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Organometallics 2011, 30, 974–985 DOI: 10.1021/om101023e
Cage Phosphinites: Ligands for Efficient Nickel-Catalyzed Hydrocyanation of 3-Pentenenitrile Igor S. Mikhel,† Michael Garland,† Jonathan Hopewell,† Sergio Mastroianni,‡ Claire L. McMullin,† A. Guy Orpen,† and Paul G. Pringle*,† †
School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TS, U.K., and ‡Rhodia, Centre de Recherches et Technologies de Lyon, 85 Rue des Fr eres Perrets, 69192 Saint-Fons Cedex, France Received October 29, 2010
The cage monophosphinites CgPOR {where CgP=6-phospha-2,4,8-trioxa-adamantane and R= C6H5 (La); 2-C6H4CH3 (Lb); 2,4,6-C6H2(CH3)3 (Lc); 2,4-C6H3tBu2 (Ld); CH3 (Le); CH2CF3 (Lf)} and diphosphinites CgPZPCg {where ZH2=2,20 -biphenol (Lg) or 1,2-benzenedimethanol (Lh)} have been made from CgPBr and the corresponding alcohol or phenol. The cage phosphinites are remarkably stable to water. All the ligands La-h have been tested for nickel(0)-catalyzed hydrocyanation of 3-pentenenitrile in the presence of Lewis acids (ZnCl2, Ph2BOBPh2, or iBu2AlOAliBu2), and tentative structure-activity relationships are suggested. The hydrocyanation activities obtained with catalysts derived from monophosphinite Lf (with iBu2AlOAliBu2) and diphosphinite Lh (with ZnCl2) are comparable with the commercial catalyst based on P(OTol)3. The complexes trans-[PtCl2(L)2] where L=La (1a), Le (1e), and Lf (1f) and the chelate cis-[PtCl2(Lh)] (1h) are reported. From the νCO values for the complexes trans-[RhCl(CO)(La-f)2] (2a-f), it is concluded that ligand Lf is the most phosphite-like of the monophosphinites. Treatment of [Ni(cod)2] (cod = 1,5-cyclo-octadiene) with Lh leads to a mixture of products, one of which was characterized as the binuclear [Ni2(Lh)2(μ-cod)] (3h). The crystal structures of Lh, 1a, 1e, 1f, 1h 3 2CH2Cl2, and 3h 3 3C6H5CH3 are reported. Introduction Phosphite ligands, P(OR)3, have been at the center of developments in coordination chemistry and homogeneous catalysis for over 50 years.1 The π-acceptor capacity of phosphites makes them good ligands for metals in low oxidation states2 and can increase the electrophilicity of metal centers.3 Tolman’s landmark article4 on the systematization of ligand stereoelectronic effects was driven by a need to understand the ligand effects in DuPont’s nickelphosphite-catalyzed hydrocyanation of butadiene to adiponitrile (ADN) (summarized in Scheme 1), which remains one *To whom correspondence should be addressed. E-mail: paul.
[email protected]. (1) (a) Hartwig, J. F. Organotransition Metal Chemistry; University Science Books: Sausalito, CA, USA, 2010. (b) van Leeuwen, P. W. N. M. Homogeneous Catalysis: Understanding the Art; Springer-Verlag: New York, 2004. (c) B€ orner, A. Phosphorus Ligands in Asymmetric Catalysis; Wiley-VCH: Weinheim, Germany, 2008. (2) (a) Dias, P. B.; Minas de Piedade, M. E.; Martinho Sim~ oes, J. A. Coord. Chem. Rev. 1994, 135-136, 737. (b) Baker, M. J.; Harrison, K. N.; Orpen, A. G.; Pringle, P. G.; Shaw, G. J. Chem. Soc., Dalton Trans. 1992, 2607. (c) Crispini, A; Harrison, K. N.; Orpen, A. G.; Pringle, P. G.; Wheatcroft, J. R. J. Chem. Soc., Dalton Trans. 1996, 1069, and references therein. (3) Yamakawa, T.; Fujita, T.; Shinoda, S. Chem. Lett. 1992, 21, 905. (4) Tolman, C. A. Chem. Rev. 1977, 77, 313. (5) Krill, S. In Applied Homogeneous Catalysis with Organometallic Compounds, 2nd ed.; Cornils, B., Ed.; Wiley-VCH: Weinheim, Germany, 2002. (6) Tolman, C. A. J. Chem. Educ. 1986, 63, 199. (7) (a) Chia, Y.; Drinkard, W. C.; Squire, E. N. (Du Pont) U.S. Patent 3766237, 1973. (b) Tolman, C. A.; McKinney, R. J.; Seidel, W. C.; Druline, J. D.; Stevens, W. R. Adv. Catal. 1985, 33, 1, and references therein. pubs.acs.org/Organometallics
Published on Web 02/10/2011
of the most successful industrial applications of homogeneous catalysis.5,6 Tritolylphosphite (I) was the industrial ligand of choice6,7 for this process before the emergence in the 1990s of the generation of catalysts based on bulky diphosphites such as II (Chart 1).8 The great majority of academic studies on butadiene hydrocyanation have concerned the conversion of butadiene to 3-pentenenitrile (3PN), the first step in Scheme 1, although the hydrocyanation (8) (a) Tam, W.; Kreutzer, K. A.; McKinney, R. J. (DuPont) World Patent 14659, 1995. (b) Baker, M. J.; Harrison, K. N.; Orpen, A. G.; Pringle, P. G.; Shaw, G. J. Chem. Soc., Chem. Commun. 1991, 803. (9) Vallee, C.; Chauvin, Y.; Basset, J.-M.; Santini,a, C. C.; Galland, J.-C. Adv. Synth. Catal. 2005, 347, 1835–1847. (10) (a) Birkholz, M. N.; Freixa, Z.; van Leeuwen, P. W. N. M. Chem. Soc. Rev. 2009, 38, 1099. (b) Gillespie, J. A.; Dodds, D. L.; Kamer, P. C. J. Dalton Trans. 2010, 39, 2751, and references therein. Birkholz, M. N.; Freixa, Z.; van Leeuwen, P. W. N. M. Chem. Soc. Rev. 2009, 38, 1099. (11) Bini, L.; M€ uller, C.; Wilting, J.; von Chrzanowski, L.; Spek, A. L.; Vogt, D. J. Am. Chem. Soc. 2007, 129, 12622. (12) van der Vlugt, J. I.; Hewat, A. C.; Neto, S.; Sablong, R.; Mills, A. M.; Lutz, M.; Spek, A. L.; M€ uller, C.; Vogt, D. Adv. Synth. Catal. 2004, 346, 993. (13) G€ othlich, A. P. V.; Tensfeldt, M.; Rothfuss, H.; Tauchert, M. E.; Haap, D.; Rominger, F.; Hofmann, P. Organometallics 2008, 27, 2189. (14) Some representative examples: (a) Saha, B.; RajanBabu, T. V. Org. Lett. 2006, 8, 4657. (b) Wilting, J.; Janssen, M.; M€uller, C.; Vogt, D. J. Am. Chem. Soc. 2006, 128, 11374. (c) Horiuchi, T.; Shirakawa, E.; Nozaki, K.; Takaya, H. Tetrahedron: Asymmetry 1997, 8, 57. (d) RajanBabu, T. V.; Casalnuovo, A. L. J. Am. Chem. Soc. 1996, 118, 6325. (e) Baker, M. J.; Pringle, P. G. J. Chem. Soc., Chem. Commun. 1991, 1292. (f) Elmes, P. S.; Jackson, W. R. Aust. J. Chem. 1982, 35, 2041. (g) Wilting, J.; Janssen, M.; M€uller, C.; Lutz, M.; Spek, A. L.; Vogt, D. Adv. Synth. Catal. 2007, 349, 350. (h) de Greef, M.; Breit, B. Angew. Chem., Int. Ed. 2009, 48, 551. (i) Goertz, W.; Kamer, P. C. J.; Van Leeuwen, P. W. N. M.; Vogt, D. Chem.; Eur. J. 2001, 7, 1614. (j) Casalnuovo, A. L.; RajanBabu, T. V.; Ayers, T. A.; Warren, T. H. J. Am. Chem. Soc. 1994, 116, 9869. r 2011 American Chemical Society
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Organometallics, Vol. 30, No. 5, 2011 Scheme 1
of 3-PN to ADN is a more challenging reaction, requiring isomerization to 4-pentenenitrile (4-PN) followed by hydrocyanation. 9 Rigid-backboned diphosphines10,11 such as III and diphosphonites12,13 such as IV have been shown to be effective ligands for the hydrocyanation of butadiene to 3-PN. The hydrocyanation of other alkenes (particularly asymmetric hydrocyanation) has been a topic of continuing interest,14 and all of these processes use the same classes of phosphorus ligands that are used for butadiene hydrocyanation. The rational design of improved ligands for hydrocyanation has been an enduring goal.10,15,16 A significant limitation for the application of phosphites (and other P-Ocontaining ligands) in hydrocyanation and other catalyses is their susceptibility to hydrolysis, and this has led to research to design phosphines, with kinetically inert P-C bonds, which could mimic phosphites.10,17 Tertiary phosphines based on the phospha-adamantane cage (denoted CgPR in Chart 2) have been shown to be kinetically inert and stereoelectronically similar to a bulky phosphonite (RO)2PR.18,19 Moreover, ligands incorporating the CgP moiety have proved to be effective in many catalytic reactions.20 We reasoned that cage phosphinites of the type CgPOR (Chart 2) should have greater π-acceptor capacity than CgPR and show here that this expectation is realized and that air-stable, monodentate and bidentate cage phosphinite ligands are effective for nickel(0)-catalyzed hydrocyanation of 3-pentenenitrile to ADN. Some of this work has formed part of a patent.21
Results and Discussion Ligand Synthesis. The two-step routes shown in Scheme 2 gave generally good yields (>60% overall) of cage monophosphinites La-f and diphosphinites Lg,h. The new ligands (15) Maldonado, A. G.; Hageman, J. A.; Mastroianni, S.; Rothenberg, G. Adv. Synth. Catal. 2009, 351, 387. (16) (a) Tauchert, M. E.; Kaiser, T. R.; Goethlich, A. P. V.; Rominger, F.; Warth, D. C. M.; Hofmann, P. ChemCatChem 2010, 2, 674. (b) Bini, L.; Muller, C.; Vogt, D. Chem. Commun. 2010, 46, 8325. (17) Clarke, M. L.; Ellis, D.; Mason, K. L.; Orpen, A. G.; Pringle, P. G.; Wingad, R. L.; Zaher, D.; A. Baker, R. T. Dalton Trans. 2005, 1294. (18) Baber, R. A.; Clarke, M. L.; Heslop, K. M.; Marr, A. C.; Orpen, A. G.; Pringle, P. G.; Ward, A.; Zambrano-Williams, D. E. Dalton Trans. 2005, 1079. (19) Downing, J. H.; Floure, J.; Heslop, K.; Haddow, M. F.; Hopewell, J.; Lusi, M.; Phetmung, H.; Orpen, A. G.; Pringle, P. G.; Pugh, R. I.; Zambrano-Williams, D. Organometallics 2008, 27, 3216. (20) Hopewell, J.; Jankowski, P.; McMullin, C. L.; Orpen, A. G.; Pringle, P. G. Chem. Commun 2010, 46, 100, and references therein. (21) Mastroianni, S.; Mikhel, I.; Pringle, P. (Rhodia) World Patent WO 2010/102962, 2010.
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Chart 1
Chart 2
were purified by flash chromatography and have been characterized by a combination of high-resolution mass spectrometry and 31P, 13C, 1H, and 19F NMR spectroscopy (see Experimental Section for the data). The 31P NMR spectrum of the o-xylene-linked diphosphinite Lh showed two singlets in a 1:1 ratio consistent with the presence of rac and meso diastereoisomers, associated with the R- and β-enantiomeric forms of the CgP moiety.20 The 31P NMR spectrum in CD2Cl2 of a recrystallized sample of the 2,20 -biphenyl-linked diphosphinite Lg at 25 °C showed two singlets at δ 83.3 and 80.5 in a ratio of ca. 22:1, corresponding to the rac and meso isomers. However at þ20 °C, a single broad resonance (w1/2 ca. 100 Hz) at δ 82.7 was observed that resolved at -60 °C into four singlets at δ 89.8, 83.8, 82.4, and 78.6 in the approximate ratio of 35:1:10:1, respectively. This is consistent with the presence of three diastereoisomers, resulting from arrested rotation about the biphenyl C-C bond, introducing an extra element of chirality (labeled R and S for the biaxial dissymmetry) and giving rise to two singlets (δ 89.8 and 82.4) for the two C2-symmetric isomers (RRR/βSβ and RSR/βRβ forms) and two singlets (δ 83.8 and
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Scheme 2a
Figure 1. Thermal ellipsoid plot of Lh. Displacement ellipsoids are shown at the 50% probability level, and all hydrogen atoms have been removed for clarity. Selected bond lengths (A˚) and angles (deg): P-C2, 1.8656(12); P-C9, 1.8720(12); P-O4, 1.6516(8); C2-P-C9, 93.03(5).
Conditions: (i) Br2 in CH2Cl2 at 0 °C. (ii) ROH þ BuLi in THF at 0 °C. (iii) Diol (2,20 -biphenol or 1,2-benzenedimethanol) þ 2 BuLi in THF at 0 °C. a
78.6, with 7JPP not resolved) for the inequivalent P atoms in the C1-symmetric isomer (RRβ/RSβ form). Phosphinites are reputed to be sensitive to hydrolysis22 (eq 1), and indeed we found that a sample of Ph2POMe stirred in 2% aqueous MeCN at ambient temperature was 50% hydrolyzed to Ph2P(O)H after 55 min. By contrast, under similar conditions, the CgPOMe showed no detectable signs (by 31P NMR) of hydrolysis after 17 days. The inertness of CgPOMe to hydrolysis may be due to the methyl groups on the cage (which are held rigidly in the vicinity of the P atom), providing a hydrophobic environment and steric protection to the phosphorus.
Crystals of Lh were grown from a saturated solution in a mixture of CH2Cl2, ethyl acetate, and hexane. It crystallized in space group C2/c, and the molecule has crystallographic 2-fold symmetry (Figure 1). Hydrocyanation Catalysis. A simplified scheme for the commercialized hydrocyanation of butadiene to ADN catalyzed by nickel(0) complexes is shown in Scheme 1. We have studied the conversion of 3-PN to ADN catalyzed by nickel(0) (22) Pryjomska, I.; Bartosz-Bechowski, H.; Ciunik, Z.; Trzeciak, A. M.; Zi ozkowski, J. J. Dalton Trans. 2006, 213.
complexes of the cage phosphinites La-h in the presence of a Lewis acid using acetone cyanohydrin as the source of HCN; the results are given in Table 1. The process involves an isomerization of 3-PN to 4-PN followed by hydrocyanation (Scheme 3). The byproduct arises from isomerization of 3-PN to 2-PN and addition of HCN to 3-PN to give ethylsuccinonitrile (ESN) or 2-methylglutaronitrile (2-MGN). The yield in Table 1 refers to the conversion of 3-PN to dinitriles (ADN, 2-MGN, and ESN) under the standard reaction conditions (see Experimental Section), and the linearity is the proportion of the dinitrile product that is the desired ADN. The yields obtained with all the aryl phosphinites, CgPOAr (entries 1-4) were very low but were greater than 1%, making the CgPOAr ligands more effective for this catalysis than CgPPh, where only traces of dinitriles were detected (entry 17). Increasing the steric bulk of the CgPOAr ligand appears to be detrimental to the yield. The catalyst derived from CgPOMe (entries 5-7) gave significantly higher yields (up to 19%) than the CgPOArderived catalysts. The yields were higher still (up to 36%) using the CgPOCH2CF3 ligand (entries 8-10). The tentative conclusions from the catalysis results for the monodentate CgPOR ligands are that the hydrocyanation activity is (1) greater when R=alkyl than when R=aryl; (2) greater with smaller R groups when R=aryl; (3) greater with more electron-withdrawing R groups when R=alkyl. Diphosphites have many advantages over monophosphites in terms of catalyst stability, activity, and selectivity, which has led to their adoption for commercial hydrocyanation catalysis.8 The cage diphosphinites Lg and Lh were screened for hydrocyanation catalysis, and the results are given in Table 1. The results with Lg (entries 11-13) were disappointingly similar to those with the monodentate analogue La (entries 1-3). The 2,20 -biphenyl linking group is a common structural element in the highly successful diphosphite ligands8 but evidently does not improve the catalyst (23) Bite angle and flexibility differences between ligands Lg and Lh should be important in understanding the catalytic results since it has been established that even small changes in natural bite angles can have a large effect on hydrocyanation activity and selectivity with nickeldiphosphine catalysts. However, defining a bite angle for ligands with diolate backbones is frustrated by the large number of conformations with similar energy; see: van Leeuwen, P. W. N. M.; Kamer, P. C. J.; Reek, J. N. H.; Dierkes, P. Chem. Rev. 2000, 100, 2741.
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Scheme 3
Table 1. Hydrocyanation Catalysis Results with Cage Phosphinitesa entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18d
L La Lb Lc Ld Le Lf Lg Lh CgPPh P(OTol)3
Scheme 4
Lewis Acid
yield
linearity
Ph2BOBPh2 Ph2BOBPh2 Ph2BOBPh2 Ph2BOBPh2 ZnCl2 TIBAO Ph2BOBPh2 ZnCl2 TIBAO Ph2BOBPh2 ZnCl2 TIBAO Ph2BOBPh2 ZnCl2 TIBAO Ph2BOBPh2
3 2 1 1 4 19 7 3 36 15 1 2 5 40 12 6
