Hydrogen prepared in situ

Chloroplatinic acid sodium tetrahydridoborate Hydrogenation with catalyst and hydrogen prepared in situ... 

Hydroformylation s. Aldehydes from ethylene derivatives Hydrogen prepared in situ 17, 96 18,115 G-Hydrogen... 

Nitrosyl chloride (178), nitrosyl chloride—hydrogen fluoride (NOF -3HF, NOF -6HF) (179), nitrous acid—hydrogen fluoride solutions (180,181), or nitrogen trioxide (prepared in situ from nitric oxide and oxygen) (27) can be used in place of sodium nitrite in the dia2oti2ation step. 

Later, a completely different and more convenient synthesis of riboflavin and analogues was developed (34). It consists of the nitrosative cyclization of 6-(A/-D-ribityl-3,4-xyhdino)uracil (18), obtained from the condensation of A/-D-ribityl-3,4-xyhdine (11) and 6-chlorouracil (19), with excess sodium nitrite in acetic acid, or the cyclization of (18) with potassium nitrate in acetic in the presence of sulfuric acid, to give riboflavin-5-oxide (20) in high yield. Reduction with sodium dithionite gives (1). In another synthesis, 5-nitro-6-(A/-D-ribityl-3,4-xyhdino) uracil (21), prepared in situ from the condensation of 6-chloro-5-nitrouracil (22) with A/-D-ribityl-3,4-xyhdine (11), was hydrogenated over palladium on charcoal in acetic acid. The filtrate included 5-amino-6-(A/-D-ribityl-3,4-xyhdino)uracil (23) and was maintained at room temperature to precipitate (1) by autoxidation (35). These two pathways are suitable for the preparation of riboflavin analogues possessing several substituents (Fig. 4). 

Nitrosyl chloride [55], nitrosyl fluoride-hydrogen fluoride liquid complexes (NOF3HF, NOF 6HF) [56], nitrous acid-hydrogen fluoride solutions [57, 5 ] nitrogen trioxide (prepared in situ from nitric oxide and oxygen) [59] and rert-butyl nitrite-hydrogen fluoride-pyndine [60] have been substituted for sodium nitrite in the diazotization step... 

Difluoroalkenyl boranes, which are prepared in situ, are converted to difluoromethyl ketones by oxidation with hydrogen peroxide m alkaline media [720] (equation 107)... 

The isomerization of an allylic amine to an enamine by means of a formal 1,3-hydrogen shift constitutes a relatively small structural change. However, this transformation could be extremely valuable if it could be rendered stereoselective. In important early studies, Otsuka and Tani showed that a chiral cobalt catalyst, prepared in situ from a Co(ii) salt, a chiral phosphine, and diisobutylaluminum hydride (Dibal-H), can bring about the conversion of certain pro-chiral olefins to chiral, isomeric olefins by double bond migra-... 

Cobalt monoboride crystals are deposited on a graphite substrate by simultaneous reduction with hydrogen of a mixture of CoClj (prepared in situ by chlorination of Co powder at 800°C) and BClj in an Ar flow ... 

Finally, Jessop and coworkers describe an organometalhc approach to prepare in situ rhodium nanoparticles [78]. The stabilizing agent is the surfactant tetrabutylammonium hydrogen sulfate. The hydrogenation of anisole, phenol, p-xylene and ethylbenzoate is performed under biphasic aqueous/supercritical ethane medium at 36 °C and 10 bar H2. The catalytic system is poorly characterized. The authors report the influence of the solubility of the substrates on the catalytic activity, p-xylene was selectively converted to czs-l,4-dimethylcyclohexane (53% versus 26% trans) and 100 TTO are obtained in 62 h for the complete hydrogenation of phenol, which is very soluble in water. 

The efficiency of the new ligands was examined in enantioselective hydrogenation of some prochiral substrates. Itaconic ester hydrogenation using in situ prepared Rh-complexes was the first test reaction chosen. The best results from... 

Sodium hydrogen telluride, (NaTeH), prepared in situ from the reaction of tellurium powder with an aqueous ethanol solution of sodium borohydride, is an effective reducing reagent for many functionalities, such as azide, sulfoxide, disulfide, activated C=C bonds, nitroxide, and so forth. Water is a convenient solvent for these transformations.28 A variety of functional groups including aldehydes, ketones, olefins, nitroxides, and azides are also reduced by sodium hypophosphite buffer solution.29... 

When the conditions are controlled properly, Zn can mediate the reduction of the C-C double bond of a, (3-unsaturated carbonyl compounds in the presence of a nickel catalyst in aqueous ammonium chloride (Eq. 10.7). The use of ultrasonication enhances the rate of the reaction.15 Sodium hydrogen telluride, (NaTeH), prepared in situ from the reaction of... 

Prochiral aryl and dialkyl ketones are enantioselectively reduced to the corresponding alcohols using whole-cell bioconversions, or an Ir1 amino sulfide catalyst prepared in situ.695 Comparative studies show that the biocatalytic approach is the more suitable for enantioselective reduction of chloro-substituted ketones, whereas reduction of a,/ -unsaturated compounds is better achieved using the Ir1 catalyst. An important step in the total synthesis of brevetoxin B involves hydrogenation of an ester using [Ir(cod)(py) P(cy)3 ]PF6.696... 

The fused heterocycles 58 have been synthesized via cycloaddition of 1-phenylisobenzofuran 467 (prepared in situ from 466) and methyl a-phenylselenoacrylate. The cycloaddition was carried out in toluene at 80 °C for 2 h to give adduct 468. Hydrogen peroxide on reaction with 468 at —40 °C provided product 469. The [3+2] cycloaddition between 469 and an amine-derived dipole has been accomplished under acidic conditions <1985CPB2762> to provide the heterocycle 58 as a single diastereomer (Scheme 105) <20000L923>. 

Keurentjes et al. performed a continuous hydrogenation of 1-butene in supercritical carbon dioxide.[9,10] A fluorous derivative of Wilkinson s catalyst was prepared in situ by mixing the ligand with [(COD)RhCl]2 under hydrogen / carbon dioxide pressure (Figure 4.37). 

Hydrogen bromide is prepared in situ by heating sodium bromide and concentrated sulphuric acid ... 

The stereoselective isomerization of allyl silyl ethers to (E)- or (Z)-silyl enol ethers was carried out in the presence of a cationic iridium(i) catalyst. The complex, prepared in situ by treating [Ir(cod)2]PFf,/2PPi3 with hydrogen was... 

The water-soluble analogue of Wilkinson s catalyst, [RhCl(TPPMS)3] [TPPMS = PPh2(C6H4S03Na)], prepared in situ from Rh(/<-Cl)(diene)]2 and TPPMS, reacts with hydrogen in aqueous solution to yield [RhH(TPPMS)3], instead of [RhH2(TPPMS)3], according to Eq. (6) ... 

The hydrogenation catalysts can be prepared in situ, starting from the surface alkyl complex. In terms of catalytic performances, these catalysts are highly effective (Table 6.21) [150]. The best hydrogenation systems are based on silica supported dinuclear complexes, for which the structures of the active sites have not been investigated. Hydrogenation of toluene and xylenes can be achieved under similar conditions. 

Water-soluble complexes constitute an important class of rhodium catalysts as they permit hydrogenation using either molecular hydrogen or transfer hydrogenation with formic acid or propan-2-ol. The advantages of these catalysts are that they combine high reactivity and selectivity with an ability to perform the reactions in a biphasic system. This allows the product to be kept separate from the catalyst and allows for an ease of work-up and cost-effective catalyst recycling. The water-soluble Rh-TPPTS catalysts can easily be prepared in situ from the reaction of [RhCl(COD)]2 with the sulfonated phosphine (Fig. 15.4) in water [17]. 

Generally, the imine substrates are prepared from the corresponding ketone and amine and are hydrogenated as isolated (and purified) compounds. However, reductive animation where the C = N function is prepared in situ is attractive from an industrial point of view, and indeed there are some successful examples reported below [18, 19]. It is reasonably certain that most catalysts described in this chapter catalyze the addition of H2 directly to the C=N bond and not to the tautomeric enamine C = C bond, even though enamines can also be hydrogenated enantioselectively. 

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