Synthesis and Applications of 2-Quinoxalinol Salens and Their Metal Complexes

Abstract

Salen ligands have been of interest to a wide variety of chemists for several reasons. They have been investigated in large part due to their ease of preparation and their ability to form stable metal complexes. Derivatives of 2-quinoxalinol are key intermediates as bioactive agents in agriculture, dyes, and have been key pharmaceutical intermediates. The combination of salen and 2-quinioxalinol, 2-quinioxalinol salens (salqu) are expected to be of use in the development of new catalysts, sensor and pharmaceutical agents. In the first portion of this work, a solution-phase parallel method for the synthesis of salqu ligands was designed and optimized. Simple laboratory techniques with low sensitivity to moisture and air for solution-phase parallel reactions were coupled with convenient workup and purification procedures to give high-purity and good yields for a small salqu ligands library of 20 compounds. During preparation of this library, it was found that upon reaction with salicylaldehydes, 2-quinoxalinol imines were obtained as byproducts. Diamino-2-quinoxalinols produce 2-quinoxalinol imines regioselectively as the only isomer in good yield. The regioselectivity of this reaction was determined by use of isotopic 15N labeling experiments. The 2-quinoxalinol imines may then be reacted to yield unsymmetrical substituted salqu ligands. Salqu ligands can be further reacted with different metal salts to form salqu metal complexes. Salqu metal complexes were usually applied as pharmaceutical agents and catalysts for many asymmetric reactions. They can be prepared with simple workup process and are usually stable to both air and moisture condition. These complexes present one option for the extraction of metal ions from wastes or waste streams. To take further advantage of these properties, solid phase catalysts have become popular in organic synthetic methods because of increased ease of workup after reactions. Recently, SPE (solid phase extraction) technology has been well developed for use in many purposes. Based on these, salqu ligands and salqu metal complexes, solid phase salqu ligands and their complexes demonstrate promise in new research fields for their catalytic application. Finally, salqu copper complexes have been found to be a kind of high efficient catalysts for the oxidization of C-H bond of aryl methylene compounds.