Isomerization of Decalins

The complexity of skeletal isomerization of decalin is depicted in Figures 8A and 8B. It shows that methylbicyclo[4.3.0]nonanes (MBCN1) are the... 

Use ) gui to produce the cis and i/rms forms of decalin. Run a PM.f calculation of the energies of these two forms. What is the cis-lmns isomerization energy as calculated by PMIT ... 

The changes in the product concentrations are more pronounced in case of Pt-zeolites. Particularly the rate of decalin isomerization is considerably enhanced by the addition of platinum. The ratio of Iso/ROP decreases from the value of 12 after 1 h to the value of 1.4 after 9 h. The ratio of ROP/CP decreases from the values close to 3 within the first 3 h to the value of 1.3 after 9 h. This is in contrast to H-Beta-25, where the ratio ROP/CP is almost constant during the entire experiment and its values do not exceed 1.5. The same trends are... 

Fig. 15 A proposed reaction network of direct ring opening of decalin reaction over acidic zeolites. PC Protolytic cracking HeT Hydride transfer HT Hydrogen transfer I Isomerization P /i-scission DS Desorption TA Transalkylation. Adapted from ref. 47.

The first precise evaluation of the anomeric effect was realized by Descotes and co-workers in 1968 (22). These authors have studied the acid catalyzed isomerization of the cis and trans bicyclic acetals 6 and 6 and found that, at equilibrium, the mixture contains 57% ci s and 43% trans at 80°C. The cis isomer is therefore more stable than the trans by 0.17 kcal/mol. The cis isomer 5 has one (stabilizing) anomeric effect whereas the trans isomer 6 has none. Steric interactions in cis acetal 5 were estimated tobel.65 kcal/mol (one gauche form of ri-butane, 0.85 kcal/mol and an OR group axial to cyclohexane, 0.8 kcal/mol). By subtracting an entropy factor (0.42 kcal/ mol at 80°C) caused by the fact that the cis acetal S exists as a mixture of two conformations (cis decalin system), they arrived at a value of 1.4 kcal/mol for the anomeric effect. 

Another example is the thermal and photochemical cis-trans isomerization of Cp2Fe2(CO)2( -CO)( -Sip-TolH).25 In this case, both cis(H) and trans isomers can be isolated at full purity by flash chromatography. Interconversion between these isomers occurs both thermally and photochemi-cally in cyclohexane-d12, and the composition in the thermal equilibrium state (cis(H) trans = 2 98 at 25°C) is extremely different from that in the photostationary state (cis(H) trans = 70 30). Kinetics of the thermal isomerization in decalin afforded the activation parameters shown in Eq. (58). The large negative activation entropies imply that this reaction also... 

The isomerization of the decalins is most easily explained as a free radical reaction initiated by hydrogen abstraction at one of the tertiary carbons. Inversion at the free radical center followed by hydrogen abstraction would yield the other isomer of decalin. Sufficient repetition of the process would bring about equilibrium between the cis and trans forms. 

Naphthalene is reduced to 1,4-dihydronaphthalene by sodium and alcohol. Isomerization of this product to 3,4-dihydronaphthalene occurs with sodamide in liquid ammonia. Tetrahydronaphthalene (tetralin) is formed from naphthalene by sodium in amyl alcohol or by reduction with nickel-aluminum alloy and aqueous alkali. Catalytic hydrogenation of naphthalene can be stopped at the tetralin stage over copper chromite, Raney nickel, or alkali metal catalysts. cis-Decahydronaphthalene is produced by high-pressure hydrogenation of tetralin over Adams catalyst, whereas a mixture of cis- and trans-decalins is obtained from naphthalene under the same conditions. ... 

The solution to this problem is quite easy to visualize if the tricyclic structure II is looked upon as a c/s-decalin skeleton to which an extra C-C bond between carbons in different rings has been added. That this bond is made at the expense of the cyclopropane ring of I becomes obvious not only because of the absence of this three membered unit in II, but also because the other functions (C=C and C=0) appear in the product without any change other than the isomerization of the double bond. 

The direct synthesis of anthraquinone from phthalic anhydride and benzene has been reported to proceed over zeolite Beta [50] in a shape selective manner. In a conventional anthraquinone synthesis, anthracene is used as a feedstock for oxidation. Once there is a shortage of it in the market, additional anthracene could be produced by isomerization of its isomer, viz. phenanthrene. This, however, is not possible by direct isomerization of the trinuclear aromatic system but involves the partially (symmetrically) hydrogenated species. Consequently, isomerization of symmetrical octahydrophenanthrene to symmetrical octahydro-anthracene was studied by Song and Moffatt [51]. As sketched in Figure 3, a high yield of symmetrical octahydroanthracene can be obtained over zeolite H-mordenite (ngj/nyy = 8) at 250 °C (liquid phase, decalin as solvent). These examples show that (shape selective) catalysis on zeolites is more and more expanding into the conversion of polycyclic aromatics, and we foresee continued interest and success in this field of zeolite catalysis. 

Isomerization. With Pd catalysts, and, to a lesser extent, with Pt catalysts, a mixture of isomeric products may be obtained due to positional isomerization of double bonds during hydrogenation. As illustrated below, 5yn-addition of H2 to either face of the double bond in alkene A furnishes cw-decalin C. However, 5yn-addition of Hj to the isornerized alkene B can produce the cw-decalin C and/or trans-decalin D, depending on which face of the double bond undergoes addition by H2. In fact, hydrogenation of A in the presence of Pt furnishes 80% of the thermodynamically more stable trans-decalin and only 20% of cw-decalin. 

Johnson et al. ° prepared rra s-decalin-l,4-dione by hydrogenation of 1,4-naphthoquinone over RUO2, followed by chromic acid oxidation of the resulting diol and isomerization of the cis- to the rrnn.v-dione. 

Prior to 1960 the only perhydronaphthalenes for which physical properties were available were the cis and trans isomers of decalin. Soon thereafter, our investigation of the hydrogenation of the two monomethylnaphthalenes led to the isolation, identification, and measurement of the physical properties of all eight of the resulting isomeric monomethyldecalins (5). The 68 dimethyldecalins, obtainable by hydrogenation of the 10 dimethylnaphthalenes, will be described in this contribution... 

Haa of Isomerization of Axial Methyl to Equatorial Methyl, in Methyl-trans-decalins, Liquid Phase... 

In contrast with the cyclohexanes and the decalins, evaluation of the conformational energies of the octalins seems not to have been attempted by either experimental or theoretical approaches. The experimental equilibria in Table IX permit calculation of the free energy changes for isomerization of the various isomers, but are of little help for estimating the enthalpies unless the corresponding entropy changes can be assessed. 

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