Hydrogenation homogeneous reactions

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. ... 

By homogeneous reaction of the conjugated double bond system selectively the C=C double bond is hydrogenated [63-66] the ester function is not affected. Moreover, by action of the chiral catalyst, a chiral hydrogenated product is created with good enantioselectivity. 

In 1906 Bodenstein and Lind (24) investigated the gas phase homogeneous reaction between molecular bromine and molecular hydrogen at pressures in the neighborhood of 1 atm. They fitted their experimental data with a rate expression of the form... 

Model flame studies in hydrogen-oxygen flames show that vaporized Mg, Cr, Mn, Sn or U salts are active as radical recombination catalysts at about 1 ppm (4). It was not determined whether this effect was produced by homogeneous reactions in the vapor phase or by the presence of Gne particles. 

The scope of hydrogen transfer reactions is not limited to ketones. Imines, carbon-carbon double and triple bonds have also been reduced in this way, although homogeneous and heterogeneous catalyzed reductions using molecular hydrogen are generally preferred for the latter compounds. 

The hydration number (the number of water molecules intimately associated with the salt) of the quaternary ammonium salt is very dependent upon the anion. The change in the order of reactivity is thus believed to be due to the hydration of the anion the highly hydrated chloride and cyanide ions are less reactive than expected, and the poorly hydrated iodide fares better under phase transfer conditions than in homogeneous reactions. Methanol may specifically solvate the anions via hydrogen bonding, and this effect is responsible for the low reactivity of more polar nucleophiles in that solvent. 

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 reaction of an olefin with hydrogen may be analogous to the related homogeneous reaction (44, 46). The olefin may form a n complex with a single center and combine with hydrogen in the sequence shown in Fig. 22A. Besides the reverse of the preceding sequence, an alternative... 

The steric effects may be more pronounced in heterogeneous catalysts than in homogeneous reactions in solution. The rigid, solid surface restricts the approach of the reactants to the active centers and interaction between the reactants. The steric requirements are quite stringent when a two-point adsorption is necessary and when, in consequence, the internal motion of the adsorbed molecules is limited. In this way, the stereoselectivity of some heterogeneous catalytic reactions, for example, the hydrogenation of alkenes on metals (5) or the dehydration of alcohols on alumina and thoria (9), have been explained. 

The reaction is formally a hydrosilylation process analogous to the homogeneous reactions described in Chapter 5. Scheme 8.11 shows the proposed H—Si(lll) surface-propagated radical chain mechanism [48]. The initially formed surface silyl radical reacts with alkene to form a secondary alkyl radical that abstracts hydrogen from a vicinal Si—H bond and creates another surface silyl radical. The best candidate for the radical translocation from the carbon atom of the alkyl chain to a silicon surface is the 1,5 hydrogen shift shown in Scheme 8.11. Hydrogen abstraction from the neat alkene, in particular from the... 

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