Pyrazole is a 5-membered aromatic N-heteroatom with its importance widely found in pharmaceuticals and bioactives. Our group has recently developed a Ti-mediated oxidative N-N coupling pyrazole synthesis reaction, in which the N-N coupling is achieved by an oxidation-induced formal reductive elimination. Ti has been found to be at its highest formal oxidation state during reductive elimination with the absence of redox non-innocent ligand. Instead, the reduction elimination is triggered by a one-electron oxidant, followed by a disproportionation. However, this reaction is limited by the stoichiometric use of Ti and oxidant. In order to develop a catalytic reaction based on the Ti-mediated pyrazole synthesis, we are driven to gain a deeper understanding of the oxidation-induced N-N reductive elimination process. A series of kinetic studies have been performed on TEMPO oxidation of Ti complexes monitored by nuclear magnetic resonance (NMR) spectroscopy, from which an empirical rate law has been determined. A Ti-mediated photooxidative pyrazole synthesis with Ru(bpy)3Cl2 photooxidant has been developed as a model reaction undergoing solely on the outer sphere electron transfer pathway. This mechanistic study will provide new understandings of the role of oxidant in oxidation-induced N-N reductive elimination and lead to the development of novel catalysis methods.

Oxidation-Induced Formal N-N Reductive Elimination: An Inner-Sphere or Outer-Sphere Process?

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