Dimethylcyclohexenes isomerization

Palladium also efficiently catalyzes the isomerization of cyclic olefins (15, 17, 19) 1,1-dimethylcyclohexene isomerizes to a small extent to the less stable 2,3-isomer 17), but 2-methylmethylenecyclohexane is rapidly isomerized, and has indeed disappeared after 30% hydrogenation, It is significant that the 2,3-dimethylcyclohexene is the major initial product, and this subsequently passes to the stabler 1,2-isomer (17). 4-fer<-Butylmethylenecyclohexane similarly is converted rapidly to the stabler 4-palladium-charcoal, 4-methylmethylenecyclohexane isomerizes completely to 1,4-dimethylcyclohexene, which itself does not isomerize (19). Table XII shows the olefin compositions which result after partial reduction of a number of related olefins there are two comparably stable isomers (C and D), and these are both popular, but it is likely that neither of the two kinds of distributions shown are true equilibrium distributions. J -octalin isomerizes to J -octalin over palladium-charcoal (19). The stereochemical analysis of hydrogenations over palladium is substantially complicated by this extensive isomerization. 

In comparison, the ratio of 2,4-/l,3-dimethylcyclohexene obtained from m-xylene is close to unity (1.2) and the ratio changes little during the course of the reaction. An initial random distribution should yield equal amounts of these isomeric cycloalkenes and the relative constancy of the ratio is consistent with the fact that, in competition with one another, 1,3- and 2,4-dimethylcyclohexene are reduced at comparable rates. The failure to observe the other postulated dimethylcyclohexenes is to be expected, because none would have substituents attached to the double bond in the cycle. Consequently, because of their expected greater reactivity in competition, their maximum concentration should be no more than a few per cent of the most reactive of the cycloalkenes actually observed in these experiments. 

Metals differ in their ability to catalyze isomerizations. Both the relative rates of isomerization of individual alkenes and the initial isomer distribution vary with the metal. The rates of isomerization of the three n-butenes on ruthenium and osmium, for example, are cis-2- > trans-2- > 1-butene,175 whereas on platinum and iridium they are cis-2- > 1- > trans-2-butene.176 These observations are in accordance with the fact that the rates of formation of the 1- and 2-butyl intermediates are different on the different metals. The order of decreasing activity of platinum metals in catalyzing the isomerization of dimethylcyclohexenes was found to be Pd Rh,... 

In the transformation of 1,2-dimethylcyclohexene the fraction of the cis isomer was observed to increase with increasing hydrogen pressure. This is a clear indication that the hydrogen partial pressure affects step 3 (equation 5) in the Horiuti-Polanyi mechanism by shifting the equilibrium to the formation of the half-hydrogenated state, therefore suppressing isomerization and increasing selectivity. 

Dimethylcyclohexene and 1,5-dimethylcyclohexene yield an identical mixture of cis- and trans-1,3-dimcthylcyclohcxanc on Pd153 (Table 7). On Pt and Rh, in contrast, the two isomeric dimethylcyclohexenes lead to products of different composition (Table 7). The adsorption modes of 1,3-dimethylcyclohexene is illustrated by the -adsorbed species 13 and 14 leading to cis- and trans-1,3-dimcthy Icyclohexane, respectively (equations 14 and 15)155. A comparison of 13 and 14 clearly indicates that the high cis/trans ratio... 

The jr-adsorbed species of 1,5-dimethylcyclohexene are those depicted as 15 and 16. The nearly equal amount of the isomeric products formed on Pt and Rh (Table 7) testifies to the almost equal degree of hindrance of the homoallylic methyl group with the catalyst surface in the alternate adsorption modes. 

The RCM of (+)-/l-citronellene (38, 2,6-dimethylocta-2,7-diene) in toluene (0.75 M), induced by 8, proceeds quantitatively at 20 °C to yield isomerically pure (S)-3-methylcyclopentene (39) see equation 29226. The same result can be achieved with rrans-WOCl2(OC6H3-Br2-2,6)2/Et4Pb as catalyst78. This is a remarkable improvement on the previous, very difficult synthesis of this compound. In concentrated solution in toluene (5 m), 39 undergoes ROMP when initiated by 8 at — 30 °C226. (S)-4-methylcyclohexene can be made in a similar way, as can 3,3-dimethylcyclohexene and 2-methylcyclohexene78. 

Therefore, for either antipode, the diastereomeric activated complex controlling optical yield could be either the one corresponding to the formation of the x-complex or the one corresponding to the olefin insertion into the metal-hydrogen bond. In the case of rhodium, it appears from the results of the hydroformylation of 1,2-dimethylcyclohexene and of 2-methylmethylidencyclohexane, that the second case is more probable 10). In the case of platinum, the fact that isomerization of the substrate, which is very likely to occur via metal alkyl-complex formation, proceeds at a rate similar to or even higher than the hydroformylation rate seems to indicate that the same situation can also be assumed. 

TABLE 3.14 The Stereoselectivity and Isomerization Ability of the Platinum Metals in the Hydrogenation of 1,6-Dimethylcyclohexene and 2-Methyl met hvlenecyclohexanet... 

The hydrogenation of methyl-substituted 1-ethoxycyclohexenes 75-79 over palladium catalyst affords predominantly less stable saturated ethers (Scheme 3.19).15,170,171 As described previously, the hydrogenation of the corresponding di-methylcyclohexenes over palladium always leads to predominant formation of more stable isomers, the proportions of which are also shown in Scheme 3.19 for comparison. Since extensive isomerization occurs prior to hydrogenation, isomeric ethoxycy-clohexenes 75 and 76, 77 and 78, as well as the corresponding dimethylcyclohexenes afford nearly the same results between the isomer pairs even at rather initial stages of... 

The hydrogenation of 1,2-dimethylcyclohexene (17 Pt02, HOAc, 1 atm) yields trans-(19) as well as cis-1,2-dimethylcyclohexane (18), but Ir and Os in t-butyl alcohol are much more selective (equation lb). The addition of hydrogen to (17) competes with isomerization to 1,6-dimethylcyclohexene (20), which not only reacts more rapidly but yields trans (19) as well as the cis product (18) via syn addition of the two hydrogen atoms (equation 17). In the Pt-catalyzed reaction, the intermediate (20) can be observed, although its steady-state concentration remains low (0.21% of the 1,2-isomer 17) because of its... 

A comparison of the proportion of the saturated stereoisomers, formed under the same conditions (Pt02, AcOH) from the isomeric xylenes and the derived dimethylcyclohexenes, leads to the proposal that the reaction proceeds through the desorbed cyclohexenes. Low concentrations of the intermediate cyclohexenes were detected later. The effect of the metal on the stereoselectivity of hydrogenating o-xylene follows closely the effect of the metal on the hydrogenation of 1,2- and 1,6-dimethylcyclo-hexene. The highest selectivity for the conversion of o-xylene to the cis isomer is given by iridium... 

Cg Aromatic Reactions with Hydrogen. The mild acid nature of the family of aluminophosphate based sieves renders them selective for a number of rearrangements as observed in the reactions of olefins and paraffins described above. This property as well as their apparent low disproportionation activity observed in the alkylation of toluene suggests that they be evaluated as the acid function in bifunctional Cg aromatic isomerization. As described above, cyclo-olefins are most likely involved in the conversion of ethylbenzene to xylenes. Strong acid functions, such as in mordenite, actively isomerize cyclo-olefinic intermediates but also catalyze ring-opening reactions which lead to loss of aromatics. A more selective acid function must still effectively interconvert ethyl cyclohexene to dimethylcyclohexenes but must leave the cyclohexene rings intact. 

However, such stereoselectivity is not always observed. For example, hydrogenation of 1,2-dimethylcyclohexene (3) yields trans-4 as well as cis-4, with the product ratio dependent on the metal and the reaction conditions employed 6 19. In contrast to osmium and iridium, which give nearly exclusively the expected co-product, hydrogenation in the presence of platinum results in a higher proportion of the tra .v-product. In the latter case hydrogenation competes with isomerization to 1,6-dimethylcyclohexene. 

In the experimental work which follows, the isomeric xylenes and the related tetrahydro derivatives, the isomers of dimethylcyclohexene, were hydrogenated to obtain more detailed stereochemical information than is presently available. 

If the hydrogenation reaction consisted solely of the one-sided addition of hydrogen to a double bond, then pure cfs-1,2-dimethylcyclohexane should be obtained from 1,2-dimethylcyclohexene however, only 77% of the expected isomer was formed. Conceivably, the introduced olefin might isomerize to one in which the groups are trans or to an olefin which can yield the trans isomer by a one-sided addition of hydrogen, for example,... 

The hydrogenation of substituted cyclic alkenes is anomalous in many cases in that substantial amounts of rran -addition products are formed, particularly with palladium catalysts. For example, the alkene 19 on hydrogenation over palladium in acetic acid gives mainly frans-decalin (7.12), and the alkene 1,2-dimethylcyclohexene 20 gives variable mixtures of cis- and trans-, 2-dimethylcyclohexane depending on the conditions (7.13). Similarly, in the hydrogenation of the isomeric dimethylbenzenes (xylenes) over platinum oxide, the cis-dimethylcyclohexanes are the main products, but some trans isomer is always produced. 

The Lewis acid catalysed addition of propiolate esters to olefins gave cyclobutenes stereospecifically. Thus, cis-but-2-ene and frans-but-2-ene gave 35 % and 31 % yields of isomerically pure cyclobutenes (90) and (9IX respectively, on treatment with methyl propiolate and aluminium trichloride at 25 °C. With 1,2-dimethylcyclohexene, the bicyclo[4,2,0]octene (92) was formed in 72% yield. Use of unsymmetrical olefins such as propene and but-l-ene gave mixtures of regioisomeric cyclobutenes, and 1,1-dialkylated olefins gave ene-addition products. 

As shown in Scheme 6.13, if the catalyst for the reduction is changed from Pt to Pd, the stereochemistry of the reduction is apparently altered. However, once it is recognized that Pd on alumina (aluminum oxide [AI2O3]) causes isomerization—a rearrangement vide infra) from one alkene to another—faster than reduction occurs, the result is explicable. Thus, (Scheme 6.13) 1,2-dimethylcyclohexene... 

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