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Charging and Deforming the Pybox Ligand: Enantiomerically Pure Double Helices and Their Interconversion

Gade, Lutz H.
Published in Chemistry - A European Journal. 2002, vol. 8, no. 18, p. 4308-4318
Abstract Reaction of pyrrole-2,5-biscarbonitrile (1) with an excess of (S)- or (R)-valinol in boiling chlorobenzene selectively yielded the two enantiomeric bis(oxazolinyl)pyrroles (S,S)-bis[2-(4,4-diisopropyl-4,5-dihydrooxazolyl)]pyrrole (S,S-iproxpH, 2 a) and (R,R)-bis[2-(4,4-diisopropyl-4,5-dihydrooxazolyl)]pyrrole (R,R-iproxpH, 2 b), respectively. Lithiation of 2 a and 2 b at -78 °C and reaction with an equimolar amount of [PdCl2(cod)] (cod=1,5-cyclooctadiene) gave the helical dinuclear palladium complexes (M)-[PdCl(S,S-iproxp)]2 (3 a) and (P)-[PdCl(S,S-iproxp)]2 (3 b) as well as (P)-[PdCl(R,R-iproxp)]2 (4 a) and (M)-[PdCl(R,R-iproxp)]2 (4 b). Reaction of a 1:1 mixture of lithiated 2 a and 2 b with an equimolar amount of [PdCl2(cod)] gave a mixture of the homochiral complexes 3 a,b and 4 a,b along with the racemic mixture of the heterochiral complex [Pd2Cl2(S,S-iproxp)(R,R-iproxp)] (5). The double helical structure as well as the absolute configuration of these neutral dinuclear palladium complexes was confirmed by X-ray diffraction studies of all five complexes. One of the oxazolyl units and the anionic pyrrolide occupy two coordination sites in an approximately square-planar ligand arrangement at the Pd centers whereas the second oxazolyl ring is twisted out of this plane and binds to the second metal center. The heterochiral complex 5 does not possess any element of molecular symmetry. The P-helical complexes 3 b and 4 a display a positive CD at 310 nm and a weaker negative CD at 350 nm, while the compounds possessing M-helicity have the corresponding mirror image CD spectra. Complexes 3 a and 4 a have an additional weak long wavelength CD feature between 380 and 420 nm which is absent in the spectra of 3 b and 4 b. Upon heating a solution of 3 b, interconversion to the diastereomer of opposite helicity 3 a sets in, for which a first-order rate law with respect to the concentration of the complex was established; activation parameters: δH(diff.)=68 kJ mol-1, δS(diff.)=-99 J mol-1 K-1. A cross-over experiment monitored by 1H NMR spectroscopy also gave the racemate of the mixed-ligand complex 5: (P)-[Pd2Cl2(S,S-iproxp)(R,R-iproxp)] and (M)-[Pd2Cl2(S,S-iproxp)(R,R-iproxp)] indicating an intermolecular exchange involving mononuclear {PdCl(iproxp)} complex fragments.
Keywords chiral ligandshelical structuresN ligandspalladium
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MAZET, Clement, GADE, Lutz H. Charging and Deforming the Pybox Ligand: Enantiomerically Pure Double Helices and Their Interconversion. In: Chemistry - A European Journal, 2002, vol. 8, n° 18, p. 4308-4318.

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Deposited on : 2010-06-21

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