Selective Oxidation of an Active Intramolecular Amine/Borane Frustrated Lewis Pair with Dioxygen

Oxidation reactions are rarely encountered in frustrated Lewis pair (FLP) chemistry. We describe the selective reaction of an amine/borane FLP with molecular oxygen. The trimethylene-bridged amine/borane frustrated Lewis pair was prepared by hydroboration of N-allyl-tetramethylpiperidine with Piers' borane [HB(C6F5)(2)]. The intramolecular N/B system undergoes a variety of typical FLP reactions. It is a very active hydrogen splitting reagent at ambient conditions. Its reaction with dioxygen takes place rapidly at ambient conditions to give a tetrahydroisoxazolium/borate salt. A possible pathway of its formation is proposed and supported by DFT calculations.