Charging and deforming the pybox ligand: enantiomerically pure double helices and their interconversion

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 degrees C and reaction with an equimolar amount of [PdCl(2)(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 [PdCl(2)(cod)] gave a mixture of the homochiral complexes 3 a,b and 4 a,b along with the racemic mixture of the heterochiral complex [Pd(2)Cl(2)(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: DeltaH( not equal )=68 kJ mol(-1), DeltaS( not equal )=-99 J mol(-1) K(-1). A cross-over experiment monitored by (1)H NMR spectroscopy also gave the racemate of the mixed-ligand complex 5: (P)-[Pd(2)Cl(2)(S,S-iproxp)(R,R-iproxp)] and (M)-[Pd(2)Cl(2)(S,S-iproxp)(R,R-iproxp)] indicating an intermolecular exchange involving mononuclear [PdCl(iproxp)] complex fragments.