Dehydrogenation proposed mechanism

Figure 12.4 Formamide synthesis via methanol dehydrogenation proposed mechanism.

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. 

In the dehydrogenation of C2H6 to produce C2H4, CH is a minor coproduct this is also reflected in the second step of the mechanism hence, both the overall reaction and the proposed mechanism do not strictly represent a simple system. 

The proposed mechanisms are similar in both cases and involve in particular an (aryl)(hydrido)ruthenium intermediate in which the ruthenium is additionally coordinated by an in. ( ////-generated /V-phenylimine moiety tethered to the same Ru-bound aromatic ring. The C-C bond-forming step for the construction of the corresponding heterocyclic framework proceeds via insertion of the C=N double bond into the C-Ru bond with transfer of the (hydrido) ruthenium complex to the now phenylamine nitrogen. The desired heterocycles 158 and 159 were obtained after successive reductive elimination, deamination, and dehydrogenation. 

Figure 12 Proposed mechanism of decarboxylation and dehydrogenation of the C-terminal Cys of EpiA catalyzed by EpiD. Adapted with permission from C. Chatterjee M. Paul L. Xie W. A. van der Donk, Chem. Rev. 2005, 105, 633-684.

Scheme 1 Proposed mechanism of alkane dehydrogenation with acceptors...

Two different mechanisms have been proposed for this dehydrogenative silylation process. The first mechanism proposed by Oro, Esteruelas and coworkers includes the oxidative addition of 1-alkyne to the Ir—Si bond, followed by the reductive elimination of 151 (equation 61)117,118. The proposed mechanism is supported by the identification of [IrH(C=CPh)( j2-( -Pr)2PCH2CH20Me)]BF4 in stoichiometric as well as catalytic conditions by 31P 1H NMR analyses118. The other mechanism proposed by Jun and Crabtree includes the insertion of 1-alkyne into the Ir—Si bond, followed by isomerization and /J-hydride elimination (equation 62)113, which is consistent with the mechanism proposed for the highly selective formation of (Z)-l-silyl-l-alkenes (see Section IILB)115. 

Schemes. Proposed mechanisms for the hydrosilylation and dehydrogenative silylation of phenylaceylene by an iridium catalyst, m/z values are given for cationic species observed by ESI(+)-MS.

Scheme 2 A general proposed mechanism for the rhodium-catalyzed dehydrogenative borylation of alkenes using HBcat...

Scheme 8.4 Proposed mechanism for the metal catalyzed dehydrogenation of ammonia borane.

Scheme 8.5 Proposed mechanism for the acid-initiated dehydrogenation of ammonia borane in solution.

Equation (2) explains the transient deactivation with a model describing reversible and irreversible coke. We can see that the partial pressure of propane in the reactor does not influence the deactivation. This has also been demonstrated in an earlier study of the same system [8]. This observation is consistent with kinetic models for propane dehydrogenation proposed by Loc et al. [13]. They suggested that the rate-determining step is the dissociative adsorption of propane. From this mechanism it follows that the deactivation will be... 

The proposed mechanism for the dehydrogenative silylation includes dialkyl-M (M = Ru or Rh) species as the key intermediate ". 

Mechanism. A proposed mechanism for the paring reaction is given below. The first step is the formation of a carbonium ion intermediate, either by dehydrogenation followed by protonation of the double bond or by hydride ion transfer to another carbonium ion. The adsorbed... 

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