Molecule-Induced Radical Formation (MIRF) Reaction of Et2BOOEt with Et3B is Key to the Et3B/O2 Radical Initiation

Open-to-air triethylborane (Et3B) is a well-known room-temperature initiator of free radical reactions. Current consensus is that the initiating species in this system are ethyl radicals formed in the triethylborane autoxidation cycle, although some early studies suggested that reactions of the oxidation products can also contribute to the initiation. We report a quantitative mechanistic study which demonstrates that most ethyl radicals in the triethylborane initiator system are not formed in the autoxidation cycle but instead are generated by a Molecule-Induced Radical Formation (MIRF) reaction between triethylborane and its main oxidation product, ethylperoxyborane Et2BOOEt (secondary initiation). A simplified kinetic model allowed us to predict optimal conditions for the initiation, and propose the MIRF reaction as an alternative, homogeneous initiator system. The high efficiency of this initiator has been demonstrated in several model radical reactions.