Isomerization olefin effect

Successive hydrogen transfers within 60, followed by coordination of olefin and then H2 (an unsaturate route), constitute the catalytic cycle, while isomerization is effected through HFe(CO)3(7r-allyl) formed from 59. Loss of H2 from 60 was also considered to be photoinduced, and several hydrides, including neutral and cationic dihydrides of iridium(III) (385, 450, 451), ruthenium(II) (452) and a bis(7j-cyclopentadienyltungsten) dihydride (453), have been shown to undergo such reductive elimination of hydrogen. Photoassisted oxidative addition of H2 has also been dem-... 

Not only can acids catalyze olefin isomerization, but strong bases can also effect isomerization. These base-catalyzed isomerizations proceed through proton abstraction of an allylic hydrogen atom followed by protonation of the allylic anion to regenerate either the original or the isomeric olefin ... 

Table VI shows that regardless of the starting isomeric olefin, selectivity is lower with Co2(CO)8 than with rhodium this is especially true when the starting isomer is conjugated. Moreover, when hydroformylation is done with Co2(CO)8, two aromatic ring effects are observed ...

As indicated in Scheme VII/32, cyclononanone (VII/165) is transformed into hydroperoxide hemiacetal, VII/167, which is isolated as a mixture of stereoisomers. The addition of Fe(II)S04 to a solution of VII/167 in methanol saturated with Cu(OAc)2 gave ( )-recifeiolide (VII/171) in quantitative yield. No isomeric olefins were detected. In the first step of the proposed mechanism, an electron from Fe2+ is transferred to the peroxide to form the oxy radical VII/168. The central C,C-bond is weakened by antiperiplanar overlap with the lone pair on the ether oxygen. Cleavage of this bond leads to the secondary carbon radical VII/169, which yields, by an oxidative coupling with Cu(OAc)2, the alkyl copper intermediate VII/170. If we assume that the alkyl copper intermediate, VII/170, exists (a) as a (Z)-ester, stabilized by n (ether O) —> <7 (C=0) overlap (anomeric effect), and (b) is internally coordinated by the ester to form a pseudo-six-membered ring, then only one of the four -hydrogens is available for a syn-//-elimination. [111]. This reaction principle has been used in other macrolide syntheses, too [112] [113]. 

Several larger membered ring azocompounds have also been synthesized and their decomposition kinetics studied . Reported kinetic data are collected in Table 8. The decomposition products are the corresponding diaryl cycloparaffins and isomeric olefins. The cis isomers decompose with considerably lower activation energies than the trans isomers. This feature of the reaction has been attributed to the greater ease with which both aryl substituents achieve coplanarity with the C-N=N-C linkage in the transition state-and thereby stabilize the incipient diradical — in the cis than in the trans isomer. A similar effect was noted for the... 

On heating, alkylborons may undergo isomerization by migration of the boron to a less sterically encumbered position by a series of reversible j8-hydride elimina-tion-hydroboration steps. Displacement of the isomerized olefin may be effected by addition of a second olefin, which forms a more stable alkylboron than the one displaced. This reaction has been used to effect the contrathermodynamic isomerization of olefins. 

The backbone isomerizations of cholest-5-ene to give a A -olefin, and of androst-5-ene and D-homoandrost-5-ene to give mixtures of isomeric olefins, are now reported in full. Trifluoroacetic acid effects these rearrange-... 

Only three studies of the reaction of higher olefins with deuterium over nickel catalysts have been reported in which mass-spectrometric analysis of the products was performed (32, 48, 49). There have been other separate studies of the isomerization reactions, to be described in the next section, but no simultaneous studies of both exchange and isomerization. Thus when results have been given for the deuterated butenes formed by olefin exchange (48), it is uncertain to what extent deuterated isomerized olefins are contributing to the total effect. There is room here for much further work. 

Various kinds of olefin isomerizations are effectively promoted by Fe(CO)5. However, because the isomerized product is usually released from the iron complex by oxidation, these isomerizations require a stoichiometric amount of pentacarbonyliron as described below (Scheme 10.4, eq (1)) [16-17]. 

Silver ions cause perturbation of the (E)-(Z) photoisomerization pathway for both stilbene and azobenzene . The efficiency of silver ions in this respect is compared with the effect of Nal which can only induce a heavy atom effect. Ag+ clearly forms complexes with both compounds. Observation of cis-trans conversion in olefin radical cations shows that electron transfer can bring about isomerization of stilbene derivatives. The efficiency of such processes obviously depends on the presence and nature of any substituents. Another study deals with photochemical generation, isomerization, and effects of oxygenation on stilbene radicals. The intermediates examined were generated by electron transfer reactions. Related behaviour probably occurs through the effect of exciplex formation on photoisomerization of styrene derivatives of 5,6-benz-2,2 -diquinoyE. ... 

Moreover, the quantum amplification effect of the one-way isomerizing olefins could be utilized for practical purposes particularly in the solid state (28). 

The quantum chain process is not limited to the one-way isomerizing olefins but could be observed in olefins two-way isomerizing in the triplet state where the energy minimum exists at t. 6a, 7b, and 8b undergo two-way isomerization in the triplet state [63-66], However, the quantum yields of cis trans isomerization of 6a, 7b, and 8b increase with increasing cis isomer concentration and exceed unity the value of 8b was reported to be 42 on benzU sensitization at [cis-8b] = 2.7 X 10 M [66], Here are mentioned 1.) porphyrin-sensitized isomerization of stilbene and 2.) effects of additives on the quantum chain process of styrylstilbene. 

Reaction rate depends on the structure of the hydrocarbon olefin (see Table 1). Internal olefins are less reactive than terminal olefins. Branching in remote locations has little effect, whereas branching at one of the olefinic carbons reduces the reactivity by another order of magnitude. In addition, the product derives from isomerized olefin. Essentially no quaternary aldehydes are formed by hydroformylation. For example, the product from 2,3-dimethyl-2-butene is 3,4-dimethylvaleraldehyde . 

An active metal supported on a zeolite has a promotional effect in increasing the ratio of isomerization to disproportionation which could be due to the introduction of a bifunctional path which is composed of the steps of hydrogenation of m-xylene on the metal to form an olefin, isomerization of the formed olefin and dehydrogenation of the isomerized olefin to o-xylene. Also, the rate of disproportionation reaction is reduced due to hydrogen spilling over, and reacting with the carbocation intermediates in the disproportionation reaction and consequently decreasing the rate of the disproportionation reaction." ... 

Isomerization of the Olefin. The formation of isomeric olefins by double bond shifts and as a consequence the formation of aldehydes other than those expected on the basis of attachment of the formyl group to one of the two carbon atoms of the original double-bond is most pronounced with cobalt catalysts. Unmodified rhodium catalyst is also effective in olefin isomerization. 

Base catalysis is most effective with alkali metals dispersed on solid supports or, in the homogeneous form, as aldoxides, amides, and so on. Small amounts of promoters form organoalkali comnpounds that really contribute the catalytic power. Basic ion exchange resins also are usebil. Base-catalyzed processes include isomerization and oligomerization of olefins, reactions of olefins with aromatics, and hydrogenation of polynuclear aromatics. 

The Nenitzescu process is presumed to involve an internal oxidation-reduction sequence. Since electron transfer processes, characterized by deep burgundy colored reaction mixtures, may be an important mechanistic aspect, the outcome should be sensitive to the reaction medium. Many solvents have been employed in the Nenitzescu reaction including acetone, methanol, ethanol, benzene, methylene chloride, chloroform, and ethylene chloride however, acetic acid and nitromethane are the most effective solvents for the process. The utility of acetic acid is likely the result of its ability to isomerize the olefinic intermediate (9) to the isomeric (10) capable of providing 5-hydroxyindole derivatives. The reaction of benzoquinone 4 with ethyl 3-aminocinnamate 35 illustrates this effect. ... 

High thermodynamic selectivity (7) demands that the initially formed cis olefin be displaced rapidly relative to its saturation or to its isomerization. As the reaction nears completion and the acetylene concentration diminishes, its effectiveness in displacing olefin will diminish and selectivity will fall. Displacement by acetylene is also impeded through depletion of acetylene in the vicinity of the catalyst owing to intra- or interpartile diffusion resistance (53a). A change in a reaction parameter thus can have different influences... 

The cyclopropanation of 1-trimethylsilyloxycyclohexene in the present procedure is accomplished by reaction with diiodomethane and diethylzinc in ethyl ether." This modification of the usual Simmons-Smith reaction in which diiodomethane and activated zinc are used has the advantage of being homogeneous and is often more effective for the cyclopropanation of olefins such as enol ethers which polymerize readily. However, in the case of trimethylsilyl enol ethers, the heterogeneous procedures with either zinc-copper couple or zinc-silver couple are also successful. Attempts by the checkers to carry out Part B in benzene or toluene at reflux instead of ethyl ether afforded the trimethylsilyl ether of 2-methylenecyclohexanol, evidently owing to zinc iodide-catalyzed isomerization of the initially formed cyclopropyl ether. The preparation of l-trimethylsilyloxybicyclo[4.1.0]heptane by cyclopropanation with diethylzinc and chloroiodomethane in the presence of oxygen has been reported. "... 

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