Cycloalkenes, hydrogenation cyclohexene

As mentioned earlier (see Section 11.1.4), cycloalkenes (primarily cyclohexene) can be used to carry out transfer hydrogenations. Similar processes during which the alkene itself serves as hydride ion donor may be observed under ionic hydrogenation conditions 223... 

Hydrogenation of cyclododecene using the polymeric rhodium catalyst II occurs at a rate 5 times slower than does cyclohexene [Mathur and Williams, 1976 Mathur et al., 1980]. The low-molecular-weight homolog III shows no difference in catalytic activity toward the two 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... 

Most of the studies to date have employed either palladium [222—229] or platinum [220,224,226,228—235], commonly as Adams reduced platinum oxide, although nickel [228,236,237], rhodium [238,239], ruthenium [239], iridium [239], iron [237] and tungsten [237] have also been used. Many of these studies have been concerned with the stereochemistry of the hydrogenation of disubstituted cycloalkenes. Table 32 shows some typical results for the platinum- and palladium-catalysed hydrogenation of disubstituted cyclohexenes. Table 33 shows comparative results for the hydrogenation of 1,4-dialkylcyclohexenes over palladium, platinum and rhodium catalysts. 

The oxidation of alkenes and cycloalkenes and their halogen derivatives with at least one hydrogen or halogen atom at the double bond leads to carboxylic acids. Ozonolysis usually requires the oxidative decomposition of the ozonide. The oxygen content of the ozonide is not sufficient for the formation of two molecules of acids or one dicarboxylic acid. The nonoxidative decomposition of cyclohexene ozonide gives an aldehyde-acid or its derivatives [1108]. It comes, therefore, as a surprise that carboxylic acids are claimed as products of the decomposition of ozonides by hydrogenation over the Lindlar catalyst [55] (equation 108). 

The literature of biphasic hydrogenations contains plenty of substrates (al-kenes and cycloalkenes, arylaliphatic olefins, carbonyl compounds, etc.), mainly with TPPMS as water-soluble ligand (solubility approx. 200 g/1 [150] as compared with 1100 g/1 with TPPTS [37]). So far, no industrial process has been derived from these smdies. Besides the development of the basics of biphasic operation, the research concentrates on fundamental work concerning the question of where the reaction takes place phase boundary, organic phase, or aqueous phase. Wilkinson [29] concluded from his hydrogenation tests with hexenes or cyclohexenes in the presence of TPPMS that the somewhat lower rate of hydrogenation as compared with monophasic conversion should be due to the necessary diffusion of the hydrogen to the alkene/water interface. In this way the iso-... 

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]. 

A similar large reduction in rate was observed in going from cyclohexene to 1-nitro-l-cyclohexene. Similarly, substituting a hydrogen atom by an electron-donating methyl group at the double bond site in cyclopentene and cyclohexene enhances the reactivity in comparison to the unsubstituted cycloalkene. The presence of substituents at sites remote from the double bond has little effect on the reactivity as can be seen by comparing the Arrhenius parameters for cyclohexene and 4-methyl-1-cyclohexene. 

The discussion to this point has concerned only cycloalkanes. Additional considerations come into play with cycloalkenes. For example, cyclohexene is said to adopt a half-chair conformation having methylene hydrogen atoms in four different environments. The hydrogen atoms are considered to be in axial or equatorial positions on C4 and C5, but those on C3 and C6 are said to be pseudoaxial and pseudoequatorial (Figure 3.26). 

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. 

Both aikenes and alkynes quench the excited state Pt2(pop)4. Aikenes such as trans-stilbcnc that have triplet energies of less than -57.7 kcal/mol quench the A2u State of Pt2(pop)4 by energy transfer. This quenching reaction is followed by emission from the excited state of stilbene, and also by photoisomerization of the stilbene. For cycloalkenes such as cyclohexene and cyclopentene that contain allylic C-H bonds, quenching reactions with Pt2(pop) proceed by hydrogen-atom abstraction ... 

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