Next-Generation D2 -Symmetric Chiral Porphyrins for Cobalt(II)-Based Metalloradical Catalysis: Catalyst Engineering by Distal Bridging.

Novel D2 -symmetric chiral amidoporphyrins with alkyl bridges across two chiral amide units on both sides of the porphyrin plane (designated "HuPhyrin") have been effectively constructed in a modular fashion to permit variation of the bridge length. The CoII complexes of HuPhyrin, [Co(HuPhyrin)], represent new-generation metalloradical catalysts where the metal-centered d-radical is situated inside a cavity-like ligand with a more rigid chiral environment and enhanced hydrogen-bonding capability. As demonstrated with cyclopropanation and aziridination as model reactions, the bridged [Co(HuPhyrin)] functions notably different from the open catalysts, exhibiting significant enhancement in both reactivity and stereoselectivity. Furthermore, the length of the distal alkyl bridge can have a remarkable influence on the catalytic properties.

Synthesis of chiral N-phosphoryl aziridines through enantioselective aziridination of alkenes with phosphoryl azide via Co(II)-based metalloradical catalysis.

The Co(II) complex of a new D 2-symmetric chiral porphyrin 3,5-DiMes-QingPhyrin, [Co(P6)], can catalyze asymmetric aziridination of alkenes with bis(2,2,2-trichloroethyl)phosphoryl azide (TcepN3) as a nitrene source. This new Co(II)-based metalloradical aziridination is suitable for different aromatic olefins, producing the corresponding N-phosphorylaziridines in good to excellent yields (up to 99%) with moderate to high enantioselectivities (up to 85% ee). In addition to mild reaction conditions and generation of N2 as the only byproduct, this new metalloradical catalytic system is highlighted with a practical protocol that operates under neutral and non-oxidative conditions.

Metalloradical activation of carbonyl azides for enantioselective radical aziridination.

Organic azides have been increasingly employed as nitrogen sources for catalytic olefine aziridination due to their ease of preparation and generation of benign N2 as the only byproduct. Among common organic azides, carbonyl azides have not been previously demonstrated as effective nitrogen sources for intermolecular olefin aziridination despite the synthetic utilities of N-carbonyl aziridines. As a new application of metalloradical catalysis, we have developed a catalytic system that can effectively employ the carbonyl azide TrocN3 for highly asymmetric aziridination of alkenes at room temperature. The resulting enantioenriched N-Trocaziridines have been shown as valuable chiral synthons for stereoselective synthesis of other chiral aziridines and various chiral amines. The Co(II)-based metalloradical system, which proceeds with distinctive stepwise radical mechanism, may provide a general method for asymmetric synthesis of chiral aziridines from alkenes with organic azides.

Cobalt(II)-catalyzed intramolecular C-H amination with phosphoryl azides: formation of 6- and 7-membered cyclophosphoramidates.

A highly effective Co(II)-based system has been developed for catalytic intramolecular C-H amination with phosphoryl azides without the need of terminal oxidant or other additives, resulting in the high-yielding production of cyclophosphoramidates with nitrogen gas as the byproduct. Additional features of this new catalytic system include the amination of primary C-H bonds and formation of 7-membered-ring structures.