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Charging and Deforming the Pybox Ligand: Enantiomerically Pure Double Helices and Their Interconversion
Author(s) -
Mazet Clément,
Gade Lutz H.
Publication year - 2002
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20020916)8:18<4308::aid-chem4308>3.0.co;2-t
Subject(s) - chemistry , palladium , ligand (biochemistry) , enantiomer , pyrrole , stereochemistry , ring (chemistry) , chlorobenzene , racemic mixture , medicinal chemistry , absolute configuration , crystallography , catalysis , organic chemistry , biochemistry , receptor
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 [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: Δ H ≠ =68 kJ mol −1 , Δ S ≠ =−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.