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.

Supramolecular Chirality in Porphyrin Chemistry

“…This ability utilizes a new principle of enantioselectivity, which is based upon only a two-point host-guest interaction mode in combination with the coupling electronic transitions of the chromophoric host, the relative orientation of which is totally controlled by the guest stereochemistry. Bis-porphyrinoids on the basis of C21-alkylated N-confused porphyrin served as other examples in this category [52]. Hence, the methylene-and o-xylene-linked bis-porphyrinoids, 56-58 (Figure 17), the binding modes of which involved one or two C21 atoms, were obtained and optically resolved.…”

Optically Active Porphyrin and Phthalocyanine Systems

“…540 nm). The ortho-xylene linked system 62 consists of a C21-alkylated Ni(II) ion and an N2-alkylated metal-free NCP ligand (Figure 98), 121 the most pronounced CD band does not lie near the Soret-type band of the absorption spectrum at approximately 430 nm; therefore, the CD spectrum did not indicate excitonic coupling between the chromophores but rather exhibited a band morphology similar to that observed in the spectrum of the monomer. This result indicated that the NCP chromophores were relatively far from each other, which was confirmed by analysis of the solid-state structure, which revealed that the mean planes of the NCP were nearly perpendicular (Figure 99).…”

“…121 For example, it has been demonstrated that the dimeric systems of 66a−66b, which are comprised of coordinating linkages between the optically active moieties via the Pt(II) ion, are diastereoselective (Figure 104). 121,225 The system remains chiral due to differing coordination modes for the subunits and the rigid helical structure. The enantiomers have CD spectra that are near perfect mirror images (Figure 105).…”

Blooming of confused porphyrinoids—fusion, expansion, contraction, and more confusion

“…15d), 90,91 and so on. [92][93][94][95][96][97][98][99] Additionally, oxidation of Ni II (NCTTP 2À ) in pyridine gives Ni III (21-oxo-NCTTP 3À )(py) (Fig. 15b).…”