N-Heterocyclic carbenes (NHCs), [1] one class of stable and isolable carbenes, [2] are a well-studied ligand that can often replace phosphines in organometallic complexes, which have found extensive use in catalysis. [3,4] NHCs are also utilized as organocatalysts in their own right, [5][6][7] and there are growing number of reports of unique and unexpected reactivity of a variety of stable carbenes. For example, recent studies from Bertrand and co-workers showed that certain (amino)-(alkyl)carbenes react directly with dihydrogen and ammonia, [8] mimicking metalloid oxidative addition processes (Scheme 1 a). Small main-group species, such as P 4
[9, 10] also react with carbenes. Current work from our group involves the reactivity of "frustrated Lewis pairs" (FLPs), [11,12] a combination of Lewis acids and bases in which steric encumbrance prevents or limits adduct formation. These unquenched sites can have unique reactivity. For example, mixtures of bulky phosphines and boranes react directly with dihydrogen [13,14] and olefins (Scheme 1 b). [15,16] Furthermore, the compound (2,4,6-Me 3 C 6 H 2 ) 2 P(C 6 F 4 )B(C 6 F 5 ) 2 is the first example of a main group system that can reversibly react with dihydrogen (Scheme 1 c). [17] The hydrogen-activating ability of these species has been exploited in metal-free catalytic reduction of imines, nitriles, and aziridines. [18,19] However, the
[*] Dr.