Regioselective 1,2-additions of alcohols to allenamides mediated by N-Iodosuccinimide: Synthesis of N,O-aminals

N,O-Aminals are synthesized through the N-iodosuccinimide-mediated regioselective 1,2-additions of alcohols to allenamides. These novel reactions proceed rapidly and exhibit broad substrate scope for a variety of allenamides. The reaction demonstrates that N-Iodosuccinimide is effective in activating the terminal CC bond of allenamides generating conjugated sulfamide ion species. It is noteworthy that the alcohol served as both solvent and nucleophile in this transformation.     

New regio and stereoselective intermolecular Pauson-Khand reactions of allenamides

Intermolecular Pauson-Khand reactions are still quite limited in scope and yields. There are also few reports using allenes as the olefinic part in this version. We report herein new regio and stereoselective reactions of allenamides with several alkynes. These reactions give functionalized cyclopentenones bearing an exocyclic enamide, which can be useful synthetic intermediates.     

Acid-catalyzed cyclization of acyliminium ions derived from allenamides. A new entry to protoberberines

Under mild acidic conditions, N-(ω-phenylalkyl)allenamides can yield stabilized acyliminium ions which through intramolecular aromatic electrophilic substitution furnish 1-vinylisoindolines and isoquinolines. Stereoelectronic and entropic effects in these cyclizations have been evaluated by DFT computations. The 1-vinylisoquinolines obtained have been employed as key intermediates in the synthesis of the protoberberine skeleton.     

Inverse electron-demand aza-[4+2] cycloaddition reactions of allenamides

An inverse electron-demand aza-[4+2] cycloaddition reaction of allenamides with 1-azadiene is described here. Effects of solvents on diastereoselectivity along with synthetic scopes and mechanistic insights are illustrated. Despite some synthetic limitations, this aza-[4+2] cycloaddition does provide a useful template for the synthesis of aza-glycoside related heterocycles.     

Mechanism of the rhodium-catalyzed asymmetric isomerization of allylamines to enamines

A theoretical study of the mechanism of the rhodium-catalyzed asymmetric isomerization of allylamines to enamines by using density functional theory with the B3LYP functional leads us to discard the so far accepted nitrogen-triggered mechanism, in which the isomerization occurs on N-bonded intermediates and transition states, in favor of a variation of the classical allylic mechanism for olefin isomerization. The modified allylic mechanism consists of four main steps: 1) N-coordination of the allylamine to Rh(I); 2) intramolecular isomerization from kappa(1)-(N)-coordination to eta(2)-(C,C)-coordination of the allylamine; 3) oxidative addition of C(1)--H to form a distorted octahedral eta(3)-allyl complex of Rh(III); and 4) hydrogen transfer to C(3) (reductive C(3)--H elimination). The two hydrogen transfer steps (oxidative addition and reductive elimination) have the highest barriers of the overall process. The oxidative addition barrier, which includes solvent effects, is 28.4 kcal mol(-1). For the reductive elimination, the value in solvent is 28.6 kcal mol(-1), very similar to the oxidative addition barrier.     

The influence of distal substitution on the base-induced isomerization of long-chain terminal alkynes

When compared to a long-straight chain terminal alkyne, a long chain terminal alkyne with a distal isopropyl unit (isobranched) isomerizes about two times faster when treated with strong base under identical conditions, and appears to follow pseudo first order kinetics. In both cases, equilibration to a 95–97:5–3 mixture of terminal:internal alkyne accompanies isomerization. The difference in rate may be due to an unusual folding of both long-chain alkynes, bringing the distal substituent close to the carbon-carbon-triple bond moiety. The distal isopropyl moiety may provide unanticipated steric hindrance that disrupts such folding, making the propargylic proton more available for reaction with base.     

硅杂环丙烯异构化反应的理论研究

用量子化学理论方法研究了硅杂环丙烯单重态的异构化反应。结果表明:该异构化反应的过渡态为三元环结构,该反应为氢迁移反应;反应的势垒高度为276.67kJ·mol^-^1[MP2/6-31G(d)]。通过内禀反应坐标(IRC)计算,获得了沿反应途径的势能剖面。     

Palladium-catalyzed isomerization of 1-hexene

The isomerization of 1-hexene over Pd(acac)₂ + BF₃OEt₂ and Pd(acac)₂ + PPh₃ + BF₃OEt₂ catalyst systems proceeds by the consecutive-parallel scheme. Tetracoordinate square-planar Pd hydrides bearing two vacant coordination sites are likely to be catalytically active species for the isomerization of hexene-1. This revised version was published online in June 2006 with corrections to the Cover Date.     

Selective isomerization of α-pinene

a-Pinene isomerization was studied in gaseous and liquid phases on g-zirconium phosphate and rhodium g-zirconium phosphate. The first showed higher activity but lower reaction selectivity than the second, that led to tricyclic terpenes formation.     

Skeletal isomerization of 1-pentene

Series of ZSM-5 and Y zeolite catalysts have been prepared. Zeolites were submitted to ion exchange, drying, formation with 50 wt.% of aluminium hydroxide as a binder and then to final thermal treatment. Determination of catalysts activity in 1-pentene isomerization reaction was carried out in the temperature range of 175–325°C. The highest activity in isomerization reaction was attained in the presence of ZSM-5 zeolite catalysts with strong and medium strength acidity.     

Kinetic studies on Brook-type isomerization of acylpolysilanes to silenes

First-order rate constants of Brook-type isomerization of acylpolysilanes (Me₃Si)₃SiCOR (R = iso-Pr, tert-Bu, Ad, 2,6-xylyl, and Mes) leading to silenes (Me₃Si)₂SiC(OSiMe₃)R at various temperatures were determined. Their Eyring plots gave kinetic parameters of ΔH^‡ = 26.6-29.4 kcal mol⁻¹ and ΔS^‡ = −11.5 to −14.6 cal mol⁻¹ K⁻¹. The isomerization was accelerated by introducing an electron-donating alkyl substituent on the carbonyl carbon. These results are in accordance with a concerted mechanism involving a four-centered transition state.     

Skeletal isomerization of butene over natural ferrierite

The conversion of butenes to isobutene was compared on chemically modified natural and synthetic H-ferrierite catalysts. The proper balance of extra-framework species together with properly chosen reaction conditions allowed 100% isobutene selectivity.     

Kinetic study on isomerization of verbenol to isopiperitenol and citral

Thermal isomerization of verbenol in the gas and liquid phases is discussed. The effect of temperature and residence time on the reaction mixture composition was studied (at 460-600°C and a residence time t of about 10^-3 min for the gas phase conditions; at 200-350°С and t = 10-80 min for the liquid phase process). The kinetic behavior and the reaction mechanisms are presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fragmentation and isomerization of 1,2-dimethylcyclobutane

The potential surface of 1,2-dimethylcyclobutane is investigated with respect to fragmentation and isomerization by the semiempirical MO method SINDO1. Energy and geometry of eight transition states and four intermediates are determined for the nonconcerted fragmentations under optimization of all internal coordinates. We find that symmetric cleavage ofcis-dimethylcyclobutane is the most favored process leading to fragmentation. Whereas the symmetric elimination involves two different barrier heights, the asymmetric elimination involves only two equal barrier heights. The possibility of isomerization ofcis- totrans- dimethylcyclobutane was also studied and revealed two different barrier heights lower than the corresponding heights for fragmentation.     

Tunnelling and reaction path curvature effects in the isomerization of the methoxy radical

The reaction path hamiltonian is used to investigate the isomerisation CH₃O CH₂OH; the reaction path, the frequencies along it and the coupling coefficients describing reaction path curvature were calculated by ab-initio methods. Correlation effects were included by configuration interaction using a double-zeta-plus-polarisation basis set. Including both tunnelling and curvature gives temperature dependent rate constants in broad agreement with experiment, whereas previous results were in error by several orders of magnitude.     

Butane isomerization over platinum promoted sulfated zirconia catalysts

The isomerization of n-butane to i-butane has been studied at 11 bar in a microflow reactor over sulfated zirconia (SZ) and platinum containing sulfated zirconia (Pt-SZ) catalysts. In the presence of H₂ a significantly higher temperature is required for isomerization over SZ than in its absence. The rate over SZ is higher with n-butane containing 33 ppm butene as an impurity than with a feed that is pre-equilibrated over a Pt/SiO₂ catalyst to a much lower butene content. Over Pt-SZ the reaction rate is higher, because any butene consumed is rapidly regenerated; the conversion is perfectly stable in 83 h runs, selectivity to i-butane is 95%; i-pentane and propane are the main byproducts. The activation energy is 53 kJ mol⁻¹. Upon increasing the pressure of H₂ from 1.1 to 6.6 bar, the reaction rate was found to decrease in a perfectly reversible fashion. Kinetic analysis reveals that the reaction order is negative in H₂ (−1.1 to −1.3 depending on the temperature) and positive in n-butane (+ 1.3 to +1.6), indicating that the mechanism of this isomerization is intermolecular: butene is formed and reacts with adsorbed C₄-carbenium ions to adsorbed C₈ intermediates which isomerize and undergo β-fission to fragments with i-C₄ structure. This mechanism is confirmed over Pt-SZ by isotopic labelling experiments, though at much lower pressure, using double labelled ¹³CH₃---CH₂---CH₂---¹³CH₃. The primary reaction product consists of i-butane molecules, containing zero, one, two, three and four ¹³C atoms in a binomial distribution.     

Photocatalytic isomerization of norbornadiene over Y zeolites

K, Zn, Cs and La ion-exchanged Y zeolites were prepared and used as photocatalysts for the isomerization of norbornadiene. The prepared catalysts were characterized by EDS, XRD, ²⁷Al- and ²⁹Si-NMR, UV-vis and IR. The La-exchanged sample showed the highest degree of exchange, but accompanied with some dealumination. La-exchange generated both B-acid and L-acid sites, whereas other metals produced only L-acid sites. For the photocatalytic isomerization reaction, the activity order was LaY > CsY > ZnY > KY. The heavy atom effect and B-acidity are expected to play important roles in the reaction.     

无机锡催化δ-酮腈异构化成吡啶酮反应的研究

本文分别对三种无机锡(SnCl~2,SnO和SnO~2)催化δ-酮腈异构化成吡啶酮的条件进行了考察。实验结果表明:SnCl~2和SnO具有很高的催化活性。