Aza-Michael Addition of Dinitrogen to α,β-Unsaturated Carbonyl Compounds in a Dititanium Framework

The direct use of dinitrogen (N₂) as a building block for the synthesis of NN-containing organic compounds is of fundamental interest and practical importance but has remained a formidable challenge to date. Here, we report an unprecedented 1,4-conjugate (aza-Michael) addition of N₂ to α,β-unsaturated carbonyl compounds in a dititanium framework. The resulting hydrazinopropenolate products could be easily converted to diverse NN-containing organic compounds such as β-hydrazine-functionalized esters and amides, pyrazolidinones, and pyrazolines depending on the types of Michael acceptors through protonation with MeOH. Further transformations of a hydrazinopropenolate titanium complex through C-C and N-C bond formations with electrophiles such as CO₂ and benzaldehyde have also been achieved. The mechanistic details of the N₂ addition reaction have been elucidated by computational studies, revealing the importance of redox-active metal centers in this event. This work showcases the potential of using N₂ as a building block for the synthesis of NN-containing organic compounds through activation and functionalization in a molecular metal framework.