Organometallics 2000, 19, 2065-2072
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New Chiral Phosphine-Phosphite Ligands in the Enantioselective Rhodium-Catalyzed Hydroformylation of Styrene Sirik Deerenberg, Paul C. J. Kamer, and Piet W. N. M. van Leeuwen* Institute for Molecular Chemistry, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands Received September 28, 1999
A series of chiral phosphine-phosphite ligands 1a-g have been synthesized from monophosphines 2-4, enantiomerically pure propene oxide or styrene oxide, and 3,3′,5,5′tetra(tert-butyl)-2,2′-bisphenol phosphorochloridite or enantiomerically pure 3,3′-bis(trimethylsilyl)-2,2′-binaphthol phosphorochloridites. These phosphine-phosphites have been used in the rhodium-catalyzed asymmetric hydroformylation of styrene. The structures of the active catalysts, [HRh(L-L)(CO)2] complexes (L-L ) ligands 1a-g), have been studied using high-pressure NMR and IR spectroscopy. The obtained spectroscopic data show that the ligands coordinate in an equatorial-apical fashion to the rhodium center with the phosphine in apical position. Systematic variation in configuration of the stereocenters at both the ligand bridge and the phosphine moiety revealed a remarkable cooperative effect on the selectivity of the hydroformylation reaction. Under mild reaction conditions ee's of 63% and regioselectivities up to 92% toward 2-phenylpropanal were obtained (25-60 °C, 20 bar of syn gas CO:H2 [1:1]) for ligands 1. The absolute configuration of the product is governed by the stereogenic center of the backbone of the ligand. There is a large cooperative effect, however, from the phosphine moiety. Spectroscopic data, in combination with the obtained results in catalysis, suggest that phosphine-phosphite ligands (L-L) containing the conformationally flexible and axially chiral biphenyl moiety exist predominantly as single atropisomers in the HRh(L-L)(CO)2 complexes. Comparison of the bisphenol and binaphthol substituents suggests that the high enantiomeric excesses obtained with the former are caused by the preferential formation of the most selective diastereomer. Introduction Hydroformylation is an important and thus extensively studied industrial process.1-4 Improvement of rates and selectivities and mechanistic aspects receive a great deal of attention.5,6 Asymmetric hydroformylation is a potentially powerful synthetic tool for the synthesis of several chiral building blocks that can be used as precursors for high-value-added compounds such as pharmaceuticals, agrochemicals, flavors, and fragrances.7-11 (1) Pruett, R. L. Adv. Organomet. Chem. 1979, 17, 1. (2) Tolman, C. A., Faller: York and London, 1983; Chapter 2, pp 88-89. (3) Beller, M.; Cornils, B.; Frohning, C. D.; Kohlpaintner, C. W. J. Mol. Catal. A 1995, 104, 17-85. (4) Frohning, C. D.; Hohlpaintner, C. W. In Applied Homogeneous Catalysis with Organometallic Compounds; Cornils, B., Herrmann, W. A., Eds.; Springer-Verlag: New York, 1996; pp 29-90. (5) van Leeuwen, P. W. N. M.; van Santen, R. A. In Surface Science and Catalysis; Moulijn, J. A., Ed.; 1993; Vol. 79. (6) Horiuchi, T.; Shirakawa, E.; Nozaki, K.; Takaya, H. Organometallics 1997, 16, 2981-2986. (7) Gladiali, S.; Bayon, J. C.; Claver, C. Tetrahedron: Asymmetry 1995, 6, 1453-1474. (8) Agbossou, F.; Carpentier, J. F.; Mortreaux, A. Chem. Rev. 1995, 95, 2485-2506. (9) Rieu, J.-P.; Boucherle, A.; Cousse, H.; Mouzin, G. Tetrahedron 1986, 42, 4095. (10) Botteghi, C.; Paganelli, S.; Schionato, A.; Marchetti, M. Chirality 1991, 3, 355. (11) Botteghi, C.; Delponte, G.; Marchetti, M.; Paganelli, S. J. Mol. Catal. 1994, 93, 1-21.
In general, chiral diphosphine-Rh(I) complexes show high catalytic activity and chemoselectivity. The enantiomeric excesses obtained so far, however, do not exceed 60%.12 Chiral diphosphite-Rh(I) complexes show high enantioselectivities,13,14 but the scope is limited. Takaya et al. have reported a hydroformylation catalyst precursor based on [Rh(acac)(CO)2] modified with a chiral phosphine-phosphite ligand, BINAPHOS,15,16 which combines the advantages of both ligand types; high ee’s were obtained in the hydroformylation of styrene and other olefins, combined with excellent chemo- and regioselectivity.17
(12) Gladiali, S.; Pinna, L. Tetrahedron: Asymmetry 1990, 1, 693. (13) Babin, J. E.; Whiteker, G. T. WO 93/03839, U.S. Pat. US 911,518, 1992. (14) Buisman, G. J. H.; van der Veen, L. A.; Klootwijk, A.; de Lange, W. G. J.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Vogt, D. Organometallics 1997, 16, 2929-2939. (15) Sakai, N.; Mano, S.; Nozaki, K.; Takaya, H. J. Am. Chem. Soc. 1993, 115, 7033.
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Most of the research published to date is dedicated to ligands having stereogenic elements attached to the donor atom(s). Ligands having a stereogenic donor atom, for instance a stereogenic phosphine, are rare, and DIPAMP is one of the few examples.18 Juge´ et al. have developed a method for the synthesis of enantiomerically pure, borane-protected stereogenic phosphines of the type P(Ph)(R)(Me)(BH3).19 It is a useful chiral building block, which can be applied in the synthesis of chiral ligands containing stereogenic P atoms. Following this method, Mezzetti et al. synthesized (S,S)-Me2Si(CH2P(1-Np)(Ph))2, which gave ee’s up to 97.7% in the catalytic hydrogenation of dehydroamino acids.20 Although high enantioselectivities have been obtained, catalytic applications of bidentate ligands containing stereogenic phosphorus atoms are still rare.21-31 Previous work in our group and in the literature shows the possibility of chiral cooperativity between stereocenters in a ligand. For instance, diphosphite ligands containing biaryl moieties with bulky substituents show hindered rotation around the biaryl axis.14,17 Although the additional stereocenter originating from the atropisomeric biaryl substituents may lead to several diastereomers, high enantioselectivities are obtained in the hydroformylation reaction. It was found that the low-energy barrier for interconversion in atropisomers resulted in the formation of a single diastereomeric HRh(CO)2(diphosphite) complex. This complex also gives rise to high enantioselectivities. Herrmann et al. have published molecular modeling studies of the BINAPHOS-rhodium complex,32 but the mechanistic aspects of the asymmetric hydroformylation reaction are still not understood well enough to be used for the prediction of the enantioselectivities. Encouraged by the success of BINAPHOS, the success of ligands containing stereogenic phosphorus atoms, and the cooperative effect found in bulky phosphite ligands, (16) Horiuchi, T.; Ohta, T.; Shirakawa, E.; Nozaki, K.; Takaya, H. J. Org. Chem. 1997, 62, 4285-4292. (17) Nozaki, K.; Sakai, N.; Nanno, T.; Higashijima, T.; Mano, S.; Horiuchi, T.; Takaya, H. J. Am. Chem. Soc. 1997, 119, 4413-4423. (18) Brunner, H.; Zettlmeier, W. Handbook of Enantioselective Catalysis; VCH: Weinheim, Germany, 1993. (19) Juge´, S.; Ste´phan, M.; Laffitte, J. A.; Genet, J. P. Tetrahedron Lett. 1990, 31, 6357. (20) Stoop, R. M.; Mezzetti, A.; Spindler, F. Organometallics 1998, 17, 668. (21) Nettekoven, U.; Widhalm, M.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. Tetrahedron: Asymmetry 1997, 8, 3185-3188. (22) Geneˆt, J. P.; Pinel, C.; Ratovelomanana-Vidal, V.; Mallart, S.; Pfister, X.; Can˜o De Andrada, M. C.; Laffite, J. A. Tetrahedron: Asymmetry 1994, 5, 665. (23) Geneˆt, J. P.; Pinel, C.; Ratovelomanana-Vidal, V.; Mallart, S.; Pfister, X.; Bischoff, L.; Can˜o De Andrada, M. C.; Darses, S.; Galopin, C.; Laffite, J. A. Tetrahedron: Asymmetry 1994, 5, 675. (24) Geneˆt, J. P.; Juge´, S.; Laffite, J. A.; Pinel, C.; Mallart, S. Chem. Abstr. 1994, 121, P156763d. (25) Nettekoven, U.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Widhalm, M.; Spek, A. L.; Lutz, M. J. Org. Chem. 1999, 64, 39964004. (26) Yamanoi, Y.; Imamoto, T. Rev. Heteroatom. Chem. 1999, 20, 227. (27) Yamanoi, Y.; Imamoto, T. J. Org. Chem. 1999, 64, 2988. (28) Ohff, M.; Holz, J.; Quirmbach, M.; Bo¨rner, A. Synthesis 1998, 1391. (29) Imamoto, T.; Watanabe, J.; Wada, Y.; Masuda, H.; Yamada, H.; Tsurata, H.; Matsukawa, S.; Yamaguchi, K. J. Am. Chem. Soc. 1998, 120, 1635. (30) Miura, T.; Imamoto, T. Tetrahedron. Lett. 1999, 40, 4833. (31) Maienza, F.; Worle, M.; Steffanut, P.; Mezzetti, A.; Spindler, F. Organometallics 1998, 18, 1041. (32) Gleich, D.; Schmid, R.; Herrmann, W. A. Organometallics 1998, 17, 2741.
Deerenberg et al.
we designed a series of chiral phosphine-phosphite ligands that allowed a systematic approach to study the asymmetric induction of a ligand in the hydroformylation reaction.
In this paper we describe the synthesis of novel chiral phosphine-phosphite ligands. The designed ligands consist of a phosphine moiety with a stereogenic phosphorus atom. The configuration of the phosphorus atom is expected to be important for the course of the catalytic reaction, since it is in close vicinity of the metal center. Furthermore, the ligands contain a bridge having a stereocenter and a bulky phosphite moiety. The stereogenic phosphorus moiety, the stereocenter in the backbone, and the configuration of the biaryl moiety can result in several diastereomers. Systematic variation of the stereocenters provides information about the effect of the different stereocenters on the enantioselectivity. The presence of bulky substituents on aromatic biphenyl positions has already shown to have a significant effect on the catalyst performance.13,14,33-38 Having two different donor atoms, the ligand is anticipated to coordinate as follows: the phosphine, the better σ-donor, will coordinate in an apical position, and the phosphite, the better π-acceptor, will coordinate in an equatorial position.39 This was confirmed by high-pressure NMR and IR studies of RhH(CO)2(phosphine-phosphite) complexes under catalytic conditions.40-45 Results and Discussion Ligand Synthesis. Ligands 1a-e were synthesized in three steps starting from the corresponding monophosphines 2-4 (Scheme 1). Methyldiphenylphosphine was protected by reaction with BH3‚SMe2 to obtain (33) Buisman, G. J. H.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. Tetrahedron: Asymmetry 1993, 4, 1625. (34) van Leeuwen, P. W. N. M.; Buisman, G. J. H.; van Rooy, A.; Kamer, P. C. J. Recl. Trav. Chim. Pays-Bas 1994, 113, 61. (35) Buisman, G. J. H.; Vos, E. J.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. J. Chem. Soc., Dalton Trans. 1995, 409-417. (36) van Rooy, A.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Veldman, N.; Spek, A. L. J. Organomet. Chem. 1995, 494, C15-C18. (37) Buisman, G. J. H.; Martin, M. E.; Vos, E. J.; Klootwijk, A.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. Tetrahedron: Asymmetry 1995, 6, 719-738. (38) Buisman, G. J. H.; van der Veen, L. A.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. Organometallics 1997, 16, 5681-5687. (39) Unruh, J. D.; Christenson, J. R. J. Mol. Catal. 1982, 14, 19. (40) Elsevier, C. J. J. Mol. Catal. 1994, 92, 285. (41) Roe, D. C. J. J. Magn. Reson. 1985, 63, 388. (42) Horva´th, I. Organometallics 1986, 5, 2333. (43) Brown, J. M.; Kent, A. G. J. Chem. Soc., Perkin Trans. 2 1987, 1597. (44) Castellanos Pa´ez, A.; Castillo´n, S.; Claver, C.; van Leeuwen, P. W. N. M.; de Lange, W. G. J. Organometallics 1998, 17, 2543-2552. (45) van Rooy, A.; Burgers, D.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. Recl. Trav. Chim. Pays-Bas 1996, 115, 492-498.
New Chiral Phosphine-Phosphite Ligands Scheme 1
monophosphine 2 as a crystalline product.46-49 The boronato group was used both to avoid oxidation of the phosphine and to activate the methylphosphine toward lithiation.50 Stereogenic monophosphines 3 and 4 were synthesized as enantiomerically pure, crystalline products using the methodology of Juge´19 and Mezzetti,20 respectively. After metalation of these phosphines, 2-4, using sec-BuLi in THF at -40 °C, they reacted with enantiomerically pure propene oxide or styrene oxide to yield phosphino alcohols 5a-e.46-49 Phosphino alcohols 5a-e were coupled with bisphenol phosphorochloridite in the presence of triethylamine.33,51 Filtration over a short silica column, to remove hydrolyzed phosphorochloridite, gave pure phosphine-phosphites 6a-e as colorless oils. Compounds 2-6 show broad signals in the 31P NMR indicative of a phosphine-borane complex.21,52,53 Phosphine-phosphite borane adducts 6 were immediately deprotected overnight at 50 °C using diethylamine. After filtration, the phosphine-phosphites 1 were obtained in moderate to good yields (3795%). From 1H NMR and 31P NMR we concluded that in none of the reactions were the stereocenters in the ligands affected, since no formation of diastereomers was observed. The existence of a single peak for each phosphorus atom of 1a-e in the 31P NMR suggests the rapid isomerization in the biphenyl moiety, which results in an averaged configuration on the NMR time scale.14,17 In none of the preceding reactions is the configuration at the P atom affected. As borane decomplexation reactions are known to occur with retention of configuration at the phosphorus atom,19,54,55 the absolute configuration at the phosphorus atom is assigned as S. To study the influence of the biaryl moiety on the catalyst, bulky binaphthol-based ligands 1f,g were (46) Imamoto, T.; Oshiki, T.; Onozawa, T.; Kusumoto, T.; Sato, K. J. Am. Chem. Soc. 1990, 112, 5244. (47) Imamoto, T. Pure Appl. Chem. 1993, 65, 655. (48) Pellon, P. Tetrahedron Lett. 1992, 33, 4451. (49) Imamoto, T.; Kusumoto, T.; Suzuki, N.; Sato, K. J. Am. Chem. Soc. 1985, 107, 5301. (50) Schmidbauer, H. J. J. Organomet. Chem. 1980, 200, 287. (51) Jongsma, T. Thesis, Polymer-bound rhodium hydroformylation catalysts, University of Groningen, 1992. (52) Wolfe, B.; Livinghouse, T. J. Am. Chem. Soc. 1998, 120, 5116. (53) Uziel, J.; Ste´phan, M.; Kaloen, E.-B.; Geneˆt, J.-P.; Juge´, S. Bull. Soc. Chim. Fr. 1997, 134, 379. (54) Juge´, S.; Merde`s, R.; Ste´phan, M.; Genet, J. P. Phosphorus, Sulfur Silicon Relat. Elem. 1993, 77, 199. (55) Juge´, S.; Ste´phan, M.; Merde`s, R.; Genet, J. P.; Halut-Desportes, D. J. Chem. Soc., Chem. Commun. 1993, 531.
Organometallics, Vol. 19, No. 11, 2000 2067 Scheme 2
prepared (Scheme 2). The interconversion around the binaphthyl bond is energetically highly unfavorable, and stable diastereomeric phosphine-phosphite ligands have been obtained in diastereomerically pure form. Ligands 1f and 1g were synthesized in a similar way as described for ligands 1a-e. A literature method was applied for the synthesis of enantiomerically pure (R)and (S)-3,3′-bis(trimethylsilyl)-2,2′-binaphthol phosphorochloridites.14 Reaction with phosphino alcohol 5a and decomplexation of the BH3 group gave phosphinephosphites 1f and 1g as white solids. [RhH(CO)2(L-L)] Complexes. The coordination mode of the ligand in the active complex has been studied using IR and NMR under 20 bar of syn gas.34-37,44,56,57 Hydridorhodium phosphine-phosphite dicarbonyl complexes [RhH(CO)2(L-L)] were prepared from Rh(acac)(CO)2 and ligands 1. The spectroscopic data are shown in Table 1. The IR spectrum of RhH(CO)2(1a) in cyclohexane shows absorptions at 2021 (medium) and 1974 (strong) cm-1 due to a symmetric and an antisymmetric stretch vibration of two equatorially coordinated CO ligands. The Rh-H vibration was not observed in the spectrum. Upon deuteration, there is no shift observed in the CO absorptions.44,58,59 This is characteristic of a coordination fashion in which the hydride and both CO ligands of RhH(CO)2(L-L) are orientated cis to one another. Similar data were observed for ligands 1b-g. The 31P NMR spectrum of RhH(CO)2(1a) shows signals at 6.91 and 163.89 ppm attributed to the phosphine and the phosphite, respectively. For the Rh-H coupling constant a value of 9 Hz is found, whereas J{P1-H} and J{P2-H} are found to be 102 and
