Cis/trans-Isomerization of alkenes

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.

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. 

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. 

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... 

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. 

Although cis-trans isomerization of epoxides is not formally associated with this section, it is clearly a potential problem in the conversion of an alkene to an epoxide. There are several catalysts for this process. 

Physical details relating to the isomerization of the stilbenes (12) have been determined.A study of the photophysical properties of the styrylstiIbenes (13) has shown that quantum yields for trans-cis isomerism are low from the singlet but high from the triplet state. The photochemical isomerization of the alkene (14) in an ethanol glass affords the trans-isomer with high efficiency even at liquid helium temperatures. Photochemical cis-trans-isomerization of cis-1,2-di-l-naphthylethylene (14) has also been studied in the crystalline phase. A study of the photochemical isomerization of a series of styryl phenanthrenes has been reported. The mechanism of the reaction involved was discussed. ... 

Isomerization of drugs typically involves racemization or epimerization of optical isomers or cis-trans isomerization of compounds containing an alkene functional group. Examples of epimerization include pilocarpine (55), rolitetracycHne (56), and ergotamine (57). Drugs susceptible to racemization include benzodiazepines, penicillins, and cephalosporins. The rearrangement of amphotericin B is an example of cis-trans isomerization (58). 

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