Tridentate nitrogen ligands derived from 2,6-bis-hydrazinopyridine (BHP) : preparation and study of the 2,6-bis-hydrasonopyridines, 2, 6-bis-pyrazolylpytidines, and 2,6-bis-indazolylpyridines.
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Duncan, Nathan C.
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The development of ligands for asymmetric catalysis has been a focal point in our research group. Tridentate nitrogen ligands have been used in a variety of asymmetric catalytic reactions. Of these, the 2,6-bis-pyrazolylpyridine class of ligands has found only limited use, due to difficulties in the normal synthetic route that limit the synthesis of ligands with bulky chiral groups attached to the pyrazole rings. The chiral derivatives that have been used in catalysis have shown modest to poor results, due in part to limitations in the synthesis of these ligands that prevent the development of bulky, chiral ligands. A new synthetic route has been developed using 2,6-bis-hydrazinopyridine (BHP). This route allows for a facile, one-pot synthesis of 2,6-bis-pyrazolylpyridines that is not limited by the bulkiness of the groups that become attached to the pyrazole ring. The primary focus of this research has been the development of new chiral 2,6-bis-pyrazolylpyridines and an investigation into the limitations of steric bulk on chelation ability in this class of ligands. For this purpose, several novel chiral ligands have been synthesized. The effect of sterics and electronics on the regioselectivity of the formation of the pyrazole ring was also studied in order to develop more regioselective routes to this class of ligands. The new bulky pyrazolylpyridine ligands that have been synthesized using this route have also been tested for the ability to sucessfully coordinate a transition metal. This study lead to a better understanding of the limitations the size of the substitutents attached to pyrazole ring have on the ligands’ ability to chelate metals. While BHP was developed primarily for the synthesis of the 2,6-bis-pyrazolylpyridine class of ligands, its use has now been expanded to the synthesis of other classes of tridentate-nitrogen ligands, many of which would be diffiuclt or impossible to synthesize through any other route. Using this new methodolgy, the syntheses of two novel classes of ligands, each with unique properties, the 2,6-bis-hydrazonopyridines and 2,6-bis-indazolylpyridines, have now been accomplished. The previously unknown 2,6-bis-indazolylpyridine class of ligands is offers the possibility to synthesize more robust catalysts than is possible using the bis-pyrazole ligands because of the electronic nature of the indazole ring.