The cis- trans isomerization of alkenes

The fact that a reaction is favourable (that is, AG° is negative) does not mean that the reaction will go at any appreciable rate the rate is determined by the activation energy barrier that must be 

In order to interconvert the cis and trans isomers we must use a different strategy. One method is to shine light on the molecule. If UV light is used it is of the right wavelength to be absorbed by the C=C 7t bond exciting one of the n electrons into the anti bonding 7t orbital. There is how no 7t bond and the molecule can rotate freely. 

The quantum symbols hv are conventionally used for light in a reaction and the excited state diagram is the best we can do for a molecule with one electron in the n orbital and one in the ji orbital. 

Another approach to alkene isomerization would be to use a catalyst. Base catalysis is of no use as there are no acidic protons in the alkene. Acid catalysis can work (Chapter 19) if a carbocation is formed by protonation of the alkene. 

Lowering the energy of the transition state means stabilizing it in some way or other. For example, if there is a separation of charge in the transition state, then a more polar solvent that can solvate this will help to lower the energy of the transition state. Catalysts generally work by stabilizing the transition states or intermediates in a reaction. We shall return to this point when we have introduced kinetic and thermodynamic products. 

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A similar selectivity was obtained by Nakazaki and coworkers for the cis-trans isomerization of doubly bridged alkenes in diethyl-( 4- )-tartrate. Upon irradiation at room temperature in this chiral solvent, m-bicyclic a,p-unsaturated ketone 13 was transformed into optically active (-)-trans-ketone 14 (Scheme... 

Catalytic antibodies have been produced to catalyze an impressive variety of chemical reactions. For instance, stable phosphonic esters (or lactones, respectively) have been used as transition state mimics for the hydrolysis of esters and amides [555-557], acyl transfer [558] and lactonization [559] reactions, which all proceed via a tetrahedral carbanionic intermediate. In some instances, the reactions proved to be enantiospecific [560]. Elimination reactions [561, 562], reductions [563], formation and breakage of C-C bonds [564,565] and even photochemical reactions [566] can be catalyzed. The cleavage of an ether [567] and cis-trans isomerization of alkenes have been reported [568]. 

An interpretation of activation parameters has led to the conclusion that the bromination transition state resembles a three-membered ring, even in the case of alkenes that eventually react via open carbocation intermediates. It was foimd that for cis trans pairs of alkenes tiie difference in enthalpy at the transition state for bromination was greater than the enthalpy difference for the isomeric alkenes, as shown in Fig. 6.2. This... 

Figure 6.4 The requirement for cis-trans isomerism in alkenes. Compounds that have one of their carbons bonded to two identical groups can t exist as cis-trans isomers. Only when both carbons are bonded to two different groups are cis-trans isomers possible.

Some care must be taken in drawing conclusions from the E/Z or syn/anti selectivity of a given catalyst/alkene combination. The intrinsic stereoselectivity may be altered in some cases by subsequent isomerizations initiated by the catalyst. For example, epimerization of disubstituted vinylcyclopropanes is effectively catalyzed by palladium compounds the cis - trans rearrangement of ethyl chrysanthemate or of chrysanthemic acid occurs already at room temperature in the presence of PdCl2 L2 (L = MeCN, EtCN, PhCN)96 Oxycyclopropane carboxylic esters undergo metal-... 

An important problem in many syntheses is to produce the desired isomer of a cis-trans pair of alkenes. The problem would not arise if it were possible to isomerize the undesired isomer to the desired isomer. In many cases such isomerizations can be carried out photochemically. A typical example is afforded by cis- and /raw.v-1,2-diphenylethene (stilbene) ... 

Cis-trans isomerism in alkenes arises because the electronic structure of the carbon-carbon double bond makes bond rotation energetically unfavorable at normal temperatures. Were it to occur, rotation would break the pi part of the double bond by disrupting the sideways overlap of two parallel p orbitals (Figure 23.2). In fact, an energy input of 240 kj/mol is needed to cause bond rotation. 

In isomerization reactions, an alkene is deprotonated to form an allyl anion, which is reprotonated to give the more stable alkene (double-bond migration). The most simple example is the isomerization of 1-butene producing a mixture of cis- and trans-2-butene (Scheme 3). Because the stability of the cis-allyl anion formed as an intermediate is greater than for the trans form, a high cis/trans ratio is observed for base-catalyzed reactions whereas for acid-catalyzed reactions the ratio is close to unity. Thus, the cis/trans ratio of the products has frequently been used as an indication of base-catalyzed reaction mechanisms. The carbanions formed in the course of such superbase reactions are not freely mobile in solution,... 

Let s begin by considering a simple disubstituted cyclohexane, 1,2-dimethyl-cyclohexane. If the ring is drawn flat, in the plane of the page, then one substituent on each carbon projects above the page and the other projects below the page. The situation is somewhat similar to the cis-trans isomerism that occurs with alkenes. Both... 

Carbon-carbon double bonds cannot rotate, and many alkenes show geometric (cis-trans) isomerism (Sections 2-7B and 2-8B). The following are the cis-trans isomers of some simple alkenes ... 

The photoprotonation of cycloalkenes, described in this procedure, is believed to proceed via initial light-induced cis —> trans isomerization of the alkene.4 The resulting highly strained trans isomer undergoes facile protonation. This procedure permits the protonation of cyclohexenes and cycloheptenes under neutral or mildly acidic conditions.5 Since the process is irreversible, high levels of conversion to addition products can be achieved. 

Evidence for the two-step nature of the dihydropyran formation follows from the observation that both cis- and fraws-dibenzoylethene gave the same dihydropyran96, under conditions where cis-trans isomerism of the electrophilic alkene did not occur. On heating, the dihydropyrans rearrange into a mixture of the corresponding alkylated enamines (44 and 45) (Scheme 29). This is kinetically rather than thermodynamically controlled, since the equilibrium composition was obtained only after treatment with acid96, and can therefore be regarded as irreversible in an aprotic solvent (benzene) at 80°C. 

The stereoselective synthesis of unsaturated oxetanes has recently been achieved by Feigenbaum and coworkers.Previous studies have indicated that photochemical cis-trans isomerization of enals is rapid and results in the formation of equivalent amounts of stereoisomeric alkene adducts. " For example, irradiation of rran.r-crotonaldehyde and 2,6-dimethylfuran produced a 1 1 mixture of alkenic isomers (174) and (175) in 64% yield. Irradiation of 4-trimethylsilylbutyn-2-one and furan provided with S 1 stereoselectivity the bicyclic oxetane (176) in which the methyl group occupies the exo position, presumably because of the small steric requirement of the triple bond. Desilyation of the protected al-kyne produced an alkynic oxetane which was hydrogenated under Lindlar conditions to bicyclic vinyl-oxetane (177) attempts to use the unprotected butyn-2-one gave low isolated yields of oxetane because of extensive polymerization. The stereochemical outcome thus broadens previous alkynyloxetane syn-theses and makes possible the preparation of new oxetane structures that may be synthetically useful. 

Tokumaru and his co-workers have demonstrated that sensitized irradiation of the cis-alkene (35) yields the trans-isomer in 100% yield. There is no evidence for the formation of the cis- from the trans-isomer. The sensitized trans-cis isomerization of the alkene (36) has been studied. An investigation of the photoreactions of the alkene (37) and their dependence on oxygen concentration has been reported. A detailed study of the singlet-state cis-trans isomerization of cis-cycloheptene at -78 °C induced by methyl benzoate as sensitizer has been reported. 

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