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.