Reaction scheme dehydrogenation

The dimer of 1-methyl- -pyrroline (39) was obtained by reduction of N-methylpyrrole with zinc and hydrochloric acid (132) and, together with the trimer, by mercuric acetate dehydrogenation of N-methylpyrrolidine (131). J -Pyrroline-N-oxides form dimers in a similar manner (302). Treatment of 1,2-dimethyl-zl -piperideine with formaldehyde, producing l-methyl-3-acetylpiperidine (603), serves as an example of a mixed aldol reaction (Scheme 18). 

Bischler-Napieralski reaction of 139 to a 3,4-dihydroisoquinoline, oxidation, dehydrogenation and reduction of the nitro to the amino function gave 140 which was subjected to a Pschorr reaction (Scheme 49). Quaternization was accomplished by methyl iodide to furnish the isoquinolininium salt 141 which underwent an ether cleavage on heating a solid sample or benzene or DMF solution to Corunnine (127) (73TL3617). 

The reaction scheme is rather complex also in the case of the oxidation of o-xylene (41a, 87a), of the oxidative dehydrogenation of n-butenes over bismuth-molybdenum catalyst (87b), or of ethylbenzene on aluminum oxide catalysts (87c), in the hydrogenolysis of glucose (87d) over Ni-kieselguhr or of n-butane on a nickel on silica catalyst (87e), and in the hydrogenation of succinimide in isopropyl alcohol on Ni-Al2Oa catalyst (87f) or of acetophenone on Rh-Al203 catalyst (87g). Decomposition of n-and sec-butyl acetates on synthetic zeolites accompanied by the isomerization of the formed butenes has also been the subject of a kinetic study (87h). 

The most frequent synthetic approaches, summarized in Scheme 4, are towards the primary photophores. The preparation of aryl azide derivatives follows the typical retro-synthetic pathway in the majority of the reported cases (Scheme 4 A), and, practically, diazotation is the most commonly used procedure [24 - 29]. In the case of diazirines only one major synthetic sequence is repeated ammonolysis of oximes followed by dehydrogenation (Scheme 4B) [30-32]. There are various ways of preparing diazo- or diazocarbonyl-compounds most frequently the Forster and Bamford-Stevens reactions (Scheme 4C) are employed [33-37]. 

Cycloisomerization represents another approach for the construction of cyclic compounds from acyclic substrates, with iridium complexes functioning as efficient catalysts. The reaction of enynes has been widely studied for example, Chatani et al. reported the transformation of 1,6-enynes into 1-vinylcyclopentenes using [lrCl(CO)3]n (Scheme 11.26) [39]. In contrast, when 1,6-enynes were submitted in the presence of [lrCl(cod)]2 and AcOH, cyclopentanes with two exo-olefin moieties were obtained (Scheme 11.27) [39]. Interestingly, however, when the Ir-DPPF complex was used, the geometry of olefinic moiety in the product was opposite (Scheme 11.28) [17]. The Ir-catalyzed cycloisomerization was efficiently utilized in a tandem reaction along with a Cu(l)-catalyzed three-component coupling, Diels-Alder reaction, and dehydrogenation for the synthesis of polycyclic pyrroles [40]. 

Attention will be focussed on three typical chemical reaction schemes. For the first illustration, two parallel competing reactions are considered. For instance, it may sometimes be necessaru to convert into a desired product only one component in a mixture. The dehydrogenation of six-membered cycloparaffins in the presence of five-membered cycloparaffins without affecting the latter is one such example of a selectivity problem in petroleum reforming reactions. In this case, it is desirable for the catalyst to favour a reaction depicted as... 

VII. Generalized Reaction Scheme for Oxidative Dehydrogenation of Alkanes... 

The data presented above showed that the oxidative dehydrogenation reactions of the various alkanes share many common features. Thus it is tempting to discuss selectivity for alkane oxidative dehydrogenation with a common scheme. The reaction scheme for ethane oxidation [Eqs. (5)-(7)] provides a useful basis for such a discussion. It shows that the primary reaction of alkane oxidation can take on three different pathways depending on the reaction temperature (Scheme I). The first step in all three pathways is breaking a C—H bond, which is the rate-limiting step. The three pathways are described below. 

In a more recent study of the dehydrogenation of cyclohexane to benzene over a chromium oxide catalyst at 450°C., Balandin and coworkers (Dl) concluded that benzene was formed by two routes. One of these, the so-called consecutive route, involves cyclohexene as a gas phase intermediate, while the other proceeds by a direct route in which intermediate products are not formed in the gas phase. It was concluded that the latter route played a larger role in the reaction than did the former. These conclusions were derived from experiments on mixtures of cyclohexane and Cl4-labeled cyclohexene, which made it possible to evaluate the individual rates Wi, BY, Wt, and Wz in the reaction scheme... 

During studies on the oxidative dehydrogenation of complexes such as (69), it was discovered that ring bromination could occur when bromine was used as the oxidizing agent. Reaction conditions could be varied so as to optimize either reaction (Scheme 30).157 Oxidative dehydro-... 

Because of the conjugation of the free electron pair on the nitrogen atom with the double bond, enamines react very readily with double bonds activated by electronegative groups. Addition of acrolein to l-methyl-2-ethylidenepyrrolidine, followed by dehydrogenation, led to 1,7-dimethylindole.260 In a similar addition to l-methyl-2-alkyl-d2-piperideines, l-methyl-8-alkyl-l,2,3,4-tetrahydroquinolines (75) were obtained.38 In the reaction scheme, the starting bases are considered to react as the tautomeric l-methvl-2-alkvlidenepiperidines (74). 

Kinetic Parameters for the Horiuti-Polanyi Reaction Scheme for Isobutane Dehydrogenation and Isobutylene Hydrogenation... 

Fig. 19 Reaction scheme for segmented PPV (3), by dehydrogenation of soluble precursor polymer (2), using DDQ. (2) is deposited by CVP of (1)7 Tpi pyrolysis temperature, T substrate temperature during CVD.

The same condensation-cyclization sequence with 4-methoxy-phenylhydrazine (292), however, proceeded in only 17% yield (113), so an alternative synthesis was devised for the important 8-oxygenated derivatives, such as 293 and 294, which employed a more conventional Fischer indole reaction (Scheme 47) (113). In the event, condensation of keto ketal 295 with hydrazine 280 gave hydrazone 296. Cyclization in hot diphenyl ether gave ketone 297, which, upon dehydrogenation, protection of the phenol as the benzoate, and chlorination gave 298. Deprotection and/or methylation afforded the target chloro-y-carbolines (299-301). 

Another mechanism for olefin disproportionation had been proposed earlier by these same authors (29). In this reaction scheme, the olefins first undergo dehydrogenation to acetylenes which subsequently cyclize to cyclobutadiene surface intermediates. The cyclobutadiene intermediate is suggested as the quasi-cyclobutane proposed by Bradshaw et al. (27). There is, however, no evidence supporting a mechanism involving acetylene or cyclobutadiene intermediates. 

The paraffin dehydrogenation reaction scheme is shown in Fig. 2. Paraffins are dehydrogenated to form mono-olefins with the double bond distributed according to thermodynamics (less than 10% in the a position). The extent of the reaction is largely controlled by thermodynamic equilibrium, and typical paraffin conversion levels are limited to 10-15%. The reaction is typically carried out at low pressure to enhance the equilibrium in favor of olefin production. 

The reaction scheme involves initial formation of an ammine complex that can provide amide ions for reaction with benzyl carbonium ions formed by hydride ion abstraction from toluene. The resulting coordinated benzylamine is displaced from the complex by NH3, and subsequently rapidly dehydrogenated to benzonitrile. In support of this mechanism, it has been demonstrated that the events depicted in Eq. (6) occur very rapidly over ZnX catalyst at 500°. In addition, the use of an olefin to provide carbonium ions for abstraction of hydride ions from toluene enhances the overall reaction rate. 

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