Alkenes interconversions

The isomerization of alkenes in acid is probably a good part of the reason why El eliminations in acid generally give E-aikenes. in Chapter 19, we explained how kinetic control could lead to E-alkenes interconversion of E- and Z-aikenes under the conditions of the reaction allows the thermodynamic product to prevail. 

There must then be some mechanism by which the quickly formed -alkene is converted into the more stable Z-alkene, presumably through another intermediate that is more stable than the transition state for alkene interconversion. This information is summarized on a reaction profile diagram, transition state... 

Attack on oxiranes by trivalent phosphorus (64HC(19-l)43l) provides a method of deoxygenation to alkenes with inversion (c/. Section 5.05.3.4.3(hY)) and this makes possible the interconversion of (Z)- and (f)-alkenes (Scheme 58) (B-74MI50505). Silicon nucleophiles behave analogously (76JA1265, 76S199). 

Aromatic compounds such as toluene, xylene, and phenol can photosensitize cis-trans interconversion of simple alkenes. This is a case in which the sensitization process must be somewhat endothermic because of the energy relationships between the excited states of the alkene and the sensitizers. The photostationary state obtained under these conditions favors the less strained of the alkene isomers. The explanation for this effect can be summarized with reference to Fig. 13.12. Isomerization takes place through a twisted triplet state. This state is achieved by a combination of energy transfer Irom the sensitizer and thermal activation. Because the Z isomer is somewhat higher in energy, its requirement for activation to the excited state is somewhat less than for the E isomer. If it is also assumed that the excited state forms the Z- and -isomers with equal ease, the rate of... 

In principle, cri-2-butene and fran5-2-butene may be interconveited by rotation about the C-2=C-3 double bond. However, unlike rotation about the C-2—C-3 single bond in butane, which is quite fast, interconversion of the stereoisomeric 2-butenes does not occur under normal circumstances. It is sometimes said that rotation about a car bon-carbon double bond is restricted, but this is an understatement. Conventional laboratory sources of heat do not provide enough energy for rotation about the double bond in alkenes. As shown in Figure 5.2, rotation about a double bond requues the p orbitals of C-2 and C-3 to be twisted from their stable parallel alignment—in effect, the tt component of the double bond must be broken at the transition state. 

Another interesting biooxygenation reaction with alkenes, recently identified, represents an enzymatic equivalent to an ozonolysis. While only studied on nonchiral molecules, so far, this cleavage of an alkene into two aldehydes under scores the diversity of functional group interconversions possible by enzymatic processes [121,122]. 

The reactions of olefins with non-organometallic Tc(VII) compounds behaved similarly. In a recent study, [Tc03C1(AaA)] (86a) (in which AA stands for aromatic diamine derivatives) was shown to react quantitatively with olefins, and produce the corresponding Tc(V) diolato-complex [TcOC1(OaO)(AaA)] (87a). The process could not be run catalytically, as Tc(V) complexes tend to undergo disproportionation rather than reoxidation in the presence of water [97]. These alkene-glycol interconversions could not be performed with the analog Re(VII) compound. Rhenium displays completely contrary behaviour, in that alkenes can... 

Molybdenum catalysts that contain enantiomerically pure diolates are prime targets for asymmetric RCM (ARCM). Enantiomerically pure molybdenum catalysts have been prepared that contain a tartrate-based diolate [86], a binaph-tholate [87], or a diolate derived from a traris-1,2-disubstituted cyclopentane [89, 90], as mentioned in an earlier section. A catalyst that contains the diolate derived from a traris-1,2-disubstituted cyclopentane has been employed in an attempt to form cyclic alkenes asymmetrically via kinetic resolution (inter alia) of substrates A and B (Eqs. 45,46) where OR is acetate or a siloxide [89,90]. Reactions taken to -50% consumption yielded unreacted substrate that had an ee between 20% and 40%. When A (OR=acetate) was taken to 90% conversion, the ee of residual A was 84%. The relatively low enantioselectivity might be ascribed to the slow interconversion of syn and anti rotamers of the intermediates or to the relatively floppy nature of the diolate that forms a pseudo nine-membered ring containing the metal. 

Similarly to alkenes, alkynes react with various titanium-methylidene precursors, such as the Tebbe reagent [13,63], titanacydobutanes [9b, 64], and dimethyltitanocene [65] to form the titanium-containing unsaturated cyclic compounds, titanacydobutenes 67 (Scheme 14.29). Alternatively, 2,3-diphenyltitanacydobutene can be prepared by the reaction of the complex titanocene(II) bis(trimethylphosphine) with 1,2-diphenylcyclopropene [66]. Substituent effects in titanacydobutenes [67], the preparation of titanocene-vinylke-tene complexes by carbonylation of titanacydobutenes [68], and titanacyclobutene-vinylcar-bene complex interconversion [69] have been investigated. 

When alkenes are allowed to react with certain catalysts (mostly tungsten and molybdenum complexes), they are converted to other alkenes in a reaction in which the substituents on the alkenes formally interchange. This interconversion is called metathesis 126>. For some time its mechanism was believed to involve a cyclobutane intermediate (Eq. (16)). Although this has since been proven wrong and found that the catalytic metathesis rather proceeds via metal carbene complexes and metallo-cyclobutanes as discrete intermediates, reactions of olefins forming cyclobutanes,... 

Diastereoisomers are stereoisomers which do NOT have a mirror image of one another. Figure 11.20 shows the diastereoisomers of 2-butene (alkenes such as this are sometimes called geometric isomers and are a consequence of the prohibition of rotation about double bonds). If a vertical mirror was placed between the two structures in Fig. 11.20 they would not reflect onto one another. If the functionality is on the same side then the isomer is the cis-form, if on the opposite side then it is the trans- form. The chemical properties are very similar because the functional groups are identical. However, as they have different shapes their physical properties are different. Interconversion requires breaking and remaking bonds so these isomers are also stable under normal conditions. 

Describe the structure of the vertical and nonvertical excited states of alkenes and show how the interconversion and deactivation of these states leads to stereochemical isomerisation. 

Even if hydrides of transition metals play an important role as intermediates in catalytic processes such as hydrogenation and hydro-formylation of alkenes and the Fischer-Tropsch reaction,71 in order to follow with the bioinorganic subject, we refer to the interconversion process ... 

However, (TMS)3Si radicals are found to add to a variety of double bonds reversibly and therefore to isomerize alkenes [19]. An example is shown for the interconversion of ( )- to (Z)-3-hexen-l-ol and vice versa by (TMS)3Si radicals (Reaction 5.1). Figure 5.1 shows the time profile of this reaction under standard experimental conditions (AIBN, 80 °C). The equilibration of the two geometrical isomers is reached in ca 10 h, and the percentage of Z/E = 18/82 after completion corresponds to an equilibrium constant of = 4.5. The difference in the stability of the two isomers in 2-butenes, i.e., AG°( -isomer) - AG° (Z-isomer) = — 3.1kJ/mol, corresponds to K = 3.5, since... 

Although (TMS)3Si has been proven to isomerize alkenes (see above), the post-isomerization of the hydrosilylation adduct could not be observed due to steric hindrance. Only with Ph3Ge radical, the (Z)-( ) interconversion of (TMS)3Si substituted alkenes was achieved [39]. 

Much work has been done since the early 1980s on the detailed investigation of the azirine-nitrile ylide interconversion using pulsed-laser photolysis. Thus the azirines 103 (R =R =Ph, R =H R =Me, R = R =Ph R = p-napthyl, R = Me, R = H), on irradiation in isooctane, gave intense long-hved absorptions (250-400 nm) attributed to the nitrile ylides 104 (44). Quenching studies with electon-deficient alkenes led to the determination of absolute rate constants that were similar to those reported earlier for steady-state trapping experiments. The nitrile ylide-olefin reactions are discussed in more detail in Section 7.3.1. 

The stereochemistry of 1,3-dipolar cycloadditions of azomethine ylides with alkenes is more complex. In this reaction, up to four new chiral centers can be formed and up to eight different diastereomers may be obtained (Scheme 12.4). There are three different types of diastereoselectivity to be considered, of which the two are connected. First, the relative geometry of the terminal substituents of the azomethine ylide determine whether the products have 2,5-cis or 2,5-trans conformation. Most frequently the azomethine ylide exists in one preferred configuration or it shifts between two different forms. The addition process can proceed in either an endo or an exo fashion, but the possible ( ,Z) interconversion of the azomethine ylide confuses these terms to some extent. The endo-isomers obtained from the ( , )-azomethine ylide are identical to the exo-isomers obtained from the (Z,Z)-isomer. Finally, the azomethine ylide can add to either face of the alkene, which is described as diastereofacial selectivity if one or both of the substrates are chiral or as enantioselectivity if the substrates are achiral. 

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