Catalyst hydrogenation effects, olefins

Product stereochemistry is a function of the specific catalyst used for hydro-genation. For example, palladium generally gives more of the thermodynamically stable product than other catalysts. This effect has been attributed to an increased rate of equilibration of the steps in the hydrogenation process. Consequently, palladium should not be used to hydrogenate readily isomerizable olefins such as A - and A -steroids. ... 

Table 3 summarizes the scope and limitation of substrates for this hydrogenation. Complex 5 acts as a highly effective catalyst for functionalized olefins with unprotected amines (the order of activity tertiary > secondary primary), ethers, esters, fluorinated aryl groups, and others [27, 30]. However, in contrast to the reduction of a,p-unsaturated esters decomposition of 5 was observed when a,p-unsaturated ketones (e.g., trans-chalcone, trans-4-hexen-3-one, tra s-4-phenyl-3-buten-2-one, 2-cyclohexanone, carvone) were used (Fig. 3) [30],... 

In contrast to normal trisubstituted aryl olefins, a strong pressure effect was observed in this case. Interestingly, for substrate 44a catalysts hydrogenation with Ir(13d) demonstrated opposite pressure-dependant enantioselectivity of substrate 45a with catalyst Ir(46). Poor enantioselectivity was obtained for substrates that did not contain an aromatic ring adjacent to the olefin. 

Even in an excess of ligands capable of stabilizing low oxidation state transition metal ions in aqueous systems, one may often observe the reduction of the central ion of a catalyst complex to the metallic state. In many cases this leads to a loss of catalytic activity, however, in certain systems an active and selective catalyst mixture is formed. Such is the case when a solution of RhCU in water methanol = 1 1 is refluxed in the presence of three equivalents of TPPTS. Evaporation to dryness gives a brown solid which is an active catalyst for the hydrogenation of a wide range of olefins in aqueous solution or in two-phase reaction systems. This solid contains a mixture of Rh(I)-phosphine complexes, TPPTS oxide and colloidal rhodium. Patin and co-workers developed a preparative scale method for biphasic hydrogenation of olefins [61], some of the substrates and products are shown on Scheme 3.3. The reaction is strongly influenced by steric effects. 

In conclusion, the peculiarities of hydrogenation of olefins in aqueous solutions show that by shifting acid-base equilibria the aqueous environment may have important effects on catalysis through changing the molecular state ofthe substrate or the catalyst or both. 

The discovery of Wilkinson complex, RhCl[P(C6H5)3]3, acting as an effective catalyst for hydrogenation of olefins opened the door for developing asymmetric reaction catalyzed by rhodium complexes with a chiral phosphine ligand. 

Researchers performed the biphasic hydrogenation of cyclohexene with Rh(cod)2 BF4 (cod = cycloocta-1,5-diene) in ILs. They observed roughly equal reaction rates, reported as turnover frequencies of ca. 50 h in either [bmim][BF4] or [bmim][PF6]. The presumption here was that the [bmim][BF4] was free from chloride. In a separate report, the same group showed that RuCl2(Ph3P)3 in [bmim][BF4] was an effective catalyst for the biphasic hydrogenation of olefins, with turnover frequencies up to 540 h Similarly, (bmim)3-Co(CN)5 dissolved in [bmim][BF4] catalyzed the hydrogenation of butadiene to but-l-ene, with 100% selectivity at complete conversion. 

Rhodium catalysts generated from the sulfonated phosphine 23 (Table 2) were effective in the hydrogenation of olefins in an aqueous/organic or in a homogeneous methanol system, substantially higher rates being observed in the latter system.131 For example, the TOF observed in the hydrogenation of 1-hexene in the biphasic system was 220 h 1 compared to 7860 h 1 in methanol.131... 

This complex, along with its iridium analog, has proven to be an effective catalyst for the isomerization and hydrogenation of olefins (80), for H—D exchange at BH vertices (60), and for other organic reactions. Catalysis by metallocarboranes is discussed in detail in Section X. 

In this paper, the effect of lead adatoms on the activity of platinum catalysts, in liquid phase hydrogenation of olefinic compounds, is presented. 

The test-reaction, used in order to evaluate the effect of lead, was the hydrogenation of olefinic diacids (maleic, methylmaleic and dimethyl maleic acids), carried out in an aqueous solution of 0.5 M sulphuric acid. The characterization and the modification of the catalyst were performed in the same solution, before starting the hydrogenation experiments. More experimental details were reported in (13,14). 

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