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Organometallics 2003, 22, 2464-2471
Phosphine-Rh2[(R)-MTPA]4 Adducts in Solution: Characterization by NMR Spectroscopy and Chiral Discrimination Damian Magiera,§ Jan Omelanczuk,† Kamil Dziuba,‡ K. Michał Pietrusiewicz,*,‡,# and Helmut Duddeck*,§ Institut fu¨ r Organische Chemie, Universita¨ t Hannover, Schneiderberg 1B, D-30167 Hannover, Germany, Centre of Molecular and Macromolecular Studies, Department of Organic Sulfur Compounds, Polish Academy of Sciences, PL-90-363 Ło´ dz´ , Sienkiewicza 112, Poland, Department of Organic Chemistry, Maria Curie-Sklodowska University, Lublin, Poland, and Institute of Organic Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, PL-01-224 Warsaw, Poland Received November 18, 2002
The adducts of dirhodium tetraacylates and phosphines are characterized in solution by H and 31P NMR spectroscopy at room temperature. A differentiation of the enantiomers of chiral phosphine ligands is easily performed by NMR signal integration after adduct formation with the enantiopure Rh2[(R)-(+)-MTPA]4 complex (Rh*). Stereochemical aspects are discussed in terms of chiral discrimination and adduct diastereomerism. A second type of chiral recognition was discovered, namely, that between two ligand molecules in 2:1 adducts across the Rh2[(R)-(+)-MTPA]4 building block. Conditions for optimizing the experiment for the determination of enantiomeric composition of chiral phosphines by the “dirhodium method” are presented. The possibility of determining absolute configurations of chiral phosphines is briefly discussed. 1
Introduction Dirhodium and other dinuclear complexes have been studied intensively during the last few decades.1 They were applied frequently as homogeneous catalysts2 and as auxiliaries in the chirality determination of compounds with various functional groups by circular dichroism.3 It should be mentioned that dirhodium complexes are even in the focus of medicinal interest.4 In a series of papers5 we have studied the potential of the dirhodium complex Rh2[(R)-(+)-MTPA]4 [Rh*, * To whom correspondence should be addressed. (H.D.) Tel: +49 511 762 4615. Fax: +49 511 762 4616. E-mail:
[email protected]. (M.P.) Tel: +48 81 537 5679. Fax: +48 81 524 2251. E-mail:
[email protected]. § Hannover University. † Centre of Molecular and Macromolecular Science. ‡ Maria Curie-Sklodowska University. # Polish Academy of Sciences. (1) )(a) Multiple Bonds between Metal Atoms, 2nd ed.; Cotton, F. A., Walton, R. A., Eds.; Clarendon: Oxford, 1993. (b) Boyar, E. B.; Robinson, S. D. Coord. Chem. Rev. 1983, 50, 109-208. (2) ) (a) Mertis, C.; Kravaritoy, M.; Chorianopoulou, M.; Koinis, S.; Psaroudakis, N. Top. Mol. Org. Eng. 1994, 11, 321-329. (b) Modern Catalytic Methods for Organic Synthesis with Diazo Compounds: From Cyclopropanes to Ylides; Doyle, M. P., McKervey, M. A., Ye, T., Eds.; Wiley: New York, 1998. (c) Endres, A.; Maas, G. Tetrahedron 2002, 58, 3999-4005, and references therein. (3) For example: (a) Snatzke, G.; Wagner, U.; Wolff, H. P. Tetrahedron 1981, 37, 349-361. (b) Gerards, M.; Snatzke, G. Tetrahedron: Asymmetry 1990, 1, 221-236. (c) Cotton, F. A.; Falvello, L. R.; Gerards, M.; Snatzke, G. J. Am. Chem. Soc. 1990, 112, 8979-8980. (d) Frelek, J.; Szczepek, W. J. Tetrahedron: Asymmetry 1999, 10, 1507-1520. (4) Clarke, M. J.; Zhu, F.; Frasca, D. R. Chem. Rev. 1999, 99, 25112533. (5) (a) Wypchlo, K.; Duddeck, H. Tetrahedron Asymmetry 1994, 5, 27-30. (b) Wypchlo, K.; Duddeck, H. Chirality 1997, 9, 601-603. (c) Hameed, S.; Ahmad, R.; Duddeck, H. Magn. Reson. Chem. 1998, 36, S47-S53. (d) Meyer, C.; Duddeck, H. Magn. Reson. Chem. 2000, 38, 29-32. (e) Rockitt, S.; Duddeck, H.; Omelanczuk, J. Chirality 2001, 13, 214-223. (f) Malik, S.; Duddeck, H.; Omelanczuk, J.; Choudhary, M. I. Chirality 2002, 14, 407-411.
MTPA-H ) methoxytrifluoromethylphenylacetic acid ≡ Mosher’s acid; Scheme 1, top] as a solvating agent for the determination of enantiomeric ratios of various chiral monofunctional ligands. It has been shown that the “dirhodium method” is particularly suitable for softbase functionalities where the classical method of chiral lanthanide shift reagents (CLSR)6 usually fail. Typically, Rh* and ligands form kinetically labile adducts so that only averaged NMR signals can be observed for the ligand molecules in the room-temperature equilibria (in analogy to the CLSR method). In contrast to previously studied ligands (olefins, epoxides, nitriles, sulfoxides, iodides, phosphine chalcogenides, and selenides),5,7 phosphines form adducts8 in solution that are kinetically stable on the NMR timescale (Scheme 1, center), even at room temperature.9 Such unusual behavior of phosphine ligands has been noticed before by Drago et al.10 using achiral molecular systems (see below). Results and Discussion The assignments of the 1H and 13C NMR signals of the phosphine ligands 1-7 (Schemes 1, bottom, and 2) (6) (a) Sullivan, G. R. Top. Stereochem. 1978, 10, 287-329. (b) Rinaldi, P. L. Prog. NMR Spectrosc. 1983, 15, 291-352. (c) Parker, D. Chem. Rev. 1991, 91, 1441-1457. (d) Rothchild, R. Enantiomer 2000, 5, 457-471. (7) Duddeck, H.; Malik, S.; Ga´ti, T.; To´th, G.; Choudhary, M. I. Magn. Reson. Chem. 2002, 40, 153-156. (8) For the solid state structures of the 2:1 adducts of Rh-TFA with PPh3 and P(OPh)3 ligands see: Cotton, F. A.; Felthouse, T. R.; Klein, S. Inorg. Chem. 1981, 20, 3037-3042. (9) Magiera, D.; Baumann, W.; Podkorytov, I. S.; Omelanczuk, J.; Duddeck, H. Eur. J. Inorg. Chem. 2002, 3253-3257. (10) 0) (a) Telser, J.; Drago, R. S. Inorg. Chem. 1984, 23, 25992606. (b) Telser, J.; Drago, R. S. Inorg. Chem. 1986, 25, 2989-2992.
10.1021/om0209529 CCC: $25.00 © 2003 American Chemical Society Publication on Web 05/16/2003
Phosphine-Rh2[(R)-MTPA]4 Adducts Scheme 1. Structures of the Dirhodium Complexes (top), Equilibria of Free Rh* and Its Phosphine Adducts (center), and the Achiral Phosphines 1-3 (bottom)a
Organometallics, Vol. 22, No. 12, 2003 2465 Table 1. 1H, 13C, and 31P Chemical Shifts (δ, in ppm), Coupling Constants nJ(31P,X) (in Hz) of Free PPh3 (1) and of the Adduct Rh*r1 (1:1 molar ratio of the components), and Adduct Formation Shifts (∆δ, in ppm), in CDCl3 free ligand X 1H
13C
δ 2/6 3/5 4 1 2/6 3/5 4
31P a
arom. H: 7.35-7.28
nJ(31P,X)
∆δ
) 10.7
∼+0.3
n.d. 5J ) 0-2 1J ) 33.4 2J ) 10.6 3J ) 9.8 4J e 3 1J RhP ) 96.1 2J RhP ) 23.3
∼-0.1 ∼+0.1 -7.1 +0.7 +0.4 +1.9 -31.1
δ
n.d.a n.d. n.d. 1J ) 10.8 2J ) 19.4 3J ) 7.0 4J e 1
137.3 133.7 128.5 128.7 -4.2
ligand in the adduct
nJ(31P,X)
7.59 ≈ 7.23 7.42 130.2 134.4 128.9 130.6 -35.3
3J
n.d.: not determined due to signal overlap or signal complex-
ity.
Table 2. 1H, 13C, and 31P Chemical Shifts (δ, in ppm), Coupling Constants nJ(31P,X) (in Hz) of Free (m-Tol)3P (2) and of the Adduct Rh*r2 (1:1 molar ratio of the components), and Adduct Formation Shifts (∆δ, in ppm), in CDCl3 free ligand X 1H
a
In the formula schemes the chiral dirhodium complex Rh* is represented by “[Rh-Rh]” and the phosphine ligands by “P” or by their compounds numbers 1-7.
were straightforward on the basis of literature data.11 The 1H signals were sometimes complex and overlapping, especially for phenyl groups attached to P. A particularly severe case is triphenylphosphine (1), so that we included tris(3-methylphenyl)phosphine (2) in our study in which the signals are much more dispersed but the spin-spin coupling parameters are expected to be very similar to those of 1. Likewise, 13C signals of pentafluorophenyl groups were obscured due to the manifold 19F,13C couplings. All NMR data of the free ligands and their 1:1 adducts are collected in Tables 1-7. (I) Achiral Phosphines: Adduct Formation. As noted before,9,10 phosphines are unique in forming adducts with dirhodium tetraacylates whose lifetimes are long enough to observe the individual adducts by solution NMR at room temperature (“frozen equilibria”). Moreover, they provide 31P signal splittings due to scalar 103Rh,31P coupling.9,10b Therefore, we regarded it necessary first to explore suitable experimental conditions for recording the NMR data of adducts with a variety of structurally different phosphines (e.g., 1-7). To this end, we started by investigating the mode of adduct formation and performed NMR titration experiments varying the molar ratios of the phosphines 1 and 3 (symbol P in Scheme 1, center) and the dirhodium complex Rh* (symbol [Rh-Rh]) from 1:1 (excess of (11) Pretsch, E.; Bu¨hlmann, P.; Affolter, C. Structure Determination of Organic Compounds, Tables of Spectra Data, 3rd ed.; SpringerVerlag: Berlin, Heidelberg, 2000.
13C
δ 2 4 5 6 CH3 1 2 3 4 5 6 CH3
31P
7.17 7.13 7.21 7.06 2.29 137.1 134.5 137.9 129.5 128.3 130.7 21.4 -3.9
ligand in the adduct
nJ(31P,X)
) 8.5 5J e 1 4J ) 1.5 3J ) 7.4 5J e 1 1J ) 9.8 2J ) 22.3 3J ) 7.7 4J ≈ 0 3J ) 6.5 2J ) 16.6 4J ≈ 0 3J
δ 7.44 7.21 7.07 7.36 2.18 129.9 134.4 138.3 131.0 128.3 131.2 21.2 -34.0
nJ(31P,X)
∆δ
) 11.1 5J e 1 4J ) 2.3 3J ) 9.1 5J e 1 1J ) 33.3 2J ) 10.8 3J ) 9.8 4J ) 2.6 3J ) 10.3 2J ) 9.8 4J ≈ 0 1J RhP ) 96.3 2J RhP ) 23.1
+0.27 +0.08 -0.14 +0.30 -0.11 -7.2 -0.1 +0.4 +0.5 0 +0.5 -0.2 -31.1
3J
Table 3. 1H, 13C, and 31P Chemical Shifts (δ, in ppm), Coupling Constants nJ(31P,X) (in Hz) of Free P(OPh)3 (3) and of the Adduct Rh*r3 (1:1 molar ratio of the components), and Adduct Formation Shifts (∆δ, in ppm), in CDCl3 free ligand X 1H
13C
31P a
δ 2/6 3/5 4 1 2/6 3/5 4
7.14 7.31 7.12 151.6 120.7 129.7 124.2 129.2
ligand in the adduct
nJ(31P,X)
δ
nJ(31P,X)
∆δ
< 1.0 4J < 1.0 4J < 1.0 2J ) 3.5 3J ) 6.9 4J < 1.0 5J ) 1.3
7.00 to 7.25 151.1 120.5 129.1 124.7 8.0
n.d.a n.d. n.d. 2J ≈ 0 3J ≈ 0 4J ≈ 0 5J ≈ 0 1J RhP ) 143.1 2J RhP ) 25.3
n.d. n.d. n.d. -0.5 -0.2 -0.6 +0.5 -121.2
4J
n.d.: not determined due to signal overlap or signal complex-
ity.
rhodium sites) to 2:1 (equal number of phosphorus and rhodium sites) and further to 3:1 (excess of phosphorus sites). In the case of PPh3 (1) the adducts are formed consecutively. Using a 1:1 molar ratio, only the 31P signal of the 1:1 adduct appears at δ ) -35.3 as a double doublet due to the one- and two-bond couplings of 31P to 103Rh (Figure 1, top). The second molar equivalent of 1 is used for converting the 1:1 into the 2:1 adduct (Figure 1, center), the 31P signal forming the AA′ part
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Organometallics, Vol. 22, No. 12, 2003
Magiera et al.
Table 4. 1H, 13C, and 31P Chemical Shifts (δ), Coupling Constants nJ(31P,X) (in Hz) of Free Nonracemic 4 (S:R ) 2.45:1) and of the Adduct Rh*r4 (1:1 molar ratio of the components),a Adduct Formation Shifts (∆δ, in ppm), and Dispersion Effects [∆ν ) ν(R) - ν(S), in Hz], in CDCl3 free ligand δ
nJ(31P,X)
1′
1.59
2J
) 4.1
2/6
7.45
3J
) 7.6
3/5
7.33 n.d.c
4
7.35
5J
≈ 1.5
3′′
6.85
4J
) 4.1
4′′
7.30
5J
≈ 1.5
5′′
6.91
4J
