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is so much different in water than in most organic solvents that one shouldalways keep this warning in mind.HYDROGENOLYSIS Transfer hydrogenation is a reaction in which hydrogen is catal

was more active (2 turnovers/h) Similar butwater soluble tungsten and molybdenum complexes are known [223-226]which would allow the use of water as solvent for such reactions It isnoteworthy, though, that ionic hydrogenation of ketones by dihydrogencomplexes has so far been observed only in non-aqueous solutions[223,227]; perhaps the coordination of a ketone is disfavoured in water due

to competition by

ORGANIC SUBSTRATES

3.4.1 Hydrogenation of nitro compounds and imines

Amines are extremely important intermediates and end products of thechemical industry and are often obtained by hydrogenation of thecorresponding nitro compounds or imines A search of the literature reveals,that hydrogenation of nitro compounds catalyzed by well-defined molecularcomplexes in aqueous solutions is rare One reason may line in the fact, thatreduction of the function proceeds in one-electron steps, while manysoluble 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 surprisingtherefore, that the catalysts of nitro-hidrogenations are either metal centeredradicals themselves or are capable of delocalizing thetemporary surplus of electron(s) on their large conjugated system

or on a cluster framework Catalysts, operatingthrough formation of intermediate monohydrides, which does not require thechange of the oxidation state of the metal, are good candidates of nitro-reduction (see also 3.8.2) on reductions with ) Unfortunately,other functional groups in a molecule are usually even more reactivetowards hydrogenation than the nitro functionality - therefore selectiveHydrogenations of nitro compounds catalyzed by arebriefly mentioned in 3.1.1.2 Some other hydrogenation catalysts with Pt(II),Pd(II) or Rh(III) central ions contain ligands with extended conjugated

such as 1-phenyl-azo-2-naphtol [228], indigosulfonic acid [229]and the sodium salts of 1,2-dioxy-9,10-anthraquinone-3-sulfonic acid(Alizarin red, QS) [73,74] It was established by EPR and NMRinvestigations [73,74], that with the Pd(II) complex of Alizarin red,

activation of dihydrogen takes place at the metal ion but theligand also takes important part in the redox reaction: it is reduced to areduction of groups is not an easy task

semiquinone radical Such radicals are rapidly oxidized by nitro compounds,and, indeed, this complex is an active catalyst for hydrogenation of nitrogroups at room temperature and 1 bar Other characteristics of reductionsinvolving i.e the very slight temperature dependence of the rate,hydrogenolysis of carbon-halogen bonds, and sensitivity to radicalscavangers, are also in accord with the formation of radicals during thehydrogenation process In addition of being capable of reduction ofgroups, is also a very effective catalyst for hydrogenations atlow temperatures and this property made it the catalyst of choice for thehydrogenation of many model- and biomembranes (see 3.7).

Catalytic hydrogenation of chloro-nitroaromatics is usually accompanied

by dehalogenation However, the water-soluble complex prepared from

and TPPTS in DMSO-containing water reduced nitrophenol to 2-amino-5-chlorophenol with high selectivity [231] (Scheme3.25)

5-chloro-2-The selectivity of the hydrogenation halo-nitro aromatic compounds can

be influenced by cyclodextrins, as additives, or by using derived catalysts [232] (see Ch 10)

cyclodextrin-Asymmetric hydrogenation of imines was studied in aqueous/organic

biphasic systems and presented a puzzle which is still not solved completely

It was first discovered by Bakos et al [233], that acetophenone benzylimineswere hydrogenated to the corresponding amines with unprecedentedenantioselectivity up to 96% under very mild conditions with the Rh-

complexes of sulfonated BDPP, 36, provided the degree of sulfonation of

BDPP was close to 1 (in fact it was 1.41-1.65) (Scheme 3.26) With

increasing number of sulfonate substituents in 36 the enantioselectivity

decreased sharply

This “monosulfonation effect” was investigated in detail by de Vries et

al [234,235], who isolated the sulfonated BDPP with one, two, three andfour sulfonate groups (each phenyl ring carries only one) In hydrogenation

of acetophenone benzylimine it was confirmed, that indeed, the highestenantioselectivity (94 %) could be achieved by using monosulfonated BDPP

as ligand in the in situ prepared Rh-catalyst, whereas with the bissulfonatedligand a practically racemic product (2 % e.e.) was obtained Note, thatmonosulfonated BDPP is chiral at one of the phosphorus atoms, and it wasdetermined by HPLC that it contained a 1:1 ratio of the two epimers Nowthe puzzle is in that how can a ligand, which is a 1:1 mixture of twodiastereomers, induce such outstandingly high enantioselectivity what wasfound with the Rh-complex of monosulfonated BDPP in the hydrogenation

of imines It is also important to add, that under comparable conditions, the

its methyl ester decreased monotonously with increasing degree ofsulfonation (from 87 % to 65 % and from 74 % to 45 %, respectively).However, in case of itaconic acid there was a slight “monosulfonationeffect” (Table 3.9)

It is clear from Table 3.9, that the effect is not related to the difference inelectron density on the two phosphorus atoms since this should be the samewith and Still the catalyst with trisulfonated BDPP gave

miserable enantioselectivity with both substrates It is also important, thatthe Rh-complex with the monosulfonated BDPP is well soluble in ethylacetate and moves completely to the organic phase during hydrogenation,while the other three sulfonated BDPP-s yield exclusively water solublecomplexes Presumably, one of the sulfonate groups acts as the anion of thecationic 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 It was found by Buriak andOsborn [146,147] that in microemulsions, prepared with the surfactant AOT(Scheme 3.11) the sulfonate group of AOT did coordinate to rhodium in the

complex It was suggested that this led to an easierexample in case of (Scheme 3.28), i.e to a switch from a dihydride route

of hydrogenation to a monohydride pathway How this would lead to highenantioselection still remains elusive

Nevertheless, these studies nicely emphasized the warning of the authors

of [146]: “ large changes in enantioselectivity result from small energydifferences (well below 5 kcal/mol) which can arise from apparently minoreffects which are difficult to evaluate, such as solvation energies” Solvationdeprotonation of an intermediate dihydride species in case of than for

is so much different in water than in most organic solvents that one shouldalways keep this warning in mind.

HYDROGENOLYSIS

Transfer hydrogenation is a reaction in which hydrogen is catalyticallytransferred from a suitable hydrogen donor to a reducible substrate(S) yielding the hydrogenated product and the oxidized form of thedonor molecule (D) [236-238]

Several of the most common hydrogen donors, such as formic acid andformates, ascorbic acid, EDTA or 2-propanol are well or at least sufficientlysoluble in water In addition, itself can serve as a source of hydrogen.Frequently, hydrogenation of unsaturated substrates is achieved by using

mixtures; such reactions are discussed in 3.8 As written in eq.(3.11) the hydrogen transfer reaction is often reversible, an obvious examplebeing the reduction of ketones using 2-propanol as donor

Reductions with hydrogen transfer are attractive for at least two reasons.First, the concentration of in the reaction mixture can be much higherthan that of under high pressure (cf for example and

in water at 1 bar pressure) This may be beneficial for afaster reaction Second, the use of a soluble or liquid hydrogen donor alsoeliminates the safety hazard of handling high pressure hydrogen

Formic acid and formates were among the most effective donors used forthe reduction of olefins with or catalysts innon-aqueous systems [239-241] No wonder, the water soluble analogues ofthese catalysts became widely used in aqueous solutions In a series ofinvestigations [242-245] with Ru/TPPMS and Rh/TPPMS catalysts olefins(such as 1-heptene) were hydrogenated in mixtures of HCOOH/HCOONa.Crotonaldehyde was selectively reduced to butyraldehyde by the

catalyst [245] It was also established that (unfiltered)ultraviolet irradiation accelerated the reactions [245]

Dimethyl itaconate was reduced by hydrogen transfer from aqueoussodium formate under mild conditions (Scheme 3.29) This reaction servedalso as one of the model processes in development of new reactors, such asthe centrifugal partition chromatograph, for high throughput catalyst testing[246-248]

Based on isotope labelling experiments in and

mixtures it was suggested, that the reaction mechanism involved arhodacyclobutane intermediate (Scheme 3.30) In this respect the reactionpathway differs substantially from those of hydrogenations with

Water-soluble Rh(I) complexes containing TPPTS catalyzed the transferhydrogenation of itaconic, mesaconic, citraconic and tiglic acids as well as

[235] The reactions were run at 50 °C for 15-67 h,during which 48-100 % conversions were achieved Use of the chiral

tetrasulfonated cyclobutanediop, 37, led to an enantiomeric excess of up to

43 %, which is close to the value obtained in biphasic hydrogenations

catalyzed by the same rhodium complex [100]

Aqueous sodium formate served as hydrogen donor in the reduction ofaldehydes catalyzed by [202] Since in this case both thecatalyst and the substrate reside in the organic phase, a phase transfer agentwas necessary to carry from the aqueous into the organic phase;

were applied for this purpose An important feature of thereaction is the strong substrate inhibition which does not allow the reduction

of e.g benzaldehyde in solutions with higher than 0.8 M aldehydeconcentration The precise nature of this substrate inhibition is not clear; itmay be due to formation of catalytically unreactive intermediates either via

or coordination of the substrate aldehydes

The same reaction was investigated in a reverse experimental setup, i.e

and the hydrogen donor HCOONa in the aqueous phase and the substratealdehyde together with the products in the organic (chlorobenzene) phase[249,250] Unsaturated aldehydes, such as cinnamaldehyde (Scheme 3.18)

and citral (Scheme 3.31) were reduced to the corresponding unsaturated

alcohols with high selectivity No cis-trans isomerization was observed

around the double bond

It is important to note, that in this arrangement of an aqueous/organic

biphasic reaction the substrate inhibition discussed above was hardly

observable 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] Under comparable conditions, Ru(II) and Rh(I) complexes

of PTA behaved very similar to their TPPMS-containing analogues in that

led to exclusive formation of unsaturated alcohols [27,204]

while catalysis by selectively produced saturated aldehydes inreduction of unsaturated aldehydes with [27,28,204]

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) The reaction was aided by phase transfer catalysis using

Aliquat-336 and required a large excess of to prevent reduction of

rhodium into inactive metal Substrates like acetophenone, butyrophenone,

cyclohexanone and dibenzyl-ketone were reduced to the corresponding

secondary carbinols with turnover frequencies of [251]

It is not easy to rationalize this difference in the selectivity provided by

aqueous phase One reason may be in that when a solution of

in a water-immiscible organic solvent is stirred with an aqueoussolution of a mild base (HCOONa in this case), formation of

can be assisted by extraction of chloride into the aqueous phase (Scheme3.33)

Although there is no evidence for this process taking place in the

reduction of ketones with hydrogen transfer from formate [251], in a related system the rate of hydrogenation of acetophenone, catalyzed by the same

catalyst in the presence of was substantially increased upon mixing the

organic solution with water (Figure 3.4) [252] Most of the chloride wasfound in the aqueous phase which means that the equilibrium depicted onScheme 3.33 was largely shifted to the right This is supported by thefinding that when a 0.5 M aqueous was added instead of asolution, the reaction proceeded with the original low rate On the other

The water-soluble iridiurn(III) complex,

was found a suitable catalyst precursor for reduction of aldehydesand ketones by hydrogen transfer from aqueous formate [254] Under theconditions of Scheme 3.34 turnover frequencies in the range of

were determined Of the several water-soluble substrates the cycliccyclopropanecarboxaldehyde reacted faster than the straight-chainbutyraldehyde, and aldehydes were in general more reactive than the onlysimple ketone studied (2-butanone) While glyoxylic acid was reduced fast,pyruvic acid did not react at all

The reaction rate of the reduction of these carbonyl compounds showed asharp 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 thecatalytically inactive hydroxide-bridged trimer, which,however, is in equilibrium with the starting catalyst precursor at theoptimum pH of the reaction The active form of the catalyst is most probably

hand, is known to be a good catalyst for ketone hydrogenation

in the presence of amines [253]

It is instructive to see, that in biphasic aqueous organometallic catalysis aseemingly minor change (dissolving the catalyst in the aqueous or, contrary,

in the organic phase) may lead to major changes in the rate and/or theselectivity of the catalyzed reaction under otherwise identical conditions

highest extent at pH 3.2; the compound was characterized in solution and inisolated from, as well It is supposed that reduction of the carbonylcompounds takes place on this dimer (Scheme 3.35).

In the presence of aqueous NaOH, palladium(II) chloride was effectivefor the transfer hydrogenation of unsaturated acids, azlactones andphenylpyruvic acid (Scheme 3.36) at 65 °C although in quite long reactiontimes (typically 16 h) [255] For these water-soluble substrates organicsolvents were not required No attempt was made to clarify the nature of theactive catalytic species, which -under these conditions- may well be a finecolloid of Pd metal.

Hydrogen transfer to ketones from 2-propanol was developed into anextremely efficient method of obtaining secondary alcohols [256,257] andthe use of chiral N-(p-tolylsulfonyl)diamines allow the reduction ofprochiral ketones with extraordinary stereoselectivity [257-259] In general,water is not well tolerated in such processes, and several studies showed thatboth the rate and the enantioselectivity of transfer hydrogenations from 2-propanol decrease substantially in increasingly aqueous mixtures even in thepresence of water soluble catalysts [260,261] However, in a recent studythe opposite effect was found Using the water soluble Rh- and Ir-complexes

of the aminosulfonic acid ligands, depicted on Scheme 3.37, a series ofacetophenones were reduced with high enantioselectivity in 2-propanolcontaining 15 % water Unexpectedly, raising the water concentration to 34and then to 51 % further increases in both the rate and enantioselectivitywas observed [262,263].

Simple alkenes [264] as well as unsaturated phospholipids [265] werehydrogenated in photochemically assisted hydrogen transfer reactions fromaqueous solution of ascorbic acid The reactions took place in solutionsbuffered to pH 5-6 upon illumination with visible light using

produced in the absence of reducible substrates [266] Interestingly, other

were ineffective in this reaction [265]

Hydrogenolysis of the C-Halogen bond is a valuable technique in organicchemistry and also a potential method for destroying halogen-containingwastes (polychlorinated aromatics belong to the most notorious pollutants).Catalysis in water is particularly important since it gives hope for processessuitable for environmental cleanup Complexes of various metals, such as

Rh, Ru but most of all Pd can catalyze this reaction, with or withoutphosphine ligands Most of the reactions studied so far are based onhydrogen transfer from a water-soluble donor molecule; gaseous (molecular)being less favoured

Chloroarenes were efficiently hydrodechlorinated with a

( or ) catalyst in biphasic systems under mild conditions [267].The catalyst tolerates the presence of a variety of functional groups (R, OR,COAr, COOH, ) Some chloro heterocycles (e.g 5-chloro-l-ethyl-2-methylimidazole) can be readily dehalogenated, but 2-chlorotiophene doesnot react at all

Several aliphatic and benzylic halides were dehalogenated by hydrogentransfer from sodium or ammonium formate with

3.10, carbon tetrachloride and benzyl chloride were particularly reactive Asexpected, the order of reactivity was and chlorobenzeneremained unchanged Interestingly, of the two ruthenium complexes

was a less effective catalyst for the reactions of carbontetrachloride and chloroform, however, it showed appreciably highercatalytic activity in the dehalogenation of hexyl halides The turnovernumbers (TON-s) in Table 3.10 were obtained in 3 h reactions and there is aremarkable difference to an analogous system, with ascatalyst, where benzyl chloride was reduced by HCOOLi in refluxingdioxane with only 26 turnovers in 6 h [269]

Under the conditions of Table 3.10, HCOOH is decomposed and theyield of reaches only 3.5% (in contrast to HCOONa, where the 478TON corresponds to 60 % conversion) The use of 5 bar instead offormate as hydrogen donor leads to 1.8 % conversion The reason for thislow conversion can be the low concentration of dissolved hydrogen (<0.004

M under the reaction conditions) as opposed to that of formate (1.67 M);such a limited reactivity has also been observed in related non-aqueoussystems [270] It is worth mentioning that also decomposes duringthe reaction as indicated by the pressure rise in the reactor, however, theconversion of still reaches 31 %

3-Chloro-1-phenylpropene (cinnamyl chloride) was reductivelydehalogenated in water/n-heptane biphasic systems by hydrogen transferfrom formates using Pd(II) complexes with sulfonated phosphine ligands of

the type 21 (Scheme 3.38) [271] Addition of polyether detergents increased

the rate of hydrogenolysis and supressed the formation of alcohol

byproducts, probably acting as a phase transfer catalyst for formate salts.The best selectivity (90 %) for 1-phenylpropene was achieved by the use oftriethylene glycol.

The usefulness of a “built-in” phase transfer catalyst function wasdemonstrated in the hydrogenolysis of 1-chloromethyl-naphtalene (Scheme3.39) catalyzed by complexes having a crown-ether moiety in thephosphine ligand [67] Both solid HCOOK and its aqueous solution could

be used as hydrogen donor with the bifunctional catalyst By comparison,

was catalytically inactive with solid potassium formate andgave only low conversions with aqueous HCOOK solutions [66]

A “counter phase transfer catalysis” effect was observed in reduction ofallyl chloride with sodium formate in water/n-heptane systems with watersoluble palladium(II) catalysts having phosphine ligands with polyetherchains [274]; it was demonstrated that the catalyst transported the substrate

to the aqueous phase where it reacted with the formate

Under “ligandless” conditions catalyzed the hydrogenolysis ofseveral 4-substituted aryl chlorides in alkaline aqueous solutions using

as reductant (Scheme 3.40) [275] In case of certain

ortho-substituted substrates, such as 2-chlorophenolate and 2-chloroaniline,strong chelation in the intermediate palladacycle completely inhibited thereaction On the other hand, in case of 2-chlorobenzoic acid addition ofiodide led to 86 % yield of benzoic acid

could also be used for the hydrogenolytic removal of phenolichydroxy groups However, in this case phenols, e.g 4-methoxyphenol, had

to be transformed first into monoaryl sulfates which, in turn, could be

[276] Again, no phosphines or other organic ligands were required for anefficient reaction

Homogeneous catalytic asymmetric hydrogenolysis of epoxysuccinatesoffers a route to the preparation of chiral malic acid derivatives [277] whichare useful building blocks in natural product synthesis The reaction wasstudied in aqueous solution using a catalyst prepared from

and sulfonated BDPP (36) with varying degree of sulfonation [278] (Scheme

3.41) 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

no “monosulfonation effect” was observed (see 3.4.1) The reason probably

is in that the sodium salt of cis-epoxysuccinic acid dissolves well in water

where the catalysis takes place, in contrast to imines and esters ofdehydroamino acids Therefore the exceptional solubility of the Rh(I)-complex of monosulfonated BDPP in organic solvents does not play a rolehere

In a related reaction, racemic sodium trans-phenylglycidate was

hydrogenolyzed with kinetic resolution (Scheme 3.41) With tetrasulfonated

(S,S)-BDPP as ligand, the Rh-complex preferentially catalyzed the reaction

of the (2R,3S)-epoxide yielding a product mixture rich in

(2R)-2-hydroxy-3-phenylpropionate

Cyclodextrins are used in order to influence the rate and/or selectivity ofhydrogenolytic reactions, too A few such reactions are discussed in Chapter10

AQUEOUS SOLUTION

Carbon dioxide has acquired a rather controversial status recently It isthe carbon source of life on earth through green plant photosynthesis, yet,through its major contribution to the “greenhouse effect” its increasingconcentration in the atmosphere poses a serious threat to the stability ofglobal climate In order to avoid abrupt climatic changes and all the dangersthey may bring, in addition to many other measures, anthropogenicemission must be decreased One possibility is to recover fromindustrial end-gases or gaseous intermediates, which has already beenpracticed on large scale in the natural gas industry or in case of the

mixtures obtained by the water gas shift reaction (see 3.8) It would behighly desirable to make use of such recovered (often in form of anaqueous solution) as a C1 building block in organic synthesis instead offinding ways of long-term sequestration Of course, concentrated, cheap

is also available from natural sources For all these reasons the possibleapplication of carbon dioxide as a raw material has attracted much interestand several outstandingly active catalysts have been discovered for itshydrogenation, especially under supercritical conditions, in the presence ofvarious amines The chemistry of the “fixation of ” by metal complexeshas been reviewed quite frequently [279-285] therefore this short chaptercovers only the partially or fully aqueous systems

Since carbon dioxide is a thermodynamically stable, highly oxidizedcompound, its synthetic utilization requires some kind of a reduction -reaction with molecular hydrogen is a distinct possibility Stepwisereduction of with may yield formic acid, formaldehyde, methanoland finally methane, together with CO or Fischer-Tropsch-type derivatives

as shown on Scheme 3.42 In aqueous organometallic catalysis the mostcommon product of such a reduction is formic acid Formation of carbonmonoxide, formaldehyde, and methane has already been reported, however,methanol and Fischer-Tropsch type products were not observed

Reaction of two gaseous compounds resulting in a liquid product arebiased by a decrease in enthropy which –depending on the temperature–may make the whole process thermodynamically unfavourable This is alsothe case for the hydrogenation of to HCOOH (eq 3.12) with

However, in aqueous solution hydration of the solutes makes the overallenthropy difference smaller, and reaction (3.13) becomes slightly exergonicwith

Thermodynamics tells, therefore, that in water this reaction is likely toproceed; we must not forget, though, that these data refer to standardconditions, and in order to eliminate the kinetic activation barrier at 25 °Chighly active catalysts are needed Unfortunately, the same catalysts can also

be active in the reverse process, i.e in the decomposition of formic acid toand at low pressures; decomposition to CO and (i.e the reversewater gas shift) is rarely observed

Equation (3.13) can also be shifted to the right by further reactions ofHCOOH which may be simply its neutralization with a base, or reactionswith amines or alcohols, yielding formamides or formate esters,respectively In this context it is worth recalling the

equilibrium (3.14) in water; the distribution of the possible reactive species

is highly dependent on the actual pH, temperature and pressure [286]:

The beneficial effect of water was observed in several experiments onreduction of Inoue et al were the first to discover that in the presence

of a base (NaOH, etc.) transition metal phosphine

complexes of l,2-bis(diphenylphosphino)ethane (diphos) and such as

and

catalyzed the formation of HCOOH from and (25 bareach, room temperature, benzene) with 12-87 turnovers in 20 h [287] Thereaction was substantially accelerated by very small amounts of water(already 0.1 mmol water had the same effect in 10 mL benzene as the 500mmol used routinely), therefore it is unlikely that the rate increasewould reflect the physical change of the bulk solvent Interestingly, when

was used as a base, formic acid was obtained, albeit with very lowyield (3 turnovers in 20 h), even in the absence of

presence of water It was supposed, that was reduced first by toformic acid, decarbonylation of which by afforded the targetcompound [288]

Homogeneous catalytic synthesis of formic acid (formate salts) fromunder catalysis by in alkaline aqueous ethanol was reported[289] Typically, the solvent contained 20 % v/v water, the base wasand the reactions were run at 60 °C, under 60 bar (1:1) for 5 h.Under such conditions, itself catalyzed the formation of HCOOH with

while addition of increased the reaction rate toFrom the reaction mixture could be isolated,which was also supposed to be the key catalytic intermediate in the reaction.Although the role of water was not specifically addressed it is worth notingthat exclusively formic acid i.e no ethyl formate was produced despite thepresence of 80 % ethanol in the solvent

catalyst precursor for hydrogenation in THF and also observedacceleration of the reaction in the presence of water [290] With carefulspectroscopic measurements they could detect the formation of the

bidentate formato complex, It was therefore suggestedthat the mechanism of the reaction involved the insertion of into theRh-H bond of the dihydride yielding a hydridorhodium-formatointermediate, followed by reductive elimination of formic acid thenoxidative addition of to regenerate the dihydride (Scheme 3.43)

It was also suggested [290] that the rate accelerating effect of water wasdue to formation of an intermolecular hydrogen bond between theligand in and the incoming within the insertiontransition state, such as depicted (A) on Scheme 3.44.

The existence of such or closely related intermediates received support

from studies on the water effect in the hydrogenation of carbon dioxide

[291] This complex catalyzes the reduction of to formic acid with an

15 mL/5 mL, 2 mL) Since addition of inhibited the

reaction, it was concluded, that the catalytic cycle probably does not involve

key catalytic intermediate, capable of simultanous transfer of its hydride,

and a proton from the coordinated to the incoming as depicted (B)

in Scheme 3.44 Elimination of formic acid this way probably generates a

transient hydroxo species, which then coordinates a dihydrogen molecule

suggestion is supported by the observation, that the catalyst precursor,

is converted to under

8 bar/8 bar in anhydrous THF in 10 min at 80 °C

Several patents describe the production of formic acid or formates by

hydrogenation of bicarbonates or carbonates [292,293] It is disclosed,

that in water/2-propanol mixtures the yield of formic acid was a function of

the molar composition of the solvent While in water the yield was 13 %, it

increased sharply to 54.5 % in 2-propanol/water 20/80, passed through a

maximum at 60/40 (60.7 %) and fall back to 43.4 % in neat 2-propanol In

this particular case the reaction conditions were the following:

80 °C, 27 bar 54 bar It seems that greaterdifficulties 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]

Based on the results discussed above, it can be concluded, that in many

cases water is advantageous for the hydrogenation of carbon dioxide It is

interesting to note, therefore, that although attempts had been made to react

and water-soluble phosphine complexes of transition metals already in

1975 [48], the first successful hydrogenation of catalyzed by a

transition metal compound in a fully aqueous system was reported only in

conditions were sufficient to provide good conversions of the reactants in

the aqueous solution, e.g at 40 °C, with 3 bar and 17 bar a turnover

frequency was achieved, without a need for any other

additive It is also interesting, that the primary products of the reaction were

formic acid and formaldehyde, which later decomposed to give CO and

(and ) Although without any spectroscopic or other evidence, the

catalytic cycle was suggested to involve formation of a metallocarboxylic

acid (via “abnormal insertion” [279] of into the Ru-H bond), as shown

on Scheme 3.45

Water-soluble rhodium complexes, such as or the ones

Leitner et al [282,295] for the hydrogenation of in aqueous solutions in

the presence of amines or aminoalkanols In this system no other products of

carbon dioxide reduction, such as formaldehyde or methanol could be

detected There was no formation of HCOOH in the absence of an amine,

however, a formic acid concentration of 3.63 M was obtained in an aqueous

solution containing 3.97 M (well soluble in water as compared to

) and 5.4 mM Initial turnover frequencies were

substantially higher than any other before, e.g at 81 °C and 40 bar total

an overall activation barrier was determined Interestingly,

under the same conditions the ruthenium complex, proved

much inferior to the Rh-TPPTS catalysts with a TOF of only In the

supposed catalytic cycle key role was assigned to a monohydrido-rhodium

complex (Scheme 3.46) which at that time could

not be supported by spectroscopic methods but which later became

characterized by and NMR spectroscopy [86]

It was found recently [203,296,298,299], that water-soluble transitionmetal phosphine complexes catalyze the hydrogenation of bicarbonatewith much higher rate than that of For example, with the

solution of the complex in was pressurized with 20 bar and 60bar at 24 °C Conversely, the same catalyst hydrogenated bicarbonate

Based on similar observations, a detailed study of the hydrogenation ofbicarbonate was undertaken with catalysts such as

and Highpressure and NMR measurements revealed, that in aqueous

converted to various hydrido-ruthenium complexes, such as

) and in phosphine excess:

Interestingly, the reaction rate of reduction was further increased

by applying pressure (Figure 3.5) Note, that in the absence of

i.e in aqueous solutions under pressure (pH 4.5) the rate ofhydrogenation was miserable

It was concluded, that the effect of increasing pressure was that of adecreasing pH, facilitating the formation of which exists inacidic solutions (vide supra) Based on this assumption, the suggestedcatalytic cycle involved insertion of bicarbonate to the Ru-H bond in

(Scheme 3.47) followed by protonation to liberate HCOOHfrom the complex

The same water-soluble catalysts were suitable for the hydrogenation ofaqueous suspensions of under pressure leading to formatesolutions with concentrations up to 0.93 M [296]

It should be mentioned here, that the heterogeneous catalytichydrogenation of bicarbonate in aqueous solution is a well-known process[300,301] Coupled to a catalytic decomposition of formate back to and

this reaction was even suggested as a method for storage andtransport of hydrogen [272] The best catalysts of bicarbonate hydrogenationconsist of metallic palladium, either supported, such as Pd/C, or colloidal,

photochemically assisted reduction of

A very interesting finding was published by Pruchnik et al who studiedthe hydrogenation of with catalysts prepared from

[302] When a mixture of and was passed through aflow 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 inthe production of methane which had not been observed before withhomogeneous catalysts Unfortunately, no further results have beenpublished and no suggestion concerning the catalytic cycle have been madeyet

The iridium cluster with = pyridylphosphines 67

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] Again, no mechanistic details of these extraordinary

reactions are known

In conclusion it seems, that many catalysts and processes are developed

for the catalytic hydrogenation of carbon dioxide, both homogeneous and

heterogeneous Aqueous systems are well suited for this process but

particular advantages can be gained also from using supercritical as a

solvent for its own reactions The major factor is certainly the economics of

this transformation which hinges on the availability of cheap hydrogen

However, the day may come when the use of as a Cl source will be

competitive with the utilization of fossil fuels; in addition to hydrogenation,

much depends on the success of research on selective carbon-carbon bond

formation reactions involving carbon dioxide [304,305]

INTEREST

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 These

membranes consist of a bilayer of polar lipids to which is attached

(“floating”, embedded or penetrating) a myriad of other constituents

(non-polar lipids, proteins, carbohydrates, etc.) Many fundamental processes of

life (e.g 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]