Study of the hydroformylation of 2,5-dihydrofuran catalyzed by

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Study of the hydroformylation of 2,5-dihydrofuran catalyzed by rhodium diphosphine complexes Inmaculada del Río, Piet W.N.M. van Leeuwen, and Carmen Claver

Abstract: Rhodium diphosphine systems, including chiral ones, were used as catalyst precursors for the hydroformylation of 2,5-dihydrofuran. The effect of the reaction conditions and the bite angle on the activity and selectivity of the reaction were studied. The solution structures of the species present have been investigated spectroscopically through HP-NMR experiments during the hydroformylation reaction. Key words: hydroformylation, dihydrofuran, rhodium complexes, diphosphines, HP-NMR. Résumé : On a utilisé des systèmes diphosphines de rhodium, y compris des systèmes chiraux, comme précurseurs de catalyseurs pour l’hydroformylation du 2,5-dihydrofurane. On a étudié l’effet des conditions de la réaction et l’angle d’accès sur l’activité et la sélectivité de la réaction. On a étudié les structures des espèces présentes en solution à l’aide de la spectroscopie, impliquant des expériences de RMN du phosphore et de l’hydrogène au cours de la réaction d’hydroformylation. Mots clés : hydroformylation, dihydrofurane, complexes du rhodium, diphosphines, RMN du phosphore et de l’hydrogène. [Traduit par la Rédaction]

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Introduction Considerable effort has been made to synthesize derivatives of heterocyclic compounds such as tetrahydrofuran and tetrahydropyran (1–3). These derivatives are of interest for the preparation of intermediates for the synthesis of natural products or pharmaceuticals (4). The great development of the rhodium-catalyzed hydroformylation is shown for the amount of modified systems using monofunctional and chelate phosphorus ligands. Furthermore, the hydroformylation reactions have been applied for different substrates to obtain intermediates in organic syntheses (5–7). The hydroformylation of heterocyclic olefins would provide a potential synthetic tool for preparing derivatives of tetrahydrofuran and tetrahydropyran. Few systems have proved to be successful in the hydroformylation of internal cyclic alkenes. In most cases, selectivities are only moderate, probably because of the drastic reaction conditions required (8, 9). In the case of the hydroformylation of dihydrofurans with rhodium/phosphine catalysts, high pressures and temperaReceived August 8, 2000. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on May 23, 2001. Dedicated to Professor Brian James on the occasion of his 65th birthday. I. del Rio and C. Claver.1 Departament de Química Física i Inorgànica, Universitat Rovira i Virgili, Pl. Imperial Tarraco I, 43005 Tarragona, Spain. P.W.N.M. van Leeuwen. Institute of Molecular Chemistry, Nieuwe Achtergracht 166, NL-1018 WV Amsterdam, The Netherlands. 1

Corresponding author (telephone: +34-977-559574; fax: +34977-559563; e-mail: [email protected]).

Can. J. Chem. 79: 560–565 (2001)

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tures have been used and low selectivities towards tetrahydrofuran-2-carbaldehyde were obtained (10, 11). In recent years, the use of bulky phosphites in rhodium– phosphorus catalytic systems has been found to be useful in the hydroformylation of cycloalkenes (12–15). We reported on the hydroformylation of 2,3-dihydrofuran and 2,5-dihydrofuran under mild conditions using rhodium systems with PPh3 and tris(o-tert-butylphenyl)phosphite (P(OC6H4-2-t-Bu)3) as catalyst precursors (16, 17). By modifying the reaction conditions we succeeded in selectively introducing a formyl group in the 2- or 3-position of tetrahydrofuran, starting from 2,3- and 2,5-dihydrofuran (16). The influence of the phosphorus co-catalyst, pressure, and temperature on the reaction selectivity has also been studied and a reaction mechanism proposed (17). Although only the tetrahydrofuran-3-aldehyde would be expected to form in the hydroformylation of 2,5-dihydrofuran, it has been shown (16) that the tetrahydrofuran-2-aldehyde can also form through an isomerization process (Scheme 1). This isomerization is taking place simultaneously with the hydroformylation reaction. When the 2,5-dihydrofuran reacts with the rhodium hydride complex, the 3-alkyl intermediate is formed, which can evolve to the 2,3-dihydrofuran via the β-hydride elimination reaction. This new substrate can evolve to give either of the metal alkyl intermediates, 2alkyl or 3-alkyl. The 3-alkyl intermediate is thermodynamically favoured according to Ojima et al. (18) because the metal—carbon bond is formed at the carbon with the highest electron density regardless of the phosphorus ligand used. However, because the C-2—metal bond in the 2-alkyl intermediate is more polarized than the C-3—metal bond in the 3-alkyl intermediate, the acyl complex forms more quickly from the 2-alkyl than from the 3-alkyl rhodium intermediate (Scheme 1).

DOI: 10.1139/cjc-79-5/6-560

© 2001 NRC Canada


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Scheme 1.

O

H-[Rh]

O

[Rh]-H

2,5-DHF

O

O

H-[Rh]

3-alkyl

CO

CHO 3-ald

[Rh]

O

O

[Rh]

O

CO

O

CHO

[Rh]-H 2,3-DHF

Therefore, according to the proposed model, there is a fast preequilibrium between the metal alkyl species, via the βelimination process. The selectivity is dominated by the rate of formation of the acyl complex, provided that the hydrogen pressure is high enough for the hydrogenolysis of the metal–acyl complex not to be the rate-determining step (Scheme 1). The asymmetric hydroformylation of heterocyclic olefins has been studied very little and enantiomeric excesses below 10% ee have been published (19, 20). Only a successful enantioselective hydroformylation of oxygen- and nitrogencontaining internal cyclic olefins was reported in which phosphine-phosphite ((R,S)-BINAPHOS and (R,S)BIPHEMPHOS) were used as ligands (21). Enantiomeric excesses between 60–70% were obtained in the hydroformylation of 2,5-dihydrofuran, using these rhodium phosphine-phosphite systems. If diphosphines are used as ligands in homogeneous catalysis, not only is there the possibility of chiral control but also the possibility of stabilizing a specific geometry that depends on the bite angle of the diphosphine. In recent years a correlation has been observed between the bite angle of the diphosphines and the catalytic activity or selectivity in rhodium-catalyzed hydroformylation (22–25), nickel hydrocyanation (26, 27), palladium-catalyzed allylic alkylation (28), cross coupling reactions of Grignard reagents with organic halides (29, 30), and palladium hydroxycarbonylation (31). In hydroformylation, ligands with wider bite angles tend to give little isomerization of the alkenes at relatively low ligand-to-metal ratios, while retaining a high activity. Nowadays high-pressure NMR is often used to identify organometallic compounds under high pressure, although the reaction intermediates during rhodium-catalyzed hydroformylation have scarcely been characterized (32–37). We report here a study of the hydroformylation of 2,5dihydrofuran using rhodium diphosphine systems as the catalytic precursor. We studied the effect of the bite angle on the activity of the system and tested different chiral diphosphines. We performed in situ HP-NMR experiments during the catalysis to determining the nature of the catalytic species.

Experimental section General techniques Rhodium complexes were synthesized using standard Schlenk techniques under a nitrogen atmosphere. The

2-alkyl

2-ald

diphosphines DPEphos and Xantphos were synthesized via published methods (23). Solvents were distilled and deoxygenated before use. All other reagents were commercial samples and were used as purchased. Gas chromatography analyses were performed on a Hewlett–Packard 5840A, a gas chromatograph with a flame ionization detector and an Ultra-2 (5% diphenylsilicone:95% dimethylsilicone) (25 m × 0.2 mm Ø) capillary column. Catalysis Hydroformylation experiments were carried out in a specially designed autoclave with magnetic stirring and electrical heating. The catalytic solution was contained in a glass vessel. The inside part of the cover was made from Teflon® to protect the solution from direct contact with the stainless steel. These experiments were not performed at constant pressure, but for the amount of substrate used the drop of pressure was never more than 3 bar. Standard experiment A solution of the substrate (5 mmol) (previously stirred with alumina for 24 h), the catalyst (0.05 mmol), and the diphosphine were placed in the evacuated autoclave. The gas mixture (CO–H2) was introduced, the system was heated, and the stirring initiated when the thermal equilibrium was reached. Conversion and regioselectivities were determined by GC analysis of the crude samples. Determination of the enantiomeric excess The reaction mixture was treated to obtain the corresponding carboxylic acids as has been previously reported (21). The crude acid was dissolved in ether and treated with diazomethane in an Aldrich MNNG diazomethane generation apparatus (38) to afford the methyl ester. The enantiomeric excess was determined by 1H NMR spectroscopy using Eu(hfc)3 as the chiral shift reagent. The absolute configuration of the enantiomers was previously determined by comparing the optical rotation of the corresponding alcohol by reduction of the methyl ester (21). The in situ HP-NMR experiments In a typical experiment, the in situ HP-NMR experiments were carried out in a sapphire tube (φ = 10 mm). The rhodium complex [Rh(µ-OMe)(COD)]2 (0.04 mmol), dppb (0.08 mmol), and substrate (0.05 mmol) were dissolved in toluene-d8 (2 mL) under nitrogen, and the sapphire tube was © 2001 NRC Canada



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closed. After pressurizing the mixture with H2–CO, the tube was placed in the NMR spectrometer and the spectra were recorded.

Results and discussion Catalysis experiments The catalytic species were prepared in situ by mixing [Rh(µ-OMe)(COD)]2 with the corresponding diphosphine. After a screening with different diphosphines, the diphosphine dppb (bis(diphenylphosphino)butane) was chosen to study the effect of the parameters and the reaction conditions on the hydroformylation of 2,5-dihydrofuran with Rh–diphosphine systems. Preliminary experiments were undertaken to establish how the P-P:[Rh] molar ratio affected the hydroformylation of 2,5-dihydrofuran with dppb (Table 1). It is important to note that when the rhodium precursor was used without the phosphorus ligand, conversion into aldehyde was high but the reaction was not regioselective (Table 1, entry 1). A P-P:[Rh] molar ratio of 1 was required to obtain high conversion and regioselectivity in the tetrahydrofuran-3-aldehyde (Table 1, entry 3). When the PP:[Rh] ratio was increased the regioselectivity increased to 100% but the conversion decreased (Table1, entries 4, 5). This effect contrasts with previous results in the hydroformylation styrene where an excess of phosphine is required to allow the active species to be formed (39, 40). The fact that the regioselectivity increased as the P-P:[Rh] molar ratio increased can be explained in terms of ligand dissociation phenomena (Scheme 1). Since an excess of ligand shifts the equilibrium toward coordinatively saturated species, the β-elimination processes are not favoured. The effects of pressure and temperature were examined using dppb as the ligand (Table 2). At 30 bar, the conversion decreased when the temperature decreased. However, the regioselectivity in tetrahydrofuran-3-carbaldehyde increased (Table 2, entries 6–9). To obtain total regioselectivity in the 3-formyl derivative of the tetrahydrofuran, the temperature had to be 40°C but conversion was only 51% (Table 2, entry 9). There was a slightly increase in conversion when the total pressure of the system was increased (Table 2, entries 9, 10). Finally, conversion and of regioselectivity were 96 and 99%, respectively at 34 bar of CO–H2 pressure, 50°C of temperature, and substrate:Rh = 100. The substrate:Rh ratio was increased to 200 but the conversion fell to 84% and the regioselectivity to 90:10. The effect of pressure and temperature on the hydroformylation of this substrate can be explained in terms of the proposed mechanism for the hydroformylation of 2,5dihydrofuran (Scheme 1). When the temperature was decreased, selectivity for the formation of the tetrahydrofuran3-aldehyde was higher (Table 2, entries 6–9). Thus, in the range of temperatures explored it seems that only the β-elimination is significantly affected. Therefore by increasing the temperature, a fast preequilibrium is established between the metal alkyl species, via the β-elimination process, and the regioselectivity towards the tetrahydrofuran-3-aldehyde decreases. At constant temperature (Table 2, entries 9, 10) the total pressure increased from 30 to 60 bar, and the rate of formation of the rhodium acyl species increased because of

Can. J. Chem. Vol. 79, 2001 Table 1. Influence of the P-P:Rh molar ratio on conversion and regioselectivity in the hydroformylation of 2,5-dihydrofuran with dppb.a Run

P-P:Rh

P (bar)

Conv. (%)

3-Ald. (%)b

1 2 3 4 5

— 0.5 1 1.5 2.5

60 60 60 60 80

96 84 97 37 5

51 71 90 100 100

a Conditions: 0.05 mmol [Rh(µ-OMe)(COD)]2, 5 mmol 2,5-dihydrofuran, 15 mL of dichloroethane, T = 80°C, t = 20 h. b % Tetrahydrofuran-3-carbaldehyde.

Table 2. Effect of the pressure and temperature on the hydroformylation of 2,5-dihydrofuran with the [Rh]–dppb system.a Run

P (bar)

T (°C)

Conv. (%)

3-Ald. (%)b

6 7 8 9 10 11

30 30 30 30 60 34

80 60 50 40 40 50

97 97 90 51 59 96

84 94 99 100 100 99

a Conditions: 0.05 mmol [Rh(µ-OMe)(COD)]2, 5 mmol 2,5-dihydrofuran, in 15 mL of dichloroethane, t = 20 h, P-P:Rh = 1. b % Tetrahydrofuran-3-carbaldehyde.

Table 3. Effect of the bite angle on the hydroformylation of 2,5dihydrofuran with diphosphines.a Run

Phosphine

Bite angleb (°)

12 13 14 15 16

dppe dppp dppb dppf Xantphos

78 86 98 99 112

Conv. (%) (6 h)

Conv. (%) (20 h)

3-Ald. (%)c

26 22 42 56 79

71 79 96 96 97

97 99 99 99 96

a Conditions: 0.05 mmol [Rh(µ-OMe)(COD)]2, 5 mmol 2,5-dihydrofuran, in 15 mL of dichloroethane, P = 34 bar, T = 50°C, P-P:Rh = 1. b Calculated by molecular modelling (46). c % Tetrahydrofuran-3-carbaldehyde.

the higher partial pressure of CO. In this situation, CO insertion is much faster than β-elimination. In our case, the temperature was relatively low (40°C) and the β-elimination was not predominant so the regioselectivity was not affected, and there was only a slight increase in conversion into aldehydes. Once the conditions allowing high conversion and regioselectivity had been selected, the effect of the bite angle of different diphosphines was studied (Table 3). Diphosphines with bite angles smaller than 90° such as dppe and dppp (Table 3, entries 12, 13) provided low conversions to aldehyde. Systems containing diphosphines with a wider bite angle such as dppb, 1,1′-bis(diphenylphosphino)ferrocene (dppf), and 9,9-dimethyl-4,6-bis(diphenylphosphino)xanthene (Xantphos) provided high conversions (Table 3, entries 14–16). After 6 h of reaction activity was seen to increase with the bite angle (Table 3). © 2001 NRC Canada



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Scheme 2.

Scheme. 3

O Ph2P

PPh2 PPh2

PPh2

(S-S)-CHIRAPHOS

H

H PPh2 PPh2

O

P Rh

N(CH3)2 PPh2 PPh2

(–)-(R)-BINAP

CH H 3 PPh2

Fe

P P Rh P P

CO O P P Rh Rh P P CO O

Conv. (%)

3-Ald. (%)

17 18 19 20 21

CHIRAPHOS BDPP DIOP BINAP BPPFA

98 91 96 100 100

98 98 94 88 99

X

4

Table 5. 31P and 1H NMR data of the species formed in the reaction of [Rh(µ-OMe)(COD)]2 with dppb under hydroformylation conditions.a

Table 4. Hydroformylation of 2,5-dihydrofuran with chiral diphosphines.a b

P

2

3

(–)-(R)-BPPFA

Phosphine

P

P

1

PPh2

Run

P Rh

P

H (S-S)-DIOP

(S-S)-BDPP

H

CO CO

31

P NMR

ee (%)

Complex

δ

1

3 14 3 5 4

1 2 3 4

26.7 19.6 16.7 13.7

116 147 131 155

R S R R S

a

Conditions: 0.05 mmol [Rh(µ-OMe)(COD)]2, 5 mmol 2,5-dihydrofuran, in 15 mL of dichloroethane, P = 34 bar, T = 50°C, t = 20 h, P-P:Rh = 1. b % Tetrahydrofuran-3-carbaldehyde.

Xantphos-type diphosphines have been shown to give excellent regioselectivities in the rhodium-catalyzed hydroformylation of 1-octene (23). In the hydroformylation of 2,5-dihydrofuran under the conditions chosen, it seems that the bite angle has no influence on the regioselectivity (Table 3) as for all ligands tested the selectivity is high. Within the Xantphos series of ligands one also observes an increase of the rate at wider bite angles, which is ascribed to the stabilization of the unsaturated species obtained after CO dissociation (25). We tested several chiral diphosphines in the asymmetric hydroformylation of 2,5-dihydrofuran (Scheme 2). Conversions into aldehydes were higher than 90% with regioselectivities as high as 99% in the tetrahydrofuran-3aldehyde. However, only in the case of BDPP the enantiomeric excess was 14% (Table 4). It has been previously found that [Rh]–BDPP systems provide enantiomeric excesses that are higher than other rhodium diphosphine systems in the hydroformylation of styrene (39, 40). Rhodium systems with DIOP and related ligands have previously been used in the hydroformylation of 2,5dihydrofuran at higher pressures and temperatures. The enantiomeric excesses observed were in the same range (3 to 4%) (19, 20). It has been reported that an excess of chiral ligand may increase the enantiomeric excess (33–37). However, when the same experiments were performed with P-P:Rh = 1.25 conversions into aldehydes were very low. It seems that an excess of ligand quenches the system forming species that are not active in the hydroformylation reaction.

JRh-P (Hz)

1H NMR δ

1

–8.8 –9.3 — —

11 — — —

JH-Rh (Hz)

2

JP-H (Hz)

49 — — —

a Conditions: 0.04 mmol [Rh(µ-OMe)(COD)]2, in 2 mL of toluene-d8, P = 30 bar CO–H2, T = 25°C, dppb:Rh = 1.

High-pressure NMR study HP-NMR experiments were performed with dppb under hydroformylation conditions with and without 2,5dihydrofuran to observe the species present during the hydroformylation reaction. The HP-NMR spectroscopic study was performed at 30 bar H2–CO, room temperature, and a dppb:Rh ratio of 1 in toluene-d8, which has been found to provide the same catalytic behaviour as dichloroethane. The 31P NMR spectrum of the solution has four doublets at δ 26.7 (1JRh-P = 116 Hz), 19.6 (1JRh-P = 147 Hz), 16.7 (1JRh-P = 131 Hz), and 13.7 (1JRh-P = 155 Hz) (Table 5). The 1H NMR of this solution revealed a double triplet in the hydride region at –8.8 ppm (1JRh-H = 11 Hz, 2JP-H = 49 Hz), which is indicative of the coupling of a hydride with the rhodium and with two equivalent phosphorus nuclei. The 1H NMR spectrum also shows a broad signal at –9.3 ppm (Table 5). We assigned the doublet at 26.7 ppm and the double triplet in the hydride region to the mononuclear rhodium complex [HRh(dppb)(CO)2] (1) (Scheme 3) in accordance with data for BDPP from the literature (32). We suggest that this species has trigonal–bipyramidal geometry with the dppb coordinated to the rhodium centre. In the present case, the intermediate phosphorus-hydride coupling constant (2JP-H = 49 Hz) and the averaged coupling constant (1JRh-P = 115 Hz) suggest that there is an apical–equatorial coordination of the diphosphine with an exchange of the phosphorus atoms (32, 34, 41, 42), although small amounts of the equatorial–equatorial isomer cannot be discounted. The doublet at 19.6 ppm with 1JRh-P = 147 Hz and the broad hydride observed in the 1H NMR were attributed to the mononuclear rhodium complex 2, which has four equivalent phosphorus © 2001 NRC Canada



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Fig. 1. 31P NMR spectrum during hydroformylation of 2,5dihydrofuran at room temperature. (a) Hydride region of the 1H NMR.

Fig. 2. 31P NMR spectrum during hydroformylation of 2,5dihydrofuran at 80°C.

1

1

(a)

1

2 P=O

2

dppb

3

dppb

3

atoms with an average coupling constant (43). The doublet at 16.7 ppm is attributed to a square-planar rhodium complex 3 with two dppb coordinating to the metal (Scheme 3) (44). Finally, the doublet at 13.7 ppm had previously been observed by James et al. (45) and assigned to the carbonyl diphosphine dimer [Rh(CO)2dppb]2 (4) which has a Rh—Rh bond and two CO groups bridging the metal atoms. When the temperature was increased to 50 and 90°C, the 31 P NMR spectrum showed a major signal at 26.7 ppm, which was attributed to complex 1. Two minor doublets were observed at 19.6 and 16.7 ppm and assigned to compounds 2 and 3. The hydrides of complexes 1 and 2 were also observed in the 1H NMR. No formation of the dinuclear rhodium complex 4 was observed in the 31P NMR spectrum. The HP-NMR tube was then depressurized at room temperature and the 2,5-dihydrofuran was added (0.05 mL) under nitrogen. The HP-NMR spectrum at 30 bar CO–H2 and room temperature remained unchanged (Fig. 1). Three doublets at δ 26.7, 19.6, and 16.7 ppm were assigned to complexes 1, 2, and 3, respectively. The resonance of the free dppb was observed at –17 ppm and three singlets were attributed to oxidized phosphine at δ 29 ppm. In the 1H NMR spectra the double triplet was assigned to complex 1 (Fig. 1). The temperature was increased to 80°C to reach hydroformylation conditions, and the formation of the aldehydes was observed in the 1H NMR during the hydroformylation reaction. Simultaneously the species observed in the 31P NMR were the ones that had previously been observed without the addition of substrate at these temperatures (Fig. 2). Complex 1 was the major compound observed and the other two doublets were assigned to complexes 2 and 3. Since complexes 2 and 3 are not active under hydroformylation conditions, complex 1 can be considered as one of the species present in the catalytic cycle of the hydroformylation reaction. It is well known that diphosphines containing at least one alkyl substituent are prone to oxidation. The three signals for oxidized diphosphine observed at room temperature (Fig. 1) indicate the possible presence of rhodium species with partially oxidized monocoordinated diphosphine. This oxidation

can be due to the difficulty to avoid the presence of oxygen in these HP-NMR experiments. At higher P-P:Rh ratios, conversion into aldehydes decreases drastically. In the case of dppb, this can be because in the presence of high P-P:[Rh] ratios inactive species 2 and 3 can be easily formed, as was observed in the HP-NMR experiments.

Conclusions The effect of pressure and temperature was explained in terms of the described isomerization process via β-elimination during the hydroformylation of 2,5-dihydrofuran. The rhodium–diphosphine systems provide high activity by using diphosphines with wider bite angles. The regioselectivity depends on the pressure and temperature conditions. High pressures and low temperatures favour the selective formation of the tetrahydrofuran-3-carbaldehyde. HP-MR experiments performed with and without the substrate during the hydroformylation of 2,5-dihydrofuran show that the rhodium hydride [HRh(dppb)(CO)2] (1) is the predominant species during the hydroformylation. The addition of an excess of ligand decreased conversion because inactive species 2 and 3 were formed with two coordinated diphosphines in the case of dppb.

Acknowledgements This work was supported by Spanish Ministerio de Educación y Ciencia (DGES PB-97-0407-C05-01) and the Generalitat de Catalunya (CIRIT) with a research grant to I. del Río.

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