DEHYDROGENATION CENTER

In the above diagram, H-D refers to hydrogenation-dehydrogenation centers and A to acidic centers on the catalyst. The reaction sequence involves successive ring contraction and expansion steps, similar to the mechanism proposed by Pines and Shaw (P4) to account for transfer of tagged carbon from the side chain to the ring when ethylcyclohexane was contacted with a nickel-silica-alumina catalyst. 

In discussing the mechanism of hydrocarbon transformations on bifunctional catalysts, it is useful to refer to a reaction scheme originally proposed by Mills et al. (16) to describe the reforming of C6 hydrocarbons. The scheme is shown in Figure 5.2. The vertical reaction paths in the figure take place on the hydrogenation-dehydrogenation centers of the catalyst and the horizontal reaction paths on the acidic centers. 

In the reaction scheme in Figure 5.2, the dehydrocyclization of n-hexane proceeds via formation of n -hexene on dehydrogenation centers, followed by cyclization of the n-hexene to methylcyclopentane on acidic centers. The methylcyclopentane then is converted to benzene in the manner already described. Alternatively, it seems possible that a hexadiene may be an intermediate in the reaction sequence. Such a sequence would involve formation of a hexadiene on platinum sites, followed by cyclization on acidic centers to form a cyclic olefin, methylcyclopentene (1). 

When additional substituents ate bonded to other ahcycHc carbons, geometric isomers result. Table 2 fists primary (1°), secondary (2°), and tertiary (3°) amine derivatives of cyclohexane and includes CAS Registry Numbers for cis and trans isomers of the 2-, 3-, and 4-methylcyclohexylamines in addition to identification of the isomer mixtures usually sold commercially. For the 1,2- and 1,3-isomers, the racemic mixture of optical isomers is specified ultimate identification by CAS Registry Number is fisted for the (+) and (—) enantiomers of /n t-2-methylcyclohexylamine. The 1,4-isomer has a plane of symmetry and hence no chiral centers and no stereoisomers. The methylcyclohexylamine geometric isomers have different physical properties and are interconvertible by dehydrogenation—hydrogenation through the imine. 

This impressive reaction is catalyzed by stearoyl-CoA desaturase, a 53-kD enzyme containing a nonheme iron center. NADH and oxygen (Og) are required, as are two other proteins cytochrome 65 reductase (a 43-kD flavo-protein) and cytochrome 65 (16.7 kD). All three proteins are associated with the endoplasmic reticulum membrane. Cytochrome reductase transfers a pair of electrons from NADH through FAD to cytochrome (Figure 25.14). Oxidation of reduced cytochrome be, is coupled to reduction of nonheme Fe to Fe in the desaturase. The Fe accepts a pair of electrons (one at a time in a cycle) from cytochrome b and creates a cis double bond at the 9,10-posi-tion of the stearoyl-CoA substrate. Og is the terminal electron acceptor in this fatty acyl desaturation cycle. Note that two water molecules are made, which means that four electrons are transferred overall. Two of these come through the reaction sequence from NADH, and two come from the fatty acyl substrate that is being dehydrogenated. 

This hypothesis would agree also with the results of the study of the influence of added substances on some other reactions (124). As an example, dehydration of cyclohexanol on alumina at 220°C is retarded by cyclohexanone, the dehydrogenation of cyclohexanol to cyclohexanone (the second reaction branch) not occurring with this catalyst at all. Hence, cyclohexanone is adsorbed on dehydration centers, on which the reaction which would lead to its formation does not take place at all. A similar result was obtained also for the second reaction branch, the dehydrogena-... 

The proposed mechanism of H2 evolution by a model of [FeFeJ-hydrogenases based upon DFT calculations [204-206] and a hybrid quanmm mechanical and molecular mechanical (QM/MM) investigation is summarized in Scheme 63 [207]. Complex I is converted into II by both protonation and reduction. Migration of the proton on the N atom to the Fe center in II produces the hydride complex III, and then protonation affords IV. In the next step, two pathways are conceivable. One is that the molecular hydrogen complex VI is synthesized by proton transfer and subsequent reduction (Path a). The other proposed by De Gioia, Ryde, and coworkers [207] is that the reduction of IV affords VI via the terminal hydride complex V (Path b). Dehydrogenation from VI regenerates I. 

Most in vitro studies of xanthines have centered around the enzyme xanthine oxidase. Bergmann and co-workers 40-4)) have examined the main oxidative pathways in the xanthine oxidase catalyzed oxidation of purines. The mechanism proposed by these workers 41 > is that the enzyme binds a specific tautomeric form of the substrate, regardless of whether or not that form represents the major structure present in solution. It is then proposed that the purine, e.g., xanthine, undergoes hydration at the N7=C8 double bond either prior to or simultaneously with dehydrogenation of the same position. Accordingly, the process would involve either pathway a or b. Fig. 15. Route a would give a lactim form of the oxidized purine, while b would give the cor-... 

Considerable interest in the subject of C-H bond activation at transition-metal centers has developed in the past several years (2), stimulated by the observation that even saturated hydrocarbons can react with little or no activation energy under appropriate conditions. Interestingly, gas phase studies of the reactions of saturated hydrocarbons at transition-metal centers were reported as early as 1973 (3). More recently, ion cyclotron resonance and ion beam experiments have provided many examples of the activation of both C-H and C-C bonds of alkanes by transition-metal ions in the gas phase (4). These gas phase studies have provided a plethora of highly speculative reaction mechanisms. Conventional mechanistic probes, such as isotopic labeling, have served mainly to indicate the complexity of "simple" processes such as the dehydrogenation of alkanes (5). More sophisticated techniques, such as multiphoton infrared laser activation (6) and the determination of kinetic energy release distributions (7), have revealed important features of the potential energy surfaces associated with the reactions of small molecules at transition metal centers. 

A hydrogen termination of the crystallite surface is assumed in most studies. Calculations show that a small part of the hydrogen can be removed without the formation of DBs, as shown for the transformation of Si29H36 to SijjH [Hi-4-]. Further dehydrogenation, however, leads to the formation of DBs that are efficient recombination centers and thus effectively suppress the luminescence [De2, Hi4, De5]. The observed IR luminescence has been proposed to be due to recombination via DB centers [De3]. 

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