Showing papers on "Cyclopropane published in 1987"

Activation of the C–C bond provides a molecular basis for structure sensitivity in metal catalysis

Abstract: The catalytic activation of hydrocarbon C–H and C–C bonds converts petroleum into fuels and chemicals. Understanding of the structural and electronic effects that control the catalysis has been a goal of researchers for decades. A focal issue concerns the number of metal atoms in a catalytic site required for C–C bond rupture1,2. Metal surfaces catalyse alkane hydrogenolysis (C–C bond breaking), but the catalytic sites are poorly defined3,4; soluble, structurally defined transition metal complexes are not known to catalyse this reaction. Here we present evidence demonstrating that isolated mononuclear rhenium complexes on the surface of MgO catalyse alkene hydrogenation, but not C–C bond rupture; in contrast, ensembles consisting of three of the same rhenium complexes catalyse both alkene hydrogenation and rupture of the C–C bond of cyclopropane. These results provide a molecular foundation for structure sensitivity in surface catalysis and point the way to design of multicentre catalytic sites on surfaces5–7.

Formation of carbocations from C3 compounds in zeolites of different acidities

Abstract: Generation of enylic carbenium ions from compounds possessing C3 carbon skeletons has been investigated on mordenites of different acidities by ultraviolet–visible spectroscopy. The formation of similar carbocations from allene, propene, cyclopropane, isopropanol and acetone has been identified. The rate of formation of allylic and polyenylic carbocations markedly depends on the number of acidic centres in the zeolite. The order of capability for carbenium ion formation found for organic compounds of similar structures was in good agreement with that observed previously in sulphuric acid solutions.

Cyclopropane stereomutation catalyzed by one-electron oxidants

Abstract: Le cis-(methoxy-4 phenyl)-1 vinyl-2 cyclopropane est rapidement isomerise par pBrPh 3 N + SbF 6 − a des temperatures de l'ordre de −90°C

Decay Reactions of Aryldiazenyl Radicals in Solution

Abstract: The decay processes of aryldiazenyl radicals were studied by using a time-resolved ESR method. The first order rate constants of decomposition were determined at −117–−48 °C in cyclopropane solutions in the presence or absence of olefins. Aryldiazenyl radicals were relatively persistent and the k1 of phenyldiazenyl-d5 at −96 °C in cyclopropane was 34 s−1. The k1 of 20 aryldiazenyl radicals at −96 °C afforded a ρ value of +1.53 (r=0.91) in a Hammett plot. Activation energies ranged from 46.2 to 25.2 kJ mol−1 and the frequency factors, log A, from 13.2 to 8.2 s−1. The second-order decay rates were measured at the initial period of the decay process at lower temperatures, in a batch system (cyclopropane) and in a flow system (isohexane). The rate constant of phenyldiazenyl-d5 in cyclopropane was 9.2·10−5 M−1 s−1 at −93 °C and the Arrhenius parameters were Ea=23.5 kJ mol−1 and logA=12.77 M−1 s−1, respectively.

Desulfurization of saturated C3S molecules on Mo(110): the effect of ring strain

Abstract: The reactions of trimethylene sulfide (c-C3H6S) and 1-propanethiol (C3H7SH) have been investigated on Mo(110) under ultrahigh vacuum using temperature-programmed reaction spectroscopy and Auger electron spectroscopy. Deuterium preadsorption experiments were conducted in conjunction with temperature-programmed reaction spectroscopy to deduce some mechanistic details of the reactions. Desulfurization reactions of both molecules to produce propane and propene were observed in the temperature range of 300-350 K, with propane production preceding propene production. In addition, trimethylene sulfide decomposed to form cyclopropane at 190 K. Both trimethylene sulfide and 1-propanethiol reacted on Mo(110) to produce gaseous dihydrogen in two peaks at approximately 350 and 540 K, as well as surface carbon and sulfur. Small amounts of reversibly adsorbed 1-propanethiol desorbed from Mo(110) between 175 and 200 K. Auger electron spectroscopy measurements suggest that approximately 50% of chemisorbed trimethylene sulfide decomposed to form hydrocarbons, while 70% of irreversibly chemisorbed 1-propanethiol decomposed to form hydrocarbons. The decomposition of trimethylene sulfide to cyclopropane is postulated to occur by one of three pathways. One of these pathways is entirely intramolecular, and the other two involve metallacycle transition states or intermediates. Trimethylene sulfide and 1-propanethiol are proposed to form propane and propene by way of a surface propyl thiolate intermediate,more » in a fashion similar to the reactions of tetrahydrothiophene and 1-butanethiol on Mo(110). The possible contributions of ring strain to the energetics and selectivity of the desulfurization reactions are discussed.« less

An O .fwdarw. N acyl transfer. An important activation step for a formal nucleophilic substitution in a cyclopropane derivative

Abstract: Etude des reactions a partir de piperidino-1' cyclopropyl-2 dimedon O-acyle via le cyclopropylidene-2 dimedon

A high level ab initio study of corner-protonated cyclopropane

Abstract: Calculations have been carried out at the MP4SDO//6-31G* level for corner-protonated cyclopropane that show it to have an unsymmetrical π complex structure, the formal charges on the basal atoms being in the ratio 3:2; implications concerning the norbornyl cation are discussed.

Fungicidal pyridyl cyclopropane carboxamidines

Abstract: Novel fungicidal pyridyl cyclopropane carboxamidines having the general structural formula and tautomers thereof wherein R is selected from the group consisting of hydrogen, pyridyl, substituted pyridyl, C₁-C₃ alkoxy, C₁-C₆ alkyl, acyl, carbamoyl, sulfonyl and cyano; R₁ is selected from the group consisting of halogen, C₁-C₃ alkoxy and C₁-C₃ alkenyloxy; R₂ is selected from the group consisting of hydrogen and C₁-C₃ alkyl; R₃ is selected from the group consisting of hydrogen, alkyl, aryl or arylalkyl; and fungicidally acceptable organic and inorganic salts thereof.

Evidence of carbene formation in oxidative coupling of methane over lithiumpromoted magnesium oxide

Abstract: The presence of carbene in oxidative coupling of methane over Li–Mg oxide is revealed by introducing ethylene, which is transformed into cyclopropane.

The aza-di-π-methane rearrangement of O-acetyl 2,2-dimethyl-4,4-diphenylbut-3-enal oxime

Abstract: The photochemical reaction of O-acetyl 2,2-dimethyl-4,4-diphenylbut-3-enal oxime is described: the reaction yields two cyclopropane derivatives, O-acetyl 3,3-dimethyl-2,2-diphenylcyclopropane carboxaldehyde oxime as the primary photoproduct, formed by an aza-di-π-methane reaction previously unobserved in such compounds, and 1-cyano-3,3-dimethyl-2,2-diphenylcyclopropane formed by thermal elimination of acetic acid from the primary product.

Insertion of methylene into ethane and cyclopropane

Abstract: The insertions of methylene into the CH and CC bonds of ethane and into a CC bond of cyclopropane are calculated by using third-order perturbation theory with the 6-31G(d) basis set. At this level of theory, the barriers for these reactions are predicted to be 0.2, 46.0, and 2.2 kcaljmol, respectively. Thus, the introduction of strain has a dramatic effect on the barrier to insertion into a heavy atom-heavy atom bond. In their closed shell singlet states, the dominant reaction of CH2 and SiH2 is believed to be insertion into available bonds. There is experimental1 and theoretical2 evidence that methylene inserts into H-H and C-H bonds with no energy barrier. Likewise, the most recent experimental3 and theoretical4 evidence suggests that the insertion of silylene into H-H to form silane occurs with little or no barrier. The most recent estimates5•6 place the barriers to insertion of methylene into both the C-H bond of methane and the Si-H bond of silane and of silylene into the Si-H bond of silane at close to zero. The barrier for insertion of silylene into the methane C-H bond is believed to be about 20 kcaljmol.6•7 The rates of insertions of CH2 and SiH2 into single bonds between heavy atoms X,Y are apparently much slower than those for X-H insertions. The reason for this could be statistical or a higher barrier for the X-Y insertions. X-Y insertions might be facilitated by introducing strain into the system, thereby weakening the X-Y bond. In this paper, we present the results of preliminary ab initio calculations on the insertions of methylene into the C-H and C-C bonds of ethane and into a CC bond of cyclopropane. Optimized geometries for RHF stationary points were obtained by using the 6-31G(d) 8 basis set and the Schlegel optimization method9 in GAUSSIAN82. 10 Minina and transition states were verified by establishing that the matrices of energy second derivatives have zero and one negative eigenvalue, respectively. For the prediction of reaction energetics secondand third-order Moller-Plesset perturbation theory corrections11 (MP2 and MP3) were added. The 6-31G(d) structures for methylene, ethane, propane, cyclopropane, and cyclobutane are available elsewhere. 12 The 6-31G(d) transition states for the three reactions of interest are shown in Figure 1, and the energetics for the reactions are summarized in Table I. All transition-state optimizations were carried out in cl symmetry; however, the saddle-point structures for the insertions into the CC bonds have essentially Cs symmetry. For all three reactions, the approach of methylene to the substrate is skewed, with the methylene hydrogens avoiding steric interactions with substrate atoms. This is easiest for the attack at the ethane CH bond (Figure 1a) and most difficult for attack at the ethane CC bond (Figure 1b). This has a dramatic effect on the internuclear distances at these two saddle points. The newly forming bonds (CC and CH for the CH insertion; CC and CC for the CC insertion) are stretched by roughly 25% relative to their final equilibrium values for the CH insertion and 31 and 42% for the CC insertion. In contrast, while the breaking CH bond has only stretched 11% by the CH insertion transition state, the analogous CC bond in the CC insertion has stretched by 28%, t North Dakota State University. t Minot State College. 0002-7863/87/1509-1323$01.50/0 Table I. Energetics (kcaljmol) for Insertion Reactions•

Elimination and addition reactions. Part 41. Nucleophilic eliminative fission of cyclopropanes: the coiled spring effect of ring strain on nucleofugality and its evaluation

Abstract: Rates have been measured of sulphonyl-activated eliminative ring fissions of a series of six cyclopropanes in which the leaving group is stabilised by alkoxycarbonyl, cyano, or sulphonyl groups. The measurements allow assignment of ranks (nucleofugalities) to carbon leaving groups in systems in which the connection to the leaving group is strained by incorporation in a cyclopropane ring. The values obtained are compared with those obtained for a unstrained (acyclic) analogues. Rank enhancements of about 9 (log) units are obtained; these enhancements suggest that free energies of activation for leaving-group expulsion are reduced by about 53 kJ mol–1, or about 46% of the excess of enthalpy of the strained ring, notwithstanding the small degree of ring fission in the transition structure. The effect of phenyl substitution at the leaving group suggests that cleavage of the ring is very little advanced in the transition structure, although this is variable with the nature of the leaving-group stabilisation. This is the first direct determination of the effect of strain on nucleofugality.