Metal complexes of a biconcave porphyrin with D4-structure--versatile chiral shift agents.

Representative metal complexes of a biconcave D4-symmetric porphyrin were synthesised by metalion insertion into the porphyrin ligand 1. The NMR spectra suggested D4-symmetry for the ZnII and dioxo-RuVI complexes of 1 and C4-symmetry for the unsymmetrically ligated RuII and RhIII complexes. Metal complexes of 1 proved to be versatile chiral 1H NMR shift agents for a broad spectrum of organic amines, alcohols, carboxylic acids, esters, nitriles and nonpolar fullerene derivatives. A practical analysis of chiral substrates with 1 covers enantiomeric excesses beyond 99%. An X-ray structure of (1:1)-cocrystals of an achiral, biconcave CoII porphyrinate and C60 provided the first detailed insights into the structure of such a biconcave metallo-porphyrinate. It also showed remarkable packing of the carbon sphere against the main concave units of the porphyrin and gave clues about the relevant interactions between biconcave porphyrins and fullerenes.

A highly enantiopure biconcave porphyrin with effective D4-structure

A first representative of an effectively D4-symmetric biconcave porphyrin (1) was prepared from a tetramerizing condensation of a C2-symmetric pyrrole (2). The chiral pyrrole 2 was synthesized in a six-step reaction sequence starting from the C2h-symmetric 2,6-di-tert-butylanthracene. The relevant stereochemistry was introduced in a highly diastereo-discriminating Diels-Alder reaction with fumaric acid di(-)menthyl ester, catalyzed by aluminum chloride. X-ray analyses of two of the dimenthyl esters prepared unambiguously secured their tentatively assigned absolute configuration and that of the pyrrole 2 (as the S,S isomer). The enantiomeric purity of the pyrrole 2 was determined as 99% ee, using the Co11 complex of the porphyrin 1 as a chiral shift reagent. The pyrrole 2 lent itself to a stereochemically nearly uniform preparation of the chiral, biconcave porphyrin 1. Applying Horeau's principle, 1 was calculated to be present in an enantiomeric excess of about 10(9):1. The validity of the statistical considerations relevant for this estimate were verified by examination of the results from preparative tetramerization experiments in which the enantiomeric purity of the pyrrole 2 was deliberately lowered.