A Frustrated and Confused Lewis Pair


We report a new class of frustrated Lewis pairs (FLPs) by the hydroboration of bulky isocyanates iPr2ArNCO (iPr2Ar=2,6-iPr2C6H3) and Ph2tBuArNCO (Ph2tBuAr=2,6-Ph2-4-tBuC6H2) with Piers’ borane (HB(C6F5)2). While hydroboration of smaller isocyanates such as iPr2ArNCO leads to isocyanate—N/B FLP adducts, hydroboration of the bulkier Ph2tBuArNCO 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.

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