Alkenes and dienes

Migration of the double bond of terminal alkenes to internal position is favored by the equilibria. Thus 1-butene in the presence of activated clay, silica gel, alumina, or phosphoric acid on pumice may yield equilibrium product mixtures comprised of about 20% 1-butene and 80% 2-butenes.91 The main transformation of branched 1-alkenes under mild conditions is also double-bond migration. For example, 2,4,4-trimethyl-1-pentene is isomerized to the equilibrium mixture92 with 20% 2,4,4-trimethyl-2-pentene when treated with silica gel at 25°C. 

The isomerization is believed to occur by a carbocation mechanism90 initiated by a proton furnished by the catalyst or by an ion (such as an alkyl cation formed by cracking) present in the reaction mixture  

Skeletal isomerization requires higher temperature and stronger acid catalysts than do double-bond migration and cis-trans isomerization. Butylenes, for example, are transformed to isobutylene over supported phosphoric acid catalysts.98 The equilibrium mixture at 300°C contains approximately equal amounts of straight-chain and branched butenes. Similar studies were carried out with pentene isomers.99 Side reactions, however, may become dominant under more severe conditions.100 

The acid-catalyzed isomerization of cycloalkenes usually involves skeletal rearrangement if strong acids are used. The conditions and the catalysts are very similar to those for the isomerization of acyclic alkenes. Many alkylcyclohexenes undergo reversible isomerization to alkylcyclopentenes. In some cases the isomerization consists of shift of the double bond without ring contraction. Side reactions, in this case, involve hydrogen transfer (disproportionation) to yield cycloalkanes and aromatics. In the presence of activated alumina cyclohexene is converted to a mixture of 1-methyl- and 3-methyl-1-cyclopentene 103 

Similar isomerization occurs in the presence of silica-alumina-thoria.104 As it might be expected, this reaction is similar to the isomerization of cyclohexane to methylcyclopentane. Both processes involve the same intermediate cyclohexyl car-bocation, which is formed, however, in different reactions. It may be formed from cyclohexane by hydride ion transfer, or by protonation of cyclohexene. Bicyclic alkenes undergo complex interconversions via carbocations over acidic catalysts.105 

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The mechanism of the Diels-Alder reaction is best understood on the basis of a molecular orbital approach To understand this approach we need to take a more detailed look at the rr orbitals of alkenes and dienes... 

Uses. Magnesium alkyls are used as polymerization catalysts for alpha-alkenes and dienes, such as the polymerization of ethylene (qv), and in combination with aluminum alkyls and the transition-metal haUdes (16—18). Magnesium alkyls have been used in conjunction with other compounds in the polymerization of alkene oxides, alkene sulfides, acrylonitrile (qv), and polar vinyl monomers (19—22). Magnesium alkyls can be used as a Hquid detergents (23). Also, magnesium alkyls have been used as fuel additives and for the suppression of soot in combustion of residual furnace oil (24). 

Acid catalysis using strong acid catalysts, especially 2eohtes which enhance selectivity because of pore si2e restrictions, has been used for a variety of alkenes and dienes (9—11). /-Butyltoluenediamine [106398-83-8] (/-BTDA) (C H gN2) is available on a semicommercial basis (12). 

Fig. 1. Vapor-pressure ratios of the alkanes, alkenes, and dienes with respect to / -butane A, isobutane B, isobutylene C, 1-butene D, 1,3-butadiene E,...

The photochemistry of alkenes and dienes has already been mentioned in connection with the principles of orbital symmetry control in electrocyclic and cycloaddition processes in Section 13.2. Cycloadditions are considered, from a synthetic viewpoint, in Chapter 6 of Part B. This section will emphasize unimolecular photoreactions of alkenes and dienes. 

Overman has extended his tethered Biginelli reaction to include alkenes and dienes instead of p-keto esters to deliver 51 diastereoselectively over 52 in the presence of Cu(OT02. 

Table 14.1 Heats of Hydrogenation for Some Alkenes and Dienes...

The alkene and diene polymers discussed in Sections 7.10 and 14.6 are called chain-growth polymers because they are produced by chain reactions. An initiator adds to a C=C bond to give a reactive intermediate, which adds to a second alkene molecule to produce a new1 intermediate, which adds to a third molecule, and so on. By contrast, polyamides and polyesters are called step-growth polymers because each bond in the polymer is formed independently of the others. A large number of different step-growth polymers have been made some of the more important ones are shown in Table 21.2. 

S+1C] Cycloaddition Reactions Cydopropanation of Alkenes and Dienes with Fischer Carbene Complexes... 

Dissociation energies D values) of R—H bonds provide a measure of the relative inherent stability of free radicals Table 5.4 lists such values. The higher the D value, the less stable the radical. Bond dissociation energies have also been reported for the C—H bond of alkenes and dienes and for the C—H bond in radical precursors XYC—H, where X,Y can be H, alkyl, COOR, COR, SR, CN, NO2, and so... 

Cycloaddition reactions result in the formation of a new ring from two reactants. A concerted mechanism requires that a single transition state, and therefore no intermediate, lie on the reaction path between reactants and adduct. The most important example of cycloaddition is the Diels-Alder (D-A) reaction. The cycloaddition of alkenes and dienes is a very useful method for forming substituted cyclohexenes.1... 

Photocycloaddition of Alkenes and Dienes. Photochemical cycloadditions provide a method that is often complementary to thermal cycloadditions with regard to the types of compounds that can be prepared. The theoretical basis for this complementary relationship between thermal and photochemical modes of reaction lies in orbital symmetry relationships, as discussed in Chapter 10 of Part A. The reaction types permitted by photochemical excitation that are particularly useful for synthesis are [2 + 2] additions between two carbon-carbon double bonds and [2+2] additions of alkenes and carbonyl groups to form oxetanes. Photochemical cycloadditions are often not concerted processes because in many cases the reactive excited state is a triplet. The initial adduct is a triplet 1,4-diradical that must undergo spin inversion before product formation is complete. Stereospecificity is lost if the intermediate 1,4-diradical undergoes bond rotation faster than ring closure. 

Photodimerization reactions of some other simple alkenes and dienes follow/39-30 36-182 Although not a dimerization reaction, photochemical ring closures to yield cyclobutane derivatives are analogous and are included in this section 31-35 ... 

The Pd/MOP combination has proved active for the asymmetric hydrosilylation of cyclic alkenes and dienes. Thus treatment of norbornene with HSiCl3 at 0 °C for 24 h in the presence of 0.01 mol.% of M eO-MOIV[Pd(// -C3H5)Cl]2 gave quantitative yield of evo-2-(trichlorosilyl)norbornane oxidation produced the corresponding alcohol in 93% ee (Equation (12)). Lowering the temperature (to — 20 °C) increased this to a 96% ee. Both mono- and difunctionalization of nbd has proved possible, depending upon the quantity of trichlorosilane used (Scheme 22). In both reactions, extremely good enantioselectivities are observed 113... 

TABLE 2. Relative rate constants for the addition of the trifluoromethyl radical and the diethyl a-benzylmalonyl radical to simple alkenes and dienes... 

TABLE 9. Propagation rate constants (kprop) for the polymerization of selected alkenes and dienes at 60 °C6,22,23... 

Lee and Squires determined the gas-phase acidities of a number of cyclic alkenes and dienes including the bicyclic compounds 4, 5, 6 and 715. Their values are summarized in Table 5 and have estimated uncertainties of 1-2 kcal mol 1. The relatively high acidity of 4 was attributed to bishomoconjugation of the double bond with the allyl anion, as shown in 815. 

A variety of cyclic ethers, 410, have been obtained via both, solution-phase and polymer-supported methods in the [3 + 2] cycloadditions of nitrile oxides to alkenes and dienes to give isoxazolines (Scheme 1.50). Both simple and substituted dienes have been found suitable for polymer-supported formation of cyclic ethers of ring sizes five through seven (449). 

Hydrogenation of Alkenes and Dienes with Croups V-VII Transition-Metal... 

The only other alkenyl carbenoid with a proton trans to the halide that can readily be generated by deprotonation is the parent 1-lithio-l-chloroethene 57 [43] (Scheme 3.13). Insertion into organozirconocenes arising from hydrozirconation of alkenes and alkynes, followed by protonation, affords terminal alkenes and ( )-dienes 59, respectively [38]. The latter provides a useful complement to the synthesis of 54 in Scheme 3.12 since the stereocontrol is >99%. 

The well-known Diels-Alder reaction [95,104-106] is a standard method for forming substituted cyclohexenes through the thermally allowed 4s + 2s cycloaddition of alkenes and dienes. In particular, the reaction between ethene and 1,3-butadiene to yield cyclohexene is the prototype of a Diels-Alder reaction (Scheme 28.4). It is now well recognized that this reaction takes place via a synchronous and concerted mechanism through an aromatic boatlike TS [105]. 

Although cycloaddition reactions have yet to be observed for alkene radical cations generated by the fragmentation method, there is a very substantial literature covering this aspect of alkene radical cation chemistry when obtained by one-electron oxidation of alkenes [2-16,18-26,28-31]. Rate constants have been measured for cycloadditions of alkene and diene radical cations, generated oxidatively, in both the intra- and intermolecular modes and some examples are given in Table 4 [91,92]. 

Oxidation of alkenes and dienes involving an allene substituent as a formal nucleophile is a conceptually new reaction. Allcnc-substitutcd 1,3-cyclohexadienes 34 undergo a pal-ladium(II)-catalyzed oxidation to give bicydic compounds 35 or 36 in good yields (Scheme 17.14) [14]. When y-alkenylallenes, e.g. 37, 41 and 43, were treated with 1 mol% palladium trifluoroacetate, a similar oxidative carbocyclization took place [15]. In both reactions the new stereocenters are formed with high stereoselectivity. 

In the presence of cuprous chloride, trichloromethylphosphonic dichloride (38) adds to a variety of alkenes and dienes via a free-radical process.40 There have been... 

In view of the stereochemical behavior in the additions to alkenes and dienes, the authors suggest that the reaction proceeds via a stepwise electrophilic addition126. However, in this case the two sulfur atoms of the dithioether dication are positively charged. In the reaction with multiple bonds, therefore, one of these sulfur atoms should be an electrophilic center whereas the other one should simultaneously be a nucleophilic center. In... 

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