Isomerization of cycloalkenes

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... 

A review of photochemistry carried out in ionic liquids and a monograph dealing with a variety of aspects of photochromism have been published Reviews have also highlighted the photosensitized enantiodifferentiating isomerization of cycloalkenes other than cyclooctene and the area of asymmetric photochemistry in general Mechanisms for the photosensitized isomerization of alkenes have been discussed and calculations have been carried out to assess the electron-transfer processes in the tetramethylethene-tetracyanoethene system ... 

Highly constrained (E)-cycloalkenes, generated readily through the photoisomerization of (Z)-isomers, are known to possess unique physical and (chir)optical properties. These properties are of particular interest and importance from both the experimental and theoretical points of view. For example, the experimental barriers of thermal E-to-Z isomerization of cycloalkenes are compared with the theoretical values obtained by semiempirical calculations. Among the calculations reported, the PM3 method appears to give good fits to the experimental results e.g., the experimental strain energy of (Z)-cyclooctene (20Z) is 11.37 kcal/mol, which is close to the value (10.55 kcal/mol) obtained by PM3. The relevant experimental (chir)optical properties, such as specific rotations and absorption maxima and thermodynamic parameters for isomerization, are summarized in Table 16.2. 

Crotonaldehyde, hydrogenation of, 43-48 Cubane, isomerization of, 148 Cyclic dienes, metathesis of, 135 Cyclic polyenes, metathesis of, 135 Cycloalkenes, metathesis of, 134-136 kinetic model, 164 ring-opening polymerization, 143 stereoselectivity, 158-160 transalkylation, 142-144 transalkylidenation, 142-144 Cyclobutane configuration, 147 geometry of, 145, 146 Cyclobutene, metathesis of, 135 1,5,9-Cyclododecatriene, metathesis of, 135... 

Bis(diamino)alanes (R2N)2A1H were used for the hydroalumination of terminal and internal alkenes [18, 19]. TiCb and CpjTiCb are suitable catalysts for these reactions, whereas CpjZrCb exhibits low catalytic activity. The hydroaluminations are carried out in benzene or THF soluhon at elevated temperatures (60°C). Internal linear cis- and trans-alkenes are converted into n-alkylalanes via an isomerization process. Cycloalkenes give only moderate yields tri- and tetrasubstituted double bonds are inert. Hydroaluminahon of conjugated dienes like butadiene and 1,3-hexa-diene proceeds with only poor selechvity. The structure of the hydroaluminahon product of 1,5-hexadiene depends on the solvent used. While in benzene cyclization is observed, the reaction carried out in THF yields linear products (Scheme 2-10). 

ZJE isomerization of the alkene bond in A -enecarboxyIic acids occurs during Kolbe electrolysis because the intermediate radical reacts reversibly with this function to form a cyclopropane [92], This process leads to a partial loss of stereochemistry in the synthesis of long chain alkenes [93]. However, it does not present stereochemical problems during the synthesis of cycloalkenes such as chaul-moogric acid [94]. 

The growing interest in enantioselective isomerization of meso oxiranes to allylic alcohols arises from the ready availabihty of starting materials and the synthetic value of the homochiral products. First apphed to simple meso cycloalkene oxides, this methodology has been successfully exteuded to fuuctioualized meso oxiranes, and even to the kinetic resolution of racemic oxiranes, demonstrating its potential in accessing highly advanced synthons. 

Earlier examples of copper(I)-photocatalyzed cycloalkene cyclodimerizations have been summarized in Houben-Weyl, Vol. 4/5a, pp 280-292 as was the copper(I) chloride photocata-lyzed isomerization of cycloocta-1,5-diene to tricyclo[3.3.0.02 6]octane (see Houben-Weyl, Vol. 4/5 a, p 231). This same reaction has recently been used for the preparation of 4-oxatctra-cyclo[6.3.0.02,(,.07,1 L]undecanes.10... 

Oxidation with lead tetraacetate is a far less selective process.490,491 Studied mainly in the oxidation of cycloalkenes, it gives stereoisomeric 1,2-diol diacetates, but side reactions (allylic acetoxylation, skeletal rearrangement) often occur. A change in reaction conditions in the oxidation of cyclopentadiene allows the synthesis of different isomeric mono- and diesters.492... 

Despite earlier observations,624 625 ozonolysis of cyclic olefins in MeOH or in mixed solvents (ethers or esters and MeOH) followed by isomerization in the presence of Lindlar Pd and hydrogen does not give directly dicarboxylic acids. Instead, mainly the corresponding dialdehydes are formed.626 The best method of synthesis of dicarboxylic acids is ozonolysis of cycloalkenes in a mixture of acetic acid and formic acid followed by further oxidation with oxygen 626... 

C chemical shifts of cycloalkenes given in Table 4.12 [229, 233, 238-241] again reflect the special bonding state of three-membered rings, characterized by the smallest shift values in the series. As shown for cyclooctene in Table 4.12, the relation 6x /) < Sx(E) can also be applied to distinguish ris-frans-isomeric cycloalkenes. 

Isomerization of substituted styrene oxides allows the synthesis of aldehydes in high yields726 [Eq. (5.275)]. Cycloalkene oxides do not react under these conditions, whereas 2,2,3-trimethyloxirane gives isopropyl methyl ketone (85% yield). Isomerization of oxiranes to carbonyl compounds is mechanistically similar to the pinacol rearrangement involving either the formation of an intermediate carbocation or a concerted mechanism may also be operative. Glycidic esters are transformed to a-hydroxy-/3,y-unsaturated esters in the presence of Nafion-H727 [Eq. (5.276)]. 

Kropp PJ. Photochemistry of cycloalkenes. III. Ionic behaviour in protic media and isomerization in aromatic hydrocarbon media. J Am Chem Soc 1967 89 5199-5208. 

A convenient photochemical procedure for obtaining ( )-cycloalkenes and the products derived therefrom is the photosensitized isomerization of (Z)-isomers, where more conventional instrumentations, such as low/high pressure mercury lamps and quartz/Vycor/Pyrex vessels, can be used. 

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. 

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