Facile Activation of Zinc. Preparation of Cyclobutanones via Dichloroketene and Cyclopropanes Using the Simmons-Smith Reaction

Zinc powder activated by heating in an inert atmosphere gave good yields when used for the title reactions. The very easy work up procedure for the cyclobutanones further increases the potential of the method.     

An inexpensive modification of the Simmons-Smith reaction: The formation of bromomethylzinc bromide as studied by NMR spectroscopy

Bromomethylzinc bromide, prepared from zinc metal and methylene bromide in tetrahydrofuran, gives moderate to good yields of cyclopropanation product when treated with olefins, thus providing an inexpensive alternative to the Simmons-Smith procedure. An NMR study of the reaction between metallic zinc and methylene bromide has been carried out: in addition to bromomethylzinc bromide and dimethylzinc, two further species were observed and tentatively identified as bis(bromomethyl)zinc and a di- or tri-zinc compound.     

Asymmetric Simmons-Smith cyclopropanation of unfunctionalized olefins

This paper describes a promising asymmetric cyclopropanation system for unfunctionalized olefins using readily available dipeptide N-Boc-l-Val-l-Pro-OMe as ligand.     

Highly diastereoselective Simmons-Smith cyclopropanation of allylic amines

[reaction: see text] Cyclopropanation of allylic tertiary amines using the Simmons-Smith reagent has been achieved by employing chelating groups in close proximity to the amine. The chelating groups promote cyclopropanation at the expense of N-ylide formation. Using pseudoephedrine as the chelating group, high diastereoselectivity is observed.     

The Double Simmons-Smith Reactions VIA Silatropies

The cyclopropanation of ethyl 3-trimethylsiloxy-2-alkenoates with the Simmons-Smith reagent, followed by ring cleavage and hydrolysis with an alkali solution, gave the corresponding 3-methyl-4-oxoalkanoic acids and 5-oxoalkanoic acids.     

Reaction pathways from single-molecule trajectories

Signal-pair histograms map transitions between different states of a single molecule and open new opportunities for the reconstruction of kinetic pathways.     

Catalytic asymmetric Simmons-Smith cyclopropanation of unfunctionalized olefins

This paper describes a catalytic asymmetric cyclopropanation system for unfunctionalized olefins using readily available dipeptide N-Boc-l-Val-l-Pro-OMe (1) as ligand.     

Diastereoselective Simmons-Smith cyclopropanations of allylic amines and carbamates

Cyclopropanation of 3-(N,N-dibenzylamino)cyclohexene with either Zn(CH(2)I)(2)(Wittig-Furukawa reagent) or CF(3)CO(2)ZnCH(2)I (Shi's reagent) gives the corresponding syn-cyclopropane as a single diastereoisomer, whilst cyclopropanation of 3-(N-tert-butoxycarbonylamino)cyclohexene with CF(3)CO(2)ZnCH(2)I gives the corresponding anti-cyclopropane exclusively; facile N-deprotection gives access to either diastereoisomer of the trifluoroacetic acid salt of 2-aminobicyclo[4.1.0]heptane.     

Fluctuations in the ensemble of reaction pathways

The dominant reaction pathway is a rigorous framework to microscopically compute the most probable trajectories, in nonequilibrium transitions. In the low-temperature regime, such dominant pathways encode the information about the reaction mechanism and can be used to estimate nonequilibrium averages of arbitrary observables. On the other hand, at sufficiently high temperatures, the stochastic fluctuations around the dominant paths become important and have to be taken into account. In this work, we develop a technique to systematically include the effects of such stochastic fluctuations, to order k(B)T. This method is used to compute the probability for a transition to take place through a specific reaction channel and to evaluate the reaction rate.     

Normal,abnormal and pseudo-abnormal reaction pathways for the imine-peroxyacid reaction

Steric and mesomeric effects have a marked influence upon the formation of oxaziridine (normal pathway) or nitrone (abnormal pathway) products from the imine-peroxyacid reaction; n.m.r. studies of the thermal isomerization of oxaziridines to nitrones provide evidence of a pseudo-abnormal oxidation pathway.     

Reaction pathways and mechanisms of photodegradation of pesticides

The photodegradation of pesticides is reviewed, with particular reference to the studies that describe the mechanisms of the processes involved, the nature of reactive intermediates and final products. Potential use of photochemical processes in advanced oxidation methods for water treatment is also discussed. Processes considered include direct photolysis leading to homolysis or heterolysis of the pesticide, photosensitized photodegradation by singlet oxygen and a variety of metal complexes, photolysis in heterogeneous media and degradation by reaction with intermediates generated by photolytic or radiolytic means.     

Iterated reaction graphs: simulating complex Maillard reaction pathways

This study investigates a new method of simulating a complex chemical system including feedback loops and parallel reactions. The practical purpose of this approach is to model the actual reactions that take place in the Maillard process, a set of food browning reactions, in sufficient detail to be able to predict the volatile composition of the Maillard products. The developed framework, called iterated reaction graphs, consists of two main elements: a soup of molecules and a reaction base of Maillard reactions. An iterative process loops through the reaction base, taking reactants from and feeding products back to the soup. This produces a reaction graph, with molecules as nodes and reactions as arcs. The iterated reaction graph is updated and validated by comparing output with the main products found by classical gas-chromatographic/mass spectrometric analysis. To ensure a realistic output and convergence to desired volatiles only, the approach contains a number of novel elements: rate kinetics are treated as reaction probabilities; only a subset of the true chemistry is modeled; and the reactions are blocked into groups.     

Reaction pathways of zirconocene-catalyzed silylation of alkenes with chlorosilanes

Reaction pathways as well as stereochemistries and stoichiometries of zirconocene-catalyzed silylation of olefins with chlorosilanes in the presence of ⁿBuMgCl were studied and discussed in detail. Rate determining steps were examined by kinetic studies and labeling experiments.     

Exploring two-state reaction pathways in the photodimerization of cyclohexadiene.

The ground- (S0) and lowest triplet-state (T1) pathways associated with dimerization of cyclohexadiene to give [2+2] and [4+2] cycloadducts have been theoretically studied at the UBLYP and UB3LYP levels of theory with the 6-31G* basis set. The DFT energies were validated by CCSD(T) single-point energy calculations. These cycloaddition reactions follow stepwise mechanisms with formation of bis-allylic biradical (BB) intermediates. In the S0 ground state, the interaction between two cyclohexadiene molecules with formation of BB intermediate IN(S0) has a large activation enthalpy of 32.0 kcal mol(-1). On the other hand, C-C bond-formation in the lowest triplet state (T1) leading to BB intermediate IN(T1) has a low activation enthalpy of 5.0 kcal mol(-1), but the subsequent ring closure involves a very large activation enthalpy of 43.4 kcal mol(-1). Triplet-to-singlet intersystem crossing from IN(T1) to IN(S0) favors cyclization to give the corresponding [2+2] and [4+2] cycloadducts.     

Mechanistic pathways for the reaction of quercetin with hydroperoxy radical

The extensive theoretical study of the interaction of one of the most abundant and reactive flavonols, quercetin, with hydroperoxy radical (HOO·), using the M052X/6-31 + Gd, p level of theory, was performed. Results indicating that quercetin is not a planar molecule are in accord with the X-ray analysis. The applied method successfully reproduces the bond dissociation enthalpy, and reveals that the reaction of quercetin with the hydroperoxy radical is governed by a hydrogen atom transfer mechanism. It is confirmed that the 3′OH and 4′OH are the most reactive sites, and that the reaction in the 3′OH position is faster than that in the 4′OH position.     

Divergent pathways in the reaction of Fischer carbenes and palladium

[reaction: see text] The Pd-catalyzed reaction of beta-arylaminochromium(0) carbene complexes produces by transmetalation the first isolated and X-ray structurally characterized bis-Pd(II) carbene complex, as well as other alternative reaction pathways, such as the oxidative addition-transmetalation sequence, not seen before in this chemistry.     

Theoretical study of the reactions of AlF with HCl: Reaction pathways

The mechanisms for the reactions of AlF with HCl have been characterized in detail using DFT as well as the ab initio method, including zero-point corrections. From the potential energy profile it can be predicted that the reaction process of forming the new Al(III) hydride HAlFCl compound for this reaction is spontaneous with a low activation energy barrier. The reaction yielding to AlFCl₂ and molecular hydrogen was calculated to be highly exothermic. The present calculations also show that the possible routes to the trihalides species start more favorable with the primary insertion products.