1.4- cyclohexadiene dehydrogenation

Cyclohexadiene and benzene form identical structures on Pt(l 1 1) at low pressures (Figures 7.23 and 7.24). 1,3-Cyclohexadiene dehydrogenates to form benzene on the surface, while benzene adsorbs molecularly. Figure 7.24b schematically shows the adsorbed benzene structure at low pressure. The STM images of the C6 cyclic hydrocarbons show three different adsorbed structures on Pt(l 1 1). Cyclohexene and cyclohexane partially dehydrogenate to form rc-allyl, 1,4-cyclohexadiene adsorbs in a boat configuration, and both 1,3-cylohexadiene and benzene adsorb as molecular benzene on the surface. 

The use of base catalysis for various reactions, such as olefin isomerization, cyclohexadiene dehydrogenation, aromatic alkylation, and olefin polymerization, has been demonstrated. In some cases, compounds are formed which are difficult to prepare by other means, and some highly selective reactions have been found. 

Of special interest for petrochemical and organic synthesis is the implementation of thermodynamically hindered reactions, among which incomplete benzene hydrogenation or incomplete cyclohexene and cyclohexadiene dehydrogenation should be mentioned. Cost-effective methods of cyclohexene production would stimulate the creation of new processes of phenol, cyclohexanol, cyclohexene oxide, pyrocatechol synthesis, cyclohexadiene application in synthetic rubber production, and a possibility for designing caprolactam synthesis from cyclohexene and cyclohexadiene via combined epoxidation. At present, the most... 

The adsorption of 1,3-cyclohexadiene on Pt(lll) leads to irreversible dehydrogenation to benzene below 400 K, the majority of which is further dehydrogenated at higher temperatnres to form a carbonaceons residue [48], No 1,3-cyclohexadiene desorbed dnring TPD from the Sn/Pt(l 11) alloys, but the monolayer was converted with 100% selectivity to prodnce gas-phase benzene. No carbon was left on the alloy snrfaces after TPD as determined by AES. The ensemble requirement for cyclohexadiene dehydrogenation on these alloys is at most a few (4-5) Pt atoms (see Pig. 2.1). 

The formed methylcyclohexane carbocation eliminates a proton, yielding 3-methylcyclohexene. 3-Methylcyclohexene can either dehydrogenate over the platinum surface or form a new carbocation by losing H over the acid catalyst surface. This step is fast, because an allylic car-bonium ion is formed. Losing a proton on a Lewis base site produces methyl cyclohexadiene. This sequence of carbocation formation, followed by loss of a proton, continues till the final formation of toluene. 

An alternative new synthetic approach to chrysene 1,2-dihydro-diol based on Method IV has recently been developed (60). This method (Figure 12) entails synthesis of 2-chrysenol via alkylation of 1-1ithio-2,5-dimethoxy-1,4-cyclohexadiene with 2-(1-naphthyl) e-thyl bromide followed by mild acid treatment to ge nerate the diketone 12. Acid-catalyzed cyclization of 12 gave the unsaturated tetracyclic ketone 13 which was transformed to 2-chrysenol via dehydrogenation of its enol acetate with o-chloranil followed by hydrolysis. Oxidation of 2-chrysenol with Fremy s salt gave chrysene... 

More convenient synthetic access to 15 is provided by the sequence in Figure 13 (68). Alkylation of the potassium salt of 2,6-dimethoxy-1,4-cyclohexadiene with 2-(2-naphthyl)ethy1 bromide in liquid ammonia followed by mild acidic hydrolysis generated the diketone (16). Cyclization of 16 in polyphosphoric acid took place smoothly in the desired direction to afford the partially saturated ketone which underwent dehydrogenation with DDQ to 15. 

A totally different route based on dehydrogenation of a saturated polymer precursor was introduced by Francois et al. [49] (Scheme 2.9). The method is based on anionic copolymerization of cyclohexadiene with styrene, followed by oxidation with chloranil. Due to possible coupling of two styrene (or two cyclohexadiene) molecules, a block copolymer, containing oligo(phenylene vinylene) units separated by oligo(phenylacetylene) and oligo(phenylene) blocks, is obtained. To the best of our knowledge, it was, so far, used only in the synthesis of phenyl-substituted PPV 10. 

There seems to be no great difference in the free energy between acyclic triene and the cyclic diene. This is because of smaller strain in the six-membered ring as compared with the four-membered one. On the other hand in 6n electron system in electrocyclic process there is more efficient absorption in the near regions of u.v. spectrum. This is why under both thermal and photochemical conditions, the (1, 6) electrocyclic reactions are reversible. Side reactions are more frequent in reversible. Side reactions are more frequent in reversible transformations of trienes than in dienes. The dehydrogenation of cyclic dienes to aromatic compounds may also occur in the thermal process. On heating cyclohexadiene yields benzene and hydrogen. 

Cyclohexaamylose-N-methylacetohydroxamic acid, preparation of, 23 254 Cyclohexadienes, 20 293 reaction with HCN, 33 19, 20 on silica, reactions of, 34 54-56-34 64 cracking, 34 55, 72 vibrational spectra, 42 243 Cyclohexadienyl radicals, ESR of, 22 300 1,4-Cyclohexanediols, conversion of ethers, mechanism, 35 361-364 Cyclohexanes, 33 101, 102, 103 autoxidauon of, 25 303 conformational analysis of, 18 9-17 dehydrogenation, 31 14, 21-22 benzene accumulation over platinum, 36 18... 

The dehydrogenation of the cyclohexadiene species into aromatics should be catalytic. 

Cyclohexadienes are available by the methodology of Birch, and the reactions of l-methoxy-, 1,3-dimethoxy-, l,3-bis(trimethyl-silyloxy)-, and l-methoxy-4-methyl-l,3-cyclohexadiene with a number of 1,4-benzoquinones have been investigated. Acid treatment of the adducts and subsequent dehydrogenation provides a synthesis of 2-dibenzofuranols. Thus the adduct 159 (Scheme 41) from 1,4-benzoquinone and 1,3-dimethoxy-1,3-cyclohexadiene, on treatment with a trace of concentrated hydrochloric acid in ethanol at room temperature, affords the tetrahydrodibenzofuranone 161. When the adduct 159 is heated under reflux in aqueous methanol, the reaction can be arrested at the dihydrodibenzofuran 160. The tetrahydrodibenzofuranone 161 on dehydrogenation with palladized charcoal affords 2,7-dibenzofurandiol. ... 

The adsorption of cyclohexadiene on the Pt(l 11) surface produces the same two surface structures that were found during the adsorption of benzene on this crystal face Thus, this molecule readily dehydrogenates on this platinum surface to benzene at 300 K. 

The natural product, anhydrolycorinone was prepared in the palladium mediated intramolecular Heck reaction of a tetrahydroindole derivative. The coupling, which is run in air and requires the use of a stoichiometric amount of palladium, is accompanied by the dehydrogenation of the cyclohexadiene moiety and the oxidation at the benzylic position to a lactam (4.11.).12... 

Ozonolysis of cyclic olefins in the presence of carbonyl compounds gives the corresponding cross-ozonides.1329 In the ozonation of 1,2,4,5-tetramethyl-1,4-cyclohexadiene, oxidative dehydrogenation (formation of 1,2,4,5-tetramethylben-zene) was found to compete with oxidative cleavage because of steric hindrance.1330 Secondary ozonides (the 76 1,2,4-trioxolanes) are formed in high yields in the gas-phase, low-temperature ozonation of terminal and disubstituted alkenes.1331... 

The reaction of indolizines with dialkyl acetylenedicarboxylates in the presence of a dehydrogenating catalyst leads to 1,2-dicarbalkoxycycl-[3,2,2]azines.22 23 Methyl phenylpropiolate may be used instead, although attempts to effect reaction between indolizine and certain other dienophiles including diphenylacetylene, diethyl azodicarboxylate, and 1,3-cyclohexadiene were unsuccessful. Hydrolysis of the diesters yielded the corresponding acids. Subsequent decarboxylation proceeded in high yield using copper chromite in quinoline [Eq. (5)]. 

PV2M010O40 has usually been used in the acid form. H5PV2M010O40 catalyzes aerobic oxidative cleavage of cycloalkanes, 1-phenylalkanes, and ketones. For example, the oxidation of 2,4-dimethyl cyclopentanone and 2-methylcyclo-hexanone gives 5-oxo-3-methylhexanoic acid and 6-oxoheptanoic acid, respectively, in yields higher than 90% [285, 286). Bromination of arenes with HBr [287), oxidative dehydrogenation of cyclohexadiene [288, 289) and a-terpinene [290), oxidation of 2,4-dimethylphenol [291) and sulfides [292) are other examples. 

Much experimental evidence indicates that Pd mediates the formation of a donor-acceptor complex followed by a direct hydrogen transfer36. This process, however, was disproved in the disproportionation of 1,4-cyclohexadiene (a special case of transfer hydrogenation) over colloidal nickel40. Another possibility is a consecutive dehydrogenation-hydrogenation process. 

It was found that in these reactions free hydrogen is not extracted. The results of these works allowed the authors to conclude that two processes proceeded in the reaction system dehydrogenation leading to benzene formation and hydrogenation responsible for cyclohexane accumulation. Both these reactions proceed simultaneously and are conjugated. The catalyst action is reduced to distribution of mobile hydrogen between three cyclohexadiene molecules. 

Figure 4.5 Kinetic curves of cyclohexane conjugated dehydrogenation (1 cyclohexane 2 cyclohexene 3 benzene and 4 cyclohexadiene).

Minachev et al. [76] studied oxidative dehydrogenation of cyclohexane on zeolite cationic forms at 300-475 °C, the main reaction product of which is cyclohexene. Cyclohexadiene and C02 are also formed, and at long-term contacts benzene is detected. Cyclohexene yield and selectivity of the reaction depend on zeolite structure and composition, reaction temperature and oxygen cyclohexane ratio in the reaction mixture. Among alkaline cationic forms of zeolite, the highest cyclohexene yield (21%) is observed for NaA zeolite (66% selectivity). 

One may suggest that the reaction products (cyclohexene, cyclohexadiene and benzene) are synthesized in a step-by-step dehydrogenation of higher saturated analogs. In this connection,... 

The first polymer was shown to be active for the dehydrogenation of several substrates, while the second one was entirely inactive under the same conditions. A free radical mechanism could be proven, using the substrate 5-ethyl-5-methyl-1,3-cyclohexadiene. 

The 18 7r-electron structure has the stability of an aromatic ring according to the Hiickel 4n + 2 rule, while the others may be considered to be active like the semiquinone or quinone. So, when an oxidizing agent can be found to oxidize the aromatic molecule, a structure is formed which may abstract a hydrogen atom, by which it is reduced again. A mixture of nitrobenzene and cyclohexadiene-1,4 suited this purpose, and complexes of phthalocyanine and tetraphenylporphyrin were found to catalyze the oxidative dehydrogenation of cyclohexadiene by nitrobenzene t00> ... 

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