Competitive 1,2- and 1,5-hydrogen shifts following 2-vinylbiphenyl photocyclization

[reaction: see text] The photocyclization of 2-vinylbiphenyl and its derivatives has been proposed to occur via a two-step mechanism: photocyclization to form an unstable 8a,9-dihydro-phenanthrene intermediate, followed by exothermic unimolecular isomerization to a 9,10-dihydrophenanthrene. The mechanism of the hydrogen shift process has been investigated using deuterated derivatives of 2-isopropenylbiphenyl and 2,6-diphenylstyrene. 1H NMR analysis of the photoproducts indicates that the thermally allowed 1,5-hydrogen or deuterium shift is a minor product-forming pathway and that an unusual double 1,2-hydrogen or deuterium shift is the major product-forming pathway. The potential energy surface for photocyclization and hydrogen shift processes has been explored computationally. The calculated barrier for the 1,5-shift is predicted to be significantly lower than that for the 1,2-shift. Alternative mechanisms for the occurrence of 1,2-hydrogen or deuterium migration are presented.     

Detection of 1,2-hydride shifts in the formation of euph-7-ene by the squalene-tetrahymanol cyclase of Tetrahymena pyriformis

Incubation of samples of 2,3-dihydrosqualene, specifically labeled with deuterium at either carbon position 7 or 11, with an enzyme extract from Tetrahymena pyriformis, containing a squalene-tetrahymanol cyclase, provided specimens of euph-7-enes displaying deuterium patterns consistent with the biosynthetic operation of two consecutive 1,2-hydride shifts.     

1,2-aryl and 1,2-hydride migration in transition metal complex catalyzed diazo decomposition: a novel approach to alpha-aryl-beta-enamino esters

N-Tosyl diazoketamines were prepared by addition of the ethyl alpha-diazoacetate anion to N-sulfonylimines. The diazo decomposition of the diazoketamines with Rh(2)(OAc)(4) complex resulted in aryl migration to give alpha-aryl-beta-enamino esters in good yields and high stereoselectivity. The effect of the catalysts on the migratory aptitude of 1,2-aryl over 1,2-hydride migration was studied. A reaction mechanism involving a "bridged" phenonium ion is proposed. Reaction: see text.     

Kinetics of the thermal decomposition of trans-1,2-dicyanocyclobutane

The kinetics of the thermal decomposition of trans-1,2-dicyanocyclobutane, which yields only vinyl cyanide, have been studied in the temperature range of 570°-660°K using a stirred-flow reactor. The reaction was found to be first order and homogeneous with rate constants represented by the Arrhenius equation where θ = 2.303 RT kcal/mol. The Arrhenius parameters are considerably higher than previously reported. On the assumption of a biradical mechanism the results are consistent with a cyano stabilization energy of ～5 kcal/mol, in good agreement with the results of recent studies of related systems.     

Ring-expansion reactions in the thermal decomposition of tert-butyl-1,3-cyclopentadiene

The thermal decomposition of tert-butyl-1,3-cyclopentadiene has been investigated in single-pulse shock-tube studies at shock pressures of 182-260 kPa and temperatures of 996-1127 K. Isobutene (2-methylpropene), 1,3-cyclopentadiene, and toluene were observed as the major stable products in the thermolysis of dilute mixtures of the substrate in the presence of a free-radical scavenger. Hydrogen atoms were also inferred to be a primary product of the decomposition and could be quantitatively determined on the basis of products derived from the free-radical scavenger. Of particular interest is the formation of toluene, which involves the expansion of the ring from a five- to a six-membered system. The overall reaction mechanism is suggested to include isomerization of the starting material; a molecular elimination channel; and C-C bond fission reactions, with toluene formation occurring via radical intermediates formed in the latter pathway. These radical intermediates are analogous to those believed to be important in soot formation reactions occurring during combustion. Molecular and thermodynamic properties of key species were determined from G3MP2B3 quantum chemistry calculations and are reported. The temperature dependence of the product spectrum was fit with a detailed chemical kinetic model, and best-fit kinetic parameters were derived using a Nelder-Mead simplex minimization algorithm. Our mechanism and rate constants are consistent with and provide experimental support for the H-atom-assisted routes to the conversion of fulvene to benzene that have been proposed in the literature on the basis of theoretical investigations.     

Preparation of polymer-bound 1,2 - and 1,3 - diamines

Two modified copolymers were used for the preparation of polymer bound symmetrical aliphatic diamines using phase transfer catalysis (P.T.C.). These compounds are potential ligands for transition metal ions.     

1,2- and 1,3-diphosphetanes from alkylidenephosphines

Abstract

Alkyl- or arylbis(trimethylsilyl)phosphines as well as tris(trimethylsilyl)phosphine and the corresponding arsines react with acyl chlorides to give [1-(trimethylsiloxy)alkylidene]phosphines 1 and -arsines 2; most of their 2,2-dimethylpropylidene derivatives are thermally stable at room temperature. With the same class of phosphines as starting compounds and carbon disulfide [bis(trimethylsilylsulfano)methylidene]phosphines 3 are formed, whereas [(dialkylamino)methylidene]-4 and [diarylmethylidene]phosphines 5 or the corresponding arsines 6 and 7 can be obtained from acyl amides or ketones.¹     

Mass spectra of methylsubstituted monosila- and monogermacyclobutanes, 1,2- disila- and 1,2-digermacyclopentanes. Some correlations with thermal decomposition

The mass spectra of 1,1-dimethyl-1-silacyclobutane (I—as reported by Cherniak et al.),⁶, 1,1-dimethyl-1-germacyclobutane (II), 1,1,2,2-tetramethyl-1,2-disilacyclopentane (III) and 1,1,2,2-tetramethyl-1,2-digermacyclopentane (IV) are compared and some correlations between electron-impact fragmentation and thermal decomposition are derived. The mass spectra of the germanium compounds with respect to the silicone compounds are enriched by light fragment ions and exhibit lower intensities of odd-electron ions. The composition of some ions and apparently of neutral fragments coincides with that of the unstable intermediates which are suggested in the thermal decomposition mechanism of some related compounds. The loss of C₂H₄ is more characteristic under electron-impact as well as in thermal decomposition of Si-compounds, while C₃H₆ is preferable eliminated by the Ge-compounds.     

Investigations on the thermal decomposition of 1,2-dioxetanes via DSC

Reproducible measurements, reliable results and 1^st order kinetics for the whole reaction are obtained during the thermal decomposition of dioxetanes only, if inclusions of impurities in commercial sample pans are blocked by additional thick (magnitude of 100 ?m) and close Al₂O₃ protective layers. As a rule, nearly the same activation parameters are then found both for the decomposition of solvent-free dioxetanes and diluted solutions in several solvents. Mixtures of different dioxetanes in the same solvent contribute independently to the overall heat flow rate.

     

Anodic azolation of 1,2- and 1,3-dimethoxybenzenes

Electrochemical azolation of 1,2-and 1,3-dimethoxybenzenes with tetrazole, pyrazole and triazole derivatives in MeCN in an undivided cell with Pt electrodes proceeds through the formation of intermediate arenonium cations of the ipso-structure. Nature of the starting arenes and the corresponding intermediate arenonium cations determine composition and yields of the target products.     

Regioselective synthesis of 1,2- vs 1,3-squaraines

Squaraines have been known for many decades as very stable and versatile vis-NIR absorbing dyes. They have found applications for example as sensitizers in organic photovoltaics and photodetectors. The most common squaraine structure is the 1,3-regioisomer. Their 1,2-regioisomers are seldom mentioned and unanimously regarded as side products. A facile direct synthesis of 1,2-squaraines, highlighting the role played by reaction conditions and electronic factors, is described. The first electrochemical characterization of these dyes is also shown.     

Kinetics of the thermal decomposition of 1,2-dioxaspiro[2,5]octane

The products and kinetics of the thermolysis of 1,2-dioxaspiro[2,5]octane in cyclohexanone and cyclohexanone-CCl₄ mixtures are studied. 1,2-Dioxaspiro[2,5]octane is consumed via two parallel routes: isomerization to oxepan-2-one and solvent (cyclohexanone) oxidation with the partial escape of radicals from the cage (17% at 25 °C). Under an inert atmosphere, the alkyl radicals formed by solvent oxidation initiate the chain radical decomposition of 1,2-dioxaspiro[2,5]octane. The mechanism of 1,2-dioxaspiro[2,5]octane thermolysis is discussed on the basis of the results obtained. The activation parameters of 1,2-dioxaspiro[2,5]octane isomerization to oxepan-2-one and reactions of dioxaspiro[2,5]octane with cyclohexanone are discussed.