Hydroformylation and Carbonylation Reactions in Ionic Liquids

As mentioned above, the poor solubility of higher olefins in water still hampers their hydroformylation in aqueous media. Since 2003, ILs have entered the hydroformylation field as an alternative to the aqueous phase thanks to the higher solubilities of long-chain olefins in these non-conventional solvents.124 Of course, as a prerequisite, the transition metal catalyst has to dissolve in the IL, too. Moreover, an efficient confinement in the IL phase has to occur to prevent catalyst leaching under the conditions of intense mixing characteristic of continuous liquid-liquid biphasic operations. Important advances were made in ligand design, addressing in particular the ionic tagging strategy. A vast number of publications have shown improved potential of ILs with regard to product separation, catalyst recovery, improved catalyst stability and selectivity.125

Since the pioneering results by Chauvin et al. in 199 5,126 the story of the struggle against problems such as catalyst leaching, deactivation, etc., working in ILs, has been excellently outlined in a review by Haumann and Riisager.124 The use of triphenylphosphine as ligand led to acceptable rates in ILs, but with high rhodium leaching into the organic phase. Recourse to sulfonated phosphines such as monosulfonated triphenylphosphine retained the catalyst in the ionic liquid phase but decreased its activity significantly. This drawback was surmounted by the use of 47 (Table 1.5), which was derived from a simple cation metathesis reaction between TPPTS (37) and 1-butyl-2,3-dimethylimi-dazolium chloride [bdmim][Cl] in acetonitrile.127

Hydroaminomethylation reactions (Scheme 1.25) were also successfully performed in [pmim][BF4] using a rhodium/sulfoxantphos system by reacting piperidine with different n-alkenes. The resulting amines were obtained in more than 95% yield with TOF of up to 16 000 h-1, along with high regioselectivity for the linear amines, with n/i ratios up to 78, and quantitative catalyst recovery.128 Analogous hydroaminomethylation reactions were also reported to be catalysed by Rh(CO)2acac/BISBIS in [bmim][p-CH3-C6H4-SO3] with high conversions and n/i ratios ranging from 5 to 36.129

The known cobaltocenium complexes 48 and 49 were proposed as ligands for rhodium (Table 1.5). They conferred a lower activity to the catalyst, but Rh leaching was less than 0.2% for both ionic ligands, and recycling experiments showed unchanged activity and selectivity for several runs.

The concept of ionic tagging in aryl phosphine design is made explicit by the structures of ligands collected in Table 1.5.

Pyridinium and guanidinium-based ligands 50 and 51, respectively, have been applied to the hydroformylation of 1-hexene in [bmim][BF4] and

Table 1.5 Ligands for hydroformylation reactions in ILs.

Ligand

Ref.

Hydroformylation Reaction

Ligand

Ref.

PPho

Hydro Formalistion Reactions
Scheme 1.25

Scheme 1.26

[bmim][PF6]. Their use resulted in moderate selectivity, affording heptanal in 72-80% yield with TOFs ranging between 180 and 240 h"1. Minor rhodium leaching was observed when using the guanidinium-derived ligand 51 in [bmim][BF4].131

Ligands 52 and 53 were tested in the biphasic hydroformylation of 1-octene in [bmim][PF6]. With 52 a higher activity (TOF of 552 h"1) was balanced by a lower selectivity (52% nonanal), whereas in the case of 53 the lower TOF (51 h"1) was compensated by an improved selectivity (74%).132

Compared to TPPTS, which allowed good immobilization of rhodium but provided much lower rates, the bidentate xanthene-based ligand 54 showed an excellent selectivity towards nonanal of about 95% during all recycling experiments, with no Rh leaching detected.133 In analogy, ligand 55 gave excellent results in the rhodium-catalysed hydroformylation of 1-octene in [bmim][PF6], with no Rh and P leaching detected throughout seven recycling experiments.134

Much more recently, enantiocontrol issues have been addressed in hydroformylation reactions. The first study on stereoselective biphasic hydroformylation in IL appeared in 2007, using the chiral sulfonated ligand 56 in [bmim][BF4] for the enantioselective hydroformylation of vinyl acetate and styrene (Scheme 1.26). The hydroformylation of vinyl acetate resulted in the predominant formation of 2-acetoxypropanal with ee > 50%. In the hydrophilic IL [bmim][BF4], the 56 derived catalyst showed 79% conversion with high selectivity for the branched product, while the ee was moderate at around 22%.135

The last contribution quoted in this section is a phosphine-free hydro-formylation process based on a liquid triphasic system consisting of isooctane, water and trioctylmethylammonium chloride (TOMAC). The hydroformyla-tion of model olefins required neat RhCl3 only as catalyst precursor. In the triphasic system, the catalyst is confined in the TOMAC phase, likely in the form of an ion pair. Products are obtained in excellent yields (>90% at 80 °C) and high regioselectivity (>98%) in favour of the branched aldehyde in the case of styrene, while the exo isomer was obtained in >90% selectivity in the case of norbornene. The products were easily removed and the catalyst was recycled several times, with no leaching of rhodium into the organic phase.136

Carbonylation of olefins and aryl halides to the corresponding esters or amides find in ILs an ideal media for the palladium-catalysed process.

(E/Z)-Isomeric vinyl bromides were stereoselectively carbonylated to the corresponding (E)-a,b-unsaturated carboxylic acids in [bmim][PF6], while vinyl dibromides underwent hydroxycarbonylation to give monoacids (Scheme 1.27). The catalyst-containing IL phase could be recycled five times in the second reaction of Scheme 1.27.137

A group of imidazolium-based ILs with different substituents has been tested as solvent in the hydroethoxycarbonylation of olefins catalysed by PdCl2(PPh3)2. Beside the nature of the IL, the presence of 1,10-bis(diphenyl-phosphino)ferrocene (dppf) strongly affected the regioselectivity. For example, using [acetonyl-mim][PF6] as the solvent the branched ester was obtained only, while in the presence of dppf the regiochemistry was completely inverted in favour of the linear ester (Scheme 1.28).138

A drawback was partial metal leaching in the product separation process, which prevents recycling with identical performance of the catalyst-containing IL phase.

COOEt

.COOEt

EtOH, CO

Scheme 1.28

An efficient carbonylation of aniline to the corresponding methyl carbamate was reported to be catalysed by Pd(phen)Cl2 in [bmim][BF4]. The reaction was carried out by introducing successively, in an autoclave, 1 mg Pd complex catalyst, 1 mL ionic liquid, 1 mL amine, 10 mL methanol and 5MPa of a mixture of gases (CO 4.5 MPa and O2 0.5 MPa), followed by heating at 175 °C for 1 h. The reaction took place homogeneously thanks to the good solubility of Pd(phen)Cl2 in the IL (0.11-0.12 g per 100 mL). The N-phenyl methyl carba-mate was eventually precipitated by adding water to the resulting mixture and filtered off (conversion = 99%, selectivity = 98%, TOF = 4540 h"1). The catalyst-containing IL solution could be reused with slight loss of catalytic activity.139

1.3.3 Hydroformylation Reactions in scCO2 and in Ionic Liquids/scCO2 Systems

Supercritical carbon dioxide (scCO2) has been used as the reaction medium for hydroformylation reactions. The most attractive feature of SCFs is that its properties (density, polarity, viscosity, diffusivity, etc.) can be varied over a wide range by small changes in P/T values.

Catalysts soluble in scCO2 are examined first. In the late 1990s Leitner and co-workers reported that the hexafluoroacetylacetonate ligand (hfacac) confers its metal complexes a good CO2-philicity - a necessary requisite to promote an efficient catalysis in scCO2. Thus, the rhodium complex [(cod)Rh(hfacac)] exhibited improved activity for hydroformylation in scCO2 compared with that observed in toluene solution; however, the linear/branched selectivity was poor. Terminal and internal double bonds with alkyl and aryl substituents were hydroformylated in scCO2 almost quantitatively at 40 °C at substrate/catalyst ratios of b 800 :1.140 Triaryl phosphines are the most important class of ligands for hydroformylation catalysts; however, most of them, including triphenyl-phosphine, are not soluble in scCO2 and cannot be used as ligands for the hydroformylation reaction. Since the installation on a molecule of per-fluoroalkyl tags confers it with good solubility in scCO2, ligands 59 and 60 (Scheme 1.29) were proposed as modifiers of [(cod)Rh(hfacac)]. With catalysts containing the CO2-philic phosphine (59a,b) and phosphite (60) ligands, hydroformylation reactions compared favourably with analogous reactions in

(cod)Rh(hfacac)

(cod)Rh(hfacac)

Rp--CH2CH2(CF2)5CF3

Rp--CH2CH2(CF2)5CF3

cat.

Scheme 1.29

other reaction media, linear selectivity improved up to a maximum n/i ratio of 5.6 : 1 and an efficient separation and recycling was possible, with rhodium leaching in products being limited to <1 ppm.140

Ligand design led to the development of the CO2-philic chiral ligand 61 (Scheme 1.30). Rhodium-catalysed asymmetric hydroformylation of styrene in the presence of 61 could be performed for eight successive runs - the system being still active after a total turnover number of more than 12 000 catalytic cycles per rhodium centre. Both the catalytic reaction and the extraction step made use of scCO2, allowing quantitative recovery of the product free of solvent, with rhodium content in the products ranging from 0.36 to 1.94 ppm.141 The branched aldehyde was obtained in up to 93.6% ee and with more than 1:9 inverse n/i regioselectivity.

A simple way to overcome the insolubility of TPPTS in scCO2 is based on the use of supercritical mixtures containing water. For example, the hydroformylation of propene has also been studied in supercritical CO2/H2O and in supercritical propene/H2O mixtures using Rh(acac)(CO)2 and TPPTS as the catalytic system.142 Visual observation of the reaction in both systems revealed that the reaction occurred under homogeneous conditions since a single phase was present at supercritical T and P. The work-up was very simple, indeed when the pressure and temperature were lowered to the ambient value a biphasic system was formed. Products and the catalyst were easily separated, the products being in the organic phase and the rhodium catalyst in the aqueous phase. Rhodium leaching in the organic phase was ~1.0x 10"6mgmL"1. Table 1.6 compares results of the hydroformylation of propene in scCO2/H2O, in sc propene/H2O and under classic aqueous biphasic conditions (see Section 1.3.1).

ScCO2 has also found an interesting application not as the solvent for a hydroformylation reaction but as a switch to halt the reaction, induce a phase separation and separate products with negligible metal leaching. Briefly, the hydroformylation of 1-octene was conducted neat in the presence of a catalyst

Hydrofomulation

Scheme 1.30

Table 1.6 Comparison of catalytic properties for biphasic and supercritical reaction systems

Table 1.6 Comparison of catalytic properties for biphasic and supercritical reaction systems

Reaction system

TON [g-aldehyde (g-Rh h)

n/i Ratio

Biphasica

76.3

3.2

scCO2/H2Ob

190.1

4.3

sc propene /H2Oc

601.4

8.4

a[Rh] = 30ppm, P/Rh = 50, propylene = 2g, H2O = 5mL, cyclohexane = 5mL, CO/H2 = 1:1 (3.54 MPa), reaction pressure = 4.0-6.0 MPa, T = 100 °C, t = 8h.

b[Rh] = 15 ppm, P/Rh = 20, CO2 = 13 g, propylene = 2g, H2O = 0.4 mL, ethanol = 0.4 mL, CO/ H2 = 1:1 (3.5-4 MPa), reaction pressure = 12.0-14.0 MPa, T = 55 °C, t = 6h. c[Rh] = 15Pppm, /Rh = 20, propylene = 13g, H2O = 0.4mL, ethanol = 0.4mL, CO/H2 = 1:1 (3.54 MPa), reaction pressure = 6.0-10.0 MPa, T = 117 °C, t = 4h.

a[Rh] = 30ppm, P/Rh = 50, propylene = 2g, H2O = 5mL, cyclohexane = 5mL, CO/H2 = 1:1 (3.54 MPa), reaction pressure = 4.0-6.0 MPa, T = 100 °C, t = 8h.

b[Rh] = 15 ppm, P/Rh = 20, CO2 = 13 g, propylene = 2g, H2O = 0.4 mL, ethanol = 0.4 mL, CO/ H2 = 1:1 (3.5-4 MPa), reaction pressure = 12.0-14.0 MPa, T = 55 °C, t = 6h. c[Rh] = 15Pppm, /Rh = 20, propylene = 13g, H2O = 0.4mL, ethanol = 0.4mL, CO/H2 = 1:1 (3.54 MPa), reaction pressure = 6.0-10.0 MPa, T = 117 °C, t = 4h.

formed in situ from [Rh(acac)(CO)2] (P/Rh = 5:1) and the scCO2-insoluble ligand MeO-PEG750PPh2.143 After 2 h at T = 70 0C and p(H2/CO) = 50 bar, CO2 was introduced. The reaction stopped completely when the CO2 density reached 0.57 g mL"1, a yellow-orange solid formed, the catalyst, and the aldehydes (n/i = 2.5:1) were isolated quantitatively in solvent-free form with low levels of metal contamination (ca. 5 ppm) by extraction with scCO2. The strength of this procedure was that the reaction/separation sequence was

Products scC02

Products scC02

Scheme 1.31

repeated successfully six times with no significant changes in conversion or selectivity (conversion > 99%, n/i = 2.3:1-2.5:1).

A technologically highly attractive approach to the design of new hydro-formylation processes exploits CO2-insoluble catalysts under continuous flow conditions or using microreactor technology.144 A hydroformylation process of 1-octene was developed in a continuous flow system using scCO2 as the transport vector for substrates and products under relatively mild conditions for this kind of reaction, namely at 100 °C and 125 bar, with TOFs up to 240 h"1 (Scheme 1.31).145 The catalytic system was prepared by mixing [Rh(acac)(CO)2] and [Rmim][Ph2P-p-C6H4SO3]. The R group in the imidazo-lium ion was optimized for a correct balance of solubility in 1-octene (the highest solubility, the fastest reaction) and in scCO2 (the lowest solubility, the least catalyst leaching into product). The pentyl group was the best compromise, and the process operated at 100 °C, 140 bar with the following typical flow rates: CO2 (0.65 nLmin"1), syngas (1:1; 3.72mmolmin_1), 1-octene (0.2mL min"1 = 1.27mmolmin_1). Observed rates were on the order of 160-240 catalyst turnovers per hour with low rhodium leaching over a 12 h period at a total pressure of 125-140 bar.

The authors pointed out that this process is potentially emissionless since the scCO2 can, in principle, be recycled.145

1.3.4 Hydroformylation and Carbonylation Reactions Promoted by SILP Catalysts

To apply continuous flow technologies, the choice of solid catalysts is highly recommended not only for the easier separation process involved but also for the use of fixed bed reactors.146 In this section attention is focused only on the most recent examples of solid-phase assisted catalysis using ionic liquids as the catalyst-containing bed and scCO2 as carrier gas. Examples prior to 2006 are covered in recent reviews and are not discussed here.146'147 The concept of a Supported Ionic Liquid Phase (SILP) was very fruitful to the development of new gas-phase applications using flow reactor technologies.148 The preparation of a SILP catalyst requires the addition to a support material (SiO2, Al2O3, TiO2, ZrO2, etc.) of a methanolic solution of a catalyst precursor, e.g. [Rh (acac)(CO)2], a ligand, e.g. 62, and an IL, e.g. [bmim][n-C8H17OSO3] (Scheme 1.32). After methanol is removed in vacuo, the resulting solid material is characterized. A SILP is defined by (i) its ionic liquid loading a, defined as the IL volume/support pore volume ratio and correlated to the film thickness, (ii) the metal content, i.e. the Rh/support mass ratio, and (iii) the ligand/metal molar ratio. Scheme 1.32 gives a schematic representation of the surface cross section of a SILP catalyst in a fixed bed. The IL film is physically adsorbed on the surface of the solid support and the catalyst is dissolved in this microlayer. Since the film has the size of the diffusion layer, all metal complexes are involved in the catalytic reaction, which takes place under homogeneous

conditions. When SILP particles are used as the fixed bed of a flow reactor, reagents enter the IL film, they react under homogeneous conditions and products, eventually, are desorbed into the carrier gas stream.

Reactions are run for a time that depends on catalyst half-life. Stabilities exceeding 700 h time-on-stream have been recorded.149 Wasserscheid and coworkers optimized the hydroformylation of propene catalysed by a silica-supported phosphane 62-Rh complex in the IL [bmim][n-C8H17OSO3]. Using helium or He/CO2 mixtures as carrier gas, TOF values ranged from 16 to 46 h-1 under different reaction conditions (reagent partial pressures, support pre-treatment, etc.), while selectivity in favour of the linear aldehyde was constantly around 94-95%.

1-Butene has been also successfully hydroformylated under these conditions using the SILP catalyst, which exhibited a higher activity and selectivity with respect to propene.150

Among various conditions examined in a continuous fixed bed reactor, when 0.1 wt% of Rh is used, the TOF is 17 and 324 h-1 at 80 and 120 °C, respectively, while the % of linear aldehyde decreases from 99.9% to 97.7%. From the determination of the full rate law, a first order with respect to Rh is found, meaning that mass transport from the gas to the IL phase is not limiting the reaction rate; moreover, the overall kinetics picture is consistent with a homogeneous Rh-catalysed reaction. This observation confirms the potentiality of SILP-catalysis, which combines the kinetic advantages of homogeneous conditions (the reaction takes place in the IL liquid film confined on the support surface with high specific area) with the practicality of solid heterogeneous catalysis. Indeed recyclability and the reduced amount of IL needed with respect to liquid-liquid processes make the SILP strategy competitive from an economic and an environmental point of view.

Higher alkenes, too, can be hydroformylated over a fixed bed SILP catalyst. In this case the carrier gas is scCO2, which is soluble in ILs but does not dissolve ionic compounds.151 For example, 1-octene, CO and H2 are mixed in scCO2 and flowed through a tubular reactor. The reactor was packed with the catalytic system generated from [Rh(acac)(CO)2] and [propylmim][Ph2P-p-C6H4SO3] dissolved in [omim][Tf2N] which, in turn, is supported on silica gel.

Under optimized conditions, rates corresponding to 800molsubst (mol cata-lyst-1) h-1 are observed, with n/i ~ 3. The catalyst remains stable over a 40 h reaction time with less than 0.5 ppm of rhodium leaching.151

A fixed-bed SILP process has the major advantage over existing batch technology of demanding smaller amounts of expensive metal catalyst and ionic liquids. This approach proved useful also for conducting continuous, fixed-bed gas-phase methanol carbonylation at industrially relevant reaction conditions.152 The SILP rhodium iodide complex catalyst used was [bmim][Rh(CO)2I2]-[bmim][I]-SiO2, prepared by one-step impregnation of a silica support with a methanolic solution of the [bmim][I] and the dimer [Rh(CO)2I]2. The reactant gas stream consisting of CO and vaporized methanol/methyl iodide feed (3:1 wt%) passed through the SILP catalyst bed at 180 °C. At P = 20bar and after 2h, 99% of conversion was obtained with a

Scheme 1.33

production rate of 21 mol L 1 h 1 and a product selectivity for methyl acetate of 75%.

SILP catalysts are versatile materials that can be also used under standard batch conditions. Thus, allyl alcohol was hydroformylated with a SILP Rh/ PPh3 system, affording 80-90% of linear aldehydes with n/i up to 31 (Scheme 1.33).153

The SILP catalyst was prepared simply by adding to a CH2Cl2 solution of HRhCO(PPh3)3 and PPh3, in a 1:7 ratio, [bmim][PF6] and then silica previously calcined at 450 °C for 24 h, and finally removing the solvent under vacuum. The catalyst was suspended in water used as the reaction solvent and allyl alcohol; the mixture was charged into a 100 mL stainless steel high-pressure autoclave and finally the reactor was flushed with a CO/H2 (1:1) mixture at 25 °C, then heated to 80 °C and pressurized up to 4MPa of CO/H2 (1:1) for 5h. After depressurization, the catalyst was simply recovered by filtration. Yields of linear aldehyde reached 90% with a n/i ratio in the 10-30 range.

Another SILP catalyst used under batch conditions employed mesoporous MCM-41 as the solid support. The catalyst was derived from [Rh(CO)2(acac)] and TPPTS (1:5 mol ratio) in the desired IL. The excellent catalytic performance of this SILP catalyst in the hydroformylation of C6-C12 linear alkenes (TOF up to 500 h"1) was determined by the large surface area and uniform mesopore structure of MCM-41 and was almost independent of the type of IL used: [bmim][BF4], [bmim][PF6] and 1,1,3,3-tetramethylguanidinium lactate.154

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