Alkenes, temperature dependence

Materials that typify thermoresponsive behavior are polyethylene—poly (ethylene glycol) copolymers that are used to functionalize the surfaces of polyethylene films (smart surfaces) (20). When the copolymer is immersed in water, the poly(ethylene glycol) functionaUties at the surfaces have solvation behavior similar to poly(ethylene glycol) itself. The abiUty to design a smart surface in these cases is based on the observed behavior of inverse temperature-dependent solubiUty of poly(alkene oxide)s in water. The behavior is used to produce surface-modified polymers that reversibly change their hydrophilicity and solvation with changes in temperatures. Similar behaviors have been observed as a function of changes in pH (21—24). 

As for cyclopropanation of alkenes with aryldiazomethanes, there seems to be only one report of a successful reaction with a group 9 transition metal catalyst Rh2(OAc)4 promotes phenylcyclopropane formation with phenyldiazomethane, but satisfactory yields are obtained only with vinyl ethers 4S) (Scheme 2). Cis- and trans-stilbene as well as benzalazine represent by-products of these reactions, and Rh2(OAc)4 has to be used in an unusually high concentration because the azine inhibits its catalytic activity. With most monosubstituted alkenes of Scheme 2, a preference for the Z-cyclopropane is observed similarly, -selectivity in cyclopropanation of cyclopentene is found. These selectivities are the exact opposite to those obtained in reactions of ethyl diazoacetate with the same olefins 45). Furthermore, they are temperature-dependent for example, the cisjtrcms ratio for l-ethoxy-2-phenylcyclopropane increases with decreasing temperature. 

Various other biphasic solutions to the separation problem are considered in other chapters of this book, but an especially attractive alternative was introduced by Horvath and co-workers in 1994.[1] He coined the term catalysis in the fluorous biphase and the process uses the temperature dependent miscibility of fluorinated solvents (organic solvents in which most or all of the hydrogen atoms have been replaced by fluorine atoms) with normal organic solvents, to provide a possible answer to the biphasic hydroformylation of long-chain alkenes. At temperatures close to the operating temperature of many catalytic reactions (60-120°C), the fluorous and organic solvents mix, but at temperatures near ambient they phase separate cleanly. Since that time, many other reactions have been demonstrated under fluorous biphasic conditions and these form the basis of this chapter. The subject has been comprehensively reviewed, [2-6] so this chapter gives an overview and finishes with some process considerations. 

In an extension of this work, the reuse of the polymeric catalyst was addressed and several new PE-poly(alkene) glycol copolymers were prepared [68]. Commercially available oxidized polyethylene (CO2H terminated, both high and low molecular weight) was converted to the acid chloride and reacted with Jeffamine D or Jeffamine EDR, and subsequently converted to the tributylammonium bromide salt with butyl bromide. These new quaternary salts were shown to catalyze the nucleophihc substitution of 1,6-dibromohexane with sodium cyanide or sodium iodide. While none of the polymeric quaternary salts catalyzed the reaction as well as tetrabutylammonium bromide, the temperature-dependent solubility of the polymers allowed removal of the polymer by simple filtration. 

A number of cis/trans 4,6-dialkyl-2,2-dimethyl-l,3-dioxanes were studied by C NMR spectroscopy (93JOC5251). The C NMR shifts of C -Me groups (Scheme 8) were found to be very sensitive to the 1,3-dioxane conformation [chair form Me(ax) ca. 19 ppm and Me(eq) ca. 30 ppm— pure 30.89 ppm in the twist-boat form both methyl carbons resonate at ca. 25 ppm (pure 24.70 ppm)]. With these values, AG° of the chair to twist-boat equilibrium was calculated (Table IV). For 13a (nitrile), 13b (alkyne), and 13e (methyl ester) (Scheme 8) in CH2CI2, the temperature dependence of the AG° values was determined. Depending on the substituent, small negative or positive entropy terms were found generally the enthalpy term dominates the -AG° value. In the tram isomers 13, the cyano and alkyne substituents favor the chair conformation, but CHO, ester, alkene, and alkyl substituents, respectively, clearly favor the twist-boat conforma-... 

As for other organics in the atmosphere, the OH radical is a major oxidant for alkenes. Table 6.8 gives the rate constants for some OH-alkene reactions as well as their temperature dependence in Arrhenius form. Several points are noteworthy (1) the reactions are very fast, approaching 10-l() cm3 molecule-1 s-1 for the larger alkenes (2) the rate constants have a pressure dependence (3) the apparent Arrhenius activation energies are negative. ... 

The interaction of metal atoms with monoalkenes has been investigated on both a spectroscopic and preparative scale. It appears that the primary interaction between a metal atom and an alkene at low temperature is the formation of a ir-complex. This may subsequently lead to a thermally stable 7r-alkene complex or to rearrangement products by hydrogen abstraction or reaction with another alkene moiety, depending on the electronic requirements of the metal and the particular alkene considered. 

In this article (Part I) we have comprehensively reviewed the structural implications of the vibrational spectroscopic results from the adsorption of ethene and the higher alkenes on different metal surfaces. Alkenes were chosen for first review because the spectra of their adsorbed species have been investigated in most detail. It was to be expected that principles elucidated during their analysis would be applicable elsewhere. The emphasis has been on an exploration of the structures of the temperature-dependent chemisorbed species on different metal surfaces. Particular attention has been directed to the spectra obtained on finely divided (oxide-supported) metal catalysts as these have not been the subject of review for a long time. An opportunity has, however, also been taken to update an earlier review of the single-crystal results from adsorbed hydrocarbons by one of us (N.S.) (7 7). Similar reviews of the fewer spectra from other families of adsorbed hydrocarbons, i.e., the alkynes, the alkanes (acyclic and cyclic), and aromatic hydrocarbons, will be presented in Part II. 

The temperature dependence of the percentage conversion of reactant is depicted in Figure 1 which shows data for both propene and butene. At low temperatures butene is significantly less reactive than propene, but at higher temperatures (essentially above ca 210°C) these differences in reactivities are much less pronounced and both alkenes react at similar rates. With propene as feed, the major oligomers are C6, C9 with a small fraction of C12. 

Wiberg and coworkers investigated the OR temperature dependence of hydrocarbons. Different studies explored how the presence of an alkene chromophore, the conformer distribution, and vibrational modes affected the OR in small molecules. 

An AA BB proton spectrum was observed in solutions at low temperature for the ethylene moieties in r -C5H5)Rh(C2H4)2. The coalescence ofthis pattern upon raising the temperature led Cramer to ascribe these variations in the spectra to complete rotation of the alkene ligands. The analysis of the temperature dependence of the NMR spectra, which was carried out in 1964, implied barriers... 

The selectivity of carbenes has been qualitatively estimated by a series of competition reactions between various carbenes and mixtures of different alkenes it is found that electrophilic carbenes react preferentially with the most electron-rich alkene present [87, 92]. Fluorocarbenes, being less reactive, give rise to fewer products from C—H insertion reactions than CCI2 [91] (Figure 6.60). However, selectivity may be temperature-dependent [93, 94]. 

IS the most popular, one-step method for m ag fluorinated cyclopropanes and cyclopropenes a-Fluorocarbenes are particularly well behaved, because they all have singlet ground states [/, 2] and therefore usually add stereospecifically to alkenes and do not insert into C-H bonds competitively with addition Moreover, quantitative competition studies of carbene additions to alkenes near room temperature show that a-fluorocarbenes are more selective than other a-halocarbenes, with difluorocarbene being the most selective electrophihc carbene known [3, 4] The relative selectivities, however, can be quite temperature dependent [5, d] The numerous preparations and cycloaddmons of fluorocarbenes have been reviewed thoroughly [7, 8 9,10 ... 

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