- Amines are extremely important intermediates and end products of the chemical industry and are often obtained by hydrogenation of the corresponding nitro compounds or imines. - A search of the literature reveals, that hydrogenation of nitro compounds catalyzed by well-defined molecular complexes in aqueous solutions is rare. - One reason may line in the fact, that reduction of the function proceeds in one-electron steps, while many soluble hydrogenation catalysts act in the “oxidative addition of. - elimination of the product” cycles in which the central metal ion (formally) looses or gains two electrons at a time. - It is not surprising therefore, that the catalysts of nitro-hidrogenations are either metal centered radicals themselves or are capable of delocalizing the temporary surplus of electron(s) on their large conjugated system or on a cluster framework Catalysts, operating through formation of intermediate monohydrides, which does not require the change of the oxidation state of the metal, are good candidates of nitro- reduction (see also 3.8.2) on reductions with. - the very slight temperature dependence of the rate, hydrogenolysis of carbon-halogen bonds, and sensitivity to radical scavangers, are also in accord with the formation of radicals during the hydrogenation process. - The selectivity of the hydrogenation halo-nitro aromatic compounds can be influenced by cyclodextrins, as additives, or by using cyclodextrin- derived catalysts [232] (see Ch .10).. - could be achieved by using monosulfonated BDPP as ligand in the in situ prepared Rh-catalyst, whereas with the bissulfonated ligand a practically racemic product (2 % e.e.) was obtained Note, that monosulfonated BDPP is chiral at one of the phosphorus atoms, and it was determined by HPLC that it contained a 1:1 ratio of the two epimers. - It is also important to add, that under comparable conditions, the enantiomeric excess of the hydrogenation of acid and its methyl ester decreased monotonously with increasing degree of sulfonation (from 87 % to 65 % and from 74 % to 45. - Presumably, one of the sulfonate groups acts as the anion of the cationic rhodium center and in case of the monosulfonated BDPP this gives an organosoluble 1:1 zwitter-ionic product (Scheme 3.27).. - Coordination of the group of the ligand to the rhodium may, indeed, be important in the observed effect. - Nevertheless, these studies nicely emphasized the warning of the authors of [146]: “..large changes in enantioselectivity result from small energy differences (well below 5 kcal/mol) which can arise from apparently minor effects which are difficult to evaluate, such as solvation energies”. - Transfer hydrogenation is a reaction in which hydrogen is catalytically transferred from a suitable hydrogen donor to a reducible substrate (S) yielding the hydrogenated product and the oxidized form of the donor molecule (D . - Several of the most common hydrogen donors, such as formic acid and formates, ascorbic acid, EDTA or 2-propanol are well or at least sufficiently soluble in water. - This reaction served also as one of the model processes in development of new reactors, such as the centrifugal partition chromatograph, for high throughput catalyst testing [246-248].. - Use of the chiral tetrasulfonated cyclobutanediop, 37, led to an enantiomeric excess of up to 43. - Aqueous sodium formate served as hydrogen donor in the reduction of aldehydes catalyzed by [202]. - An important feature of the reaction is the strong substrate inhibition which does not allow the reduction of e.g. - or coordination of the substrate aldehydes.. - Although the aldehydes are sufficiently soluble in water to allow a fast reaction, still most of the substrate is found in the organic phase at all times. - Therefore the concentration of the aldehydes in the catalyst- containing aqueous phase is not high enough to cause efficient inhibition of catalysis [250]. - In contrast to the case of the water soluble complexes (P = PTA, TPPMS or TPPTS) which did not promote the reduction of function in aldehydes or ketones in biphasic systems, was found an active catalyst for reduction of ketones with aqueous HCOONa (Scheme 3.32). - Most of the chloride was found in the aqueous phase which means that the equilibrium depicted on Scheme 3.33 was largely shifted to the right. - Under the conditions of Scheme 3.34 turnover frequencies in the range of. - Of the several water-soluble substrates the cyclic cyclopropanecarboxaldehyde reacted faster than the straight-chain butyraldehyde, and aldehydes were in general more reactive than the only simple ketone studied (2-butanone). - The reaction rate of the reduction of these carbonyl compounds showed a sharp maximum at pH 3.2, which coincides with the value of HCOOH in the studied concentration, and there was no reaction above pH 5. - The lack of reactivity at higher pH can be attributed to the formation of the catalytically inactive hydroxide-bridged trimer, which, however, is in equilibrium with the starting catalyst precursor at the optimum pH of the reaction. - The active form of the catalyst is most probably. - It is instructive to see, that in biphasic aqueous organometallic catalysis a seemingly minor change (dissolving the catalyst in the aqueous or, contrary, in the organic phase) may lead to major changes in the rate and/or the selectivity of the catalyzed reaction under otherwise identical conditions.. - It is supposed that reduction of the carbonyl compounds takes place on this dimer (Scheme 3.35).. - No attempt was made to clarify the nature of the active catalytic species, which -under these conditions- may well be a fine colloid of Pd metal.. - of the aminosulfonic acid ligands, depicted on Scheme 3.37, a series of acetophenones were reduced with high enantioselectivity in 2-propanol containing 15 % water. - The same system produced in the absence of reducible substrates [266]. - Hydrogenolysis of the C-Halogen bond is a valuable technique in organic chemistry and also a potential method for destroying halogen-containing wastes (polychlorinated aromatics belong to the most notorious pollutants).. - Most of the reactions studied so far are based on hydrogen transfer from a water-soluble donor molecule. - Interestingly, of the two ruthenium complexes was a less effective catalyst for the reactions of carbon tetrachloride and chloroform, however, it showed appreciably higher catalytic activity in the dehalogenation of hexyl halides. - 3-Chloro-1-phenylpropene (cinnamyl chloride) was reductively dehalogenated in water/n-heptane biphasic systems by hydrogen transfer from formates using Pd(II) complexes with sulfonated phosphine ligands of the type 21 (Scheme . - In contrast to the hydrogenation of prochiral imines (Table 3.9) the enantioselectivity of the hydrogenolysis of sodium cis-epoxysuccinate decreased monotonously with the increasing number of sulfonate groups, i.e.. - Therefore the exceptional solubility of the Rh(I)- complex of monosulfonated BDPP in organic solvents does not play a role here.. - One possibility is to recover from industrial end-gases or gaseous intermediates, which has already been practiced on large scale in the natural gas industry or in case of the. - The chemistry of the “fixation of ” by metal complexes has been reviewed quite frequently [279-285] therefore this short chapter covers only the partially or fully aqueous systems.. - However, in aqueous solution hydration of the solutes makes the overall enthropy difference smaller, and reaction (3.13) becomes slightly exergonic with. - the distribution of the possible reactive species is highly dependent on the actual pH, temperature and pressure [286]:. - The reaction was substantially accelerated by very small amounts of water (already 0.1 mmol water had the same effect in 10 mL benzene as the 500 mmol used routinely), therefore it is unlikely that the rate increase would reflect the physical change of the bulk solvent. - was prepared from and in the. - catalyst precursor for hydrogenation in THF and also observed acceleration of the reaction in the presence of water [290]. - With careful spectroscopic measurements they could detect the formation of the. - dihydrides, and and also that of the. - bidentate formato complex, It was therefore suggested that the mechanism of the reaction involved the insertion of into the Rh-H bond of the dihydride yielding a hydridorhodium-formato intermediate, followed by reductive elimination of formic acid then oxidative addition of to regenerate the dihydride (Scheme 3.43).. - It is disclosed, that in water/2-propanol mixtures the yield of formic acid was a function of the molar composition of the solvent. - 80 °C, 27 bar 54 bar It seems that greater difficulties are in the separation of the product formic acid from the reaction mixture than in the chemistry of its production - ingenious approaches are also found in the patent literature [292,293].. - Mild conditions were sufficient to provide good conversions of the reactants in the aqueous solution, e.g. - It is also interesting, that the primary products of the reaction were formic acid and formaldehyde, which later decomposed to give CO and (and. - [282,295] for the hydrogenation of in aqueous solutions in the presence of amines or aminoalkanols. - It was found recently that water-soluble transition metal phosphine complexes catalyze the hydrogenation of bicarbonate with much higher rate than that of For example, with the catalyst a was determined when the solution of the complex in was pressurized with 20 bar and 60 bar at 24 °C. - Based on similar observations, a detailed study of the hydrogenation of bicarbonate was undertaken with catalysts such as. - Note, that in the absence of. - When a mixture of and was passed through a flow reactor ( each) containing an aqueous solution of the Rh- PTA catalyst, the major product was CO accompanied by a few % of methane (Scheme 3.48). - At 70 °C the activity of the catalyst for CO production reached a The peculiarity of this system is in the production of methane which had not been observed before with homogeneous catalysts. - and acac = acetylacetonate, have been claimed recently as catalysts for the removal of C oxides from mixtures of or CO and by passing the gas mixture through a homogeneous aqueous acid solution of the complexes [303]. - Biological membranes are important constituents of living cells, separating and at the same time connecting the inside of the cell and the extracellular space as well as the different cellular compartments. - photosynthesis) are catalyzed by membrane-bound enzymes, and such processes are very sensitive to changes in the properties of the environment in which they take place. - Much effort is devoted to the understanding of the relationships between the properties of membranes and the activity of proteins in enzymic and transport phenomena [306,307].. - An example is shown on Scheme 3.49, in which one of the hydroxy groups of glycerol is esterified by phosphoric acid, while the other two by long chain fatty acids. - Certain lipids, including phosphatidyl cholines form vesicles (liposomes) in water upon dispersion by ultrasound, and such liposomes are often used in studies of the basic characteristics of membrane hydrogenations.. - In most (but not all) cases, lipids of natural membranes contain unsaturated acids in the cis-configuration and the steric bulk created by such a cis-double bond will not allow tight packing of the lipid molecules even when temperature is decreased. - heat or cold, etc.) can be correlated to the fluidity of the membrane [309].. - The water soluble (polar) catalyst should act on double bonds which are buried into the hydrophobic middle region of the lipid bilayer - no clear picture exists on how this happens. - Then, as hydrogenation of the membrane advances, the bilayer becomes more and more rigid which reduces the availability of the double bonds to the catalyst. - Diversity of the components means that in addition to lipids the catalyst shall encounter a variety of other molecules which may even inhibit hydrogenation (as happens with some proteins, amino acids, and carbohydrates [120]). - cis) hydrogenation of the lipid fatty acids have not yet been studied detail. - The lipids in a domain surrounding a specific protein, or in the inner half of the bilayer are less accessible by the catalyst arriving from the intercellular solution and therefore are hydrogenated to a smaller extent than the bulk lipids on the outer “surface” of the membrane.. - In a simple way, one kind of spatial selectivity can be achieved by appropriate timing of the reaction. - It was shown that saturation of lipids of the thylakoid (inner) membrane in the blue-green alga, Anacystis nidulans could be avoided by interrupting the hydrogenation at a proper time. - Topological selectivity requires the modification of only one side of the bilayer and selective surface hydrogenation of cells was demonstrated by colloidal catalysts [317].. - Finally, one also has to consider, that hydrogenation of live cells creates un unnatural composition and physical state (increased rigidity) of the membranes. - As can be seen from this short overview of the hydrogenation of biomembranes, one has to be very careful in the experimental work (choice of catalyst, conditions, reaction time, etc.) and even more careful in the interpretation of the changes such a manipulations bring in the properties of live cells. - A selection of the various systems which have already been studied by this method is given in Table 3.11 Further details can be obtained from the references in the Table.. - First, because the conditions are unprecedentedly mild, second, because it involves a sequential deprotonation of the complex and hydride transfer by by the resulting [Ru-H] hydride (Scheme 3.54). - bifunctional behaviour is supposed in case of the hydrogenase enzymes, too [1].. - It is suggested, that the mechanism of the reaction involves a six-coordinate transition state depicted on Scheme 3.56, which facilitates the hydride transfer to the 4-position. - Coordination of the benzylnicotinamide substrate requires an available coordination site on the. - metal, which is provided by ring slippage of the pentamethylcyclopentadienyl ligand. - In the latter system, containing also the NADH-dependent horse liver alcohol dehydrogenase (HLADH), cyclohexanone was reduced to cyclohexanol, and 2-methylthiacyclohexan-4-one to a mixture of the stereoisomers of the corresponding alcohol with moderate yields and good optical purity (Scheme 3.57).. - 3.17) in which the proper ratio of carbon monoxide and hydrogen is adjusted by making use of the water gas shift reaction (WGSR) (eq. - much has been learned about the mechanistic details of the WGSR, which may bring us closer to the discovery of an efficient and robust water soluble catalyst. - Based on the study of the chemistry of metal carbonyls under WGSR conditions, it is generally accepted that the reaction involves the following steps.. - in the following we shall see a few such compounds [359,360]. - These, too, can be intermediates in the WGSR [346]. - The best known catalysts of the water gas shift reaction are the binary metal carbonyls ( M = Cr, Mo, W and. - In the basic. - One of the most active catalysts is - in acetic acid solution [351]. - The mechanism of the reaction with catalyst is rather unexpected. - What makes this mechanistic suggestion sound (and the investigations elegant) is in that all intermediates were isolated or characterized, and all steps of the reaction were studied individually, too.. - Upon lowering the CO pressure, an increased yield of the diazaoxide (azoxybenzene) byproduct was observed.. - The primary product of the reaction is ethene (Scheme 3.62) which is reduced further to ethane in a separate catalytic cycle.. - The dehydroxylation of the carboxylic acid on Scheme 3.63 may be related to the above dehalogenation in that it is supposed to take place via formation of an intermediate chloro-compound in the acidic reaction mixture [370].. - The activity of the water soluble catalyst strongly varied with the pH of the solution. - °C) in the presence of diamines [372] (Scheme 3.65). - Conversely, only saturated aldehydes were obtained in the presence of 4-dimethylaminopyridine.
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