Cycloalkenes Cyclohexenes, Cyclopentenes

A systematic study has confirmed the low activity of EHs toward cycloalkene oxides (1,2-epoxycycloalkanes, 10.123) [184], In the presence of mouse liver microsomal EH, activity was very low for cyclopentene oxide and cyclohexene oxide (10.123, n = 1 and 2, respectively), highest for cyclo-heptene oxide (10.123, n = 3), and decreased sharply for cyclooctene oxide (10.123, n = 4) and higher homologues. Mouse liver cytosolic EH showed a different structure-activity relationship in that the highest activity involved cyclodecene oxide (10.123, n = 6). With the exception of cyclohexene oxide, which exhibited an IC50 value toward microsomal EH in the p.M range, cycloalkene oxides were also very weak inhibitors of both microsomal and cytosolic EH. 

Cycloalkenes such as cyclohexene, 1-methylcyclohexene, cyclopentene, and nor-bornene are hydrosilylated with triethylsilane in the presence of aluminum chloride catalyst in methylene chloride at 0 °C or below to afford the corresponding hydrosilylated (triethylsilyl)cycloalkanes in 65-82% yields [Eq. (23)]. The reaction of 1-methylcyclohexene with triethylsilane at —20 °C occurs regio- and stereoselectively to give c/i-l-triethylsilyl-2-methylcyclohexane via a tra x-hydrosilylation pathway. Cycloalkenes having an alkyl group at the double-bonded carbon are more reactive than non-substituted compounds in Lewis acid-catalyzed hydrosilylations. ... 

Hoveyda et al. reported a novel method for synthesizing of chromene 71 by ROM-RCM of cycloalkene 70 bearing the phenyl ether at the 3-position [Eq. (6.48)]." ° The yield is improved when the reaction is carried out under ethylene gas. In the case of cyclopentene 70a (n = 0) or cyclohexene 70b (n = 1), the yield is poor because the starting cycloalkene is in a state of equihbrium with the product and a thermodynamic product should be formed under these reaction conditions. They obtained enantiomeric ally pure cycloheptene derivative (5)-70e using zirconium-catalyzed kinetic resolution of 70e developed by their group, and chromene 71c was synthesized as a chiral form via ROM-RCM using lb [Eq. (6.49)] ... 

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... 

Cleavage of epoxides. As with simpler haloboranes, these reagents cleave epoxides of cycloalkenes to give, after nonoxidative workup, halohydrins in 65-90% yield. When carried out at -78 to -100°, the cleavage can show high en-antioselectivity. Thus the halodiisopinocampheylboranes derived from (+ )-pinene react with the oxide of cyclohexene or of cyclopentene to furnish (1R,2R) halo-... 

Oxo-cyclopentene and 3-oxo-cyclohexene react with diisobutylaluminum benzene-tellurolate to produce diisobutylaluminum 3-phenyltelluro-l-cycloalken-l-olates that condensed with butanal and benzaldehyde. The resulting 3-oxo-2-[(organo)hydroxymeth-yl]-l-phenyltcllurocycloalkanes were converted by 3-chloroperoxybenzoic acid to 3-oxo-2-[(organo)hydroxymethyl]-cycloalkenes2. 

The few available data for carbocation additions to cycloalkenes (Scheme 41) show an analogous reactivity order Cyclopentenes are more reactive than the acyclic analogs, and the only cyclohexene derivative shown in Scheme 41 is less reactive. Because of the paucity of data, this analogy should not be overinterpreted. The location of norbornene between the compounds which give secondary and tertiary carbocations has already been mentioned (Scheme 36 in Section III.D.4.a.). 

One of the most popular ways of using the double bond cleavage sequence envisages its utilization for the preparation of l, -dicarbonyl compounds via oxidation of the respective cycloalkene derivatives. Thus oxidation of cyclohexene represents the easiest way to prepare the 1,6-dialdehyde 460. Intramolecular aldol condensation of460 proceeds with ease to give the respective cyclopentene derivatives 461 or 462 (Scheme 2.149). 

Oxidations. A widely used method for allylic oxidation is the Kharash-Sosnovsky reaction using a peroxide and a copper(I) salt system. Enantioselective allylic oxidations of cycloalkenes such as cyclopentene, cyclohexene and cycloheptene with tert-butyl peibenzoate were investigated with a variety of catalysts derived from bis(oxazoline) ligands and copper(I) triflate complexes (eq 18). The ligand-copper(I) complexes from the /-Bu-... 

The equilibrium constants for a series of cycloalkenes decrease in the order norbomene > c -cyclooc-tene > cyclopentene > cycloheptene > cyclohexene, which correlates with the calculated strain energies as well as the kinetically determined relative adsorption constants on Pt (Table 2). Tolman states that electron donation from a filled metal rf-orbital to an empty alkene Tr -orbital is extremely important in determining the stability of these complexes. Steric effects of substituents are relatively unimportant compared to electronic effects, and resonance is more important than inductive interactions. The ability of the metal to back bond is lowered progressively in the series Ni° > Pt° > Rh > Pt" > Ag which reduces the importance of resonance and decreases the selectivity of the metal for different substituted alkenes. 

In another investigation, Alder and Stein found that cyclopentene forms aphenyl azide adduct but is considerably less reactive than the bridged compounds of type (6) cyclohexene formed no adduct. Ziegler later found the reaction with phenyl azide in ether very useful for the characterization of cis- and tranj-cycloalkenes of medium-size rings. Thus cis- and tranj-cyclooctene form crystalline adducts melting at 87 and 111, respectively. Approximate relative reaction rates of frans-cyclo-alkenes with phenyl azide are shown under the formulas ... 

Table 1 provides data on the hydrogenation of a range of cycloalkenes. Curiously, there is no obvious trend in the hydrogenation yields of the olefins, cyclopentene and cyclooctene being reduced faster than cyclohexene and cycloheptene. These variations are, therefore, not on account of any size selectivity of the catalyst. The heats of hydrogenation (AH) of the cycloalkenes (Cs-Cg) are different from each other [12]. However, the difference in the AH supports the variation we have observed in the hydrogenation yields of cycloalkenes. The orientation of the substrate towards the catalyst could be interpreted on the basis of the Horiuti-Polanyi mechanism [13]. 

In the case of cycloalkenes, it is found that the size of the ring is an important factor in product distribution. Photo [2 -I- 2] cycloaddition of cyclohexenone derivatives (4) to carbomethoxy cyclobutene [47], cyclopentene [48], and cyclohexene [49] (see Scheme 5) demonstrates a gradual reversal of regioselectivity from head-to-head to head-to-tail adducts as indicated in Table 2 [50]. This result of head-to-tail products is not consistent with the dipole-dipole interaction theory. Stability of biradical intermediates is suggested to explain the reversed regioselectivity. 

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