Other Cycloalkenes

Heats of hydrogenation decrease progressively as the size of the ring 

The hydrogenation of these compounds has attracted much interest for the following reason 1,2-dimethylcyclohexene (for example) is likely to chemisorb either in the tt- or the di-cr form, with the two methyl groups pointing away from the 

To appreciate how important alkyl reversal is in the reactions being considered, the reactions of monosubstituted Ce cycles with deuterium are informative the results obtained with carbon-supported metals after 25% addition are given in Table 7.7 Remembering that deuterium numbers M of the cycloalkane greater than two mean more hydrogen exchange than cycloalkene exchange, and vice versa, the results are broadly in line with the characteristics of the three metals as exposed in the earlier sections. A certain amount of alkyl reversal must occur in all cases, but alkene desorption is only important with palladium. With Pt/C 

In most cases the substituent is a methyl group, but in the cyclohexene series ijopropyl and terf-butyl groups have also been used,as have compounds where alkyl groups are in the 1,3-, 1,4- and 2,4-positions. 

Reverting to the 1,2-dimethylcycloalkenes, we find that on nickel film at 273 K the C4 and C5 cycles gave respectively 84 and 66% of the Z-isomer the 

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Placement occurs through an isomerization process similar to that responsible for 3,1-placement in propene polymerization (Sec. 8-5c-l). 1,3-Placement is also observed with nickel and palladium a-diimine initiators [Sacchi et al., 2001] (Sec. 8-8b). 1,3-Placement has not been reported for other cycloalkene polymerizations. 

We have once earlier used the term cycloethene for acetylene in the context of defining the strain energy of acetylene in terms of ethylene, i.e. AHf(g, C2H2) vs 2AH g, C,H4) AH g, C2H6)], in order to compare that quantity with the strain energies of cyclopropene and other cycloalkenes cf Reference 4, p. 94. 

Cyclopropene has rather exceptional properties compared to the other cycloalkenes. It is quite unstable and the liquid polymerizes spontaneously although slowly, even at —80°. This substance, unlike other alkenes, reacts... 

In poly(l-pentenylene) the chemical shifts of the -carbons are about 0.5 ppm upfield from those in the polymers of the other cycloalkenes, an effect which is attributed to a higher proportion of gauche conformations about the CH2—CH2 bonds arising from the influence of the y-olefinic carbons347. 

The reaction can be extended to other cycloalkenes such as cyclooctene, 1,5-cyclooctadiene, or 1,5-cyclooctadiene resulting in ozonides that have, in addition to the geminal 3-methyl-3-carbomethoxy substituents, a heptanal or cis-heptenal group at position 5 (in the case of 1,3-cyclooctadiene this side chain has the double bond in the non-conjugated position relative to the aldehydic group) <1993TL6591>. 

The ozonolysis of cyclohexene to 1,6-dioxygenated compounds is shown in Figure 17.28. Other cycloalkenes similarly afford other l,y-dioxygenated cleavage products. With the three methods for workup (Figure 17.27), this ozonolysis provides access to 1,6-hexanediol, 1,6-hexanedial, or to 1,6-hexanedicarboxylic acid. Each of these compounds contains two functional groups of the same kind. 

The dipole moment is a manifestation of the overall charge distribution, and the calculated moments are generally in good agreement with the experimental values. Among cyclopropene derivatives, cyclopropene is the most interesting case. Here, the sign of the dipole moment is reversed from that found in cyclobutene or other cycloalkenes. The... 

Metathesis catalysts with one chiral A-substituent (e.g., 17) are useful for ROMP. In alternating copolymerization of norbomene with other cycloalkenes the steric interactions of the growing polymer chain dictate selectivity. 

The reaction with other cycloalkenes also proceeds with good enantioselectivity (Scheme 12). 

Photolysis in hydrocarbon solvents leads to rearranged products similar to those from other cycloalkenes, as well as hydrogen abstraction and coupling products. ... 

Other cycloalkenes give mixed results. Cyclopropenes, unless heavily substituted, tend to react with each other rather than with acetylenes [lOS]. Cyclohexenes (and larger rings) lack useful reactivity in general, although an interesting exception is the homoallylic amine derivative shown in Eq. (45)]. Evidence from this and related systems suggests that prior com-plexation of the heteroatom to a cobalt atom in complex 12 (Scheme 5.2) facilitates complexa-tion and, ultimately, cycloaddition of the double bond [106]. 

Cyclopentene yields mixtures of ROMP and double-bond polymerization with some Ti and V initiators. ROMP occurs exclusively with molybdenum and tungsten initiators, as well as Re, Nb, and Ta initiators. The relative amounts of cis and trans structures vary with the initiator and temperature [DalTAsta et al., 1962 Pampus and Lehnert, 1974]. Metallocene initiators polymerize cyclopentene through the double bond, but the polymer structure consists of cis 1,3-placement (Coates, 2000 Kaminsky, 2001 Kelly et al., 1997]. 1,3-Placement occurs through an isomerization process similar to that responsible for 3,1-placement in propene polymerization (Sec. 8-5c-l). 1,3-Placement is also observed with nickel and palladium a-diimine initiators [Sacchi et al., 2001] (Sec. 8-8b). 1,3-Placement has not been reported for other cycloalkene polymerizations. 

Some years ago, we found that the combination of manganese dioxide and a catalytic amount of benzoquinone (or hydroquinone) led to efQcient conversion of cyclohexene into cyclohexenyl acetate if palladium acetate was used as catalyst. With the exception of cyclooctene, other cycloalkenes also gave cycloalkenyl acetates in fair to good yieids[30].[3i] (Table 1). 

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