Cleavage of N–H Bond of Ammonia via Metal–Ligand Cooperation Enables Rational Design of a Conceptually New Noyori–Ikariya Catalyst

The asymmetric transfer hydrogenation (ATH) of ketones/imines with Noyori–Ikariya catalyst represents an important reaction in both academia and fine chemical industry. The method allows for the preparation of chiral secondary alcohols/amines with very good to excellent optical purities. Remarkably, the same chiral Noyori–Ikariya complex is also a precatalyst for a wide range of other chemo- and stereoselective reductive and oxidative transformations. Among them are enantioselective sulfonamidation of acrylates (intramolecular aza-Michael reaction) and carboxylation of indoles with CO₂. Development of these catalytic reactions has been inspired by the realized cleavage of the N–H bond of sulfonamides and indoles by the 16e^– amido derivative of the 18e^– precatalyst via metal–ligand cooperation (MLC). This paper summarizes our efforts to investigate N–H bond cleavage of gaseous ammonia in solution via MLC and reports the serendipitous discovery of a new class of chiral tridentate κ³[N,N′,N″] Ru and Ir metallacycles, derivatives of the famous M–FsDPEN catalysts (M = Ru, Ir). The protonation of these metallacycles by strong acids containing weakly coordinating (chiral) anions generates ionic complexes, which were identified as conceptually novel Noyori–Ikariya precatalysts. For example, the ATH of aromatic ketones with some of these complexes proceeds with up to 99% ee.