A new approach to dendritic supported NIXANTPHOS-based hydroformylation catalysts
Graphical abstract
Introduction
Over the last few years catalysis saw various efforts to support metal complexes on soluble high-loading polymers and dendrimers as potential alternatives to insoluble solid-phase supports. This is because heterogenization of soluble metal complexes on solid supports often leads to mechanically sensitive systems with lower activity of the catalyst, which is less likely if the catalyst is supported on a soluble polymer and homogeneous conditions are maintained [1]. Soluble polymers may be easily separated from the reaction mixture via precipitation or membrane filtration [2]. Occasionally these systems are able to perform reactions in membrane flow reactors. The concept of soluble polymer support has been successfully employed in a wide variety of catalytic reactions [3].
For many years several groups have been investigating new applications of the hydroformylation reaction. This most important homogeneously catalysed reaction produces a mixture of linear and branched aldehydes from olefins, carbon monoxide, and hydrogen [4]. In order to achieve high selectivity towards the formation of one product, appropriate ligands have to be employed.
The phenoxazine-based ligand NIXANTPHOS (1) evolved from van Leeuwen's initial studies of xanthene-based diphosphine ligands [5] and proved to be a superior ligand with regard to its selectivity towards the linear aldehydes. Moreover, 1 has recently been successfully immobilized on silica [6] (2) and polystyrene (3) (Fig. 1) [7], providing, after metal complexation, a catalyst suitable for recycling by simple filtration.
On the other hand, soluble polymers with special ligands for asymmetric induction [8], water [9], and fluorous [10] solubility have been used in hydroformylation. Similarly catalysts on hyperbranched polyelectrolytes are under investigation [11]. Inspired by these achievements, we envisaged employing a soluble polymer support instead of a silica or polystyrene support in order to obtain a homogenous, highly active and selective hydroformylation catalyst that is recyclable via membrane filtration.
As a polymer support, dendritic polyglycerol was chosen (Fig. 2). This material is easily obtained from a controlled polymerization of glycidol [12]. In general, hyperbranched polymers, e.g., polyglycerol are excellent platforms for catalysts due to their easy accessibility and high degree of flexible functionality for the attachment of ligands [13].
Section snippets
Results and discussion
First, we wanted to evaluate different attachment routes by connecting the hydroxy groups of polyglycerol with the nitrogen anchor in the ligand. As expected, the phenoxazine nitrogen in 1 was found to be quite unreactive. Acylation of the nitrogen function with succinic anhydride failed despite the fact that many reaction conditions were tested. Alkylation of 1 with ethyl γ-bromobutyrate also failed to give the desired product.
With methyl acrylate and acrylonitrile, however, 1 reacts to give
General remarks
Hyperbranched polyglycerol was prepared according to a published procedure [12] with a molecular weight of 8000 g mol−1 and analyzed by NMR and GPC. Dialysis (benzoylated cellulose tubing, Sigma, MWCO 1000) was performed in 1 L beaker charged with chloroform and stored over 24 h, and after one day solvent was exchanged.
Polystyrene supported NIXANTPHOS (3) [7]
Dry toluene (15 mL) was added to a mixture of polystyrene isocyanate resin (1.412 g, 2.4 mmol, novabiochem: 200–400 mesh, 1.7 mmol g−1), and 4,6-bis(diphenylphosphino)phenoxazine 1 (0.653 g, 1.185 mmol). The suspension was stirred and heated at 115 °C overnight under Ar. To the resulting mixture was added anhydrous dipropylamine (0.5 mL, 3.56 mmol) in order to neutralize the unreacted isocyanate groups. The reaction mixture was stirred at room temperature for 1 h under Ar. After filtration, the collected
Methyl 3-[10-(4,6-bis(diphenylphosphino))phenoxazinyl]propionate (4)
To a stirred suspension of 2.5 g of 1 (4.54 mmol) and 8 mL of methyl acrylate (97 mmol, 21 equiv.) a solution of 0.2 g of NBu4Br (0.62 mmol) and 40 mg of NaOMe (0.7 mmol, 16%) in 2 mL of methanol was added though a syringe. The mixture was heated and stirred under reflux for 2 h. After that TLC showed very little substrate left (cyclohexane–acetone (4:1)). Fifty millilitres of water was added and the mixture was extracted with 3 × 20 mL of DCM. The combined organic phases were dried over MgSO4, filtered,
3-[10-(4,6-bis(Diphenylphosphino))phenoxazinyl] propionitrile (5)
Prepared analogously as 5, yield 77% of pale yellow seeds, m.p. 219–220 °C.
1H NMR (500 MHz, C6D6), δ: 1.60 (t, 2H, J = 7.8), 3.02 (t, 2H, J = 7.8), 5.78 (d, 2H, J = 7.5), 6.42 (d, 2H, J = 7.5), 6.48 (t, 2H, J = 7.5), 7.1 (bs, 12H), 7.5 (bs, 8H); 13C NMR (125 MHz, C6D6), δ: 14.3 (CH2), 41.4 (CH2), 113.0 (CH), 118.3 (C), 125.4 (CH), 127.8 (CH), 129.9 (m,CH), 133.1 (C), 135.7 (m, CH), 138.7 (C), 138.8 (C); 31P NMR (81 MHz, C6D6), δ: −17.7; IR (KBr plate): ν (cm−1) 696 (vs), 744 (s), 1227 (s), 1279 (s), 1381
4,6-bis(Diphenylphosphino)-10-tertbutyldimethylsilyl-phenoxazine (6)
At 0 °C, 16 mL of n-butyllithium (2.5 M in hexanes, 39 mmol) was added dropwise to a stirred solution of 4.84 g of 10-(tertbutyldimethylsilyl)phenoxazine (16.3 mmol) and 5.9 mL of TMEDA (39 mmol) in 250 mL of diethyl ether. The reaction mixture was slowly warmed to room temperature and stirred for 16 h. The maize yellow suspension was then cooled to 0 °C and a solution of 7.0 mL of chlorodiphenylphosphine (39 mmol) in 25 mL of hexanes was added dropwise. The reaction mixture decolorized and a light brown
Z-4,6-bis(Diphenylphosphino)-10-propenyl-phenoxazine (7)
A solution of 50 mg of 1 (0.09 mmol) and 5 mg of NaH (60% oil suspension, 10 equiv.) dissolved in 10 mL of DMF was heated to 70 °C for 1 h and then 7 mg (0.09 mmol) of allyl chloride dissolved in 1 mL of DMF was added though a syringe and the mixture was heated for an additional hour. 50 mL of water was added, the resulting mixture was extracted with 3 × 20 mL of ethyl acetate. Combined organic phases were dried on MgSO4 and the solvent was removed. The residue was purified by chromatography
Ethyl 6-[(4,6-bis(diphenylphosphino)phenoxazine-10-carbonyl)-amino]-hexanoate (8)
2.3 g of 1 (4.17 mmol) and ethyl 6-isocyanatohexanoate (6.26 mmol, 1.16 g, 1.12 mL, 1.5 equiv.) were mixed in toluene and heated under reflux for 5 days. The solvent was evaporated and the residue was purified by chromatography (cyclohexane–ethyl acetate (5:1)). Yield 1.4 g of substrate (61%) and 0.92 g (30%) of expected product, oil, after standing solidifies to give pale yellow microcrystalls, m.p. 75–76 °C.
1H NMR (500 MHz, C6D6), δ: 1.06 (t, 3H, J = 7.0), 1.08 (m, 4H), 1.49 (tt, 2H, J = 7.5, J = 7.0), 2.12
4,6-bis-Diphenylphosphanyl-phenoxazine-10-carboxylic acid allylamide (9)
0.5 g of 1 (0.906 mmol) and allylisocyanate (0.5 g, 6 mmol) were mixed in toluene and heated under reflux for 1 day under Ar. Next the reaction mixture was stirred at room temperature for 6 h, the solvent was evaporated and the residue was purified by chromatography (from cyclohexane–dichlormethane (1:1) to dichlormethane) followed by crystallization from DCM\ethanol. Yield 0.472 g (82%) of colorless crystals, m.p. 178–179 °C.
1H NMR (500 MHz, CDCl3), δ: 3.92 (t, 2H, J = 5.5), 5.15 (d v. d, 2H, J = 25.2 and
bis-4,6-bis-Diphenylphosphanyl-10H-phenoxazine-methane (10)
A solution of 551 mg of 1 (1 mmol) and 200 mg of NaH (60% oil suspension, 10 equiv.) dissolved in 30 mL of abs. DMF was heated at 70 °C for 1 h, then 150 mg (1.1 mmol) of 4-bromo-1-butene dissolved in 1 mL of DMF was added though a syringe and the mixture was heated for 15 h. Thirty millilitre of water and 40 mL of ethyl acetate was added and the resulting mixture was extracted. The organic phase was evaporated up to 10 mL and methanol was added. The formed crystals was filtered, evaporated, and dried with
10-But-3-enyl-4,6-bis-diphenylphosphanyl-10H-phenoxazine (11)
1H NMR (500 MHz, CDCl3), δ: 2.45 (q, 2H, J = 7.2), 3.59 (t, 2H, J = 7.7), 5.19 (d v. d, 2H, J = 10.2 and 17.2), 5.89 (m, 1H), 6.02 (d, 2H, J = 6.7), 6.48 (d, 2H, J = 7.7), 6.68 (t, 2H, J = 7.7), 7.15–7.31 (bs, 20H); 13C NMR (125 MHz, CDCl3), δ: 29.1 (CH2), 44.0 (CH2), 111.8 (CH), 117.3 (CH2), 123.7 (CH), 124.7 and 124.9 (C), 125.3 (CH), 128.1 (m, CH), 132.9 (C), 133.8 (CH), 133.9 (m, CH), 136.9 (C), 147.1 (C); 31P NMR (81 MHz, CDCl3), δ: −18.0; IR (KBr plate): ν (cm−1) 694 (s), 746 (s), 918 (w), 1232 (m),
Acryloyl modified polyglycerol
To an ice-cooled mixture of 0.4 g of polyglycerol (M = 8000, 5 mmol of free OH groups) and 0.9 mL of triethylamine (6.5 mmol) dissolved in 15 mL of DMF, 0.4 mL of acryloyl chloride (4.9 mmol) dissolved in 2 mL of DMF was added dropwise. The mixture was stirred overnight at room temperature. The solvent was evaporated with an oil pump and the residue dissolved in CHCl3, filtered and put into a dialysis tube and kept in 1 L beaker filled with CHCl3 under stirring. The solvent was replaced after one day and
Attaching of NIXANTPHOS (1) to polyglycerol (12)
To a stirred suspension of 1.0 g of 1 (181 mmol) and 0.31 g of acrylylated polyglycerol (2.3 mmol of vinyl functions) in 3 mL of methanol, a solution of 0.2 g of NBu4Br (0.62 mmol) and 40 mg of NaOMe (0.7 mmol) in 2 mL of methanol was added through a syringe. The mixture was heated and stirred at reflux for 2 h. The solvent was evaporated, the residue was dissolved in CHCl3 and filtered, placed into a dialysis tube, and kept in a 1 L beaker filled with CHCl3 that was stirred. The solvent was replaced after
Micro-dendrimeric NIXANTPHOS trimer (13)
Six-hundred milligram of NIXANTPHOS (1.09 mmol; 3.1 equiv.) and 26 mg of sodium hydride (1.09 mmol; 3.1 equiv.) was dissolved in 2 mL of DMF and stirred for 30 min at 70 °C. The mixture was cooled down and 126 mg of 1,3,5-tris-bromomethyl-benzene [27] (0.35 mmol, 1 equiv.) was added to the solution and stirred for 16 h at 90 °C in a sealed tube. The crude product was cleaned by dialysis in chloroform and heated in ethylacetate, filtered, and solvent was removed under reduced pressure.
Yield 335 mg (54%) of
1,3,5-tris-Boc-piperazinomethyl-benzene (14)
Three gram of 1,3,5-tris-bromomethyl-benzene acid (8.4 mmol), 5.5 g of boc-piperazine (29.5 mmol, 3.5 equiv.) and 4 mL of triethylamine were stirred in 50 mL dioxane at 80 °C for 3 h. The solvent was evaporated and 30 mL of dichloromethane was added. The formed solid was removed and the solution was extracted with 2 × 20 mL of a thinned HCl (pH 5) solution. The organic phases were dried on MgSO4 and the solvent was removed. Yield 5.61 g (99%) of colorless crystals.
1H NMR (500 MHz, CDCl3), δ: 1.43 (s, 27H),
1,3,5-tris-Piperazinomethyl-benzene (15)
5.61 g of 1,3,5-tris-boc-piperazinomethyl-benzene (14) (8.34 mmol) were stirred in 30 mL dioxane and 20 mL 3 molar HCl at room temperature for 1 day. The solvent was evaporated and a sodium hydroxide solution was added. The solution was extracted with 3 × 20 mL of chloroform. Combined organic phases were dried on MgSO4 and the solvent was removed. Yielding 2.8 g of the 1,3,5-tris-piperazinomethyl-benzene as an oil (90%).
1H NMR (400 MHz, MeOD), δ: 2.29–2.61 (12H), 2.76–2.92 (12H), 3.51 (s, 6H), 7.22 (s,
Micro-dendrimeric NIXANTPHOS with a long spacer (17)
A solution of 551 mg of 1 (1 mmol) and 1.6 mL of hexamethylenediisocyanate (1.682 g, 10 mmol, 10 equiv.) dissolved in 5 mL of toluene was heated to reflux for 1 day. Next the solvent and hexamethylenediisocyanate was removed by bulb to bulb distillation. The crude product (16) was washed with dry heptane and analyzed by MS and NMR.
1H NMR (500 MHz, CDCl3), δ: 1.31–1.45 (4H), 1.52–1.65 (4H), 3.26–3.31 (4H), 5.35 (t, 3H, J = 5.5), 6.50 (d, 2H, J = 7.7), 6.98 (t, 2H, J = 7.7), 7.19–7.29 (20H), 7.49 (d, 2H, J =
Polyurea NIXANTPHOS (18)
Eighty milligram of polyglycerol (M = 8000, 1 mmol of free OH groups) dissolved in 2 mL of pyridine and 420 mg of 16 (6.5 mmol), which remain traces of hexamethylenediisocyanate, dissolved in 3 mL of dry pyridine was stirred overnight at 80 °C. The solvent was evaporated with an oil pump and the received solid was washed with dichloromethane and dried with an oil pump to leave 171 mg of 18 as a yellowish solid.
General hydroformylation procedure
Fifty milligram of olefin (A or B), 1 mol% of Rh(CO)2acac and 5 mol% of diphosphine ligand were placed in an autoclave with a magnetic stirrer and dissolved in 4 mL of CH2Cl2. The autoclave was charged with 20 bar CO–H2 (1:1) and heated at 80 °C. After 20 h the pressure was released and the crude reaction mixture was analyzed by NMR (and GC). Data of purified aldehydes:
Hydroformylation of A, linear product.
Nonanal
1H NMR (500 MHz, CDCl3), δ: 0.76–0.96 (m, 3H), 1.21–1.51 (m, 10H), 1.51–1.71 (m, 2H), 2.24–2.38 (m, 2H), 9.72 (s, 1H). 13C NMR (125 MHz, CDCl3), δ: 15.4 (CH3), 24.0 (CH2), 26.1 (CH2), 28.5 (CH2), 30.5 (CH2), 30.6 (CH2), 33.2 (CH2), 54.8 (CH2), 201.2 (CH).
Hydroformylation of A, branched product.
2-Methyl-octanal
1H NMR (500 MHz, CDCl3), δ: 0.76–0.96 (m, 3H), 1.06–1.20 (d, 3H, J = 6.7), 1.21–1.51 (m, 6H), 1.51–1.71 (m, 4H), 2.38–2.55 (m, 1H), 9.58 (d, 1H). 13C NMR (125 MHz, CDCl3), δ: 15.4 (CH3), 18.8 (CH3), 30.5 (CH2), 30.6 (CH2), 33.1 (CH2), 35.0 (CH2), 35.6 (CH2), 40.9 (CH), 201.7 (CH).
Hydroformylation of B, linear product.
4-Phthalimidylbutanal
1H NMR (200 MHz, CDCl3), δ: 1.98 (p, J = 7.1, 2H), 2.51 (dt, J = 7.1, J = 1.2, 2H), 3.70 (t, J = 7.1, 2H), 7.6–7.9 (m, 4H), 9.76 (t, J = 1.2, 1H); 13C NMR (125 MHz, CDCl3), δ: 21.1 (CH2,), 37.0 (CH2,), 41.0 (CH2), 123.2 (CH), 131.9 (C), 133.9 (CH), 168.3 (C), 200.8 (CH).
Hydroformylation of B, branched product.
2-Methyl-3-phthalimidylpropanal
1H NMR (500 MHz, CDCl3), δ: 1.16 (d, J = 7.2, 3H), 2.7–3.0 (m, 1H), 3.80 (dd, J = 14.1, J = 6.5, 1H), 4.02 (dd, J = 14.1, J = 7.2, 1H), 7.6–7.9 (m, 4H), 9.73 (d, J = 1.6, 1H).
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