Resolution of topologically chiral molecular objects

Abstract

The enantiomers of topologically chiral molecular objects cannot be interconverted by a continuous deformation. It must be noted that this behavior is opposite to that of molecules showing classical or Euclidian chirality. Interlocked oriented rings and the trefoil knot are prototypical topologically chiral objects. We have been designing a transition-metal templated route to the corresponding real molecules (i.e., [2]-catenanes and molecular knots) using copper(I) as the template and 2,9-diphenyl-1,10-phenanthroline (dpp) based ligands. The precursor to the catenane species was a tetrahedral Cu(dpp)2+ complex in which the two ligands fit in around Cu(I), thereby allowing the interlocking process. In the case of the knot, the precursor was a double-stranded helical complex in which two bis-phenanthroline ligands wrap around two Cu(I) ions. The compounds were obtained as racemates and were resolved as their Cu(I) complexes either by HPLC on chiral stationary phases or by diastereoselective crystallization. For the trefoil knot (K · 2Cu+), the latter method proved to be the most efficient. It took advantage of the dicationic nature of the species to be resolved. The original triflate anion was exchanged by an optically pure anion (S)-(+)-1,1′-binaphthyl-2,2′-diyl phosphate ((+)-BNP-) and the diastereomer (+)-K · 2Cu+ · 2(+)BNP- was selectively crystallized in a mixture of nitromethane and benzene. Removal of the Cu(I) ions by treatment with cyanide afforded an enantiomerically pure molecular knot (K), showing an optical rotatory power close to +2,000° · mol-1 · L · dm-1. Chirality 10:125–133, 1998. © 1998 Wiley-Liss,Inc.