Cyclohexene disproportionation

Summarizing all of the above, it would appear that 1,3- and 1,4-cyclohexadiene have nearly identical enthalpies of formation. Does this mean that the 1,3-isomer is destabilized and/or that the 1,4-isomer is stabilized Let us accept an enthalpy of formation of ca 105 kJmoD1 for the enthalpy of formation of both isomers. In the absence of any stabilization or destabilization, we would expect the cyclohexene disproportionation reaction 25... 

The results obtained in this study indicate that in Al-ffee H-boralite (BOR 1) only weak BrOnsted acid sites (Si—OH—B) are present. They are active only in cyclohexanol dehydration. Their catalytic activity is, however, relatively low. The insertion of A1 into the framework results in the creation of strong Bronsted acid sites. Most probably they are Si—OH—Al, the same as in zeolites. The IR band which could be characteristic of such Si—OH—Al (at about 3610 cm ) was not seen in the spectrum because of the very low concentration of these hydroxyls. The catalytic activity of Si—OH—Al is much higher that of Si—OH - B. Contrary to Si—OH -B, Si—OH— A1 are active in consecutive reactions of cyclohexene (isomerization and disproportionation). Cyclohexene isomerization (to methylcyclopentenes), a typical carbenium ion reaction is catalysed by strong Brdnsted acid sites even at temperatures as low as 450 K. The same strong Bronsted acid sites catalyse also cyclohexene disproportionation (to cyclohexane, methylcyclopentane and coke). Our earlier... 

Grubbs and coworkers (35) while examining Rh and Co catalysts derived from 14 reported the loss of infrared CO stretches and visual darkening of the catalysts after use for hydrogenation of olefins, aldehydes or ketones, cyclohexene disproportionation to benzene and cyclohexane or the cyclotrimerization of a wide variety of acetylenes. Stille (36) using a rhodium catalyst prepared from 14 observed activity for the hydrogenation of benzene that increased with reuse, a phenomenon usually associated with metal crystallite formation. Rhodium catalysts of 15 and 16 used to hydroformylate octene-1 revealed a loss of carbonyl adsorptions and a loss in catalytic activity upon reuse (37). 

It can be seen that primary and secondary R02 radicals disproportionate with the participation of the a-C—H bond. This explains why the substitution of D in the a-position for H retards the recombination of R02 [/tn//tD =1.9 for ethylbenzene, h/ d = 2.1 for styrene, and h/ d=1-37 for diphenylmethane [179]). Because of this, R02 radicals of unsaturated compounds with a double bond in the a-position to the peroxyl free valence disproportionate more rapidly than structurally analogous aliphatic peroxyl radicals (at 300 K, 2kt = 2x 107 and 3.8 x 106 L mol-1 s-1 for R02 radicals of cyclohexene and cyclohexane, respectively [180]). Among the products of secondary peroxyl radicals disproportionation, carbonyl compound and alcohol were found in a ratio of 1 1 at room temperature (in experiments with ethylbenzene [181], tetralin [103], and cyclohexane [182-184],... 

The disproportionation of 1,3-cyclohexadiene to benzene and cyclohexene in dioxan is catalysed by KOt-Bu (El-Kholy et al, 1977) according to reactions (31)-(33). The hydride transfer step (32) was found to be rate-determining. The presence of 18-crown-6 increased the overall rate of disproportionation by... 

The H atom detachment in Eq. (6) is followed by H atom abstraction from another cyclohexane molecule in Eq. (8) and the cyclohexyl radicals disappear in disproportionation and combination reactions giving in nearly equal amounts cyclohexene and dicyclohexyl end products. In the liquid phase, the secondary decomposition of the primarily formed energy-rich reaction products is of minor importance because of the effective collisional deactivation. This is in contrast to the gas phase reaction, where the primarily formed products readily undergo decomposition in the absence of deactivation (at low pressures), e.g. ... 

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... 

Ethylene coordinates too strongly to the ruthenium center to act as a hydrogen acceptor in this system, but slightly larger olefins do show activity. However, reaction selectivity is reduced compared with f-butyl ethylene. Thus, the use of cis and trans 2-pentene as hydrogen acceptor leads to products consisting of 90% carbosilane and 10% hydrosilylated olefin, and 1-hexene yields a carbosilane/hydrosilylation ratio of 4 6. In comparison, no hydrosilylation products are observed for dehydrocoupling in the presence of cyclohexene, but carbosilane formation is accompanied by disproportionation to benzene and cyclohexane as a side reaction. 

The reactivity of polyfluorinated cyclohexadienes toward antimony or niobium pentafluorides has some peculiarities. The equilibrium mixture of 1,4- and 1,3-cyclohexadienes is formed from pure isomers of each and a catalytic amount of MF5 at room temperature where the non-conjugated diene predominates [167], With an excess of SbF5 both isomers disproportionate to give a mixture of aromatic compounds and cyclohexene polyfluoroaromatics and the corresponding cyclohexene [167,168] ... 

Dehydrogenation catalysts.2 Reduction of di(benzene)chromium with potassium sand in DME generates a species that dehydrogenates 1,4-cyclohexadiene to benzene. The catalyst prepared from benzene(l,3-cyclohexadiene)iron(0) promotes dehydrogenation and disproportionation to cyclohexene and benzene. 

Figure 5.1 illustrates the key reactions implied in the manufacturing of cyclohexanone by phenol hydrogenation. The reactions are of consecutive type, in which the desired product is an intermediate. Small amounts of cyclohexene might appear at higher temperature by cyclohexanol dehydration. Additional reactions can lead to heavies by polymerization or benzene and cyclohexane by disproportionation. 

Collins et al. have studied a number of reactions in excess tetralin at 400°C (15). They reported 99% conversion of indene to indane after 1 hour and conversion of cyclohexene and 1-cyclohexenylbenzene to cyclohexane and cyclohexylbenzene after 18 hours. At 400°C values in Table V predict nearly complete (>90%) hydrogenation of both indene and 1-cyclohexenylbenzene after 1 hour and a conversion of cyclohexene to cyclohexane at a rate of 40% per hour. Molecular disproportionation is a feasible pathway for these reactions. 

Disproportionation is a special form of double bond migration in which the double bond is transferred from one molecule to another. Reactions of this type are especially liable to occur over palladium, and for this reason palladium sometimes is best avoided in olefin hydrogenation when the double bond is contained in an incipient aromatic system. Disproportionation activity in the hydrogenation of cyclohexene (and presumably other incipient aromatic systems will follow the same order) decreases with the metal in the order palladium >> platinum > rhodium (16). An example of the complication that can be caused by disproportionation during hydrogenation is found in the attempted reduction of... 

In the field of hydrocarbon conversions, N. D. Zelinskii and his numerous co-workers have published much important information since 1911. Zelinskii s method for the selective dehydrogenation of cyclohexanes over platinum and palladium was first applied to analytical work (155,351,438,439), but in recent years attempts have been made to use it industrially for the manufacture of aromatics from the cyclohexanes contained in petroleum. In addition, nickel on alumina was used for this purpose by V. I. Komarewsky in 1924 (444) and subsequently by N. I. Shuikin (454,455,456). Hydrogen disproportionation of cyclohexenes over platinum or palladium discovered by N. D. Zelinskii (331,387) is a related field of research. Studies of hydrogen disproportionation are being continued, and their application is being extended to compounds such as alkenyl cyclohexanes. The dehydrocyclization of paraffins was reported by this institute (Kazanskil and Plate) simultaneously with B. L. Moldavskil and co-workers and with Karzhev (1937). The catalysts employed by this school have also been tested for the desulfurization of petroleum and shale oil fractions by hydrogenation under atmospheric pressure. Substantial sulfur removal was achieved by the use of platinum and nickel on alumina (392). 

Petrov and Shchekin (297) showed that below the cracking temperature (250-316°C.) cyclohexene undergoes over silica-alumina hydrogen disproportionation and dimerization. Identical results were obtained from 1-methyl-l-cyclopentene. Ring expansion of lower alkylated cyclopentanes occurs simultaneously with polymerization. However, no bicyclic compounds with similar rings were formed. 

Complex 29 catalyzes the disproportionation of hydrogen peroxide to oxygen and water (48). In the absence of readily oxidizable substrates, it degrades. In the presence of olefins, however, it catalyzes the formation of epoxides. This catalysis was demonstrated for cyclohexene (1.6 turnovers), styrene (3.2 turnovers), and ds-stilbene (2.5 turnovers). The formation of the epoxides is not exclusive, because allylic oxidation... 

The related Nbra 1,3-hexadiene compound (ArO Nb QHs) (Ar = C6H3PrJ2) catalyzes the hydrogenation of cyclohexadiene to cyclohexane, as well as its disproportionation to cyclohexene and benzene.24... 

The use of water, which has strong OH-bonds, should eliminate hydrogen abstraction from the matrix. However, the yield of cyclohexane was still higher than that of cyclohexene. This result can be attributed to the formation of hydrogen atoms by reaction of some alkali metal atoms with water instead of with cyclohexyl bromide. These hydrogen atoms could then either combine with cyclohexyl radicals to give cyclohexane or add to cyclohexene (formed from a previous disproportionation reaction) to regenerate cyclohexyl radicals. 

The nature of the product varies with the olefin (1) cyclopentene gave the olefinic product only (2) cyclohexene and cycloheptene gave the saturated products only and (3) 1-heptene and 1-dodecene gave a mixture of the disproportionation products. 

The oxidation of cyclohexene has been the subject of considerable discussion, and it is now apparent that it behaves differently from the straight-chain olefins. Cyclohexene was originally reported to yield both cyclohex-2-en-l-yl acetate, structure (VII), and cyclohex-3-en-l-yl acetate, structure (VIII), in chloride-containing acetic acid (76) and only the allylic isomer with Pd(OAc)a in chloride-free acetic acid (6). However, it has now been demonstrated that if no oxidants are present to regenerate the Pd(0) to Pd(II) in neutral or basic HOAc, the Pd(0) formed will disproportionate the cyclohexene to give benzene (22, 295). In acetic acid containing perchloric acid, cyclohexanone (structure VIII) and cyclohex-1-en-l-yl acetate are formed (22). If Pd(0) is prevented from precipitating by use of oxidants in neutral or basic acetic acid, the allylic and homoallylic acetates are formed. 

The conversion of cyclohexanol on acid sites in zeolites and boralites is composed of two steps dehydration (to cyclohexene and water) and consecutive reactions of cyclohexene skeletal isomerization and disproportionation. Our IR and catalytic studies have shown that the dehydration occurs on both strong and weak Bronsted sites. On the other hand, only the strong Bronsted acid sites are required for isomerization and disproportionation. This observation may be used to propose a new ipethod for investigation of heterogeneity of acid sites in zeolites by a simple catalytic test. 

In the presence of 1 the catalytic alkoxylation of cyclohexene oxide with secondary and tertiary alcohols can be performed. This catalyst is known to cause disproportionation of epoxides, yielding olefins and diols. FT-IR spectroscopy indicated the formation of an active intermediate composed of 1 and epoxide. The carboca-tionic intermediate species is highly reactive with respect to nucleophilic compounds [28]. 

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