Dehydrogenative catalytic cycle

Catalysis (qv) refers to a process by which a substance (the catalyst) accelerates an otherwise thermodynamically favored but kiaeticahy slow reaction and the catalyst is fully regenerated at the end of each catalytic cycle (1). When photons are also impHcated in the process, photocatalysis is defined without the implication of some special or specific mechanism as the acceleration of the prate of a photoreaction by the presence of a catalyst. The catalyst may accelerate the photoreaction by interaction with a substrate either in its ground state or in its excited state and/or with the primary photoproduct, depending on the mechanism of the photoreaction (2). Therefore, the nondescriptive term photocatalysis is a general label to indicate that light and some substance, the catalyst or the initiator, are necessary entities to influence a reaction (3,4). The process must be shown to be truly catalytic by some acceptable and attainable parameter. Reaction 1, in which the titanium dioxide serves as a catalyst, may be taken as both a photocatalytic oxidation and a photocatalytic dehydrogenation (5). 

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. 

Scheme 15 Proposed catalytic cycle for dehydrogenation of alcohols to ketones...

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). 

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). 

The essential features of the catalytic cycle (see Figure 10) involve the binding of NAD, the displacement of the water molecule by alcohol, the deprotonation of the coordinated alcohol to give a zinc alkoxide intermediate, the hydride transfer from the alkoxide to NAD to give a zinc-bound aldehyde, the displacement of the aldehyde by water and the release of NADH. The principal role of the zinc in the dehydrogenation reaction is, therefore, to promote deprotonation of the alcohol and thereby enhance hydride transfer... 

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. 

The polymerization of compounds having active methyne groups has also been reported [81] (Eq. 8). The oxidative coupling polymerization of these monomers follows a mechanism similar to that of phenols. The catalytic cycle observed in the polymerization of / -phcnylcncdiaminc with Fe(edta) as the catalyst in an aqueous solution differs from that in the polymerization of phenols as follows The activation of monomers usually involves either electron transfer from the anion or elimination of a hydrogen atom from the monomer. The oxidative polymerization of phenols uses the former mechanism of the electron transfer. In contrast, in the case of the polymerization of aromatic diamines as monomers, the neutral amines are coordinated to the catalyst, followed by the subsequent electron transfer and dehydronation. The dehydronation proceeds by the reaction with 02. Another mechanism has also been proposed where dehydrogenation... 

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-... 

Moreover, we believe that the azo form helps in stabilizing several of the reactive copper complexes involved in this catalytic cycle such as the hydroxy copper complex 17. Thus, we surmise that this novel catalytic, aerobic oxidation procedure for alcohols into carbonyl derivatives proceeds via a dehydrogenation mechanism and relies on the effective role of hydrazine or azo compounds as hydrogen shuttles and stabilizing ligands for the various copper complexes (20). 

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.)... 

The aerobic oxidation of alcohols catalysed by low-valent late-transition-metal ions, particularly those of group VIII elements, involves an oxidative dehydrogenation mechanism. In the catalytic cycle (Fig. 5) ruthenium can form a hydridometal species by /1-hydride elimination from an alkoxymetal intermediate, which is reoxidized by dioxygen, presumably via insertion of 02 into the M-H bond with formation of H202. Alternatively, an alkoxymetal species can decompose to a proton and the reduced form of the catalyst (Fig. 5), either directly or via the intermediacy of a hydridometal intermediate. These reactions are promoted by bases as cocatalysts, which presumably facilitate the formation of an alkoxymetal intermediate and/or /1-hydride elimination. 

Fig. 10.4. Peroxidase catalytic cycle. Four pathways are shown for the return of Compound I to the resting state (1) oxidative dehydrogenation, where RH is the substrate (2) oxidative halogenation, where X" is a halogen ion and RH is the substrate (3) peroxide disproportionation, and (4) oxygen transfer, where R is the substrate.

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. 

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 use of successive cycling pulses, both sinusoidal and otherwise, in a chromatographic reactor has been treated and analyzed by Gore (35). An experimental treatment in which pulsed frequencies were used to optimize the cyclohexane dehydrogenation (catalytic) reaction has also been presented (36). 

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