Selective hydrogenation Second step

Kroutil et al. have recently reported [18] an elegant one-pot oxidation/reduction sequence for the deracemization of a chiral secondary alcohol using a single biocatalyst. LyophiUzed cells of a Rhodococcus sp. CBS IVJ.Ti converted racemic 2-decanol into the (S)-enantiomer in 82% yield and 92% enantiomeric excess (e.e.). via a non-specific oxidation followed sequentially by an (S)-selective reduction (Scheme 6.5). Acetone was used as the hydrogen acceptor in the first step and isopropanol as the hydrogen donor in the second step. 

The observed dependence of the N 2 selectivity on temperature may suggest that the reduction of stored nitrates by H2 occurs via an in-series two-step pathway. The first step is fast even at low temperatures and is responsible for the consumption of hydrogen and for the formation of ammonia. The second step is slower and implies the reduction of residual nitrates with ammonia to form nitrogen this reaction occurs to a significant extent only at higher temperatures. 

The first step is catalytic hydrogenation to the corresponding alcohol over a 5% palladium-on-charcoal catalyst in methanol solvent at 30°C and 7 bar. The selectivity is 97% at >99% conversion. In the second step the alcohol is carbonylated at 35 bar CO and 125-130°C in the presence of a PdCl2(Ph3P)2 and HC1 as catalyst. Selectivities to ibuprofen are ca. 70% at 99% conversion. 

Application The KLP process selectively hydrogenates acetylenes in crude butadiene streams from steam crackers to their corresponding diene or olefin to recover 1,3-butadiene. The KLP process can be used in new installations to eliminate the costly second-stage extractive distillation step or as a retrofit to increase product quality or throughput. 

In the second step, the triple bond in 63 is selectively reduced to the cz -alkene using the Lindlar catalyst to form 64. In this case, the Lindlar catalyst is a poisoned heterogeneous palladium catalyst on barium sulfate. The deactivation of the catalyst with quinoline is responsible for the selective hydrogenation to the alkene and not through to the alkane. The reason for the highly stereoselective reduction with syn-addition to the cw-alkene is that one face of the triple bond is shielded by the catalyst surface. 

The formation of a C-C bond resulting from the coupling of two C-H bonds is a particularly attractive target, since the only formal by-product would be hydrogen, or water in an oxidative system. However, substantial hurdles impede the conception of a catalytic arene cross-coupling process that does not involve any substrate pre-activation at all. Aside from issues of reactivity and regioselectivity, the prevention of homo-coupling is a key factor for the development of this important class of reaction. The catalyst must be able to react with one arene in the first step of the catalytic cycle and then invert its selectivity in the second step to react exclusively with a different arene (Scheme 29). 

The second step of the process, the aldol hydrogenation, is in principle more straightforward, since no selectivity problems appeared with suitable metal... 

The reaction of oxygen with most radicals is very fast because of the triplet character of molecular oxygen (see Table 11.2, Entries 42 and 43). The ease of autoxidation is therefore governed by the rate of hydrogen abstraction in the second step of the propagation sequence. The alkylperoxyl radicals that act as the chain carriers are fairly selective. Positions that are relatively electron rich or provide particularly stable radicals are the most easily oxidized. Benzylic, allylic, and tertiary positions are the most susceptible to oxidation. This selectivity makes radical chain oxidation a preparatively useful reaction in some cases. 

From a synthetic point of view it is important to point out that over amorphous Ni catalysts only the first hydrogenation step takes place at 373 K (C=C reduction) and higher temperature is necessary for the second step (C=0 reduction). Thus, an appropriate selection of reaction temperature guarantees the corresponding product with excellent selectivity. 

Barrault and co-workers studied the selective hydrogenation of methyl oleate into oleyl alcohol over RuSnB/alumina catalysts. The yield of oleyl alcohol was 75 % at 90 % methyl oleate conversion over a RuSnB/AbOa catalyst for a bulk atomic ratio Sn/Ru of 4. Over such catalysts the reaction involves three steps (1) the hydrogenation of the methyl oleate into the oleyl alcohol, (2) the transesterification between the methyl oleate and the oleyl alcohol with the formation of the heavy oleyl oleate ester, (3) the hydrogenolysis of this heavy ester into oleyl alcohol. The first and the third steps could involve mixed ruthenium-tin sites with two SnOx (x = 2) species, while the second could require tin species without... 

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