Hydrogenation, catalytic alkenes catalyst reactivity

The structure of Os3(/r-H)2(CO)10 has been established by X-ray8 and neutron diffraction.9 The 46-electron complex displays a relatively high reactivity under mild conditions, associated with a stable triosmium framework and has been extensively studied as a model for the chemisorption of alkenes and alkynes on surfaces and in the catalytic isomerization and hydrogenation of alkenes.10 When supported onto alumina it is a catalyst for the methanation of CO and C02 slightly less efficient than NiOs3(/r-H)3(CO)9(,5-CsH5)>... 

This method ensures the deposition of very reactive metal nanoparticles that require no activation steps before use. We shall review here the following examples of catalytic reactions that are of interest in line chemical synthesis (a) the hydrogenation of substituted arenes, (b) the selective hydrogenation of a, 3-unsaturated carbonyl compounds, (c) the arylation of alkenes with aryl halides (Heck reaction). The efficiency and selectivity of commercial catalysts and of differently prepared nanosized metal systems will be compared. 

However, the pathways for these reactions, particularly in the gas phase, have been only -.rtially characterized. In a wide variety of these reactions, coordinatively unsaturated, highly reactive metal carbonyls are produced [1-18]. The products of many of these photochemical reactions act as efficient catalysts. For example, Fe(C0)5 can be used to generate an efficient photocatalyst for alkene isomerization, hydrogenation, and hydrosilation reactions [19-23]. Turnover numbers as high as 3000 have been observed for Fe(C0)5 induced photocatalysis [22]. However, in many catalytically active systems, the active intermediate has not been definitively determined. Indeed, it is only recently that significant progress has been made in this area [20-23]. 

The chiral sites which are able to rationalize the isospecific polymerization of 1-alkenes are also able, in the framework of the mechanism of the chiral orientation of the growing polymer chain, to account for the stereoselective behavior observed for chiral alkenes in the presence of isospecific heterogeneous catalysts.104 In particular, the model proved able to explain the experimental results relative to the first insertion of a chiral alkene into an initial Ti-methyl bond,105 that is, the absence of discrimination between si and re monomer enantiofaces and the presence of diastereoselectivity [preference for S(R) enantiomer upon si (re) insertion]. Upon si (re) coordination of the two enantiomers of 3-methyl-l-pentene to the octahedral model site, it was calculated that low-energy minima only occur when the conformation relative to the single C-C bond adjacent to the double bond, referred to the hydrogen atom bonded to the tertiary carbon atom, is nearly anticlinal minus, A- (anticlinal plus, A+). Thus one can postulate the reactivity only of the A- conformations upon si coordination and of the A+ conformations upon re coordination (Figure 1.16). In other words, upon si coordination, only the synperiplanar methyl conformation would be accessible to the S enantiomer and only the (less populated) synperiplanar ethyl conformation to the R enantiomer this would favor the si attack of the S enantiomer with respect to the same attack of the R enantiomer, independent of the chirality of the catalytic site. This result is in agreement with a previous hypothesis of Zambelli and co-workers based only on the experimental reactivity ratios of the different faces of C-3-branched 1-alkenes.105... 

The TEAF system can be used to reduce ketones, certain alkenes and imines. With regard to the latter substrate, during our studies it was realized that 5 2 TEAF in some solvents was sufficiently acidic to protonate the imine (p K, ca. 6 in water). Iminium salts are much more reactive than imines due to inductive effects (cf. the Stacker reaction), and it was thus considered likely that an iminium salt was being reduced to an ammonium salt [54]. This explains why imines are not reduced in the IPA system which is neutral, and not acidic. When an iminium salt was pre-prepared by mixing equal amounts of an imine and acid, and used in the IPA system, the iminium was reduced, albeit with lower rate and moderate enantioselectivity. Quaternary iminium salts were also reduced to tertiary amines. Nevertheless, as other kinetic studies have indicated a pre-equilibrium with imine, it is possible that the proton formally sits on the catalyst and the iminium is formed during the catalytic cycle. It is, of course, possible that the mechanism of imine transfer hydrogenation is different to that of ketone reduction, and a metal-coordinated imine may be involved [55]. 

The promotion effects of Mn on unsupported Fe-based F-T catalysts were also studied by Jensen and Massoth. " These authors concluded that the incorporation of Mn chemically and electronically promotes the active Fe surface. More particularly, it appears to alter the CO hydrogenation reaction path by suppressing the direct formation of paraffins from the reactive intermediate, leading to the increased production of higher olefins. Finally, Das et al. also observed that the addition of moderated amounts of Mn promoter to unsupported Fe F-T catalysts promotes the catalytic activity as well as the selectivity towards lower alkenes. ... 

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