Vibrational spectra of cis- and trans-1,2-dimethyl-3-bromocyclopropane

The infrared and Raman spectra of cis- and trans-dimethylbromocyclopropane have been recorded from 4000 to 50 cm⁻¹. An assignment of the majority of the fundamentals is proposed and compared to those of related molecules. Definite and consistent trends in a number of normal modes of the ring and the methyl groups with the nature, position and number of the substituents have been found. Clear evidence has been obtained for steric interaction between the three substituents in cis-position.     

Transition structures, energetics, and secondary kinetic isotope effects for cope rearrangements of cis-1,2-divinylcyclobutane and cis-1,2-divinylcyclopropane: a DFT study

The minimum energy reaction paths and secondary kinetic isotope effects (KIE) for the Cope rearrangements of cis-1,2-divinylcyclobutane and cis-1,2-divinylcyclopropane obtained by (U)B3LYP calculations are reported. Both reactions proceed through endo-boatlike reaction paths, and have aromatic transition states. The predicted activation energies are in agreement with the experimental data. The reaction paths of the rearrangements are intervened by enantiomerization saddle points of the products (and the reactant in the case of divinylcyclobutane). The calculated KIEs are similar in the two systems, and consistent with the geometries of the transition structures. There is computational evidence that the isotope effect associated with the conversion of a pure sp(2) C-H bond into a pure sp(3) one might be the same in all molecules. The predicted KIEs agree with experiment for divinylcyclopropane, but not for divinylcyclobutane.     

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.     

A comparison of the Cope rearrangements of cis-1,2-divinylcyclopropane, cis-2,3-divinylaziridine, cis-2,3-divinyloxirane, cis-2,3-divinylphosphirane, and cis-2,3-divinylthiirane: a DFT study

Transition structures, energetics, and nucleus-independent chemical shifts (NICS) for Cope rearrangements of cis-2,3-divinylaziridine (1N), cis-2,3-divinyloxirane (1O), cis-2,3-divinylphosphirane (1P), and cis-2,3-divinylthiirane (1S), leading to 4,5-dihydro-1H-azepine (3N), 4,5-dihydrooxepine (3O), 4,5-dihydro-1H-phosphepine (3P), and 4,5-dihydrothiepine (3S), respectively, are reported at the (U)B3LYP/6-31G level and compared to those of cis-1,2-divinylcyclopropane (1C). The minimum energy path for all rearrangements proceeds through an endo-boatlike, aromatic transition structure. The predicted activation barriers increase in the order of 1C < 1N < 1O < 1P < 1S, which agrees qualitatively with the decreasing ring strain order of reference compounds (cyclopropane > aziridine > oxirane > phosphirane > thiirane). The exothermicities for these rearrangements decrease in the order of 1N > 1O > 1C > 1P > 1S. If the place of 1C in this sequence is ignored, the decreasing reaction exothermicity order correlates well with the increasing activation barrier order and with decreasing strain order of reference compounds. NICS values calculated for transition structures are typical of highly aromatic transition structures of thermally allowed pericyclic reactions.     

Kinetics of thermal gas-phase isomerizations and fragmentations of cis- and trans-1-(E)-propenyl-2-methylcyclobutanes at 275 degrees C

Kinetic studies of the thermal isomerization and fragmentation reactions exhibited by cis- and trans-1-(E)-propenyl-2-methylcyclobutanes at 275 degrees C in the gas phase have provided first-order rate constants for cis,trans interconversions of the cyclobutanes, 1,3-carbon migrations leading to 3,4- and 3,6-dimethylcyclohexenes, isomerizations providing directly and indirectly four acyclic dienes, and fragmentations to ethylene, propene, and mixtures of pentadienes and hexadienes. Both cis and trans isomers of 1-(E)-propenyl-2-methylcyclobutane form trans-3,4-dimethylcyclohexene faster than they are converted to cis-3,4-dimethylcyclohexene; the trans reactant gives rise to cis-3,6-dimethylcyclohexene in preference to its trans isomer, while the cis starting material gives neither at measurable rates; both form the relatively minor product 1,6-(Z)-octadiene. The rate constants derived from 35 kinetic runs starting with four distinct 1-(E)-propenyl-2-methylcyclobutane samples are consistent to within narrow error limits. The stereomutations, isomerizations, and fragmentations of the 1-(E)-propenyl-2-methylcyclobutanes are interpreted in terms of competitive processes involving conformationally flexible short-lived 2-(E)-octene-4,7-diyl and 3-methyl-5-(E)-heptene-1,4-diyl diradicals.     

Delocalization energy of the cyanomethyl radical: Kinetics of thermal diastereoisomerization of cis- and trans-1,2-dicyano- and 1 ,2-dicyano-1-methylcyclopropanes

Arrhenius parameters for the thermal first-order geometrical isomerization of 1,2-dicyanocylopropanes(I) have been determined in naphthalene solution over the range 208.0–259.5° in both directions:

where θ = 4.575T × 10^?3 and k is in sec^?1. Since this enthalpy of activation is lower than that of the geometrical isomerization of 1,2 - dideuterocyclopropane by 17.8±0.4 kcal, it may be concluded that replacement of hydrogen by the cyano group leads to an energy lowering of 8.9$\text{kcal}\text{mol}$.Kinetic parameters have been determined in the gas-phase at two temperatures, 217.8° and 259.5°: log k_t,c = 13.73– 45.64/θ; log k_c,t =13.86–44.43/θ.The rates of cis-trans interconversion of 1,2 - dicyano - 1 - methyl - cyclopropane(II) relative to those of I have been obtained by examination of mixtures of both substances in t-butylbenzene solution at 259.5°: 1.2-dicyano, k_t,c= 1.25 and k_c,t = 3.53; 1,2 - dicyano - 1 - methyl, k_t,c = 8.09 and k_c,t = 22.35 × 10^?5 sec^?1. The rate acceleration by methyl amounts to a factor of 6.4, corresponding to ΔΔG^≠ = 1.96$\text{kcal}\text{mol}$. A preliminary examination of optically active material leads to a minimum R_A = 1.37 favoring rotation of (CN)(H) over (CN)(CH₃).     

Synthesis,stereochemistry, and isomeric transformations of cis- and trans-1,2-dimethyl-4-aryl-5-aroyl-2-imidazolines

The corresponding trans- and cis-1,2-dimethyl-4-aryl-5-aroyl-2-imidazolines were obtained from complexes of cis- and trans-1-methyl-2-aryl-3-aroylaziridines with BF₃ by heating with acetonitrile. The reaction proceeds with inversion of the configuration of the starting 3-aroylaziridines. In the presence of bases the complexes of cis-1,2-dimethyl-4-aryl-5-aroyl-2-imidazolines readily undergo isomerization to the corresponding trans analogs. The structures of the products were established on the basis of the IR, PMR, and mass spectra and the results of elementary analysis. The configurations of the compounds were determined by means of the Overhauser nuclear effect.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 952–957, July, 1981.     

The molecular structure of cis- and trans-1,2-difluoro-ethene: An electron diffraction study

The molecular structure of cis- and trans-1,2-difluoroethene was studied in the gas phase by electron diffraction, using the sector-microphotometer technique. The molecules are planar. For trans-difluoroethene the geometrical parameters are: C-F bond: 1.338(0.003) Å; CC bond: 1.320(0.009) Å; C-H bond: 1.088(0.004) Å, ∠CCF 119.8°(0.2°); ∠CCH: 125° (1.2°). For cis-difluoroethene: C-F bond: 1.332(0.003) Å; CC bond: 1.311(0.008) Å; C-H bond: 1.100(0.003) Å; ∠CCF: 122.5°(0.2°); ∠CCH:127.0 °(2.3°).     

Temperature dependence of the rate constants of the reactions of oxygen atoms with trans-2-butene,cis-2-butene, 2-methylpropene, 2-methyl-2-butene,and 2,3-dimethyl-2-butene

The kinetics of the reactions of ground state oxygen atoms with trans-2-butene, cis-2-butene, 2-methylpropene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene was investigated in the temperature range 200 to 370K. In this range, the rate constants are (in units 10^?11 cm³ s^?1): (1.1 ± 0.1) exp[+(180 ± 24)K/T]; (0.98 ± 0.09) exp[+(149 ± 23)K/T]; (1.14 ± 0.13) exp[+(128 ± 33)K/T]; (2.34 ± 0.16) exp[+(250 ± 16)K/T]; and (3.31 ± 0.50) exp[+(257 ± 36)K/T], respectively. The atoms were generated by the H₂ laser photolysis of NO and detected by the time resolved chemiluminescence in the presence of NO. The concentrations of the O(³P) atoms were kept so low that secondary reactions with products are unimportant. © 1995 John Wiley & Sons, Inc.     

Magnetic isotope effects in biradicals: Substantial 13C enrichment and a novel mechanistic probe

Photolysis of 2,n-diphenylcycloalkanones (n-membered ring, n = 10, 11, 12, 15) produces products that are significantly enriched in ¹³C. The enrichments are different for each product, and this allows assignment of the dynamic pathways through which each product is formed.     

Lipase-mediated kinetic resolution of cis-1,2-indandiol and the Ritter reaction of its mono-acetate

Lipase-mediated kinetic resolution of cis-1,2-indandiol 5 in the presence of lipase PS was examined. Enantiomerically enriched (1S,2R)-2-acetoxy-1-indanol 6a was obtained when cis-1,2-indandiol 5 was treated with one equivalent of vinyl acetate. Treatment of 5 with two equivalents of vinyl acetate furnished a mixture of (1R,2S)-2-acetoxy-1-indanol 6a and (1R,2S)-1-acetoxy-2-indanol 6b. A route to both enantiomers of 1 was also developed by using the enantiomerically enriched mono-acetate thus obtained.     

Synthesis of cis-6,6,9-trimethyl-Δ3,4-octalin-2-one and cis-6,9-dimethyl-6-carbomethoxy-Δ3,4-octalin-2-one : Potential de synthons for the construction of pentacyclic triterpenes

Synthesis of the bicyclic enones (1 and 2) which are precursors for the rings DE of the pentacyclic triterpenes, β-amyrin and glycyrrhetinic acid has been reported from the readily available 6 - methoxy - 2- naphthoic acid.     

Ab initio coupled cluster determination of the equilibrium structures of cis- and trans-1,2-difluoroethylene and 1,1-difluoroethylene

The equilibrium structures of cis- and trans-1,2-difluoroethylene and 1,1-difluoroethylene, C(2)H(2)F(2), have been determined with high-level coupled cluster techniques combined with large basis sets, explicit consideration of core/valence, and scalar relativistic and higher order correlation effects. Excellent agreement was found with new semiexperimental structures, increasing the level of confidence in both approaches. Differences in bond lengths among ethylene and the fluoroethylenes are discussed.