Hydrogenation of simple olefins

Dror and Manassen hydrogenated a- and cyclic olefins such as 1-octene, 1 -dodecene and cyclohexene in micellar systems using rhodium catalysts modified with water-insoluble carboxylated tenside phosphines 45 (Table 3 n=3,5,7,9,l 1) in the presence of conventional tensides such as sodium dodecylsulfate (SDS) or cetyltrimethylammonium bromide (CTAB) and cosolvents e.g. dimethyl sulfoxide. Linear olefins were more reactive than cyclic olefins. Maximum efficiency was observed in the presence of the anionic surfactant SDS when 45 contained a chain of 5-7 carbon atoms. In contrast, using the cationic tenside CTAB the ligand 45 with 5 C atoms was almost inactive but became active again with dodecyl trimethylammonium bromide. 

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. For example, the TOF observed in the hydrogenation of 1-hexene in the biphasic system was 220 h compared to 7860 h in methanol.  

Rhodium catalysts modified with carboxylated phosphines 45 (Table 3 n=5, n=7)229 phosphonium phosphines 103 (Table 5 n=2,3,6,10) form very active catalytic systems for the hydrogenation of olefins in aqueous/organic two phase systems. 

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A study concerning the lowermost particle size necessary for admitting a reaction to occur is provided by Amiridis et al. [55] in their investigations of propene hydrogenation. For metal particle sizes greater than 2nm, hydrogenation of simple olefins is considered to be structure insensitive... 

Hydrogenation of Simple Olefins with Iridium Catalysts... 

Figure 1.12. Asymmetric hydrogenation of simple olefins catalyzed by chiral Ir complexes.

Background. The Wilkinson Rh complex, RhCl[P(C6H5)3]3, catalyzes the hydrogenation of simple olefins in organic solvents under mild conditions. The mechanism which involves the oxidative addition of H2 to Rh(I) is shown in Scheme 2 (5-7). 

Table I summarizes the application of various low-valent titanium metallocenes as catalysts for olefin hydrogenation. Compounds 10 and 37 are very effective hydrogenation catalysts for C2H4 and cyclohexene. Since different researchers have used widely varying conditions, we can only estimate that the polystyrene-supported (7j-C8H8)2Ti (142) is comparable in activity to compounds 10 and 37. When one recalls that 37 was prepared by a formal oxidation of the Ti centers in 10, it is remarkable that 37 is as good a catalyst as 10. Solutions prepared by reaction of l-methyl-17-allylbiscyclopentadienyltitanium (54) with H2 do appear to be more active hydrogenation (126) catalysts than 10 and 37. The dicarbonyl complex, (17-CsH5)2Ti(CO)2 (39), has been shown to be a catalyst for the hydrogenation of acetylene at —50 atm of H2 (143). It does not catalyze the hydrogenation of simple olefins. However, Floriani and Fachinetti discovered that if...

The hydrogenation of simple olefins by lr(0) NPs dispersed in ILs depends on steric hindrance at the C=C double bond, the reactivity follows the order terminal > disubstituted > tri-substituted > tetrasubstituted (Fig. 6.9). Such an order of reactivity for olefins is the same as that of classical iridium complexes in homogeneous conditions [128]. 

Hydridoarene clusters of Rh and Ru are moderately active catalysts of hydrogenation of simple olefins [20]. Conversely, benzene and monosubstituted benzenes can be efficiently hydrogenated in aqueous biphasic systems with hydridoareneruthe-nium cluster catalysts, such as [Ru3(p2-H)2(p2 OH) (pj-O) (ri -C5Hg)(ri "-C5Me5)2] [21]. 

Iridium-Phosphinooxazoline Catalysts. Asymmetric hydrogenation of simple olefins with chiral Rh or Ru catalysts generally shows low reactivity and unsatisfactory enantioselectivity (198,248,249). However, several unfunctionalized olefins can be hydrogenated in high yields and excellent enantioselectivity by using iridium catalysts with chiral phosphinooxazoline ligands (60,186,187, 189,191-194,250) (Fig. 56). To avoid catalyst deactivation, the extremely weakly coordinating anion tetrakis[3,5-bis(trifiuoromethyl)phenyl]borate has to be used (182,251). 

The same research group recently characterized key intermediates in the Ir-catalyzed asymmetric hydrogenation of simple olefins. When a... 

Figure 1.27 Four possible mechanisms of the Ir-catalyzed hydrogenation of simple olefins.

Both recent computational studies agree that the most feasible pathway for the hydrogenation of simple olefins is Ir "/Ir migratory insertion mechanism with migratory insertion as the rate-determining and stereoregulating step. ° ... 

The origins of enantioselection in the Ir-catalyzed asymmetric hydrogenation of simple olefins are now reasonably well understood as a result of intensive experimental and computational studies. This helps in development of new effective catalysts and synthetic methods. Similar progress is wanted for the recently discovered highly effective Ir-catalyzed asymmetric hydrogenations of aromatic heterocycles, ketones, and keto-esters. 

Finally, these aqueous colloidal solutions were used as catalysts for the hydrogenation of simple olefins and arenes with Roucoux et al. [107], These nanocatalysts appeared efficient in mild conditions, evidencing that these materials are active despite the evolution of their environment related to their dissolution into water. For example, octene and dodecene were fully converted into the corresponding alkanes (r.t 1 bar H2) with moderate activities. An increase in the hydrogen pressure (P(H2) = 10 bar) was not detrimental for the stability of the colloidal suspension. The complete hydrogenation of toluene into cyclohexane was observed overnight with Ru/PTA, whereas 60% of /n-methoxymethylcyclohexane was formed from methoxymethylanisole. Nevertheless, preliminary recycling tests showed that an improvement of the stability and recovery of these materials was necessary to increase their interest in catalysis. 

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