Catalyst hydrogen transfer reaction

Propylene. Propylene alkylation produces a product that is rich in dimethylpentane and has a research octane typically in the range of 89—92. The HF catalyst tends to produce somewhat higher octane than does the H2SO4 catalyst because of the hydrogen-transfer reaction, which consumes additional isobutane and results in the production of trimethylpentane and propane. 

A new process developed by Institut Francais du Petrole produces butene-1 (1-butene) by dimerizing ethylene.A homogeneous catalyst system based on a titanium complex is used. The reaction is a concerted coupling of two molecules on a titanium atom, affording a titanium (IV) cyclic compound, which then decomposes to butene-1 by an intramolecular (3-hydrogen transfer reaction. ... 

A rare-earth-exchanged zeolite increases hydrogen transfer reactions. In simple terms, rare earth forms bridges between two to three acid sites in the catalyst framework. In doing so, the rare earth protects... 

Changing to a catalyst, which minimizes hydrogen transfer reactions... 

Catalyst activity. An increase in catalyst activity will increase della coke. As catalyst activity increases so does the number of adjaceni sites, which increases the tendency for hydrogen transfer reactions to occur. Hydrogen transfer reactions are bimolecular and require adjacent active sites. 

Certain catalyst properties appear to increase coke formation. Catalysts with high rare earth content tend to promote hydrogen transfer reactions. Hydrogen transfer reactions are bimolecular reactions that can produce multi-ring aromatics. 

The hydrogen transfer reaction (HTR), a chemical redox process in which a substrate is reduced by an hydrogen donor, is generally catalysed by an organometallic complex [72]. Isopropanol is often used for this purpose since it can also act as the reaction solvent. Moreover the oxidation product, acetone, is easily removed from the reaction media (Scheme 14). The use of chiral ligands in the catalyst complex affords enantioselective ketone reductions [73, 74]. 

Figure 1 Productivities of different copper catalysts (mmol prod/gcat h) in hydrogen transfer reactions.

We have already reported that the use of cyclohexanol at 140°C in the hydrogenation of ergosterol over Cu/A1203 gives the 5(5 derivative with an 81% selectivity owing to an intramolecular hydrogen transfer reaction, whereas direct H2 addition over the same catalyst gives the 5a isomer with 89% selectivity (10) (Scheme 1). 

For the hydrogen transfer reactions, the substrate (0.100 g, 0.64 mmol) was dissolved in anhydrous n-heptane (8 mL) and the solution transferred under N2 into a glass reaction vessel where the catalyst (0.100 g) had been previously treated. Catalytic tests were carried out with magnetic stirring under N2 at boiling point temperature with 2-propanol and 90°C or 140°C with other donor alcohols. 

The termination of Ziegler-Natta synthesis is obtained by neutralisation of the catalytic site using, for instance, alcohol. The reaction can also simply be stopped (with no clear termination) by embedding of the catalyst into the polymer. Industrially, the control of final molecular mass is often accomplished by a hydrogen transfer reaction (see Figure 28). 

The principle of hydrogen transfer reactions has been applied to a variety of oxidative transformations of alcohols with Ru11 catalysts.72 Among them, one interesting application is the aerobic oxidation of alcohols developed by Backvall,153-157 which can be performed with a catalytic... 

Hydrogen transfer reactions are reversible, and recently this has been exploited extensively in racemization reactions in combination with kinetic resolutions of racemic alcohols. This resulted in dynamic kinetic resolutions, kinetic resolutions of 100% yield of the desired enantiopure compound [30]. The kinetic resolution is typically performed with an enzyme that converts one of the enantiomers of the racemic substrate and a hydrogen transfer catalyst that racemizes the remaining substrate (see also Scheme 20.31). Some 80 years after the first reports on transfer hydrogenations, these processes are well established in synthesis and are employed in ever-new fields of chemistry. 

Since the first use of catalyzed hydrogen transfer, speculations about, and studies on, the mechanism(s) involved have been extensively published. Especially in recent years, several investigations have been conducted to elucidate the reaction pathways, and with better analytical methods and computational chemistry the catalytic cycles of many systems have now been clarified. The mechanism of transfer hydrogenations depends on the metal used and on the substrate. Here, attention is focused on the mechanisms of hydrogen transfer reactions with the most frequently used catalysts. Two main mechanisms can be distinguished (i) a direct transfer mechanism by which a hydride is transferred directly from the donor to the acceptor molecule and (ii) an indirect mechanism by which the hydride is transferred from the donor to the acceptor molecule via a metal hydride intermediate (Scheme 20.3). 

In Figure 13.19 we have shown a route to L-699,392 published by Merck involving three steps based on homogeneous catalysts, viz. two Heck reactions and one asymmetric hydrogen transfer reaction, making first an alcohol and subsequently a sulphide [21], Stoichiometric reductions for the ketone function have been reported as well [22] and the Heck reaction on the left-hand side can be replaced by a classic condensation reaction. L-699,392 is used in the treatment of asthma and related diseases. 

The mechanism for the iridium-catalyzed hydrogen transfer reaction between alcohols and ketones has been investigated, and there are three main reaction pathways that have been proposed (Scheme 4). Pathway (a) involves a direct hydrogen transfer where hydride transfer takes place between the alkoxide and ketone, which is simultaneously coordinated to the iridium center. Computational studies have given support to this mechanism for some iridium catalysts [18]. 

Thus, a large focus of FCC catalyst research involves control of the density and location of acid sites in order to control product selectivity, product quahty and coke make. In a landmark publication Pines [30] demonstrated how widely spaced framework A1 sites may be utilized to produce high octane gasolines by decreasing hydrogen transfer reactions which in turn results in preservation of olefins. 

Pines and Kolobielski (18) have shown that phenylcyclohexene, although it is not a cyclic diolefin, will also undergo reactions similar to those that cyclic diolefins undergo when treated with base catalysts. When heated to 200-220 with a sodium-benzyl-sodium catalyst, it underwent a hydrogen transfer reaction resulting in the formation of biphenyl and of phenyl-cyclohexane molecular hydrogen was not produced. The mechanism of this reaction may be pictured as an elimination of sodium hydride from one molecule with the hydride ion being accepted by another molecule (A"-E"). 

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