A Frustrated and Confused Lewis Pair

Abstract

We report a new class of frustrated Lewis pairs (FLPs) by the hydroboration of bulky isocyanates ArNCO ( Ar=2,6-iPr C H ) and ArNCO ( Ar=2,6-Ph -4-tBuC H ) with Piers' borane (HB(C F ) ). While hydroboration of smaller isocyanates such as ArNCO leads to isocyanate-N/B FLP adducts, hydroboration of the bulkier 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.

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 the system reported here, however, a reaction with one substrate molecule leads to cleavage of the B−C bond, in return furnishing a boron center with a free coordination site. In case of addition of PhNCO to 1 , the resulting molecule 6 even has a very similar structural motif to the postulated FLP structure of the “confused” Lewis pair reported by Grimme and Warren et al . It also has to be noted that the reactivity of 1 with the (hetero)cumulenes X=C=Y is similar to known P/B‐based FLP systems and systems with adjacent p‐orbitals like the phosphinoboronate ester Ph 2 PBpin (pin=1,2‐O 2 C 2 Me 4 ) reported by Stephan and Westcott et al …”

Hydride oxidation from a titanium–aluminum bimetallic complex: insertion, thermal and electrochemical reactivity

“…Such isocyanate insertion products are uncommon: a related example was recently reported with a boron-based Lewis acid. 62 Despite isocyanate being a weak oxidizing agent, the product retains the Ti( iii ) center; one hydride reduced the backbone carbon and two hydrides were transferred to an isocyanate fragment which was presumably the source of the oxo moiety, however no other products could be identified. Bulky isocyanates such as bis(2,6-diisopropylphenyl)isocyanate did not lead to isolable products.…”

On 1,3-phosphaazaallenes and their diverse reactivity

“…Heteroallenes like carbodiimides, isocyanates, and isothiocyanates have shown a diverse reactivity towards the per-uorinated boranes B(C 6 F 5 ) 3 and HB(C 6 F 5 ) 2 (Pier's borane), ranging from the development of new heterocycles to the formation of classic and frustrated Lewis pairs (FLPs) and 1,2hydroboration reactions. 41,[55][56][57][58] Treatment of 3a with B(C 6 F 5 ) 3 in toluene afforded a colorless suspension (Scheme 6, top). Aer stirring for 16 h and subsequent workup, 20 the isolated colorless solid was hardly soluble in aromatic hydrocarbons and started to polymerize tetrahydrofuran within minutes.…”