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Rhodium complexes possessing S-phosphinite ligands with or without an amino group: application to hydroformylation of styrene
Inorganica Chimica Acta 357 (2004) 2850–2854 www.elsevier.com/locate/ica
Rhodium complexes possessing S-phosphinite ligands with or without an amino group: application to hydroformylation of styrene Ioannis D. Kostas a
a,*
, Barry R. Steele a, Fotini J. Andreadaki a, Vladimir A. Potapov
b
Institute of Organic and Pharmaceutical Chemistry, National Hellenic Research Foundation, 48 Vas. Constantinou Ave., 11635 Athens, Greece b Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences, 1 Favorsky Street, 664033 Irkutsk, Russia Received 4 December 2003; accepted 7 February 2004 Available online 27 February 2004
Abstract A chelate cationic rhodium(I) complex with a hemilabile amino- and sulfur-containing phosphinite ligand has been synthesized and, according to the NMR data, the ligand is bound to the metal in a P ; S-bidentate coordination mode without any Rh–N interaction. This complex efficiently catalyzes the hydroformylation of styrene. The chelate rhodium complex with the analogous ligand without the amino group has also been synthesized and examined as a catalyst for the same hydroformylation reaction. The reaction rate is higher using the former complex compared to the latter one without the amino group, with, however, a slightly lower regioselectivity towards the formation of the branched aldehyde. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Hemilabile ligands; Phosphinites; P ; S-ligands; Rhodium complexes; Hydroformylations; Homogeneous catalysis
1. Introduction Rhodium-catalyzed hydroformylation as an attractive synthetic transformation is a very well-established reaction with enormous academic and industrial interest, and represents one of the most important homogeneously catalyzed reactions world-wide [1–4]. The design and synthesis of new ligands with improved catalytic activity and selectivity towards hydroformylation is still a research subject of great significance. The stereoelectronic properties of the ligands are crucial for their catalytic activity and selectivity, and since the introduction of the concept of the ‘‘natural bite angle’’ for metal complexes with bidentate phosphines by Casey and Whiteker [5], the influence of the bite angle as well as steric and electronic effects on the hydroformylation has received much attention [6–9]. Ligands with mixed donors of different coordination abilities, in particular those which contain ‘‘soft’’ and ‘‘hard’’ donor atoms, so-called hemilabile ligands, have *
Corresponding author. Tel.: +30-210-7273878; fax: +30-2107273877. E-mail address: [email protected] (I.D. Kostas). 0020-1693/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2004.02.005
been of great interest due to the improved catalytic activity of their transition-metal complexes in certain homogeneously catalyzed reactions [10–13]. Particular attention has focused on phosphine ligands containing nitrogen [14–23] or oxygen [10,24–27]. The use of P ; Sligands in catalysis, on the other hand, is relatively unexplored compared to P ; N - and P ; O-ligands due to the fear of metal poisoning by sulfur [28–32]. We have previously reported several functionalized hemilabile P ; N -, P ; N ; P - and N ; P ; N -ligands and their application in rhodium-catalyzed hydroformylation, hydroaminomethylation and hydrogenation [33–37]. Recently, we reported a hemilabile amino- and sulfurcontaining phosphinite ligand (1) and the analogous sulfur-containing phosphinite (2) without the amino group, as catalyst precursors for the palladium-catalyzed Heck reaction of aryl bromides with styrene (Fig. 1) [38]. Treatment of PdCl2 (NCPh)2 with one equivalent of ligand 1 yielded the corresponding chelate palladium complex, in which the ligand is bound to the metal in a P ; S-bidentate coordination mode without any Pd–N interaction. Attempts to synthesize a monomeric chelate palladium complex with ligand 2 were unsuccessful and we found that the role of the amino
I.D. Kostas et al. / Inorganica Chimica Acta 357 (2004) 2850–2854
Fig. 1.
group in the formation of a P ; S-chelate complex as well as in the Heck reaction was decisive. Within our ongoing research, we wish to report here the coordination behavior of the two above-mentioned ligands towards rhodium. Rhodium complexes with both of these ligands were synthesized and applied to the hydroformylation of styrene, in order to investigate the influence of the amino group on the catalyst behavior of the complex in this reaction. Although phosphinites have been widely used in homogeneous catalysis [16,35,37–43], to our knowledge, sulfur-containing phosphinites have not been applied to rhodium-catalyzed hydroformylation until now.
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which the ligand is bound to rhodium via phosphorus and sulfur atoms, while nitrogen is situated away from the coordination sphere around the metal. Complex 4 displays similar behavior. Its 31 P NMR spectrum (d 135.06, d, JRhP ¼ 164:03 Hz) is essentially the same as that of complex 3, indicating an analogous chelation. It is clear, therefore, that the coordination ability of ligand 2 towards rhodium is in contrast to that observed towards palladium, in which the analogous chelation could not be achieved [38]. In the 1 H- and 13 C NMR spectra of 3 and 4, the olefinic COD resonances are non-equivalent. In complex 3, the olefinic COD resonances are split into four signals, two signals for the olefinic protons or carbons trans to the phosphorus atom (1 H NMR: 5.68, 5.60–5.47; 13 C NMR: 113.40, 108.45–108.25), and two signals for the olefinic protons or carbons trans to the sulfur atom (1 H NMR: 5.15, 5.02–4.89; 13 C NMR: 86.99, 84.56). The COD behavior in complex 4 is quite similar to that observed in complex 3 for the olefinic protons and carbons (1 H NMR: 5.76, 5.61–5.49, 5.29, 5.05–4.92; 13 C NMR: 112.68–112.45, 106.89–106.69, 85.26, 82.65).
2. Results and discussion
2.2. Hydroformylation
2.1. Synthesis and characterization of the rhodium complexes
The rhodium complexes 3 and 4 were applied to the hydroformylation of styrene using a high substrate:catalyst ratio of 1455:1 (Scheme 2, Table 1). The reaction was first catalyzed by complex 3 under variable conditions of pressure and temperature. The catalyst displays a high chemoselectivity for aldehydes (over 98%), and the variation observed in the regioselectivities towards the branched aldehyde 6 is as expected for hydroformylation of styrene using rhodium systems. At 100 bar total pressure of syngas, the conversion of styrene was almost quantitative after 22 h at 30 °C, with a 96.0% regioselectivity towards 6 (entry 1). Increasing the temperature dramatically increases the reaction rate but with a lower regioselectivity. On lowering the syngas pressure to 50 or 30 bar, the reaction rate as well as the regioselectivity is decreased. However, even under these mild reaction conditions, the activity of complex 3 is still acceptable. The catalytic activity of complex 4 was tested for a representative range of reaction conditions (entries 2, 4, 7). Under identical conditions, it displays lower TONs compared to those obtained for complex 3. The higher activity observed with the amino-substituted complex 3 compared to that observed with complex 4 is not surprising, since it is widely accepted that ligands containing electron-withdrawing substituents lead to higher
Treatment of [Rh(COD)2 ]BF4 in dichloromethane solution with one equivalent of the ligand 1 or 2 yielded the cationic rhodium complexes 3 or 4, respectively, in high yields (Scheme 1). The coordination mode in the complexes was determined by spectroscopic techniques. In the 31 P NMR spectrum, complex 3 shows a doublet at d 135.33 (JRhP ¼ 167:3 Hz) as a result of Rh–P interaction (dPðligand 1Þ 112:21Þ. The observation of the [1Rh(COD)]þ ion (m=z 620) and the absence of higher aggregates in the ESI MS spectrum suggest the monomeric nature of complex 3. In the 13 C NMR spectrum of 3, the SCH3 resonance is shifted to low field compared to that in the free ligand, in contrast to the N(CH3 )2 resonance which is the same in both the complex and the free ligand (SCH3 : dCðligand 1Þ 14.38, dCðcomplex 3Þ 18.80; N(CH3 )2 : dCðligand 1Þ 45.56, dCðcomplex 3Þ 45:60Þ. These data indicate the formation of a seven-membered chelate complex in
Scheme 1.
Scheme 2.
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Table 1 Hydroformylation of styrene catalyzed by rhodium complex 3 or 4 Entry
Catalysta
T (°C)
P b (bar)
Time (h)
Conversion (%)
Rc c (%)
Rbr d (%)
TONe
1 2 3 4 5 6 7 8 9 10
3 4 3 4 3 3 4 3 3 3
30 30 40 40 40 60 60 60 40 40
100 100 100 100 100 100 100 100 50 30
22 22 4 4 8 1 1 4 22 22
98.8 89.0 85.7 62.8 99.2 98.4 81.0 99.9 95.9 75.5
98.8 99.1 99.2 99.7 99.3 98.0 99.3 99.3 98.8 99.2
96.0 96.3 94.7 96.0 95.3 88.5 90.6 88.9 92.4 87.2
1420 1283 1237 911 1433 1403 1170 1443 1379 1090
a
A 4 mM solution in CH2 Cl2 . Styrene:catalyst ¼ 1455:1. P ¼ initial total pressure of CO/H2 (1/1). c Rc ¼ chemoselectivity towards aldehydes. d Rbr ¼ regioselectivity towards branched aldehyde. e Turnover number (TON) ¼ aldehydes fraction substrate:catalyst ratio. b
reaction rates [2]. For Rh-catalyzed hydroformylation with ligands possessing additional nitrogen donors, it has been noted also that, despite the absence of a Rh–N interaction, the existence of an additional coordination site as a stabilizing group during the course of a metalmediated reaction can improve the catalytic efficiency of the complexes [35,44]. Although complex 3 exhibits a higher activity compared to 4, the regioselectivity towards the branched aldehyde 6 observed with complex 3 was slightly lower than that observed with 4. Although mechanistic studies have not been performed yet and the reasons are not understood, this may perhaps be due to steric effects, since it is known that the regioselectivity of hydroformylation is governed by steric factors [8].
chloride hydrate according to a literature procedure [45– 47]. Syntheses of the complexes and catalysis were carried out under argon by using dry and degassed reagents and solvents. Hydroformylation was performed in a stainless steel autoclave (300 ml) with magnetic stirring. Syngas: CO/H2 (1/1). NMR: Bruker AC 300 (300.13 MHz, 75.47 MHz, and 121.50 MHz for 1 H, 13 C and 31 P, respectively); 1 H and 13 C NMR shifts were referenced to the solvent and the 31 P NMR shifts were referenced to external 85% H3 PO4 . The distinction of the CH, CH2 and CH3 carbons in the 13 C NMR spectra was performed by DEPT NMR experiments. ESI MS: Finnigan MAT TSQ 7000. GC–MS (EI): Varian Saturn 2000 with a 30 m 0.25 mm DB5-MS column. GC: Varian Star 3400 CX with a 30 m 0.53 mm DB5 column.
2.3. Conclusion In summary, we have synthesized two seven-membered chelate rhodium complexes with S-phosphinite ligands, one of which contains an amino group with the nitrogen situated away from the coordination sphere around the metal. The amino-substituted complex efficiently catalyzes the hydroformylation of styrene and exhibits a higher activity compared to the complex without the amino group, presumably due to the existence of nitrogen as a stabilizing group at an additional coordination site during the course of the metal-mediated reaction. However, the regioselectivity towards the branched aldehyde was slightly lower when the amino group was present on the aryl ring of the ligand, and this may perhaps be due to steric effects. 3. Experimental 3.1. General Ligands 1 and 2 were prepared as reported recently [38]. [Rh(COD)2 ]BF4 was prepared from rhodium tri-
3.2. Rhodium(1+)-[(1,2,5,6)-1,5-cyclooctadiene]-[2[(1-methylthio-S)-3[(diphenylphosphino-P)oxy]propyl]N,N-dimethyl-benzenamine]-tetrafluoroborate(1)) (3) A solution of the ligand 1 (0.1942 g, 0.475 mmol) in dichloromethane (15 ml) was added dropwise to the dark red solution of [Rh(COD)2 ]BF4 (0.1933 g, 0.476 mmol) in dichloromethane (5 ml) with a dry ice/acetone cooling bath. The reaction mixture was warmed slowly to room temperature within 1 h and stirred at this temperature for an additional 2 h. The resulting orange solution was evaporated under reduced pressure to a volume of 1 ml and addition of ether (20 ml) caused the precipitation of a solid. The solvents were decanted and the remaining solid was washed with ether (2 10 ml) and dried, yielding 3 (0.3009 g, 90%) as a mustard-yellow solid, m.p. (dec.) 132–140 °C. 1 H NMR (CD2 Cl2 ): d 8.06–7.99 (m, 2H, Ar), 7.76–7.67 (m, 3H, Ar), 7.44–7.39 (m, 4H, Ar), 7.28–7.06 (m, 5H, Ar), 5.68 (br m), 5.60– 5.47 (m), 5.15 (br m) and 5.02–4.89 (m) (4 1H, COD– CH), 4.79–4.73 (m, 1H, CHS), 4.05 and 3.78 (2 br m, 2 1H, CH2 O), 2.68–2.23 (m, 10H, CH2 and COD–
I.D. Kostas et al. / Inorganica Chimica Acta 357 (2004) 2850–2854
CH2 ), 2.18 (s, 6H, N(CH3 )2 ), 1.78 (s, 3H, SCH3 ). 13 C{1 H} NMR (CD2 Cl2 ): d 154.00–121.01 (Ar), 113.40 (dd, JRhC ¼ 11:6 Hz, JPC ¼ 5:2 Hz, COD–CH), 108.45– 108.25 (m, COD–CH), 86.99 (d, JRhC ¼ 10:7 Hz, COD– CH), 84.56 (d, JRhC ¼ 10:7 Hz, COD–CH), 71.61 (s, CH2 OP), 45.60 (s, N(CH3 )2 ), 44.47 (s, CHS), 34.78, 32.92, 31.92, 29.95 and 28.66 (s, CH2 and COD–CH2 ), 18.80 (s, SCH3 ). 31 P{1 H} NMR (CD2 Cl2 ): d 135.33 (d, JRhP ¼ 167:3 Hz). ESI MS: m=z 620 ([M–BF4 ]þ ). Anal. Calc. for C32 H40 BF4 NOPRhS (707.42): C, 54.33; H, 5.70; N, 1.98. Found: C, 53.68; H, 5.74; N, 1.56%. 3.3. Rhodium(1+)-[(1,2,5,6)-1,5-cyclooctadiene]-[[(1methylthio-S)-3-[(diphenylphosphino-P)oxy]propyl]benzene]-tetrafluoroborate(1)) (4) Treatment of [Rh(COD)2 ]BF4 (0.0499 g, 0.123 mmol) in dichloromethane (5 ml) with ligand 2 (0.0451 g, 0.123 mmol) in dichloromethane (10 ml), as described above for the synthesis of 3, yielded complex 4 (0.0725 g, 89%) as a yellow solid, m.p. (dec.) 147–156 °C. 1 H NMR (CDCl3 ): d 8.06–8.01 (m, 2H, Ar), 7.69 (br m, 3H, Ar), 7.43 (br m, 3H, Ar), 7.25 (br m, 5H, Ar, obscured with the signal of CH Cl3 ; clearly observed in CD2 Cl2 solution), 6.84 (br m, 2H, Ar), 5.76 (br m), 5.61–5.49 (m), 5.29 (br m) and 5.05–4.92 (m) (4 1H, COD–CH), 3.98 (br m, 1H, CHS), 3.74–3.71 (m, 2H, CH2 O), 2.77–2.13 (m, 10H, CH2 and COD–CH2 ), 1.92 (s, 3H, SCH3 ). 13 C{1 H} NMR (CDCl3 ): d 137.16–128.20 (Ar), 112.68–112.45 (m, COD–CH), 106.89–106.69 (m, COD–CH), 85.26 (d, JRhC ¼ 12:2 Hz, COD–CH), 82.65 (d, JRhC ¼ 12:2 Hz, COD–CH), 71.89 (s, CH2 OP), 50.57 (s, CHS), 32.58, 31.68, 31.16, 29.28 and 28.54 (s, CH2 and COD–CH2 ), 14.86 (s, SCH3 ). 31 P{1 H} NMR (CD2 Cl2 ): d 135.06 (d, JRhP ¼ 164:03 Hz). Anal. Calc. for C30 H35 BF4 OPRhS (664.35): C, 54.24; H, 5.31. Found: C, 53.74; H, 5.19%. 3.4. Hydroformylation of styrene In a typical experiment, styrene (2 ml, 17.456 mmol) and a 4 mM solution of rhodium complex 3 or 4 in dichloromethane (3 ml, 0.012 mmol) were placed under argon in an oven-dried autoclave, which was then closed, pressurized with syngas (CO/H2 ¼ 1:1) and brought to the appropriate temperature. After the required reaction time, the autoclave was cooled to room temperature, the pressure was carefully released and the solution was passed through Celite and analyzed by GC and GC–MS. Conversions were determined by GC. Acknowledgements The investigation was supported by the Greek General Secretariat of Research and Technology and the Russian Foundation for Basic Research.
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References [1] C.D. Frohning, C.W. Kohlpaintner, Hydroformylation, Chapter 2.1.1, in: B. Cornils, W.A. Herrmann (Eds.), Applied Homogeneous Catalysis with Organometallic Compounds, vol. 1, VCH, Weinheim, 1996. [2] P.W.N.M. van Leeuwen, C. Claver (Eds.), Rhodium Catalyzed Hydroformylation, Kluwer Academic Publishers, Dordrecht, 2000. [3] B. Breit, Acc. Chem. Res. 36 (2003) 264. [4] Survey of hydroformylation for 2002: F. Ungvary, Coord. Chem. Rev. 241 (2003) 295. [5] C.P. Casey, G.T. Whiteker, Isr. J. Chem. 30 (1990) 299. [6] L.A. van der Veen, M.D.K. Boele, F.R. Bregman, P.C.J. Kamer, P.W.N.M. van Leeuwen, K. Goubitz, J. Fraanje, H. Schenk, C. Bo, J. Am. Chem. Soc. 120 (1998) 11616. [7] L. Gonsalvi, H. Adams, G.J. Sunley, E. Ditzel, A. Haynes, J. Am. Chem. Soc. 124 (2002) 13597. [8] Z. Freixa, P.W.N.M. van Leeuwen, Dalton Trans. (2003) 1890. [9] R.P.J. Bronger, P.C.J. Kamer, P.W.N.M. van Leeuwen, Organometallics 22 (2003) 5358. [10] A. Bader, E. Lindner, Coord. Chem. Rev. 108 (1991) 27. [11] C.S. Slone, D.A. Weinberger, C.A. Mirkin, Prog. Inorg. Chem. 48 (1999) 233. [12] A. B€ orner, Eur. J. Inorg. Chem. (2001) 327. [13] P. Braunstein, F. Naud, Angew. Chem. Int. Ed. 40 (2001) 680. [14] K.N. Gavrilov, A.I. Polosukhin, Russ. Chem. Rev. 69 (2000) 661. [15] G. Helmchen, A. Pfaltz, Acc. Chem. Res. 33 (2000) 336. [16] F. Agbossou-Niedercorn, I. Suisse, Coord. Chem. Rev. 242 (2003) 145. [17] M.L. Clarke, A.M.Z. Slawin, M.V. Wheatley, J.D. Woollins, Dalton Trans. (2001) 3421. [18] J. Andrieu, P. Richard, J.-M. Camus, R. Poli, Inorg. Chem. 41 (2002) 3876. [19] G. Franci o, D. Drommi, C. Graiff, F. Faraone, A. Tiripicchio, Inorg. Chim. Acta 338 (2002) 59. [20] T. Bunlaksananusorn, K. Polborn, P. Knochel, Angew. Chem. Int. Ed. 42 (2003) 3941. [21] L. Barloy, G. Malaise, S. Ramdeehul, C. Newton, J.A. Osborn, N. Kyritsakas, Inorg. Chem. 42 (2003) 2902. [22] P. Braunstein, J. Zhang, R. Welter, Dalton Trans. (2003) 507. [23] C. Borriello, M.E. Cucciolito, A. Panunzi, F. Ruffo, Inorg. Chim. Acta 353 (2003) 238. [24] N.J. Hovestad, E.B. Eggeling, H.J. Heidb€ uchel, J.T.B.H. Jastrzebski, U. Kragl, W. Keim, D. Vogt, G. van Koten, Angew. Chem. Int. Ed. 38 (1999) 1655. [25] X. Bei, H.W. Turner, W.H. Weinberg, A.S. Guram, J. Org. Chem. 64 (1999) 6797. [26] R. Weber, U. Englert, B. Ganter, W. Keim, M. M€ othrath, Chem. Commun. (2000) 1419. [27] W.-M. Dai, K.K.Y. Yeung, J.-T. Liu, Y. Zhang, I.D. Williams, Org. Lett. 4 (2002) 1615. [28] J.C. Bay on, C. Claver, A.M. Masdeu-Bult o, Coord. Chem. Rev. 193–195 (1999) 73. [29] E. Hauptman, P.J. Fagan, W. Marshall, Organometallics 18 (1999) 2061. [30] O. Pamies, M. Dieguez, G. Net, A. Ruiz, C. Claver, Organometallics 19 (2000) 1488. [31] J.R. Dilworth, D. Morales, Y. Zheng, Dalton Trans. (2000) 3007. [32] D. Morales-Morales, R. Red on, Y. Zheng, J.R. Dilworth, Inorg. Chim. Acta 328 (2002) 39. [33] I.D. Kostas, C.G. Screttas, J. Organomet. Chem. 585 (1999) 1. [34] I.D. Kostas, J. Chem. Res. (S) (1999) 630. [35] I.D. Kostas, J. Organomet. Chem. 626 (2001) 221.
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Rhodium complexes possessing S-phosphinite ligands with or without an amino group: application to hydroformylation of styrene Ioannis D. Kostas a
a,*
, Barry R. Steele a, Fotini J. Andreadaki a, Vladimir A. Potapov
b
Institute of Organic and Pharmaceutical Chemistry, National Hellenic Research Foundation, 48 Vas. Constantinou Ave., 11635 Athens, Greece b Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences, 1 Favorsky Street, 664033 Irkutsk, Russia Received 4 December 2003; accepted 7 February 2004 Available online 27 February 2004
Abstract A chelate cationic rhodium(I) complex with a hemilabile amino- and sulfur-containing phosphinite ligand has been synthesized and, according to the NMR data, the ligand is bound to the metal in a P ; S-bidentate coordination mode without any Rh–N interaction. This complex efficiently catalyzes the hydroformylation of styrene. The chelate rhodium complex with the analogous ligand without the amino group has also been synthesized and examined as a catalyst for the same hydroformylation reaction. The reaction rate is higher using the former complex compared to the latter one without the amino group, with, however, a slightly lower regioselectivity towards the formation of the branched aldehyde. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Hemilabile ligands; Phosphinites; P ; S-ligands; Rhodium complexes; Hydroformylations; Homogeneous catalysis
1. Introduction Rhodium-catalyzed hydroformylation as an attractive synthetic transformation is a very well-established reaction with enormous academic and industrial interest, and represents one of the most important homogeneously catalyzed reactions world-wide [1–4]. The design and synthesis of new ligands with improved catalytic activity and selectivity towards hydroformylation is still a research subject of great significance. The stereoelectronic properties of the ligands are crucial for their catalytic activity and selectivity, and since the introduction of the concept of the ‘‘natural bite angle’’ for metal complexes with bidentate phosphines by Casey and Whiteker [5], the influence of the bite angle as well as steric and electronic effects on the hydroformylation has received much attention [6–9]. Ligands with mixed donors of different coordination abilities, in particular those which contain ‘‘soft’’ and ‘‘hard’’ donor atoms, so-called hemilabile ligands, have *
Corresponding author. Tel.: +30-210-7273878; fax: +30-2107273877. E-mail address: [email protected] (I.D. Kostas). 0020-1693/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2004.02.005
been of great interest due to the improved catalytic activity of their transition-metal complexes in certain homogeneously catalyzed reactions [10–13]. Particular attention has focused on phosphine ligands containing nitrogen [14–23] or oxygen [10,24–27]. The use of P ; Sligands in catalysis, on the other hand, is relatively unexplored compared to P ; N - and P ; O-ligands due to the fear of metal poisoning by sulfur [28–32]. We have previously reported several functionalized hemilabile P ; N -, P ; N ; P - and N ; P ; N -ligands and their application in rhodium-catalyzed hydroformylation, hydroaminomethylation and hydrogenation [33–37]. Recently, we reported a hemilabile amino- and sulfurcontaining phosphinite ligand (1) and the analogous sulfur-containing phosphinite (2) without the amino group, as catalyst precursors for the palladium-catalyzed Heck reaction of aryl bromides with styrene (Fig. 1) [38]. Treatment of PdCl2 (NCPh)2 with one equivalent of ligand 1 yielded the corresponding chelate palladium complex, in which the ligand is bound to the metal in a P ; S-bidentate coordination mode without any Pd–N interaction. Attempts to synthesize a monomeric chelate palladium complex with ligand 2 were unsuccessful and we found that the role of the amino
I.D. Kostas et al. / Inorganica Chimica Acta 357 (2004) 2850–2854
Fig. 1.
group in the formation of a P ; S-chelate complex as well as in the Heck reaction was decisive. Within our ongoing research, we wish to report here the coordination behavior of the two above-mentioned ligands towards rhodium. Rhodium complexes with both of these ligands were synthesized and applied to the hydroformylation of styrene, in order to investigate the influence of the amino group on the catalyst behavior of the complex in this reaction. Although phosphinites have been widely used in homogeneous catalysis [16,35,37–43], to our knowledge, sulfur-containing phosphinites have not been applied to rhodium-catalyzed hydroformylation until now.
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which the ligand is bound to rhodium via phosphorus and sulfur atoms, while nitrogen is situated away from the coordination sphere around the metal. Complex 4 displays similar behavior. Its 31 P NMR spectrum (d 135.06, d, JRhP ¼ 164:03 Hz) is essentially the same as that of complex 3, indicating an analogous chelation. It is clear, therefore, that the coordination ability of ligand 2 towards rhodium is in contrast to that observed towards palladium, in which the analogous chelation could not be achieved [38]. In the 1 H- and 13 C NMR spectra of 3 and 4, the olefinic COD resonances are non-equivalent. In complex 3, the olefinic COD resonances are split into four signals, two signals for the olefinic protons or carbons trans to the phosphorus atom (1 H NMR: 5.68, 5.60–5.47; 13 C NMR: 113.40, 108.45–108.25), and two signals for the olefinic protons or carbons trans to the sulfur atom (1 H NMR: 5.15, 5.02–4.89; 13 C NMR: 86.99, 84.56). The COD behavior in complex 4 is quite similar to that observed in complex 3 for the olefinic protons and carbons (1 H NMR: 5.76, 5.61–5.49, 5.29, 5.05–4.92; 13 C NMR: 112.68–112.45, 106.89–106.69, 85.26, 82.65).
2. Results and discussion
2.2. Hydroformylation
2.1. Synthesis and characterization of the rhodium complexes
The rhodium complexes 3 and 4 were applied to the hydroformylation of styrene using a high substrate:catalyst ratio of 1455:1 (Scheme 2, Table 1). The reaction was first catalyzed by complex 3 under variable conditions of pressure and temperature. The catalyst displays a high chemoselectivity for aldehydes (over 98%), and the variation observed in the regioselectivities towards the branched aldehyde 6 is as expected for hydroformylation of styrene using rhodium systems. At 100 bar total pressure of syngas, the conversion of styrene was almost quantitative after 22 h at 30 °C, with a 96.0% regioselectivity towards 6 (entry 1). Increasing the temperature dramatically increases the reaction rate but with a lower regioselectivity. On lowering the syngas pressure to 50 or 30 bar, the reaction rate as well as the regioselectivity is decreased. However, even under these mild reaction conditions, the activity of complex 3 is still acceptable. The catalytic activity of complex 4 was tested for a representative range of reaction conditions (entries 2, 4, 7). Under identical conditions, it displays lower TONs compared to those obtained for complex 3. The higher activity observed with the amino-substituted complex 3 compared to that observed with complex 4 is not surprising, since it is widely accepted that ligands containing electron-withdrawing substituents lead to higher
Treatment of [Rh(COD)2 ]BF4 in dichloromethane solution with one equivalent of the ligand 1 or 2 yielded the cationic rhodium complexes 3 or 4, respectively, in high yields (Scheme 1). The coordination mode in the complexes was determined by spectroscopic techniques. In the 31 P NMR spectrum, complex 3 shows a doublet at d 135.33 (JRhP ¼ 167:3 Hz) as a result of Rh–P interaction (dPðligand 1Þ 112:21Þ. The observation of the [1Rh(COD)]þ ion (m=z 620) and the absence of higher aggregates in the ESI MS spectrum suggest the monomeric nature of complex 3. In the 13 C NMR spectrum of 3, the SCH3 resonance is shifted to low field compared to that in the free ligand, in contrast to the N(CH3 )2 resonance which is the same in both the complex and the free ligand (SCH3 : dCðligand 1Þ 14.38, dCðcomplex 3Þ 18.80; N(CH3 )2 : dCðligand 1Þ 45.56, dCðcomplex 3Þ 45:60Þ. These data indicate the formation of a seven-membered chelate complex in
Scheme 1.
Scheme 2.
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Table 1 Hydroformylation of styrene catalyzed by rhodium complex 3 or 4 Entry
Catalysta
T (°C)
P b (bar)
Time (h)
Conversion (%)
Rc c (%)
Rbr d (%)
TONe
1 2 3 4 5 6 7 8 9 10
3 4 3 4 3 3 4 3 3 3
30 30 40 40 40 60 60 60 40 40
100 100 100 100 100 100 100 100 50 30
22 22 4 4 8 1 1 4 22 22
98.8 89.0 85.7 62.8 99.2 98.4 81.0 99.9 95.9 75.5
98.8 99.1 99.2 99.7 99.3 98.0 99.3 99.3 98.8 99.2
96.0 96.3 94.7 96.0 95.3 88.5 90.6 88.9 92.4 87.2
1420 1283 1237 911 1433 1403 1170 1443 1379 1090
a
A 4 mM solution in CH2 Cl2 . Styrene:catalyst ¼ 1455:1. P ¼ initial total pressure of CO/H2 (1/1). c Rc ¼ chemoselectivity towards aldehydes. d Rbr ¼ regioselectivity towards branched aldehyde. e Turnover number (TON) ¼ aldehydes fraction substrate:catalyst ratio. b
reaction rates [2]. For Rh-catalyzed hydroformylation with ligands possessing additional nitrogen donors, it has been noted also that, despite the absence of a Rh–N interaction, the existence of an additional coordination site as a stabilizing group during the course of a metalmediated reaction can improve the catalytic efficiency of the complexes [35,44]. Although complex 3 exhibits a higher activity compared to 4, the regioselectivity towards the branched aldehyde 6 observed with complex 3 was slightly lower than that observed with 4. Although mechanistic studies have not been performed yet and the reasons are not understood, this may perhaps be due to steric effects, since it is known that the regioselectivity of hydroformylation is governed by steric factors [8].
chloride hydrate according to a literature procedure [45– 47]. Syntheses of the complexes and catalysis were carried out under argon by using dry and degassed reagents and solvents. Hydroformylation was performed in a stainless steel autoclave (300 ml) with magnetic stirring. Syngas: CO/H2 (1/1). NMR: Bruker AC 300 (300.13 MHz, 75.47 MHz, and 121.50 MHz for 1 H, 13 C and 31 P, respectively); 1 H and 13 C NMR shifts were referenced to the solvent and the 31 P NMR shifts were referenced to external 85% H3 PO4 . The distinction of the CH, CH2 and CH3 carbons in the 13 C NMR spectra was performed by DEPT NMR experiments. ESI MS: Finnigan MAT TSQ 7000. GC–MS (EI): Varian Saturn 2000 with a 30 m 0.25 mm DB5-MS column. GC: Varian Star 3400 CX with a 30 m 0.53 mm DB5 column.
2.3. Conclusion In summary, we have synthesized two seven-membered chelate rhodium complexes with S-phosphinite ligands, one of which contains an amino group with the nitrogen situated away from the coordination sphere around the metal. The amino-substituted complex efficiently catalyzes the hydroformylation of styrene and exhibits a higher activity compared to the complex without the amino group, presumably due to the existence of nitrogen as a stabilizing group at an additional coordination site during the course of the metal-mediated reaction. However, the regioselectivity towards the branched aldehyde was slightly lower when the amino group was present on the aryl ring of the ligand, and this may perhaps be due to steric effects. 3. Experimental 3.1. General Ligands 1 and 2 were prepared as reported recently [38]. [Rh(COD)2 ]BF4 was prepared from rhodium tri-
3.2. Rhodium(1+)-[(1,2,5,6)-1,5-cyclooctadiene]-[2[(1-methylthio-S)-3[(diphenylphosphino-P)oxy]propyl]N,N-dimethyl-benzenamine]-tetrafluoroborate(1)) (3) A solution of the ligand 1 (0.1942 g, 0.475 mmol) in dichloromethane (15 ml) was added dropwise to the dark red solution of [Rh(COD)2 ]BF4 (0.1933 g, 0.476 mmol) in dichloromethane (5 ml) with a dry ice/acetone cooling bath. The reaction mixture was warmed slowly to room temperature within 1 h and stirred at this temperature for an additional 2 h. The resulting orange solution was evaporated under reduced pressure to a volume of 1 ml and addition of ether (20 ml) caused the precipitation of a solid. The solvents were decanted and the remaining solid was washed with ether (2 10 ml) and dried, yielding 3 (0.3009 g, 90%) as a mustard-yellow solid, m.p. (dec.) 132–140 °C. 1 H NMR (CD2 Cl2 ): d 8.06–7.99 (m, 2H, Ar), 7.76–7.67 (m, 3H, Ar), 7.44–7.39 (m, 4H, Ar), 7.28–7.06 (m, 5H, Ar), 5.68 (br m), 5.60– 5.47 (m), 5.15 (br m) and 5.02–4.89 (m) (4 1H, COD– CH), 4.79–4.73 (m, 1H, CHS), 4.05 and 3.78 (2 br m, 2 1H, CH2 O), 2.68–2.23 (m, 10H, CH2 and COD–
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CH2 ), 2.18 (s, 6H, N(CH3 )2 ), 1.78 (s, 3H, SCH3 ). 13 C{1 H} NMR (CD2 Cl2 ): d 154.00–121.01 (Ar), 113.40 (dd, JRhC ¼ 11:6 Hz, JPC ¼ 5:2 Hz, COD–CH), 108.45– 108.25 (m, COD–CH), 86.99 (d, JRhC ¼ 10:7 Hz, COD– CH), 84.56 (d, JRhC ¼ 10:7 Hz, COD–CH), 71.61 (s, CH2 OP), 45.60 (s, N(CH3 )2 ), 44.47 (s, CHS), 34.78, 32.92, 31.92, 29.95 and 28.66 (s, CH2 and COD–CH2 ), 18.80 (s, SCH3 ). 31 P{1 H} NMR (CD2 Cl2 ): d 135.33 (d, JRhP ¼ 167:3 Hz). ESI MS: m=z 620 ([M–BF4 ]þ ). Anal. Calc. for C32 H40 BF4 NOPRhS (707.42): C, 54.33; H, 5.70; N, 1.98. Found: C, 53.68; H, 5.74; N, 1.56%. 3.3. Rhodium(1+)-[(1,2,5,6)-1,5-cyclooctadiene]-[[(1methylthio-S)-3-[(diphenylphosphino-P)oxy]propyl]benzene]-tetrafluoroborate(1)) (4) Treatment of [Rh(COD)2 ]BF4 (0.0499 g, 0.123 mmol) in dichloromethane (5 ml) with ligand 2 (0.0451 g, 0.123 mmol) in dichloromethane (10 ml), as described above for the synthesis of 3, yielded complex 4 (0.0725 g, 89%) as a yellow solid, m.p. (dec.) 147–156 °C. 1 H NMR (CDCl3 ): d 8.06–8.01 (m, 2H, Ar), 7.69 (br m, 3H, Ar), 7.43 (br m, 3H, Ar), 7.25 (br m, 5H, Ar, obscured with the signal of CH Cl3 ; clearly observed in CD2 Cl2 solution), 6.84 (br m, 2H, Ar), 5.76 (br m), 5.61–5.49 (m), 5.29 (br m) and 5.05–4.92 (m) (4 1H, COD–CH), 3.98 (br m, 1H, CHS), 3.74–3.71 (m, 2H, CH2 O), 2.77–2.13 (m, 10H, CH2 and COD–CH2 ), 1.92 (s, 3H, SCH3 ). 13 C{1 H} NMR (CDCl3 ): d 137.16–128.20 (Ar), 112.68–112.45 (m, COD–CH), 106.89–106.69 (m, COD–CH), 85.26 (d, JRhC ¼ 12:2 Hz, COD–CH), 82.65 (d, JRhC ¼ 12:2 Hz, COD–CH), 71.89 (s, CH2 OP), 50.57 (s, CHS), 32.58, 31.68, 31.16, 29.28 and 28.54 (s, CH2 and COD–CH2 ), 14.86 (s, SCH3 ). 31 P{1 H} NMR (CD2 Cl2 ): d 135.06 (d, JRhP ¼ 164:03 Hz). Anal. Calc. for C30 H35 BF4 OPRhS (664.35): C, 54.24; H, 5.31. Found: C, 53.74; H, 5.19%. 3.4. Hydroformylation of styrene In a typical experiment, styrene (2 ml, 17.456 mmol) and a 4 mM solution of rhodium complex 3 or 4 in dichloromethane (3 ml, 0.012 mmol) were placed under argon in an oven-dried autoclave, which was then closed, pressurized with syngas (CO/H2 ¼ 1:1) and brought to the appropriate temperature. After the required reaction time, the autoclave was cooled to room temperature, the pressure was carefully released and the solution was passed through Celite and analyzed by GC and GC–MS. Conversions were determined by GC. Acknowledgements The investigation was supported by the Greek General Secretariat of Research and Technology and the Russian Foundation for Basic Research.
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