Obtain a stable N₂ Reduction intermediate from an azo compound containing a Boc protecting group

Boc is the abbreviation of t-Butyloxycarbonyl, which is an amino protecting group commonly used in organic synthesis, especially in peptide synthesis. In chemical research, Boc reagents are often used as pharmaceutical intermediates, protective agents, and amino acid reagents, etc. These uses make it a great contribution to the chemical and pharmaceutical industries.

Nitrogen is the most important component of the atmosphere, but its reactivity is low. Except for a few fungi and algae, most organisms cannot directly use nitrogen. The nitrogen fixation intermediate model can be used to study the reduction mechanism from nitrogen to ammonia, thereby guiding the development of new nitrogen fixation systems.

Some researchers have pioneered a series of transition metal nitrogen fixation center models, which have attracted widespread attention. In recent years, a variety of nitrogen fixation intermediate models based on transition metal centers (such as Fe, Ru, Mo, etc.) have been applied to the mechanism study of nitrogen reduction. However, there is still a lack of easily characterized main group models to study the interconversion process of nitrogen fixation intermediates. Therefore, building a nitrogen fixation intermediate model based on main group elements can provide a new platform to study the reaction properties of active dinitrogen species such as diazene, which will help expand the frontier of nitrogen fixation reactions and promote the development of non-metallic nitrogen fixation systems.

No matter in the process of ammonia synthesis by Haber-Bosch method or in the process of nitrogen fixation by biological nitrogenase, the generation and reduction of active diazene and a series of dinitrogen intermediate species are considered to be the key to the conversion of nitrogen into ammonia. Recently, a series of Lewis-acid-stabilized nitrogen fixation intermediates have been constructed using an in situ deprotection-hydrogenation-coordination strategy.

Starting from an azo compound containing a Boc protecting group, the Boc protecting group was removed under the action of a Lewis acid, and the dinitrogen species generated were hydrogenated in situ and stabilized by the Lewis acid coordination, thereby isolating and fully characterizing the first Examples of Lewis acid - diazene complexes [(C₆F₅)₃B-N₂H₂-B (C₆F₅)₃]. Due to the highly active nature of diazene, this intermediate is unstable at room temperature and will further disproportionate and decompose into Lewis acid-hydrazine complex [(C₆F₅)₃B-N₂H₄-B (C₆F₅)₃] and nitrogen.

The researchers further used the complex reacting with a phosphorus-centered Lewis base, and a proton transfer reaction can occur to obtain the corresponding deprotonated product. Through theoretical calculations, the researchers confirmed that the Lewis acid promoted the departure of the Boc group and the hydrogenation of dinitrogen species through the coordination with the carbonyl group on the Boc group, and simultaneously generated CO₂ and isobutene. These findings lay the foundation for further expansion of the model of metal-free nitrogen fixation intermediates to correlative electron transfer, electron-coupled proton transfer reactions, etc.

Reference

1. Diazene Chemistry: Metal-Free Models of N₂ Reduction Intermediates
   J. Am. Chem. Soc., 2023