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Organometallics 2009, 28, 4886–4889 DOI: 10.1021/om900530q
Metal Effects on the Asymmetric Cycloaddition Reaction between 3,4-Dimethyl-1-phenylarsole and Diphenylvinylphosphine Oxide Mengtao Ma, Sumod A. Pullarkat, Mingjun Yuan, Na Zhang, Yongxin Li, and Pak-Hing Leung* Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371 Received June 19, 2009 Summary: The organopalladium complex containing orthometalated (S)-[1-(dimethylamino)ethyl]naphthalene as the chiral auxiliary has been used successfully to promote the asymmetric cycloaddition reaction between 3,4-dimethyl-1phenylarsole and diphenylvinylphosphine oxide in high stereoselectivity (only one isomer). However, the neutral dichloro palladium complex obtained upon removal of the auxiliary is very unstable and instantly decomposed to produce the arsenic elimination reaction product (3,4-dimethyl-2,4-cyclohexadienyl)diphenylphosphine oxide. However when the same reaction was promoted by the analogous platinum complex, only one stereoisomer was again obtained, but the resulting dichloro platinum complex is very stable in the solid state as well as in solution and coordinates to the platinum center as a As-O bidentate ligand, as confirmed by 1H NMR and single-crystal X-ray analysis. The enantiomerically pure As kPdO heterobidentate ligand could be readily liberated from the dichloro platinum complex as an air-sensitive solid by treatment of the complex with aqueous potassium cyanide. It is well known that enantiomerically pure phosphines such as BINAP are very important chiral auxiliaries in asymmetric catalysis.1 Similarly, their corresponding oxides are also very useful for asymmetric synthesis and catalysis.2 For instance, due to the presence of both the soft (P) and hard (O) Lewis base centers on one molecule, the hemilabile ligand3 bis(phosphine) monoxide can stabilize a lot of transition metals in various oxidation state; at the same time, its weak chelation with metals can easily generate reactive, coordinatively unsaturated species, which provides low activation energy paths for various transformations at the metal center. A number of bis(phosphine) monoxides have shown widespread application in the field of catalysis such as carbonylation of alcohol and hydroformylation of olefins.4 A recent example is that of Charette et al., who found the *To whom correspondence should be addressed. E-mail: pakhing@ ntu.edu.sg. (1) (a) Noyori, R.; Takaya, H. Acc. Chem. Res. 1990, 23, 345. (b) Esquivias, J.; Arrayas, R. G.; Carretero, J. C. Angew. Chem., Int. Ed. 2006, 45, 629. (c) Nishimura, T.; Katoh, T.; Takatsu, K.; Shintani, R.; Hayashi, T. J. Am. Chem. Soc. 2007, 129, 14158. (2) (a) Faller, J. W.; Parr, J. Organometallics 2000, 19, 1829. (b) Faller, J. W.; Lavoie, A. R.; Grimmond, B. J. Organometallics 2002, 21, 1662. (c) Cyr, P. W.; Rettig, S. J.; Patrick, B. O.; James, B. R. Organometallics 2002, 21, 4672. (3) (a) Jeffrey, J. C.; Rauchfuss, T. B. Inorg. Chem. 1979, 18, 2658. (b) Bader, A.; Lindner, E. Coord. Chem. Rev. 1991, 108, 27. (c) Slone, C. S.; Weinberger, D. A.; Mirkin, C. A. Prog. Inorg. Chem. 1999, 48, 233. (d) Braunstein, P.; Naud, F. Angew. Chem., Int. Ed. 2001, 40, 681. (4) For a recent detailed review of bis(phosphine) monoxide ligands see: Grushin, V. V. Chem. Rev. 2004, 104, 1629. pubs.acs.org/Organometallics
Published on Web 07/31/2009
chiral Me-DuPHOS monoxide to be a very effective ligand in the enantioselective copper-catalyzed addition of dialkylzinc to N-phosphinoylimines (mostly in more than 90% yield and 90% ee) and that the resulting chiral amines are very important synthons for the preparation of chiral drugs.5 Compared to the phosphorus-based hemilabile ligands, especially bis(phosphine) monoxide bidentate ligands, the arsenic-based hemilabile ligands are rare.3c To our knowledge, no asymmetric synthesis of an enantiomerically pure AskPdO hetero-bidentate ligand has been reported hitherto. In this paper, we report metal effects on the preparation of an optically pure As(III) kP(V)dO hetero-bidentate ligand by means of an asymmetric cycloaddition reaction between 3,4dimethyl-1-phenylarsole (DMPA) and diphenylvinylphosphine oxide.
Results and Discussion Asymmetric Cycloaddition between DMPA and Diphenylvinylphosphine Oxide Promoted by a Palladium Complex. Diphenylvinylphosphine oxide is not a chemically reactive dienophile in this reaction because no reaction was observed when the neutral complex (þ)-1 was treated directly with diphenylvinylphosphine oxide despite strong reaction conditions (80 °C) being employed. When the complex (þ)-1 was treated with aqueous silver perchlorate in dichloromethane, however, the resultant perchlorate complex is chemically reactive toward diphenylvinylphosphine oxide. Temperature has a large effect on this Diels-Alder reaction. For example, the reaction took 3 months to complete when it was carried out at room temperature. However, at higher temperature (60 °C), it can complete in 6 days. The 31P{1H} NMR spectrum of the crude cycloaddition reaction mixture in CDCl3 exhibited only one singlet at δ 49.5 ppm (Scheme 1). The cycloadduct (þ)-2 was isolated by column chromatography to give a pale yellow solid in 76% yield, [R]D þ83.8 (c 0.8, CH2Cl2). Attempts to get crystals suitable for single-crystal X-ray determination were however not successful. In order to confirm the structure, coordination mode, and absolute configuration of the generated cycloadduct, we sought to obtain the corresponding neutral dichloro complex. When the complex (þ)-2 was treated with concentrated hydrochloric acid for 10 min at room temperature, the (5) (a) Boezio, A. A.; Pytkowicz, J.; C^ ote, A.; Charette, A. B. J. Am. Chem. Soc. 2003, 125, 14260. (b) Cte, A.; Boezio, A. A.; Charette, A. B. Angew. Chem., Int. Ed. 2004, 43, 6525. (c) Bonnaventure, I.; Charette, A. B. J. Org. Chem. 2008, 73, 6330. r 2009 American Chemical Society
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Scheme 1
Scheme 2
Scheme 3
31
P{1H} NMR spectrum of the crude mixture in CDCl3 exhibited one singlet at δ 34.0 ppm. The compound was isolated by column chromatography using a silica gel column with dichloromethane-acetone as eluent in 67% yield, [R]D -72.3 (c 0.1, CH2Cl2). However from the 1H and 13 C NMR spectra and other characterizations, it was evident that it was not the desired dichloro complex and was in fact the arsenic elimination reaction product (3,4dimethyl-2,4-cyclohexadienyl)diphenylphosphine oxide, (-)-3. The possible reason is that upon acid treatment, initially the naphthylamine was removed chemoselectively to give the dichloro complex, but due to the fact that the resulting dichloro complex is very unstable, an arsenic elimination process occurred and the resultant intermediate (with η2 and oxygen coordinated on Pd) dissociated to produce the organic compound (-)-3 and other unidentified side products.6 Metal Effect: Asymmetric Cycloaddition between DMPA and Diphenylvinylphosphine Oxide Promoted by a Platinum Complex. As aforementioned, the Diels-Alder reaction between DMPA and diphenylvinylphosphine oxide promoted (6) (a) Ma, M.; Pullarkat, S. A.; Li, Y.; Leung, P. H. J. Organomet. Chem. 2008, 693, 3289. (b) Ma, M.; Pullarkat, S. A.; Li, Y.; Leung, P. H. Inorg. Chem. 2007, 46, 9488.
by the palladium complex cannot produce the desired enantiomerically pure AskPdO ligand. However we found that the metal employed had a considerable effect on the final outcome of the reaction, especially on the stability of the formed cycloadduct in the neutral complex. Similar to the palladium complex,6 the DMPA can readily split the chloro bridges of the platinum complex (þ)-4 regiospecifically to give the neutral complex (þ)-5 in 54% yield, [R]D þ140.0 (c 0.6, CH2Cl2) (Scheme 2). It was recrystallized with chloroform-n-hexane-diethyl ether to yield yellow crystals. The molecular structure and absolute configuration of complex (þ)-5 have been resolved by single-crystal X-ray structural analysis (Figure 1). As in the case of the palladium complex (þ)-1, the bridgehead arsenic in (þ)-5 is trans to the NMe2 group. The geometry at the platinum in complex (þ)-5 is distorted square planar with angles at platinum in the range 80.6(1)96.9(1)° and 173.8(1)-177.1(1)°. Selected bond lengths and angles are listed in the caption of Figure 1. The bond angle at the bridgehead arsenic [C21-As1-C26, 87.5(2)°] is almost the same as that of the palladium complex (þ)-1 [87.4(1)°]. The chloro ligand in complex (þ)-5 and similar complexes is well known to be both kinetically and
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Figure 1. Molecular structure of complex (þ)-5. Selected bond lengths (A˚) and angles (deg): Pt1-C1 = 1.995(3), Pt1-N1 = 2.134(3), Pt1-Cl1 = 2.411(1), Pt1-As1 = 2.342(1), As1-C21 = 1.912(4), As1-C26 = 1.915(4); C1-Pt1-N1 = 80.6(1), C1Pt1-As1 = 96.9(1), N1-Pt1-As1 = 177.1(1), C1-Pt1-Cl1 = 173.8(1), N1-Pt1-Cl1 = 95.4(1), As1-Pt1-Cl1 = 87.2(1), C21-As1-C26 = 87.5(2).
thermodynamically stable.7 This terminal ligand, however, can be replaced efficiently by treatment of the complex with aqueous silver perchlorate so that the incoming dienophile, diphenylvinylphosphine oxide, can coordinate to the platinum center, thus facilitating an intramolecular reaction with DMPA. The 31P{1H} NMR spectroscopic studies indicated that the cycloaddition reaction was completed within one month at 60 °C (which is slower compared to the reaction involving palladium complex (þ)-2), and only one diastereomer at δ 51.3 ppm was produced. The complex (þ)-6 was subsequently isolated by column chromatography as a solid in 68% yield, [R]D þ67.2 (c 0.6, CH2Cl2). The chiral naphthylamine auxiliary on complex (þ)-6 could be removed chemoselectively by the treatment of the complex with concentrated hydrochloric acid for 10 min at room temperature. The 31P{1H} NMR spectrum of the crude mixture showed a singlet at δ 57.0 ppm. The dichloro platinum complex (þ)-7 was obtained as yellow crystals in 86% yield, [R]D þ122.2 (c 0.1, CH2Cl2). The X-ray structural analysis confirmed that the desired cycloadduct had formed via the exo-cycloaddition reaction (Figure 2). The cycloadduct in complex (þ)-7 was coordinated to the platinum center as a bidentate ligand via the bridgehead arsenic and the oxygen atom of diphenylvinylphosphine oxide. It needs to be noted that complex (þ)-7 is a monomer unlike the bridged dimeric structure of the phosphorus analogue.8 The geometry at the platinum center is distorted square planar with angles at platinum in the range 88.1(1)-92.5(1)° and 176.1(1)-176.8(1)°. Selected bond lengths and angles are listed in the caption of Figure 2. In comparison to the unstable dichloro palladium complex, complex (þ)-7 is stable, and this can be attributed to the stronger Pt-O (7) Aw, B. H.; Hor, T. S. A.; Selvaratnam, S.; Mok, K. F.; White, A. J. P.; Williams, D. J.; Rees, N. H.; McFarlane, W.; Leung, P. H. Inorg. Chem. 1997, 36, 2138. (8) Teo, T. W.; Selvaratnam, S.; Vittal, J. J.; Leung, P. H. Inorg. Chim. Acta 2003, 352, 213.
Ma et al.
Figure 2. Molecular structure of complex (þ)-7. Selected bond lengths (A˚) and angles (deg): Pt1-O1 = 2.070(3), Pt1-As1 = 2.320(1), Pt1-Cl1 = 2.276(1), Pt1-Cl2 = 2.344(1), As1-C7= 1.974(3), As1-C12 = 1.978(3), O1-P1 = 1.508(3); O1-Pt1Cl1 = 176.8(1), As1-Pt1-Cl2 =176.1(1), O1-Pt1-As1 = 89.5(1), Cl1-Pt1-As1 = 90.0 (3), O1-Pt1-Cl2 = 88.1 (1), Cl1-Pt1-Cl2 = 92.5(1), C7-As1-C12 = 77.1(1).
[2.070(3) A˚] bond when compared with the Pd-O bond [2.124(5) and 2.138(5) A˚].8,3c The optically active ligand (-)-8 can be stereospecifically liberated from the dichloro complex (þ)-7 by treatment with aqueous potassium cyanide at room temperature. Compound (-)-8 was obtained as a white solid in quantitative yield, [R]D -18.6 (c 0.7, CH2Cl2). The 31P{1H} NMR spectrum of the liberated ligand in CDCl3 exhibited a singlet at δ 30.9 ppm. Owing to the instability of the noncoordinated bridgehead arsenic, the liberated ligand cannot be stored for a long duration, as it will convert to the arsenic elimination reaction product (-)-3. This process [(-)-8 to (-)-3] was monitored by using the 31P{1H} NMR spectrum and was found to be completed in 4 h. Therefore the liberated ligand (-)-8 must be recoordinated to selected metal ions. Due to the aforementioned reason and also in order to determine the optical purity of (-)-8, it was recoordinated to the bis(acetonitrile) complex (þ)-9 (Scheme 3). The 31P{1H} NMR spectrum of the crude recoordinated product showed a singlet at δ 51.3, which is identical to that recorded from the original cycloaddition reaction. No other 31P{1H} signals could be detected, thus confirming that the liberated ligand is optically pure. In summary, the asymmetric cycloaddition reactions between DMPA and diphenylvinylphosphine oxide promoted by chiral palladium and platinum complexes both showed high stereoselectivity with only one isomer being formed. The reaction rate using the chiral palladium promoter is much faster than that using the platinum template (6 vs 30 days at 60 °C). However the corresponding dichloro palladium complex is very unstable and hence could not yield the desired optically active ligand since it instantly decomposed to give the arsenic elimination reaction product, (3,4-dimethyl-2,4-cyclohexadienyl)diphenylphosphine oxide,
Note
(-)-3. It was noted that the corresponding dichloro platinum complex (þ)-7 is very stable and subsequently can lead to the liberation of the desired enantiomerically pure AskPdO ligand (-)-8. These studies highlight the difference in chelate stabilization by different metal centers, and further investigations are in progress especially with the aim of utilizing these ligands in transition metal complex promoted catalysis.
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Acknowledgment. We thank the Nanyang Technological University for support of this research and for a scholarship to M.M. Supporting Information Available: Complete experimental section as well as crystallographic data in CIF format for complexes (þ)-5 and (þ)-7. This material is available free of charge via the Internet at http://pubs.acs.org.
