摘要:
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", 2a) and (R,R)-bis[2(4,4'-diisopropyl-4,5-dihydrooxazolyl)]- pyrrole ("RR-iproxpH", 2b), respectively. Lithiation of 2a and 2b at -78degreesC 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) (3a) and (P)-[PdCl(S,S-iproxp)](2) (3b) as well as (P)-[PdCl(R,R-iproxp)](2) (4a) and (M)-[PdCl(R,Riproxp)](2) (4b). Reaction of a 1:1 mixture of lithiated 2 a and 2 b with an equimolar amount of [PdCl,(cod)] gave a mixture of the homochiral complexes 3a,b and 4a,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 3b and 4a display a positive CD at 3 10 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 3b, interconversion to the diastereomer of opposite helicity 3a sets in, for which a first-order rate law with respect to the concentration of the complex was established; activation parameters: DeltaHdouble dagger=68kJmol(-1), DeltaSdouble dagger= -99 J mol(-1) K-1. A cross-over experiment monitored by H-1 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.