Tetrahydrodicyclopentadiene, isomerization

Isomerization and oligomerization of olefins are important reactions in chemical industry and they have been studied extensively in acidic IL systems. For purpose of easy catalyst separation and recovery, the supported chloroaluminate IL-phase catalyst was applied in endo-tetrahydrodicyclopentadiene isomerization. Scheme 2.26 [108]. As a typical operation, sodium montmorillonite (Na-MMT) was first transformed to l-hexadecyl-3-methyl-imidazolium (HDMI) montmorillonite (I-MMT) through ion exchange with [HDMIjCl and used as the support. Subsequently, chloroaluminate IL [HDMIjCl was supported onto the I-MMT by impregnation ([HDMIjCl-AlClj/I-MMT). Under optimized reaction conditions, >99% yield to exo-tetrahydrodicyclopentadiene was obtained over [HDMIjCl-AlClj/I-MMT catalyst at 50 °C in 2 h. The conversion of endo-tetrahydrodicyclopentadiene was 82.7% when Na-MMT was used as the support directly without IL modification. 

Preparation B. Ault and Kopet2 describe as an introductory organic experiment a procedure for the preparation of adamantane by isomerization of endo-tetrahydrodicyclopentadiene by the method of Schleyer but the product is isolated from a hexane extract of the reaction mixture as the beautifully crystalline inclusion complex with thiourea.3 As noted in 1, 1164, the ratio in this complex is 3.4 molecules of host per molecule of guest hydrocarbon. Although the reaction time is only 1 hour, the yield (15%) is about the same as that reported for a reaction time of 8-12 hours. 

On the other hand, R-exchanged zeolites have also been used in combination with a metal function, for carrying out the isomerization and hydrocracking of paraffins and cycloalkanes. This is the case for isomerization of n-hexane to isohexane and 2,2-dimethyl-butane (Rabo et al. 1961) the isomerization of n-undecane to mixed Ci 1 isomers at 275°C on Pt/Ce-Y zeolite (Weitkamp et al. 1985) the isomerization of c (7o-exo-tricyclo[5.2.1.02,6]-decane or exo-tricyclo[6.2.1.02,7]-undecane into adamantane or 1-methyladamantane, respectively, on R-Y at 150-270°C (Lau and Maier 1987) the isomerization of tetrahydrodicyclopentadiene into adamantane on Re-Y in a H2/HC1 atmosphere at 250°C (Honna et al. 1986) or the double bond relocation of 2-alkyl acrolein into fran.j-2-methyl-2-alkenals over Ce,B-ZSM-5 (Fisher et al. 1986). Recently, it has been reported that Ce-promoted Pd/ZSM-5 is an active and selective catalyst in the dehydroisomerization of a-limonene to / -cymene (Weyrich et al. 1997). 

The Lewis-acid-catalysed rearrangement of polycyclic hydrocarbons C H2 4 c (x = 1,2,3,...) into their thermodynamically more stable diamond-like isomers is usually referred to as the adamantane rearrangement . The archetype of this unique isomerization is the AlCl3-catalyzed transformation of en /o-tricyclo[5.2.1.0 ]decane (11) (tetrahydrodicyclopentadiene) into adamantane (Ad-H) (Scheme 4), which was first discovered by Schleyer " more than 30 years ago. This process is thermodynamically controlled and it involves carbenium ion intermediates which rearrange via successive 1,2-C bond and 1,3 hydride shifts. Numerous theoretical and experimental investigations have established the most direct thermodynamically feasible pathway (exo-11 -> 12 13 -> 14 -> 15 Ad-H) for the isomerization of endo-11 to adamantane. 

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