Protic Media Enhanced Protodeboronation for a Potential H2O2-Sensitive Ligand System

The detection of elevated concentrations of reactive oxygen species (ROS) such as H2O2 represents a promising route to identify oxidative misregulation in cells, a hallmark of carcinoma and neurodegeneration. Magnetic resonance imaging (MRI) presents an opportunity to detect ROS over a greater portion of the body compared to strategies such as fluorescence. To sense hydrogen peroxide, potential Fe2+ and Co2+ MRI sensors were synthesized by appending pinacolborane to two amino-pyridine ligand scaffolds. These were designed to expose phenol upon reaction with H2O2, turning on PARACEST (Paramagnetic Chemical Exchange Saturation Transfer) and allowing detection by MRI. Instead of yielding a phenol upon exposure to oxidative conditions, protodeboronation was observed as the major product even under mild conditions. Optimization of oxidation conditions allowed access to the phenolic analogue of the two ligands studied in 11% and 17% conversion under organic conditions. Treatment of the Fe2+ and Co2+ complexes with H2O2 resulted in rapid, partial demetallation with no evidence of the bound or free ligand undergoing an oxidative transformation. These results stress the importance of establishing stability of potential MRI contrast agents in aqueous conditions.