Hydrogenation of Alkenes and Ketones

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]. 

The olefin hydrogenation by the lr(0) NPs in ILs follows the classical monomo-lecular surface reaction mechanism v= fecK[S]/l -i- K[Sj. The reaction rate is a mass controlled process under hydrogen pressure 4 atm. The catalytic kinetic constant 

Interestingly, in multiphase catalytic processes the primary products can be extracted during the reaction thus modulating the product selectivity (using different substrates and reaction products solubility with the catalyst containing phase, such as dienes/monoenes and arenes/cycloalkenes). Indeed, this approach can constitute a suitable method for avoiding consecutive reactions of primary products and has been used in the partial hydrogenation of dienes and arenes by transition-metal NPs dispersed in ILs. 

In as much as 1,3-butadiene is at least four times more soluble in BMI.BF4 than butenes, the selective partial hydrogenation could be performed by Pd(0) NPs embedded in the IL. Selectivities up to 72% in 1-butene were achieved at 99% 1,3-butadiene conversion, 40°C and 4atm of constant pressure of hydrogen 

Entry Pt(o) medium preparation Pt(o) size (nm) Reaction medium TOF (h- )  

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SYNPHOS AND DIFLUORPHOS AS LIGANDS FOR RUTHENIUM-CATALYZED HYDROGENATION OF ALKENES AND KETONES... 

The water-soluble Ru(II) complex [Ru(i76-C6H6)(CH3CN)3](BF4)2 catalyzed the biphasic hydrogenation of alkenes and ketones with retention of the catalyst in the aqueous phase (87). However, the ruthenium complex moved to the organic phase when benzaldehyde was hydrogenated. In a benzene-D20 system, H-D exchange was observed between H2 and D20. Both monohydridic pathway and a dihydridic pathway are possible for hydrogen activation, and these two different catalytic cycles influence the yield and product distribution. 

Ruthenium complexes of chiral phosphines notably BINAP, 2,2 -bis(diphenyl-phosphino)-l,l -binaphthyl are very useful for industrial hydrogenations and H-transfer hydrogenation of alkenes and ketones. This is due to high turnover numbers and enantiomeric excesses (ee) of the products. A specific example41 of an... 

Most reactions involving asymmetric catalysis are based around the conversion of a planar sp carbon atom into a tetrahedral sp carbon atom. This category of reactions includes asymmetric hydrogenation of alkenes and ketones, as well as the addition of other reagents to these groups as identified in Figure 1.1. 

The most obvious way to reduce an aldehyde or a ketone to an alcohol is by hydro genation of the carbon-oxygen double bond Like the hydrogenation of alkenes the reac tion IS exothermic but exceedingly slow m the absence of a catalyst Finely divided metals such as platinum palladium nickel and ruthenium are effective catalysts for the hydrogenation of aldehydes and ketones Aldehydes yield primary alcohols... 

Asyimnetric hydrogenation of prochiral ketones is an important method for the preparation of chiral secondary alcohols. Until recently, however, such reactions were limited to substrates with pendent metal binding sites, like /3-keto esters. Many of the catalysts that efficiently hydrogenate C-C double bonds exhibit little or no reactivity with isolated ketones. This discrepancy may be ascribed to the different binding modes of alkenes and ketones, and the chemoselectivity of catalysts for these groups. While substrates with C-C double bonds can form metal... 

In cluster catalysis there is always the possibility that the cluster breaks down and fragments or monomers are the actual catalysts. Os3H2(CO)io [45] in the hydrogenation of alkenes, and Ru4H4(CO)i2 [46] and its phosphine derivatives in the hydrogenation of alkynes, alkenes, and even ketones, seem to catalyze as clusters. The dimer Rh2(OAc)4 [47] is an active hydrogenation calatyst for alkenes. 

Hydrogenation. With bis(dibenzotropyl)amine and a phosphine as coligands to modify [(cod)RhCl]2 a hydrogenation catalyst is formed. Reduction of alkenes and ketones with this system employs EtOH as hydrogen source. ... 

Slurry reactors are widely used for metal-catalyzed hydrogenations of alkenes and alkynes, including the important process of converting vegetable oils to margarine and other fats. Hydrogenations of ketones, aldehydes. 

Ligand-metal bifunctional catalysis provides an efficient method for the hydrogenation of various unsaturated organic compounds. Shvo-type [83-85] Ru-H/OH and Noyori-type [3-7] Ru-H/NH catalysts have demonstrated bifimctionality with excellent chemo- and enantioselectivities in transfer hydrogenations and hydrogenations of alkenes, aldehydes, ketones, and imines. Based on the isoelectronic analogy of H-Ru-CO and H-Re-NO units, it was anticipated that rhenium nitrosyl-based bifunctional complexes could exhibit catalytic activities comparable to the ruthenium carbonyl ones (Scheme 29) [86]. 

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