Abstract
Selective amination of σ and π entities such as C-H and C=C bonds of substrates remains a challenging endeavor for current catalytic methodologies devoted to the synthesis of abundant nitrogen-containing chemicals. The present work addresses an approach toward discriminating aromatic over aliphatic alkenes in aziridination reactions, relying on the use of anionic metal reagents (M = Mn, Fe, Co, Ni) to attenuate reactivity in a metal-dependent manner. A family of Mn(II) reagents bearing a triphenylamido-amine scaffold and various pendant arms has been synthesized and characterized by various techniques, including cyclic voltammetry. Aziridination of styrene by PhI=NTs in the presence of each Mn(II) catalyst establishes a trend of increasing yield with increasing Mn(II/III) anodic potential. The Fe(II), Co(II), and Ni(II) congeners of the highest-yielding Mn(II) catalyst have been synthesized and explored in the aziridination of aromatic and aliphatic alkenes, exhibiting good to high yields with para-substituted styrenes, low to modest yields with sterically congested styrenes, and invariably low yields with aliphatic olefins. Co(II) mediates faster styrene aziridination in comparison to Mn(II) but is less selective than Mn(II) in competitive aziridinations of conjugated versus nonconjugated olefins. Indeed, Mn(II) proved to be highly selective even versus well-established copper and rhodium aziridination reagents. Mechanistic investigations and computational studies indicate that all metals follow a two-step styrene aziridination pathway (successive formation of two N-C bonds), featuring a turnover-limiting metal-nitrene addition to an olefinic carbon, followed by product-determining ring closure. Both steps exhibit activation barriers in the order Fe > Mn > Co, most likely stemming from relevant metal-nitrene electrophilicities and M(II/III) redox potentials. The aziridination of aliphatic olefins follows the same stepwise path, albeit with a considerably higher activation barrier and a weaker driving force for the formation of the initial N-C bond, succeeded by ring closure with a miniscule barrier.