Isomerization, of alkenes

Pyrazine 1,4-dioxides are available by the direct self-condensation of 1,2-hydroxyaminooximes (70JOC2790). 1,2-Nitrooximes are obtained by the isomerization of alkene initrogen trioxide adducts, which are reduced with palladium on charcoal to the hydroxyaminooximes which undergo acid-catalyzed auto-condensation to the pyrazine 1,4-dioxides (Scheme 19). 

Electrophilic attack on the sulfur atom of thiiranes by alkyl halides does not give thiiranium salts but rather products derived from attack of the halide ion on the intermediate cyclic salt (B-81MI50602). Treatment of a s-2,3-dimethylthiirane with methyl iodide yields cis-2-butene by two possible mechanisms (Scheme 31). A stereoselective isomerization of alkenes is accomplished by conversion to a thiirane of opposite stereochemistry followed by desulfurization by methyl iodide (75TL2709). Treatment of thiiranes with alkyl chlorides and bromides gives 2-chloro- or 2-bromo-ethyl sulfides (Scheme 32). Intramolecular alkylation of the sulfur atom of a thiirane may occur if the geometry is favorable the intermediate sulfonium ions are unstable to nucleophilic attack and rearrangement may occur (Scheme 33). 

The primary and secondary products of photolysis of common diazirines are collected in Table 4. According to the table secondary reactions include not only isomerization of alkenes and hydrogen elimination to alkynes, but also a retro-Diels-Alder reaction of vibrationally excited cyclohexene, as well as obvious radical reactions in the case of excited propene. 

The isomerization of alkenes is believed to take place via an excited state in which the two sp carbons are twisted 90° with respect to one another. This state is referred to as the p (perpendicular) state. This geometry is believed to be the minimum-energy geometry for both the singlet and triplet excited states. 

Silver(I)-alkene complexation is implicated in the silver-catalysed isomerization of alkenes [152] an example is shown in Figure 4.32. 

The strict geometrical requirements for elimination can be put to further use, as illustrated by elegant procedures for the geometrical isomerization of alkenes. Trimethylsilyl potassium (10) and phenyldimethylsilyl lithium (11) both effect smooth conversion of oxiranes into alkenes, nucleophilic ring opening being followed by bond rotation and spontaneous syn fi-elimination ... 

The hydrogenation and isomerization of alkenes can usually be described by the classical Horiuti-Polanyi mechanism. According to that mechanism, in a deuterium atmosphere, double bond migration incorporates deuterium into the allylic position. 

The monosulfonated PPh derivative, Ph2P(m-C6H4S03K) (DPM) and its rhodium complex, HRh(CO)(DPM)3 have been synthesized and characterized by IR and NMR spectroscopic techniques. The data showed that the structure was similar to [HRh(CO)(PPh3)3]. The catalytic activity and selectivity of [HRh(CO)(DPM)3] in styrene hydroformylation were studied in biphasic catalytic systems.420 421 Rh1 complexes [Rh(acac)(CO)(PR3)] with tpa (131), cyep (132), (126), ompp (133), pmpp (134), tmpp (135), PPh2(pyl), PPh(pyl)2, and P(pyl)3 were characterized with NMR and IR spectra. Complexes with (131), (132), and (126) were catalysts for hydrogenation of C—C and C—O bonds, isomerization of alkenes, and hydroformylation of alkenes.422 Asymmetric hydroformylation of styrene was performed using as catalyst precursor [Rh(//-0 Me)(COD)]2 associated with sodium salts of m-sulfonated diarylphosphines.423... 

The [Co(CN)5]3 complex is an effective catalyst for some reactions, particularly the isomerization of alkenes. Newer and more efficient catalysts have been developed for some of the processes, but the catalytic behavior of the pentacyanocobalt(II) ion is also significant from a historical perspective. In reactions such as that shown in Eq. (22.10), two Co2+ ions increase one unit in oxidation state, instead of the more common situation in which one metal ion increases by two units in oxidation state. The cobalt complex also reacts with CIT3I, Cl2, and H202, which are indicated as X-Y in the equation... 

The isomerization of alkenes is an industrially important process that can be used to prepare specific isomers used as monomers in polymerization. One step in the isomerization process involves a change in bonding mode of an alkene. For example, isomerization of 1-alkenes to produce 2-alkenes may take place as the alkene changes from rf to if in the transition state. This mechanism can be shown as... 

One of the effective catalysts for isomerization of alkenes is a square planar complex of Rh1 +. That... 

Various transition metal complexes, in particular of late transition metals, were reported to be effective catalysts for such double bond isomerization. Because organic synthesis is the focus of this volume, this section will cover the transition metal-catalyzed isomerization of alkenes, which has the significant synthetic and industrial utilities. This chapter will also include the synthetic application, asymmetric reactions,4-6 and isomerization of alkynes, in particular, that of propargylic alcohols. 

Rhodium species in oxidation states I and III are involved in the process. Rhodium-catalyzed hydrogenations generally involve oxidative addition reactions, followed by the reverse process of reductive elimination in the final step. Another common elimination process is the so-called (l-elimination, which accounts for the frequent side reaction of isomerization of alkenes, according to Eq. (1) ... 

Molybdenum complexes A (Figure 3.46) react efficiently with terminal and internal alkenes in toluene (e.g. 500 eq. Z-2-pentene are metathesized in 2 min at 25 °C 20 eq. of styrene in 2 h at 25 °C). These catalysts also oligomerize 2,4-hexadiene [808] and 1,5-hexadiene [809] and promote RCM of enol ethers. Isomerization of alkenes by catalysts A is a potential catalytic side-reaction [810-812]. 

Molybdenum catalysts such as 1 can also lead to the isomerization of alkenes [810-812]. Care is due in particular if enantiomerically pure olefins with the stereogenic center near the C-C double bond are to be metathesized, or when strained rings are to be formed [811]. 

Kijenski and Malinowski reported that Na/MgO is a highly active catalyst for the isomerization of alkenes at 293 K and that the basic have base strengths stronger than those corresponding to H — 35 (240,241). 

At higher temperatures, C—H and C—C bonds may be similarly broken. Thus, zeolite catalysts may be used for (i) alkylation of aromatic hydrocarbons (cf. the Friedel-Crafts reactions with AICI3 as the Lewis acid catalyst), (ii) cracking of hydrocarbons (i.e., loss of H2), and (Hi) isomerization of alkenes, alkanes, and alkyl aromatics. 

The majority of observations concerning the isomerization of alkenes are in harmony with the Horiuti-Polanyi associative mechanism,170 171 which involves the reversible formation of the 24 half-hydrogenated state (Scheme 4.11) (see in more detail in Section 11.1.2). 

While the last step [Eq. (11.4)] is virtually irreversible under hydrogenation conditions, both the adsorption of alkene [Eq. (11.2)] and the formation of alkyl intermediate (half-hydrogenated state) [Eq. (11.3)] are reversible. The reversibility of these steps accounts for the isomerization of alkenes accompanying hydrogenation (see Section 4.3.2). Isomerizations, either double-bond migration or cis-trans isomerization, may not be observable, unless the isomer is less reactive, or the isomerization results in other structural changes in the molecule, such as racemization. 

Metals differ in their ability to catalyze isomerization. Both the relative rate of transformation of the individual isomers and the initial isomer distribution vary with the metal. The order of decreasing activity of platinum metals in catalyzing the isomerization of alkenes was found to be Pd >- Rh, Ru, Pt > Os > Ir.53 Platinum is generally the preferable catalyst if isomerization is to be avoided. The most active Raney nickel preparations rival palladium in their activity of isomerization. 

The hydrogenation mechanisms discussed above indicate the reversible formation of intermediates. The degree of reversibility depends on the nature of the catalyst and the alkene, and on reaction conditions. Some nonreducible internal alkenes undergo slow hydrogen exchange and isomerization, indicating that reversible steps can occur in these cases. The reversible formation of alkylmetal intermediates provides a ready explanation for the isomerization of alkenes in the presence of certain metal complexes (see Section 4.3.2). 

One may well ask why the isomerization of alkenes discussed in the preceding section requires a sensitizer. Why cannot the same result be achieved by direct irradiation One reason is that a tt — tt singlet excited state (5,) produced by direct irradiation of an alkene or arene crosses over to the triplet state (Ij) inefficiently (compared to n —> it excitation of ketones). Also, the Si state leads to other reactions beside isomerization which, in the case of 1,2-diphenyl-ethene and other conjugated hydrocarbons, produce cyclic products. For example, cw-l,2-diphenylethene irradiated in the presence of oxygen gives phenanthrene by the sequence of Equation 28-8. The primary photoreaction is cyclization to a dihydrophenanthrene intermediate, 6, which, in the presence of oxygen, is converted to phenanthrene ... 

The complexes [PdCl2(Me2SO)2] and [RhCl3(Me2SO)3] can be reduced by NaBH4 to give catalysts for the isomerization of alkenes and the hydrogenation of alkenes, dienes and alkynes.101... 

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