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98
Chapter 3
was more active (2 turnovers/h). Similar but
water soluble tungsten and molybdenum complexes are known [223-226]
which would allow the use of water as solvent for such reactions. It is
noteworthy, though, that ionic hydrogenation of ketones by dihydrogen
complexes 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
3.4 HYDROGENATION OF MISCELLANEOUS
ORGANIC SUBSTRATES
3.4.1
Hydrogenation of nitro compounds and imines
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 ). Unfortunately,
other functional groups in a molecule are usually even more reactive
towards hydrogenation than the nitro functionality - therefore selective

Hydrogenations of nitro compounds catalyzed by are
briefly 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 NMR
investigations [73,74], that with the Pd(II) complex of Alizarin red,
activation of dihydrogen takes place at the metal ion but the
ligand also takes important part in the redox reaction: it is reduced to a
reduction of groups is not an easy task.
Previous Page
Hydrogenation
99
semiquinone radical. Such radicals are rapidly oxidized by nitro compounds,
and, indeed, this complex is an active catalyst for hydrogenation of nitro
groups at room temperature and 1 bar Other characteristics of reductions
involving i.e. 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. In addition of being capable of reduction of
groups, is also a very effective catalyst for hydrogenations at
low temperatures and this property made it the catalyst of choice for the
hydrogenation 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 5-chloro-2-
nitrophenol to 2-amino-5-chlorophenol with high selectivity [231] (Scheme
3.25).
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).
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 benzylimines
were hydrogenated to the corresponding amines with unprecedented
enantioselectivity 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.
100
Chapter 3
This “monosulfonation effect” was investigated in detail by de Vries et
al. [234,235], who isolated the sulfonated BDPP with one, two, three and
four sulfonate groups (each phenyl ring carries only one). In hydrogenation
of acetophenone benzylimine it was confirmed, that indeed, the highest
enantioselectivity (94 %) 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. Now
the puzzle is in that how can a ligand, which is a 1:1 mixture of two
diastereomers, induce such outstandingly high enantioselectivity what was
found with the Rh-complex of monosulfonated BDPP in the hydrogenation
of imines. 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 %, respectively).

However, in case of itaconic acid there was a slight “monosulfonation
effect” (Table 3.9).
It is clear from Table 3.9, that the effect is not related to the difference in
electron density on the two phosphorus atoms since this should be the same
with and Still the catalyst with trisulfonated BDPP gave
Hydrogenation
101
miserable enantioselectivity with both substrates. It is also important, that
the Rh-complex with the monosulfonated BDPP is well soluble in ethyl
acetate and moves completely to the organic phase during hydrogenation,
while the other three sulfonated BDPP-s yield exclusively water soluble
complexes. 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. It was found by Buriak and
Osborn [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 easier
example 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 high
enantioselection still remains elusive.
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”. Solvation
deprotonation of an intermediate dihydride species in case of than for
102
Chapter 3
is so much different in water than in most organic solvents that one should

always keep this warning in mind.
3.5 TRANSFER HYDROGENATION AND
HYDROGENOLYSIS
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) [236-238].
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. 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 example
being 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 higher
than that of under high pressure (cf. for example and
in water at 1 bar pressure). This may be beneficial for a
faster reaction. Second, the use of a soluble or liquid hydrogen donor also
eliminates the safety hazard of handling high pressure hydrogen.
Formic acid and formates were among the most effective donors used for
the reduction of olefins with or catalysts in
non-aqueous systems [239-241]. No wonder, the water soluble analogues of
these catalysts became widely used in aqueous solutions. In a series of
investigations [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 aqueous

sodium formate under mild conditions (Scheme 3.29). 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].
Hydrogenation
103
Based on isotope labelling experiments in and
mixtures it was suggested, that the reaction mechanism involved a
rhodacyclobutane intermediate (Scheme 3.30). In this respect the reaction
pathway differs substantially from those of hydrogenations with
Water-soluble Rh(I) complexes containing TPPTS catalyzed the transfer
hydrogenation of itaconic, mesaconic, citraconic and tiglic acids as well as
that of and acids from HCOOM
[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 of
aldehydes catalyzed by [202]. Since in this case both the
catalyst and the substrate reside in the organic phase, a phase transfer agent
was necessary to carry from the aqueous into the organic phase;
were applied for this purpose. An important feature of the
reaction is the strong substrate inhibition which does not allow the reduction
of e.g. benzaldehyde in solutions with higher than 0.8 M aldehyde
concentration. The precise nature of this substrate inhibition is not clear; it
may 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.
having the water-soluble catalyst excess TPPMS,
and the hydrogen donor HCOONa in the aqueous phase and the substrate
aldehyde together with the products in the organic (chlorobenzene) phase
[249,250]. Unsaturated aldehydes, such as cinnamaldehyde (Scheme 3.18)
104
Chapter 3
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 in
reduction 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].

Hydrogenation
105
It is not easy to rationalize this difference in the selectivity provided by
dissolved in the organic phase and by in the
aqueous phase. One reason may be in that when a solution of
in a water-immiscible organic solvent is stirred with an aqueous
solution of a mild base (HCOONa in this case), formation of
can be assisted by extraction of chloride into the aqueous phase (Scheme
3.33).
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
106
Chapter 3
organic solution with water (Figure 3.4) [252]. 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. This is supported by the
finding that when a 0.5 M aqueous was added instead of a
solution, 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 aldehydes
and ketones by hydrogen transfer from aqueous formate [254]. Under the
conditions of Scheme 3.34 turnover frequencies in the range of
were determined. 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). While glyoxylic acid was reduced fast,
pyruvic acid did not react at all.

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
the dimeric which happens to form to the
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 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.
Hydrogenation
107
highest extent at pH 3.2; the compound was characterized in solution and in
isolated from, as well. It is supposed that reduction of the carbonyl
compounds takes place on this dimer (Scheme 3.35).
108
Chapter 3
In the presence of aqueous NaOH, palladium(II) chloride was effective
for the transfer hydrogenation of unsaturated acids, azlactones and
phenylpyruvic acid (Scheme 3.36) at 65 °C although in quite long reaction
times (typically 16 h) [255]. For these water-soluble substrates organic
solvents were not required. 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.
Hydrogen transfer to ketones from 2-propanol was developed into an
extremely efficient method of obtaining secondary alcohols [256,257] and

the use of chiral N-(p-tolylsulfonyl)diamines allow the reduction of
prochiral ketones with extraordinary stereoselectivity [257-259]. In general,
water is not well tolerated in such processes, and several studies showed that
both the rate and the enantioselectivity of transfer hydrogenations from 2-
propanol decrease substantially in increasingly aqueous mixtures even in the
presence of water soluble catalysts [260,261]. However, in a recent study
the opposite effect was found. Using the water soluble Rh- and Ir-complexes
Hydrogenation
109
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. Unexpectedly, raising the water concentration to 34
and then to 51 % further increases in both the rate and enantioselectivity
was observed [262,263].
Simple alkenes [264] as well as unsaturated phospholipids [265] were
hydrogenated in photochemically assisted hydrogen transfer reactions from
aqueous solution of ascorbic acid. The reactions took place in solutions
buffered to pH 5-6 upon illumination with visible light using
as photosensitizer and as catalyst. The same system
produced in the absence of reducible substrates [266]. Interestingly, other
olefin hydrogenation catalysts such as or
were ineffective in this reaction [265].
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).
Catalysis in water is particularly important since it gives hope for processes
suitable for environmental cleanup. Complexes of various metals, such as
Rh, Ru but most of all Pd can catalyze this reaction, with or without
phosphine ligands. Most of the reactions studied so far are based on
hydrogen 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 does
not react at all.
Several aliphatic and benzylic halides were dehalogenated by hydrogen
transfer from sodium or ammonium formate with
or catalysts [268]. As it is seen from Table
3.10, carbon tetrachloride and benzyl chloride were particularly reactive. As
expected, the order of reactivity was and chlorobenzene
remained unchanged. 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. The turnover
numbers (TON-s) in Table 3.10 were obtained in 3 h reactions and there is a
remarkable difference to an analogous system, with as
catalyst, where benzyl chloride was reduced by HCOOLi in refluxing
dioxane with only 26 turnovers in 6 h [269].
110
Chapter 3
Under the conditions of Table 3.10, HCOOH is decomposed and the
yield of reaches only 3.5% (in contrast to HCOONa, where the 478
TON corresponds to 60 % conversion). The use of 5 bar instead of
formate as hydrogen donor leads to 1.8 % conversion. The reason for this
low 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-aqueous
systems [270]. It is worth mentioning that also decomposes during
the reaction as indicated by the pressure rise in the reactor, however, the
conversion of still reaches 31 %.
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 3.38) [271]. Addition of polyether detergents increased
the rate of hydrogenolysis and supressed the formation of alcohol
Hydrogenation
111
byproducts, probably acting as a phase transfer catalyst for formate salts.
The best selectivity (90 %) for 1-phenylpropene was achieved by the use of
triethylene glycol.
The usefulness of a “built-in” phase transfer catalyst function was
demonstrated in the hydrogenolysis of 1-chloromethyl-naphtalene (Scheme
3.39) catalyzed by complexes having a crown-ether moiety in the
phosphine 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 and
gave only low conversions with aqueous HCOOK solutions [66].
A “counter phase transfer catalysis” effect was observed in reduction of
allyl chloride with sodium formate in water/n-heptane systems with water
soluble palladium(II) catalysts having phosphine ligands with polyether
chains [274]; it was demonstrated that the catalyst transported the substrate
to the aqueous phase where it reacted with the formate.
112
Chapter 3

Under “ligandless” conditions catalyzed the hydrogenolysis of
several 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 the
reaction. On the other hand, in case of 2-chlorobenzoic acid addition of
iodide led to 86 % yield of benzoic acid.
could also be used for the hydrogenolytic removal of phenolic
hydroxy groups. However, in this case phenols, e.g. 4-methoxyphenol, had
to be transformed first into monoaryl sulfates which, in turn, could be
reduced by into the corresponding arenes (Scheme 3.40)
[276]. Again, no phosphines or other organic ligands were required for an
efficient reaction.
Homogeneous catalytic asymmetric hydrogenolysis of epoxysuccinates
offers a route to the preparation of chiral malic acid derivatives [277] which
are useful building blocks in natural product synthesis. The reaction was
studied 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 of
dehydroamino acids. Therefore the exceptional solubility of the Rh(I)-
complex of monosulfonated BDPP in organic solvents does not play a role
here.
In a related reaction, racemic sodium trans-phenylglycidate was

hydrogenolyzed with kinetic resolution (Scheme 3.41). With tetrasulfonated
Hydrogenation
113
(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 of
hydrogenolytic reactions, too. A few such reactions are discussed in Chapter
10.
3.6 HYDROGENATION OF CARBON DIOXIDE IN
AQUEOUS SOLUTION
Carbon dioxide has acquired a rather controversial status recently. It is
the carbon source of life on earth through green plant photosynthesis, yet,
through its major contribution to the “greenhouse effect” its increasing
concentration in the atmosphere poses a serious threat to the stability of
global climate. In order to avoid abrupt climatic changes and all the dangers
they may bring, in addition to many other measures, anthropogenic
emission must be decreased. 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
mixtures obtained by the water gas shift reaction (see 3.8). It would be
highly desirable to make use of such recovered (often in form of an
aqueous solution) as a C1 building block in organic synthesis instead of
finding ways of long-term sequestration. Of course, concentrated, cheap
is also available from natural sources. For all these reasons the possible
application of carbon dioxide as a raw material has attracted much interest
and several outstandingly active catalysts have been discovered for its
hydrogenation, especially under supercritical conditions, in the presence of
various amines. 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.
Since carbon dioxide is a thermodynamically stable, highly oxidized
compound, its synthetic utilization requires some kind of a reduction -
reaction with molecular hydrogen is a distinct possibility. Stepwise
reduction of with may yield formic acid, formaldehyde, methanol
and finally methane, together with CO or Fischer-Tropsch-type derivatives
as shown on Scheme 3.42. In aqueous organometallic catalysis the most
common product of such a reduction is formic acid. Formation of carbon
monoxide, formaldehyde, and methane has already been reported, however,
methanol and Fischer-Tropsch type products were not observed.
114
Chapter 3
Reaction of two gaseous compounds resulting in a liquid product are
biased by a decrease in enthropy which –depending on the temperature–
may make the whole process thermodynamically unfavourable. This is also
the case for the hydrogenation of to HCOOH (eq. 3.12) with
However, in aqueous solution hydration of the solutes makes the overall
enthropy difference smaller, and reaction (3.13) becomes slightly exergonic
with
Thermodynamics tells, therefore, that in water this reaction is likely to
proceed; we must not forget, though, that these data refer to standard
conditions, and in order to eliminate the kinetic activation barrier at 25 °C
highly active catalysts are needed. Unfortunately, the same catalysts can also
be active in the reverse process, i.e. in the decomposition of formic acid to
and at low pressures; decomposition to CO and (i.e. the reverse
water gas shift) is rarely observed.
Equation (3.13) can also be shifted to the right by further reactions of

HCOOH which may be simply its neutralization with a base, or reactions
with 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 on
reduction of Inoue et al. were the first to discover that in the presence
of a base (NaOH, etc.) transition metal phosphine
Hydrogenation
115
complexes of l,2-bis(diphenylphosphino)ethane (diphos) and such as
and
catalyzed the formation of HCOOH from and (25 bar
each, room temperature, benzene) with 12-87 turnovers in 20 h [287]. 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. Interestingly, when
was used as a base, formic acid was obtained, albeit with very low
yield (3 turnovers in 20 h), even in the absence of
was prepared from and in the
presence of water. It was supposed, that was reduced first by to
formic acid, decarbonylation of which by afforded the target
compound [288].
Homogeneous catalytic synthesis of formic acid (formate salts) from
under catalysis by in alkaline aqueous ethanol was reported
[289]. Typically, the solvent contained 20 % v/v water, the base was
and 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 to

From 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 noting
that exclusively formic acid i.e. no ethyl formate was produced despite the
presence of 80 % ethanol in the solvent.
Tsai and Nicholas used as a
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).
116
Chapter 3
It was also suggested [290] that the rate accelerating effect of water was
due to formation of an intermolecular hydrogen bond between the
ligand in and the incoming within the insertion
transition state, such as depicted (A) on Scheme 3.44.
Hydrogenation
117
The existence of such or closely related intermediates received support
from studies on the water effect in the hydrogenation of carbon dioxide
catalyzed by Tp = hydridotris(pyrazolyl)borate
[291]. This complex catalyzes the reduction of to formic acid with an
average (100 °C, 50 bar total pressure,
15 mL/5 mL, 2 mL). Since addition of inhibited the
reaction, it was concluded, that the catalytic cycle probably does not involve

an acetonitrile-containing complex. was suggested a
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
and undergoes metathesis to regenerate This
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 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].
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
1989; in that case the catalyst was [294]. Mild
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
118
Chapter 3
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
prepared in situ from and TPPTS and from
and TPPTS were succesfully used by
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
pressure a was observed. For this reaction
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].
Hydrogenation

119
It was found recently [203,296,298,299], 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. Conversely, the same catalyst hydrogenated bicarbonate
(1 M ) with a under comparable conditions.
Based on similar observations, a detailed study of the hydrogenation of
bicarbonate was undertaken with catalysts such as
and High
pressure and NMR measurements revealed, that in aqueous
solutions under hydrogen part of is
converted to various hydrido-ruthenium complexes, such as
(at or ), or (at
) and in phosphine excess:
120
Chapter 3
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 of
hydrogenation was miserable.
It was concluded, that the effect of increasing pressure was that of a
decreasing pH, facilitating the formation of which exists in
acidic solutions (vide supra). Based on this assumption, the suggested
catalytic cycle involved insertion of bicarbonate to the Ru-H bond in
(Scheme 3.47) followed by protonation to liberate HCOOH
from the complex.
The same water-soluble catalysts were suitable for the hydrogenation of
aqueous suspensions of under pressure leading to formate

solutions with concentrations up to 0.93 M [296].
Hydrogenation
121
It should be mentioned here, that the heterogeneous catalytic
hydrogenation 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 and
transport of hydrogen [272]. The best catalysts of bicarbonate hydrogenation
consist of metallic palladium, either supported, such as Pd/C, or colloidal,
stabilized by The latter [273] actively catalyzes the
photochemically assisted reduction of
A very interesting finding was published by Pruchnik et al. who studied
the hydrogenation of with catalysts prepared from
or other water soluble small phosphines, such as and
[302]. 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. Unfortunately, no further results have been
published and no suggestion concerning the catalytic cycle have been made
yet.
122
Chapter 3
The iridium cluster with = pyridylphosphines 67
( 1, or 2) and the rhodium complex with
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].
3.7
HYDROGENATIONS OF BIOLOGICAL
INTEREST
3.7.1 Hydrogenation of biological membranes
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].