Alkenes In Solution Research Articles

Evidence for Ruthenium(II) Peralkene Complexes as Catalytic Species during the Isomerization of Terminal Alkenes in Solution.

The isomerization (chain-walking) reaction of terminal to internal alkenes is catalyzed by part-per-million amounts of practically any Ru source when the reaction is carried out with a neat terminal alkene. Here, we provide evidence that the soluble starting Ru sources evolve to catalytically active peralkene Ru(II) species under reaction conditions. These species may also explain the isomerization products found during other Ru-catalyzed alkene processes, i.e., alkene metathesis reactions. A Finke-Watzky mechanism for catalyst formation is consistent with the evidence obtained.

Supramolecular Template-Assisted Catalytic [2+2] Photocycloaddition in Homogeneous Solution

Supramolecular Template-Assisted Catalytic [2+2] Photocycloaddition in Homogeneous Solution

Brönsted Basicities and Nucleophilicities of N-Heterocyclic Olefins in Solution: N-Heterocyclic Carbene versus N-Heterocyclic Olefin. Which Is More Basic, and Which Is More Nucleophilic?

A Brönsted basicity scale comprising nine representative N-heterocyclic olefins (NHOs) was established by measuring the equilibrium acidities of their corresponding precursors in DMSO using an ultraviolet-visible spectroscopic method. The basicities (pKaHs) of the investigated NHOs cover a range from 14.7 to 24.1. The basicities of unsaturated NHOs are stronger than those of their N-heterocyclic carbene (NHC) analogues; however, the basicities for the saturated ones are much weaker than those of their NHC analogues, which is largely due to the aromatization effect that intrinsically influences the acidic dissociations of NHC and NHO precursors. The nucleophilicities of four NHOs were measured photometrically by monitoring the kinetics of reactions of these NHOs with common reference electrophiles for quantifying nucleophilic reactivities. In general, the nucleophilicity of the NHOs is much stronger than that of commonly used Lewis bases such as Ph3P or DMAP [4-(dimethylamino)pyridine] but weaker than that of their NHC analogues; however, caution should be taken when generalizing this conclusion to a wide range of electrophiles with distinctively electronic and structural properties.

Trans‐Stilbene epoxidation by He+O2 atmospheric pressure plasma: Epoxidation without oxidant waste stream

AbstractPlasmas interacting with organic liquids can lead to novel synthetic processes that are not feasible with conventional vacuum systems due to vapor pressure limitations. Of particular interest are processes that eliminate the generation of waste streams. Here, we show that He+O2 plasma can drive the epoxidation of alkenes in solution, in a process that generates epoxides without oxidant waste streams, runs at room temperature and atmospheric pressure, and requires no catalyst. The reactions between different reactive oxygen species generated in the plasma and the target alkene, trans‐stilbene in this study, have been identified and optimization of the plasma conditions within the constraints of the current experimental setup has led to yields of ~70%, which are of preparative interest.

Photoinduced Tetrazole‐Based Functionalization of Off‐Stoichiometric Clickable Microparticles

We report the preparation of tetrazole‐containing step‐growth microparticles and the subsequent use of photoinduced nitrile imine‐mediated tetrazole‐ene cycloaddition (NITEC) reactions on the particles with spatiotemporal control. Microparticles with an average diameter of 4.1 µm and with inherent tetrazole‐ene dual functionality are prepared by a one‐pot off‐stoichiometric thiol‐Michael addition dispersion polymerization. The NITEC reaction is performed efficiently in the solid phase by UV irradiation, leading to the formation of fluorescent pyrozoline adducts, with an estimated quantum yield of 0.7. Particle concentration‐independent reaction kinetics are observed and full conversion is reached within 10 min of UV exposure at an intensity of 8 mW cm−2. Temporal control is demonstrated with either UV or rooftop sunlight irradiation of variable duration. By using two‐photon writing with a laser centered around 700 nm wavelength, spatial control is demonstrated with micrometer‐scale resolution via surface patterning of the microparticles. Further, microparticles with exclusive tetrazole functionality are prepared by a one‐pot, two‐step thiol‐Michael addition dispersion polymerization. The NITEC reaction between tetrazole‐functional particles and acrylates in solution is examined at various tetrazole/alkene molar ratios, and a 10:1 excess of alkenes in solution is found necessary for efficient functionalization.

Supramolecular Dimerization and [2 + 2] Photocycloaddition Reactions of Crown Ether Styryl Dyes Containing a Tethered Ammonium Group: Structure-Property Relationships.

Molecular self-assembly is an effective strategy for controlling the [2 + 2] photocycloaddition reaction of olefins. The geometrical properties of supramolecular assemblies are proven to have a critical effect on the efficiency and selectivity of this photoreaction both in the solid state and in solution, but the role of other factors remains poorly understood. Convenient supramolecular systems to study the structure-property relationships are pseudocyclic dimers spontaneously formed by styryl dyes containing a crown ether moiety and a remote ammonium group. New dyes of this type were synthesized to investigate the effects of structural and electronic factors on the quantitative characteristics of supramolecular dimerization and [2 + 2] photocycloaddition in solution. Variable structural parameters for the styryl dyes were the size and structure of macrocyclic moiety, the nature of heteroaromatic residue, and the length of the ammonioalkyl group attached to this residue. Quantum chemical calculations of the pseudocyclic dimers were performed in order to interpret the relationships between the structure of the ammonium dyes and the efficiency of the supramolecular photoreaction. One of the dimeric complexes was obtained in the crystalline state and studied by X-ray diffraction. The results obtained demonstrate that the photocycloaddition in the pseudocyclic dimers can be dramatically affected by the electronic structure of the styryl moieties, as dependent on the electron-donating ability of the substituents on the benzene ring, and by the conformational flexibility of the pseudocycle, which determines the mobility of the olefinic bonds. The significance of electronic factors is highlighted by the fact that the photocycloaddition quantum yield in geometrically similar dimeric structures varies from ≤10(-4) to 0.38. The latter value is unusually high for olefins in solution.

Volume demanding geometric isomerization of cis-4-stilbazole. HCl salts in the crystalline state: Probing the role of a metastable dimer

Abstract Cis-trans isomerization is one of the most common and well-investigated photoreaction of olefins in solution. This occurs via 180° rotation of one of the substituents on C C bond requiring large space around the double bond. One would expect that in crystals the neighboring molecules would prohibit such a process. In spite of this in early 1960s Schmidt and co-workers reported occurrence of such a process in crystals via 2 + 2 ‘meta cyclobutane’ formation. In this study by examining the photochemistry of four cis-stilbazolium salts we show that the photoconversion to the corresponding trans isomers does not occur via 2 + 2 addition. Large separation that ensures considerable space between neighboring olefins favors cis-trans isomerization in crystals. An aspect that is puzzling is that the product trans isomer phase separates and recrystallizes within the parent cis crystals and photodimerizes to the cyclobutane dimer. Details of this phenomenon are to be understood.

Dehydrogenative Homocoupling of Terminal Alkenes on Copper Surfaces: A Route to Dienes

AbstractHomocouplings of hydrocarbon groups including alkynyl (sp1), alkyl (sp3), and aryl (sp2) have recently been investigated on surfaces with the interest of fabricating novel carbon nanostructures/nanomaterials and getting fundamental understanding. Investigated herein is the on‐surface homocoupling of an alkenyl group which is the last elementary unit of hydrocarbons. Through real‐space direct visualization (scanning tunneling microscopy imaging) and density functional theory calculations, the two terminal alkenyl groups were found to couple into a diene moiety on copper surfaces, and is contrary to the common dimerization products of alkenes in solution. Furthermore, detailed DFT‐based transition‐state searches were performed to unravel this new reaction pathway.

Dehydrogenative homocoupling of terminal alkenes on copper surfaces: a route to dienes.

Homocouplings of hydrocarbon groups including alkynyl (sp(1) ), alkyl (sp(3) ), and aryl (sp(2) ) have recently been investigated on surfaces with the interest of fabricating novel carbon nanostructures/nanomaterials and getting fundamental understanding. Investigated herein is the on-surface homocoupling of an alkenyl group which is the last elementary unit of hydrocarbons. Through real-space direct visualization (scanning tunneling microscopy imaging) and density functional theory calculations, the two terminal alkenyl groups were found to couple into a diene moiety on copper surfaces, and is contrary to the common dimerization products of alkenes in solution. Furthermore, detailed DFT-based transition-state searches were performed to unravel this new reaction pathway.

Visible light induced photo-oxidation of highly branched isoprenoid (HBI) alkenes: Significant dependence on the number and nature of double bonds

Visible light induced, type II (i.e. involving singlet oxygen, 1O2) photosensitized oxidation of mono-, di-, tri- and tetraunsaturated C25 highly branched (HBI) alkenes in solution was investigated. The photodegradation rate was shown to be dependent on the number and nature of the double bonds, especially the number of trisubstituted ones. HBI alkenes with at least one trisubstituted double bond were photodegraded at similar or higher rates than some other lipids (e.g. vitamin E) known to be very reactive towards 1O2. The 1O2 mediated photo-oxidation of HBI alkenes likely involves direct reaction of 1O2 with the trisubstituted double bond via a concerted ‘ene’ addition, leading to formation of a hydroperoxide at each carbon of the original double bond. The enhanced photoreactivity of HBI alkenes with at least one trisubstituted double bond was confirmed in dead cells of the diatom Haslea ostrearia. In addition, since HBI alkenes with such double bonds were photodegraded at similar or higher rates compared to polyunsaturated fatty acids, vitamin E and chlorophyll a, they should be significantly photochemically altered within the euphotic zone of the ocean. In contrast, HBIs with only di- and monosubstituted double bonds seem to be relatively inert with respect to type II photoprocesses. The results support the apparent good preservation of the monoene IP25 in sediments, while the interpretation of sedimentary occurrences of HBI trienes may need further consideration.

High surface area Pd, Pt and Ni ion-exchanged Zr, Ti and Sn(IV) phosphates and their application to selective heterogeneous catalytic hydrogenation of alkenes

Amorphous acidic metal(IV) phosphates of zirconium, titanium and tin have been prepared and hydrogen-exchanged for bivalent Pd, Pt and Ni. These bivalent metals were returned to the zero valent state by reducing them with either hydrogen at 400 °C or with sodium tetrahydroborate at room temperature. The resulting Pd 0, Pt 0 and Ni 0 phosphates were investigated as selective catalysts for heterogeneous hydrogenation of alkenes in solution at normal temperatures and pressures and, for Ni, also in the vapour phase. Quantitative studies on rates of hydrogenation are discussed. The usual methods for preparing metal(IV) phosphates give either crystalline or amorphous solids having low specific surfaces areas. A method has been developed, by which metal(IV) phosphates having large surface areas (lsa) may be prepared easily. These lsa supports take up large amounts of transition metal cations by simple exchange. The enhanced incorporation of Pd, Pt or Ni and increases in surface areas of the phosphate supports have provided some active, selective catalysts. Pd/Ti phosphates were the most active and compared well with commercial Pd on carbon. Ni/Ti phosphate is a very selective catalyst for vapour phase hydrogenation of alkenes and, at slightly higher temperatures, it is an efficient hydrocarbon cracking catalyst.

Silyl‐Substituted Tetrathia[7]helicenes: Synthesis, X‐ray Characterization and Reactivity (Eur. J. Org. Chem. 27/2007)

AbstractThe cover picture shows the photochemical cyclization of silyl‐substituted trans‐1,2‐bis(benzodithiophen‐2‐yl)ethenes into the corresponding silylated helicenes and the transformation of these into new halogenated helicenes. The picture underlines a peculiar aspect of the paper, that is, the high solubility of the above silylated trans alkenes, which is a consequence of the presence of the two silyl groups and results in a very fast and easy photocyclization that can be perfomed in solution by using a glass reactor with an immersion Hg lamp. The molecular packing of the trans alkenes in solution shows a long interplanar distance between two molecules, which accounts for its high solubility. The work is a collaboration between Milan and Parma Universities, whose Cathedrals are shown. Details are discussed in the article by E. Licandro et al. on p. 4499 ff.

Absolute Rate Constants for the Addition of the 1‐(tert‐Butoxy)carbonylethyl Radical onto Cyclic Alkenes in Solution

AbstractAbsolute rate constants are reported for the addition of the 1‐[(tert‐butoxy)carbonyl]ethyl (= 2‐(1,1‐dimethylethoxy)‐1‐methyl‐2‐oxoethyl) radical .CHMeCO2(t‐Bu) to several cyclic and monosubstituted alkenes in MeCN as obtained by time‐resolved electron paramagnetic resonance (EPR). The activation energies for the addition of this alkyl radical are mainly governed by the addition enthalpy but are also substantially lowered by the ambiphilic effect and by relief of cyclic strain.

Cross Metathesis on Olefin-Terminated Monolayers on Si(111) Using the Grubbs' Catalyst

This paper reports the functionalization and patterning of olefin-terminated monolayers on Si(111) through cross metathesis. A simple, one-step synthesis of a diolefin--CH2=CH(CH2)9O(CH2)9CH=CH2--was developed from commercially available starting materials. Mixed partially olefin-terminated monolayers of this novel diolefin and 1-octadecene on hydrogen-terminated Si(111) were obtained. The olefins are raised above the rest of the monolayer and thus sterically accessible for further functionalization. Olefin-terminated monolayers were reacted with the Grubbs' first generation catalyst and olefins in solution that were terminated with fluorines, carboxylic acids, alcohols, aldehydes, and alkyl bromides. Characterization of these monolayers using X-ray photoelectron spectroscopy and horizontal attenuated total reflection infrared spectroscopy demonstrated that olefins on the surface had reacted via cross metathesis to expose fluorines, carboxylic acids, aldehydes, alcohols, and bromides. Through calibration experiments, we demonstrated a simple 1:1 correspondence between the ratio of olefins in solution used in the assembly and the final composition of the mixed monolayers. Finally, these monolayers on silicon were patterned on the micrometer-size scale by soft lithography using microfluidic channels patterned into poly(dimethylsiloxane) (PDMS) stamps. Micrometer-wide lines of polymer brushes were synthesized on these monolayers and characterized by scanning electron microscopy. In addition, olefin-terminated monolayers were patterned into micrometer-sized lines exposing carboxylic acids by cross metathesis with olefins in solution. This method of patterning is broadly applicable and can find applications in a variety of fields including the development of biosensors and nanoelectronics.

Absolute Rate Constants for the Addition of the 1-(tert-Butoxy)carbonylethyl Radical to Alkenes in Solution

Absolute rate constants and some of their Arrhenius parameters are reported for the addition of the 1-[(tert-butoxy)carbonyl]ethyl radical (MeC. HCO2Me3) to several mono- or 1,1-disubstituted alkenes in acetonitrile as obtained by time-resolved electron spin resonance spectroscopy. At 295 K, the rate constants range from 470 M−1 s−1 (but-1-ene) to 2.4⋅105 M−1 s−1 (1,1-diphenylethene), the experimental activation energies range from 26.8 kJ/mol (but-1-ene) to 14.7 kJ/mol (styrene), and the frequency factors obey on the average log (A/M−1 s−1)=7.9±0.5. The rate constants of the secondary 1-[(tert-butoxy)carbonyl]ethyl radical are close to the geometric mean of those of the related primary [(tert-butoxy)carbonyl]methyl and the tertiary 2-(methoxycarbonyl)propan-2-yl radicals. The activation energies for addition of these three carboxy-substituted alkyl radicals are mainly governed by the addition enthalpy but are also substantially lowered by ambiphilic polar effects. The results support a previously derived predictive analysis, and relations to rate constants of acrylate polymerizations are discussed.

Absolute Rate Constants for the Addition of the 2-(Methoxycarbonyl)propan-2-yl and the 3,3.3-Trifluoracetonyl Radicals to Alkenes in Solution

Absolute rate constants and some of their Arrhenius parameters were obtained by time-resolved electron spin resonance (ESR) spectroscopy for the addition of the 2-(alkoxycarbonyl)propan-2-yl and 3,3,3-trifluoroacetonyl (=3,3,3-trifluoro-2-oxopropyl) radicals to a variety of mono- and 1,1-disubstituted alkenes. Their analysis shows that the addition of 2-(alkoxycarbonyl)propan-2-yl is mainly governed by the exothermicity of the reaction with slight modifications by nucleophilic and electrophilic effects giving rise to an overall ambiphilic behavior. In contrast, large electrophilic polar effects dominate the addition of the 3,3,3-trifluoroacetonyl (=3,3,3-trifluoro-2-oxopropan-2-yl) radical, as it is expected from its large electron affinity. For both radicals, the activation energies are well-predicted by analytic equations for the enthalpic and polar terms. A comparison of the rate data of 2-(alkoxycarbonyl)propan-2-yl with the homo- and copolymerization rate constants of the propagating radical of methyl methacrylate shows that the additions of these structurally related low- and high-molecular-weight radicals to alkenes are governed by very similar effects.

Alkyl Rearrangement Processes in Organozirconium Complexes. Observation of Internal Alkyl Complexes during Hydrozirconation

Isotopically labeled alkyl zirconocene complexes of the form (CpRn)2Zr(CH2CDR‘2)(X) (CpRn = alkyl-substituted cyclopentadienyl; R‘ = H, alkyl group; X = H, D, Me) undergo isomerization of the alkyl ligand as well as exchange with free olefin in solution under ambient conditions. Increasing the substitution on the Cp ring results in slower isomerization reactions, but these steric effects are small. In contrast, changing X has a very large effect on the rate of isomerization. Pure σ-bonding ligands such as methyl and hydride promote rapid isomerization, whereas π-donor ligands inhibit β-H elimination and hence alkyl isomerization. For (η5-C5H5)2Zr(R)(Cl), internal alkyl complexes have been observed for the first time. The rate of isomerization depends on the length of the alkyl group: longer alkyl chains (heptyl, hexyl) isomerize faster than shorter chains (butyl). The transient intermediate species have been identified by a combination of isotopic labeling and 1H, 2H, and 13C NMR experiments. The solid-state structure of the zirconocene cyclopentyl chloride complex, Cp2Zr(cyclo-C5H9)(Cl), has been determined by X-ray diffraction.

Absolute rate Constants for the Addition of Malonyl Radicals to Alkenes in Solution

AbstractAbsolute rate constants and their Arrhenius parameters are obtained for the addition of a cyclic malonyl radical derived from Meldrum's acid to 20 mono‐ or 1,1‐disubstituted alkenes in (±)‐propylene oxide and for the addition of the open‐chain di(tert‐butyl)malonyl radical to six mono‐ or 1,1‐disubstituted alkenes in 1,1,2‐trichloro‐1,2,2‐trifluoroethane by time‐resolved electron spin resonance spectroscopy. At room temperature, the radicals add at the unsubstituted C‐atoms with rate constants ranging from 1.1. 105 M−1S−1 (acrolein) to 2.4. 106 M−1S−1 (1,1‐diphenylethene). The frequency factors are in the narrow ranges of log (A/M−1S−1) = 8.7 ± 0.1 for the cyclic and log (A/M−1S−1) = 8.2 ± 0.2 for the open‐chain malonyl species, whereas the activation energies vary from 12.9 kJ/mol (1,1‐diphenylethene) to 21.7 kJ/mol (acrylonitrile). They correlate with the alkene ionization potentials and, more weakly, with the reaction enthalpy. No correlation was found between the activation energies and the alkene electron affinities. Hence, the results confirm and quantify the electrophilic nature of malonyl radicals in addition reactions.

ChemInform Abstract: Rate Constants for the Addition of 2‐Hydroxy‐2‐propyl Radicals to Alkenes in Solution Studied by Laser Flash Photolysis.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Rate Constants for the Addition of 2‐Hydroxy‐2‐propyl Radicals to Alkenes in Solution studied by laser flash photolysis

AbstractAbsolute rate constants for the addition of the highly nucleophilic 2‐hydroxy‐2‐propyl radical to eight fast‐reacting 1‐ and 1,1‐disubstituted alkenes in MeOH at room temperature have been determined by laser flash photolysis. Also the absorption spectra of the 2‐hydroxy‐2‐propyl and the benzylic and alkyl‐type adduct radicals are presented. The rate constants were obtained using various methods for the analysis of the kinetic traces and support earlier findings.