A Frustrated and Confused Lewis Pair

We report a new class of frustrated Lewis pairs (FLPs) by the hydroboration of bulky isocyanates ^iPr2 ArNCO (^iPr2 Ar=2,6-iPr₂ C₆ H₃ ) and ^Ph2tBu ArNCO (^Ph2tBu Ar=2,6-Ph₂ -4-tBuC₆ H₂ ) with Piers' borane (HB(C₆ F₅ )₂ ). While hydroboration of smaller isocyanates such as ^iPr2 ArNCO leads to isocyanate-N/B FLP adducts, hydroboration of the bulkier ^Ph2tBu ArNCO allows isolation of the substrate-free aminoborane with a short, covalent N-B bond. This confused FLP reversibly binds unsaturated substrates such as isocyanates and isocyanides, suggesting the intermediacy of a "normal" FLP along the reaction pathway, supported by high-level DFT studies and variable-temperature NMR spectroscopy. These results underscore the possibility of FLP behavior in systems that possess no obvious frustrated Lewis acid-base interaction.