Hydrogenation, catalytic, alkene selectivity

The used Pd/ACF catalyst shows a higher selectivity than the fresh Lindlar catalyst, for example, 94 1% versus 89 + 2%, respectively, at 90% conversion. The higher yield of 1-hexene is 87 + 2% with the used catalyst versus 82 + 3% of the Lindlar in a 1.3-fold shorter reaction time. Higher catalyst activity and selectivity is attributed to Pd size and monodispersity. Alkynes hydrogenation is structure-sensitive. The highest catalytic activity and alkene selectivity are observed with Pd dispersions <20% [26]. This indicates the importance of the Pd size control during the catalyst preparation. This can be achieved via the modified ME technique. 

One of the most famous catalysts, which operates through a mechanism involving formation of a mono-hydride (M—H), is [RuCl2(PPh3)3].38-40 In the catalytic hydrogenation of alkenes (Equations (1) and (2)) it shows very high selectivity for hydrogenation of terminal rather than internal C=C bonds. 

Another example of selective C=C bond hydrogenation has arisen from mechanistic studies on an iron m-hydride dihydrogen complex, [Fe(PP3)(FI)(H2)](BF4) [PP3 = P(CH2CH2PPh2)3], a catalyst inactive with alkene substrates. Scheme 6 shows that no decoordination of dihydrogen is required in any step of the cycle and that the vacant site is created by unfastening of one of the P-donor atoms (species (16)).50 Extensive studies on catalytic alkene hydrogenation by analogous tripodal (polyphosphine) Rh, Os, and Ir complexes have been carried by Bianchini and co-workers.51,52... 

The time is ripe for truly exciting developments in the reactivity of dinuclear transition metal compounds. The potential for cyclic sequences of reactions, as is required for catalytic reactions, has already been realized. (1) It has been shown, by Muetterties, et al. (53), that alkynes can be selectively hydrogenated to alkenes (cU 2H-addition) by Cp2Mo2(C0) the rate determining step involves CO dissociation from the acetylene adducts Cp2MO2(C0) (R2C2). (2) We have found that... 

Ruthenium complexes have been used for the hydrogenation of alkenes but they had not been used for hydrogenations of carbonyl compounds until recently, when several highly selective catalytic hydrogenations of carbonyl compounds (P-keto esters, a-diketones, a-amino ketones etc., ee > 95%) were reported using BINAP-Ru" complexes (equation 18). ° ... 

The partial reduction of substrates containing triple bonds is of considerable importance not only in research, but also commercially for stereoselectively introducing (Z)-double bonds into molecular frameworks of perfumes, carotenoids, and many natural products. As with catalytic hydrogenation of alkenes, the two hydrogen atoms add syn from the catalyst to the triple bond. The high selectivity for alkene formation is due to the strong absorption of the alkyne on the surface of the catalyst, which displaces the alkene and blocks its re-adsorption. The two principal metals used as catalysts to accomplish semireduction of alkynes are palladium and nickel. 

RuCl2(PPh3)3 and related Ru(II) complexes serve usefully in the catalytic hydrogenation of alkenes as shown in equations 9.22 and 9.23,61 showing very high selectivity for terminal over internal double bonds. 

Eisen and Ivanov [60] elaborated a pulse, micro-catalytic gas chromatographic method of selective hydrogenation of alkenes and cyclohexenes at 90°C on 5% palladium on silica gel as the catalyst. Under these conditions aromatic hydrocarbons are not hydrogenated, and cyclopentenes are hydrogenated to an insignificant extent. 

Catalytic hydrogenation of alkene 62 proceeded in a completely facially selective manner to give the C7-arylated perhydroindole 63 (quant.) that was then subjected to treatment with phosphoms oxytrichloride and so effecting a Bischler-Napieralski cyclisation reaction to give the lactam 64 (81%). Upon reduction with lithium aluminium hydride this lactam provided ( )-y-lycorane [( )-54] in 84% yield. 

Although Pd-N complexes sometimes have catalytic activities that are comparable to that of the Pd-P complexes,they often lead to significantly different reactivity profiles as exemplified by the Pd-catalyzed regioselective carbonylation (Scheme 9) or the selective activation of a switchable bisnucleophile (Scheme 10). Other examples can be found in the catalytic hydrogenation of alkenes, or the carbacyclization of enynes, as well as in cross-coupling and allylic substitution reactions. ... 

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