Catalytic cycle for the dehydrogenation

The proposed catalytic cycle for the dehydrogenation of alcohols to ketones is shown in Scheme 15. The initial reaction of 17 with H2O affords the hydride complex a and C02- Dehydrogenation of a by acetone gives the active species b and 2-propanol. The subsequent reaction of b with the alcohol yields the corresponding ketone and regenerates a to complete the catalytic cycle. 

The generation of methane in the reaction was evidenced by the 111 NMR spectrum of the reaction mixture. It was also shown that the newly obtained complex 29 reacts catalytically with silanol 28 to give the trimer 30 (presumably from trimerization of 31) with the evolution of hydrogen gas. In the presence of MesSiOMe the same reaction resulted in the formation of an insertion product of the intermediate silanone 31 as shown in the lower part of Scheme 12. The proposed catalytic cycle for the dehydrogenation of 28 with 29 is shown in Scheme 13. It should be noted, however, that spectroscopic evidence for the proposed silanones was not presented. 

Proposed catalytic cycle for the dehydrogenation of McjNH BHj with a Zr-Ru heterobimetal-... 

Fig. 2. Postulated catalytic cycle for the dehydrogenation of alkanes by L2ReH7 [35,37].

Figure 3 Catalytic cycle for the (I) hydroxylation of monophenols and (II) dehydrogenation of o-diphenols to o-quinones by tyrosinase. M = monophenol and D = diphenol bound forms. (Adapted from Ref. 42.)... 

Considering that (methyldichlorosilyl)bis(dichlorosilyl)methane 139 has two Si-H bonds, these workers first attempted to prepare the 1,3-disilacyclopentane. Surprisingly, l,3-disilacyclopent-4-ene 141 was obtained in very good yield when tetrakis(triphenylphosphine)palladium was used as a catalyst. On the basis of the results, a plausible catalytic cycle for this dehydrogenative double-silylation reaction was proposed (Scheme 17). 

The combination of Co Kbpy)2 -O2 with these substrates yields product profiles that are similar, which indicates that the reaction path probably involves the same reactive intermediate (species 2, Scheme 6-2). Scheme 6-2 outlines catalytic cycles for the demethylation of N-methyl anilines, and the dehydrogenation of benzylamine and benzyl alcohols. In the case of the latter catalyst [Con(bpy> ] is inactive and must be neutralized with one equivalent of HO-... 

The Cu(II) complex with polyaniline (emeraldine base) exhibits a higher catalytic efficiency for the dehydrogenative oxidation of cinnamyl alcohol into cin-namaldehyde. Iron(III) chloride is similarly used instead of copper(II) chloride. The catalytic system is applicable to the decarboxylative dehydrogenation of man-delic acid to give benzaldehyde. The cooperative catalysis of polyaniline and cop-per(II) chloride operates to form a reversible redox cycle under oxygen atmosphere as shown in Scheme 3.4. The copper salt contributes to not only oxidation process but also metallic doping. The reduced phenylenediamine anionic species appear to be stabilized by the metallic dopants. 

The postulated catalytic cycle for the direct amidation by reaction of alcohols and amines could be envisioned as similar to the catalytic cycle for esterificatiOTi reaction of alcohols with intermediates being the hemiaminal instead of hemiacetal (Fig. 5). The intermediate aldehyde, generated in the alcohol dehydrogenatimi cycle, reacts with the primary amine to form an intermediate hemiaminal, which tmdergoes dehydrogenation to form an amide. 

Our collective mechanistic studies are consistent with the indicated catalytic cycle. Notably, the catalyst engages primary alcohols in rapid and reversible dehydrogenation, yet the coupling products, which are homoallylic alcohols, are not subject to oxidation as coordination of the homoallylic olefin to the catalyst provides a hexa-coordinate 18-electron complex lacking an open coordination site for p-hydride elimination (Scheme 14). 

Whilst the enhancement of unwanted side reactions through excessive distortion of the concentration profiles is an effect that has been reported elsewhere (e.g., in reactive distillation [40] or the formation of acetylenes in membrane reactors for the dehydrogenation of alkanes to olefins [41]), the possible negative feedback of adsorption on catalytic activity through the reaction medium composition has attracted less attention. As with the chromatographic distortions introduced by the Claus catalyst, the underlying problem arises because the catalyst is being operated under unsteady-state conditions. One could modify the catalyst to compensate for this, but the optimal activity over the course of the whole cycle would be comprised as a consequence. 

A further method for the synthesis of the title compounds with only hydrogen as byproduct is the base-catalyzed dehydrogenative coupling (index D) of ammonia and tris(hydridosilylethyl)boranes, B[C2H4Si(R)H2]3 (R = H, CH3). Initially, the strong base, e.g. n-butyl lithium, deprotonates ammonia. The highly nucleophilic amide replaces a silicon-bonded hydride to form a silylamine and lithium hydride, which then deprotonates ammonia, resuming the catalytic cycle. Under the conditions used, silylamines are not stable and by elimination of ammonia, polysilazane frameworks form. In addition, compounds B[C2l-L Si(R)H2]3 can be obtained from vinylsilanes, H2C=CHSi(R)H2 (R - H, CH3), and borane dimethylsulfide. 

During the early years of development of the multiplet theory, attention was paid chiefly to the correspondence of the structure of reacting molecules and catalyst, especially in relation to the sextet model of dehydrogenation of six-membered cycles on metal catalysts. This work permitted the determination of the group of metals that can act as catalysts for the dehydrogenation of cyclohexane (the so-called Blandin s square of activity ) and the prediction of catalytic activity, e.g., for Re which was unknown as a catalyst for this reaction. 

Fig. 2.6. The catalytic cycle proposed for the dehydrogenation of alkane RCH2CH3 to give the alkene RCH=CH2 by an iridium complex.

The same cycle is followed during the reactions of linear alkanes to form linear alk-enes. Although the thermod)mamics for dehydrogenation of cyclooctene are more favorable than those for the dehydrogenation of linear alkanes, primary C-H bonds typically undergo oxidative addition faster than secondary C-H bonds, as discussed in Chapter 6. Thus, linear alkanes react faster than cyclic alkanes. However, the accumulation of a-olefin inhibits the catalytic process. An T) -olefin complex formed from the a-olefin becomes the resting state of the catalytic cycle for reactions catalyzed by the POCOP system, instead of the vinyl hydride complex that is the resting state of the PCP system. The accumulation of the olefin complex that lies off the cycle leads to a lower concentration of the iridium complexes within the cycle and slower reactions as the concentration of a-olefin product increases. 

New TOFs for ethanol were also obtained using 3.1 ppm catalyst (Ru L= 1 1) without base 1,483 h at 2 h became the highest TOF to date for dehydrogenation of this primary alcohol. The authors also propose a mechanism (Scheme 6), which suggests an outer-sphere process for the dehydrogenative step (step C) of the catalytic cycle, whereby a Ru-amide intermediate undergoes H-H addition across the Ru-N bond. 

A mechanism involving a Pd(ll)/Pd(lV) catalytic cycle is proposed for the dehydrogenative double addition (Scheme 8). Isolation of dimeric Pd(ll) complex 3 upon treatment of HP(0)(pin) with (77 -allylPdCl)2 and its reaction with 1-octyne forming 76% of 2 support the mechanism. A model complex of a structure somewhat different from 3 shows phosphopalladation with 1-octyne, which is provided to rationalize the transformation from 3 to 2 via the elemental steps shown in Scheme 8. 

Computational and catalytic studies of the hydrosilylation of terminal alkynes have been very recently reported, with the use of [ Ir( r-Cl)(Cl)(Cp ) 2] catalyst to afford highly stereoselectively P-Z-vinylsilanes with high yields (>90%) [35]. B-isomers can be also found among the products, due to subsequent Z —> E isomerization under the conditions employed. The catalytic cycle is based on an lr(lll)-lr(V) oxidahve addition and direct reductive elimination of the P-Z-vinylsilane. Other iridium complexes have been found to be active in the hydrosilylation of phenylacetylene and 1-alkynes for example, when phenylacetylene is used as a substrate, dehydrogenative silylation products are also formed (see Scheme 14.5 and Table 14.3). 

This observation was not so obvious on coke yields because the coke production is a contribution of mnltiple mechanisms and reactions. Thus, the coke yields are quite similar, probably because the catalytic coke is decreased while the contaminant coke is increased. The coke remarks are also observed on the CPS samples taking into account that the dehydrogenation degree is not strongly affected by the extended ReDox cycles, becanse the lower catalysts decay is limiting the effect of the required mass of catalyst (C/0 ratio). Thus, the small decrement of the coke yield on the CPS samples is possibly related to the descent of the catalyst (less specific area) leaving less available space for coke adsorption and less activity for catalytic coke production. It is clear that prolonging the deactivation procednres is not beneficial as far as the metal effects are concerned. 

Note that each of these simple elementary reactions is reversible, and so the entire catalytic cycle is also reversible. This is known as the principle of microscopic reversibility. Consequently, if platinum is a good hydrogenation catalyst, then it must also be a good dehydrogenation catalyst. In fact, as we will see later, catalysts change only the reaction rate, not the equilibrium. Every catalyst catalyzes both the forward and the reverse reactions in the same proportions. In the above example, the reverse reaction is actually more interesting for industry, because propene is a valuable monomer for making poly(propylene) and other polymers. 

The first activation of an alkane C-H bond was described in 1969 [29]. Three decades were to pass until the development of the current catalytic procedures for dehydrogenation and C-O, C-C, and C-B bond-forming reactions. Progress has been slow. Nevertheless, significant advances in catalyst research were achieved in the 1990s, aided by the development of improved metal ligands and the increased understanding of the mechanism of transition metal-catalyzed C-H activation reactions. Further improvements of catalytic cycles are nec-... 

---