A Frustrated and Confused Lewis Pair

Abstract

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.

α‐Borylated Phosphorus Ylides (α‐BCPs): Electronic Frustration within a C−B π‐Bond Arising from the Competition for a Lone Pair of Electrons

“…In case of addition of PhNCO to 1,t he resulting molecule 6 even hasavery similars tructural motif to the postulated FLP structureo ft he "confused" Lewis pair reported by Grimme and Warren et al [24] It also has to be noted that the reactivity of 1 with the (hetero)cumulenes X=C=Yi ss imilart ok nown P/Bbased FLP systems and systemsw ith adjacent p-orbitals like the phosphinoboronate ester Ph 2 PBpin (pin = 1,2-O 2 C 2 Me 4 )r eported by Stephan and Westcott et al [25] Overall, the presented a-borylated phosphorus ylide (a-BCP) demonstrates that it is indeed possible to generate frustration or at least ah ighly polarized, very reactive bond in aC /Bbased p-bond by "restraining" the electron pair by competition for the lone pair of electrons. This is emphasized by the fact that reactivity of 1 with CO and other small molecules is found, even thoughn oe lectron-withdrawings ubstituents at the boron atom or ring strain effects are employed.…”