Cycloalkenes ring contraction

A cationic mechanism is responsible for the ring contraction of cycloalkenes with thallium(III) salts in the presence of diluted perchloric acid resulting in the formation of formylcycloalkanes. This method was unsuccessful for cyclopentene, whereas 1-methylcyclopentene gave acetyicy-clobutane (32) in 16-24% yield98 depending on the better stabilization of the intermediate cationic species. 

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... 

The electrophilic character of iodine in IOB is greatly increased by the addition of boron trifluoride, probably because of the in situ formation of the monomeric dipole Phl + OBF3 . Some cycloalkenes reacted with IOB.BF3 undergoing ring-contraction, for example cyclohexene gave formylcyclopentane (60%), and 3,4-dihydro-27/-pyran gave tetrahydrofurfural [8] ... 

When 2-methoxycycloalkyl phenyl telluriums, prepared from cycloalkenes and phenyl tellurium trichloride in methanol, are treated with 3-chloroperoxybenzoic acid, the elimination of the phenyltelluro group is accompanied by ring-contraction and formation of the dimethyl acetals of formylcycloalkanes2 3. 

Phenyl cycloalkyl tellurium compounds were also oxidized by sodium periodate in aqueous methanol to the corresponding tellurium oxides. These compounds are unstable at 20° and eliminate benzenetellurenic acid with formation of cycloalkenes and ring-contracted cycloalkyl aldehydes2. 

RSeCJIi — ROCHi. Oxidation of an alkyl phenyl selenide with m-chloroperben-zoic acid (2-5 equiv.) in methanol affords the corresponding alkyl methyl ethers in high yield. Oxidation of selenides with a vicinal phenyl group is accompanied by rearrangement of the phenyl group. vtc-Methoxy selenides derived from cycloalkenes are oxidized under these conditions to dimethyl acetals of ring-contracted aldehydes. 

Cycloalkenes to Acetals of Ring-Contracted Cycloalkyl Aldehydes... 

Hydroxymercuration and thallium(III) oxidation of cycloalkenes, 1-methylcycloalkenes and methylenecycloalkanes have been reported to yield ring-contracted aldehydes, ring-contracted ketones and ring-expanded cycloalkanones, respectively. 

Photoexcitation of cycloalkenes introduces additional features because the ring limits the extent to which the double bond can twist. Cyclohexene, cycloheptene, and cyclooctene give rise to ring contraction and carbene insertion products. 

There is interest in cycloaddition of diazoalkanes to cycloalkenes for various reasons. Small rings (cyclopropene and cyclobutene) react easily and often in good yield as a result of their angle-strained double bond. Their products with diazomethane, 2,3-diazabicyclo[3.1.0]hex-2-ene (6.56) and 2,3-diazabicyclo[3.2.0]hept-2-ene (6.57) and their substituted derivatives are interesting for synthetic purposes, e. g., by azo-extrusion leading to ring contraction. 

The amount of work involved in the deamination of ten different starting materials becomes evident from the fact that a total of 17 and 12 products was detected and analyzed for the experiments in the trans- and the cw-series, respectively. The corresponding tables contain 116 analyses The products are cycloalkenes (from elimination), acetates and butyrates from external substitution, and products of internal substitution (ArNH, AcO, OH, depending on the procedure used). Two isolated products, obtained in 0.3-0.5% and 2.5% yield, were identified later by Whiting s group (Cooper et al., 1982), as cis- and m 5-3-butylcyclopentylmethyl esters, i. e., products of ring contraction. 

The reaction of o>-halogeno-l-(NJV-dialkylamino)cycloalkenes with amines is known to result in a Favorskii-type ring contraction, giving rise to cm-diamino-bicyclo[n,l,0]alkanes, but, under the same conditions, the acyclic analogue (28) yields predominantly acyclic products. However, by employing (28), AgBF, and MejNH in a molar ratio of 1 1 2, cyclization is almost quantitative. Starting from... 

ENOLETHER/ /ISOL/ /0-FUNCT/ /CYLOHEXANE/ /CYCLOALKADIENE/ /AND/ /N-SULFONYL/ /AZIDE/ GIVES CYCLOPENTANE ISOL CYCLOALKENE IMINOESTER+ N-SULFONYL C-FUNCT RING-CONTRACTION,II ULTRASONIC PRESSURE=4 TEMP=4 SOLV=6... 

The reaction is extremely useful for preparing strained cycloalkenes, particularly cyclobutenes. The success of such reactions stems from the fact that they are not ring forming, but involve contraction of a less-strained saturated ring. 

In cyclohexene the double bond in a cis configuration [Fig. 4.6(f)] forms one bond of the ring. If we leave off hydrogens and proceed as before, we can approximate the C C vibration with the linear model in Fig. 4.6(h). Note that when the middle bond is stretched by an amount S the end bonds are each contracted by an amount 5/2, so the restoring force on each mass is FiS -h F2SI2 or (Fj + 2/2)5. This means that the linear model, [Fig. 4.6(h)] can be replaced by the free diatomic model [Fig. 4.6(i)] which will have the same frequency if its force constant is F + F2(cos a)/2. Approximate frequencies for cycloalkenes can be calculated using Eq. (4.2) with both masses equal to 12 and with a force constant equal to 8.9 + 4.5 (cos a)/ 2 mdyne/A. These results are shown in Table 4.5. 

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