Metal-Catalyzed Reactions in Water (Dixneuf/Metal-Catalyzed Reactions in Water) || Water-Soluble...

109

4Water-Soluble Hydroformylation CatalysisDuc Hanh Nguyen, Martine Urrutigoıty, and Philippe Kalck

4.1Introduction

The hydroformylation reaction, shown in Scheme 4.1, is the functionalization ofalkenes by CO and H2 to produce aldehydes containing one more carbon atom. Thisreaction was discovered by Otto Roelen [1–3] in 1938, during his investigations onthe Fisher–Tropsch transformation of the CO/H2 couple into alkanes, alkenes, andoxygen-containing compounds. Indeed, by recycling ethylene to improve the yields,he observed the formation of a C3 aldehyde, and instead of rejecting this unexpectedresult, he studied in depth this carbonylation reaction, which introduces a hydrogenatom on one ethylenic carbon atom and a formyl group on the other one. In fact,this cobalt-catalyzed formation of propanal involves the [Co(H)(CO)4] complex asa precatalyst and homogeneously operates under harsh conditions at 140–180 ◦Cand 200–300 bar [4]. During the period 1945–1951, several processes using cobaltcatalysts were developed, as these primary oxo products give access to a multitudeof industrially important secondary products such as alcohols, acids, diols, amines,or esters [5]. Recently, the cobalt-catalyzed hydroformylation reaction has beenreviewed, as well as the cobalt recycling process [6]. In the 1950s, rhodium complexeswere discovered to be highly active hydroformylation catalysts. By incorporation oforganophosphines into the metal coordination sphere, both Wilkinson and Pruett’sresearch groups independently obtained high regioselectivities for linear aldehydeunder mild conditions of temperature and pressure [7, 8]. However, because ofhigh thermal stress sensibility of the rhodium complex, the catalyst recycling isgenerally not possible. Therefore, the development of this reaction on an industrialscale is limited to light alkenes, especially propene, as the corresponding aldehydesare exhausted in the gas phase. The largest requisite product for the industry is2-ethylhexanol, obtained by aldolization of the linear butanal, its dehydration into2-ethylhex-2-enal, and its further hydrogenation on Raney nickel. The most efficientUnion Carbide process to hydroformylate propene using rhodium progressivelydisplaced cobalt since the 1970s.

During the development of coordination chemistry, a generally accepted conceptwas that the use of water should be avoided until Chatt et al. [9] published, in 1973,

Metal-Catalyzed Reactions in Water, First Edition. Edited by Pierre H. Dixneuf and Victorio Cadierno. 2013 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2013 by Wiley-VCH Verlag GmbH & Co. KGaA.

110 4 Water-Soluble Hydroformylation Catalysis

R RCHO

R

CHO+

Co or Rh

CO / H2

Scheme 4.1 General equation for the reaction providing an aldehyde (linear, n, andbranched, iso) starting from a terminal alkene.

the synthesis of phosphine ligands containing acetoxymethyl or hydroxymethylsubstituents and showed that the corresponding complexes are water soluble witha modest catalytic activity. In 1974, Kuntz was able to prepare the trisodium saltof tris(m-sulfophenyl)phosphine, now called TPPTS, by smooth sulfonation oftriphenylphosphine with oleum followed by neutralization with sodium hydroxide[10]. The synthetic process was patented [11] and later the extraction [12], sulfonation[13], and hydrolysis [14] steps were improved. In order to reduce the amounts ofthe corresponding oxide OTPPTS, improvements in the procedures were doneto reach directly TPPTS/OTPPTS ratios around 94 : 6 in the crude material [15].Many explorations of the catalytic activity of transition-metal complexes containingthis highly water-soluble ligand for certain reactions, such as hydrogenation,hydroformylation, hydrodimerization of butadiene, and condensation of ethylacetoacetate with myrcene to produce geranylacetone via a C-C coupling, gave highlypromising results [16]. Such performances led the scientific community to considerthat water does not introduce a deleterious effect in catalysis and opened the way toexplore the coordination catalysis in this medium. As underlined in the introductorypart of the book dedicated to the aqueous-phase organometallic catalysis by Cornilsand Herrmann [17], the concept of such catalysis was simultaneously developedby Joo and Beck in Hungary [18] and Manassen [19]. The successful developmentof the Ruhrchemie/Rhone-Poulenc (RCH/RP) process by the setup for the firstrhodium-TPPTS-catalyzed hydroformylation unit of propene (100 000 T a−1) inOberhausen in July 1984 [20] substantiated the success of this creative researcharea and initiated a large research effort to expand the concept.

4.2Hydroformylation of Light C2 –C5 Alkenes in the RCH/RP Process

Interestingly, water was already present in the historical cobalt-catalyzed hydro-formylation of propene. It was coproduced by undesired aldehyde condensationgiving rise to heavy compounds. Moreover, in the metal recovery steps, as the[Co(H)(CO)4] resting state complex is not particularly sensitive to the presence ofwater, this cobalt(I) is either reduced into metallic cobalt or oxidized in Co2+ salts,or transformed in the water-soluble Na[Co(CO)4] species by addition of NaOH asdescribed in the three BASF, RCH, and PCUK (now Exxon) recycling processes.After decantation, the active cobalt complex is regenerated either in a high CO/H2

pressure tank or by addition of sulfuric acid under CO pressure. Classically, oper-ating conditions are 130–150 ◦C and 220–250 bar of CO/H2 mixture with a molarratio of 1. The conversion of propene is 75% and the linear/branched (n/iso) ratio

4.2 Hydroformylation of Light C2 –C5 Alkenes in the RCH/RP Process 111

L

L

L

H

Rh

CO SO3Na

P

3

L =

Figure 4.1 General structure of the [Rh(H)(CO)(TPPTS)3] catalyst; TPPTS is thetris(m-sulfophenyl)phosphine ligand.

is 80 : 20, and a significant amount of heavy-end (6–10%) C4 alcohols and butylformate is produced [4, 16].

The RCH/RP oxo process is largely simpler and more cost effective than theformer cobalt one. It involves the [Rh(H)(CO)(TPPTS)3] complex (Figure 4.1)containing the sodium salt of the tris(m-sulfophenyl)phosphine, which is highlysoluble in water (1100 g l−1) and insoluble in usual organic solvents. Indeed thealkenes and produced aldehydes stay in the organic phase, whereas the hydrosolublecatalyst remains in the aqueous phase [20].

The process combines a homogeneous catalysis in the aqueous phase wherethe rhodium complex is in contact with propene, hydrogen, and carbon monoxidedissolved in water and rigorously a heterogeneous sequence by which the productsare macroscopically immediately separated from the aqueous phase as soon as theyare formed to feed the organic phase. It has been considered at the beginning thatthis reaction was occurring in the bulk of the aqueous phase, especially because theaddition of a cosolvent such as an alcohol increases the solubility of the alkene inwater [21, 22]. The reaction presumably involves mainly the interface region [23].

In the presence of an excess of the TPPTS ligand, the active species is[Rh(H)(CO)(TPPTS)2]. In this complex, the hydridic character of the H ligandis significantly pronounced so that after coordination of propene the hydride mi-gration mainly occurs on the C2 carbon atom (Figure 4.2). The selectivity toward thelinear aldehyde is about 96% and the by-products (about 1%) are n-butanol (0.5%),isobutanol (0.1%), butylformates (traces), and heavy ends (0.4%). The reaction isoperated at 122 ◦C under a 50 bar total pressure, with CO/H2 and water/organicphase ratios of 1.01 and 6, respectively. The unit constitutes a reactor, a decanterwhere the separation between the two aqueous and organic phases is very fast, astripping column, and a distillation column [23]. Although the cost of the rhodiummetal is largely higher than that of cobalt (about 1300 times), the much higher activ-ity of the [Rh(H)(CO)(TPPTS)3] catalyst makes the industrial process economicallyviable. In particular, the recycling of rhodium is so efficient that the loss of preciousmetal, calculated for a period of 15 production years, is 1 kg of rhodium for 106

metric tons of n-butanal, that is, in the ppb range [20, 24]. This RCH/RP processaccounted for around 12% (800 000 tons per year in 2004 [25]) of the world capacityfor producing butanal and pentanal. The overall cost advantage is estimated tobe around 10% better than that of the classical Union Carbide Company (UCC)process using the [Rh(H)(CO)(PPh3)3] catalyst.


[Rh(H)(CO)L3]

RhOC

L H

L

RhL

LCO

H

RhL

OCL

H

Rh

OC

LLRh

OC

L

L

O

RhOC

L H

H

L O

Rh

OC

LL

O

H

O

H

CO

H2

Figure 4.2 Main steps of the catalytic cycle of the hydroformylation of propene by[Rh(H)(CO)(TPPTS)3], the main cycle is related to the linear aldehyde.

It is worth to underline that the general advantages of this modern oxo processare (i) the use of water as a nontoxic, nonflammable, eco-friendly environmentalsolvent; (ii) an excellent atom economy; (iii) an efficient recovery of catalyst, whichdoes not suffer any thermal stress; and (iv) minimized energy consumption [26].Nevertheless, a small amount of the TPPTS ligand is oxidized into OTPPTS, whichrequires along the continuous process a reduction step. It has been demonstratedthat in the absence of dioxygen, a redox reaction occurs involving rhodium(III)complex, water, and the TPPTS ligand [27]. Water is the source of the oxygen,and, for instance, the intermediate species [RhCl(H)(OH)(CO)(TPPTS)x], resultingfrom the formal oxidative addition of H2O, has been identified leading to the[RhCl(TPPTS)y] complex.

Moreover, traces of propyldi(sulfophenyl)phosphine are produced presumably bythe direct oxidative addition of a phosphorus–phenyl bond to the rhodium–propylintermediate species, through a similar mechanism leading to the formation ofpropyldiphenylphosphine demonstrated by Garrou et al. [28] for the UCC process.In both cases, the stronger electron donating ligand leads to a poorly active rhodiumcomplex.

4.2 Hydroformylation of Light C2 –C5 Alkenes in the RCH/RP Process 113

CO/H2CO/H2

Unreacted

CHOVapors CHO

CHO

Rea

ctor

Decanter

Cat

alys

t wat

er-p

hase

recy

clin

g

Dis

tilla

tion

unit

Str

ippi

ng c

olum

nReboiler

Vent

Figure 4.3 Flow diagram of the Ruhrchemie/Rhone-Poulenc biphasic hydroformylationprocess.

In the industrial unit [26, 29], which produces around 150 000 tons per year,the hydroformylation reaction is carried out in a continuously stirred tank reactorcontaining the aqueous rhodium catalyst and is fed with propene and syngas(CO/H2 = 1.01) as represented in Figure 4.3. The crude aldehyde, taken in thetop part of the reactor, is sent to the decanter, where it is degassed and separatedinto the aqueous catalyst solution and the organic phase. The catalyst solutionreturns to the reactor through the heat exchanger producing process steam. Theorganic aldehyde phase is sent to a stripping column, in which fresh syngas isinjected, acting as the contercurrent stripping agent to move unreacted propeneback to reactor. At the base of this column, the crude aldehyde is collected andthen fractionally distilled into n-butanal and 2-methyl-propanal in the distillationcolumn. The reboiler of this column is a falling film evaporator incorporated intothe reactor. Thus, owing to the large exothermicity of this carbonylation reaction(28 kcal mol−1), the heat of the reaction is recovered so that the process is a netsteam producer. It is noteworthy that the full process is largely simplified withregard to the former oxo-cobalt one, and the use of water as solvent mediumsuppresses any thermal stress of the rhodium catalyst and any corrosion concernsin the unit.

When starting from alkenes with a longer chain length, lower reaction ratesare obtained. The hydroformylation reaction has been applied to but-1-ene, justusing higher concentrations of rhodium catalyst to ensure significant space-time


yields in the industrial unit and not modifying the temperature to maintain theligand stability [26, 30]. In the petroleum industry, after extraction of butadiene andconversion of isobutene into methyl tert-butyl ether (MTBE), the C4 cut, namely,‘‘raffinate II,’’ contains 50–65% of but-1-ene that can be selectively transformed inthe hydroformylation reaction, while the remaining cis- and trans-but-2-enes anda few butanes are unreactive. Thus, provided some flexibility is ensured in theprocess for having compatibility with the various concentrations in the feedstock,it is possible to convert but-1-ene into n-pentanal with 95% selectively. Then thealdehyde is immediately oxidized to pentanoic (or valeric) acid, which is the basisof a new generation of ester-type lubricants for substitution of CFC in refrigerationsystems [30]. The linear aldehyde, n-valeraldehyde, is also a fragrant molecule. Afew modifications of the propene plant can be performed [31], so that a 12 000tons per year unit went in stream in 1995 at the Oberhausen site [24]. It isworth to mention that the activity of [Co(H)(CO)3(TPPTS)] generated in situ from[Co2(CO)6(TPPTS)2] [32] has been explored in the cascade isomerization of Z- andE-pentenes into pent-1-ene, which cannot be done by rhodium, and its furtherhydroformylation [33]. Yields up to 75% and n/iso selectivities up to 75 : 25 canbe gained at 190 ◦C and 100 bar CO/H2 in 12 h, but the leaching of cobalt variesbetween 9 and 60 ppm.

Other sulfonated ligands have been further synthesized by Herrmann and hisresearch group in connection with the RCH factory (Hoechst AG, for the patentssee Ref. [20]) in order to further improve the performances of the catalysts [34–37].Among BISBIS-Na (Na for indicating the cation associated with the sulfonategroups), NORBOS-Na, and BINAS-Na (Figure 4.4), the last one is particularlyinteresting because when operated at 110–130 ◦C, 20–60 bar, and a P/Rh molarratio of about 10, turnover frequencies (TOFs) of up to 10 000 h−1 were reached,with a 99% linearity of the aldehyde [36]. Despite these impressive performancesin terms of reactivity and linear selectivity, these ligands have not been usedindustrially, presumably because of their very high production costs with regard tothat of TPPTS [38]. Nevertheless, pilot plant tests for more than two months haveshown that the conversion of propene and productivity are constant, and owing tothe thermal stability of the ligand until 135 ◦C, the catalytic system could operatewith no major problem [36]. Hydroformylation of but-1-ene can be operated at130 ◦C and 25 bar of CO/H2 adding the BISBIS ligand to [RhCl(CO)(TPPTS)2](BISBIS/Rh = 5) with a 98% regioselectivity in pentanal and a 2987 h−1 TOF [39].

From an economical point of view, transformation of internal alkenes into theterminal ones and their selective hydroformylation into the linear aldehyde is aninteresting strategy. Indeed, Beller and coworkers [40] have shown that using thesterically hindered BINAS-Na ligand and low CO partial pressures, pent-2-ene canbe converted into the linear aldehyde with an excellent regioselectivity (99%). It isessential to carefully control the pH of the aqueous medium for successful tandemisomerization/hydroformylation.

Several reviews have appeared to analyze how to retain the transition-metalcomplexes in the aqueous phase, thanks to the coordination of water-solubleligands in order to avoid the rhodium leaching concerns [41–44]. Indeed, other

4.3 Hydroformylation of Alkenes Heavier than C5 115

P

Ph

P

Ph

NaO3S

NaO3S

SO3Na

SO3NaBISBIS−Na

P

P SO3Na

SO3Na

NaO3S

SO3Na

SO3Na

NaO3S

BINAS

P

MeMe

NaO3S SO3Na

SO3Na

NORBOS

Figure 4.4 Representation of the new water-soluble phosphine ligands. (Source: Adaptedfrom Refs. [34–37].)

water-soluble phosphines bearing polar groups such as carboxylates, ammonium,phosphonium, and hydroxyl rests have been synthesized [45–47]. Functionalizationof tris(hydroxymethyl)phosphine by the Mannich-type condensation with aminoacids have produced novel water-soluble ligands [48]. For the hydroformylationreaction of propene, it is necessary to adjust the pH of the medium around theisoelectric point of the aminoacids used, in order to reach the optimum n/iso ratio.However, all the results present modest regioselectivities with n/iso ratios between1.5 and 1.9. A significant decrease in the activity and selectivity is observed alongthe recycling, using the glycine-derived ligand.

4.3Hydroformylation of Alkenes Heavier than C5

The solubility of the alkenes in water is progressively reduced as the carbonchain length increases. In purely biphasic systems, pent-1-ene is not carbonylatedindustrially. For higher alkenes, it is necessary to overcome the problems of the lowreactivity due, in a first analysis, to the poor mass transfer rates in the water phase.In fact, it is better to consider that an interfacial layer also called an interphase witha continuous variation in the composition going from the organic to the aqueouslayer – and not in the strict sense an interface – is progressively organized moreor less rapidly during catalysis. In this privileged volume, the contact betweenthe reactants and the active complex within its solvation sphere is improved. Thevarious methods explored to introduce auxiliary agents or systems are analyzed inthe following sections.


4.3.1Water-Soluble and Amphiphilic Ligands

Although with higher alkenes the selectivity of the hydroformylation reaction inthe linear aldehyde remains at a high level, the reactions rates are poor. A fewstudies have been reported on the activity of cobalt/TPPTS systems for hex-1-ene[49, 50]. Starting from [Co2(CO)6(TPPTS)2] or [CoCl2(TPPTS)2], the catalysis occursat elevated temperatures and pressures (60–120 ◦C and 60–90 bar) and a 90%conversion rate with 68% selectivity in aldehydes (n/iso = 3 for the aldehydes) canbe obtained. Thus, rhodium has been favored in the studies because of the milderconditions of reaction and high conversion rates as well as high selectivities to endaldehydes.

In order to obtain better results toward linearity, the ionic strength of theaqueous phase has been studied [51, 52]. For instance, similar to the first patents[15], addition of Na2HPO4 significantly decreases the reaction rate but leads tohigher regioselectivity in linear aldehyde. This observation has been extended[53] to various sulfate salts such as Li2SO4, Na2SO4, Cs2SO4, or Al2(SO4)3. Insolutions where TPPTS, [Rh(H)(CO)(TPPTS)3], and the sulfate salt are strongelectrolytes, NMR studies [51, 54] have shown that the ionic strength increasesthe activation barrier for the dissociation of a TPPTS ligand from the rhodiumcoordination sphere. In these conditions, the active species [Rh(H)(CO)(TPPTS)2]giving rise to a higher hydridic character for the Rh–H ligand and thus to thehigher selectivity in linear aldehyde is favored over the [Rh(H)(CO)2(TPPTS)] activespecies, although it is more difficult to be formed. It is suggested [52] that inthe rhodium [Rh(H)(CO)(TPPTS)3] complex, the sulfonate groups would lie onthe surface of a sphere that is 8 A in diameter and therefore with nine negativecharges. Their close proximity should promote the formation of a complex networkof hydrogen bonds and sodium–sulfonate ion pairs. Instead of having a liganddissociation to minimize electrostatic repulsions between sulfonate groups, thehigh concentration of cations induces a high ionic strength that stabilizes thehydration sphere. In more recent studies, hydroformylation of pent-4-en-1-ol at35 ◦C provides around 75% selectivity in the 6-hydroxy-hexan-1-al linear aldehyde,whereas at higher temperatures ranging from 45 to 75 ◦C and in the presence ofNa2SO4, regioselectivities as high as 98% are reached in the branched aldehydewhose cyclization provides the hemiacetal 2-hydroxy-3-methyltetrahydropyran [55].Presumably, in this case, the coordination of the alcohol function of the substrateis strongly dependent on the ionic strength.

Solutions to the problems of the poor solubility of the alkenes in water, andeven their insolubility, can be found at least partially by addition of cosolvents orby performing the reaction in micellar systems. In the TPPTS-containing ligand[Rh2(µ-StBu)2(CO)2(TPPTS)2] complex, oct-1-ene is fully hydroformylated with a97% selectivity in the linear aldehyde [56, 57]. However, the conversion rates remainas low as 18% after 15 h at 80 ◦C and 5 bar. Addition of a cosolvent, particularly alower alcohol, improves the substrate solubility in the water-rich phase and thusincreases the reaction rates, still keeping a biphasic system. Indeed, introduction of


ethanol to the former catalytic system (22 wt% to water) allows reaching conversionrates around 90% after 8 h [58]. Nevertheless, there is a loss of selectivity, as someisomerization of oct-1-ene occurs (5–10%). Moreover, the regioselectivity decreasesto 83%. Full analysis of the gas-liquid-liquid system shows, for instance, that fora 50/50 water–ethanol solvent, the solubility of oct-1-ene is enhanced by a factorof 104 and that the solubility of hydrogen and carbon monoxide is significantlyimproved [59].

The introduction of an organosoluble ligand such as PPh3 to maintain thecatalyst in the interfacial region and to improve the issue of mass transfer has alsobeen explored [60]. Addition of 0.166 equiv of PPh3 to the [Rh(H)(CO)(TPPTS)2]complex generated in situ from [RhCl(COD)]2 results in a conversion rate ofoct-1-ene 10 times faster (PPh3/TPPTS = 0.0069) and the 0.666 PPh3/Rh ratio56 times faster (PPh3/TPPTS = 0.028) [61]. Further investigations by Froningand Kohlpaintner [31] using 31P-NMR on the rhodium species present in themedium have shown that the mixed [Rh(H)(CO)(TPPTS)(PPh3)2] complex can beproduced but in very low amounts when compared to [Rh(H)(CO)(TPPTS)2] in theaqueous and [Rh(H)(CO)(PPh3)3] in the toluene phase [62]. Moreover, the 97%selectivity for nonanal in a pure water system [58, 63] shifting to 75% when bothligands are introduced (PPh3/TPPTS molar ratio = 0.33) is rather consistent witha specific activity of [Rh(H)(CO)(PPh3)3] in the organic phase. The redistributionphenomenon is not in favor of the mixed species, which would maintain it in theinterfacial area. This concept has been further explored with a different approachwhere the ligand acts itself as a surfactant. Thus, the carboxylate analog of thesulfonate TPPTS ligand, m-TPPTC (lithium salt), provides a much more effective[Rh(H)(CO)(TPPTC)3] precursor, as the conversion rate is 94% with an aldehydeselectivity of 84% for oct-1-ene, these values being 43 and 86% for dec-1-ene and16 and 83% for dodec-1-ene [64].

Reaction in micellar systems is another strategy to improve the solubil-ity of the substrate in water by introducing surfactants or amphiphilic lig-ands [65]. Adding directly RC(C6H4-p-SO3Na) surfactants, where the R sub-stituent is a long carbon chain, to Rh(III)salts/TPPTS systems generate efficientcolloidal hydrogenation catalysts [66]. Using the powerful effect of ion pairs,the association of cationic platinum complexes of the type [(dppb)Pt(H2O)2]2+

(dppb being the bis(diphenyl)phosphinobutane ligand) with anionic surfactantsleads to interesting results [67]. Indeed, at 70 ◦C and under 80 bar, aldehy-des are obtained with an n/iso ratio of 99 : 1. Therefore, this concept has beenextended to the preparation of surface active ligands, in which, for instance, thetris(2-pyridyl)phosphine is functionalized by sulfoalkylation with alkyl-1,2-sulfonesto generate the amphoteric P[C5H4N+CH(CH2SO−

3 )(CH2)nCH3]3 ligand [68]. The[Rh(H)(CO)(PPh3){P(2-py)3}2] complex is slightly soluble in water [69], whereas therhodium complex containing the new amphoteric ligands (n = 0, 3, 5, 7, 9, 11)generates micellar hydroformylation [70]. For instance, tetradec-1-ene is convertedto pentadecanal, achieving 79% conversion in 3 h (n = 5). These performances arelower for a longer chain length in the ligand. Recovering the catalyst is achieved bysimple phase separation (n = 0–7). In contrast, longer tails (n = 9, 11) induce the


formation of very stable emulsions, which do not break into the expected biphasicsystem even after one year.

More generally speaking, the water/organic interfacial area are increased by theformation of micelles in which hydrophobic substrates can be sequestered [47]. Forinstance, the promotion effect of the cationic surfactant cetyltrimethylammoniumchloride (CTAC) and its optimal concentration increase significantly the conver-sion rate of myrcene when compared to the pure toluene/water biphasic systemcontaining the Rh/TPPTS catalyst [71].

Another approach is to provide surfactant properties to rhodium and cobaltcatalysts synthesizing anionic tenside phosphine ligands. The interest of suchamphiphilic ligands is to produce a highly catalytic active species concen-tration inside the hydrophobic core of the micelles formed in the aqueousmedia. Thus, the tris(ω-phenyl)alkylphosphines, where the alkyl chain lengthvaries from one to six carbon atoms, generate from [Rh(acac)(CO)2] the cor-responding rhodium [Rh(acac)(CO)(P{(CH2)xC6H4-p-SO3Na}3)] species and then[Rh(H)(CO)(P{(CH2)xC6H4-p-SO3Na}3)3]. They are good precursors of the hydro-formylation of oct-1-ene with TOFs of 335 h−1 when x = 3 and 360 h−1(x = 6),provided an equal solvent mixture of methanol and water is used [72–74]. These per-formances, reaction rates, and selectivities (around 75% in 1-nonanal) are slightlyhigher than those obtained with the rhodium/TPPTS catalyst (TOF 260 h−1, n/iso =58%). The authors observe that the catalytic activity increases as micelles formationbecomes more likely. In this way, chelating 1,1′-biphenyl- or binaphthyl-phosphinesbearing the sulfonated pendants – [C6H4(CH2)x-p-C6H4-SO3Na] groups give moresatisfactory activities under same conditions. In addition, no rhodium leachinghas been detected. Owing to these promising results, two new phosphines witheven longer pendants with a C10 chain have been synthesized as shown inFigure 4.5 [75].

Concerning the yields of 1-nonanal and the TOF values, we can note that theresults are always higher than those obtained with TPPTS. For example, for an L/Rhratio of 9, the first ligand gives 435 h−1 instead of 220, and 89% 1-nonanal comparedto 78%. With the second ligand, the 1-nonanal selectivity is 91%. Attempts to extendthis reaction to tetradecene were not successful as low conversions are observedfor this highly insoluble substrate [75].

These previous catalytic systems somewhat differ from the use of amphiphilicligands to perform the hydroformylation reaction in an organic phase and recyclingby switching the rhodium catalyst between the organic and the aqueous phase.Indeed, after the reaction, the organic phase is washed with water of the appropriatepH in order to transform the catalyst and the excess of water-soluble ligand, eitherby protonation or deprotonation. Decantation followed by reintroduction of a newcharge of substrate and neutralization of the aqueous phase represents the differentsteps to perform another catalytic cycle within the new organic phase. The drawbackof this method is the rapid loss of activity, as after the second run, it is reduced to86%. The ligands explored are a series of triphenylphosphines bearing hydroxyl-,carboxy-, diethylamino-, diphenylamino-, or pyridylphenyl groups, and the moresuccessful one is phenylbis(4-diethylaminophenyl)phosphine. It is expected that


PP

NaO3S SO3Na

SO3NaNaO3S

P

NaO3S

NaO3S SO3Na

Figure 4.5 Tris[p-(10-p-sulfonatophenyl-decyl)phenyl]phosphine and 2,2′-bis(di[p-(10-phenyldecyl)phenylphosphinomethyl]-1,1′-biphenyl lig-ands. (Source: Adapted from Ref. [75].)


the deactivation arises from the protonation of the nitrogen functions during theacidic extraction steps [76].

4.3.2Phase-Transfer Agents: Cyclodextrins and Calixarenes

As catalysis was considered occurring within the aqueous phase, cyclodextrins(CDs) were early largely used as shuttles to transport the organic substrate. Theyare characterized by high solubility in water and form inclusion compounds with alarge variety of substrates transferring them into the aqueous phase, so that masstransfer limitations are reduced [77]. They are cyclic oligosaccharides resulting fromα-d-glucopyranosyl moieties with 6, 7, or 8 units for α-, β-, or γ-CDs, respectively(Figure 4.6). Their molecular geometries are represented by a truncated cone withthe 3-OH and the 6-OH groups of the respective glucose units occupying the widerand the narrower rim of the cone, respectively [78].

The CD cavity is thus mainly hydrophobic and forms with the organic substratean inclusion compound (Figure 4.7). Owing to the main hydrophilic properties ofthe CD external face, the inclusion compound can migrate into the aqueous phaseallowing the substrate to come into contact with the hydrosoluble catalyst.

After reaction, the final product is released into the organic phase, so thatthe CD can go on its shuttle role [79, 80]. It appears that CDs can act ascounter-phase-transfer agents in a reverse of phase-transfer catalysis [81]. Moreover,methylation of OH functions to give permethylated CDs (in fact 14 methyl functionsfor β-CD, called DM-β-CD [82]) provides higher solubility in water.

The first use of such agents has been reported in the Wacker-type catalyzedoxidation of terminal alkenes into the corresponding methyl ketones [83, 84]. Manystudies have been carried out for CD-based hydroformylation since the first reportin 1991, even if it describes some inhibiting effect of an α-CD for this reaction [79].

O

OHHO

OH

O

OOH

HO OHO

OOH

OH

OH

O

OO

OH

OH

HO

OOH

OHHO

O

OOH

HO

HO

O

O

HO OH

OO

OH

1

23

45

6

n

Figure 4.6 Schematic representation of classical cyclodextrins (n = 1, α-CD; 2, β-CD; and3, γ-CD) and their cone geometry. (Source: Adapted from Ref. [78].)


+

Host Invited molecule Inclusion complex

Figure 4.7 Representation of the incorporation of a host molecule into a cyclodextrin.

S

Substrate S Product P

P

S P

Interfacial layer

Organic phase

Aqueous phase

[Rh]

H2/CO

R RCHO

Figure 4.8 Improved contacts between reactants and the rhodium catalyst/cyclodextrin sys-tem to provide the hydroformylation products at the interfacial layer. (Source: Adapted fromRef. [85].)

CDs are especially used for the functionalization of heavy alkenes, and Figure 4.8shows how the contact between the rhodium catalyst and the substrate is enhancedat the interface [85].

The length of the terminal alkene plays an unexpected role in the reactionrate. In the presence of the [Rh2(µ-StBu)2(CO)2(TPPTS)2] complex, oct-1-ene istransformed into n-nonanal with 26.2% yield in 18 h (5 bar, 80 ◦C), whereas for thehigher alkenes such as C10, C12, C14, and C16, the yields are decreased to 6.2, 2.1,1.6, and 1.4%, respectively [86]. Thus, it is important to consider the availability ofthe terminal double bond outside the CD rim for coordination to the water-solublerhodium center. According to the length of the carbon chain, the alkene can bemore or less included in the cavity and thus the C=C double bond more or lesshidden.

Interestingly, in presence of randomly methylated β-CD (with an average of12.6 methyl groups per CD and is called RAME-β-CD), dec-1-ene has been effi-ciently hydroformylated in terms of conversion (100 vs 10%) and chemoselectivity


O

PPh2 PPh2

SO3NaNaO3SFigure 4.9 Sulfoxantphos ligand.

(95 vs 60%) but the linear to branched aldehydes ratio was lower (1.8 vs 2.7) [87].This decrease is due to the inclusion of TPPTS ligand, resulting in the lowerlocal concentration of extra ligand around the rhodium center, which favors theformation of the catalytically less regioselective [Rh(H)(CO)2(TPPTS)] species [88].In addition, 31P{1H} NMR studies under such hydroformylation conditions re-vealed that significant quantities of inactive dimeric species are formed. FurtherNMR investigations showed that one aromatic moiety of TPPTS is trapped intothe hydrophobic cone of RAME-β-CD reducing the coordination capacity of thephosphine ligand [89]. Chemoselectivity can be further improved to 98–99% using1,3,5-triaza-7-phophaadamantane (PTA) or its benzylated chloride, thanks to theabsence of interaction between these two ligands and the CD [90]. Similarly, thebulky diphosphine sulfoxantphos ligand (Figure 4.9) can be used and its absenceof dissociation from the rhodium center allows to reach more than 99% C9 and C11

aldehyde selectivity and high n/iso ratios close to 26 [91].In addition, as α-CD derivatives cannot form inclusion complexes with

TPPTS because of their too small cavity size, their functionalization with2-hydroxy-3-trimethylammoniopropyl substituents and their application tohydroformylation of dec-1-ene have been explored [92]. A supramolecularorganization occurs via ionic interactions between the cationic groups on the α-CDand the anionic charges of the TPPTS ligand. This beneficial effect presumablyimproves the approach of the substrate to the metal center and results in anincrease in the catalytic performances.

On the other hand, a study using DM-β-CD in the [Rh2(µ-StBu)2(CO)2(TPPTS)2]catalyzed hydroformylation of C8, C10, C12, C14, and C16 terminal alkenes showshigher rates than those presented above with β-CD, the yields being respectively59.1, 50.7, 30.1, 8.3, and 5.3 (DM-β-CD being 2.6 mmol and [Rh] 0.125 mmol)[86]. While β-CD tediously converts an alkene higher than oct-1-ene, DM-β-CDhas the capacity to ensure the transfer of a wider range of alkenes, presumablybecause of the higher flexibility of its cavity. The direct recycling of the catalyticaqueous phase containing the rhodium complex, the excess of TPPTS ligand(P/Rh = 10), and the DM-β-CD significantly increases the conversion rates ofoct-1-ene or dec-1-ene. For each recycling, a higher yield is obtained, from 66 to91% for oct-1-ene and from 65.5 to 85% for dec-1-ene, the linearity being roughlyconstant at about 88%. While a constant activity, or its gradual loss, is generallyobserved along recycling runs, the present catalytic system should progressivelyadopt a higher level of organization allowing a better catalytic activity along thesuccessive recycling runs. Therefore, the transfer agent induces a more efficientcontact between the reactants, and its local organization is not disrupted during thetwo phases decantation followed by the reinjection of the reactants. An attractive


Rh

TPPTS

HCO

Organic volume

Aqueous bulk

SO3Na

P 3

Figure 4.10 Organization of the interphase around the cyclodextrin. (Source: Adapted fromRef. [86].)

hypothesis is that the {catalyst/CD/substrate/solvent} system is formed to associatethe rhodium complex to the external part of the CD in the water-rich area, whereasthe alkene stands in the organic-rich part. Thus, an organization of the CD involvingan organic volume inside the aqueous bulk constitutes the building block of aninterphase as represented in Figure 4.10. After the reductive elimination of thealdehyde, the [Rh(H)(CO)(DM-β-CD)] part can coordinate easily with the alkenefrom the organic phase, the [Rh(TPPTS)2] fragment being maintained in theaqueous phase.

Extension to other functionalized CDs has been done by introduction of variousgroups in the place of the methyl substituents [93]. In addition to 2-hydroxypropyl,acetyl, and sulfonate groups, phosphine [94] or diphosphine ligands [95] withvarious lengths of spacers between receptor and catalytically active centers canbe grafted onto the CD. The aim is to keep the catalyst near the mass transferagent [96]. For instance, with the CH2-S-(CH2)nN(CH2PPh2)2 substituent onβ-CD (n = 2–4) and starting from the [Rh(COD)2]+ precursor, the quantitativeconversion of oct-1-ene can be achieved at 80 ◦C and 100 bar within 18 h with a99% selectivity for the C9 aldehyde and 76% regioselectivity for n-nonanal [97]. In acontrol experiment, the classical biphasic Rh/TPPTS system has been tested, andit is necessary to operate at 120 ◦C to obtain a 4% conversion; thus the catalyticsystem involving the β-CD functionalized with the diphosphine ligand (n = 2) isestimated to be 150 times more active. The aqueous phase contains 30% DMF,which significantly reduces the surface tension. Recovery of the water-rich phaseto perform a second run results in only 50% of the original activity. Moreover, aninternal alkene such as hex-3-ene can be successfully carbonylated at 60 ◦C and100 bar into 2-ethyl-pentanal (90%) and 1-methyl-hexanal (4%). No reaction occurs


HOHO HO

SO3NaSO3Na

OH

Ar2P

Ar2P

NaO3S SO3Na

NaO3S SO3Na

[Rh]n n

Figure 4.11 General scheme of sulfonated calix[4]arenes bearing two phosphine ligands,a, n = 1 and b, n = 2.

when using β-CD/Rh/TPPTS. This supramolecular rhodium catalytic system isable to hydroformylate various unreactive alkenes.

Very recently, in the cobalt-catalyzed hydroformylation reaction of heavyalkenes, the introduction of RAME-β-CD or partially methylated β-CD to the[Co(H)(CO)(TPPTS)2] active species generated in situ allowed to improve the 28%conversion of oct-1-ene to 98% and the 64% selectivity in aldehydes to 96%, the 2.8n/iso ratio decreasing to 1.3 [98]. The reaction occurs for 5 h, at 100 ◦C and underan 80 bar CO/H2 pressure. Introduction of the sodium salt of the more donatingtrisulfonated tris(biphenyl)phosphine ligand (BiphTS) in the coordination sphereof the cobalt center produces the [Co(H)(CO)2(BiphTS)] less active species. Infact, under similar conditions but at 120 ◦C, a 95% conversion and a lower (85%)selectivity in aldehydes are obtained, the n/iso ratio being still at 1.3 [99].

With a conical shape similar to CDs, calix[n]arenes (n = 4, 6, 8) can also actas mass transport promoters when their conformations are stabilized. They aremacrocyclic compounds with a hydrophobic core sandwiched between a highlyhydrophobic large rim and a more hydrophilic small rim. The stabilization can beachieved by tethering bulky substituents on the narrow rim, and a diphosphiniteligand has been attached at the entrance of the conical cavity to coordinate ametal center and drive the catalysis, especially the contact to the reactants [100].The structure of such calixarenes would enable an olefin to be included in thehydrophobic cavity and to simultaneously interact with a catalytic transition-metalcenter coordinated to the phosphine moieties (Figure 4.11). Catalytic activity inhydroformylation in homogeneous systems has been demonstrated, especiallyusing diphosphite ligands in toluene/n-decane medium [101].

Moreover, metal complexes with water-soluble calix[4]arenes containing twophosphine ligands on the upper rim are able to function as inverse phase-transfercatalysts in aqueous biphasic hydroformylation reaction [102, 103]. Complexesproduced from [Rh(acac)(CO)2] and calixarenes a (n = 1) and b (n = 2) drawn inFigure 4.11 have been tested for the hydroformylation of oct-1-ene and dec-1-ene.They give better catalytic performances in terms of conversions and selectivity inaldehyde compared to the {DM-β-CD, TPPTS, [Rh(acac)(CO)2]} catalytic system


Organic phase

SiO2 supportAqueous film

[H2/CO] gaseous phase

[H2/CO]org.

R

RCHO

[H2/CO]w.

[Rh]

Figure 4.12 View of a hydrophilic support supporting a film of water containing thewater-soluble catalyst at the interface of the organic phase.

under the same conditions: a 73% yield of aldehyde is obtained instead of 26%.The activity and selectivity are retained after three consecutive recycling runs [102].

4.3.3Supported Aqueous-Phase Catalysis

In 1989, a new approach was developed for increasing the interfacial contactby immobilizing the water-soluble coordination complex in a thin film of watermaintained on a high-surface-area hydrophilic solid [104–108]. It has been calledsupported aqueous-phase catalysis, and sometimes by its acronym ‘‘SAPC,’’ and ithelps overcome the possible limitation of biphasic catalysis because of low substratesolubility in water and/or significant catalyst loss [107, 108].

The technique consists in adsorbing onto the high-surface-area support a thinfilm of water containing the catalyst precursor and an excess of hydrophilic ligands,which allows the catalytic reaction to take place very efficiently at the water–organic(containing the reactants and products) interface. Eligible inorganic supports arehydrophilic porous solids with highly specific surface area to give an optimal spreadout of catalyst, which forms a large interfacial area. It is the case, for instance, forsilica or mesoporous glass beads. A general scheme is given in Figure 4.12.

Generally, the support is impregnated with the catalyst precursor such as[HRh(CO)(TPPTS)3] with eventually an excess of TPPTS. The content of water is


adjusted by the addition of controlled amounts taking into account water originallypresent on the support, the catalyst looking as a dry yellow colored solid. It has beenshown by SAPC applications of rhodium–TPPTS [104, 105, 109], platinum–TPPTS[110], and cobalt–TPPTS [111] complexes in the hydroformylation reaction ofolefins that these catalysts show a constant catalytic activity along the recyclingruns. Moreover, extensive experiments have been carried out to measure the lossof metal in the organic solution and the analyses have shown that no rhodiumwas detectable with a sensitivity of 1 ppb [112]. In separate experiments, afterone catalytic cycle, the organic solution presents no activity in hydrogenationor hydroformylation reactions, so that it can be concluded that rhodium is notleached into the organic phase either as a soluble species or as colloids. The highlyhydrophilic sulfonate groups belonging to the ligands of the SAP catalysts are mostlikely strongly associated with adsorbed water and surface hydroxyl groups, whilethe rhodium center, which is in a relatively hydrophobic local environment, ispushed into the nonaqueous phase.

In the early experiments, hydroformylation of oleyl alcohol into the correspondingsaturated alcohol and aldehyde, all components being completely insoluble in water,has been performed with success [113]. The reaction occurs at the organic–aqueousphase film interface. Further experiments confirm this interpretation, as severalalkenes of different carbon chain length are functionalized at the same rate,more precisely at the same TOFs [109]. Alkenes with up to 17 carbon atomscan be efficiently transformed [106]. The water content of the support exerts adramatic influence on the activity of such catalysts. It has been observed thatfor poor levels of hydration, the activity remains low, presumably as being dueto important restrictions to the mobility of the organometallic complex insidethe pores. Raising the water quantity increases the conversion, whereas thenormal/branched regioselectivity in aldehyde remains unaffected; for instance,with 2.9 w/w% H2O on a controlled-pore glass CPG-240 support, a TOF of2 × 10−4 s−1 was noted and with 9 w/w% H2O, the TOF was 2 × 10−2 s−1,which is, two orders of magnitude higher [113]. A bell-shaped curve describesthe dependence of the rate with the water content and beyond a certain valuethe activity decreases. Such a phenomenon has been interpreted as being due tothe progressive filling of the pores, which favors the mobility of the complex inwater but significantly reduces the contact between the organic reactants and thecatalyst. More recently, the nature of the supports and thus the size of the poreshave been studied for the hydroformylation of oct-1-ene, starting from the catalystprecursor [Rh2(µ-StBu)2(CO)2(TPPTS)2] in the presence of a slight excess of freeligand in order to maintain a molar P/Rh = 6 ratio, in such a way the inactivespecies [Rh2(µ-StBu)2(CO)4] does not form [114]. A silica Sipernat 22, characterizedby a 100 µm mean granulometry, a 173 m2 g−1 BET(Bruauer, Emmett and Teller)surface area, and a 45 nm mean pore diameter, has been used as a support.Different hydration levels of this silica ranging from 1.3 to 47.2 w/w% of total watercontent show that a poor catalytic activity is observed until around 12.7% watercontent. Beyond this value, the conversion increases sharply to achieve 60% whenthe pores are fully filled with water (16%). However, the conversion still rises until


a plateau is reached at about 20 w/w% hydration. A rather large zone of stabilityis obtained where the activity is maintained at 80–90% yield in 18 h. This area islocated between 20 and 44% [115]. When higher hydration rates are experimented,water is no more strongly retained on the support, and the droplets of water areclearly observed in the organic solution when the stirring is stopped. Thus, someamount of complexes leaches from the support decreasing the apparent efficiencyof the SAPC. Some other supports present a similar behavior as, for instance,S200, that is, a silica with a 316 m2 g−1 BET surface area and a 704 nm meanpore diameter or an apatitic phosphate (86 m2 g−1 and 8.3 nm). In both the cases,the stability area is observed after the volume of the pores is filled. In all theexperiments, the selectivity for the conversion of oct-1-ene into the linear aldehydeis more or less the same, the highest value being 87%. Two salient features areworth noting. Indeed, when catalysis is carried out in a pure biphasic system, thelinearity is generally close to 95–97% [57], whereas the organosoluble complexcounterpart [Rh2(µ-StBu)2(CO)2(PPh3)2] in toluene solutions leads to selectivitiesin linear aldehydes of about 75% [116]. The general mean value of 80% means thatthe catalytic reaction occurs in a rich organic area but is already influenced by watermolecules to some extent. The interfacial surface presumably plays a central rolein this catalysis. The second observation of interest is related to the large field ofstability of the apatitic-phosphate support [117]. The sulfonate groups of the TPPTSligand interact with the calcium atoms of the support, so that the rhodium complexwith its solvation sphere is firmly retained. It is relevant to consider that catalysisoperates in a volume where the sulfonate groups and the apatite surface are in anaqueous environment; the phenyl groups bonded to the phosphorus atoms and therhodium metal should emerge in the organic phase to be in direct contact with thereactants (Figure 4.13).

Moreover, solid-state NMR data immediately recorded after impregnation, andseveral weeks after catalysis, suggest that an organization of the water moleculestakes place to adopt a fixed location. It can be considered that the solvation sphere ofthe complex and the extra TPPTS ligands should become more and more organized

Ca2+ Ca2+ PO43−PO4

3−

Ca2+

P

3

[Rh]

SO3− Aqueous phase

Organic phase

Figure 4.13 Organization of the interphase at the surface of an apatitic support.


along catalysis in such a way that the income of the alkene/hydrogen/carbonmonoxide reactants is more efficient. The pores can contain the extra TPPTSligands that are in equilibrium with the rhodium active species on the surface,as it is proved that they are necessary to prevent the irreversible deactivation into[Rh2(µ-StBu)2(CO)4].

4.4Innovative Expansions

In order to increase the reaction rates as well as the selectivities, many authors haveexplored innovative protocols. Thermomorphic solvents and catalyst CO2-ionizationswitching are efficient ways to recycle the catalysts. Moreover, cascade reactions,such as hydroaminomethylation (HAM), represent a successful spread of hydro-formylation in aqueous phase.

4.4.1Thermoregulated Catalytic Systems

Phosphorus-containing ligands with nonionic polyoxyethylene moieties (insteadof ionic substituents) have been designed to obtain a large solubility in waterand perform thermoregulated phase-transfer catalysis. They present the peculiarproperty to have an inverse temperature-dependent solubility in the aqueousphase, similar to the nonionic surfactants. Thus, under ambient conditions, thecatalyst is insoluble in the organic phase, but by increasing the temperature,the hydrogen bonds between the polyoxyethylene chains and water are split anda higher solubility in organic compounds is gained, beyond the so-called cloudpoint [118, 119]. Catalysis can occur in the organic phase and the phenomenonbeing reversible, coming back to the ambient conditions, the two-phase systemis restored. In fact, heating at a temperature higher than the cloud point, thecatalyst loses its hydration shell, transfers into the organic phase, and catalyzes thehydroformylation reaction, as supported by the same hydroformylation rates of a1 : 1 mixture of hex-1-ene and dec-1-ene. As soon as the temperature is loweredunder the cloud point, the complex regains its hydration shell and returns to theaqueous phase [119].

Triphenylphosphine ligands in which the one, two, or three phenyl substituentsbear a (OCH2CH2)n-OH chain (n = 8–25) in the para position have been synthe-sized, and the biphasic rhodium-catalyzed hydroformylation reaction successfullyapplied to dodec-1-ene under mild conditions (100 ◦C and 5 bar of a CO/H2 1 : 1 gasmixture for 7 h) in a water/toluene system at pH = 6 [118]. The conversion ratesare around 93%, and the selectivity in aldehydes is 85%, these values remain thesame along four recycling runs. Ethoxylation of Ph2P(C6H4SO2NH2) leads to thePh2P[C6H4SO2N{(OCH2CH2)nH)}2] ligand and the corresponding rhodium com-plex allows to perform the hydroformylation of dec-1-ene in 20 successive runs withyields and chemoselectivities higher than 94 and 99%, respectively [120]. Some

4.4 Innovative Expansions 129

other ligands have been designed [121]. In this thermoregulated phase-transfercatalysis, the issues arise from some ligand accumulation in the organic layer[120, 122] and the formation of colloidal rhodium nanoparticles, progressivelyleading to the precipitation of the metal [123].

The surface activity of an amphilic phosphine has been shown to bethermoregulated by a CD [124]. Native β-CD and RAME-β-CD can indeedincorporate into their cavity the (4-tert-butyl)phenyl substituent of the 1-(4-tert-butyl)benzyl-1-azonia-3,5-diaza-7-phosphaadamantyl bromide ligand while itsamphiphilic character is maintained. At high temperatures such as 100 or 120 ◦C,dissociation of the ligand from the CD results in an efficient mass transferby inclusion of high olefins in the cavity. At the end of the catalytic reaction,decreasing the temperature allows to reincorporate the amphiphilic ligand and toobtain very rapidly a clean decantation.

Recently, rhodium nanoparticles stabilized by the thermoregulatedPh2P(CH2CH2O)16CH3 ligand have been investigated in the hydroformylation ofstyrene, cyclohexene, oct-1-ene, dec-1-ene, or dodec-1-ene in an aqueous/1-butanolbiphasic system [125]. High conversion rates (97–99% at 70 ◦C and 5 bar) andaldehyde yields (93–97%) can be obtained, and the three successive runs showthat the reactivity is maintained, although the rhodium particles size progressivelyincreases.

This concept can be extended to soluble polymer-bound catalysts, where it isnecessary to operate above a critical solution temperature [126] and thermoregulatedmicroemulsions [127]. In order to avoid any mass transport problems and to realizean efficient recycling of the complex, temperature-dependent multicomponentsolvent systems have been explored [122, 128]. Usually, three solvents are involved:a polar one in which the complex is soluble, a nonpolar solvent immisciblewith the first one to extract the reaction products, and a third mediator solventwith intermediate polarity providing the system a homogeneous or heterogeneousnature according to the temperature. Thus, the catalytic reaction is operated at hightemperature in a single phase, and under ambient conditions, the products’ layercan be easily separated and the catalyst recycled.

4.4.2Ionic Liquids and Carbon Dioxide Induced Phase Switching

In an approach to find catalytic systems providing high reaction ratesconsistent with the industrial requirements, addition of the water-soluble1-octyl-3-methylimidazolium bromide derivative has been studied in the classicalbiphasic hydroformylation reaction of heavy terminal alkenes [129]. The TOF islargely increased for oct-1-ene from 10.5 to 1105 h−1 while operating at 100 ◦C and30 bar of CO/H2; the same effect occurs for dec-1-ene where the TOF varies from0 to 340 h−1.

Another strategy is to perform the reaction in a single aqueous phase. A rep-resentative example concerns a water-soluble allylic alcohol converted into the


N NMe23

N NMe23

HCO3−

HO HO CHO HOCHO

OCHO

CO/H2

n iso

N2

CO2Toluene phase

Water phase

[Rh]-P

[Rh]-P

Figure 4.14 Schematic view of the CO2 switching effect for homogeneous-catalystrecycling.

corresponding C4 aldehydes, the linear isomer leading to the cyclic hemiac-etal, 2-hydroxytrihydrofuran. The reaction occurs in the aqueous phase saturatedwith CO2 to transform the amidine-substituted triphenylphosphine into thewater-soluble amidinium hydrogenocarbonate ligand. After catalysis, nitrogenis bubbled into the solution to remove CO2 and return the catalyst to the addedtoluene organic phase (Figure 4.14) [130, 131].

4.4.3Cascade Reactions

Performing the HAM reaction in an aqueous two-phase system is one interestingmethod. HAM represents the tandem reaction of the hydroformylation of analkene, the condensation of the corresponding aldehydes with an amine, followedby the hydrogenation of the resulting enamines or imines (Figure 4.15) [132].

One of the first studies on the application of two-phase catalysis related tothe HAM reaction of aliphatic alkenes has been recently published [133]. Theauthors develop an elegant way to synthesize primary amines, starting from shortalkenes (C3−C5) and ammonia. Major challenges in this reaction are to avoid thenumerous side reactions producing mainly secondary and even tertiary amines.Experiments were performed with monodentate TPPTS [134] and bidentate BINASsulfonated ligands, with a high phosphorus to rhodium ratio (P/Rh = 425 and140, respectively) to maintain the rhodium catalyst in the aqueous phase, and also

CatalystCO/H2

R RCHO HNR1R2

R NR1R2

R NR2

R1 = H R NR1R2

Catalyst,H2

−H2OHydroformylation

Imine or enamine formation

Hydrogenation

Figure 4.15 Cascade reactions in the hydroaminomethylation of alkenes (only the linearproducts are shown).

4.4 Innovative Expansions 131

to keep the active species in hydroformylation as [Rh(H)(CO)L2]. However, sucha strategy leads to slow hydrogenation rates. To oppose this loss of activity, themore efficient hydrogenation iridium [IrCl(COD)]2 precursor was added to build adual transition-metal catalytic system (in this case, iridium/rhodium ratio = 8 : 1).Higher chemoselectivities with respect to the primary amine have been obtainedwith a NH3/alkene ratio of 8 : 1 and by lowering the polarity of the organic phase.These conditions allow a better extraction of the hydrophobic primary amine fromthe aqueous phase and favor the reactivity of NH3 toward aldehydes, leading to thepreferential formation of the primary amine with regard to the secondary amine.For instance, in toluene, the primary to secondary amine ratio is 82 : 18 instead of69 : 31 in methyl(tert-butyl)ether.

A similar approach concerns the HAM of high alkenes using water-solublephosphine ligands, with a dual rhodium/iridium catalytic system [135–137]. Thisfunctionalization of long-chain alkenes is of great interest because the directsynthesis of aliphatic tertiary amines is of high industrial importance. In order toimprove the poor solubility of these alkenes in water, experiments were carried outwith the cationic cetyltrimethylammonium bromide (CTAB) surfactant, as alreadyused in the hydroformylation reaction under the same conditions [138]. TPPTSand the diphosphine BISBIS ligands (Figure 4.4), the steric hindrance of whichshould improve the regioselectivity for linear amines, have been investigated [139].[RhH(CO)(TPPTS)2] is generated from the [RhCl(CO)(TPPTS)2] rhodium precursorunder H2/CO pressure, and the catalytic tests are performed without additionalorganic solvent. The most attractive results are observed at 130 ◦C and 30 bar CO/H2

(1 : 1) pressure. Dodecane, the hydrogenation product, and isomerized dodeceneare the main by-products observed in quite important amounts. The study of severalparameters, especially the cationic surfactant CTAB concentration, the phosphineligand to rhodium molar ratio, and the amine to alkene ratio, allows to optimize thereaction conditions. The best results are obtained with an amine/alkene ratio of 4and a P/Rh ratio of 30, giving rise to a 91% conversion, a 46% selectivity in amines,and an n/iso ratio of 14.6. Adding BISBIS to [RhCl(CO)(TPPTS)2] under CO/H2

generates the [RhH(CO)(BISBIS)] active species. The presence of CTAB improvesthe regioselectivity, such as to an n/iso ratio of around 70 when P/Rh = 10.The combination of [IrCl(CO)(TPPTS)2] and [RhCl(CO)(TPPTS)2] under the samecatalytic conditions leads to a better chemoselectivity in the final amines, which isin relationship with the better capacity of iridium to play a more active role in thehydrogenation step.

The micellar approach [140] gives attractive results in the rhodium-catalyzedhydroformylation of oct-1-ene involving amphiphilic triphenylphosphine function-alized poly(2-oxazoline) macroligands (Figure 4.16) [141]. The HAM study has beenextended to the reaction of the same alkene with dimethylamine, adapting thereaction conditions. The phosphine moieties are covalently linked to the hydropho-bic part of a mixed block copolymer. A higher reaction temperature is necessaryand even at 150 ◦C, where the rhodium catalyst is still stable, modest yields ofamine (22%) are observed with an n/iso ratio of 7.5 and a TOF of 461 h−1. Thedual rhodium/iridium catalytic system (Rh/Ir ratio of 2 : 1 at 130 ◦C), starting


N

O

N N

O (CH2)8CH3

(CH2)5O

NHO

PPh2

N30 4 4

n

Hydrophilic part

Hydrophobic part

Figure 4.16 Amphiphilic triphenylphosphine functionalized by poly(2-oxazoline) macro-ligands.

from the [Rh(acac)(CO)2] and [IrCl(COE)2]2 precursors (COE, cyclooctene), givesthe best results in amines (yield 24%, TOF 600 h−1), with a 62% selectivity andan n/iso ratio of 11. In this temperature range, the main problem is a very highhydrogenation activity compared to the hydroformylation activity, leading to theundesired hydrogenation of the olefin faster than the formation of the aldehydes.

Recently, the use of salts of primary and secondary amines was also reported[142] in the HAM of oct-1-ene in water. Selectivities up to 99% are obtained, mainlyin linear amines (60 bar CO/H2 (1 : 3), 130 ◦C), starting from the [RhCl(COD)]2precursor and a large excess of TPPTS (P/Rh = 64). The addition of organic orinorganic acids to perform the reaction in acidic medium appears to be an efficientway to avoid the aldol condensation and to promote the intermediate enaminehydrogenation.

4.5Conclusion

Apart from the direct use of a biphasic system in the hydroformylation of propene,and in a lesser extent but-1-ene, higher alkenes require the presence of moresophisticated means to both efficiently perform the hydroformylation reaction andthe recycling of the metal complex. Many elaborated systems have been exploredpresenting attractive solutions such as the use of CDs, supports for the aqueousphase, amphiphilic ligands to produce micelles, and pH-adjusted catalysts. In mostof the cases, a local organization occurs at the interface between the aqueous andorganic phases, so that the mass transfer is no more the limiting rate parameter.An interesting view of such an organization can be found in the structuring ofthis interface into a thin interphase containing microvolumic reactors in whichcatalysis takes place.

References 133

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