Hydrogenation in aqueous solution

In metallic form, barium is very reactive, reacting readily with water to release hydrogen. In aqueous solution it is present as an ion with a +2 charge. Barium acetate, chloride, hydroxide, and nitrate are water-soluble, whereas barium arsenate, chromate, duoride, oxalate, and sulfate are not. Most water-insoluble barium salts dissolve in dilute acids barium sulfate, however, requkes strong sulfuric acid. 

The reaction is used for the chain extension of aldoses in the synthesis of new or unusual sugars. In this case, the starting material, L-arabinose, is an abundant natural product and possesses the conect configurations at its three chirality centers for elaboration to the relatively rare L-enantiomers of glucose and mannose. After cyanohydrin formation, the cyano groups are converted to aldehyde functions by hydrogenation in aqueous solution. Under these conditions, —C=N is reduced to —CH=NH and hydrolyzes rapidly to —CH=0. Use of a poisoned palladiurn-on-barium sulfate catalyst prevents further-reduction to the alditols. 

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) ... 

Several copper, silver, ruthenium, rhodium, and cobalt compounds (e.g., Ru-Cl3 aq, [RuC h(l)ipy) (bipy=2,2 -bipyridine), RhCl3 aq, fotx(dimelbylglyoxima-to)cobalt derivatives (cobaloximes), etc.) have been found to catalyze hydrogenations in aqueous solutions [9]. Although important for the early research into homogeneous catalysis, these catalysts did not gain synthetic significance. 

The ruthenium cluster [Ru2(i76-C6H6)H6]C12 is a catalyst for fumaric acid hydrogenation in aqueous solutions, with a turnover frequency of 35 h 1 at 50°C (86). 

Buxton GV, Mulazzani QG, Ross AB (1995) Critical review of rate constants for reactions of transients from metal ions and metal complexes in aqueous solutions. J Phys Ref Data 24 1035-1349 Cheek CH, Swinnerton JW (1964) The radiation-induced chain reaction between nitrous oxide and hydrogen in aqueous solutions. J Phys Chem 68 1429-1432 Christensen H, Sehested K (1980) Pulse radiolysis at high temperatures and high pressures. Radiat Phys Chem 16 183-186... 

In accordance with this theory, it was found that the homogeneous reduction of dichromate salts by molecular hydrogen in aqueous solution, represented by the equation... 

With this in mind, we test a trickle bed reactor using the hydrogenation in aqueous solution of maleic acid into succinic acid in the presence of a supported palladium catalyst. 

Biologically important substrates can be hydrogenated in aqueous solution by using sulfonated triarylphosphine ligands that confer water solubility upon the catalyst. However, it was later reported that rhodium metal was formed from these complexes in the presence of hydrogen. It was claimed that the role of the ligands is merely to prevent aggregation of the rhodium colloid produced. 

Streptomycin trihydrochloride was hydrogenated in aqueous solution at atmospheric pressure using platinum as catalyst. - About one molar equivalent of hydrogen was absorbed to yield a product which formed a crystalline trihelianthate from which a white powder, designated... 

Polydextrose may be partially reduced by transition-metal catalytic hydrogenation in aqueous solution. It may be neutralized with any food-grade base and/or decolorized and deionized for further purification. 

Catalytic hydrogenation in aqueous solutions is discussed in several general overviews [3-12], In this section, a cross-section of the field is given, with just some examples of catalysts and reactions. 

RuCl3.aq and RhCl3.aq were among the first catalysts of hydrogenation in aqueous solution [59, 60]. Spectrophotometric experiments revealed the formation of an Ru(II)-alkene complex prior to the heterolytic activation of H2 ( unsaturated route of hydrogenation). 

Several representatives of the widely studied tertiary phosphine ligands for enantioselective hydrogenation in aqueous solutions are shown as Structures 21 -39. It is seen that the most successful ligands in this field do have their water-soluble, mostly sulfonated, derivatives [7, 11]. In case of acid-sensitive compounds, solubility in water could be achieved by attaching dimethylamino substituents to the parent arylphosphines and by their further protonation or quaternization. Monophosphines, such as 21 [88], played a minor role in comparison with the chelating diphosphines. Water-soluble phosphine-phosphinite ligands (L) were obtained from trehalose and used as components of with [Rh(cod)(L)[BF4]] catalysts for the enantioselective hydrogenation ofenamides [157]. 

The hydrogen solubility was studied by using TMP-aldol and TMP-triol in aqueous mixtures. A correlation equation was obtained for solubility of hydrogen in aqueous solutions [20]. 

In the hydrogenation of 2-aminopyridine under acidic conditions two equivalents of hydrogen were absorbed, giving tetrahydroaminopyridine as the hydrochloride salt, which on further reduction was converted to piperidine hydrochloride and ammonia (74). When 2-aminopyridine hydrochloride was hydrogenated in aqueous solution four equivalents of hydrogen were absorbed, yielding piperidine hydrochloride and ammonia. 

The species which reacts in a given system is consequently normally the species formed initially. This paper is concerned with a discussion of the species produced in systems formerly regarded as producing nascent hydrogen in aqueous solution at pH 7. It is apparent that e aq is... 

Solubility op Hydrogen in Aqueous Solutions op Calcium Chloride, Magnesium Sulphate, and Lithium Chloride at 15°. 

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