Azoarenes have several different applications in chemical industry as dyes, radical reaction initiators, indicators, food additives, therapeutic agents, electronics, drug delivery agents. Currently, azoarenes are synthesized by primarily stoichiometric pathways including azo coupling, condensation of nitroso compounds with anilines, oxidation of aniline and reductive coupling of nitro compounds. Many of the above routes produce significant amount of waste because those reactions need stoichiometric amount of chemical oxidants. My research targets catalytic formation of azoarenes via nitrene coupling, using organic azides as precursors. This route to azoarenes is more efficient and sustainable because organoazides are readily synthesizable molecules from less expensive respective anilines and the only by-product of this reaction is dinitrogen. Our lab reported catalytic formation of azoarenes from ortho substituted bulky aryl azides using iron catalyst ligated by two bulky alkoxides Fe(OR)2(THF)2 (OR = OCtBu2Ph). This complex enabled highly efficient catalytic transformation of bulky aryl azides ArN3 (Ar = 2,4,6-Me3Ph or 2,6-Et2Ph) to corresponding azoarenes. However, no azoarene formation was observed for smaller aryl azides (Ar = 2-MePh, 3,5-Me2Ph, 4-MePh). Instead, formation of mono(alkoxide) bridging imido complexes, Fe2(μ2-NAr)2(OR)2(THF)2 was observed, along the formation of unreactive iron tris(alkoxide) Fe(OR)3 by-product. The alkoxide disproportionation ([Fe(OR)2] → [Fe(OR)] + Fe(OR)3) was postulated to be responsible for the observed limitations in catalysis. More recently, we reported the synthesis of bulkier alkoxide [OR’] [OR’ = OCtBu2(3,5-Ph2Ph)] ligand and its Fe(OR’)2(THF)2 complex. We hypothesized that the larger size of the alkoxide would prevent formation of Fe(OR)3 and as a result, it would prevent alkoxide disproportionation and thereby would enable better catalytic reactivity with both non-bulky and bulky azides. According to our expectations, OR’ leads to a catalytic nitrene coupling for wider variety organoazide precursors, including various ortho- and meta-substituted aryl azides. We were also able to obtain insight into the reaction mechanism, via the isolation and characterization of iron-tetrazene inetrmediates. Recently, we synthesized a new chelating bis(alkoxide) ligand [OO] and its iron(II) complex Fe[OO](THF)2. This new system enables selective reactivity for the coupling of non-bulky (i. e. phenyl) aryl azides.

Comparison of reactivity of iron alkoxide complexes towards azoarene synthesis

2019 Fall National Meeting

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