforded bicyclic cyclobutane derivatives as a single isomer, the cycloaddition of some internal allenes with axial chirality yielded a diastereomeric mixture of cycloadducts. These results are in good accordance with the stepwise mechanism through a biradical intermediate with a coplanar allyl radical.     

Metathesis polymer based on 5-trimethylsilylbicyclo[2.2.2]oct-2-ene: Synthesis and gas-transport properties

A new silicon-containing bicyclic monomer 5-trimethylsilylbicyclo[2.2.2]oct-2-ene has been synthesized, and its metathesis polymerization and gas transport properties of the polymer based on it have been studied. The monomer is synthesized by the two-step scheme using the Diels–Alder reaction from 1,3-cyclohexadiene and vinyltrichlorosilane followed by methylation with a Grignard reagent. The resulting 5-trimethylsilylbicyclo[ 2.2.2]oct-2-ene is inactive in metathesis homopolymerization in the presence of first- and second- generation Grubbs catalysts and a Hoveyda–Grubbs catalyst, but it slowly polymerizes when norbornene is present in the reaction mixture. The high-molecular-mass copolymer (M _w = 3.0 × 10⁵, M _w/M _n = 2.8) of 5-trimethylsilylbicyclo[2.2.2]oct-2-ene and norbornene possesses good film-forming properties, and its glass transition temperature is 126°C. The gas-transport properties of the copolymer have been studied.     

Thermotropic homopolyester. I. Synthesis and characterization of homopolyesters containing the mesogenic unit,bicyclo[2.2.2]oct-2-ene

Homopolyesters based on bicyclo[2.2.2]oct-2-ene as a mesogenic group were prepared and characterized. The use of bicyclo[2.2.2]oct-2-ene in these homopolyesters lowered the symmetry of the resulting systems enough to lead to the observation of melting behavior.     

Selective fluorescence quenching of 2,3-diazabicyclo[2.2.2]oct-2-ene by nucleotides

[reaction: see text] The fluorescence quenching of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) by nucleotides has been studied. The quenching mechanism was analyzed on the basis of deuterium isotope effects, tendencies for exciplex formation, and the quenching efficiency in the presence of a molecular container (cucurbit[7]uril). Exciplex-induced quenching appears to prevail for adenosine, cytidine, and uridine, while hydrogen abstraction becomes competitive for thymidine and guanosine. Compared to other fluorescent probes, DBO responds very selectively to the type of nucleotide.