Chiral sulfinyl-functionalized pyrroles and pyrrolic tetradentate ligands

Chiral pyrroles have been interesting to chemists for their use in a wide range of fields including medicinal chemistry and asymmetric catalysis. Pyrrole-containing dipyrrins and iminopyrroles have been studied as ligands in metal complexes, but chiral examples are rare. The supramolecular arrangement of tetradentate bis(iminopyrrole)s and bis(dipyrrin)s with tetrahedrally coordinated metal ions is of interest as they often form chiral helical structures. Our group has recently reported the highly diastereoselective assembly of chiral bis(dipyrrin) ligands with the tetrahedrally coordinated metal ion zinc (II). This advance led to a further interest in the ability for tetradentate pyrrolic ligands adorned with various chiral groups to impose diastereoselectivity when self-assembling with tetrahedrally coordinated metal ions to form metallohelicates. As such, we ventured to design and synthesize bis(dipyrrin) and bis(iminopyrrole) ligands incorporating chiral moieties not previously explored.;With the ultimate aim of preparing a new ligand set for use in asymmetric catalysis, sulfinyl-affixed, pyrrole-containing bi-, tri- and tetradentate ligands was synthesized. The synthesis of (S)-5-(4-tolylsulfinyl)-1H-pyrrole-2-carbaldehyde gave access to a pyrrole bearing a homochiral sulfinyl group, and an aldehyde handle that was useful for integrating the chiral pyrrole into more complex constructions. Using this methodology, three analogues of bis(iminopyrrole)s containing sulfinyl groups were synthesized. Although the ligands, and their zinc complexes, underwent hydrolysis far more readily than their non-sulfinyl-affixed analogues, the studies suggested only one helical isomer of the corresponding metal complexes was formed when the chiral ligand is coordinated to the zinc(II) ion. Complementary aminopyrroles and bis(aminopyrrole)s bearing chiral sulfinyl groups were synthesized through the reduction of the previously mentioned bis(iminopyrrole)s.;Finally, a dipyrrin and four examples of bis(dipyrrin)s bearing the homochiral sulfinyl group were synthesized. Although the ligands were found to be prone to decomposition at room temperature, preliminary studies on the stability of their corresponding zinc complexes indicated they were much more stable at ambient conditions.;The use of sulfoxides as pyrrolic protecting groups capable of dramatically reducing the nucleophilicity of the heterocycle drew our interest to the use of a sulfoxide as a chiral directing group in our ligand design. The sulfinyl group is stereogenically stable when directly attached to pyrrole, allowing the chiral group to be in close proximity to the coordination site, thereby increasing its potential as a directing group. The sulfinyl oxygen atom has been known to take part in coordinative metal bonding, and this further intrigued us in terms of directing ability.