Enantiomer resolution of intrinsically chiral C21-alkylated N-confused porphyrin complexes

Abstract

The resolution of stereoisomers of C21-alkylated nickel(II) complexes of N-confused porphyrin (NCP) was performed by means of chiral-phase HPLC with an effectiveness of above 90 % molar ratio for each isomer. The reverse signs of the Cotton effects in the circular dichroism (CD) spectra of the separated fractions are indicative of the pair of enantiomers. The application of low-temperature 2D NMR methods to the separated diastereomers of the system comprising a chiral 2-(S)-methylbutyl substituent, in connection with the CD spectra and relative HPLC migration rates, allowed the assignment of the absolute configuration of the chiral C21-substituted complexes of NCP. The assignment was confirmed by time-dependent DFT (TDDFT) calculations of CD spectra for the C21-methylated nickel(II) complex. The system remains chiral after removal of the metal ion from the macrocyclic crevice, despite the fact that this demetalation is connected with a change of the C21 hybridization from pyramidal to trigonal. The retention of chirality was established by means of CD spectra and confirmed by TDDFT calculations for a C21-methylated NCP free base. Stereoisomers were also separated for three covalently linked bis(NCP) systems with bridges involving one or two C21 carbon atoms. The occurrence of a pair of enantiomers was established for nonsymmetrical dimers comprising only one stereogenic center. In the case of the 21,21'-(o-xylene)-linked dimer, three stereoisomers, that is, a pair of enantiomers and an optically inactive meso-form, were separated and analyzed by CD and (1) H NMR spectroscopy. The stereoisomers of a diastereoselectively formed nonsymmetrical chloroplatinum(II)-linked dimer, consisting of heterochiral C21-alkylated NCP nickel(II) subunits, after separation displayed a strong optical activity, which can be ascribed to the rigid helical structure of the complex.