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Inorg. Chem. 2001, 40, 3208-3216
Synthesis of Homochiral Tris(2-alkyl-2-aminoethyl)amine Derivatives from Chiral r-Amino Aldehydes and Their Application in the Synthesis of Water Soluble Chelators Sharad P. Hajela, Adam R. Johnson, Jide Xu, Christopher J. Sunderland, Seth M. Cohen, Dana L. Caulder, and Kenneth N. Raymond* Department of Chemistry, University of California, Berkeley, California 94720 ReceiVed September 8, 2000
A novel synthesis of 3-fold symmetric, homochiral tris(2-alkyl-2-aminoethyl)amine (TREN) derivatives is presented. The synthesis is general in scope, starting from readily prepared chiral R-amino aldehydes. The optical purity of the N-BOC protected derivatives of tris(2-methyl-2-aminoethyl)amine and tris(2-hydroxymethyl-2-aminoethyl)amine has been ascertained by polarimetry and chiral NMR chemical shift experiments. An X-ray diffraction study of the L-alanine derivative (tris(2-methyl-2-aminoethyl)amine‚3 HCl, L-Ala3-TREN) is presented: crystals grown from ether diffusion into methanol are cubic, space group P213 with unit cell dimensions a ) 11.4807(2) Å, V ) 1513.23(4) Å3, and Z ) 4. Attachment of the triserine derived backbone tris(2-hydroxymethyl-2aminoethyl)amine (L-Ser3-TREN) to three 3-hydroxy-1-methyl-2(1H)-pyridinonate (3,2-HOPO) moieties, followed by complexation with Gd(III) gives the complex Gd(L-Ser3-TREN-Me-3,2-HOPO)(H2O)2, which is more water soluble than the parent Gd(TREN-Me-3,2-HOPO)(H2O)2 and a promising candidate for magnetic resonance imaging (MRI) applications. Crystals of the chiral ferric complex Fe(L-Ser3-TREN-Me-3,2-HOPO) grown from ether/ methanol are orthorhombic, space group P212121, with unit cell dimensions a ) 13.6290(2) Å, b ) 18.6117(3) Å, c ) 30.6789(3) Å, V ) 7782.0(2) Å3, and Z ) 8. The solution conformation of the ferric complex has been investigated by circular dichroism spectroscopy. The coordination chemistry of this new ligand and its iron(III) and gadolinium(III) complexes has been studied by potentiometric and spectrophotometric methods. Compared to the protonation constants of previously studied polydentate 3,2-HOPO-4-carboxamide ligands, the sum of protonation constants (log β014) of L-Ser3-TREN-Me-3,2-HOPO (24.78) is more acidic by 1.13 log units than the parent TREN-Me-3,2-HOPO. The formation constants for the iron(III) and gadolinium(III) complexes have been evaluated by spectrophotometric pH titration to be (log K) 26.3(1) and 17.2(2), respectively.
Introduction For a variety of reasons, the synthesis of chiral, C3-symmetric ligands has recently been an area of active investigation.1,2 Ligands exhibiting many of the common donor groups have been synthesized, including phosphorus donors,3 tris(pyrazolyl)hydro-borates,4-6 alkoxides,7,8 tri- and tetraamines9,10 triamides,11,12 and tripyridines.13 Additionally, chiral tripodal ligands which do not contain 3-fold symmetry have been * To whom correspondence should be addressed. (1) Keyes, M. C.; Tolman, W. B. In AdVances in Catalytic Processes; Doyle, M. P., Ed.; JAI Press: Grenwich, CT, 1997; Vol. 2, pp 189219. (2) Moberg, C. Angew. Chem., Int. Ed. Eng. 1998, 37, 248. (3) Burk, M. J.; Harlow, R. L. Angew. Chem., Int. Ed. Eng. 1990, 29, 1462. (4) LeCloux, D. D.; Tolman, W. B. J. Am. Chem. Soc. 1993, 115, 1153. (5) LeCloux, D. D.; Tokar, C. J.; Osawa, M.; Hauser, R. P.; Keyes, M. C.; Tolman, W. B. Organometallics 1994, 13, 2855. (6) Tokar, C. J.; Kettler, P. B.; Tolman, W. B. Organometallics 1992, 11, 2737. (7) Nugent, W. A.; Harlow, R. L. J. Am. Chem. Soc. 1994, 116, 6142. (8) Lu¨tjens, H.; Wahl, G.; Mo¨ller, F.; Knochel, P.; Sundermeyer, J. Organometallics 1997, 16, 5869. (9) Ishihara, K.; Karumi, Y.; Kondo, S.; Yamamoto, H. J. Org. Chem. 1998, 63, 5692. (10) Cernerud, M.; Adolfsson, H.; Moberg, C. Tetrahedron: Asymmetry 1997, 8, 2655. (11) Hammes, B. S.; Ramos-Maldonado, D.; Yap, G. P. A.; Liable-Sands, L.; Rheingold, A. L.; Young Jr., V. G.; Borovik, A. S. Inorg. Chem. 1997, 36, 3210. (12) Tor, Y.; Libman, J.; Shanzer, A.; Felder, C. E.; Lifson, S. J. Am. Chem. Soc. 1992, 114, 6653.
prepared, including tris(2-aminoethyl)amine (TREN) derivatives with a single chiral substituent,14,15 or with three different tripodal “arms.”16 TREN is among the most widely employed 3-fold symmetric ligands, and many derivatives have been made for use as metalbinding ligands for both transition metals17 and main group elements.18 TREN has also been used as a scaffold for the synthesis of many tripodal ligands, particularly those used as models of the siderophore enterobactin or for high stability metal sequestering agents.19-21 Recently, we reported that TREN-Me3,2-HOPO forms an extremely stable Gd(III) complex which shows promise as a new magnetic resonance imaging (MRI) contrast agent.22 While investigating the synthesis of more water soluble derivatives of TREN-Me-3,2-HOPO we have developed (13) Adolfsson, H.; Wa¨rnmark, K.; Moberg, C. J. Chem. Soc., Chem. Com. 1992, 1054. (14) Utsuno, S.; Miyamae, H.; Horikoshi, S.; Endo, I. Inorg. Chem. 1985, 24, 1348. (15) Canary, J. W.; Allen, C. S.; Castagnetto, J. M.; Wang, Y. J. Am. Chem. Soc. 1995, 117, 8484. (16) Abufarag, A.; Vahrenkamp, H. Inorg. Chem. 1995, 34, 3279. (17) Schrock, R. R. Acc. Chem. Res. 1997, 30, 9. (18) Verkade, J. G. Acc. Chem. Res. 1993, 26, 483. (19) Karpishin, T. B.; Stack, T. D. P.; Raymond, K. N. J. Am. Chem. Soc. 1993, 115, 6115. (20) Cohen, S. M.; Meyer, M.; Raymond, K. N. J. Am. Chem. Soc. 1997, 120, 6277. (21) Rodgers, S. J.; Lee, C. W.; Ng, C. Y.; Raymond, K. N. Inorg. Chem. 1987, 26, 1622. (22) Xu, J.; Franklin, S. J.; Whisenhunt Jr., D. W.; Raymond, K. N. J. Am. Chem. Soc. 1995, 117, 7245.
10.1021/ic001021x CCC: $20.00 © 2001 American Chemical Society Published on Web 05/16/2001
Synthesis of Water Soluble Chelators Scheme 1a
a Reagents and Conditions: (i) BH ‚THF, THF (50-80%); (ii) 3 NaOCl, TEMPO (
