Rhodium catalyzed hydroformylation of linalool

Applied Catalysis A: General 309 (2006) 169–176 www.elsevier.com/locate/apcata

Rhodium catalyzed hydroformylation of linalool Jose´ G. da Silva, Humberto J.V. Barros, Eduardo N. dos Santos, Elena V. Gusevskaya * Departamento de Quı´mica-ICEx, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, Brazil Received 28 December 2005; accepted 1 April 2006 Available online 15 June 2006

Abstract The hydroformylation of linalool using [Rh(COD)(OAc)]2 as a catalyst precursor in the presence of triphenylphosphine or various diphosphines leads mainly to a mixture of cis and trans isomers of hemiacetal, which formally arise from the intramolecular cyclization of the primarily formed hydroxyl-aldehyde. An unexpected effect of the phosphorous ligands on the reaction rate was observed. With unmodified systems, linalool shows a very low reactivity under the hydroformylation conditions, probably due to the chelation of the substrate on rhodium. The introduction of (di)phosphine and the increase in its concentration exerts a great accelerating effect so that under optimized conditions at 40–50 8C and 20 atm of CO/H2, a virtually complete conversion of linalool has been achieved in 4–6 h. A good control of chemo and stereoselectivity was attained through the appropriate choice of reaction variables. Each of two isomers of hemiacetal can be obtained in ca. 95% chemo- and 85% stereoselectivity. # 2006 Elsevier B.V. All rights reserved. Keywords: Hydroformylation; Linalool; Rhodium catalysts

1. Introduction Hydroformylation of olefins represents one of the least expensive synthetic pathways to aldehydes [1]. Diversification of substrates by using special olefins can afford aldehydes bearing additional functional groups and other oxygen containing molecules hardly accessible by conventional synthetic routes. These hydroformylation products can be used as bi- or poly-functionalized building blocks for organic syntheses thus opening new entries to many valuable compounds [2]. Hydroformylation of allylic alcohols has been widely studied as a direct method for the synthesis of substituted tetrahydrofurans and g-butyrolactones since these heterocycles are very attractive as subunits for biologically important compounds [3–10]. These reactions can afford five-membered hemiacetals arising from a spontaneous intramolecular cyclization of primarily formed hydroxyl-aldehydes (Scheme 1) [1]. Hemiacetals can be easily converted in acetals, lactones or dehydrated giving dihydrofuran derivatives. Terpenes constitute a class of natural products that can be transformed into compounds commercially important for * Corresponding author. Fax: +55 3134995700. E-mail address: [email protected] (E.V. Gusevskaya). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.04.047

industrial production of fragrances, perfumes, flavors and pharmaceuticals as well as synthetic building blocks [11–13]. For example, linalool (1), a monoterpenic allylic alcool with a pleasant lily odor, is a key intermediate for the synthesis of various vitamins and fragrance chemicals, such as citral, geraniol, citronellol [11,14]. This compound is available from essential oils of some plants and flowers, like lavender, but most linalool is produced synthetically from a-pinene, an inexpensive major constituent of turpentine oils obtained from coniferous trees [11,14,15]. Although the hydroformylation of the most abundant monoterpenes, such as limonene, b-pinene, etc., has been quite extensively investigated [4,9,16–23], only few reports have appeared in the literature on the hydroformylation of linalool [4,24,25]. The aim of the present work was a systematic study of the hydroformylation of linalool catalyzed by rhodium complexes modified by various phosphorous ligands. 2. Experimental All chemicals were purchased from Aldrich and used as received, unless otherwise indicated. A racemic linalool from Aldrich was used as a substrate. Bis[(m-acetate)(1,5-cyclooctadiene)rhodium(I)] ([Rh(COD)(OAc)]2) was prepared by a modified procedure published in [26]. Toluene was purified

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Scheme 1.

under reflux with sodium wire/benzophenone for 8 h and then distilled under argon. The products were analyzed by gas chromatography (GC) using a Shimadzu 17B instrument fitted with a DB-1 capillary column and a flame ionization detector using dodecane as an internal standard. NMR spectra were obtained using a Bruker CXP-400 spectrometer with tetramethylsilane as an internal standard in CDCl3. Mass spectra were obtained on a HewlettPackard MSD 5890/Series II instrument operating at 70 eV. In a typical run, a toluene solution containing [Rh(COD)(OAc)]2 (0.3 mM), phosphorous ligand (3–120 mM), substrate (0.30 M) and dodecane (0.15 M) was transferred from a Schlenck tube under nitrogen into a stainless steel magnetic stirred autoclave. In some runs, a solution of pyridinium ptoluenesulfonate (PPTS) in methanol (0.01 g/ml) was also added to the reaction mixture. The reactor was pressurized to 20–80 atm total pressure (CO/H2 = 4/1–1/4), placed in an oil bath and magnetically stirred. Reactions were followed by GC using a sampling system. After carrying out the reaction and cooling to room temperature, the excess CO and H2 was slowly vented. The solution was analyzed by GC and GC/MS. The products were separated by column chromatography (silica) using mixtures of hexane and CH2Cl2 as eluents and identified by GC/MS, 1H and 13C-NMR spectroscopy. 5-Methyl-5-(4-methyl-3-pentenyl)tetrahydro-2-furanol (cis, longer CG retention time) (3a) (light yellow oil): MS (m/z/ rel.int.): 184/1 (M+); 166/5 (M+ H2O); 122/50; 109/37; 107/ 72; 101/37; 95/21; 83/60; 81/21; 69/100; 67/40; 55/71. For NMR data see Table 1. 5-Methyl-5-(4-methyl-3-pentenyl)tetrahydro-2-furanol (trans) (3b) (light yellow oil): MS (m/z/rel.int.): 184/1 (M+); 166/6 (M+ H2O); 122/40; 109/30; 107/46; 97/27; 95/32; 91/ 7; 83/44; 81/32; 70/11; 69/100; 67/49; 55/41; 53/26. For NMR data see Table 1. 5-Methoxy-2-methyl-2-(4-methyl-3-pentenyl)tetrahydrofuran (4) (light yellow oil): MS (m/z/rel.int.): 198/1 (M+); 166/ 21 (M+ CH3OH); 122/57; 115/73; 109/22; 107/64; 95/21; 83/ 34; 72/21; 69/100; 67/32; 55/41. For NMR data see Table 1. 3. Results and discussion Hydroformylation of linalool can result in hydroxy aldehyde 2 [25], hemiacetal 3 [4], its methyl ether, i.e. acetal 4 [4]

(Scheme 2) or substituted dihydrofuran 5 having a structure similar to that shown in Scheme 1 [24]. Both hemiacetal 3 and acetal 4 have a green citrus scent [4]. No data on the spectroscopic characterization of these compounds have been reported so far. Hemiacetal 3 can be dehydrated resulting in corresponding dihydrofuran 5 with an interesting woody-flower (iris) tinge [24]. We studied the hydroformylation of linalool using [Rh(COD)(OAc)]2 as a catalyst precursor in the presence of triphenylphosphine or various diphosphines as P-donor auxiliary ligands. Under all conditions used, the reaction resulted in two major products formed in higher than 90% combined selectivity in most of the runs. The GC mass balance was based on the substrate charged using dodecane as an internal standard. The difference was attributed to the formation of oligomers, which were not GC determinable. Only very small amounts of unidentified products were detected by gas chromatography. 3.1. Product characterization The two major products obtained from linalool under hydroformylation conditions were isolated from the reaction mixtures and fully characterized by GC/MS and NMR. The results are presented in Section 2 and in Table 1. Mass spectra of both compounds show the same molecular ion peak with m/ z = 184 and are very similar, indicating that these compounds seem to be closely related isomers. The observed molecular weight corresponds to the products of the hydroformylation of linalool. The analysis of their NMR spectra, which are also very similar, reveals that both compounds have a structure of hemiacetal 3 rather than hydroxy aldehyde 2 (Scheme 2). Thus, it appears that the in situ intramolecular cyclization of primarily formed aldehyde 2 occurs rapidly under the conditions used, however no detectable amounts of aldehyde 2 itself is observed in the reaction solutions. The isolated compounds could be cis and trans isomers of hemiacetal 3 regarding to the relative positions of the hydroxy and methylpentenyl groups attached to the tetrahydrofuran ring. Indeed, the greatest differences in the NMR spectra of these products are observed for the signals from the carbon atoms bound to OH (C-11) and from the corresponding hydrogens (H-11) (Table 1). The stereochemistry of the isomers has been clarified by NOESY experiments. In the isomer with

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171

Table 1 NMR data for products 3a, 3b and 4 (usual numbering for myrcene derivatives is presented in Scheme 2) C atom

H atom

d(1H) (ppm)a

d(13C) (ppm)

3a (cis)

3b (trans)

4

3a (cis)

1

1

1.90–2.05 (m, 2H

1.90–2.05 (m, 2H)

1.90–1.98 (m, 2H)

2

2

1.77–1.83 (m, 1H) 1.89–1.95 (m, 1H) 1.58-1.62 (m, 1H) 1.77-1.83 (m, 1H)

1.77–1.83 1.58–1.62 1.58–1.62 1.77–1.83

1.75-1.84 (m, 1H) 1.75–1.84 (m, 1H) 1.58–1.62 (m, 1H) 1.75–1.84 (m, 1H)

(m, (m, (m, (m,

1H) 1H) 1H) 1H)

3

4

4 5

1.44–1.48 (m, 2H) 1.58–1.62 (m, 2H) 1.95–2.05 (m, 2H)

1.43–1.49 (m, 2H) 1.66–1.68 (m, 2H) 1.95–2.05 (m, 2H)

1.46–1.51 (m, 2H) 1.58–1.61 (m, 2H) 1.98–2.08 (m, 2H)

5 6

6

5.07–5.11 (m, 1H)

5.07–5.14 (m, 1H)

5.08–5.15 (m, 1H)

7

7

8b

8

1.61 (s, 3H)

9b

9

1.60 (s, 3H) 1.62 (s, 3H) 1.67 (s, 3H) 1.68 (s, 3H) 1.15 (s, 3H) 1.16 (s, 3H) 1.29b (s, 3H) 1.30b (s, 3H)

1.61 1.62 1.68 1.69 1.18 1.33

10

11

10

11

5.39–5.42 (m, 1H

1.68 (s, 3H) 1.16 (s, 3H) 1.38 (s, 3H)

(s, (s, (s, (s, (s, (s,

3H) 3H) 3H) 3H) 3H) 3H)

a b

4

32.50 33.12 34.56 34.84 34.84

33.34 33.84 34.30 34.55 34.55

32.69 33.30 34.33 34.74 34.74

84.23 84.25 84.49 84.52 41.90 42.77 23.25 23.73 124.60 124.80 131.09 131.10 131.27 131.29

84.09 84.81

84.48 84.77

41.71 43.02 23.19 23.64 124.30 124.47 131.21 131.33 131.45

41.93 42.88 23.30 23.68 124.48 124.65 131.22 131.38

17.59

17.56

25.66 25.79 26.13

25.63

17.58 17.63 25.68 25.69 26.14 28.29

25.88 28.25

28.15b 28.18b 5.50 (br.t, 1H)

4.96 (t, 1H) 3J = 4.7

5.24–5.29 (m, 1H)

12 b

3b (trans)

12

3.32 (s, 1H) 3.33 (s, 1H)

99.81 99.86 100.23 100.32

98.39 98.75 102.34 102.41 102.80 102.84

105.14 105.37

54.21 54.33

Resonance multiplicities and coupling constants (Hz): (s) singlet, t (triplet), (m) multiplet, (br) broadened. The attributions of the correlations between carbon and hydrogen atoms can be reversed.

longer CG retention time, 3a, the methyl hydrogens H-10 (singlets at ca. 1.30 ppm) give a strong NOE correlation signal with H-11 (multiplet at ca. 5.40 ppm) showing their spatial proximity. On the other hand, in isomer 3b no NOE between the corresponding hydrogens is observed. This clearly indicates a cis configuration for 3a, in which hydrogen H-11 and methyl group C-10 are at the same side of the tetrahydrofuran ring and the methylpentenyl fragment is at the other. An analysis of NMR spectra shows that both compounds 3a and 3b exist in the solutions as a mixture of different forms, which are not GC separable under the conditions used. Only one new asymmetric center (C-11) results from the cyclocarbonylation of the racemic linalool, whose molecule already has one asymmetric center (C-3). Therefore, four diastereoisomers can be produced in this reaction, two of them having OH and methylpentenyl groups in trans positions and the other two in cis. Each one from two peaks displayed by CG attributed to

3a and 3b could represent a couple of diastereoisomers arising from ( ) and (+) linalool. However, NMR spectra of 3a and 3b show a larger number of signals. Most of the carbon resonances appear as two very close signals, but some of them ‘‘split’’ into four or even six signals (Table 1). The same occurs with some hydrogen resonances: for example, the signal from the methyl hydrogens H-10 in 3a is quadruplicated. Thus, we have to assume that these signals arise not only from different diastereoisomers of hemiacetal 3 but also from different conformations of the five-membered tetrahydrofuran ring. As it can be seen from the data in Table 1, the greatest differences in the NMR spectra of different forms of both compounds 3a and 3b are observed for the signals from C/H-10 and C/H-11. For example, one couple of the forms of 3a show the signals from methyl hydrogens H-10 at 1.15 and 1.16 ppm, whereas the other couple at 1.29 and 1.30 ppm, with the intensities of the latter signals being ca. 30% lower than the

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Scheme 2.

intensities of the signals at 1.15 and 1.16 ppm. On the other hand, the signals at 1.15 and 1.16 ppm show approximately equal intensities. The signals at 1.29 and 1.30 ppm are also almost equally intensive. As we started from the racemic linalool, it is reasonable to suggest that in each couple one signal belongs to the diastereoisomer derived from (+) linalool and the other one to that derived from ( ) linalool. Their conformers show the signals from H-10 with quite different chemical shifts: at ca. 1.15 versus ca. 1.30 ppm. According to relative intensities of these signals, the conformers appear to present in the solutions in different equilibrium concentrations, which is reasonable to expect. In the spectrum of compound 3b, we have observed two signals from hydrogens H-10: one at 1.16 ppm and another one, which is ca. 30% less intensive, at 1.38 ppm. Therefore, these signals seem to belong to different conformers of 3b. There are an infinite number of puckered ring conformations along the pseudorotation route of ring molecules of tetrahydrofuran derivatives [27]. Various theoretical and NMR spectroscopy methods were developed to determine the most stable conformations of such type of compounds [28,29]. The energy required to convert from one conformer to another is frequently high enough to make different conformers distin-

Scheme 3.

guishable by NMR even at room temperature. Two opposite envelope conformations of the isomer 3a are presented in Scheme 3. A ring inversion proceeds through the planar form shown in Scheme 2. It can be seen that in structure I the methyl group C-10 is closer to the electron rich oxygen atom than in structure II. Thus, the hydrogens H-10 in the conformers with the structures close to I should be more shielded and their Table 2 Hydroformylation of linalool catalyzed by [Rh(COD)(OAc)]2/PPh3 system: effect of the P/Rh atomic ratio and temperaturea Run

P/Rh

Temperature (8C)

Time (h)

Conversion (%)

Selectivityb (%)

3a/3b

1 2 3 4 5 6

5 20 50 200 0 5

40 40 40 40 50 50

20

50

8

50

50

9

5

70

10 11 12

20 50 200

70 70 70

13 c

50

50

47 95 61 30 12 75 100 44 98 60 98 53 100 98 98 60 100 100

89 98 96 98 85 95 93 95 95 96 92 75 73 82 94 85 81 90d

1/4.0 1.3/1 1.5/1 2.3/1 1/2.2

7

21 6 6 6 6 4 6 2 4 2 4 1.5 3 1.5 1.5 1.5 3 3

1/1.3 1.5/1 2.3/1 1/1.5 1.8/1 2.5/1 1.9/1

a Reaction conditions: linalool (0.30 M), [Rh(COD)(OAc)]2 (0.3 mM), 20 atm (CO/H2 = 1/1); conversion and selectivity were determined by GC. b Selectivity for hydroformylation products 3a and 3b. c Pyridinium p-toluenesulfonate (2.0 mM) and methanol (5.0 vol.%) were added. d Selectivity for acetal 4.

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173

signals can be shifted upfield from the corresponding signals of the conformers with the structures close to II. This consideration could explain the differences between the chemical shifts of hydrogens H-10 in the conformers of hemiacetals 3a and 3b. The analysis of the molecular model structures shows that conformation II is favorable for the spatial approximation of the CH3 group and H-11. On the other hand, conformation I is less favorable for this. Taking into account these considerations, it becomes clear why in cis isomer 3a it is observed the NOE correlation between H-11 and H-10 at 1.29–1.30 ppm (belonging to the conformers with the structure close to I) but not between H-11 and H-10 at 1.15–1.16 ppm (belonging to the conformers with the structure close to II). 3.2. Linalool hydroformylation with Rh/PPh3 systems: effects of the temperature and ligand concentration

Fig. 1. Hydroformylation of linalool catalyzed by [Rh(COD)(OAc)]2 at different P/Rh ratios. Conditions: [linalool] = 0.30 M, [Rh(COD)(OAc)]2 = 0.3 mM, 20 atm (CO/H2 = 1/1); 40 8C (Table 2, runs 1–4).

We studied the hydroformylation of linalool with the [Rh(COD)(OAc)]2/PPh3 catalytic system at different P/Rh ratios and different temperatures. The results are presented in Table 2. Since no significant changes in relative amounts of cis and trans isomers of hemiacetal 3 are observed during the reaction, only one value of the 3a/3b ratio is given for each run. At P/Rh = 5 and 40 8C (run 1), 80% of hemiacetal 3 have a trans configuration (3a/3b = 1/4). However, the reaction is very slow and only 47% of linalool is converted in 21 h. Under optimized conditions at 40–50 8C, a virtually complete conversion of linalool can be achieved in 4–6 h with an excellent combined selectivity (95–98%) for the hydroformylation products 3a and 3b, which are formed in comparable amounts (runs 2 and 7). At 70 8C, the selectivity is lower and at the end of the reaction in most of the runs ca. 20% of linalool is converted in not GC determinable oligomers. In most of the runs, cis isomer 3a predominates (60–70% of 3a + 3b), with its relative amounts being affected to a certain extent by the reaction conditions. The variations in the 3a/3b ratio show the following trends: the higher the temperature and the P/Rh ratio, the greater relative amounts of cis isomer 3a are formed as a result of the cyclocarbonylation of linalool. The fact that the isomer distribution depends on temperature is quite reasonable. Cis isomer 3a is sterically more hindered and seems to be thermodynamically less stable. On the other hand, the effect of the P/Rh ratio observed at any temperature studied is unexpected under the assumption that the cyclization of the primarily formed hydroxyl-aldehyde 2 occurs consecutively after the hydroformylation step with no participation of the rhodium catalyst (Scheme 2). It is remarkable the effect of PPh3 on the activity of the catalytic system. In the absence of PPh3, only a 12% conversion is observed at 50 8C for 6 h, while at P/Rh = 5 the reaction is virtually completed in the same time (cf. runs 5 and 6, Table 2). It is known that in most phosphine modified rhodium systems for hydroformylation, an inverse reaction rate dependence on the phosphine concentration is observed due to the competition between the phosphine and the substrate for the coordination sites on rhodium [1]. However, in the hydroformylation of linalool we have observed that the introduction of PPh3 and the

increase in its concentration to P/Rh = 20 strongly accelerate the reaction at all temperatures studied. For example, in 6-h reactions at 40 8C, only 16% of linalool is converted at P/Rh = 5 but the conversion is as much as 95% at P/Rh = 20 (Table 2, run 1 versus run 2; Fig. 1). Only at a great excess of PPh3 the reaction becomes slower (at P/Rh = 50–200 depending on reaction temperature). As an example, the kinetic curves for the reactions at 40 8C at different PPh3 concentrations are shown in Fig. 1. Most of them show an appreciable induction period, whose length decreases with rising the P/Rh ratio: ca. 3 h at P/Rh = 5; 1 h at P/Rh = 20 and 0.5 h at P/Rh = 50. At P/Rh = 200, no induction period was observed, although the reaction became quite slow. The following values for the reaction rates during the stationary period were obtained: 50 h 1 at P/Rh = 5; 190 h 1 at P/Rh = 20; 90 h 1 at P/Rh = 50 and 40 h 1 at P/Rh = 200. We tried to explain these unusual effects by the structure of the molecule of linalool. The proximity of the OH group to the olefin functionality opens the possibility of its coordination to rhodium in a p-olefin complex to form a chelate as a reaction intermediate. We suppose that in a trigonal-bipyramidal intermediate RhH(PPh3)(CO)(linalool), the diequatorial chelation will keep the substrate in an equatorial plane. This disfavors the rotation of the olefinic bond from in-plane coordination to a perpendicular coordination mode, which is necessary for the migration of the apical hydride ligand and the insertion of the olefin into a rhodium-hydride bond [1]. The introduction of PPh3 and the increase in its concentration favor the cleavage of the chelating Rh–O bonds giving active bisligand species RhH(PPh3)2(CO)(linalool), in which the insertion step can occur easier to give rhodium alkyl intermediates. Thus, the acceleration of the reaction takes place with the increase in the P/Rh ratio. On the other hand, too high PPh3 concentrations result in the formation of trisligand RhH(PPh3)3(CO) complexes, which are catalytically inactive. At the beginning of the reaction, the formation of catalytically active complexes from the chelates seems to cause the induction period. At high PPh3 concentrations, the chelates, which demand two coordination sites, are rapidly opened by phosphine coordination as no induction period is

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Table 3 Hydroformylation of linalool catalyzed by [Rh(COD)(OAc)]2/PPh3 system: effect of pressurea Run

Pressure (atm)

H2/CO

Time (h)

Conversion (%)

Selectivityb (%)

1

20

1/1

2

50

1/1

3

80

1/1

4

50

1/4

5

50

4/1

2 4 2 4 2 4 2 8 2 3

60 98 60 100 64 95 21 95 91 98

96 92 97 95 98 97 98 96 98 97

3a/3b

2.3/1 1.6.5 1/6.8 5.3/1 1/6.0

a

Reaction conditions: linalool (0.30 M), [Rh(COD)(OAc)]2 (0.3 mM), P/ Rh = 50, 50 8C; conversion and selectivity were determined by GC. b Selectivity for hydroformylation products 3a and 3b.

observed at P/Rh = 200. The fact that the reaction is very slow when no PPh3 is added, i.e. inactive rhodium-linalool complexes are mostly formed in the absence of phosphorous ligand, additionally argues for the chelation hypothesis. The formation of related chelates by the hydroxyl coordination to rhodium in rhodium alkyl intermediates has been previously suggested to explain the results obtained at the hydroformylation of other allylic and homoallylic alcohols [10]. 3.3. Linalool hydroformylation with Rh/PPh3 systems: effect of pressure We also studied the effects of the total and partial pressure of both CO and H2 on the hydroformylation of linalool (Table 3). In general, under common hydroformylation conditions (10– 30 atm, 70–120 8C), the reaction is zero order in hydrogen and

a negative order in CO concentration, while at low pressure and low temperatures a positive order in the hydrogen pressure can be observed [1]. Indeed, under the conditions we used, the decrease in the pressure of CO increases the reaction rate (Table 3, cf. runs 1 and 4). On the other hand, the increase in the total pressure of the equimolar gas mixture from 20 to 80 atm leads to no significant changes in the linalool conversion (Table 3, runs 1–3). This should reflect a net nullified result of the opposite kinetic effects of the CO and hydrogen concentrations at 50 8C. Really, the increase in the pressure of H2 accelerates the reaction (Table 3, run 1 versus run 5). The positive order in hydrogen observed at relatively high pressures used in this work suggests that, in the case of linalool, the oxidative addition of the hydrogen to rhodium acyl intermediate seems to be the most likely rate-determining step. We observed the remarkable effect of the gas pressure on the isomeric 3a/3b distribution. At 20 atm of the CO/H2 = 1/1 mixture, hydroformylation of linalool gave ca. 70% of 3a and 30% of 3b (run 1, Table 3). On increasing the total pressure, the stereoselectivity of the cyclization was switched to trans isomer 3b (Table 3, runs 2 and 3 versus run 1). At 50 and 80 atm, near complete conversions were achieved in 4 h, with 3b being formed in ca. 85% stereo and 95% chemoselectivities. On the other hand, the increase in the pressure of only CO at constant pressure of H2 enhanced the preference for cis isomer 3a to 85% (3a/3b = 5.3/1, Table 3, run 4). Thus, the appropriate choice of reaction variables allows controlling the stereoselectivity of the reaction. It is expected that hemiacetal 3 is formed as a result of the intramolecular rearrangment of aldehyde 2 via a nucleophilic attack of the hydroxylic oxygen on the carbon atom of the carbonyl group followed by the hydroxylic hydrogen transfer to the oxygen of the carbonyl group. This spontaneous cyclization should favor the formation of the thermodynamically more stable

Scheme 4.

J.G. da Silva et al. / Applied Catalysis A: General 309 (2006) 169–176

trans isomer 3b. To explain the effects of the concentration of ligands (PPh3 and CO) as well as the hydrogen pressure on the stereoselectivity of hemiacetalization we assume that similar interaction between the hydroxyl and carbonyl groups could also occur in a rhodium acyl intermediate A before its hydrogenolysis (Scheme 4). In this way, a cyclic organometallic intermediate B, which contains a tetrahydrofuran ring, could be formed. Further oxidative addition of H2 and reductive elimination of hemiacetal 3 complete the catalytic cycle. In this case, the nature of the ligands on rhodium will affect the stereochemistry of the product. Cis isomer 3a results from a less hindered isomer of intermediate B, in which the bulky rhodium and methylpentenyl fragments are at the opposite sides of the tetrahydrofuran ring. Therefore, the addition of bulky PPh3 (or diphosphine) and the increase in the P/Rh ratio should favor the formation of 3a and this really occurs. Stereoselectivity should also be dependent on the relative reactivity of rhodium acyl intermediate A towards cyclization versus hydrogenolysis via the oxidative addition of H2. Higher hydrogen pressures increase the rate of the hydrogenolysis but not the cyclization, thus favoring the formation of trans isomer 3b. We run the experiment increasing only the H2 pressure maintaining the pressure of CO at 10 atm (Table 3, run 5 versus run 1). The inversion of the stereoselectivity from cis to trans isomer (3a/3b = 1/6), similar to what has been observed at the increase in the H2/CO total pressure, additionally supports the hypothesis depicted in Scheme 4. On the other hand, the increase in only the CO pressure decelerates the reaction and favors the formation of 3a (Table 3, cf. runs 1 and 4). These observations can also be explained within the Scheme 4: at higher CO pressures the oxidative addition of H2, demanding two coordination sites on rhodium, becomes slower and the cyclization of acyl intermediate A occurs before its hydrogenolysis resulting preferably in the cis isomer of hemiacetal, 3a.

175

(dppe), 1,4-bis(diphenylphosphino)propane (dppp) and 1,4bis(diphenylphosphino)butane (dppb). As can be seen from the data presented in Table 4, the reactions with Rh/diphosphine systems are much faster than with the unmodified catalyst, but they are rather slower (ca. two times) than in the presence of PPh3 at the same P/Rh = 5 atomic ratio. Diphosphines as bidentate ligands usually give an enhanced preference for the formation of bis(phosphine)rhodium complexes. This should disfavor the formation of rhodium-linalool chelates, thus accelerating the reaction in comparison with unmodified systems. However, ligand natural bite angles of near 908 (dppe and dppp) and 988 (dppb) [30,31] would induce an apical-equatorial coordination of these ligands in a trigonalbipyramidal rhodium-olefin-hydride intermediate. Therefore, the effect in preventing the diequatorial linalool chelation and/ or cleavage the rhodium-linalool chelates is less pronounced than with the monodentate PPh3 ligand. Thus, the catalysts modified with dppe, dppp and dppb are less active in the hydroformylation of linalool than the catalyst modified with PPh3. Although the selectivity for the hydroformylation products in the Rh/diphosphine systems is lower then in the presence of PPh3, it is noticeable the enhanced preference for the formation of cis isomer 3a. We suppose that the increased steric bulk on rhodium and less flexibility of the rhodium-diphosphine complexes should favor the formation of the less hindered isomer of intermediate B with rhodium and the methylpentenyl group in the trans position, which then originates cis isomer of the hemiacetal 3a. 3.5. Hydroformylation of linalool with Rh/PPh3/pyridinium p-toluenesulfonate

Run

Ligand

P/Rh

Time (h)

Conversion (%)

Selectivityb (%)

3a/3b

1 2 3

None PPh3 dppe

– 5 5

4. Conclusions

5

3.1/1

5

dppb

5

85 93 85 73 78 70 65 60

2.0/1

dppp

12 100 58 100 71 100 68 90

1/2.2 1/1.3

4

6 6 6 12 6 12 6 12

Hydroformylation of linalool was also performed in the presence of methanol (5.0 vol.%) as a nucleophile and pyridinium p-toluenesulfonate (PPTS) as an acid co-catalyst (Table 2, run 13). The reaction leads to the formation of only one major product in 90% selectivity. This compound (4) was isolated from the reaction mixtures and characterized by GC/ MS and NMR (Section 2 and Table 1). According to the data obtained, the product 4 is methyl ether of hemiacetal 3 whose structure is depicted in Scheme 2. NMR signals from all carbon and some hydrogen atoms are duplicated, therefore compound 4 exists in the solutions as a mixture of at least two isomers (conformers). We tried to determine a stereochemistry of this compound, however no conclusive data were obtained from its NOESY spectra so far.

The hydroformylation of linalool 1 using [Rh(COD)(OAc)]2 as a catalyst precursor in the presence of triphenylphosphine or various diphosphines leads mainly to a mixture of cis and trans isomers of hemiacetal 3. With unmodified systems, linalool shows a very low reactivity under the hydroformylation conditions, probably due to the chelation which keeps the substrate in the equatorial plane disfavoring the insertion of the

3.4. Linalool hydroformylation with Rh/diphosphine systems We studied the hydroformylation of linalool in the presence of various diphosphines: 1,2-bis(diphenylphosphino)ethane Table 4 Hydroformylation of linalool catalyzed by [Rh(COD)(OAc)]2: ligand effecta

a

2.2/1

Reaction conditions: linalool (0.30 M), [Rh(COD)(OAc)]2 (0.3 mM), 50 8C, 20 atm (CO/H2 = 1/1); conversion and selectivity were determined by GC; dppe—1,2-bis(diphenylphosphino)ethane; dppp—1,4-bis(diphenylphosphino)propane; dppb—1,4-bis(diphenylphosphino)butane. b Selectivity for hydroformylation products 3a and 3b.

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apical hydride ligand. The introduction of the phosphorous ligand and the increase in its concentration favor the cleavage of the chelates and exert a great accelerating effect. A good control of chemo and stereoselectivity was achieved through the appropriate choice of the reaction conditions. The higher the temperature and the P/Rh ratio, the greater relative amounts of cis isomer 3a are formed. On the other hand, on increasing the total or hydrogen pressure, stereoselectivity is switched to trans isomer 3b. Each isomer can be obtained in ca. 95% chemo and 85% stereoselectivity at a virtually complete linalool conversion. In the presence of the acid co-catalyst (PPTS) and methanol, the main product of the hydroformylation of linalool is acetal 4. Acknowledgments We acknowledge the financial support from the CNPq and FAPEMIG (Brazil) and scholarships from CNPq (JGS) and CAPES (HJVB). References [1] P.W.N.M. Leeuwen, C. Claver (Eds.), Rhodium Catalyzed Hydroformylation, Kluwer Academic Publisher, Dordrecht, 2000. [2] C. Botteghi, M. Marchetti, S. Paganelli, in: M. Beller, C. Bolm (Eds.), Transition Metals for Organic Synthesis, vol. 1, Willey–VCH, Weinheim, 1998, p. 25. [3] M. Matsumoto, M. Tamura, J. Mol. Catal. 16 (1982) 187. [4] A.J. Chalk, in: P.N. Rylander, H. Greenfield, R.L. Augustine (Eds.), Catalysis of Organic Reactions, vol. 22, Marcel Dekker, New York, 1988, p. 43. [5] D. Anastasiou, W.R. Jackson, Aust. J. Chem. 45 (1992) 21. [6] D. Anastasiou, W.R. Jackson, Q.J. McCubbin, A.E. Trnacek, Aust. J. Chem. 46 (1993) 1623. [7] A.M. Trzeciak, E. Wolszczak, J.J. Zio´lkowski, New. J. Chem. 20 (1996) 365.

[8] K. Nozaki, W. Li, T. Horiochi, H. Takaya, Tetrahedron Lett. 38 (1997) 4611. [9] S. Sirol, P. Kalck, New J. Chem. 21 (1997) 1129. [10] J.T. Sullivan, J. Sadula, B.E. Hanson, R.J. Rosso, J. Mol. Catal. A 214 (2004) 213. [11] H. Mimoun, Chimia 50 (1996) 620. [12] K. Bauer, D. Garbe, H. Surburg, Common Fragrance and Flavor Materials: Preparation, Properties and Uses, Wiley, New York, 1997. [13] D.H. Pybus, C.S. Sell (Eds.), The Chemistry of Fragrances, RSC Paperbacks, Cambridge, 1999. [14] K.A.D. Swift, Top. Catal. 27 (2004) 143. [15] V.A. Semikolenov, I.I. Ilyna, I.L. Simakova, Appl. Catal. A 211 (2001) 91. [16] I. Cipre´s, P. Kalck, D.-C. Park, F. Serein-Spirau, J. Mol. Catal. 66 (1991) 399. [17] K. Soulantica, S. Sirol, S. Koinis, G. Pneumatikakis, P. Kalck, J. Organomet. Chem. 498 (1995) C10. [18] L. Kolla´r, G. Bo´di, Chirality 1 (1995) 121. [19] F. Azzaroni, P. Biscarini, S. Bordoni, G. Longoni, E. Venturini, J. Organomet. Chem. 508 (1996) 59. [20] E.V. Gusevskaya, E.N. dos Santos, R. Augusti, A.O. Dias, C.M. Foca, J. Mol. Catal. A 152 (2000) 15. [21] C.M. Foca, H.J.V. Barros, E.N. dos Santos, E.V. Gusevskaya, J.C. Bayon, New J. Chem. 27 (2003) 533. [22] H.J.V. Barros, E.V. Gusevskaya, E.N. dos Santos, J. Organomet. Chem. 671 (2003) 150. [23] H.J.V. Barros, B.E. Hanson, E.N. dos Santos, E.V. Gusevskaya, Appl. Catal. A 278 (2004) 57. [24] G.A. Korneeva, M.M. Minkovskii, N.A. Novikov, I.S. Istochnikova, M.M. Potarin, E.V. Slivinskii, RU 2 058 309 (1996). [25] M. Benaissa, U.J. Ja´uregui-Haza, I. Nikov, A.M. Wilhelm, H. Delmas, Catal. Today 79–80 (2003) 419. [26] G. Giordano, R.H. Crabtree, Inorg. Synth. 28 (1990) 88. [27] A. Wu, D. Cremer, J. Phys. Chem. A 107 (2003) 107. [28] A. Wu, D. Cremer, J. Phys. Chem. A 107 (2003) 107. [29] D. Gagnaire, P. Vottero, Bull. Soc. Chem. Fr. 873 (1972) 3; G. Dana, E. Touboul, O. Convert, Tetrahedron 45 (1989) 3371. [30] C.P. Casey, G.T. Whiteker, Israel J. Chem. 30 (1990) 299. [31] P. Dierkes, P.W.N.M. van Leeuwen, J. Chem. Soc., Dalton Trans. (1999) 1519.