ORGANIC LETTERS
Synthesis and Crystal Structure of a Unique Linear Homoditopic Ligand Bifacially Complexed to Lithium Picrate
2000 Vol. 2, No. 2 143-146
Leo A. Paquette,* Jinsung Tae, and Judith C. Gallucci EVans Chemical Laboratories, The Ohio State UniVersity, Columbus, Ohio 43210
[email protected] Received November 3, 1999
ABSTRACT
Preparation of the first rigid homoditopic tris(tetrahydrofuranyl) ligand with a pair of exotopic binding sites specifically tailored for effective coordination to lithium ions is described. The crystallographically defined structural parameters of the monomeric building block and a 1:2 complex of the dimer with lithium picrate reveal the unique features of these materials.
Self-assembly, that capacity of select molecules to form welloriented oligomeric networks, plays an important role in supramolecular chemistry.1 In general, ligand association develops as a direct consequence of hydrogen bonding, π-stacking, or interactions involving transition metals. Up to now, the application of group IA metal ions (Li+, Na+, etc.) to the formation of ionic supramolecules has not been accorded meaningful consideration. In earlier reports, we have described the readiness with which the sandwich complexes 1 and 2 can be generated.2,3 The unique 2:1 binding feature exhibited by the two neutral ionophores and particularly the high binding selectivity for Li+ observed in the latter instance have prompted us to consider the linkage (1) (a) Lehn, J.-M. Supramolecular Chemistry; VCH: Weinheim, 1995; p 138. (b) Philp, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35, 1154; Angew. Chem. 1996, 108, 1242. (c) Conn, M. M.; Rebek, J., Jr. Chem. ReV. 1997, 16477. (d) de Mendoza, J. Chem. Eur. J. 1998, 4, 1373. (e) Stang, P. J. Chem. Eur. J. 1998, 4, 19. (2) Paquette, L. A.; Tae, J.; Hickey, E. R.; Rogers, R. D. Angew Chem., Int. Ed. 1999, 38, 1409; Angew. Chem. 1999, 111, 1502. (3) Tae, J.; Rogers, R. D.; Paquette, L. A. Org. Lett. 2000, 2, 139. (4) After completion of this work, the preparation of rodlike ruthenium(II) coordination polymers was reported: Kelch, S.; Rehahn, M. Chem. Commun. 1999, 1123. 10.1021/ol9903436 CCC: $19.00 Published on Web 01/05/2000
© 2000 American Chemical Society
of two inositol-based scaffolds as in 3, exposure of which to Li+ can be expected to result in formation of a lithium ion-ionophore coordination polymer having the generic formula 4.4 Beyond this, methodology has presently been developed for arresting the polymerization in a way that leads to the first linear homoditopic complex involving a pair of lithium ions.
The first important consideration involves the orthoester substituent to be carried through the synthesis. The decision was made in favor of 5 because of its ready availability5 and the robustness of the p-methoxyphenyl (PMP) group. Acid-promoted coupling of 5 to myo-inositol furnished 6 in satisfactory yield (Scheme 1). The conversion of 6 to 10 via
allowed us to examine the feasibility of the key dimerization reaction. Deprotection of the PMP group with ceric ammonium nitrate in aqueous acetonitrile was completed in 10 min at 0 °C (Scheme 2). Oxidation of primary alcohol 11
Scheme 2. Scheme 1.
Synthesis of the Bifacial Dimeric Ionophore 13
Synthesis of the Trispiro Ligand 10
a Ceric ammonium nitrate, CH CN-H O (4:1) (69%). b Swern 3 2 oxidation; CH3COC(dN2)P(O)(OMe)2, K2CO3, MeOH (33% over two steps). c CuCl, TMEDA, air, CH2Cl2 (100%). a myo-Inositol, DMF, TsOH, 100 °C, 24 h (57%, 95% based on recovered starting material). b 1.0 equiv of TBSCl, imidazole, DMF (55%, 82% based on recovered starting material). c NaH, BnBr (56%, 86% based on recovered starting material). d Dess-Martin periodinane, CH2Cl2 (87%). e ClMg(CH2)3OMgCl; TsCl, Et3N, CH2Cl2, 10% Pd/C, H2 (40-50 psi), EtOAc (55% over three steps). f Dess-Martin periodinane; ClMg(CH ) OMgCl; TsCl, Et N, CH Cl ; 2 3 3 2 2 TBAF, THF (79% over four steps). g Swern oxidation; LiClO4, THF, rt, 2 h; ClMg(CH2)3OMgCl; TsCl, Et3N, CH2Cl2 (78% over three steps).
7-9 follows precedent3 and is not detailed here. Successful arrival at this aryloxy orthoacetate-protected trispiro ether (5) Voss, G.; Gerlach, H. HelV. Chim. Acta 1983, 66, 2294. (6) Muller, S.; Liepold, B.; Roth, G. R.; Bestmann, H. J. Synlett 1996, 521. 144
with the Dess-Martin periodinane or tetra-n-propylammonium perruthenate proved unworkable. The highly polar nature of the alcohol and the aldehyde precluded the use of oxidants that require significant purification during workup. The Swern process worked best, although efficiency was still sacrificed (
