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Regioselective and rapid hydroformylation of vinyl acetate catalyzed by rhodium complex modified bulky phosphite ligand
Catalysis Communications 11 (2010) 616–619
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Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Regioselective and rapid hydroformylation of vinyl acetate catalyzed by rhodium complex modified bulky phosphite ligand Aasif A. Dabbawala a, Raksh V. Jasra b,1, Hari C. Bajaj a,* a
Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute, Council of Scientific and Industrial Research (CSIR), G.B. Marg, Bhavnagar 364 021, Gujarat, India b R&D Centre, Reliance Industries Limited, Manufacturing Division, Vadodara 391 346, Gujarat, India
a r t i c l e
i n f o
Article history: Received 23 November 2009 Received in revised form 31 December 2009 Accepted 6 January 2010 Available online 11 January 2010 Keywords: Hydroformylation Vinyl acetate Monodentate phosphite High rate Regioselective
a b s t r a c t The regioselective hydroformylation of vinyl acetate catalyzed by rhodium complex of monodentate phosphite ligand, tri-1-naphthylphosphite P(ONp)3, was investigated. The P(ONp)3 ligand exhibited a considerable impact on the rate and selectivity of hydroformylation of vinyl acetate, notably high turnover frequency (up to 11,520 h 1) with excellent regioselectivity (99%) to the preferred branched aldehyde and high selectivity to aldehyde (93%). Significant results with the substrate having less reactive character toward hydroformylation and practically easy accessibility of the ligand (synthesized from inexpensive compound, 1-naphthol) make this system very attractive. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Hydroformylation of olefins represents efficient method to prepare aldehydes, and is one of the industrially most important C–C bond forming reactions catalyzed by Co and Rh metal complexes [1–3]. Hydroformylation has been mainly studied for terminal olefins. However, in recent year, there has been increased interest in the hydroformylation of functionalized olefins such as alkyl acrylate [4], allyl cyanide [5], allyl alcohol [6], enamide [7], and vinyl acetate [8]. The hydroformylation of such substrates offers the products having two functional groups widely used in organic syntheses. Particularly, vinyl acetate as substrate provides gate way to valuable building blocks for the preparation of bifunctional intermediate (Scheme 1), which can be further converted into synthetically useful compounds such as 1,2- and 1,3-propanediol, lactic acid and ethyl lactate. Vinyl acetate is generally less reactive to syngas relatively to terminal alkenes [9]. The reaction suffers from a slow rate, formation of by-products and the high pressure (200 atm) is often required to achieve high turnover frequency [10]. However, there are few reports wherein hydroformylation of vinyl acetate is carried out under mild conditions [11–13] wherein good regioselectivity was achieved by sacrificing the reaction rate. Therefore, in
* Corresponding author. Tel.: +91 278 2471793; fax: +91 278 2566970. E-mail address: [email protected] (H.C. Bajaj). 1 Present address. 1566-7367/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.01.007
the past decades, efforts have been made to increase the regioselectivity with high reaction rate and decreased undesired side products [14–16]. The high reaction rates for the hydroformylation of vinyl acetate were obtained using bidentate ligands [15] or with electron-deficient ligand such as organo phosphite ligand [16] in comparison with the electron-rich analogues. We have recently reported the synthesis and application of rhodium complex of novel monodentate bulky phosphite ligand, P(ONp)3 for the hydroformylation of olefins such as 1-hexene, styrene and cyclohexene [17]. In this context, we explore the catalytic activity of Rh/ P(ONp)3 system toward vinyl acetate hydroformylation. Herein, we report the highly regioselective and rapid hydroformylation of vinyl acetate to the branch aldehyde in the presence of Rh(CO)2(acac) and P(ONp)3 ligand. 2. Experimental 2.1. General remarks The ligand synthesis was performed using standard Schlenk technique under nitrogen atmosphere. THF was distilled from sodium/benzophenone prior to use. Toluene was purchased from Sigma–Aldrich as anhydrous grade material and used as received. Et3N was distilled from sodium and stored under N2. PCl3 and 1naphthol were obtained from Merck, India, and used as received. Rh(CO)2(acac), triphenyl phosphine (PPh3), triphenyl phosphite P(OPh)3, tricyclohexylphosphine (PCy3), 1,3-bis(diphenylphos-
617
A.A. Dabbawala et al. / Catalysis Communications 11 (2010) 616–619 Aldehydes O
O + CO + H2
O
Rh(CO)2acac/L
Vinyl acetate
CHO O
O
2-acetoxypropanal b (branched)
L=
CHO
+
O
3-acetoxypropanal l (linear)
O O
P
O
Scheme 1. Rh/tri-1-naphthyl phosphite catalyzed hydroformylation of vinyl acetate.
phino)propane (dppp), 1,4-bis(diphenylphosphino)butane (dppb), tris(4-methoxyphenyl)phosphine (p-MeO-C6H4)3P, tris(4-trifluoromethylphenyl)phosphine (p-CF3-C6H4)3P and vinyl acetate were purchased from M/s Sigma-Aldrich Chemicals, USA and used as received. The syngas (99.9%) used was from Hydro Gas India Pvt., Ltd., India. The synthesis and characterization of phosphite ligand P(ONp)3 as well as corresponding rhodium complex has been described elsewhere [17]. The hydroformylation reaction was performed in a 100 mL stainless steel autoclave (Autoclave Engineers, EZE-Seal Reactor, USA) and the reaction products were analyzed on a Shimadzu GC-17A gas chromatograph equipped with a flame ionization detector. 3. Results and discussion The rhodium catalyst of P(ONp)3 was prepared in situ by mixing P(ONp)3 with Rh(CO)2(acac). This system catalyzed the hydroformylation of vinyl acetate. The reaction products were 2-acetoxypropanal and 3-acetoxypropanal along with acetic acid and ethyl acetate as side products. The branch aldehyde (2-acetoxypropanal) was observed as the main product with high turnover frequency. The selectivity of products in hydroformylation process is mainly affected by variation in temperature. Temperature was varied from 70 to 100 °C at constant syngas pressure (3.0 MPa). With an increase of temperature from 70 to 100 °C, the reaction proceeds faster by a factor of 3.7 (Table 1) along with slight decrease in the chemo selectivity toward aldehyde. However, an excellent branch aldehyde regioselectivity (98%) was obtained in the entire range of temperatures. At lower reaction temperature (70 °C), high chemo selectivity toward aldehyde was observed (94.5%), the TOF was rather low (1800 h 1). At 100 °C, the TOF increased up to 6720 h 1 with aldehyde selectivity decreased to 89.5%. Therefore,
Table 1 Effect of temperature and syngas pressure on Rh/tri-1-naphthyl phosphite catalyzed hydroformylation of vinyl acetate.a Entry
Temp. (°C)
Press. (MPa)
Conv. (%)
TOFb (h 1)
Saldehyde (%)
b/l
1c 2 3 4 5 6 7 8
70 80 90 100 90 90 90 90
3.0 3.0 3.0 3.0 2.0 4.0 5.0 6.0
15.0 17.0 23.5 28.0 12.5 34.5 42.0 48.0
1800 4080 5640 6720 3000 8280 10,000 11,520
94.5 93.0 92.0 89.5 86.0 93.0 93.0 93.5
98/2 98/2 98/2 98/2 98/2 99/1 99/1 99/1
a Reaction conditions: sub/cat. = 4000, [Rh(CO)2acac] = 0.23 mmol/L, P/Rh = 6.0, and solvent (toluene) = 50 mL. b Turnover frequency, determined based on GC, reaction time = 10 min. c Reaction time = 20 min.
the optimum reaction temperature was 90 °C where maximum TOF and selectivity was achieved. The hydroformylation of vinyl acetate catalyzed by Rh/P(ONp)3 was studied at 90 °C under different syngas pressure (2.0– 6.0 MPa). Changes in the syngas pressure were found to affect the reaction rate and selectivity (Table 1). The results indicate that the hydroformylation of vinyl acetate proceeds slowly at low syngas pressure. At low syngas pressure, CO insertion may be difficult into the intermediate species as the ester carbonyl group form thermodynamically stable five and/or six member rings, resulting into considerably low TOF. TOF increased considerably with an increase in the syngas pressure (2.0–4.0 MPa) and above that there was a slight increase in the TOF. For example, when the syngas pressure was increase from 2.0 to 4.0 MPa, TOF increased by a factor of 2.76, whereas with further increase in the syngas pressure from 4.0 to 6.0 MPa, TOF was increased only by a factor 1.39 (see Table 1). From the above discussion one can conclude that minimum 4 MPa syngas pressure is require for achieving reasonable TOF. In all instances, regioselectivity to branch aldehyde was greater then 98%. The effect of ligand/Rh ratio on the catalytic activity/selectivity of vinyl acetate hydroformylation was studied at 90 °C, 3.0 MPa syngas pressure by varying ligand/Rh mmol ratio 3.0–18.0 (Table 2). The results indicate that TOF was improved up to ligand/ Rh of 6.0. While TOF was found to decrease as ligand/Rh increased from 6.0 to 18.0. However, chemo selectivity to aldehyde and regioselectivity to branched aldehyde were increased. This tendency is also found in enamides hydroformylation catalyzed by Rh/biphenyl based monodentate phosphite catalytic system [7]. When rhodium concentration was increased from 0.16 to 0.46 mM (ligand/Rh ratio 6) at 80 °C, 3.0 MPa syngas pressure, as expected, the rate of vinyl acetate hydroformylation reaction was found to increase with an increase in the rhodium concentration indicating linear dependency with respect to the rhodium concentration (Table 3). Similar result was also found for HRh(CO)(PPh3)3 catalyzed hydroformylation of vinyl acetate [18]. Aldehyde selectivity was low (90%) at low 0.16 mM rhodium concentration and improved up to 0.23 mM rhodium concentration, beyond that aldehyde selectivity remained unaffected. For comparison, a variety of phosphine ligands were screened for rhodium catalyzed hydroformylation of vinyl acetate under
Table 2 Effect of ligand/Rh ratio on Rh/tri-1-naphthyl phosphite catalyzed hydroformylation of vinyl acetate.a Entry
P/Rh
Conv. (%)
TOF (h
1 2 3 4
3.0 6.0 12.0 18.0
20.0 23.5 22.0 21.0
4800 5640 5280 5040
1
)
Saldehyde (%)
b/l
91.0 92.0 94.0 94.0
97/3 98/2 98/2 98/2
a Reaction conditions: sub/cat. = 4000, [Rh(CO)2acac] = 0.23 mmol/L, temp. = 90 °C, syngas pressure (1:1) = 3.0 MPa, solvent (toluene) = 50 mL, and reaction time = 10 min.
Table 3 Effect of initial rhodium concentration on Rh/tri-1-naphthyl phosphite catalyzed hydroformylation of vinyl acetate.a Entry
[Rh(CO)2acac] (mmol/L)
Rate 103 (m s
1 2 3 4
0.16 0.23 0.35 0.46
0.17 0.26 0.32 0.40
1
)
Saldehyde (%)
b/l
90.0 93.0 93.0 93.0
97/3 98/2 98/2 98/2
a Reaction conditions: sub/cat. = 4000, P/Rh = 6.0, temp. = 80 °C, syngas pressure (1:1) = 3.0 MPa, solvent (toluene) = 50 mL, and reaction time = 10 min.
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A.A. Dabbawala et al. / Catalysis Communications 11 (2010) 616–619
Table 4 Effect of various ligands on the hydroformylation of vinyl acetate.a Entry
Ligand
TOFb (h
1 2 3 4c 5d 6d 7 8 9 10 11c
– PPh3 PCy3 PCy3 dppp dppb (p-MeO-C6H4)3P P(OPh)3 (p-CF3-C6H4)3P P(ONp)3 P(ONp)3
480 2240 589 1680 2120 2600 600 4800 8000 8280 10,600
1
)
Saldehyde (%)
b/l
89.0 92.0 83.0 84.0 92.0 93.0 78.0 93.0 87.0 93.0 91.0
95/5 93/7 99/1 99/1 98/2 98/2 99/1 92/8 88/12 99/1 99/1
a Reaction conditions: sub/cat. = 4000, [Rh(CO)2acac] = 0.23 mmol/L, P/Rh = 6.0, temp. = 90 °C, syngas pressure (1:1) = 4.0 MPa, and solvent (toluene) = 50 mL. b Turnover frequency, determined based on 30–35% GC conversion. c Temp. = 100 °C. d For bidentate ligand P/Rh = 3.0.
the identical reaction conditions. It was found that at optimum condition (temp. 90 °C, syngas pressure 4.0 MPa), most of ligands gave good chemo and regioselectivity but insufficient activity (Table 4). With PPh3 ligand, TOF was 2240 h 1 and moderate selectivity (chemo as well as regioselectivity) was achieved at 90 °C and 4.0 MPa syngas pressure. Results clearly indicated that steric nature of ligands remarkably influences the regioselectivity to branched aldehyde while electronic nature of ligands greatly influences catalytic activity. Thus, as shown in entry 4, ligand PCy3, resulted in low activity and low chemo selectivity toward aldehyde. However, it gave high regioselectivity toward branched aldehyde (99%). The diphosphines ligand (dppp and dppb) exhibited high chemo selectivity and favored regioselective formation of branched aldehyde although TOF was relatively low for the both diphosphines under identical experimental conditions. Ligands having more r-donor ability slow down the reaction considerably while ligands having p-acceptor ability enhances the reaction rate.
Thus, (p-MeO-C6H4)3P gave low TOF while P(ONp)3 and (p-CF3C6H4)3P gave very high TOF whereas P(OPh)3 exhibiting TOF in between. Indeed, an excellent regioselectivity to branched aldehyde with high TOF was obtained only with P(ONp)3 (Table 4). The observed TOF is at least 3.6 times higher than those observed for the Rh/PPh3 catalyst. The bulky phosphite ligands are known to enhance the rate of the hydroformylation reaction [19–23]. The high activity of P(ONp)3 is explained by the formation of monoligated rhodium phosphite complex, HRh(CO)3L [23]. P(ONp)3 is bulkier and sterically hindered ligand as compare to PPh3 (cone angle of P(ONp)3 is 166° while for PPh3 is 145°) and it gives an active species containing only one bulkier phosphite ligand around rhodium resulting in the formation of HRh(CO)3L, (L = tri-1-naphthylphosphite) intermediate species, confirmed by in situ IR and NMR [17]. This complex easily loses CO which facilitates coordination of an alkene molecule. It is well known that during the hydroformylation of vinyl acetate, the side product, acetic acid reduces the concentration of the hydridic form of the catalyst by forming the carboxylate rhodium complex [12]. The coordination of the ester carbonyl group of vinyl acetate also influences the stability of the transient intermediate (Scheme 2) and slowdown the carbon monoxide insertion. As a result the hydroformylation activity of vinyl acetate using rhodium complex modified by phosphite is lower than that of the hydroformylation of 1-hexene [17]. However, hydroformylation activity of vinyl acetate using the bulky phosphite ligand is significantly attractive as compare to the conventional ligands. Moreover, there is a possibility of decomposition of phosphite in presence of acetic acid formed during hydroformylation. In order to check phosphite stability, 31P NMR spectra of the reaction mixture (sub/cat. = 4000, [Rh(CO)2acac] = 0.23 mmol/L, P/Rh = 6.0, temp. = 90 °C, syngas pressure (1:1) = 4.0 MPa, toluene = 50 mL) was recorded after the completion of the reaction. No decomposition of the phosphite was observed. In order to ascertain the stability of phosphite in the presence of added acetic acid, the 31P and 1H NMR of the reaction mixture containing phosphite ligand (0.7 mmol) and acetic
H OC Rh
L
OC
L= P(ONp)3
CO O
OAc -CO OAc
H
H
O
OAc OC
Rh
L
OC
Rh
Rh
OC
L
OC
Rh
L
L CO
CO
CO
CO
OAc OC
Rh
O
L
H
OAc
CO
O
H
O
O OC
Rh
L
OAc
CO Scheme 2. Coordination of the ester carbonyl group of vinyl acetate to the rhodium during hydroformylation.
A.A. Dabbawala et al. / Catalysis Communications 11 (2010) 616–619
acid (4.9 mmol) in toluene kept at 90 °C and 4 MPa syngas pressure was recorded by withdrawing the sample at regular interval. The decomposition products of phosphite, oxide of phosphite/1-naphthol were not observed till 3 h (carefully checked by 31P and 1H NMR) indicating that no decomposition of phosphite take place within 3 h. The 31P and 1H NMR spectra recoded after 17 h showed 20–25% decomposition of phosphite (indicated by the formation of oxide of phosphite/1-naphthol in 31P and 1H NMR spectra). As phosphite system exhibits high reaction rate and hydroformylation reaction is completed within short time, the influence of side products which may effect the catalyst activity will be less or trivial; and generally in hydroformylation excess ligand is used. 4. Conclusions Rhodium complex modified with monodentate bulky phosphite ligand, P(ONp)3, was shown to be effective catalytic system for hydroformylation of vinyl acetate. High TOF and excellent regioselectivity to the preferred branched aldehyde with high chemo selectivity to aldehyde were achieved by Rh/P(ONp)3 catalyst. Moreover, the ligand, P(ONp)3, is more accessible from a synthetic point of view than phosphine ligands, prepared from commercially available, inexpensive, and air stable organic compound and exhibited strong p-acceptor character than phosphine ligand. Further studies to expanding the scope of this ligand are currently underway in our laboratory. Acknowledgements Authors are thankful to analytical division of the institute for assistance with analyses and CSIR for financial support under CSIR
619
Network Project (NWP 010). AAD thanks CSIR, New Delhi, for the award of Senior Research Fellowship. References [1] B. Cornils, W.A. Herrmann, Applied Homogeneous Catalysis with Organometallic Compound, Wiley-VCH, Weinheim, 2000. p. 29. [2] A.A. Dabbawala, D.U. Parmar, H.C. Bajaj, R.V. Jasra, J. Mol. Catal. A: Chem. 282 (2008) 99. [3] M. Haumann, H. Koch, R. Schomäcker, Catal. Today 79–80 (2003) 43. [4] G. Fremy, E. Monflier, J. Carpentier, Y. Castanet, A. Mortreux, J. Mol. Catal. A: Chem. 129 (1998) 35. [5] C.J. Cobley, K. Gardner, J. Klosin, C. Praquin, C. Hill, G.T. Whiteker, A. ZanottiGerosa, J. Org. Chem. 69 (2004) 4031. [6] K.N. Bhatt, S.B. Halligudi, J. Mol. Catal 91 (1994) 187. [7] O. Saidi, J. Ruan, D. Vinci, X. Wu, J. Xiao, Tetrahedron Lett. 49 (2008) 3516. [8] S. Breeden, D.J. Cole-Hamilton, D.F. Foster, G.J. Schwartz, M. Wills, Angew. Chem., Int. Ed. 39 (2000) 4106. [9] C.K. Brown, G. Wilkinson, J. Chem. Soc. (A) (1970) 2753. [10] B. Fell, M. Barl, J. Mol. Catal 2 (1977) 301. [11] M. Matsumoto, M. Tamura, J. Mol. Catal 16 (1982) 195. [12] A.G. Abatjoglou, D.R. Bryant, L.C. D’Esposito, J. Mol. Catal. 18 (1983) 381. [13] A.M. Trzeciak, J.J. Ziolkowski, J. Mol. Catal. 43 (1987) 15. [14] Y.L. Borole, R.V. Chaudhari, Ind. Eng. Chem. Res. 44 (2005) 9601. [15] P.J. Thomas, A.T. Axtell, J. Klosin, W. Peng, C.L. Rand, T.P. Clark, C.R. Landis, K.A. Abboud, Org. Lett. 9 (2007) 2665. [16] D.B.G. Williams, M. Ajam, A. Ranwell, Organometallics 26 (2007) 4692. [17] A.A. Dabbawala, H.C. Bajaj, R.V. Jasra, J. Mol. Catal. A: Chem. 302 (2009) 97. [18] R.M. Deshpande, R.V. Chaudhari, J. Mol. Catal. 57 (1989) 177. [19] A. Van Rooy, E.N. Orij, P.C.J. Kamer, P.W.N.M. Van Leeuwen, Organometallics 14 (1995) 34. [20] A. Van Rooy, J.N.H. de Bruijn, K.F. Roobeek, P.C.J. Kamer, P.W.N.M. Van Leeuwen, J. Organomet. Chem. 507 (1996) 69. [21] P.W.N.M. Van Leeuwen, C.F. Roobeek, J. Organomet. Chem. 258 (1983) 343. [22] A. Van Rooy, E.N. Orij, P.C.J. Kamer, F. Van Den Aardweg, P.W.N.M. Van Leeuwen, J. Chem. Soc., Chem. Commun. 16 (1991) 1096. [23] T. Jongsma, G. Challa, P.W.N.M. Van Leeuwan, J. Organomet. Chem. 421 (1991) 121.
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Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Regioselective and rapid hydroformylation of vinyl acetate catalyzed by rhodium complex modified bulky phosphite ligand Aasif A. Dabbawala a, Raksh V. Jasra b,1, Hari C. Bajaj a,* a
Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute, Council of Scientific and Industrial Research (CSIR), G.B. Marg, Bhavnagar 364 021, Gujarat, India b R&D Centre, Reliance Industries Limited, Manufacturing Division, Vadodara 391 346, Gujarat, India
a r t i c l e
i n f o
Article history: Received 23 November 2009 Received in revised form 31 December 2009 Accepted 6 January 2010 Available online 11 January 2010 Keywords: Hydroformylation Vinyl acetate Monodentate phosphite High rate Regioselective
a b s t r a c t The regioselective hydroformylation of vinyl acetate catalyzed by rhodium complex of monodentate phosphite ligand, tri-1-naphthylphosphite P(ONp)3, was investigated. The P(ONp)3 ligand exhibited a considerable impact on the rate and selectivity of hydroformylation of vinyl acetate, notably high turnover frequency (up to 11,520 h 1) with excellent regioselectivity (99%) to the preferred branched aldehyde and high selectivity to aldehyde (93%). Significant results with the substrate having less reactive character toward hydroformylation and practically easy accessibility of the ligand (synthesized from inexpensive compound, 1-naphthol) make this system very attractive. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Hydroformylation of olefins represents efficient method to prepare aldehydes, and is one of the industrially most important C–C bond forming reactions catalyzed by Co and Rh metal complexes [1–3]. Hydroformylation has been mainly studied for terminal olefins. However, in recent year, there has been increased interest in the hydroformylation of functionalized olefins such as alkyl acrylate [4], allyl cyanide [5], allyl alcohol [6], enamide [7], and vinyl acetate [8]. The hydroformylation of such substrates offers the products having two functional groups widely used in organic syntheses. Particularly, vinyl acetate as substrate provides gate way to valuable building blocks for the preparation of bifunctional intermediate (Scheme 1), which can be further converted into synthetically useful compounds such as 1,2- and 1,3-propanediol, lactic acid and ethyl lactate. Vinyl acetate is generally less reactive to syngas relatively to terminal alkenes [9]. The reaction suffers from a slow rate, formation of by-products and the high pressure (200 atm) is often required to achieve high turnover frequency [10]. However, there are few reports wherein hydroformylation of vinyl acetate is carried out under mild conditions [11–13] wherein good regioselectivity was achieved by sacrificing the reaction rate. Therefore, in
* Corresponding author. Tel.: +91 278 2471793; fax: +91 278 2566970. E-mail address: [email protected] (H.C. Bajaj). 1 Present address. 1566-7367/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.01.007
the past decades, efforts have been made to increase the regioselectivity with high reaction rate and decreased undesired side products [14–16]. The high reaction rates for the hydroformylation of vinyl acetate were obtained using bidentate ligands [15] or with electron-deficient ligand such as organo phosphite ligand [16] in comparison with the electron-rich analogues. We have recently reported the synthesis and application of rhodium complex of novel monodentate bulky phosphite ligand, P(ONp)3 for the hydroformylation of olefins such as 1-hexene, styrene and cyclohexene [17]. In this context, we explore the catalytic activity of Rh/ P(ONp)3 system toward vinyl acetate hydroformylation. Herein, we report the highly regioselective and rapid hydroformylation of vinyl acetate to the branch aldehyde in the presence of Rh(CO)2(acac) and P(ONp)3 ligand. 2. Experimental 2.1. General remarks The ligand synthesis was performed using standard Schlenk technique under nitrogen atmosphere. THF was distilled from sodium/benzophenone prior to use. Toluene was purchased from Sigma–Aldrich as anhydrous grade material and used as received. Et3N was distilled from sodium and stored under N2. PCl3 and 1naphthol were obtained from Merck, India, and used as received. Rh(CO)2(acac), triphenyl phosphine (PPh3), triphenyl phosphite P(OPh)3, tricyclohexylphosphine (PCy3), 1,3-bis(diphenylphos-
617
A.A. Dabbawala et al. / Catalysis Communications 11 (2010) 616–619 Aldehydes O
O + CO + H2
O
Rh(CO)2acac/L
Vinyl acetate
CHO O
O
2-acetoxypropanal b (branched)
L=
CHO
+
O
3-acetoxypropanal l (linear)
O O
P
O
Scheme 1. Rh/tri-1-naphthyl phosphite catalyzed hydroformylation of vinyl acetate.
phino)propane (dppp), 1,4-bis(diphenylphosphino)butane (dppb), tris(4-methoxyphenyl)phosphine (p-MeO-C6H4)3P, tris(4-trifluoromethylphenyl)phosphine (p-CF3-C6H4)3P and vinyl acetate were purchased from M/s Sigma-Aldrich Chemicals, USA and used as received. The syngas (99.9%) used was from Hydro Gas India Pvt., Ltd., India. The synthesis and characterization of phosphite ligand P(ONp)3 as well as corresponding rhodium complex has been described elsewhere [17]. The hydroformylation reaction was performed in a 100 mL stainless steel autoclave (Autoclave Engineers, EZE-Seal Reactor, USA) and the reaction products were analyzed on a Shimadzu GC-17A gas chromatograph equipped with a flame ionization detector. 3. Results and discussion The rhodium catalyst of P(ONp)3 was prepared in situ by mixing P(ONp)3 with Rh(CO)2(acac). This system catalyzed the hydroformylation of vinyl acetate. The reaction products were 2-acetoxypropanal and 3-acetoxypropanal along with acetic acid and ethyl acetate as side products. The branch aldehyde (2-acetoxypropanal) was observed as the main product with high turnover frequency. The selectivity of products in hydroformylation process is mainly affected by variation in temperature. Temperature was varied from 70 to 100 °C at constant syngas pressure (3.0 MPa). With an increase of temperature from 70 to 100 °C, the reaction proceeds faster by a factor of 3.7 (Table 1) along with slight decrease in the chemo selectivity toward aldehyde. However, an excellent branch aldehyde regioselectivity (98%) was obtained in the entire range of temperatures. At lower reaction temperature (70 °C), high chemo selectivity toward aldehyde was observed (94.5%), the TOF was rather low (1800 h 1). At 100 °C, the TOF increased up to 6720 h 1 with aldehyde selectivity decreased to 89.5%. Therefore,
Table 1 Effect of temperature and syngas pressure on Rh/tri-1-naphthyl phosphite catalyzed hydroformylation of vinyl acetate.a Entry
Temp. (°C)
Press. (MPa)
Conv. (%)
TOFb (h 1)
Saldehyde (%)
b/l
1c 2 3 4 5 6 7 8
70 80 90 100 90 90 90 90
3.0 3.0 3.0 3.0 2.0 4.0 5.0 6.0
15.0 17.0 23.5 28.0 12.5 34.5 42.0 48.0
1800 4080 5640 6720 3000 8280 10,000 11,520
94.5 93.0 92.0 89.5 86.0 93.0 93.0 93.5
98/2 98/2 98/2 98/2 98/2 99/1 99/1 99/1
a Reaction conditions: sub/cat. = 4000, [Rh(CO)2acac] = 0.23 mmol/L, P/Rh = 6.0, and solvent (toluene) = 50 mL. b Turnover frequency, determined based on GC, reaction time = 10 min. c Reaction time = 20 min.
the optimum reaction temperature was 90 °C where maximum TOF and selectivity was achieved. The hydroformylation of vinyl acetate catalyzed by Rh/P(ONp)3 was studied at 90 °C under different syngas pressure (2.0– 6.0 MPa). Changes in the syngas pressure were found to affect the reaction rate and selectivity (Table 1). The results indicate that the hydroformylation of vinyl acetate proceeds slowly at low syngas pressure. At low syngas pressure, CO insertion may be difficult into the intermediate species as the ester carbonyl group form thermodynamically stable five and/or six member rings, resulting into considerably low TOF. TOF increased considerably with an increase in the syngas pressure (2.0–4.0 MPa) and above that there was a slight increase in the TOF. For example, when the syngas pressure was increase from 2.0 to 4.0 MPa, TOF increased by a factor of 2.76, whereas with further increase in the syngas pressure from 4.0 to 6.0 MPa, TOF was increased only by a factor 1.39 (see Table 1). From the above discussion one can conclude that minimum 4 MPa syngas pressure is require for achieving reasonable TOF. In all instances, regioselectivity to branch aldehyde was greater then 98%. The effect of ligand/Rh ratio on the catalytic activity/selectivity of vinyl acetate hydroformylation was studied at 90 °C, 3.0 MPa syngas pressure by varying ligand/Rh mmol ratio 3.0–18.0 (Table 2). The results indicate that TOF was improved up to ligand/ Rh of 6.0. While TOF was found to decrease as ligand/Rh increased from 6.0 to 18.0. However, chemo selectivity to aldehyde and regioselectivity to branched aldehyde were increased. This tendency is also found in enamides hydroformylation catalyzed by Rh/biphenyl based monodentate phosphite catalytic system [7]. When rhodium concentration was increased from 0.16 to 0.46 mM (ligand/Rh ratio 6) at 80 °C, 3.0 MPa syngas pressure, as expected, the rate of vinyl acetate hydroformylation reaction was found to increase with an increase in the rhodium concentration indicating linear dependency with respect to the rhodium concentration (Table 3). Similar result was also found for HRh(CO)(PPh3)3 catalyzed hydroformylation of vinyl acetate [18]. Aldehyde selectivity was low (90%) at low 0.16 mM rhodium concentration and improved up to 0.23 mM rhodium concentration, beyond that aldehyde selectivity remained unaffected. For comparison, a variety of phosphine ligands were screened for rhodium catalyzed hydroformylation of vinyl acetate under
Table 2 Effect of ligand/Rh ratio on Rh/tri-1-naphthyl phosphite catalyzed hydroformylation of vinyl acetate.a Entry
P/Rh
Conv. (%)
TOF (h
1 2 3 4
3.0 6.0 12.0 18.0
20.0 23.5 22.0 21.0
4800 5640 5280 5040
1
)
Saldehyde (%)
b/l
91.0 92.0 94.0 94.0
97/3 98/2 98/2 98/2
a Reaction conditions: sub/cat. = 4000, [Rh(CO)2acac] = 0.23 mmol/L, temp. = 90 °C, syngas pressure (1:1) = 3.0 MPa, solvent (toluene) = 50 mL, and reaction time = 10 min.
Table 3 Effect of initial rhodium concentration on Rh/tri-1-naphthyl phosphite catalyzed hydroformylation of vinyl acetate.a Entry
[Rh(CO)2acac] (mmol/L)
Rate 103 (m s
1 2 3 4
0.16 0.23 0.35 0.46
0.17 0.26 0.32 0.40
1
)
Saldehyde (%)
b/l
90.0 93.0 93.0 93.0
97/3 98/2 98/2 98/2
a Reaction conditions: sub/cat. = 4000, P/Rh = 6.0, temp. = 80 °C, syngas pressure (1:1) = 3.0 MPa, solvent (toluene) = 50 mL, and reaction time = 10 min.
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Table 4 Effect of various ligands on the hydroformylation of vinyl acetate.a Entry
Ligand
TOFb (h
1 2 3 4c 5d 6d 7 8 9 10 11c
– PPh3 PCy3 PCy3 dppp dppb (p-MeO-C6H4)3P P(OPh)3 (p-CF3-C6H4)3P P(ONp)3 P(ONp)3
480 2240 589 1680 2120 2600 600 4800 8000 8280 10,600
1
)
Saldehyde (%)
b/l
89.0 92.0 83.0 84.0 92.0 93.0 78.0 93.0 87.0 93.0 91.0
95/5 93/7 99/1 99/1 98/2 98/2 99/1 92/8 88/12 99/1 99/1
a Reaction conditions: sub/cat. = 4000, [Rh(CO)2acac] = 0.23 mmol/L, P/Rh = 6.0, temp. = 90 °C, syngas pressure (1:1) = 4.0 MPa, and solvent (toluene) = 50 mL. b Turnover frequency, determined based on 30–35% GC conversion. c Temp. = 100 °C. d For bidentate ligand P/Rh = 3.0.
the identical reaction conditions. It was found that at optimum condition (temp. 90 °C, syngas pressure 4.0 MPa), most of ligands gave good chemo and regioselectivity but insufficient activity (Table 4). With PPh3 ligand, TOF was 2240 h 1 and moderate selectivity (chemo as well as regioselectivity) was achieved at 90 °C and 4.0 MPa syngas pressure. Results clearly indicated that steric nature of ligands remarkably influences the regioselectivity to branched aldehyde while electronic nature of ligands greatly influences catalytic activity. Thus, as shown in entry 4, ligand PCy3, resulted in low activity and low chemo selectivity toward aldehyde. However, it gave high regioselectivity toward branched aldehyde (99%). The diphosphines ligand (dppp and dppb) exhibited high chemo selectivity and favored regioselective formation of branched aldehyde although TOF was relatively low for the both diphosphines under identical experimental conditions. Ligands having more r-donor ability slow down the reaction considerably while ligands having p-acceptor ability enhances the reaction rate.
Thus, (p-MeO-C6H4)3P gave low TOF while P(ONp)3 and (p-CF3C6H4)3P gave very high TOF whereas P(OPh)3 exhibiting TOF in between. Indeed, an excellent regioselectivity to branched aldehyde with high TOF was obtained only with P(ONp)3 (Table 4). The observed TOF is at least 3.6 times higher than those observed for the Rh/PPh3 catalyst. The bulky phosphite ligands are known to enhance the rate of the hydroformylation reaction [19–23]. The high activity of P(ONp)3 is explained by the formation of monoligated rhodium phosphite complex, HRh(CO)3L [23]. P(ONp)3 is bulkier and sterically hindered ligand as compare to PPh3 (cone angle of P(ONp)3 is 166° while for PPh3 is 145°) and it gives an active species containing only one bulkier phosphite ligand around rhodium resulting in the formation of HRh(CO)3L, (L = tri-1-naphthylphosphite) intermediate species, confirmed by in situ IR and NMR [17]. This complex easily loses CO which facilitates coordination of an alkene molecule. It is well known that during the hydroformylation of vinyl acetate, the side product, acetic acid reduces the concentration of the hydridic form of the catalyst by forming the carboxylate rhodium complex [12]. The coordination of the ester carbonyl group of vinyl acetate also influences the stability of the transient intermediate (Scheme 2) and slowdown the carbon monoxide insertion. As a result the hydroformylation activity of vinyl acetate using rhodium complex modified by phosphite is lower than that of the hydroformylation of 1-hexene [17]. However, hydroformylation activity of vinyl acetate using the bulky phosphite ligand is significantly attractive as compare to the conventional ligands. Moreover, there is a possibility of decomposition of phosphite in presence of acetic acid formed during hydroformylation. In order to check phosphite stability, 31P NMR spectra of the reaction mixture (sub/cat. = 4000, [Rh(CO)2acac] = 0.23 mmol/L, P/Rh = 6.0, temp. = 90 °C, syngas pressure (1:1) = 4.0 MPa, toluene = 50 mL) was recorded after the completion of the reaction. No decomposition of the phosphite was observed. In order to ascertain the stability of phosphite in the presence of added acetic acid, the 31P and 1H NMR of the reaction mixture containing phosphite ligand (0.7 mmol) and acetic
H OC Rh
L
OC
L= P(ONp)3
CO O
OAc -CO OAc
H
H
O
OAc OC
Rh
L
OC
Rh
Rh
OC
L
OC
Rh
L
L CO
CO
CO
CO
OAc OC
Rh
O
L
H
OAc
CO
O
H
O
O OC
Rh
L
OAc
CO Scheme 2. Coordination of the ester carbonyl group of vinyl acetate to the rhodium during hydroformylation.
A.A. Dabbawala et al. / Catalysis Communications 11 (2010) 616–619
acid (4.9 mmol) in toluene kept at 90 °C and 4 MPa syngas pressure was recorded by withdrawing the sample at regular interval. The decomposition products of phosphite, oxide of phosphite/1-naphthol were not observed till 3 h (carefully checked by 31P and 1H NMR) indicating that no decomposition of phosphite take place within 3 h. The 31P and 1H NMR spectra recoded after 17 h showed 20–25% decomposition of phosphite (indicated by the formation of oxide of phosphite/1-naphthol in 31P and 1H NMR spectra). As phosphite system exhibits high reaction rate and hydroformylation reaction is completed within short time, the influence of side products which may effect the catalyst activity will be less or trivial; and generally in hydroformylation excess ligand is used. 4. Conclusions Rhodium complex modified with monodentate bulky phosphite ligand, P(ONp)3, was shown to be effective catalytic system for hydroformylation of vinyl acetate. High TOF and excellent regioselectivity to the preferred branched aldehyde with high chemo selectivity to aldehyde were achieved by Rh/P(ONp)3 catalyst. Moreover, the ligand, P(ONp)3, is more accessible from a synthetic point of view than phosphine ligands, prepared from commercially available, inexpensive, and air stable organic compound and exhibited strong p-acceptor character than phosphine ligand. Further studies to expanding the scope of this ligand are currently underway in our laboratory. Acknowledgements Authors are thankful to analytical division of the institute for assistance with analyses and CSIR for financial support under CSIR
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