Selective hydrogenation First step

A number of reports have appeared concerning the details of hydrogenation reactions catalysed by [Ru(X)2(P)al (X=halide or CO2R, P=tertiary phosphine) and a feature of such catalysts is their selectivity. The first step in such reactions is generally phosphine dissociation and when X=C1 this is followed by transformation of the dichloride to [Ru(Cl)(H)(P)a] by addition of hydrogen and elimination of HCl ... 

Sulfides with widely different solubilities and solubility products can be selectively precipitated by adding S2 ions to the solution removed from the chlorides in the first step (see Fig. 11.20). Some metal sulfides (such as CuS, HgS, and Sb2S3) have extremely small solubility products and precipitate if there is the merest trace of S2" ions in the solution. Such a very low concentration of S2 ions is achieved by adding hydrogen sulfide, H2S, to an acidified solution. A higher hydronium ion concentration shifts the equilibrium... 

Haloperoxidases act as halide-transfer reagents in the presence of halide ions and hydrogen peroxide. In the first step, the halide ion is oxidized to a halonium-ion carrier, from which the positive halogen species is then transferred to the double bond. In an aqueous medium, the intermediary carbocation is trapped and racemic halohydrins are formed (Eq. 7). Selective examples of CPO-cata-lyzed formation of halohydrins are given in Table 9. In CPO-catalyzed reaction. 

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. 

An elegant four-enzyme cascade process was described by Nakajima et al. [28] for the deracemization of an a-amino acid (Scheme 6.13). It involved amine oxidase-catalyzed, (i )-selective oxidation of the amino acid to afford the ammonium salt of the a-keto acid and the unreacted (S)-enantiomer of the substrate. The keto acid then undergoes reductive amination, catalyzed by leucine dehydrogenase, to afford the (S)-amino acid. NADH cofactor regeneration is achieved with formate/FDH. The overall process affords the (S)-enantiomer in 95% yield and 99% e.e. from racemic starting material, formate and molecular oxygen, and the help of three enzymes in concert. A fourth enzyme, catalase, is added to decompose the hydrogen peroxide formed in the first step which otherwise would have a detrimental effect on the enzymes. 

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 manufacturing route of the GEM side-chain relied on a-cyanoketone 125 however, the number of chemical steps from 125 to the final side-chain was reduced by one step (Noh et ah, 2004a). The sequence began with a selective hydrogenation with Raney nickel followed by double bond migration to enamine 131 (Scheme 4.25). The amino functionality of 131 was then monoprotected, and the double bond was reduced under hydrogenation conditions to afford pyrrolidine-3-one 133. Treatment of 133 with methoxylamine yielded methoxyoxime 129. Deprotection of the carbamate functionality was achieved with methanesulfonic acid to afford the C7-side-chain as the bis-methansulfonate salt. 

Sumimoto introduced a new sebacic acid process including several catalytic hydrogenation reactions.342 The synthesis starts with naphthalene, which is first partially hydrogenated to tetralin over cobalt oxide or molybdenum oxide, then to decalin over ruthenium or iridium on carbon. The selectivity to cw-decalin is better than 90%. In a later phase of the synthesis 5-cyclododecen-l-one is hydrogenated over Raney nickel to obtain a mixture of cyclododecanone and cyclodode-canol in a combined yield of 90%. The selectivity of this step is not crucial since subsequent oxidation of either compound leads to the endproduct sebacic acid. 

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