Synthesis of Highly Stable and Reactive Nitrile N-Oxides and Their Application as Catalyst-Free Crosslinkers

Nitrile N-oxide-based click ligation is a very promising technique for polymer crosslinking due to the unique advantages of catalyst-free and byproduct-free reactions. For this technique, nitrile N-oxides that are sufficiently stable at room temperature and have high reactivity under mild conditions are very desirable. Usually, neighboring bulky substituents are introduced to enhance the stability of nitrile N-oxides, which however bring the reactivity down due to the high steric hindrance. Herein, methyl groups, small-sized substituents, were introduced into the o,o′-positions of phenyl nitrile N-oxides to enable high stability and high reactivity. Thus, several novel aromatic nitrile N-oxides were synthesized. Among them, ditopic aromatic nitrile N-oxides were prepared through three simple steps including etherification, oximation, and oxidative dehydrogenation, whereas stable tritopic aromatic nitrile N-oxides were synthesized by directly introducing the nitrile N-oxide functionality into the corresponding trifunctional compounds to avoid undesirable side reactions. These nitrile N-oxides as ligation reagents exhibited very high reactivity, and efficient catalyst-free crosslinking of natural rubber was achieved at room temperature to give elastomers with good mechanical properties, revealing the potential of such nitrile N-oxides as useful crosslinkers for unsaturated bond-containing polymers. This work provides reliable synthesis of stable ditopic/tritopic aromatic nitrile N-oxides, broadening the application of the nitrile N-oxide-based click ligation technique toward catalyst-free and byproduct-free polymer crosslinking.