Hydrogenation substrates and transition metal catalysts

Other examples include OKR of racemic secondary alcohols (Scheme 25A), oxidative desymmetrizations of meso-diols, etc. The kinetic resolution is generally defined as a process where two enantiomers of a racemic mixture are transformed to products at different rates. Thus, one of the enantiomers of the racemate is selectively transformed to product, whereas the other is left behind. This method allows to reach a maximum of 50% yield of the enantiopure remaining sec-alcohol. To overcome this fim-itation, a modification of the method, namely dynamic kinetic resolution (DKR), was introduced. In this case, the kinetic resolution method is combined with a racemization process, where enantiomers are interconverted while one of them is consumed (e.g., by esterification. Scheme 25B). Therefore, a 100% theoretical yield of one enantiomer can be reached due to the constant equifibrium shift. In most of the proposed DKR processes, several catalytic systems, e.g., enzymes and transition-metal catalysts, work together. Both reactions (transfer hydrogenation of ketones and the reverse oxidation of secondary alcohols using ketone as a hydrogen acceptor) can be promoted by a catalyst. The process can involve a temporary oxidation of a substrate with hydrogen transfer to a transition-metal complex. 

Despite fruitful results of asymmetric hydrogenation of functionalized ketones, only limited examples have been reported for reaction of ketonic substrates with no functionality near the carbonyl group [1,162,254]. Transition-metal catalysts with a bidentate chiral phosphine, successfully used for functionalized ketones, are often ineffective for reduction of simple ketones in terms of reactivity and enantioselectivity [162b,c]. However, a breakthrough in this subject has been provided by the invention of a new chiral Ru catalyst system. 

Although not exhaustive, the above summary of experiments with hydrogen chemisorbed on transition-metals serves to illustrate how neutron vibrational spectroscopy is performed with catalytic substrates and the methods used to analyze the inelastic neutron spectra. In concluding this section we note that the technique can be extended to supported catalysts such as in recent experiments with hydrogen adsorbed on both MoS and alumina supported MoSp (38). Also, as another indication of the variety of systems which can be studied, we note earlier experiments with ethylene (39) and acetylene (40) adsorbed on silver exchanged 13X zeolites. "Tn this work, deuteration of the molecules was helpful in identifying the surface vibratory modes on these ionic substrates of greater complexity. 

Heterogeneous metal catalysis is the most useful general method for the deuteration and/or tritiation of heterocyclic compounds.57 It involves exchange between the organic substrate and isotopic hydrogen (as water, usually, or gas) in the presence of a Group VIII transition metal catalyst at temperatures up to 180° [Eq. (18)]. 

The term hydrosilation (or hydrosilylation) refers to the addition of a molecule containing a Si-H bond across the multiple bond of a substrate, usually an alkene, alkyne, or carbonyl compound (equation 1). The reaction can be promoted by UV-light, radiation (y- and X rays), radical initiators, Lewis acids, nucleophiles, or, most importantly, transition metal catalysts. Hydrosilation is related to the important processes of hydrogenation (see Hydrogenation) and hydroboration (see Hydroboration), all of which belong to the general reaction class of hydroelementation. 

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