N-heterocyclic carbenes in FLP chemistry

The use of N-heterocyclic carbenes (NHCs) for the design and construction of frustrated Lewis pairs (FLPs) is outlined in this review. Stable carbene-borane adducts are briefly discussed, followed by a detailed survey of the reactivity of NHCs, in particular Arduengo-type imidazolin-2-ylidenes, towards B(C6F5)3. Structural and electronic NHC modification by variation of substituents and ring-size affords either stable normal adducts or FLPs, which undergo manifold deactivation reactions in the absence of substrates, e.g., by abnormal adduct formation, self-dehydrogenation, and other types of C-H and C-F bond activation. The degree of frustration is correlated with the calculated energies for the formation of the normal adducts NHC·B(C6F5)3, allowing one to predict and rationalize their reactivity towards small molecules such as dihydrogen, ethers, alkynes, main group elements, carbon dioxide, and nitrous oxide. Other carbene-based FLP systems with a variety of alternative Lewis acids such as boranes, alanes, and carbon-based Lewis acids are also covered, whenever "frustration" is observed. A brief introduction of structurally related bifunctional FLPs based on borylated N-heterocycles is also included.