Thermal isomerization of tricyclo[4.1.0.0(2,7)]heptane and bicyclo[3.2.0]hept-6-ene through the (E,Z)-1,3-cycloheptadiene intermediate

The thermal isomerization of tricyclo[4.1.0.0(2,7)]heptane and bicyclo[3.2.0]hept-6-ene was studied using ab initio methods at the multiconfiguration self-consistent field level. The lowest-energy pathway for thermolysis of both structures proceeds through the (E,Z)-1,3-cycloheptadiene intermediate. Ten transition states were located, which connect these three structures to the final product, (Z,Z)-1,3-cycloheptadiene. Three reaction channels were investigated, which included the conrotatory and disrotatory ring opening of tricyclo[4.1.0.0(2,7)]heptane and bicyclo[3.2.0]hept-6-ene and trans double bond rotation of (E,Z)-1,3-cycloheptadiene. The activation barrier for the conrotatory ring opening of tricyclo[4.1.0.0(2,7)]heptane to (E,Z)-1,3-cycloheptadiene was found to be 40 kcal mol(-1), while the disrotatory pathway to (Z,Z)-1,3-cyclohetpadiene was calculated to be 55 kcal mol(-1). The thermolysis of bicyclo[3.2.0]hept-6-ene via a conrotatory pathway to (E,Z)-1,3-cycloheptadiene had a 35 kcal mol(-1) barrier, while the disrotatory pathway to (Z,Z)-1,3-cyclohetpadiene had a barrier of 48 kcal mol(-1). The barrier for the isomerization of (E,Z)-1,3-cycloheptadiene to bicyclo[3.2.0]hept-6-ene was found to be 12 kcal mol(-1), while that directly to (Z,Z)-1,3-cycloheptadiene was 20 kcal mol(-1).     

Ab initio quantum dynamical study of the vinylidene-acetylene isomerization

The dynamics of vinylidene-acetylene isomerization has been studied quantum mechanically, based on a new ab initio calculated potential energy surface (PES). The grid underlying the PES and the dynamic treatment rest on planar 4-atom Jacobi-like coordinates. Three degrees of freedom, the C-C bond length and the two Jacobi-like angles are treated explicitly, while the two other coordinates relating to the C-H bond lengths have been optimized in the ab initio treatment. The energies of the resulting 3D grid have been computed with the CCSD(T) method, using a cc-pVTZ basis set, while selected stationary points have been characterized at a higher level of theory. The subsequent dynamic treatment reproduces very satisfactorily the experimental photodetachment spectrum of Lineberger and coworkers. Preliminary results are also reported for the lifetime of vinylidene. Received: 5 June 1998/Accepted: 23 July 1998 / Published online: 9 October 1998     

Derivation of a diazabicyclo[3.2.0]heptadiene from a diazatricyclo [4.1.0.02,7] heptene.

Trifluoromethylated 3,5-diazatricyclo[4.1.0.0^2,7]hept-3-ene was converted thermally or photochemically to 2,4-diazabicyclo[3.2.0]hepta-2,6-diene compound. The latter was photolyzed to an imidazole compound.     

Ab initio study on the photochemical isomerization of furan derivatives

The photochemical isomerization reactions of furan, 2-methylfuran, 2-trimethylsilylfuran, and furan-2-carbonitrile were studied using ab initio methods. The results are in agreement with the previously reported data obtained through semiempirical methods. In particular, the sensitized irradiation of furan derivatives populates the first excited triplet state of the furan, and this triplet state can evolve only through O-Ca cleavage. The selection of the bond to be broken can depend on energetic factors (furan-2-carbonitrile) or on kinetic factors (2-methylfuran, 2-trimethylsilylfuran). The direct irradiation of furan derivatives populates the singlet excited state and leads to a conversion into the Dewar isomer or into the corresponding triplet state through the usual intersystem crossing procedure. The efficiency of these processes determines the presence or the absence of isomerized furan derivatives in the reaction mixtures.     

Ab initio method study on the isomerization of 3-amino-2-pyridone

Ab initio caculations with RHF/6-31G and MP2/6-31G have been used to study the isomerization of 3-amino-2-pyridone in gas-phase and in water. The results obtained show that 3-amino-2-pyridone is isomerized into 3-amino-2-hydroxy pyridine via a four-center cyclic transition state in the gas-phase, and via a six-center cyclic transition state in water. The activation energies of this reaction are 226.3336(RHF/6-31G) and 171.2269(MP2/6-31G) in gas-phase, and 81.6398(RHF/6-31G) and 59.8668(MP2/6-31G) kJ mol⁻¹ under the condition of a single water molecule as the catalyst, respectively.     

Products and mechanism of some halogenation reactions of 1-sulfonyl-substituted tricyclo[4.1.0.0<Superscript>2,7</Superscript>]heptanes

Reactions of 1-methylsulfonyl- and 1-phenylsulfonyltricyclo[4.1.0.0^2,7]heptanes with iodine, dioxane dibromide, and dichloro-λ³-iodanylbenzene (under irradiation) gave products of stereoselective syn addition of halogen at the C¹–C⁷ central bicyclobutane bond. 7-Methyl-1-phenylsulfonyltricyclo[4.1.0.0^2,7]-heptane reacted with dioxane dibromide in carbon tetrachloride to produce a mixture of 2-bromo- and 2,3-dibromo-1-methyl-exo-7-phenylsulfonylnorcaranes at a ratio of 1: 4 as a result of cleavage of the C¹–C² bicyclobutane bond. 7-Bromo- and 7-methoxycarbonyl-1-phenylsulfonyltricyclo[4.1.0.0^2,7]heptanes take up bromine exclusively at the C¹–C⁷ central bond with strict syn stereoselectively. The regio- and stereoselectivity of the addition and their relations with the halogen nature were interpreted with account taken of structural specificities of intermediate 6-sulfonyl-substituted 6-norpinanyl radicals determined by ab initio quantumchemical calculations using 6-31G basis set.     

Ab initio calculations on the isomerization of alkene radical cations

Ab initio calculations on the isomerization of butene and pentene radical cations indicate that, for all classical ion structures, the lowest barrier for a rearrangement to the most stable ion structure is below the dissociation limit. Isomerizations of linear butene radical cations to the isobutene structure take place via the CH₃CC₂H₅^·+ structure, whereas in the pentene case the connection between linear and branched ion structures proceeds via the 1,2-dimethylcyclopropane radical cation. From the results a qualitative model is derived which suggests that for larger alkene radical cations an isomerization to structures with four alkyl substituents on the double bond may be in close competition with dissociation.     

Chemo-, regio-, and stereoselectivity in acid-catalyzed hydromethoxylation of tricyclo[4.1.0.0<Superscript>2,7</Superscript>]hept-1-yl and 7-methyltricyclo[4.1.0.0<Superscript>2,7</Superscript>]hept-1-yl phenyl sulfones

Hydromethoxylation of tricyclo[4.1.0.0^2,7]hept-1-yl and 7-methyltricyclo[4.1.0.0^2,7]hept-1-yl phenyl sulfones with methanol at 20°C in the presence of a catalytic amount of perchloric acid is initiated by the endo attack of proton at the C¹ atom, and the subsequent cleavage of the side C¹–C² bond leads to formation of mixtures of diastereoisomeric exo-7-phenylsulfonyl-2-methoxybicyclo[4.1.0]heptanes, the endo-2 isomer prevailing. Probable factors responsible for the observed chemo-, regio-, and stereoselectivity of the addition are discussed.     

Ab initio study on deactivation pathways of excited 9H-guanine

The complete active space with second-order perturbation theory/complete active space self-consistent-field method was used to explore the nonradiative decay mechanism for excited 9H-guanine. On the 1pipi* (1L(a)) surface we determined a conical intersection (CI), labeled (S0pipi*)(CI), between the 1pipi* (1L(a)) excited state and the ground state, and a minimum, labeled (pipi*)min. For the 1pipi* (1L(a)) state, its probable deactivation path is to undergo a spontaneous relaxation to (pipi*)min first and then decay to the ground state through (S0pipi*)(CI), during which a small activation energy is required. On the 1n(N)pi* surface a CI between the 1n(N)pi* and 1pipi* (1L(a)) states was located, which suggests that the 1n(N)pi* excited state could transform to the 1pipi* (1L(a)) excited state first and then follow the deactivation path of the 1pipi* (1L(a)) state. This CI was also possibly involved in the nonradiative decay path of the second lowest 1pipi* (1L(b)) state. On the 1n(O)pi* surface a minimum was determined. The deactivation of the 1n(O)pi* state to the ground state was estimated to be energetically unfavorable. On the 1pisigma* surface, the dissociation of the N-H bond of the six-membered ring is difficult to occur due to a significant barrier.     

Generation and trapping of a highly strained bicyclic alkyne: tricyclo[6.3.1.0(2,7)]dodeca-2,4,6-trien-9-yne

High-temperature bromination of 10-bromotricyclo[6.3.1.0(2,7)]dodeca-2,4,6,9-tetraene (8) resulted mainly in the formation of two isomeric tribromides (16 and 17) whose dehydrobromination with DBU afforded the corresponding 9,10-dibromotricyclo [6.3.1.0(2,7)] dodeca-2,4,6,9-tetraene (14). Treatment of 14 with tert-butyllithium in THF at -78 degrees C produces the strained bicyclic alkyne (10) which is trapped by 1,3-diphenylisobenzofuran to give two isomeric cycloadducts 12a and 12b. The adducts 12a and 12b were found to readily rearrange to the isomeric ketones 19a and 19b upon chromatography. Upon treatment with potassium tert-butoxide, the adducts 12a and 12b undergo base-catalyzed double bond isomerization to give four products 20-23. Furthermore, reaction of 14 with one and two mole of potassium tert-butoxide has been studied and the mechanism of the product formation is discussed with regard to either an allene or alkyne as a possible intermediate.